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Nessie

The recent discovery of a purported “bone” of the Milky Way, a dark cloud nicknamed “Nessie”, has provided us with new clues for mapping out the spiral structure of our galaxy. It turns out that Nessie may not be alone: a follow-up study has identified more bones, potentially making up a skeleton of the Milky Way that traces out the densest parts of its spiral arms.

Inconvenient Vantage Point

How many spiral arms does the Milky Way have? Where are they located? What does the structure look like between the arms? It may seem surprising that these fundamental questions don’t yet have clear answers. But because we’re stuck in the galaxy’s disk, we’re forced to piece together our understanding of the Milky Way’s structure based primarily on measurements of position and radial velocity of structures within the galactic plane.

The discovery of Nessie presents an intriguing new tool to identify the layout of the galaxy. Nessie is a very long, thin, infrared-dark filament that runs along the modeled position of the Scutum-Centaurus arm — and is believed therefore to trace the structure of the arm. In a new study led by Catherine Zucker (University of Virginia, Harvard-Smithsonian Center for Astrophysics), the authors have searched for additional bones like Nessie, hoping to use them to map out the skeleton of the Milky Way.

New Bones Discovered

p-v summary of bones

In this map of radial velocity vs. galactic longitude, the bone candidates are indicated by the numbered points. The colored lines indicate the positions of two of the galactic spiral arms, according to various models. Click for a closer look! [Zucker et al. 2015]

Zucker and collaborators began by using World Wide Telescope, a tool that facilitates visualization of multiple layers of data at a variety of scales, to search through Spitzer infrared data for additional structures like Nessie. Searching specifically along the predicted positions of galactic arms, they found 15 initial bone candidates.

Next, the team obtained radial-velocity data for the candidates from five separate radio surveys. Five of the candidates did not have radial velocities consistent with the galactic rotation curve at the predicted positions of the nearby arms.

Hope for a Skeleton

The authors used the remaining ten candidates to construct rough criteria for an object to be a Milky Way “bone”:

  1. Largely continuous mid-infrared extinction feature
  2. Parallel to the galactic plane, to within 30°
  3. Position within 20 pc of the galactic mid-plane
  4. Radial velocity within 10 km/s of the predicted velocity of a Milky Way arm
  5. No abrupt shifts in velocity within the extinction feature
  6. Projected aspect ratio of ≥50:1

Of the ten candidates, six meet all the criteria and are thought to mark the location of significant spiral features in the galaxy. The authors believe that this method may be used to identify hundreds of Milky Way bones in the future. Combining this skeleton with other tracers of galactic structure will ultimately help us to piece together a more accurate map of our galaxy.

Bonus

Check out this video, produced by the authors using World Wide Telescope, that shows the locations of the newly discovered bone candidates within Spitzer images of the Milky Way galactic plane. [Credit: Zucker et al. 2015]

 

Citation

Catherine Zucker et al 2015 ApJ 815 23. doi:10.1088/0004-637X/815/1/23

M-dwarf system

In an effort to learn more about how planets form around their host stars, a team of scientists has analyzed the population of Kepler-discovered exoplanet candidates, looking for trends in where they’re found.

Planetary Occurrence

Since its launch in 2009, Kepler has found thousands of candidate exoplanets around a variety of star types. Especially intriguing is the large population of “super-Earths” and “mini-Neptunes” — planets with masses between that of Earth and Neptune — that have short orbital periods. How did they come to exist so close to their host star? Did they form in situ, or migrate inwards, or some combination of both processes?

To constrain these formation mechanisms, a team of scientists led by Gijs Mulders (University of Arizona and NASA’s NExSS coalition) analyzed the population of Kepler planet candidates that have orbital periods between 2 and 50 days.

Mulders and collaborators used statistical reconstructions to find the average number of planets, within this orbital range, around each star in the Kepler field. They then determined how this planet occurrence rate changed for different spectral types — and therefore the masses — of the host stars: do low-mass M-dwarf stars host more or fewer planets than higher-mass, main-sequence F, G, or K stars?

Challenging Models

Planet radius distribution

Authors’ estimates for the occurrence rate for short-period planets of different radii around M-dwarfs (purple) and around F, G, and K-type stars (blue). [Mulders et al. 2015]

The team found that M dwarfs, compared to F, G, or K stars, host about half as many large planets with orbital periods of P < 50 days. But, surprisingly, they host significantly more small planets, racking up an average of 3.5 times the number of planets in the size range of 1–2.8 Earth-radii.

Could it be that M dwarfs have a lower total mass of planets, but that mass is distributed into more, smaller planets? Apparently not: the authors show that the mass of heavy elements trapped in short-orbital-period planets is higher for M dwarfs than for the larger F, G and K stars.

All of this goes contrary to expectation, because we know that protostellar disks, from which planets form, are more massive around larger-mass stars. So why is there more heavy-element mass trapped in planetary systems with low stellar mass?

This outcome isn’t predicted by either in situ or migration planet formation theories. The authors instead propose that the distribution could be explained if the inward drift of planetary building blocks — either dust grains or protoplanets — turns out to be more efficient around lower-mass stars.

Citation

Gijs D. Mulders et al 2015 ApJ 814 130. doi:10.1088/0004-637X/814/2/130

white dwarf

In the wake of the recent media attention over an enigmatic, dimming star, another intriguing object has been discovered: J1529+2928, a white dwarf that periodically dims. This mystery, however, may have a simple solution — with interesting consequences for future surveys of white dwarfs.

Unexpected Variability

J1529+2928 is an isolated white dwarf that appears to have a mass of slightly more than the Sun. But rather than radiating steadily, J1529+2928 dims once every 38 minutes — almost as though it were being eclipsed.

The team that discovered these variations, led by Mukremin Kilic (University of Oklahoma), used telescopes at the Apache Point Observatory and the McDonald Observatory to obtain follow-up photometric data of J1529+2928 spread across 66 days. The team also took spectra of the white dwarf with the Gemini North telescope.

Kilic and collaborators then began, one by one, to rule out possible causes of this object’s variability.

Eliminating Options

  • The period of the variability is too long for J1529+2928 to be a pulsating white dwarf with luminosity variation caused by gravity-wave pulsations.
  • The variability can’t be due to an eclipse by a stellar or brown-dwarf companion, because there isn’t any variation in J1529+2928’s radial velocity.
  • It’s not due to the orbit of a solid-body planetary object; such a transit would be too short to explain observations.
  • It can’t be due to the orbit of a disintegrated planet; this wouldn’t explain the light curves observed in different filters — plus the light curve doesn’t change over the 66-day span.

Spotty Surface

light curves

Top and middle two panels: light curves from three different nights observing J1529+2928’s periodic dimming. Bottom panel: The Fourier transform shows a peak at 37.7 cycles/day (and another, smaller peak at its first harmonic). [Kilic et al. 2015]

So what explanation is left? The authors suggest that J1529+2928’s variability is likely caused by a starspot on the white dwarf’s surface that rotates into and out of our view. Estimates show that the observed light curves could be created by a starspot at about 10,000K (compared to the white dwarf’s effective temperature of ~11,900K), covering 14% of the surface area at an inclination of 90°.

The formation of such a starspot would almost certainly require the presence of magnetic fields. Interestingly, J1529+2928 doesn’t have a strong magnetic field; from its spectra, the team can constrain its field strength to be less than 70 kG.

Given that up to 15% of white dwarfs are thought to have kG magnetic fields, eclipse-like events such as this one might in fact be common for white dwarfs. If so, then many similar events will likely be observed with future surveys of transients — like Kepler’s ongoing K2 mission, which is expected to image another several hundred white dwarfs, or the upcoming Large Synoptic Survey Telescope, which will image 13 million white dwarfs.

Citation

Mukremin Kilic et al 2015 ApJ 814 L31. doi:10.1088/2041-8205/814/2/L31

SDO solar observations

Looking for stars that “wobble” is one of the key ways by which we detect exoplanets: the gravitational pull of planets cause tiny variations in stars’ radial velocities. But our ability to detect Earth twins is currently limited by our ability to distinguish between radial-velocity variations caused by exoplanets, and those caused by noise from the star itself. A team of scientists has recently proposed that the key to solving this problem may be to examine our own star.

Precision Amid Noise

The radial-velocity technique works well for detecting large planets on close orbits, but detecting an Earth twin requires being able to detect star motion on the order of 10 cm/s! This precision is hard to reach, because activity on the stellar surface — i.e., sunspots, plages (bright spots), or granulation — can also cause variations in the measured radial velocity for the star, obscuring the signature of a planet.

Because the stars we’re examining aren’t resolved, we can’t track the activity on their surfaces — so how can we better understand the imprint that stellar activity has on radial-velocity measurements? A team of scientists has come up with a clever approach: examine the Sun as though it were a distant star.

Wealth of Information

The team, led by Xavier Dumusque (Branco-Weiss Fellow at the Harvard-Smithsonian Center for Astrophysics) and David F. Phillips (Harvard-Smithsonian Center for Astrophysics), has begun a project to observe the Sun with a ground-based solar telescope. The telescope observes the full disk of the Sun and feeds the data into the HARPS-N spectrograph in Spain, a spectrograph normally used for radial-velocity measurements of other stars in the hunt for exoplanets.

But the team has access to other data about the Sun, too: information from satellites like the Solar Dynamics Observatory and SORCE about the solar activity and total irradiance during the time when the spectra were taken. Dumusque and collaborators have combined all of this information, during a week-long test, to see if it’s possible to correct for radial-velocity perturbations due to sunspots and plages.

Reducing Variations

TSI and radial velocity

The total solar irradiance of the Sun (top panel) is correlated with the radial velocity variations measured for the Sun (bottom panel), due to the effects of surface inhomogeneities like sunspots and plages. [Dumusque et al. 2015]

By relating the total stellar irradiance (measured by SORCE) to the radial-velocity variation due to stellar noise, the team finds that — even with only a week of data — they’re already able to subtract off some of these effects. They reduce the radial-velocity variation by more than a factor of two, bringing it down to 60 cm/s.

After this initial success, the next step is to improve on this result with more extensive observations. The team plans to continue to monitor the Sun daily over the next two to three years, allowing them to further develop correction methods. They believe this will enable us to reach the precision needed to detect an Earth twin around another star.

Citation

Xavier Dumusque et al 2015 ApJ 814 L21. doi:10.1088/2041-8205/814/2/L21

NGFS

Theories of galaxy formation and evolution predict that there should be significantly more dwarf galaxies than have been observed. Are our theories wrong? Or are dwarf galaxies just difficult to detect? Recent results from a survey of a galaxy cluster 62 million light-years away suggest there may be lots of undiscovered dwarf galaxies hiding throughout the universe!

Hiding in Faintness

The “missing dwarf problem” has had hints of a resolution with the recent discovery of Ultra-Diffuse Galaxies (UDGs) in the Coma and Virgo galaxy clusters. UDGs have low masses and large radii, resulting in a very low surface brightness that makes them extremely difficult to detect. If many dwarfs are UDGs, this could well explain why we’ve been missing them!

But the Coma and Virgo galaxy clusters are similar in that they’re both very massive. Are there UDGs in other galaxy clusters as well? To answer this question, an international team of scientists is running the Next Generation Fornax Survey (NGFS), a survey searching for faint dwarf galaxies in the central 30 square degrees of the Fornax galaxy cluster.

The NGFS uses near-UV and optical observations from the Dark Energy Camera mounted on the 4m Blanco Telescope in Chile. The survey is still underway, but in a recent publication led by Roberto P. Muñoz (Institute of Astrophysics at the Pontifical Catholic University of Chile), the team has released an overview of the first results from only the central 3 square degrees of the NGFS field.

Surprising Detection

NGFS size-luminosity

Galaxy radii vs. their absolute i-band magnitudes, for the dwarfs found in NGFS as well as other stellar systems in the nearby universe. The NGFS dwarfs are similar to the ultra-diffuse dwarfs found in the Virgo and Coma clusters, but are several orders of magnitude fainter. [Muñoz et al. 2015]

In just this small central field, the team has found an astounding 284 low-surface-brightness dwarf galaxy candidates — 158 of them previously undetected. At the bright end of this sample are dwarf galaxies that resemble the UDGs found in Virgo and Coma clusters, verifying that such objects exist in environments beyond only massive clusters.

And at the faint end of the sample, the authors find additional extremely low-surface-brightness dwarfs that are several orders of magnitude fainter even than classical UDGs.

The authors describe the properties of these galaxies and compare them to systems like classical UDGs and dwarf spheroidal galaxies in our own Local Cluster. The next step is to determine which of the differences between the sample of NGFS dwarfs and previously known systems are explained by the environmental factors of their host cluster, and which are simply due to sample biases.

With much more data from the NGFS still to come, it seems likely that we will soon be able to examine an even larger sample of no-longer-missing dwarfs!

Citation

Roberto P. Muñoz et al 2015 ApJ 813 L15. doi:10.1088/2041-8205/813/1/L15

binary black hole

In June of this year, after nearly three decades of sleep, the black hole V404 Cygni woke up and began grumbling. Scientists across the globe scrambled to observe the sudden flaring activity coming from this previously peaceful black hole. And now we’re getting the first descriptions of what we’ve learned from V404 Cyg’s awakening!

Sudden Outburst

V404 Cyg is a black hole of roughly nine solar masses, and it’s in a binary system with a low-mass star. The black hole pulls a stream of gas from the star, which then spirals in around the black hole, forming an accretion disk. Sometimes the material simply accumulates in the disk — but every two or three decades, the build-up of gas suddenly rushes toward the black hole as if a dam were bursting.

The sudden accretion in these events causes outbursts of activity from the black hole, its flaring easily visible to us. The last time V404 Cyg exhibited such activity was in 1989, and it’s been rather quiet since then. Our telescopes are of course much more powerful and sensitive now, nearly three decades later — so when the black hole woke up and began flaring in June, scientists were delighted at the chance to observe it.

The high variability of V404 Cyg is evident in this example set of spectra, where time increases from the bottom panel to the top. [King et al. 2015]

The high variability of V404 Cyg is evident in this example set of spectra, where time increases from the bottom panel to the top. [King et al. 2015]

Led by Ashley King (Einstein Fellow at Stanford University), a team of scientists observed V404 Cyg with the Chandra X-ray Observatory, obtaining spectra of the black hole during its outbursts. The black hole flared so brightly during its activity that the team had to take precautions to protect the CCDs in their detector from radiation damage! Now the group has released the first results from their analysis.

Windy Disk

The primary surprise from V404 Cyg is its winds. Many stellar-mass black holes have outflows of mass, either in the form of directed jets emitted from their centers, or in the form of high-energy winds isotropically emitted from their accretion disks. But V404 Cyg’s winds — which the authors measure to be moving at a whopping ~4,000 km/s — appear to originate from much further out in the disk than what’s typical. Furthermore, the presence of disk winds and jets is normally anti-correlated, yet in V404 Cyg, both are active at the same time.

King and collaborators believe that the winds are likely associated with the disruption of the outer accretion disk due to pressure from the radiation in the central region as it becomes very luminous. V404 Cyg’s behavior is actually more similar to that of some supermassive black holes than to most stellar-mass black holes, which is extremely intriguing.

The authors are currently working to complete a more detailed analysis of the spectra and build a model of the processes occurring in this awakening black hole, but these initial results demonstrate that V404 Cyg has some interesting things to teach us.

Citation

Ashley L. King et al 2015 ApJ 813 L37. doi:10.1088/2041-8205/813/2/L37

SDO AIA 01/15/13

The Sun often exhibits outbursts, launching material from its surface in powerful releases of energy. Recent analysis of such an outburst — captured on video by several Sun-monitoring spacecraft — may help us understand the mechanisms that launch these eruptions.

Many Outbursts

Solar jets are elongated, transient structures that are thought to regularly release magnetic energy from the Sun, contributing to coronal heating and solar wind acceleration. Coronal mass ejections (CMEs), on the other hand, are enormous blob-like explosions, violently ejecting energy and mass from the Sun at incredible speeds.

But could these two types of events actually be related? According to a team of scientists at the University of Science and Technology of China, they may well be. The team, led by Jiajia Liu, has analyzed observations of a coronal jet that they believe prompted the launch of a powerful CME.

Observing an Explosion

CME gif

Gif of a movie of the CME, taken by the Solar Dynamics Observatory’s Atmospheric Imaging Assembly at a wavelength of 304Å. The original movie can be found in the article. [Liu et al.]

An army of spacecraft was on hand to witness the event on 15 Jan 2013 — including the Solar Dynamics Observatory (SDO), the Solar and Heliospheric Observatory (SOHO), and the Solar Terrestrial Relations Observatory (STEREO). The instruments on board these observatories captured the drama on the northern limb of the Sun as, at 19:32 UT, a coronal jet formed. Just eight minutes later, a powerful CME was released from the same active region.

The fact that the jet and CME occurred in the same place at roughly the same time suggests they’re related. But did the initial motions of the CME blob trigger the jet? Or did the jet trigger the CME?

Tying It All Together

In a recently published study, Liu and collaborators analyzed the multi-wavelength observations of this event to find the heights and positions of the jet and CME. From this analysis, they determined that the coronal jet triggered the release of material to form the CME, which then erupted into space — with the jet at its core — at speeds of over 1000 km/s.

Based on observed clues of the magnetic field configurations, the team has put together a theory for how this event unfolded. They believe that sudden magnetic reconnection in an active region accelerated plasma to form a large-scale coronal jet. This burst of energy also provided a push on a blob of gas, threaded with magnetic field lines, that lay above the jet. The blob then rose, and when the field lines broke, it was released as a CME with the jet at its core.

Citation

Jiajia Liu et al 2015 ApJ 813 115. doi:10.1088/0004-637X/813/2/115

Seth region

High-resolution imagery of comet 67P Churyumov–Gerasimenko has revealed that its surface is covered in active pits — some measuring hundreds of meters both wide and deep! But what processes caused these pits to form?

Pitted Landscape

ESA’s Rosetta mission arrived at comet 67P in August 2014. As the comet continued its journey around the Sun, Rosetta extensively documented 67P’s surface through high-resolution images taken with the on-board instrument NavCam. These images have revealed that active, circular depressions are a common feature on the comet’s surface.

In an attempt to determine how these pits formed, an international team of scientists led by Olivier Mousis (Laboratory of Astrophysics of Marseille) has run a series of simulations of a region of the comet — the “Seth” region — that contains a 200-meter-deep pit. These simulations include the effects of various phase transitions, heat transfer through the matrix of ices and dust, and gas diffusion throughout the porous material.

Escaping Volatiles

pitted regions

Additional examples of pitted areas on 67P’s northern-hemisphere surface include the Ash region and the Ma’at region (both imaged September 2014 by NavCam) [Mousis et al. 2015]

Previous studies have already eliminated two potential formation mechanisms for the pits: impacts (the sizes of the pits weren’t right) and erosion due to sunlight (the pits don’t have the right shape). Mousis and collaborators assume that the pits are instead caused by the depletion of volatile materials — chemical compounds with low boiling points — either via explosive outbursts at the comet’s surface, or via sinkholes opening from below the surface. But what process causes the volatiles to deplete when the comet heats?

The authors’ simulations demonstrate that volatiles trapped beneath the comet’s surface — either in icy structures called “clathrates” or within amorphous ice — can be suddenly released as the comet warms up. The team shows that the release of volatiles from these two structures can create 200-meter-deep pits within ~800 years and ~2,000 years, respectively. Since comet 67P has been around the inner solar system for about 7,000 years, both of these processes are viable explanations for the pits.

The simulations also show that direct sublimation of crystalline ices of water, carbon monoxide, and carbon dioxide can cause deep pits — but only in the absence of a surface layer of dust, known as a “dust mantle”, in that region of the comet. Direct sublimation could be a viable explanation for the pits, then, if dust grains in the area are so small that they are carried away with the released gas, rather than falling back to form a layer on the comet’s surface.

Regardless of the formation mechanism for these pits, the authors conclude that their very existence implies that the physical and chemical properties across the surface and subsurface of the comet cannot be uniform. Further observations from Rosetta will continue to help us understand comet 67P.

Citation

O. Mousis et al 2015 ApJ 814 L5. doi:10.1088/2041-8205/814/1/L5

Procyon

The binary system Procyon, located a mere 11 light-years away, consists of a bright, subgiant star and a faint white dwarf — presenting a distinct challenge for astronomers to observe. But careful analysis of two decades of precise measurements with the Hubble Space Telescope has now finally revealed some of its secrets.

Challenging Observations

Perturbations were detected as early as 1844 in the orbit of Procyon, originally thought to be a single star. Astronomers of the time suspected that this wobbling was due to the pull of a companion orbiting Procyon, but it wasn’t until five decades later that the companion was first detected visually.

Why? Because the subgiant Procyon A is the 8th brightest star in the sky. Its companion, on the other hand, is a white dwarf that’s fainter (in visual wavelengths) by a factor of nearly 16,000! And the two stars are separated by an angular distance of less than 5”.

Due to the difficulty observing the system, the measurements of its motion — and resulting estimates of the masses of the two stars — have been a subject of debate for the better part of the last century.

Led by Howard Bond (Pennsylvania State University and the Space Telescope Science Institute), a team of astronomers has now analyzed two decades of Hubble observations of the system, combined with historical, ground-based observations dating back to the 19th century. Bond and collaborators used these data to precisely measure the orbital elements of Procyon and obtain dynamical masses of the two stars.

Surprising Mixing

Procyon orbit

Relative orbit of Procyon B around Procyon A. The red curve is the authors’ fit to the orbit, and the open blue circles are positions predicted by the orbital elements found. The black dots are the HST observations of Procyon B. The open green and turquoise circles are the (significantly less precise!) historical, ground-based observations. [Bond et al. 2015]

The team reports that this system orbits once every 40.8 years. They find masses for the two stars of 1.48 solar masses for Procyon A, and 0.59 solar masses for Procyon B. Both of these masses produce very satisfying agreement with theoretical predictions of the stars’ masses based on their temperatures, luminosities, and asteroseismology of Procyon A.

But the measurements of this system also have some interesting implications. One example arises when the authors apply the standard model for the evolution of a subgiant star to Procyon A. Using the new parameters, they find that Procyon A may have an unusually high amount of mixing of material beyond its convective core. If this is confirmed, it could mean that our understanding of how stars like Procyon A evolve may need to be updated.

Thus the precise measurements of the Procyon system have allowed the authors to not only pin down the parameters of the system, but also access new information about stellar physics near the main sequence. We’ve come a long way in the 170 years since Procyon was first observed to wobble!

Citation

Howard E. Bond et al 2015 ApJ 813 106. doi:10.1088/0004-637X/813/2/106

active galactic nucleus

Who needs humans? Robotic observations made by telescopes in the Las Cumbres Observatory Global Telescope network (LCOGT) have tracked variability in the active galaxy Arp 151 over 200 days. These observations have proven to be enough information to estimate the mass of the black hole at the galaxy’s center.

Mapping Echoes

Measuring the masses of supermassive black holes is notoriously difficult. Except in the few cases where we’re able to resolve actual objects orbiting around the supermassive black hole (for instance, in the case of the black hole at the center of the Milky Way), our estimates of black-hole mass must come from indirect measurements.

One clever approach is called “reverberation mapping.” In an active galactic nucleus (AGN), continuum emission from the black hole’s accretion disk photoionizes gas clouds in the nearby broad-line region, causing the clouds to emit light. In reverberation mapping, we track the time lag between variability in the disk’s continuum emission and the clouds’ broad-line emission, obtaining a distance scale. Combining this information with a velocity (provided by the broad-line width) allows us to infer the enclosed mass — in this case, that of the black hole.

So what’s the catch? Getting this information requires a lot of man-hours and telescope-hours, because AGN need to be observed over long periods of time to see the variability and the lags needed to make these inferences. This is where LCOGT comes in.

Robotic Network

Arp 151 light curves

Arp 151 light curves. The top panel shows the continuum emission from the disk; the remaining panels show various emission lines from the broad-line-region clouds. The variability of the line emission lags slightly behind that of the continuum emission. [Valenti et al. 2015]

LCOGT is a completely robotic telescope network. Everything from the scheduling to the telescope alignment is done without human involvement. Because of this feature, the LCOGT is an ideal facility for conducting time-intensive observations of AGN.

A team of scientists led by Stefano Valenti (LCOGT, UC Santa Barbara) has published the first results from the AGN Key Project, a project which uses the LCOGT network to conduct several studies of AGN — including reverberation mapping of both local and high-redshift objects. In these first results, the team reports the outcome of a 200-day observing campaign of the galaxy Arp 151, which has a highly variable active nucleus.

The LCOGT observations successfully show a measurable lag between the continuum emission and the broad emission lines for Arp 151. They are also enough-resolved that a velocity can be measured from the broad emission lines, allowing the team to calculate the mass of the black hole enclosed. Valenti and collaborators announce a mass of 6.2 million solar masses — consistent with previously measured masses for this system.

The success of this test demonstrates the viability of this approach, as well as the powerful capabilities of robotic telescope networks for long-term AGN time domain campaigns.

Citation

S. Valenti et al 2015 ApJ 813 L36. doi:10.1088/2041-8205/813/2/L36

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