Editor’s note:Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.
Humans have always been star-gazers. Since time immemorial we have looked to the heavens and tried to make sense of what we saw there. Thousands of years ago the motions of heavenly bodies and the appearances of comets and meteors were believed to be omens that determined the fates of kings and predicted the downfall of empires. Official court astrologers were employed to read these portents in the sky and divine their (hopefully favourable) meanings.
Many of these ancient observations of celestial events still survive as written records in the form of cuneiform tablets inscribed over two millennia ago. Cuneiform was one of the earliest systems of writing and was typically inscribed on rectangular clay tablets using a blunt reed to produce wedge-shaped marks. Cuneiform tablets have been discovered at archaeological sites all across the Near and Middle East, such as at the ancient Assyrian city of Ninevah (modern-day northern Iraq) — once the largest city in the world. They were used for almost everything, including recording celestial events, documenting laws and religious beliefs, and even for entire literary works — such as the famous Epic of Gilgamesh.
Figure 1: Y. Mitsuma’s tracings of the photographs of cuneiform tablets taken by H. Hayakawa. These cuneiform tablets are preserved in the British Museum. [Mitsuma et al. 2019]
Today’s astronomers track changes in the Sun’s activity using a variety of methods, including examining the occurrence rates of sunspots. Sunspots are associated with solar flares and coronal mass ejections — huge eruptions composed of high-energy protons and electrons that can cause geomagnetic storms when they are directed towards the Earth. A well-known feature associated with particularly strong geomagnetic storms is the occurrence of aurorae, produced when high-energy particles excite atoms in the Earth’s atmosphere. Astronomers have been observing sunspots ever since the advent of the telescope, but if we want to examine the history of the Sun’s activity prior to this we need to turn to historical records of phenomena linked to solar activity, such as aurorae.
A team led by Hisashi Hayakawa, a researcher at Osaka University and the lead author of today’s paper, carried out a survey of tablets dating from the 8th and 7th centuries B.C. looking for references to aurorae, that might match evidence inferred from tree ring samples. Tree rings store information on the environmental conditions present at the time of their formation — and during periods of strong solar activity, they show enhanced concentrations of the radioactive isotope carbon-14, produced when high-energy particles interact with nitrogen atoms in the atmosphere. Studies of tree-ring samples from around 660 B.C. have been shown to contain elevated carbon-14 levels, and the researchers wondered if they could find any historical references to match this inferred spike in solar activity.
Aurora during a geomagnetic storm that was most likely caused by a coronal mass ejection, taken from the International Space Station. [ISS Expedition 23 crew]
The team found three separate Assyrian and Babylonian cuneiform tablets held in the British museum that referred to “red clouds” and a “red glow” lighting up the night sky and that date from the period 679 to 655 B.C. These descriptions are similar to the previously oldest-known reference to an aurora, found in a Babylonian tablet dating to 587 B.C. The researchers suggest that these descriptions probably refer to a type of auroral activity known as stable auroral red arcs, which are produced by the interaction of strong magnetic fields and the electrons in oxygen atoms.
Although we typically think of aurorae as being a northern-latitude (or southern in the case of the Aurora Australis) phenomenon, this has not always been true. Over time the Earth’s geomagnetic poles shift due to changes in the Earth’s magnetic field, and thousands of years ago the north pole would have been located much closer to the Middle East. Furthermore, strong solar activity, such as coronal mass ejections, can cause aurorae to become visible at much more southernly latitudes. It is therefore quite likely that aurorae would have been visible in the region of these observations during this time period.
These findings constitute the oldest-known written evidence of candidate aurorae and potentially extend the history of the Sun’s activity nearly a century beyond previous records. They could also help us to predict future solar storms, which have the capability of interrupting sensitive electronics on Earth and damaging satellites and spacecraft.
About the author, Jamie Wilson:
Jamie is a graduate student at the Astrophysics Research Centre at Queen’s University Belfast.
Editor’s note:Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.
Hubble image of UGC 5497, an example of a dwarf galaxy. Do these small galaxies also host massive black holes at their centers? [ESA/NASA]
At the centre of the most massive galaxies resides a supermassive black hole around which everything rotates. Typically, these black holes are identified by measuring the velocity and shape of the orbits of a galaxy’s innermost stars as they rotate around its centre. However, this approach is less effective when we consider much lighter galaxies like dwarf galaxies. These galaxies are much fainter than their high mass counterparts so their stellar populations can’t be sufficiently resolved by current missions. Instead, a lot of recent work in the field focuses on identifying the energetic process a massive black hole undergoes when it accretes gas and dust, turning the central region of a galaxy into an active galactic nucleus (AGN). By identifying the incidence of AGN, a lower limit on the distribution of black holes in dwarf galaxies can be obtained. Today’s authors adopt this approach to further study their black hole population and produce some surprising results.
Constructing the Sample
AGN emit across the electromagnetic spectrum, but today’s authors decide to use centimetre radio emission, as they argue it isn’t as strongly extinguished by galactic dust. An initial sample of 43,707 dwarf galaxies were identified in the NASA-Sloan Atlas. To maximise the number of AGN detections, the authors first matched this dwarf galaxy sample to archive data from the Very Large Array (VLA). 111 matches were found in the VLA “Faint Image of the Radio Sky at Twenty Centimeters” survey within 5 arcseconds of the galaxy’s optical centre. These 111 objects were re-observed by the VLA in the A-configuration. In this configuration, the antenna dishes are spread out as widely as possible along each arm of the Y-shape to create a dish with an effective diameter of 22 miles; this allows the authors to take full advantage of the VLA’s high spatial resolution. From this, they were left with 35 dwarf galaxies containing 44 significant compact radio sources.
Accounting for Stellar Birth and Death
AGN are not the only objects that can produce compact and intense radio emission; this emission could also point to the births and deaths of stars. Before they could attribute these compact radio sources to AGN, the authors tried to remove the possibility of a stellar origin for the emission.
Areas of a galaxy where large amounts of star formation have taken place, known as HII regions, are dominated by ionised atomic hydrogen. These regions are typically identified by looking for signs of Bremsstrahlung (or braking radiation), emitted when a free electron is slowed down by interaction with the copious amounts of ionised hydrogen present. The authors model the observed compact radio emission as if produced by this process, from which they calculated a thermal star formation rate. Each galaxy also has a star formation rate taken from the NASA-Sloan Atlas, calculated using the dust-corrected ultraviolet light from the whole galaxy. The dark blue and red points in the left-hand panel of Figure 1 highlight the 20 radio sources found to have thermal star formation rates that are much larger than the star formation rates from the NASA-Sloan Atlas. It doesn’t make sense for a subsection of a galaxy to be producing more stars than are being produced in the whole of the galaxy, so the authors can rule out star formation causing emission observed in these 20 radio sources.
Figure 1: Investigating alternative sources of the observed radio emission in each galaxy. The left-hand panel compares the star formation rate from the galaxy as a whole to that predicted by assuming all the radio emission is from Bremsstrahlung. Any object falling below the dashed line is considered to have emission consistent with star-formation; W49 A is a highly star-forming region of the Milky Way included as a comparison. The right-hand panel compares the expected emission from a population of supernovae to the observed emission. The dashed line describes the supernova luminosity function, with the solid lines describing observed luminosities three times brighter and dimmer than the function. Any object below the upper solid line is considered to have radio emission consistent with supernovae emission. [Reines et al. 2019]
Similarly, stars at the end of their lives — supernovae — also produce large amounts of radio emission. To model this emission the authors make use of a supernova luminosity function that describes the predicted supernovae luminosity as a function of the host galaxy’s star formation rate and the observed radio luminosity, shown as a dashed line in the right-hand panel of Figure 1. For each galaxy, the supernova luminosity function is integrated across the full range of possible supernova luminosities and compared to the observed radio luminosities. In the right-hand panel of Figure 1, we again see the emission from these same 20 compact radio sources in the upper left region. This shows, for these 20 radio sources, the observed emission is at least 3 times as bright as the expectation from supernovae emission, so the authors can rule out supernovae causing emission observed in these 20 radio sources as well.
Wandering Black Holes
Since the emission from these 20 radio sources cannot be explained by star formation or supernovae, the authors conclude that they are AGN. Figure 2 shows the positions of 13 of these AGN as red crosses within their dwarf galaxy host. The high spatial resolution of the radio measurement highlights the really interesting result of today’s paper: a number of these AGN are seen well outside the approximate optical galactic centre, indicated by the white circle. Recent simulations predict that roughly half of all massive black holes are expected to be found in the outskirts of their host galaxy. It is believed off-nuclear black holes, such as these, could have been recently stripped from a neighbouring galaxy the host recently interacted with, or perhaps we are seeing the gravitational recoil from two recently merged black holes.
Figure 2: Images of the 13 dwarf galaxies in Sample A believed to host radio-selected AGN. The red cross indicates the very secure radio AGN position; the white circle indicates the region measured by the SDSS, thus the galaxy’s approximate optical centre. Only these objects are shown as the remaining 7 objects in Sample B did not have a second measurement of mass or redshift from the VLA radio measurement thus the authors were unsure whether to consider them dwarf galaxies. [Reines et al. 2019]
Detections of AGN, and the black holes powering them, are still rare in dwarf galaxies compared to the near ubiquity identified in high-mass galaxies. Any additional detections are crucial to furthering our understanding of the true distribution of black holes in dwarf galaxies. Where today’s paper has excelled, though, is by exploiting the high spatial resolution of the VLA to identify a population of black holes outside the centres of their host galaxies. Not only is this a surprising observational result, but it represents one of the first confirmations of simulations that suggest these massive black holes tend to wander the outskirts of their host galaxies.
About the author, Keir Birchall:
Keir is a PhD student at University of Leicester studying methods to identify active galactic nuclei in various populations of galaxies to see what affects their incidence. When not doing science, he can be found behind the lens of a film camera or listening to the strangest music possible.
Editor’s note:Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.
A concept fundamental in astronomy is that stars form from cold, dense molecular gas clouds. Applied to the formation and evolution of galaxies, star-formation is understood as being directly supported by an abundance of this cold molecular gas. We find that in star-forming galaxies — typically disk-like with blue spiral arms — there is a large reservoir of this cold molecular gas from which rapid star-formation can be sustained. Quiescent galaxies, on the other hand — typically elliptical-like with red colors — do not actively form stars. Hence, they have been long thought to have only a scarce supply of cold molecular gas, likely as a result of either vigorous star-formation in the past, some sort of gas reservoir removal event, or a combination of the two.
There is overwhelming evidence that star-forming and quiescent galaxies inhabit very different environments in our universe. While star-forming galaxies are generally found alone or in small groups, quiescent galaxies are often found residing in large galaxy clusters with tens to hundreds of cluster members. This so-called morphology-density relation tells us something fundamental about the role of environment in shaping galaxy properties. In particular, it suggests that the cluster environment influences the ceasing (or quenching) of star-formation.
Figure 1. Archetypal quiescent elliptical galaxies (left) and star-forming spirals (middle) comprise the vast majority of galaxy demographics. Rare and poorly understood red disk spirals remain a puzzling mystery (right). [Masters et al. 2010]
The largest and most massive galaxies typically reside near the center of a cluster. We label them “central” galaxies to contrast them with smaller “satellite” galaxies that reside in the cluster outskirts. Most massive central galaxies are quiescent. One mechanism that has been proposed to explain this cessation of star formation in these particularly massive central galaxies is so-called hot-halo quenching. In this scenario, a massive central galaxy is kept from refreshing its supply of cold molecular gas from its surroundings because the immense gravitational force experienced by the gas as it enters the halo of the central galaxy is so great that it becomes shock heated. Although still accreted into the central galaxy, the now-hot gas becomes unsuitable to form stars.
Today’s paper discusses new and surprising findings about the gas content of these massive central galaxies, and how the galaxies are kept from forming stars.
A statistically large sample of nearby massive central galaxies was carefully selected from the Sloan Digital Sky Survey (SDSS). Instead of focusing on just typical giant central galaxies that have elliptical morphologies, the authors chose to examine the ellipticals side-by-side with little-appreciated massive central disk galaxies. These kinds of galaxy cross-breeds are extremely rare, but potentially interesting objects for understanding the complex and poorly understood mechanisms that drive star-formation cessation. Morphological classifications of elliptical and disk galaxies were taken from the citizen science project Galaxy Zoo, which crowd-sources galaxy classification through an online web platform accessible to anyone.
However, the SDSS data is only half the story. The SDSS galaxies were crossed matched with deep centimeter-wavelength surveys (ALFALFA and GASS) in order to obtain measurements of the atomic gas (H I) in these galaxies. While the atomic gas is too hot to directly form stars, given the right conditions it may cool down into molecular gas (H2).
Figure 2. (Left) the distribution of massive elliptical and disk central galaxies, with the fractional abundance of disk galaxies shown in grey. (Middle) the strength of H I detection as a function of star-formation rate for disk and elliptical central galaxies. (Right) Similarly, the atomic gas mass. [Zhang et al. 2019]
The study reports a surprising finding: nearly all massive central disk galaxies have exceedingly large atomic gas reservoirs, especially when compared to those of ellipticals, as shown in Figure 2. Not only this, but massive disk galaxies with high star-formation rates have almost the same atomic gas mass compared to those with lower star-formation rates. In other words, massive quiescent central disk galaxies are as abundant in atomic gas as star-forming ones.
Importantly, this implies that massive quiescent central disk galaxies do indeed have the raw gas supply necessary to form stars. The lack of star formation then must either be due to a marked inefficiency in converting atomic gas into molecular gas, difficulty in forming stars from the molecular gas, or both.
Figure 3. Atomic (red) and molecular (blue) gas mass as a function of star-formation rate in massive central disk galaxies. [Zhang et al. 2019]
A complementary investigation of the cold molecular gas content of these massive central disk galaxies with the COLD GASS survey suggests that reservoirs of cold molecular gas are, unsurprisingly, significantly smaller for central disk galaxies with low star-formation rates, and vice versa. While this is consistent with our current picture of star formation, it contrasts sharply with the related finding that atomic gas mass does not change with star-formation rate, as shown in Figure 3.
Taken together, this investigation into the gas content of massive central disk galaxies illustrates that although their atomic gas content is universally large compared to central ellipticals, their star-formation cessation is still driven by a dearth of cold molecular gas. Yet, the precise mechanism keeping the atomic gas from eventually giving way to star formation remains elusive.
About the author, John Weaver:
I am a second year PhD student at the Cosmic Dawn Center at the University of Copenhagen, where I study the formation and evolution of galaxies across cosmic time with incredibly deep observations in the optical and infrared. I got my start at a little planetarium, and I’ve been doing lots of public outreach and citizen science ever since.
Editor’s note:Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.
Pulsars. Fast Radio Bursts. Magnetars. The world of high-energy stellar astrophysics has no shortage of weird objects that do not always behave like we think they should. From the mysterious workings inside a neutron star to the unknown reason behind why some fast radio bursts repeat, these sources continue to surprise and mystify us. Now, the world of magnetars, stars with incredibly high magnetic fields, just got a little more interesting.
Magnetars: Not Your Average Stellar Objects
Magnetars (short for “magnetic stars”) are neutron stars with some of the strongest magnetic fields in the universe. Their magnetic field strengths are on the order of ~1015 Gauss; to put this in perspective, the magnetic field of the Earth that shields us from the Sun’s rays and produces auroras is about 0.5 Gauss. If a magnetar was at a distance from Earth equal to that of the Moon, it could strip the information off of all of the credit cards on the planet.
Artist’s illustration of a pulsar, a fast-spinning, magnetised neutron star. [NASA]
In 2006, magnetar XTE J1810-197 (which is also classified as an X-ray pulsar because in addition to having a very strong magnetic field, it intermittently emits X-rays) was found to be emitting radio pulses after a very strong outburst of energy in the radio-frequency regime. At the beginning of this outburst, the pulsar had a nearly flat spectral index. The spectral index tells you how much the total power from the source is dependent on frequency — so if the spectral index was flat, it means that the power emitted was roughly the same at all frequencies. During that burst, radio emission came in spikes that lasted about 10 milliseconds. After the outburst, the source faded in power and essentially turned off before it was re-observed 13 years later and a second radio burst was detected by the authors of today’s work. In Figure 1, you can see how the power emitted by the source declined over time but increased over frequency. Similar “spiky” short-duration radio pulses have been seen in high-energy phenomena such as giant pulses (essentially really bright radio pulses that are occasionally emitted by some sources) and fast radio bursts (FRBs). With these similarities, the bursts from this magnetar could suggest a common origin for these phenomena. Let’s look a bit further into what the authors found from this mysterious source!
Figure 1: The flux density (power) emitted by the source over observing date (top panel) and frequency (bottom panel). It can be seen that as time went on, the power of the star over time but increased over frequency. The top observations were taken between December 8, 2018 to April 27, 2019, and the bottom observations were taken during December 2018. [Maan et al. 2019]
What Are These “Spiky” Bursts?
Magnetar XTE J1810-197 was observed by the team in December of 2018 and February of 2019 with the Giant Metrewave Radio Telescope (GMRT) in India. During that time, they found four spiky bursts. When pulsars are observed, we see what is called an average profile, which maps the energy emitted over the time period that the beam crosses our sightline (the pulse phase window) — and we can make similar diagrams for a magnetar. The first panel in Figure 2 shows the regular pulse profile of the magnetar. In their observations, the authors saw that the position of the bursts roughly match that of the average profile. Unlike in the phenomenon of giant pulses, this emission is not confined to a small area of the pulse phase — which differentiates these spiky magnetar bursts from giant pulses. However, these bursts occur on a much shorter timescale than the regular emission, and they occur with different rates at different frequencies, with a width of 1–4 ms at lower frequencies and < 1 ms at higher frequencies (see second panel of Figure 2). Most of these variations are most likely caused by propagation effects in the interstellar medium.
The spectral index of XTE J1810-197’s bursts also seems to vary quite a bit with frequency. Using models of the electron density of the galaxy, the authors calculate that these variations must be intrinsic to the source and not due to the interstellar medium. The variations are also closer to the onset of the outburst, meaning something happens to the spectral index right before a burst happens. This only happens before bursts in earlier sessions, though, not in later observing sessions.
Figure 2: In the left panel, the black line represents the average profile and the blue dashed line shows the average profile from the first observing session of magnetar XTE J1810-197, with the vertical lines showing the different components. In the right panel, the pulse width distributions over time are shown by frequency (B4: 550−750 MHz and B5: 1260−1460 MHz). It is clear that the 550–750 MHz observations show a higher width which can be attributed to scatter-broadening. [Maan et al. 2019]
So What Does All of This Mean?
When magnetar XTE J1810-197’s first burst was observed in 2003, it was found that the radio spectrum was nearly flat, but after the onset of the current outburst, it was found to be much higher. It is possible that the spectrum is higher at lower frequencies. The power emitted by this object has decreased rapidly since the onset of the burst, but there are no clear physical links between spindown rate (how fast the magnetar slows down because it loses rotational energy) and emitted power.
While the outbursts have similarities to some from known objects, it is not quite clear exactly what they are yet. Giant pulses are characterized by an average energy 10 times greater than the average energy emitted by the source. Though the energies are large in the magnetar bursts, they’re not at the limit of giant pulses. The magnetar bursts are actually more similar to the giant micropulses in the Vela pulsar, as the widths are similar, so it is possible that these could be giant micropulses coming from magnetar XTE J1810-197.
Artist’s impression of a fast radio burst observed by the Parkes Radio Telescope. [Swinburne Astronomy Productions]
It has also been suggested bymultipleteams that these magnetar pulses could be connected to fast radio bursts. In January, a team led by Astrobites author Aaron Pearlman looked at magnetar J1745-2900, a magnetar that’s very close to the supermassive black hole in the center of our galaxy, and found that the magnetar pulses, just like the FRBs, show frequency structure in single pulses. This was the first time this kind of behavior had ever been observed from a magnetar. The authors of today’s paper found frequency variations of the spiky emission in XTE J1810-197. This spiky emission, which appears in both FRBs and these magnetar pulse, cannot be caused by the interstellar medium. The spikes look similar to the repeating FRB’s pulses, but the long-term frequency drift (essentially when the power emitted is detected at different frequencies) of the FRB’s pulses do not appear in the magnetar’s bursts. Though the magnetar’s brightest burst is ~5x more powerful, the FRB is nearly 10,000 times farther away, implying that it is intrinsically brighter than the magnetar bursts.
The fact that magnetars are the third type of object, after the Crab Pulsar and the repeating FRBs, to exhibit frequency structure in its bursts points to a similar emission mechanism for all three. In addition, the fact that the frequency structure is intrinsic to the star points to some kind of process similar to that of other unexplained high-energy phenomena. What will happen when the star bursts again? Will we find more bursts from other magnetars like this, or repeating FRBs that look similar? Only time will tell!
About the author, Haley Wahl:
I’m a second year grad student at West Virginia University and my main research area is pulsars! I’m currently working with the NANOGrav collaboration (a collaboration which is part of a worldwide effort to detect gravitational waves with pulsars) on polarization calibration. In my set of 45 millisecond pulsars, I’m looking at how the rotation measure (how much the light from the star is rotated by the interstellar medium on its way to us) changes over time, and also looking at other things we can learn from polarimetry. I’m mainly interested in pulsar emission and the weird things we see pulsars do! At WVU, I also work planetarium putting on shows for the public. In addition to doing research, I’m also a huge fan of running, baking, reading, watching movies, and I LOVE dogs!
Editor’s note:Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.
The gravitational constant, G, is one of the core fundamental constants of physics, appearing in Newton’s laws of gravitational motion, and therefore in the fundamental theory of gravity. While people historically questioned whether it truly is a constant, Einstein’s theory of general relativity states that G must be constant no matter where in space, or time, you find yourself. However modern work in string theory, which aims to reconcile the theory of gravity with the other fundamental forces of nature, says that the gravitational “constant” can, in fact, vary over extremely long cosmic timescales.
If the gravitational constant was changing in time, we might be able to detect it in systems whose evolution has strongly relied on gravity, such as stars. If gravity was weaker in the past, that would have affected the evolution of a star, changing how it appears today. Measurements of the rate of change of G have been performed in this way using helioseismology, white dwarfs, and globular clusters, as well as studies of the cosmic microwave background. All these experiments draw the same conclusions; that G changes at a completely negligible rate (specifically, by no more than a fraction of a trillionth a year, where the universe is only 13 billion years old).
A Stellar Archaeological Dig
Today’s authors present a new test to more closely approximate the variation of G on truly cosmic timescales. The target of the authors’ study is KIC 7970740, a low-mass, solar-like star on the main sequence that is, most importantly, roughly 11 billion years old! Thanks to high-quality measurements by the Kepler space telescope, this star also has a well measured set of clear stellar pulsations that allow for an asteroseismic analysis, and it is one of the oldest stars for which this is possible.
Because this star is so low-mass, it has had a very relaxed 11-billion-year life on the main sequence, making stellar models of the star relatively simple. This makes it a perfect candidate to study historical changes in the gravitational constant G; if G changed substantially in the the universe’s history, it will have subtly affected the evolution of this star and, as a consequence, the manner in which it pulsates today (see Figures 1 & 2). For example, if G was lower in the past, gravity will have been weaker. As a result, hydrostatic equilibrium will cause the stellar radius to be larger, which increases the star’s energy output (or luminosity). More luminous stars burn faster, changing the composition of the stellar core, which in turn affects how pulsation frequencies appear on the surface.
Figure 1: Showing the theoretical evolution on a HR-diagram of a low-mass star for varying degrees of change in the gravitational constant G. The black dots indicate the beginning of the star’s life on the main sequence, after which the star evolves up and to the left. The value β indicates the overall fractional change in G in the models. [Bellinger & Christensen-Dalsgaard 2019]
Modelling the Stellar History
To study the history of G, then, the authors study the history of KIC 7970740, by tweaking the star’s parameters in a stellar model and comparing the result to the observed pulsation frequencies, which are also modelled. Included in these models is a parameter β, representing the fractional change in the gravitational constant, and also the age of the universe, t0, which are both allowed to vary.
By fitting their evolutionary model to the asteroseismic data (in a process that took 6 months to run!), the authors find a fractional rate of change of G of (2.1 ± 2.9) × 10-12 yr-1, or two trillionths a year.
Figure 2: The same as Figure 1, now showing the evolution of the star in terms of two fundamental asteroseismic observables, Δν (the separation between oscillations of different overtones) and δν02 (the separation between oscillations of radial degree 0 and 2). [Bellinger & Christensen-Dalsgaard 2019]
Conclusions
While this result is uncertain, it is in line with previous studies on the change in G from stars — but measured over an extended age range that almost spans the full history of the universe. It is worth noting that the stellar parameters recovered for the star agree with independent studies, indicating that their model fits well and giving additional credibility to these results.
With this result, investigation of G is far from over. With the development of this technique it will become possible to apply it to an ensemble of stars, hopefully yielding a stronger result and/or highlighting any model dependencies that may have affected this result. Continuing this type of research will, hopefully, continue to improve synergies between stellar astrophysics and the most fundamental studies of the universe.
About the author, Oliver Hall:
Oliver is a final year PhD student at the University of Birmingham, UK. He’s a part of their Sun, Stars & Exoplanets research group with a focus on asteroseismology, the study of stellar pulsations, and what it can tell us about stellar populations. When not doing research he enjoys playing piano, hiking, and not moving from the sofa all weekend with a good book, show, or game.
Editor’s note:Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.
Nearly 400 years ago, it was hypothesized that the planets in our solar system formed from the leftover material that formed the Sun. This hypothesis is now widely accepted as the standard model for solar-system formation. We have even seen this process in action within other stellar systems thanks to radio telescopes like the Atacama Large Millimeter/submillimeter Array (ALMA).
We continuously focus on planets that form around stars. But what if planets could form around other astronomical bodies? After all, stars aren’t the only objects in the universe that become surrounded by a tumultuous disk of gas and dust during their lives.
Active galactic nuclei (AGN) exist at the center of galaxies. The standard model for an AGN consists of a supermassive black hole and a hot accretion disk, both of which are surrounded by a donut-shaped (or torus-shaped) region of gas and dust. This configuration is shown in Figure 1. Today’s paper takes a look at how a planet could possibly form within the dusty torus around an AGN.
Figure 1: Standard model of an active galactic nucleus. [Urry & Padovani 1995]
A Tumultuous Environment
AGN, as their name implies, are active objects. Gas fed to the AGN from an accretion disk acts as the fuel for these galactic engines, generating the high luminosities that allow us to observe these structures over vast distances. The material surrounding this central engine is where our interest lies. An AGN’s dusty torus spans a region from 0.1 parsecs (~0.3 light years) to tens of parsecs (~30 light years) away from the central supermassive black hole (SMBH), as shown in Figure 2. The inner region of the torus is heated by the central engine, and outflows of dust send material back to the interstellar medium. These structures have even been recently imaged with ALMA. However, their internal structure remains less well-understood.
Recent simulations have shown that the internal structure of a dusty torus is stratified. Figure 2 shows the disk of cold gas and dust within the dusty torus. The authors modeled the temperature distribution within this disk as a function of the AGN luminosity, finding that ices can form in regions ~1 parsec away from the central black hole, past the AGN’s snow line, and showing that the dynamics within this system could lead to these ices coalescing. The authors then took a look at how planets could grow within this environment.
Schematic diagram of an AGN, showing the central supermassive black hole, locations of different types of emission, and structure within the dusty torus. We can see the cold disk where planets could form, along with the location of the snow line, rsnow. [Wada et al. 2019]
Snowballing Planets
Stepping back to classical planet formation, as a cloud collapses to form a protostar, that star becomes surrounded by a protoplanetary disk — a disk of gas and dust that surrounds the protostar and is a source of accreting stellar material. Along with providing material for the star to grow from, a protoplanetary disk is also the site of planet growth, as its name implies. Dust particles in this disk collide with and stick to other particles to form planetesimals, larger bodies that act as the building blocks of planets. Planetesimals are large enough to attract other material via their own gravity, and they can eventually grow to planets via this accretion process.
The authors of today’s paper focus on the growth of planets from icy dust particles, which could form in the disk beyond the aforementioned AGN snow line. They tested systems of varying dust-particle sizes and analyzed the particles’ growth over time via numerical models. By analyzing the dust growth within systems of different black hole masses and disk viscosities, the authors determined that planet-sized bodies are capable of forming around low-luminosity AGN. The environments around quasars and other high-luminosity AGN, however, would not support planet formation.
Life Around a Black Hole?
Unfortunately, any planets that would form around AGN would be nearly impossible to detect using current methods. Even if we were to detect these planets, these systems would not assist humanity in its quest to understand the formation of habitable worlds. As mentioned previously, the environments around AGN are harsh, and they contain overly processed material that would make forming a habitable planet extremely difficult. Additionally, the amount of high-energy radiation from the black hole itself would cause any planet that formed there to be incapable of holding onto an atmosphere, an essential ingredient to life as we know it.
Although this study cannot be confirmed observationally and would not assist us in understanding habitable planets, this is an interesting look into where in the universe planets can form. The universe is, and will continue to be, a wild place.
About the author, Ellis Avallone:
I am a second-year graduate student at the University of Hawaii at Manoa Institute for Astronomy, where I study the Sun. My current research focuses on how the solar magnetic field triggers eruptions that can affect us here on Earth. In my free time I enjoy rock climbing, painting, and eating copious amounts of mac and cheese.
Editor’s note:Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.
Galaxies are so large that it can be hard to imagine them changing over time. However, we believe that galaxies are living and breathing entities, accreting and ejecting mass all throughout their lives. The Milky Way is no exception. Characterizing the rates of mass flow and the mass loading factor for galaxies, though, is crucial to understanding the details of this so-called galactic fountain model. In today’s paper, the authors provide new estimates of these rates for the Milky Way. They also present the first estimate of the mass loading factor (the ratio of material flowing out of the galaxy to the star formation rate) for the outflowing material from the entire Milky Way disk. Essentially, this measures how efficiently the Milky Way recycles the gas it takes from its surroundings. These are very cool results, so let’s break down exactly what they mean.
Why Is Mass Flowing In and Out of a Galaxy?
Artist’s illustration of a supernova, a type of stellar feedback that can remove mass from our galaxy. [NASA/CXC/M. Weiss]
A galaxy primarily exchanges low-density gas with its surroundings. Over time, some of this gas surrounding the galaxy will begin to clump together, and gravity will cause these clumps to fall back into the galaxy. This allows a galaxy to sustain its star-formation rate for a long period of time. Once these stars form, the most massive ones will start undergoing significant mass loss and exerting strong radiation pressure on the ambient medium. They will then end their lives as supernovae: brilliant explosions that inject more energy and momentum into their surroundings. These processes are collectively known as stellar feedback, and they are responsible for pushing gas back out of the Milky Way. In other words, the Milky Way is not an isolated lake of material; it is a reservoir that is constantly gaining and losing gas due to gravity and stellar feedback.
That Sounds Complicated. How Do the Authors Figure Out Which Gas Is Going In and Out?
Great question! Unfortunately, none of the gas travels with a bumper sticker that says ‘Milky Way Bound’. This means that the authors need to figure out which gas is likely to escape the Milky Way and which is likely to fall back in. They do this by identifying high-velocity clouds (HVCs) of gas that are traveling faster than the rotational speed of the Milky Way’s disk, meaning that they must represent inflowing or outflowing gas. Once they have identified a HVC, they check whether the cloud is moving towards the galactic disk or away from it. Finally, they ignore HVCs that are known to reside in structures (such as the Fermi Bubbles) that don’t trace the inflowing or outflowing gas. The final sample of HVCs is shown in Figure 1.
Figure 1: Plot of all HVCs identified in the paper. Inflowing clouds are shown in blue, outflowing clouds are shown in red, and the green regions show areas in which HVCs were ignored as they would not trace the inflowing or outflowing gas well. Black points show observations in which no HVCs were detected. [Fox et al. 2019]
What Were The Results?
The authors estimate the inflow rate to be 0.53 +/- 0.31 solar masses per year and the outflow rate to be 0.16 +/- 0.10 solar masses per year. This means that the Milky Way currently appears to have a net inflow of gas. HVCs only have lifetimes on the order of 100 million years, so it is important to note that this result should not be extended to very long timescales. Furthermore, the outflow rate is a lower limit; the true outflow rate could be higher if other regions like the Fermi Bubbles are included. Nevertheless, this exciting result provides evidence that the Milky Way disk may currently be gaining mass.
The paper also presents an estimate for the mass loading factor of roughly 0.1 using an independent measurement of the Milky Way’s star formation rate. This means that roughly 10% of the mass that forms stars is ejected back out of the galaxy. This result, together with the measurements of the inflow and outflow rates, can all help astronomers get closer to building a realistic model of the Milky Way.
About the author, Michael Foley:
I’m a graduate student studying astrophysics at Harvard University. My research focuses on using simulations and observations to study stellar feedback — the effects of the light and matter ejected by stars into their surroundings. I’m interested in learning how these effects can influence further star and galaxy formation and evolution. Outside of research, I’m really passionate about education, music, and free food.
Editor’s note:Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.
*just to be clear, the aliens I am talking about are rocks and not alive nor intelligent. Sorry 🙁
What if I told you that we have the opportunity to directly study other solar systems? You’d be like, “guuurrrlll, say whaaaat??” And then I’d say:
Similarly to how we can find chunks of Mars or pieces of the astroid belt on Earth, we have rocks from other solar systems flying around interstellar space — and a few just so happen to enter our solar system. This was only recently proven with the discovery of Interstellar Object (ISO) ‘Oumuamua: ‘Oumuamua was ejected from a different solar system and zoomed right into ours. Slipping between the Sun and Earth, it was detected as it started its journey back outside the solar system. ‘Oumuamua was the first object of its kind to be discovered, and it brings up the question, how many bits of other solar systems may be floating around and near us? The answer to that question can have wide implications in our understanding of solar-system formation, planet formation, and even compositions of other solar systems.
Today’s paper utilizes the ‘Oumuamua detection in addition to a recent high-resolution protoplanetary disk survey, DSHARP, to predict the number of future ISO detections. To put that number into context, the authors predict how many ISOs the new LSST survey might be able to see. To predict the average number density of ISOs (# of ISOs/volume of space) in our galaxy, astronomers have to come up with different possible methods of mass ejection from extrasolar systems. A prime opportunity to dislodge objects from a solar system takes place while the system is forming — and these dislodged objects can become ISOs. A newly forming solar system takes the form of a protoplanetary disk (see the figure below). The images below are individual protoplanetary disks shining in ~millimeter wavelengths. At these wavelengths we are most sensitive to dust of a similar size, so millimeter-sized dust grains. Each millimeter-sized dust grain is a candidate ISO; they can be flung out of their system by a newly forming massive planetesimal. These millimeter sized grains are much smaller than ‘Oumuamua, but still a good place to start in predicting how many ISOs are out there.
Three protoplanetary disks from the DSHARP survey. These images are sensitive to millimeter-sized dust grains, which show some real neat substructure like gaps (dark regions between bright regions) and rings (super bright regions, usually located next to a gap). [DSHARP collaboration]
The DSHARP images above show incredible substructure; we see gaps and rings that we are pretty sure are caused by newly forming Neptune-to-Jupiter-mass exoplanets. The authors show that the planets with the greatest ability to fling material outside their systems are those that are located far from the central star. The three disks pictured in the figure above show strong evidence for multiple planets farther than 5 AU from their central star (1AU is the distance from the Sun to Earth), so they are great examples of systems that can eject objects that become ISOs, like ‘Oumumama. For this reason, the authors use these particular disks as models in their work.
In order to come up with a mass ejection rate (the rate at which mass is lost from the system), the authors set up simulations that had the same initial conditions as the three disk systems pictured above, and for each system they created 3 random populations of dust — the locations and sizes of dust particles were randomly distributed throughout the disk. They then let these 9 total simulations run for a week on a super computer, simulating about 20 million years of the protoplanetary disks’ lives. They then determined how much mass had left the system after that time.
The authors came up with a function of mass ejected over time for millimeter-sized grains, and from that, one can calculate an average number density of these particles in and around our galaxy (you also then need to assume a certain density of stars). The authors found that, on average, for every star there is about 0.09 Earth masses worth of millimeter-sized dust. They used their data from these simulations to extrapolate up to a broader range of ISO sizes. After all, ‘Oumuamua and any interstellar interloper that we can hope to find in our solar system is going to be significantly larger than a few millimeters. If you assume some sort of power-law distribution (put very simply, the larger the ISO is the less there is of it) you can then estimate the total mass ejected from systems similar to the three protoplanetary disks for any ISO size. In this paper, the authors found that over the disks’ lifetimes, about 24 Earth masses worth of material would be ejected from these systems as ISOs with sizes ranging from a few millimeters to a few kilometers.
Photo of the LSST site taken in May 2019. Full science operations are expected to begin in 2023. [LSST Project/NSF/AURA]
So how do we then make a guess as to how many of these ISOs the LSST mission will see over its lifetime? LSST will be looking at huge swatches of the sky every night for 10 years, and its main purpose is to look at faraway things like galaxies. The authors make an estimate of how many ISOs LSST will detect that are at least 5 AU away from us, taking into account many factors like the reflectivity, size, and distance of the ISO. Their results suggest that LSST will observe several ‘Oumuamua-sized objects (greater than ~15 m) and hundreds of interstellar friendos visiting our solar system with radii of at least 1 m each year! That’s so many! It’s a much higher estimate than other papers — many of which were much more pessimistic, predicting LSST will see none at all. What this paper did differently is to utilize brand-new high-resolution images of these early systems in which ISOs form.
LSST’s main goals are to search for dark matter and answer questions about the formation and composition of our universe. But in this process, it will also be able to answer questions related to more Earthly subjects — not just answering questions like “How did the universe form?”, but also questions like “Is our solar system unique?”.
About the author, Jenny Calahan:
Hi! I am a second year graduate student at the University of Michigan. I study protoplanetary disk environments and astrochemistry, which set the stage for planet formation. Outside of astronomy, I love to sing (I’m a soprano I), I enjoy crafting, and I love to travel and explore new places. Check out my website: https://sites.google.com/umich.edu/jcalahan
Editor’s note:Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.
Disclaimer: The author was not involved in this work in any way.
If you want to figure out the fate of our universe, the value of the Hubble Constant (H0) would be handy to have. H0 tells us how fast the universe is expanding right now… I mean now… actually now — you get the picture. The Hubble Constant isn’t constant over time. Taken with other quantities, its current value can tell us a lot about the universe, such as its age and ultimate fate.
As great as H0 is, though, it’s a bit tricky to measure. And to complicate things, the measured value of H0changes with the measurement method. Currently, Planck measurements of the cosmic microwave background (CMB) return H0 = 67.4 ± 0.5 km/s/megaparsec (km/s/Mpc). This value is significantly smaller than the measurement obtained by using the distances and redshifts/velocities of distant galaxies, which is H0 = 74.03 ± 1.42 km/s/Mpc. The difference between these two measurements has been increasing since the 1990s as the measurements have been refined (see Figure 1), leading astronomers on both sides to scrutinize their methods for unaccounted errors.
The method that uses galaxy distances and velocities relies heavily on how those distances are measured. Currently, distance measurements for the purposes of determining H0 are closely tied to variable stars called Cepheids. To double-check the Cepheid-based distances, we need other objects to use as distance indicators.
One of these objects could be Mira variable stars (Miras). In this paper, the authors search for Miras in NGC 1559 using Hubble Space Telescope (HST) observations. NGC 1559 has hosted another, better understood distance indicator — a Type Ia supernovae — making it a good place to test how Miras can perform as extragalactic distance indicators.
O, See the Miras!
Miras are Asymptotic Giant Branch stars, meaning that they’ve run out of helium to burn in their cores. Their cores are inert and contain oxygen and carbon, and their outer shells consist of still-burning hydrogen. The outer shell puffs out as the star grows hotter, then cools down and shrinks. This is what causes the periodic brightness variations in a Mira: it gets bigger and brighter, then smaller and dimmer. Miras are named for their archetype, Mira, which is also known as O Ceti since it’s located in the constellation of Cetus.
Figure 2: The eponymous Mira, also known as O Ceti, as seen in different wavelengths by the HST. [M. Karovska (Harvard-Smithsonian CfA)/ NASA/ESA]
Miras have periods ranging over 100–3000 days, though they’re typically less than 400 days. In addition to having distinctive periods, Miras distinguish themselves from other variable stars with relatively dramatic changes in brightness in optical and infrared light. Miras can dredge up material from their core to their surface, so they’re often classified by their surface carbon-oxygen ratio as oxygen-rich (O-rich) or carbon-rich (C-rich).
Like Cepheids, Miras have a period–luminosity relation (PLR) that gives them their power as distance indicators. Mira PLRs are particularly distinct in the infrared. O-rich Miras with periods less than 400 days follow PLRs most tightly, so they’re currently the most reliable Mira distance indicators.
Pulling Miras Out of the Mire
To find Miras in NGC 1559, the authors use infrared observations taken by the HST’s Wide Field Camera 3. They identified the objects that were most likely to be genuinely variable. Then they determined the most likely periods of those objects, testing periods between 100 and 1000 days (since Mira periods typically lie within that range).
The authors then applied cuts based on the amplitude of the objects’ variations in brightness to select the O-rich Miras from the sample. The authors use techniques from other studies to estimate the degree of C-rich Mira contamination.
The authors end up with sample of 115 O-rich Miras. Since their sample is small, the authors draw on PLRs determined from the Large Magellanic Cloud (LMC) while fitting the NGC 1559 PLR (see Figure 3).
Figure 3: PLRs for the NGC 1559 Miras obtained using two different observed PLRs of LMC Miras from the OGLE Project (left) and Yuan et al. 2017b (right). The x-axis is the log of the period in days and the y-axis is magnitude in an infrared filter used on the HST. n = number of Miras; σ = scatter or spread in the relation; μ = distance modulus or the relation between apparent magnitude and absolute magnitude. [Huang et al. 2019]
Another Way Out
Before coming up with a measurement of H0, the authors compare the NGC 1559 PLR to the Mira PLR they obtained for NGC 4258 in a previous study. NGC 4258 hosts another reliable distance indicator: a water megamaser (a maser produces radiation by using particles that are stimulated by long wavelengths of light). The authors calibrate the NGC 1559 Miras using the megamaser distance to NGC 4258.
Using the distances to the LMC, NGC 4258, and NGC 1559 with a sample of Miras with consistent periods gives — *drumroll* — H0 = 73.3 ± 3.9 km/s/Mpc. This value is consistent with the Cepheid-based value of H0 = 74.03 ± 1.42 km/s/Mpc within reasonable expectations on measurement errors.
With this study and Huang et al. (2018) the authors have shown that Miras have great potential as extragalactic distance indicators. As tensions between measurements of H0 increase, independent distance indicators like Miras only grow in importance.
About the author, Tarini Konchady:
I’m a third-year graduate student at Texas A&M University. Currently I’m looking for Mira variables in optical to help calibrate the extragalctic distance ladder. I’m also looking for somewhere to hide my excess yarn and crochet hooks (I’m told I may have a problem).
Editor’s note:Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.
Title: LOFAR Discovery of a 23.5 s Radio Pulsar Authors: C. M. Tan et al. First Author’s Institution: Jodrell Bank Centre for Astrophysics, University of Manchester, UK Status: Published in ApJ
Pulsar Rotation Rates
Neutron stars are formed from massive stars that undergo violent supernova explosions after they run out of nuclear fuel and collapse under their own gravity. Radio pulsars are highly magnetized, rotating neutron stars that emit beams of radiation from their magnetic poles. When these beams of radio emission sweep across our line of sight, they generate radio pulses that can be detected with radio telescopes on Earth. The surface magnetic field strength, age, and internal structure of these objects can be studied through measurements of their rotational rates. Astronomers have now discovered more than 2,700 pulsars in our galaxy, and they’re constantly on the lookout for rare breeds. In today’s astrobite, we cover the discovery of the slowest known spinning radio pulsar, PSR J0250+5854, which has a rotational period of 23.5 s. This exciting finding demonstrates that radio pulsars can rotate much slower than expected and still produce radio pulsations.
Figure 1: An aerial view of the LOFAR Superterp, part of the core of the extended telescope located in the Netherlands. [LOFAR / ASTRON]
PSR J0250+5854: A Record-Setting Slow-Spinning Radio Pulsar
The authors discovered PSR J0250+5854 on 2017 July 30 using the LOw Frequency ARray (LOFAR) radio telescope (see Figure 1) as part of the LOFAR Tied-Array All-Sky Survey (LOTAAS). Additional follow-up radio observations were performed using the Green Bank,Lovell, and Nançay radio telescopes. Pulsations were detected between 120 and 168 MHz with LOFAR and at 350 MHz using the Green Bank Telescope (GBT), but no pulsed emission was detected at ~1.5 GHz using the Lovell and Nançay telescopes. The pulsar’s radio spectrum (spectral index of α = -2.6 ± 0.5, assuming its flux density follows a power-law as a function of frequency) is remarkably steep compared to the average pulsar population (<α> ≈ -1.8). This suggests that its radio emission is significantly brighter at lower frequencies (see Figure 2).
Figure 2: Radio spectrum of PSR J0250+5854 using LOFAR and GBT observations. The black line shows the fitted spectral index, with 1-σ uncertainties indicated by the shaded gray region. The circle corresponds to the measured flux density from LOFAR Two-meter Sky Survey imaging observations, and the triangles correspond to upper limits on the flux densities from LOFAR Low Band Antenna, Nançay, and Lovell radio telescope observations, respectively. [Tan et al. 2018]
Based on measurements of the pulsar’s rotation spanning more than 2 years, PSR J0250+5854 has an inferred surface dipole magnetic field strength of 26 trillion Gauss, characteristic age of 13.7 million years, and a spin-down luminosity of 8.2 x 1028 erg s-1, assuming a dipolar magnetic field configuration. PSR J0250+5854’s radio beam is very narrow according to the measured width of its pulse profile (the pulse duty cycle is < ~1% below 350 MHz, see Figure 3). Individual single pulses were routinely detected from the pulsar at low radio frequencies, except during brief periods of “pulse nulling” when the pulsar stopped emitting radio pulses. This occurred 27% of the time on average. The pulsar’s slow rotation period of 23.5 s is similar to other classes of pulsars. In particular, magnetars have high magnetic fields, spin periods ranging between roughly 2 and 12 s, and often produce X-ray emission, and X-ray Dim Isolated Neutron Stars (XDINs) have spin periods ranging between 3.4 and 11.3 s. However, no X-ray emission was detected from PSR J0250+5854 during follow-up observations with the Neil Gehrels Swift Observatory X-ray Telelescope.
Figure 3: Integrated pulse profiles of PSR J0250+5854 at observing frequencies of 350 MHz (GBT), 168 MHz (LOFAR), and 129 MHz (LOFAR). Here, only 5% of the rotational phase is shown. The inset shows the pulse profile across the whole LOFAR HBA band over a full rotation period. [Tan et al. 2018]
A Needle in a Haystack or a Haystack Full of Needles?
The P–Ṗ diagram is a key diagnostic tool for characterizing how pulsars evolve in time. Using pre-discovery LOTAAS data of PSR J0250+5854 from 2015, the authors measured a spin period derivative of Ṗ = 2.7 x 10-14 s s-1. The pulsar’s rotational parameters place it in the right region of the P–Ṗ diagram (see Figure 4) — an area where few pulsars have been found to reside. In particular, PSR J0250+5854 falls near/below many of the so-called “pulsar death lines,” beyond which pulsars are not expected to emit coherent radio emission. These models are based on assumptions about the conditions in the pulsar’s magnetosphere, such as pair production, which is thought to be essential for the generation of radio emission. Since the radio-emission mechanism in pulsars is not fully understood, searching for additional pulsars near these death regions will help to inform us about how pulsars produce radiation.
Figure 4: P–Ṗ diagram of pulsars derived from their measured rotational periods and rotational-period derivatives. The positive sloped gray lines indicate characteristic ages of 1 kyr, 100 kyr, 10 Myr, and 1 Gyr. The negative sloped gray lines correspond to inferred surface magnetic-field strengths of 10 GG, 100 GG, 10 TG, and 100 TG. Magnetars (green), XDINSs (orange), RRATs (yellow), and the 8.5-s radio pulsar PSR J2144–3933 are indicated on the plot. The colored lines show the various death-line models, where pulsars below these lines are not expected to produce radio emission. [Tan et al. 2018]
The discovery of PSR J0250+5854 begs the question: Is this a special kind of pulsar, or are there more to be found? The authors argue that more of these slow-rotating pulsars may be lurking around our galaxy, but we simply haven’t been sensitive to detecting them because commonly used Fast Fourier Transform (FFT)-based periodicity search algorithms are not well-suited to detecting slow pulsars with small duty cycles. The authors also point out that the radio emission observed from PSR J0250+5854 was much more erratic at higher frequencies. Therefore, if other slow rotating pulsars are similar to PSR J0250+5854, then this suggests that low-frequency radio telescopes, like LOFAR, may prove to be excellent observatories for searching for these slow rotators.
About the author, Aaron Pearlman:
I am a Ph.D. candidate in Physics at Caltech. My research focuses on searching for new pulsars near the center of the Galaxy using JPL’s Deep Space Network radio dishes in the southern hemisphere. I am also interested in studies of magnetars, fast radio bursts, gravitational-wave searches, and high-energy observations of compact objects. When I’m not hunting for pulsars, I can usually be found hanging out with my dogs or trying the latest vegetarian cuisine Los Angeles has to offer!
