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The Λ-cold dark matter cosmological model predicts that galaxies are assembled through the disruption and absorption of small satellite dwarf galaxies by their larger hosts. A recent study argues that NGC 4631, otherwise known as the “Whale” galaxy, shows evidence of such a recent merger — in the form of an enormous stellar stream extending from it.

Stream Signatures

According to the Λ-CDM model, stellar tidal streams should be a ubiquitous feature among galaxies. When satellite dwarf galaxies are torn apart, they spread out into such streams before ultimately feeding the host galaxy. Unfortunately, these streams are very faint, so we’re only recently starting to detect these features.

Stellar tidal streams have been discovered around the Milky Way and Andromeda, providing evidence of these galaxies’ growth via recent (within the last 8 Gyr) mergers. But discovering stellar streams around other Milky Way-like galaxies would help us to determine if the model of hierarchical galaxy assembly applies generally.

To this end, the Stellar Tidal Stream Survey, led by PI David Martínez-Delgado (Center for Astronomy of Heidelberg University), is carrying out the first systematic survey of stellar tidal streams. In a recent study, Martínez-Delgado and collaborators present their detection of a giant (85 kpc long!) stellar tidal stream extending into the halo of NGC 4631, the Whale galaxy.

Modeling a Satellite

stream simulation

The top image is a snapshot from an N-body simulation of a single dwarf satellite, 3.5 Gyr after it started interacting with the Whale galaxy. The satellite has been torn apart and spread into a stream that reproduces observations, which are shown in the lower image (scale is not the same). [Martínez-Delgado et al. 2015]

The Whale galaxy is a nearby edge-on spiral galaxy interacting with a second spiral, NGC 4656. But the authors don’t believe that the Whale galaxy’s giant tidal stellar stream is caused by its interactions with NGC 4656. Instead, based on their observations, they believe that a dwarf satellite galaxy was disrupted to make that stream.

To support their observations, the authors modeled the system using an N-body simulation. They were able to reproduce the appearance of the stream by sending a single, massive dwarf satellite onto a moderately eccentric orbit around the Whale galaxy. The team showed that, over the span of about 3.5 Gyr, the satellite became disrupted and spread into a structure very similar to the stellar tidal stream we now observe. In this simulation, the last remains of the dwarf satellite are contained within the northwest arm of the stream.

The authors point out that the Whale galaxy has additional gaseous tidal features that likely originated from a more recent, gas-rich accretion event. There are also two bright regions that may be more dwarf satellites around the galaxy (labeled DW1 and DW2 in the header image). If the authors’ interpretation of the observed stellar stream is correct, then the Whale galaxy shows evidence for multiple recent mergers. This would support the idea that hierarchical formation models apply to other galaxies similar to the Milky Way.


David Martínez-Delgado et al 2015 AJ 150 116. doi:10.1088/0004-6256/150/4/116

quadruple system

An important part of exoplanet studies is the attempt to understand how planets and solar systems form. New measurements of the lowest-mass quadruple star system ever discovered are now confirming an intriguing theory: in addition to other channels, large gas planets may form in the same way that stars do.

Formation Channels

Exoplanets have been found in an enormous variety of configurations, from hot Jupiters only 0.01 AU away from their host star, to planetary-mass companions that orbit at a whopping distance of 1,000 AU.

Formation of these gas giants could occur via a number of different theorized pathways, such as growth from rocky cores close to host star, or fragmentation from instabilities far out in the protoplanetary disk. But given that the line between giant planets and brown dwarfs is somewhat fuzzy, another theory has come under consideration as well: could gas giants form out of the collapse and fragmentation of a molecular cloud, in the same way that stars form?

In a recent study, Brendan Bowler and Lynne Hillenbrand (California Institute of Technology) argue that one star system, 2M0441+2301 AabBab, might actually be evidence that this channel works. 2M0441+2301 AabBab is a young (less than 3 million years old) quadruple system in the Taurus star-forming region, previously identified through imaging. Since photometry alone isn’t enough to be sure of the masses of the components, Bowler and Hillenbrand used the OSIRIS instrument on the Keck I telescope to obtain the first resolved spectra of each component of this system, verifying the system’s intriguing properties.

Pair of Pairs

Near-IR spectra of 2M0441+2301 Aa, Ab, Ba, and Bb. The insets shows the unresolved 2MASS image of the system and the Keck/NIRC2 images of each binary subsystem. Click for a better look! [Bowler&Hillenbrand 2015]

Near-IR spectra of 2M0441+2301 Aa, Ab, Ba, and Bb. The insets shows the unresolved 2MASS image of the system and the Keck/NIRC2 images of each binary subsystem. Click for a better look! [Bowler&Hillenbrand 2015]

2M0441+2301 AabBab is what’s known as a hierarchical quadruple system: it consists of a pair of close-binary star systems that orbit each other at an enormous distance of at least 1,800 AU — which means that, if the system is only a few million years old, the binary pairs have orbited each other no more than ~20 times.

The authors’ measurements show that the first binary pair (labeled Aab, where Aa and Ab are the two stars) consists of a 200 MJup low-mass star and a 35 MJup brown dwarf. The second binary pair (Bab) consists of a 19 MJup brown dwarf and a ~10 MJup companion. This gives 2M0441+2301 AabBab a total mass of only ~0.26 solar masses, making it the lowest-mass quadruple system yet discovered.

The hierarchical structure of this system strongly suggests that it formed from the collapse and fragmentation of a molecular cloud core. What makes this system especially interesting is the span of masses involved. The low mass of the companion in Bab indicates that it’s possible to form planetary-mass companions from a cloud-fragmentation pathway — which suggests that this may also be legitimate channel to consider for the formation of massive exoplanets.

Note: article edited to more accurately reflect the specific contributions of this study.


Brendan P. Bowler and Lynne A. Hillenbrand 2015 ApJ 811 L30. doi:10.1088/2041-8205/811/2/L30

Kappa Cygnids

Meteor showers occur when the Earth passes through a stream of debris left behind by a comet or asteroid as it loops around the Sun. For many meteor showers, we know exactly what the source is — for instance, the popular Perseid meteor shower is caused by debris from Comet 109P/Swift-Tuttle.

Another, lesser-known mid-August shower poses a mystery: the source of the Kappa Cygnid (KCG) meteor shower remains unknown. But new data from the especially strong 2014 KCG shower may help us to identify the missing parent body.

An Unpredictable Shower

The KCGs and the Perseids both occur at roughly the same time of year. Unlike the Perseids, however, the KCG shower is highly variable: it makes a strong showing some years and is completely undetected in others.

In 2014, the KCGs showed an unusual level of activity, conveniently allowing us to obtain an unprecedented number of observations of these meteors. Now a team of scientists, led by Althea Moorhead of the NASA Meteoroid Environment Office, has used these observations to better understand the KCGs and try to identify their source.

semi-major axes

One example (see the paper for the original image and more details) of the distribution of semi-major axes found for KCG meteors. Clustering of semi-major axes can be seen around Jupiter orbital resonances, which are marked by the dashed vertical lines. Click for a better look! [Moorhead et al. 2015]

Precise Trajectories

The 2014 KCGs were detected in a variety of meteor observation networks and instrument suites, from radar to video to photographic. From these observations, Moorhead’s team selected 75 meteors as belonging to the KCG shower. These observations represent the most precisely measured set of KCG meteor trajectories to date.

In characterizing the KCGs, the team found that these meteors are unusual: instead of having a single characteristic orbit, they exhibit a broad range of orbital elements like semi-major axis, eccentricity, perihelion distance, and inclination. Many of the meteors’ semi-major axes are in near-resonances with Jupiter, suggesting that the gas giant likely affects the evolution of these meteors’ orbits.

Simulating Streams

After characterizing these meteors, the authors carried out a series of N-body simulations of the meteor streams, hoping to identify a parent body. The results were mixed. The team found that two of the potential parent bodies, asteroids 2002 LV and 2001 MG1, could produce meteoroids that would intersect the Earth’s orbit. But their orbital parameters don’t quite line up with the KCGs: the meteors that these asteroids produce are slightly too slow, with too low an inclination.

Moorhead and collaborators argue that 2001 MG1 is nonetheless promising as a potential parent, because its orbit isn’t well-measured. Future refinement of these measurements might help resolve these discrepancies and solidify 2001 MG1’s claim as parent to the KCGs.


Althea V. Moorhead et al 2015 AJ 150 122. doi:10.1088/0004-6256/150/4/122

Arp 220

When two galaxies merge, the event often produces enormous galactic outflows. Though we’ve been able to study these on large scales, resolution limits in the past have prevented us from examining the launch sites, propagation, and escape of these outflows.

But recent high-resolution observations of Arp 220, a galaxy merger located a mere 250 million light years away from us, have finally provided a closer look at what’s happening in the center of this merger — and spotted something interesting.

A Curious Find

Arp 220 is an object clearly in the late stage of a galaxy merger; it has tidal tails, two distinct nuclei at its center (heavily-obscured by dust), lots of star formation, and a large-scale outflow that extends far from the galaxy.

While using Hubble observations to construct the first high-spatial-resolution optical emission line maps of Arp 220, a team led by Kelly Lockhart (Institute for Astronomy, Hawaii) discovered something unusual: evidence of a bubble-like structure, visible in the Hα+[N ii] emission. The bubble is slightly offset from the two nuclei at the galactic center, and measures ~600 pc across.

Origin Explanations

Large-scale (top) and zoomed-in (bottom) three-color Hubble observations of Arp 220: blue is optical, red is near-infrared, and green is Hα+[N ii] line emission. The bubble and the western nucleus (nuclei are marked by white circles) lie along the axis of the large-scale outflows (white vector). [Lockhart et al. 2015]

Large-scale (top) and zoomed-in (bottom) three-color Hubble observations of Arp 220: blue is optical, red is near-infrared, and green is Hα+[N ii] line emission. The bubble and the western nucleus (nuclei are marked by white circles) lie along the axis of the large-scale outflows (white vector). [Lockhart et al. 2015]

The authors propose several explanations for how the bubble was created, and examine the implications to determine which is the most likely. The explanations fall into two categories:

  1. The bubble is centered around its source.
    It could be produced by an outflow from an accreting black hole or a massive star cluster located at the bubble center.
  2. The bubble’s source is located near the two nuclei in the galactic center, but outside the bubble.
    It could be produced by a jet originating from one of the two galactic nuclei, or by a collimated outflow from a startburst concentrated near the nuclei. Either of these outflows could blow a bubble as it first interacts with the interstellar medium.

The authors show that the first category is disfavored based on observational and energetics arguments. In addition, the western-most nucleus and the bubble both align exactly with the axis of the large-scale outflows of the galaxy. Unlikely to be due to chance, this alignment is strong support in favor of the second category.

Thus, it’s probable that the bubble is blown by an outflow that originates from the inner ~100pc around one of the nuclei, either due to a jet or a starburst wind. Further observations should be able to differentiate between these two mechanisms.


Kelly E. Lockhart et al 2015 ApJ 810 149. doi:10.1088/0004-637X/810/2/149

Giant impact

In the process of searching for exoplanetary systems, we’ve discovered tens of debris disks close around distant stars that are especially bright in infrared wavelengths. New research suggests that we might be looking at the late stages of terrestrial planet formation in these systems.

Forming Terrestrial Planets

According to the widely-accepted formation model for our solar-system, protoplanets the size of Mars formed within a protoplanetary disk around our Sun. Eventually, the depletion of the gas in the disk led the orbits of these protoplanets to become chaotically unstable. Finally, in the “giant impact stage”, many of the protoplanets collided with each other — ultimately leading to the formation of the terrestrial planets and their moons as we know them today.

If giant impact stages occur in exoplanetary systems, too — leading to the formation of terrestrial exoplanets — how would we detect this process? According to a study led by Hidenori Genda of the Tokyo Institute of Technology, we might be already be witnessing this stage in observations of warm debris disks around other stars. To test this, Genda and collaborators model giant impact stages and determine what we would expect to see from a system undergoing this violent evolution.

Modeling Collisions

Impact simulation

Snapshots of a giant impact in one of the authors’ simulations. The collision causes roughly 0.05 Earth masses of protoplanetary material to be ejected from the system. Click for a closer look! [Genda et al. 2015]

The collaborators run a series of simulations evolving protoplanetary bodies in a solar system. The simulations begin 10 Myr into the lifetime of the solar system, i.e., after the gas from the protoplanetary disk has had time to be cleared and the protoplanetary orbits begin to destabilize. The simulations end when the protoplanets are done smashing into each other and have again settled into stable orbits, typically after ~100 Myr.

The authors find that, over an average giant impact stage, the total amount of mass ejected from colliding protoplanets is typically around 0.4 Earth masses. This mass is ejected in the form of fragments that then spread into the terrestrial planet region around the star. The fragments undergo cascading collisions as they orbit, forming an infrared-emitting debris disk at ~1 AU from the star.

The authors then calculate the infrared flux profile expected from these simulated disks. They show that the warm disks can exist and radiate for up to ~100 Myr before the fragments are smashed into micrometer-sized pieces small enough to be blown out of the solar system by radiation pressure.

The Spitzer Space Telescope has, thus far, observed tens of warm-debris-disk signatures roughly consistent with the authors’ predictions, primarily located at roughly 1 AU around stars with ages of 10–100 Myr. This region is near the habitable zone of these stars, which makes it especially interesting that these systems may currently be undergoing a giant impact stage — perhaps on the way to forming terrestrial planets.


H. Genda et al 2015 ApJ 810 136. doi:10.1088/0004-637X/810/2/136

giant solar tornado

On 7 November, 2012 at 08:00 UT, an enormous tornado of plasma rose from the surface of the Sun. It twisted around and around, climbing over the span of 10 hours to a height of 50 megameters — roughly four times the diameter of the Earth! Eventually, this monster tornado became unstable and erupted violently as a coronal mass ejection (CME).

Now, a team of researchers has analyzed this event in an effort to better understand the evolution of giant solar tornadoes like this one.

Oscillating Axis

In this study, led by Irakli Mghebrishvili and Teimuraz Zaqarashvili of Ilia State University (Georgia), images taken by the Solar Dynamics Observatory’s Atmospheric Imaging Assembly were used to track the tornado’s motion as it grew, along with a prominence, on the solar surface.

The team found that as the tornado evolved, there were several intervals during which it moved back and forth quasi-periodically. The authors think these oscillations were due to one of two effects when the tornado was at a steady height: either twisted threads of the tornado were rotating around each other, or a magnetic effect known as “kink waves” caused the tornado to sway back and forth.

Determining which effect was at work is an important subject of future research, because the structure and magnetic configuration of the tornado has implications for the next stage of this tornado’s evolution: eruption.

Eruption from Instability

coronal cavity

SDO/AIA 3-channel composite image of the tornado an hour before it erupted in a CME. A coronal cavity has opened above the tornado; the top of the cavity is indicated by an arrow. [NASA/SDO/AIA; Mghebrishvili et al. 2015]

Thirty hours after its formation, the tornado (and the solar prominence associated with it) erupted as a CME, releasing enormous amounts of energy. In the images from shortly before that moment, the authors observed a cavity open in the solar corona above the tornado. This cavity gradually expanded and rose above the solar limb until the tornado and prominence erupted into the space that had been opened.

Based on these observations, the authors hypothesize that the eruption could be explained using the following model:

  1. A tornado and a related solar prominence forms.
  2. Magnetic field lines within it are gradually twisted by the tornado’s rotation, until the tornado becomes unstable to the kink instability (a magnetic instability).
  3. The tornado then destabilizes the entire prominence, which expands upwards and erupts into a CME through something known as the “magnetic breakout model.”

If solar tornadoes such as this one generally cause instabilities of prominences, they could be used to predict when a related CME is about to happen — providing important information for space weather predictions.



Irakli Mghebrishvili et al 2015 ApJ 810 89. doi:10.1088/0004-637X/810/2/89

Enceladus' southern hemisphere

Enceladus, the sixth-largest moon of Saturn, is a cold, icy world — but it’s also remarkably active. Recent studies have charted over a hundred geysers venting gas and dust into space from Enceladus’ south polar region. New research addresses the question of how the moon’s extreme surface terrain influences the locations and behavior of these geysers.

Active Plumes

Saturn's E ring

Enceladus orbiting within Saturn’s E ring. Enceladus’ plumes probably created this ring. [NASA/JPL/Space Science Institute]

A decade ago, scientists discovered that Enceladus’ south polar region is home to a prominent set of four fractures known as the “tiger stripes”. This region was found to contain roughly 100 geyser jets, which form plumes of gas and dust venting into space at a combined rate of ~200 kilograms per second! These plumes are probably the source of the material in Saturn’s E ring, in which Enceladus orbits.

Recently, Carolyn Porco (UC Berkeley and CICLOPS Space Science Institute) led a study that analyzed 6.5 years of Cassini data, surveying the locations and orientations of 101 geysers. The outcome was peculiar: the geysers are distributed along the tiger stripes, but their directions are not all pointing vertically from the surface (see the video below!).

Now, Paul Helfenstein (Cornell University) has teamed up with Porco to examine whether the surface terrain surrounding the geysers affects where the jets erupt, what direction they point, and even when they’re active.

Surface Influence

Helfenstein and Porco demonstrate that the locations and behavior of the geysers are very likely influenced by Enceladus’ surface features in this region. In particular, they find:

  1. The spacing of the geyser jets on Enceladus is not random.
    The jets are roughly uniformly distributed along the three most active tiger stripes, spaced about 5 kilometers apart. This fixed spacing might be due to shear fractures — produced by fault motion along the tiger stripes — cutting across the stripes at regular intervals and providing convenient outlets for the geysers.
  2. The orientation of the geysers also isn’t random.
    Instead, the directions of jets are correlated with directions of the local terrain — be it the tiger stripes, the cross-cutting fractures, or the fine-scale tectonic fabric.

The authors further theorize that the timing of the plume activity may also be influenced by the terrain. Plume activity is thought to result from tidal flexing of Enceladus in its struggle against the gravitational forces of Saturn. The authors propose that under these stresses, the tiger stripes and fractures cutting across them might open and close at different times. The combinations of these motions may play a significant role in determining when the plumes are most active.


Check out this 3D model, based on Cassini observations, of the locations and directions of the ~100 geysers coming from the tiger stripes in Enceladus’s south polar terrain. [NASA/JPL-Caltech/Space Science Institute, Porco et al. 2014]


Paul Helfenstein and Carolyn C. Porco 2015 AJ 150 96. doi:10.1088/0004-6256/150/3/96


In 2006, pulsar PSR 1846–0258 unexpectedly launched into a series of energetic X-ray outbursts. Now a study has determined that this event may have permanently changed the behavior of this pulsar, raising questions about our understanding of how pulsars evolve.

Between Categories

A pulsar — a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation — can be powered by one of three mechanisms:

  1. Rotation-powered pulsars transform rotational energy into radiation, gradually slowing down in a predictable way.
  2. Accretion-powered pulsars convert the gravitational energy of accreting matter into radiation.
  3. Magnetars are powered by the decay of their extremely strong magnetic fields.

Astronomical classification often results in one pesky object that doesn’t follow the rules. In this case, that object is PSR 1846–0258, a young pulsar categorized as rotation-powered. But in 2006, PSR 1846–0258 suddenly emitted a series of short, hard X-ray bursts and underwent a flux increase — behavior that is usually only exhibited by magnetars. After this outburst, it returned to normal, rotation-powered-pulsar behavior.

Since the discovery of this event, scientists have been attempting to learn more about this strange pulsar that seems to straddle the line between rotation-powered pulsars and magnetars.

Unprecedented Drop

One way to examine what’s going on with PSR 1846–0258 is to evaluate what’s known as its “braking index,” a measure of how quickly the pulsar’s rotation slows down. For a rotation-powered pulsar, the braking index should be roughly constant. The pulsar then slows down according to a fixed power law, where the slower it rotates, the slower it slows down.

In a recent study, Robert Archibald (McGill University) and collaborators report on 7 years’ worth of timing observations of PSR 1846–0258 after its odd magnetar-like outburst. They then compare these observations to 6.5 years of data from before the outburst. The team finds that the braking index for this bizarre pulsar dropped suddenly by 14.5σ after the outburst — a change that’s unprecedented both in how large and how long-lived it’s been.

Why is this a problem? Many of the quoted properties of pulsars (like ages, magnetic fields, and luminosities) are determined based on models that envision pulsars as magnetic dipoles in a vacuum. But if this is the case, a pulsar’s braking index should be constant — or, in more realistic scenarios, we might expect it to change slightly over the span of thousands of years. The fact that PSR 1846–0258 underwent such a drastic change during its outburst poses a significant challenge to these models of pulsar behavior and evolution.



R. F. Archibald et al 2015 ApJ 810 67. doi:10.1088/0004-637X/810/1/67

Kepler Orrery

To date, we’ve discovered nearly 2000 confirmed exoplanets, as well as thousands of additional candidates. Amidst this vast sea of solar systems, how special is our own? A new study explores the answer to this question.

Analyzing Distributions

Knowing whether our solar system is unique among exoplanetary systems can help us to better understand future observations of exoplanets. Furthermore, if our solar system is typical, this allows us to be optimistic about the possibility of life existing elsewhere in the universe.

In a recent study, Rebecca Martin (University of Nevada, Las Vegas) and Mario Livio (Space Telescope Science Institute) examine how normal our solar system is, by comparing the properties of our planets to the averages obtained from known exoplanets.

Comparing Properties

So how do we measure up?

  1. Planet densities

    Densities of planets as a function of their mass. Exoplanets (N=287) are shown in blue, planets in our solar system are shown in red. [Martin&Livio 2015]

    Planet masses and densities
    Those of the gas giants in our solar system are pretty typical. The terrestrial planets are on the low side for mass, but that’s probably a selection effect: it’s very difficult to detect low-mass planets.
  2. Age of the solar system
    Roughly half the stars in the disk of our galaxy are younger than the Sun, and half are older. We’re definitely not special in age.
  3. Orbital locations of the planets
    This is actually a little strange: our solar system is lacking close-in planets. All of our planets, in fact, orbit at a distance that is larger than the mean distance observed in exoplanetary systems. Again, however, this might be a selection effect at work: it’s easier to detect large planets orbiting very close to their stars.
  4. Eccentricities of the planets’ orbits
    Our planets are on very circular orbits — and that actually makes us somewhat special too, compared to typical exoplanet systems. There is a possible explanation though: eccentricity of orbits tends to decrease with more planets in the system. Because we’ve got eight, it might be unsurprising that their eccentricities are so low.
  5. Super-Earths
    We don’t have any planets in the range of 1-10 times the mass of Earth, which is pretty unusual — super-Earths have a high occurrence rate among exoplanets.

In summary, the authors find that for the most part, we’re a pretty typical solar system. Our most unusual features are the lack of a super-Earth, the lack of any close-in planets, and the low eccentricities of our planets. The fact that we’re fairly average means that, from a habitability standpoint, there’s probably nothing special about our little corner of the galaxy. So perhaps life elsewhere is a possibility!


Rebecca G. Martin and Mario Livio 2015 ApJ 810 105. doi:10.1088/0004-637X/810/2/105


One of the big puzzles in astrophysics is how supermassive black holes (SMBHs) managed to grow to the large sizes we’ve observed in the very early universe. In a recent study, a team of researchers examines the possibility that they were formed by the direct collapse of supermassive stars.

Formation Mystery

SMBHs billions of times as massive as the Sun have been observed at a time when the universe was less than a billion years old. But that’s not enough time for a stellar-mass black hole to grow to SMBH-size by accreting material — so another theory is needed to explain the presence of these monsters so early in the universe’s history. A new study, led by Tatsuya Matsumoto (Kyoto University, Japan), poses the following question: what if supermassive stars in the early universe collapsed directly into black holes?

Previous studies of star formation in the early universe have suggested that, in the hot environment of these primordial times, stars might have been able to build up mass much faster than they can today. This could result in early supermassive stars roughly 100,000 times more massive than the Sun. But if these early stars end their lives by collapsing to become massive black holes — in the same way that we believe massive stars can collapse to form stellar-mass black holes today — this should result in enormously violent explosions. Matusmoto and collaborators set out to model this process, to determine what we would expect to see when it happens!

Energetic Bursts

The authors modeled the supermassive stars prior to collapse and then calculated whether a jet, created as the black hole grows at the center of the collapsing star, would be able to punch out of the stellar envelope. They demonstrated that the process would work much like the widely-accepted collapsar model of massive-star death, in which a jet successfully punches out of a collapsing star, violently releasing energy in the form of a long gamma-ray burst (GRB).

Because the length of a long GRB is thought to be proportional to the free-fall timescale of the collapsing star, the collapse of these supermassive stars would create much longer GRBs than are typical of massive stars today. Instead of the typical long-GRB length of ~30 seconds, these ultra-long GRBs would be 104–106 seconds.

Interestingly, we have already detected a small number of ultralong GRBs; they make up the tail end of the long GRB duration distribution. Could these detections be signals of collapsing supermassive stars in the early universe? According to the authors’ estimates, we could optimistically expect to detect roughly one of these events per year — so it’s entirely possible!


Tatsuya Matsumoto et al 2015 ApJ 810 64. doi:10.1088/0004-637X/810/1/64

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