An Aging Pulsar has Captured a new Companion, and it’s Spinning back up Again

When massive stars reach the end of their life cycle, they explode in a massive supernova and cast off most of their material. What’s left is a “milliscond pulsar”, a super dense, highly-magnetized neutron star that spins rapidly and emit beams of electromagnetic radiation. Eventually, these stars lose their rotational energy and begin to slow down, but they can speed up again with the help of a companion.

According to a recent study, an international team of scientists witnessed this rare event when observing an ultra-slow pulsar located in the neighboring Andromeda Galaxy (XB091D). The results of their study indicated that this pulsar has been speeding up for the past one million years, which is likely the result of a captured a companion that has since been restoring its rapid rotational velocity.

Typically, when a pulsars pairs with an ordinary star, the result is a binary system consisting of a pulsar and a white dwarf. This occurs after the pulsar pulls off the outer layers of a star, turning it into a white dwarf. The material from these outer layer then forms an accretion disk around the pulsar, which creates a “hot spot” that radiates brightly in the X-ray specturum and where temperatures can reach into the millions of degrees.

The team was led by Ivan Zolotukhin of the Sternberg Astronomical Institute at Lomonosov Moscow State University (MSU), and included astronomers from the University of Toulouse, the National Institute for Astrophysics (INAF), and the Smithsonian Astrophysical Observatory. The study results were published in The Astrophysical Journal under the title “The Slowest Spinning X-Ray Pulsar in an Extragalactic Globular Cluster“.

As they state in their paper, the detection of this pulsar was made possible thanks to data collected by the XMM-Newton space observatory from 2000-2013. In this time, XMM-Newton has gathered information on approximately 50 billion X-ray photons, which has been combined by astronomers at Lomosov MSU into an open online database.

This database has allowed astronomers to take a closer look at many previously-discovered objects. This includes XB091D, a pulsar with a period of seconds (aka. a “second pulsar”) located in one of the oldest globular star clusters in the Andromeda galaxy. However, finding the X-ray photos that would allow them to characterize XB091D was no easy task. As Ivan Zolotukhin explained in a MSU press release:

“The detectors on XMM-Newton detect only one photon from this pulsar every five seconds. Therefore, the search for pulsars among the extensive XMM-Newton data can be compared to the search for a needle in a haystack. In fact, for this discovery we had to create completely new mathematical tools that allowed us to search and extract the periodic signal. Theoretically, there are many applications for this method, including those outside astronomy.”

The slowest spinning X-ray pulsar in a globular star cluster has been discovered in the Andromeda galaxy. Credit: A. Zolotov

Based on a total of 38 XMM-Newton observations, the team concluded that this pulsar (which was the only known pulsar of its kind beyond our galaxy at the time), is in the earliest stages of “rejuvenation”. In short, their observations indicated that the pulsar began accelerating less than 1 million years ago. This conclusion was based on the fact that XB091D is the slowest rotating globular cluster pulsar discovered to date.

The neutron star completes one revolution in 1.2 seconds, which is more than 10 times slower than the previous record holder.  From the data they observed, they were also able to characterize the environment around XB091D. For example, they found that the pulsar and its binary pair are located in an extremely dense globular cluster (B091D) in the Andromeda Galaxy – about 2.5 million light years away.

This cluster is estimated to be 12 billion years old and contains millions of old, faint stars. It’s companion, meanwhile, is a 0.8 solar mass star, and the binary system  itself has a rotation period of 30.5 hours. And in about 50,000 years, they estimate, the pulsar will accelerate sufficiently to once again have a rotational period measured in the milliseconds – i.e. a millisecond pulsar.

A diagram of the ESA XMM-Newton X-Ray Telescope. Delivered to orbit by a Ariane 5 launch vehicle in 1999. Credit: ESA/XMM-Newton

Interestingly, XB910D’s location within this vast region of super-high density stars is what allowed it to capture a companion about 1 million years ago and commence the process “rejuvenation” in the first place. As Zolotukhin explained:

“In our galaxy, no such slow X-ray pulsars are observed in 150 known globular clusters, because their cores are not big and dense enough to form close binary stars at a sufficiently high rate. This indicates that the B091D cluster core, with an extremely dense composition of stars in the XB091D, is much larger than that of the usual cluster. So we are dealing with a large and rather rare object—with a dense remnant of a small galaxy that the Andromeda galaxy once devoured. The density of the stars here, in a region that is about 2.5 light years across, is about 10 million times higher than in the vicinity of the Sun.”

Thanks to this study, and the mathematical tools the team developed to find it, astronomers will likely be able to revisit many previously-discovered objects in the coming years. Within these massive data sets, there could be many examples of rare astronomical events, just waiting to be witnessed and properly characterized.

Further reading: The Astrophysical Journal, Lomonosov Moscow State University

Deepest X-ray Image Ever Made Contains Mysterious Explosion

For over sixty years, astronomers have been exploring the Universe for x-ray sources. Known to be associated with stars, clouds of super heated gas, interstellar mediums, and destructive events, the detection of cosmic x-rays is challenging work. In recent decades, astronomers have been benefited immensely from by the deployment of orbital telescopes like the Chandra X-ray Observatory.

Since it was launched on July 23rd, 1999, Chandra has been NASA’s flagship mission for X-ray astronomy. And this past week (on Thurs. March 30th, 2017), the Observatory accomplished something very impressive. Using its suite of advanced instruments, the observatory captured a mysterious flash coming from deep space. Not only was this the deepest X-ray source ever observed, it also revealed what could be an entirely new phenomenon.

Located in the region of the sky known as the Chandra Deep Field-South (CDF-S), this X-ray emission source appeared to have come from a small galaxy located approximately 10.7 billion light-years from Earth. It also had some remarkable properties, producing more energy in the space of a few minutes that all the stars in the galaxy combined.

Artist illustration of the Chandra X-ray Observatory, the most sensitive X-ray telescope ever built. Credit: NASA/CXC/NGST

Originally detected in 2014 by a team of researchers from Penn State University and the Pontifical Catholic University of Chile in Santiago, Chile, this source was not even detected in the X-ray band at first. However, it quickly caught the team’s attention as it erupted and became 1000 brighter in the space of a few hours. At this point, the researchers began gathering data using Chandra’s Advanced CCD Imaging Spectronomer.

A day after the flare-up, the X-ray source had faded to the point that Chandra was no longer able to detect it. As Niel Brandt – the Verne M. Willaman Professor of Astronomy and Astrophysics at Penn State and part of the team that first observed it – described the discovery in a Penn State press release:

“This flaring source was a wonderful surprise bonus that we accidentally discovered in our efforts to explore the poorly understood realm of the ultra-faint X-ray universe. We definitely ‘lucked out’ with this find and now have an exciting new transient phenomenon to explore in future years.”

Thousands of hours of legacy data from the Hubble and Spitzer Space Telescopes was then consulted in order to determine the location of the CDF-S X-ray source. And though scientists were able to determine that the image of the X-ray source placed it beyond any that had been observed before, they are not entirely clear as to what could have caused it.

X-ray (left) and optical (right) images of the space around the X-ray source, made with Chandra and the Hubble Space Telescope, respectively. Credit: NASA/CXC/F. Bauer et al.

On the one hand, it could be the result of some sort of destructive event, or something scientists have never before seen. The reason for this has to do with the fact that X-ray bursts also come with a gamma-ray burst (GRB), which appears to be missing here. Essentially, GRBs are jetted explosions that are triggered by the collapse of a massive star or by the merger of two neutron stars (or a neutron star with a black hole).

Because of this, three possible explanations have been suggested. In the first, the CDF-S X-ray source is indeed the result of a collapsing star or merger, but the resulting jets are not pointed towards Earth. In the second, the same scenario is responsible for the x-ray source, but the GRB lies beyond the small galaxy. The third possible explanation is that the event was caused by a medium-sized black hole shredding a white dwarf star.

Unfortunately, none of these explanations seem to fit the data. However, these research team also noted that these possibilities are not that well understood, since none have been witnessed in the Universe. As Franz Bauer – an astronomer from the Pontifical Catholic University of Chile – said: “Ever since discovering this source, we’ve been struggling to understand its origin. It’s like we have a jigsaw puzzle but we don’t have all of the pieces.”

Not only has Chandra not observed any other X-ray sources like this one during the 17 years it has surveyed the CDF-S region, but no similar events have been observed by the space telescope anywhere in the Universe during its nearly two decades of operation. On top of that, this event was brighter, more short-lived, and occurred in a smaller, younger host galaxy than other unexplained X-ray sources.

Still image of the X-ray source observed by Chandra, showing the captured flare up at bottom Credit: NASA/CXC/Pontifical Catholic Univ./F.Bauer et al.

From all of this, the only takeaway appears to be that the event was likely the result of a cataclysmic event, like a neutron star or a white dwarf being torn apart. But the fact that none of the more plausible explanations seem to account for it’s peculiar characteristics would seem to suggest that astronomers may have witnessed an entirely new kind of cataclysmic event.

The team’s study – “A New, Faint Population of X-ray Transient“- is available online and will be published in the June 2017 issue of the Monthly Notices of the Royal Astronomical Society. In the meantime, astronomers will be sifting through the data acquired by Chandra and other X-ray observatories – like the ESA’s XMM-Newton and NASA’s Swift Gamma-Ray Burst Mission – to see if they can find any other instances of this kind of event.

And of course, future surveys conducted using Chandra and next-generation X-ray telescopes will also be on the lookout for these kind of short-lived, high-energy X-ray bursts. It’s always good when the Universe throws us a curve ball. Not only does it show us that we have more to learn, but it also teaches us that we must never grow complacent in our theories.

Be sure to check out this animation of the CDF-S X-ray source too, courtesy of the Chandra X-ray Observatory:

Further Reading:  Chandra, PennState

A Black Hole’s Record Breaking Lunch

Does a distant black hole provide a new definition of pain and suffering?

The black hole, named XJ1500+0154, appears to be the real-life equivalent of the Pit of Carkoon, the nesting place of the all-powerful Sarlacc in Star Wars, which slowly digested its victims.

Over ten years ago, this giant black hole ripped apart a star and has since continued a very long lunch, feasting on the stars’ remains. Astronomers have been carefully monitoring this slow ‘digestion,’ because it is so unusual for what are called tidal disruption events (TDEs), where tidal forces from black holes tear stars apart.

“We have witnessed a star’s spectacular and prolonged demise,” said Dacheng Lin from the University of New Hampshire in Durham, New Hampshire, who led the observations of this event. “Dozens of tidal disruption events have been detected since the 1990s, but none that remained bright for nearly as long as this one.”

This artist’s illustration depicts what astronomers call a “tidal disruption event,” or TDE, when an object such as a star wanders too close to a black hole and is destroyed by tidal forces generated from the black hole’s intense gravitational forces. (Credit: NASA/CXC/M.Weiss.

This decade-long feast has gone on ten times longer than any other observed TDE.

XJ1500+0154 is located in a small galaxy about 1.8 billion light years from Earth, and three telescopes have been monitoring this X-ray event: the Chandra X-ray Observatory, the Swift satellite, and the XMM-Newton.

TDEs are different from another, more common black-hole related source of X-rays in the galaxy, active galactic nuclei (AGN). Like the digestion of the Sarlacc, AGNs really can last for thousands of years. These are supermassive black holes at the center of galaxies that pull in surrounding gas and “emit copious amounts of radiation, including X-rays,” explained Lin in a blog post on the Chandra website. “Radiation from AGNs do not vary a lot because the gas surrounding them extends over a large scale and can last for tens of thousands of years.”

In contrast, TDEs are relatively short-lived, lasting only a few months. During a TDE, some of the stellar debris is flung outward at high speeds, while the rest falls toward the black hole. As it travels inwards to be consumed by the black hole, the material heats up to millions of degrees, generating a distinct X-ray flare.

XJ1500+0154 has provided an extraordinarily long, bright phase, spanning over ten years. Lin and his team said one explanation could be the most massive star ever to be completely torn apart during a TDE.

“To have the event last so long at such high luminosity requires full disruption of a relatively massive star, about twice the mass of the sun,” Lin wrote; however, “disruption of such massive stars by the SMBH is very unlikely because stars this massive are rare in most galaxies, unless the galaxy is young and actively forming stars, as in our case.

So, another more likely explanation is that this is the first TDE observed where a smaller star was completely torn apart.

Lin also said this event has broad implications for black hole physics.

An X-ray image of the full field of view by of the region where the ‘tidal disruption event’ is taking place. The purple smudge in the lower right shows the disruption from the black hole XJ1500+0154. Credit: X-ray: NASA/CXC/UNH/D.Lin et al.

“To fully explain the super-long duration of our event requires the application of recent theoretical progress on the study of TDEs,” he wrote. “In the last two years, several groups independently found that it can take a long time after the disruption of the star for the stellar debris to settle onto the accretion disk and into the SMBH. Therefore, the event can evolve much more slowly than previously thought.”

Additionally, the X-ray data also indicate that radiation from material surrounding this black hole has consistently surpassed what is called the Eddington limit, which is defined as a balance between the outward pressure of radiation from the hot gas and the inward pull of the gravity of the black hole.

Seeing evidence of such rapid growth may help astronomers understand how supermassive black holes were able to reach masses about a billion times higher than the sun when the universe was only about a billion years old.

“This event shows that black holes really can grow at extraordinarily high rates,” said co-author Stefanie Komossa of QianNan Normal University for Nationalities in Duyun City, China. “This may help understand how precocious black holes came to be.”

Lin and his team will continue to monitor this event, and they expect the X-ray brightness to fade over the next few years, meaning the supply of ‘food’ for this long lunch will soon be consumed.

For further reading:
Paper: A likely decade-long sustained tidal disruption event
Lin’s blog post on the Chandra website
Chandra press release
Additional images and information from Chandra

Space Jellyfish Show Types Of Pulsar Wind Nebulas

Since they were first discovered in the late 1960s, pulsars have continued to fascinate astronomers. Even though thousands of these pulsing, spinning stars have been observed in the past five decades, there is much about them that continues to elude us. For instance, while some emit both radio and gamma ray pulses, others are restricted to either radio or gamma ray radiation.

However, thanks to a pair of studies from two international teams of astronomers, we may be getting closer to understanding why this is. Relying on data collected by the Chandra X-ray Observatory of two pulsars (Geminga and B0355+54), the teams was able to show how their emissions and the underlying structure of their nebulae (which resemble jellyfish) could be related.

These studies, “Deep Chandra Observations of the Pulsar Wind Nebula Created by PSR B0355+54” and “Geminga’s Puzzling Pulsar Wind Nebula” were published in The Astrophysical Journal. For both, the teams relied on x-ray data from the Chandra Observatory to examine the Geminga and B0355+54 pulsars and their associated pulsar wind nebulae (PWN).

An artist’s impression of an accreting X-ray millisecond pulsar. Credit: NASA/Goddard Space Flight Center/Dana Berry

Located 800 and 3400 light years from Earth (respectively), the Geminga and B0355+54 pulsars are quite similar. In addition to having similar rotational periods (5 times per second), they are also about the same age (~500 million years). However, Geminga emits only gamma-ray pulses while B0355+54 is one of the brightest known radio pulsars, but emits no observable gamma rays.

What’s more, their PWNs are structured quite differently. Based on composite images created using Chandra X-ray data and Spitzer infrared data, one resembles a jellyfish whose tendrils are relaxed while the other looks like a jellyfish that is closed and flexed. As Bettina Posselt – a senior research associate in the Department of Astronomy and Astrophysics at Penn State, and the lead author on the Geminga study – told Universe Today via email:

“The Chandra data resulted in two very different X-ray images of the pulsar wind nebulae around the pulsars Geminga and PSR B0355+54. While Geminga has a distinct three-tail structure, the image of PSR B0355+54 shows one broad tail with several substructures.”

In all likelihood, Geminga’s and B0355+54 tails are narrow jets emanating from the pulsar’s spin poles. These jets lie perpendicular to the donut-shaped disk (aka. a torus) that surrounds the pulsars equatorial regions. As Noel Klingler, a graduate student at the George Washington University and the author of the B0355+54 paper, told Universe Today via email:

“The interstellar medium (ISM) isn’t a perfect vacuum, so as both of these pulsars plow through space at hundreds of kilometers per second, the trace amount of gas in the ISM exerts pressure, thus pushing back/bending the pulsar wind nebulae behind the pulsars, as is shown in the images obtained by the Chandra X-ray Observatory.”

Their apparent structures appear to be due to their disposition relative to Earth. In Geminga’s case, the view of the torus is edge-on while the jets point out to the sides. In B0355+54’s case, the torus is seen face-on while the jets points both towards and away from Earth. From our vantage point, these jets look like they are on top of each other, which is what makes it look like it has a double tail. As Posselt describes it:

“Both structures can be explained with the same general model of pulsar wind nebulae. The reasons for the different images are (a) our viewing perspective, and (b) how fast and where to the pulsar is moving. In general, the observable structures of such pulsar wind nebulae can be described with an equatorial torus and polar jets. Torus and Jets can be affected (e.g., bent jets) by the “head wind” from the interstellar medium the pulsar is moving in. Depending on our viewing angle of the torus, jets and the movement of the pulsar, different pictures are detected by the Chandra X-ray observatory. Geminga is seen “from the side” (or edge-on with respect to the torus) with the jets roughly located in the plane of the sky  while for B0355+54 we look almost directly to one of the poles.”

This orientation could also help explain why the two pulsars appear to emit different types of electromagnetic radiation. Basically, the magnetic poles – which are close to their spin poles – are where a pulsar’s radio emissions are believed to come from. Meanwhile, gamma rays are believed to be emitted along a pulsar’s spin equator, where the torus is located.

“The images reveal that we see Geminga from edge-on (i.e., looking at its equator) because we see X-rays from particles launched into the two jets (which are initially aligned with the radio beams), which are pointed into the sky, and not at Earth,” said Klingler. “This explains why we only see Gamma-ray pulses from Geminga.  The images also indicate that we are looking at B0355+54 from a top-down perspective (i.e., above one of the poles, looking into the jets).  So as the pulsar rotates, the center of the radio beam sweeps across Earth, and we detect the pulses;  but the  gamma-rays are launched straight out from the pulsar’s equator, so we don’t see them from B0355.”

An all-sky view from the Fermi Gamma-ray Space Telescope, showing the position of Geminga in the Milky Way. Credit : NASA/DOE/International LAT Team.

“The geometrical constraints on each pulsar (where are the poles and the equator) from the pulsar wind nebulae help to explain findings regarding the radio and gamma-ray pulses of these two neutron stars,” said Posselt. “For example, Geminga appears radio-quiet (no strong radio pulses) because we don’t have a direct view to the poles and pulsed radio emission is thought to be generated in a region close to the poles. But Geminga shows strong gamma-ray pulsations, because these are not produced at the poles, but closer to the equatorial region.”

These observations were part of a larger campaign to study six pulsars that have been seen to emit gamma-rays. This campaign is being led by Roger Romani of Stanford University, with the collaboration of astronomers and researchers from GWU (Oleg Kargaltsev), Penn State University (George Pavlov), and Harvard University (Patrick Slane).

Not only are these studies shedding new light on the properties of pulsar wind nebulae, they also provide observational evidence to help astronomers create better theoretical models of pulsars. In addition, studies like these – which examine the geometry of pulsar magnetospheres – could allow astronomers to better estimate the total number of exploded stars in our galaxy.

By knowing the range of angles at which pulsars are detectable, they should be able to better estimate the amount that are not visible from Earth. Yet another way in which astronomers are working to find the celestial objects that could be lurking in humanity’s blind spots!

Further Reading: Chandra X-Ray Observatory

Winds of Supermassive Black Holes Can Shape Galaxy-Wide Star Formation

The combined observations from two generations of X-Ray space telescopes have now revealed a more complete picture of the nature of high-speed winds expelled from super-massive black holes. Scientist analyzing the observations discovered that the winds linked to these black holes can travel in all directions and not just a narrow beam as previously thought. The black holes reside at the center of active galaxies and quasars and are surrounded by accretion discs of matter. Such broad expansive winds have the potential to effect star formation throughout the host galaxy or quasar. The discovery will lead to revisions in the theories and models that more accurately explain the evolution of quasars and galaxies.

This plot of data from NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) and the European Space Agency's (ESA's) XMM-Newton determines for the first time the shape of ultra-fast winds from supermassive black holes, or quasars. The winds blow in every direction, in a nearly spherical fashion, coming from both sides of a galaxy (Credit: NASA/JPL-Caltech/Keele Univ.;XMM-Newton and NuSTAR Missions)
This plot of data from NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) and the European Space Agency’s (ESA’s) XMM-Newton determines for the first time the shape of ultra-fast winds from supermassive black holes, or quasars. The winds blow in every direction, in a nearly spherical fashion, coming from both sides of a galaxy (Credit: NASA/JPL-Caltech/Keele Univ.;XMM-Newton and NuSTAR Missions, [Ref])
The observations were by the XMM-Newton and NuSTAR x-ray space telescopes of the quasar PDS 456. The observations were combined into the graphic, above. PDS 456 is a bright quasar residing in the constellation Serpens Cauda (near Ophiuchus). The data graph shows both a peak and a trough in the otherwise nominal x-ray emission profile as shown by the NuSTAR data (pink). The peak represents X-Ray emissions directed towards us (i.e.our telescopes) while the trough is X-Ray absorption that indicates that the expulsion of winds from the super-massive black hole is in many directions – effectively a spherical shell. The absorption feature caused by iron in the high speed wind is the new discovery.

X-Rays are the signature of the most energetic events in the Cosmos but also are produced from some of the most docile bodies – comets. The leading edge of a comet such as Rosetta’s P67 generates X-Ray emissions from the interaction of energetic solar ions capturing electrons from neutral particles in the comet’s coma (gas cloud). The observations of a super-massive black hole in a quasar billions of light years away involve the generation of x-rays on a far greater scale, by winds that evidently has influence on a galactic scale.

A diagram of the ESA XMM-Newton X-Ray Telescope. Delivered to orbit by a Ariane 5 launch vehicle in 1999. (Illustration Credit: ESA/XMM-Newton)
A diagram of the ESA XMM-Newton X-Ray Telescope. Delivered to orbit by a Ariane 5 launch vehicle in 1999. (Illustration Credit: ESA/XMM-Newton)

The study of star forming regions and the evolution of galaxies has focused on the effects of shock waves from supernova events that occur throughout the lifetime of a galaxy. Such shock waves trigger the collapse of gas clouds and formation of new stars. This new discovery by the combined efforts of two space telescope teams provides astrophysicists new insight into how star and galaxy formation takes place. Super-massive blackholes, at least early in the formation of a galaxy, can influence star formation everywhere.

The NuStar Space Telescope launched into Earth orbit by a Orbital Science Corp. Pegasus rocket, 2012. The Wolter telescope design images throughout a spectral range from 5 to 80 KeV. (Credit: NASA/Caltech-JPL)
The NuStar Space Telescope launched into Earth orbit by a Orbital Science Corp. Pegasus rocket, 2012. The Wolter telescope design – optics in the foreground, 10 meter truss and detectors at back – images throughout a spectral range from 5 to 80 KeV. (Credit: NASA/Caltech-JPL)

Both the ESA built XMM-Newton and the NuSTAR X-Ray space telescope, a SMEX class NASA mission, use grazing incidence optics, not glass (refraction) or mirrors (reflection) as in conventional visible light telescopes. The incidence angle of the X-rays must be very shallow and consequently the optics are extended out on a 10 meter (33 foot) truss in the case of NuSTAR and over a rigid frame on the XMM-Newton.

Diagram of one of three x-ray telescopes of the XMM-Newton design. Only a few of the grazing angle concentric mirrors are shown. Inset: a simplified illustration of how a Wolter telescope works. (Credits: Wikimedia, ESA)
Diagram of one of three x-ray telescopes of the XMM-Newton design. Only a few of the grazing angle concentric mirrors are shown. Inset: a simplified illustration of how a Wolter telescope works. (Credits: Wikimedia, ESA) [click to enlarge]
The spectral ranges of the XMM-Newton and NuSTAR Telescopes. (Credits: NASA, ESA)
The spectral ranges of the XMM-Newton and NuSTAR Telescopes. (Credits: NASA, ESA)

The ESA built XMM-Newton was launched in 1999, an older generation design that used a rigid frame and structure. All the fairing volume and lift capability of the Ariane 5 launch vehicle was needed to put the Newton in orbit. The latest X-Ray telescope – NuSTAR – benefits from tens years of technological advances. The detectors are more efficient and faster and the rigid frame was replaced with a compact truss which required all of 30 minutes to deploy. Consequently, NuSTAR was launched on a Pegasus rocket piggybacked on a L-1011, a significantly smaller and less expensive launch system.

So now these observations are effectively delivered to the theorists and modelers. The data is like a new ingredient in the batter from which a galaxy and stars are formed. The models of galaxy and star formation will improve and will more accurately describe how quasars, with their active super-massive black-holes, transition into more quiescent galaxies such as our own Milky Way.




Astronomers See a Massive Black Hole Tear a Star Apart

A telescope peers into the blackness of deep space. Suddenly – a brilliant flash of light appears that wasn’t there before. What could it be? A supernova? Two massively dense stars fusing together? Perhaps a gamma ray burst?

Five years ago, researchers using the ROTSE IIIb telescope at McDonald Observatory noticed just such an event. But far from being your run-of-the-mill stellar explosion or neutron star merger, the astronomers believe that this tiny flare was, in fact, evidence of a supermassive black hole at the center of a distant galaxy, tearing a star to shreds.

Astronomers at McDonald had been using the telescope to scan the skies for such nascent flashes for years, as part of the ROTSE Supernova Verification Project (SNVP). And at first blush, the event seen in early 2009, which the researches nicknamed “Dougie,” looked just like many of the other supernovae they had discovered over the course of the project. With a blazing – 22.5-magnitude absolute brightness, the event fit squarely within the class of superluminous supernovae that the researchers were already familiar with.

But as time went on and more data on Dougie rolled in, the astronomers began to change their minds. X-ray observations made by the orbiting Swift satellite and optical spectra taken by McDonald’s Hobby-Eberly Telescope revealed an evolving light curve and chemical makeup that didn’t fit with computer simulations of superluminous supernovae. Likewise, Dougie didn’t appear to be a neutron star merger, which would have reached peak luminosity far more quickly than was observed, or a gamma ray burst, which, even at an angle, would have appeared far brighter in x-ray light.

That left only one option: a so-called “tidal disruption event,” or the carnage and spaghettification that occurs when a star wanders too close to a black hole’s horizon. J. Craig Wheeler, head of the supernova group at The University of Texas at Austin and a member of the team that discovered Dougie, explained that at short distances, a black hole’s gravity exerts a much stronger pull on the side of the star nearest to it than it does on the star’s opposite side. He explained, “These especially large tides can be strong enough that you pull the star out into a noodle.”

The team refined their models of the event and came to a surprising conclusion: having drawn in Dougie’s stellar material a bit faster than it could handle, the black hole was now “choking” on its latest meal. This is due to an astrophysical principle called the Eddington Limit, which states that a black hole of a given size can only handle so much infalling material. After this limit has been reached, any additional intake of matter exerts more outward pressure than the black hole’s gravity can compensate for. This pressure increase has a kind of rebound effect, throwing off material from the black hole’s accretion disk along with heat and light. Such a burst of energy accounts for at least part of Dougie’s brightness, but also indicates that the original dying star – a star not unlike our own Sun – wasn’t going down without a fight.

Combining these observations with the mathematics of the Eddington Limit, the researchers estimated the black hole’s size to be about 1 million solar masses – a rather small black hole, at the center of a rather small galaxy, three billion light years away. Discoveries like these not only allow astronomers to better understand the physics of black holes, but also properties of their often unassuming home galaxies. After all, mused Wheeler, “Who knew this little guy had a black hole?”

To get a simulated glimpse of Dougie for yourself, check out the amazing animation below, courtesy of team member James Guillochon:

The research is published in this month’s issue of The Astrophysical Journal. A pre-print of the paper is available here.

Moonlight Is a Many-Splendored Thing

“By the Light of the Silvery Moon” goes the song. But the color and appearance of the Moon depends upon the particular set of eyes we use to see it. Human vision is restricted to a narrow slice of the electromagnetic spectrum called visible light.

With colors ranging from sumptuous violet to blazing red and everything in between, the diversity of the visible spectrum provides enough hues for any crayon color a child might imagine. But as expansive as the visual world’s palette is, it’s not nearly enough to please astronomers’ retinal appetites.

Visible light is a sliver of light's full range of "colors" which span from kilometers-long, low-energy radio waves (left) to short wavelength, energetic gamma rays. It's all light, with each color determined by wavelength. Familiar objects along the bottom reference light wave sizes. Visible light waves are about one-millionth of a meter wide. Credit: NASA
Visible light is a sliver of light’s full range of “colors” which span from kilometers-long, low-energy radio waves (left) to short wavelength, energetic gamma rays. It’s all light, with each color determined by wavelength. Familiar objects along the bottom reference light wave sizes. Visible light waves are about one-millionth of a meter wide. Credit: NASA

Since the discovery of infrared light by William Herschel in 1800 we’ve been unshuttering one electromagnetic window after another. We build telescopes, great parabolic dishes and other specialized instruments to extend the range of human sight.  Not even the atmosphere gets in our way. It allows only visible light, a small amount of infrared and ultraviolet and selective slices of the radio spectrum to pass through to the ground. X-rays, gamma rays and much else is absorbed and completely invisible.

Earth's atmosphere blocks a good portion of light's diversity from reaching the ground, the reason we launch rockets and orbiting telescopes into space. Large professional telescopes are often built on mountain tops above much of the atmosphere allowing astronomers to see at least some infrared light that is otherwise absorbed by air at lower elevations. Credit: NASA
Earth’s atmosphere blocks a good portion of light’s diversity from reaching the ground, the reason we launch rockets and orbiting telescopes into space. Large professional telescopes are often built on mountain tops above much of the denser, lower atmosphere. This expands the viewing “window” into the infrared. Credit: NASA

To peer into these rarified realms, we’ve lofting air balloons and then rockets and telescopes into orbit or simply dreamed up the appropriate instrument to detect them. Karl Jansky’s homebuilt radio telescope cupped the first radio waves from the Milky Way in the early 1930s; by the 1940s  sounding rockets shot to the edge of space detected the high-frequency sizzle of X-rays.  Each color of light, even the invisible “colors”, show us a new face on a familiar astronomical object or reveal things otherwise invisible to our eyes.

So what new things can we learn about the Moon with our contemporary color vision?

Radio Moon
Radio Moon

Radio: Made using NRAO’s 140-ft telescope in Green Bank, West Virginia. Blues and greens represent colder areas of the moon and reds are warmer regions. The left half  of Moon was facing the Sun at the time of the observation. The sunlit Moon appear brighter than the shadowed portion because it radiates more heat (infrared light) and radio waves.

Submillimeter Moon
Submillimeter Moon

Submillimeter: Taken using the SCUBA camera on the James Clerk Maxwell Telescope in Hawaii. Submillimeter radiation lies between far infrared and microwaves. The Moon appears brighter on one side because it’s being heated by Sun in that direction. The glow comes from submillimeter light radiated by the Moon itself. No matter the phase in visual light, both the submillimeter and radio images always appear full because the Moon radiates at least some light at these wavelengths whether the Sun strikes it or not.

Mid-infrared Moon
Mid-infrared Moon

Mid-infrared: This image of the Full Moon was taken by the Spirit-III instrument on the Midcourse Space Experiment (MSX) at totality during a 1996 lunar eclipse. Once again, we see the Moon emitting light with the brightest areas the warmest and coolest regions darkest. Many craters look like bright dots speckling the lunar disk, but the most prominent is brilliant Tycho near the bottom. Research shows that young, rock-rich surfaces, such as recent impact craters, should heat up and glow more brightly in infrared than older, dust-covered regions and craters. Tycho is one of the Moon’s youngest craters with an age of just 109 million years.

Near-infrared Moon
Near-infrared Moon

Near-infrared: This color-coded picture was snapped just beyond the visible deep red by NASA’s Galileo spacecraft during its 1992 Earth-Moon flyby en route to Jupiter. It shows absorptions due to different minerals in the Moon’s crust. Blue areas indicate areas richer in iron-bearing silicate materials that contain the minerals pyroxene and olivine. Yellow indicates less absorption due to different mineral mixes.

Visible light Moon
Visible light Moon

Visible light: Unlike the other wavelengths we’ve explored so far, we see the Moon not by the light it radiates but by the light it reflects from the Sun.

The iron-rich composition of the lavas that formed the lunar “seas” give them a darker color compared to the ancient lunar highlands, which are composed mostly of a lighter volcanic rock called anorthosite.

UV Moon
UV Moon

Ultraviolet: Similar to the view in visible light but with a lower resolution. The brightest areas probably correspond to regions where the most recent resurfacing due to impacts has occurred. Once again, the bright rayed crater Tycho stands out in this regard. The photo was made with the Ultraviolet Imaging Telescope flown aboard the Space Shuttle Endeavour in March 1995.

X-ray Moon
X-ray Moon

X-ray: The Moon, being a relatively peaceful and inactive celestial body, emits very little x-ray light, a form of radiation normally associated with highly energetic and explosive phenomena like black holes. This image was made by the orbiting ROSAT Observatory on June 29, 1990 and shows a bright hemisphere lit by oxygen, magnesium, aluminum and silicon atoms fluorescing in x-rays emitted by the Sun. The speckled sky records the “noise” of distant background X-ray sources, while the dark half of the Moon has a hint of illumination from Earth’s outermost atmosphere or geocorona that envelops the ROSAT observatory.

Gamma ray Moon
Gamma ray Moon

Gamma rays: Perhaps the most amazing image of all. If you could see the sky in gamma rays the Moon would be far brighter than the Sun as this dazzling image attempts to show. It was taken by the Energetic Gamma Ray Experiment Telescope (EGRET).  High-energy particles (mostly protons) from deep space called cosmic rays constantly bombard the Moon’s surface, stimulating the atoms in its crust to emit gamma rays. These create a unique high-energy form of “moonglow”.

Astronomy in the 21st century is like having a complete piano keyboard on which to play compared to barely an octave a century ago. The Moon is more fascinating than ever for it.

End the Year with a Bang! See a Bright Supernova in Virgo

A 14th magnitude supernova discovered in the spiral galaxy NGC 4666 earlier this month has recently brightened to 11th magnitude, making it not only the second brightest supernova of the year, but an easy find in an 8-inch or larger telescope. I made a special trip into the cold this morning for a look and saw it with ease in my 10-inch (25-cm) scope at low power at magnitude 11.9.

Before the Moon taints the dawn sky, you may want to bundle up and have a look, too. The charts below will help you get there.

NGC 4666 is also known as the Superwind Galaxy. Home to vigorous star formation, a combination of supernova explosions and strong winds from massive stars in the starburst region drives a vast outflow of gas from the galaxy into space, a so-called “superwind”. Credit: ESO/J. Dietrich
NGC 4666 is also known as the Superwind Galaxy. Home to vigorous star formation, a combination of supernova explosions and strong winds from massive stars in the starburst region drives a vast outflow of gas from the galaxy into space, called a “superwind”. Credit: ESO/J. Dietrich

With the temporary name ASASSN-14lp, this Type Ia supernova was snatched up by the catchy-titled “Assassin Project”, short for  Automated Sky Survey for SuperNovae (ASAS-SN) on December 9th. Only 80 million light years from Earth, NGC 4666 is a relatively nearby spiral galaxy famous enough to earn a nickname.

Extra-planar soft X-ray emitting hot gas is observed above the most actively star-forming regions in the galactic disk of NGC 4666 and coexists together with filaments of the warm ionized medium, cosmic rays and vertical magnetic field structures channelling (or following) the outflow. Credit: M. Ehle and ESO
Hot, X-ray emitting gas in NGC 4666 billows around the main galaxy as a superwind seen here as outflows on either side of the optical image. Photo taken with the XMM-Newton telescope.  Credit: M. Ehle and ESO

Called the Superwind Galaxy, it’s home to waves of intense star formation thought to be caused by gravitational interactions between it and its neighboring galaxies, including NGC 4668, visible in the lower left corner of the photo above.

Supernovae also play a part in powering the wind which emerges from the galaxy’s central regions like pseudopods on an amoeba.  X-ray and radio light show the outflows best. How fitting that a bright supernova should happen to appear at this time. Seeing one of the key players behind the superwind with our own eyes gives us a visceral feel for the nature of its home galaxy.

Wide view map showing the location of the galaxy NGC 4666 in Virgo not far from Porrima or Gamma Virginis. This map shows the sky facing south shortly before the start of dawn in early January. Source: Stellarium
“Big picture” map showing the location of the galaxy NGC 4666 in Virgo not far from Porrima. The view faces south shortly before the start of dawn in early January. Source: Stellarium

Spectra taken of ASASSN-14lp show it to be a Type Ia object involving the explosive burning of a white dwarf star in a binary system. The Earth-size dwarf packs the gravitational might of a sun-size star and pulls hydrogen gas from the nearby companion down to its surface. Slowly, the dwarf gets heavier and more massive.

When it attains a mass 1.4 times that of the sun, it can no longer support itself. The star suddenly collapses, heats to incredible temperatures and burns up explosively in a runaway fusion reaction. Bang! A supernova.

Detailed map with stars to about magnitude 10. The galaxy is just a little more than a degree northeast of Porrima (Gamma Virginis). Source: Stellarium
Detailed map with stars to about magnitude 10. The galaxy is just a little more than a degree northeast of Porrima (Gamma Virginis). Source: Stellarium

Here are a couple maps to help you find the new object. Fortunately, it’s high in the sky just before the start of dawn in the “Y” of Virgo only a degree or so from the 3rd magnitude double star Porrima, also known as Gamma Virginis. Have at it and let us know if you spot the latest superwind-maker.

For more photos and magnitude updates, check out Dave Bishop’s page on the supernova. You can also print a chart with comparison magnitudes by clicking over to the AAVSO and typing in ASASSN-14lp in the “name” box.

New Signal May Be Evidence of Dark Matter, Say Researchers

Dark Matter Halo

Dark matter is the architect of large-scale cosmic structure and the engine behind proper rotation of galaxies. It’s an indispensable part of the physics of our Universe – and yet scientists still don’t know what it’s made of. The latest data from Planck suggest that the mysterious substance comprises 26.2% of the cosmos, making it nearly five and a half times more prevalent than normal, everyday matter. Now, four European researchers have hinted that they may have a discovery on their hands: a signal in x-ray light that has no known cause, and may be evidence of a long sought-after interaction between particles – namely, the annihilation of dark matter.

When astronomers want to study an object in the night sky, such as a star or galaxy, they begin by analyzing its light across all wavelengths. This allows them to visualize narrow dark lines in the object’s spectrum, called absorption lines. Absorption lines occur because a star’s or galaxy’s component elements soak up light at certain wavelengths, preventing most photons with those energies from reaching Earth. Similarly, interacting particles can also leave emission lines in a star’s or galaxy’s spectrum, bright lines that are created when excess photons are emitted via subatomic processes such as excitement and decay. By looking closely at these emission lines, scientists can usually paint a robust picture of the physics going on elsewhere in the cosmos.

But sometimes, scientists find an emission line that is more puzzling. Earlier this year, researchers at the Laboratory of Particle Physics and Cosmology (LPPC) in Switzerland and Leiden University in the Netherlands identified an excess bump of energy in x-ray light coming from both the Andromeda galaxy and the Perseus star cluster: an emission line with an energy around 3.5keV. No known process can account for this line; however, it is consistent with models of the theoretical sterile neutrino – a particle that many scientists believe is a prime candidate for dark matter.

The researchers believe that this strange emission line could result from the annihilation, or decay, of these dark matter particles, a process that is thought to release x-ray photons. In fact, the signal appeared to be strongest in the most dense regions of Andromeda and Perseus and increasingly more diffuse away from the center, a distribution that is also characteristic of dark matter. Additionally, the signal was absent from the team’s observations of deep, empty space, implying that it is real and not just instrumental artifact.

In a pre-print of their paper, the researchers are careful to stress that the signal itself is weak by scientific standards. That is, they can only be 99.994% sure that it is a true result and not just a rogue statistical fluctuation, a level of confidence that is known as 4σ. (The gold standard for a discovery in science is 5σ: a result that can be declared “true” with 99.9999% confidence) Other scientists are not so sure that dark matter is such a good explanation after all. According to predictions made based on measurements of the Lyman-alpha forest – that is, the spectral pattern of hydrogen absorption and photon emission within very distant, very old gas clouds – any particle purporting to be dark matter should have an energy above 10keV – more than twice the energy of this most recent signal.

As always, the study of cosmology is fraught with mysteries. Whether this particular emission line turns out to be evidence of a sterile neutrino (and thus of dark matter) or not, it does appear to be a signal of some physical process that scientists do not yet understand. If future observations can increase the certainty of this discovery to the 5σ level, astrophysicists will have yet another phenomena to account for – an exciting prospect, regardless of the final result.

The team’s research has been accepted to Physical Review Letters and will be published in an upcoming issue.

Just in Time for the Holidays – Galactic Encounter Puts on Stunning Display

At this time of year, festive displays of light are to be expected. This tradition has clearly not been lost on the galaxies NHC 2207 and IC 2163. Just in time for the holidays, these colliding galaxies, which are located within the Canis Major constellation (some 130 million light-years from Earth,) were seen putting on a spectacular lights display for us folks here on Earth!

And while this galaxy has been known to produce a lot of intense light over the years, the image above is especially luminous. A composite using data from the Chandra Observatory and the Hubble and Spitzer Space Telescopes, it shows the combination of visible, x-ray, and infrared light coming from the galactic pair.

In the past fifteen years, NGC 2207 and IC 2163 have hosted three supernova explosions and produced one of the largest collections of super bright X-ray lights in the known universe. These special objects – known as “ultraluminous X-ray sources” (ULXs) – have been found using data from NASA’s Chandra X-ray Observatory.

While the true nature of ULXs is still being debated, it is believed that they are a peculiar type of star X-ray binary. These consist of a star in a tight orbit around either a neutron star or a black hole. The strong gravity of the neutron star or black hole pulls matter from the companion star, and as this matter falls toward the neutron star or black hole, it is heated to millions of degrees and generates X-rays.

 the core of galaxy Messier 82 (M82), where two ultraluminous X-ray sources, or ULXs, reside (X-1 and X-2). Credit: NASA
The core of galaxy Messier 82 (M82), where two ultraluminous X-ray sources, or ULXs, reside (X-1 and X-2). Credit: NASA

Data obtained from Chandra has determined that – much like the Milky Way Galaxy – NGC 2207 and IC 2163 are sprinkled with many star X-ray binaries. In the new Chandra image, this x-ray data is shown in pink, which shows the sheer prevalence of x-ray sources within both galaxies.

Meanwhile, optical light data from the Hubble Space Telescope is rendered in red, green, and blue (also appearing as blue, white, orange, and brown due to color combinations,) and infrared data from the Spitzer Space Telescope is shown in red.

The Chandra observatory spent far more time observing these galaxies than any previous ULX study, roughly five times as much. As a result, the study team – which consisted of researchers from Harvard University, MIT, and Sam Houston State University – were able to confirm the existence of 28 ULXs between NGC 2207 and IC 2163, seven of which had never before been seen.

In addition, the Chandra data allowed the team of scientists to observe the correlation between X-ray sources in different regions of the galaxy and the rate at which stars are forming in those same regions.

Galaxy mergers, such as the Mice Galaxies will be part of Galaxy Zoo's newest project. Credit: Hubble Space Telescope
The Mice galaxies, seen here well into the process of merging. Credit: Hubble Space Telescope

As the new Chandra image shows, the spiral arms of the galaxies – where large amounts of star formation is known to be occurring – show the heaviest concentrations of ULXs, optical light, and infrared. This correlation also suggests that the companion star in the star X-ray binaries is young and massive.

This in turn presents another possibility which has to do with star formation during galactic mergers. When galaxies come together, they produce shock waves that cause clouds of gas within them to collapse, leading to periods of intense star formation and the creation of star clusters.

The fact that the ULXs and the companion stars are young (the researchers estimate that they are only 10 million years old) would seem to confirm that they are the result of NGC 2207 and IC 2163 coming together. This seem a likely explanation since the merger between these two galaxies is still in its infancy, which is attested to by the fact that the galaxies are still separate.

They are expected to collide soon, a process which will make them look more like the Mice Galaxies (pictured above). In about one billion years time, they are expected to finish the process, forming a spiral galaxy that would no doubt resemble our own.

A paper describing the study was recently published on online with The Astrophysical Journal.

Further Reading: NASA/JPL, Chandra, arXiv Astrophysics