Black Holes Archives

February 8, 2010December 24, 2015 by Jean Tate

Caught in the Act! Merging Galaxies Create a Binary Quasar

SDSS J1254+0846 x-ray (blue), optical (yellow)(Credits: X-ray: NASA/CXC/SAO/Green et al Optical: Carnegie Obs/Magellan/Baade Telescope/Mulchaey et al)

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Excellent teamwork by astronomers working in two different wavebands – x-ray and optical – has led to the discovery of a binary quasar being created by a pair of merging galaxies.

“This is really the first case in which you see two separate galaxies, both with quasars, that are clearly interacting,” says Carnegie astronomer John Mulchaey who made observations crucial to understanding the galaxy merger.

“The model verifies the merger origin for this binary quasar system,” Thomas Cox, now a fellow at the Carnegie Observatories, says, referring to computer simulations of the merging galaxies he produced. When Cox’s model galaxies merged, they showed features remarkably similar to what Mulchaey observed in the Magellan images. “It also hints that this kind of galaxy interaction is a key component of the growth of black holes and production of quasars throughout our universe,” Cox added.

“Just because you see two galaxies that are close to each other in the sky doesn’t mean they are merging,” says Mulchaey. “But from the Magellan images we can actually see tidal tails, one from each galaxy, which suggests that the galaxies are in fact interacting and are in the process of merging.”

As Universe Today readers know, quasars are the extremely bright centers of galaxies surrounding supermassive black holes, and binary quasars are pairs of quasars bound together by the mutual gravitation of the two host galaxies’ nuclei. Binary quasars, like other quasars, are thought to be the product of galaxy mergers. Until now, however, binary quasars have not been seen in galaxies that are unambiguously in the act of merging. But images of a new binary quasar from the Carnegie Institution’s Magellan telescope in Chile show two distinct galaxies with tails produced by tidal forces from their mutual gravitational attraction.

Supermassive black holes are to be found in the nuclei of most, if not all, large galaxies, such as our galaxy the Milky Way. Because galaxies regularly interact and merge, astronomers have concluded that binary supermassive black holes have been common in the Universe, especially during its early history (when galaxy mergers were far more common). Supermassive black holes can only be detected as quasars – which are one kind of highly luminous active galactic nucleus (AGN) – when they are actively accreting matter, a process that releases vast amounts of energy across the entire electromagnetic spectrum. A leading theory of ordinary AGNs is that galaxy mergers trigger accretion, creating quasars in both galaxies (AGNs in the hearts of the giant elliptical galaxies in rich clusters are thought to be fueled by a different mechanism, cooling flow). Because most such mergers would have happened in the distant past, binary quasars and their associated galaxies are very far away and therefore difficult for most telescopes to resolve.

The binary quasar, named SDSS J1254+0846, was initially detected by the Sloan Digital Sky Survey, a multi-year, large scale astronomical survey of galaxies and quasars. Further observations by Paul Green of the Harvard-Smithsonian Center for Astrophysics and colleagues using NASA’s Chandra’s X-ray Observatory and telescopes at Kitt Peak National Observatory in Arizona and Palomar Observatory in California strongly suggest that the object was likely a binary quasar in the midst of a galaxy merger. Carnegie’s Mulchaey then used the 6.5 meter Baade-Magellan telescope at the Las Campanas observatory in Chile to obtain deeper images and more detailed spectroscopy of the merging galaxies.

The Astrophysical Journal paper on this object is: “SDSS J1254+0846: A Binary Quasar Caught in the Act of Merging” (Paul J. Green et al 2010 ApJ 710 1578-1588; arXiv:1001.1738 is the preprint).

Source: Carnegie Institution for Science

February 4, 2010December 24, 2015 by Jean Tate

GRB Central Engines Observed in Nearby Supernovae?

SN 2009bb (Image Credit: NASA, Swift, Stefan Immler)

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Are the relativistic jets of long gamma ray bursts (GRBs) produced by brand new black holes? Do some core-collapse supernovae result in black holes and relativistic jets?

The answer to both questions is ‘very likely, yes’! And what recent research points to those answers? Study of an Ic supernova (SN 2007gr), and an Ibc one (SN 2009bb), by two different teams, using archived Gamma-Ray Burst Coordination Network data, and trans-continental Very Long Baseline Interferometry (VLBI) radio observations.

“In every respect, these objects look like gamma-ray bursts – except that they produced no gamma rays,” said Alicia Soderberg at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

Soderberg led a team that studied SN 2009bb, a supernova discovered in March 2009. It exploded in the spiral galaxy NGC 3278, located about 130 million light-years away.

The other object is SN 2007gr, which was first detected in August 2007 in the spiral galaxy NGC 1058, some 35 million light-years away (it’s one of the closest Ic supernovae detected in the radio waveband). The team which studied this supernova using VLBI was led by Zsolt Paragi at the Netherlands-based Joint Institute for Very Long Baseline Interferometry in Europe, and included Chryssa Kouveliotou, an astrophysicist at NASA’s Marshall Space Flight Center in Huntsville, Alabama.

The researchers searched for gamma-rays associated with the supernovae using archived records in the Gamma-Ray Burst Coordination Network located at NASA’s Goddard Space Flight Center in Greenbelt, Md. This project distributes and archives observations of gamma-ray bursts by NASA’s SWIFT spacecraft, the Fermi Gamma-ray Space Telescope and many others. However, no bursts coincided with the supernovae.

“The explosion dynamics in typical supernovae limit the speed of the expanding matter to about three percent the speed of light,” explained Kouveliotou, co-author of one of the new studies. “Yet, in these new objects, we’re tracking gas moving some 20 times faster than this.”

Unlike typical core-collapse supernovae, the stars that produce long gamma-ray bursts possess a “central engine” – likely a nascent black hole – that drives particle jets clocked at more than 99 percent the speed of light (short GRBs are likely produced by the collision/merger of two neutron stars, or a neutron star and a stellar mass black hole).

By contrast, the fastest outflows detected from SN 2009bb reached 85 percent of the speed of light and SN 2007gr reached more than 60 percent of light speed; this is “mildly relativistic”.

“These observations are the first to show some supernovae are powered by a central engine,” Soderberg said. “These new radio techniques now give us a way to find explosions that resemble gamma-ray bursts without relying on detections from gamma-ray satellites.”

The VLBI radio observations showcase how the new electronic capabilities of the European VLBI Network empower astronomers to react quickly when transient events occur. The team led by Paragi included 14 members from 12 institutions spread over seven countries, the United States, the Netherlands, Hungary, the United Kingdom, Canada, Australia and South Africa.

“Using the electronic VLBI technique eliminates some of the major issues,” said Huib Jan van Langevelde, the director of JIVE “Moreover it allows us to produce immediate results necessary for the planning of additional measurements.”

Perhaps as few as one out of every 10,000 supernovae produce gamma rays that we detect as a long gamma-ray burst. In some cases, the star’s jets may not be angled in a way to produce a detectable burst; in others, the energy of the jets may not be enough to allow them to blast through the overlying bulk of the dying star.

“We’ve now found evidence for the unsung crowd of supernovae – those with relatively dim and mildly relativistic jets that only can be detected nearby,” Kouveliotou said. “These likely represent most of the population.”

The 28 January, 2010 issue of Nature contains two papers reporting these discoveries: A relativistic type Ibc supernova without a detected γ-ray burst (arXiv:0908.2817 is the preprint), and A mildly relativistic radio jet from the otherwise normal type Ic supernova 2007gr (arXiv:1001.5060 is the preprint).

Sources: Newborn Black Holes May Add Power to Many Exploding Stars, Newborn Black Holes Boost Explosive Power of Supernovae

January 27, 2010December 24, 2015 by Nancy Atkinson

Extra-Galactic Whopper Black Hole Breaks Distance Record

This image composite shows the spectacular spiral galaxy NGC 300 as seen in an image from the Digitized Sky Survey 2 (DSS2), as well as the position of the stellar-mass black hole in the galaxy in an image obtained with the FORS2 instrument on the VLT. Credit: ESO/ Digitized Sky Survey 2/P. Crowther

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Not only is a newly found black huge, it also is the most far-away stellar-mass black hole ever detected. “This is the most distant stellar-mass black hole ever weighed, and it’s the first one we’ve seen outside our own galactic neighborhood, the Local Group,” said Paul Crowther, from the University of Sheffield. Using ESO’s Very Large Telescope, astronomers peered six million light-years from Earth into a spiral galaxy called NGC 300 and found a black hole with a mass above fifteen times that of the Sun. This makes it the second most massive stellar-mass black hole ever found. But soon it could get bigger. The black hole appears to have a nearby partner, a massive Wolf–Rayet star which likely will become a black hole itself, and the two black holes could merge into an even more massive object.

This image obtained with the FORS2 instrument on the VLT is centred on the position of the black hole. The image covers a field of view of about 2x2 arcminutes, or about 4000 light-years at the distance of NGC 300. Credit: ESO/P. Crowther

In 2007, an X-ray source in NGC 300 was discovered with the XMM-Newton X-ray observatory and the Swift Observatory. “We recorded periodic, extremely intense X-ray emission, a clue that a black hole might be lurking in the area,” said team member Stefania Carpano from ESA.

Subsequent observations with the VLT’s FORS2 instrument (a visual and near UV FOcal Reducer and low dispersion Spectrograph) confirmed their hunch, but also showed that the black hole and the Wolf–Rayet star circled each other every 32 hours. The astronomers also found that the black hole is stripping matter away from the star as they orbit each other.

“This is indeed a very ‘intimate’ couple,” said collaborator Robin Barnard. “How such a tightly bound system has been formed is still a mystery.”

Artists impression of the black hole and Wolf-Rayet star in NGC 300. Credit: ESO

Stellar-mass black holes are the extremely dense, final remnants of the collapse of very massive stars. These black holes have masses up to around twenty times the mass of the Sun, as opposed to supermassive black holes, found in the center of most galaxies, which can weigh a million to a billion times as much as the Sun. So far, around 20 stellar-mass black holes have been found.

Only one other system of this type has previously been seen, but other systems comprising a black hole and a companion star are not unknown to astronomers. Based on these systems, the astronomers see a connection between black hole mass and galactic chemistry.

“We have noticed that the most massive black holes tend to be found in smaller galaxies that contain less ‘heavy’ chemical elements,” said Crowther. “Bigger galaxies that are richer in heavy elements, such as the Milky Way, only succeed in producing black holes with smaller masses.”

Astronomers believe that a higher concentration of heavy chemical elements influences how a massive star evolves, increasing how much matter it sheds, resulting in a smaller black hole when the remnant finally collapses.

In less than a million years, it will be the Wolf–Rayet star’s turn to go supernova and become a black hole. “If the system survives this second explosion, the two black holes will merge, emitting copious amounts of energy in the form of gravitational waves as they combine,” said Crowther.

But this won’t happen for a few billion years. “Our study does however show that such systems might exist, and those that have already evolved into a binary black hole might be detected by probes of gravitational waves, such as LIGO or Virgo.”

Paper: NGC 300 1-X is a Wolf-Rayet/Black Hole Binary

Source: ESO

January 5, 2010December 24, 2015 by Nancy Atkinson

Chandra Stares Deep into the Heart of Sagittarius A*

Caption: Latest Chandra image of Sgr A*. Credits: X-ray: NASA/CXC/MIT/F. Baganoff, R. Shcherbakov et al.

How long can you stare at an object? This Chandra image of the supermassive black hole at the center of the Milky Way Galaxy, known as Sagittarius A* (or Sgr A* for short)Sgr A* and the surrounding region is based on data from a series of observations lasting a total of about one million seconds, or almost two weeks. Such a deep observation has given scientists an unprecedented view of the nearby supernova remnant, known as Sgr A East, and the lobes of hot gas extending for a dozen light years on either side of the black hole. These lobes provide evidence for powerful eruptions occurring several times over the last ten thousand years. But this image also provides evidence that Sgr A* isn’t a very good eater.

Astronomers have known this for quite some time. The fuel for this black hole comes from powerful winds blown off dozens of massive young stars that are concentrated nearby. These stars are located a relatively large distance away from Sgr A*, where the gravity of the black hole is weak, and so their high-velocity winds are difficult for the black hole to capture and swallow. Scientists have previously calculated that Sgr A* should consume only about 1 percent of the fuel carried in the winds.

However, it now appears that Sgr A* consumes even less than expected — ingesting only about one percent of that one percent. Why does it consume so little? The answer may be found in a new theoretical model developed using data from a very deep exposure made by NASA’s Chandra X-ray Observatory. This model considers the flow of energy between two regions around the black hole: an inner region that is close to the so-called event horizon (the boundary beyond which even light cannot escape), and an outer region that includes the black hole’s fuel source — the young stars — extending up to a million times farther out. Collisions between particles in the hot inner region transfer energy to particles in the cooler outer region via a process called conduction. This, in turn, provides additional outward pressure that makes nearly all of the gas in the outer region flow away from the black hole. The model appears to explain well the extended shape of hot gas detected around Sgr A* in X-rays as well as features seen in other wavelengths.

The image also contains several mysterious X-ray filaments, some of which may be huge magnetic structures interacting with streams of energetic electrons produced by rapidly spinning neutron stars. Such features are known as pulsar wind nebulas.

The new model of Sgr A* was presented at the 215th meeting of the American Astronomical Society in January 2009 by Roman Shcherbakov and Robert Penna of Harvard University and Frederick K. Baganoff of the Massachusetts Institute of Technology.

Source: NASA

January 4, 2010April 26, 2016 by Nancy Atkinson

Dual Black Holes Spinning in a Cosmic Dance – Complete with Disco Ball

Caption: An image of the galaxy COSMOS J100043.15+020637.2 taken with the Advanced Camera for Surveys on the Hubble Space Telescope. Image courtesy Dr. Julia Comerford.

Astronomers have discovered 33 pairs of merging black holes in cosmic dances around each other, a finding that was predicted or ‘choreographed’ by Isaac Newton. “These results are significant because we now know that these ‘waltzing’ black holes are much more common than previously known,” said Dr. Julia Comerford of the University of California, Berkeley, at the American Astronomical Society meeting in Washington, DC. “Galaxy mergers are causing the waltzing, can use this finding to determine how often mergers occur. The black holes dancing towards us are shifted towards blue light, and those moving away from us are shifted toward the red. So it is like a cosmic disco ball showing us where the black holes are dancing.”

The dances are occurring in dual black holes, which are different from binary black holes in that the distance between the two object is much larger for dual black holes.

“These black holes have a separation of a kilo parsec,” said Comerford. “You haven’t heard about lots of small binary black holes, because no one has definitively found any yet. But this is the next best thing. We know these duals are going to merge and can use models to find out how often they merge.”

The team was able to observe the black holes that have gas collapsing onto them, and this gas releases energy and powers each black hole as an active galactic nucleus (AGN), which lights up the black hole like a Christmas tree.

Astronomical observations have shown that nearly every galaxy has a central supermassive black hole (with a mass of a million to a billion times the mass of the Sun), and also that galaxies commonly collide and merge to form new, more massive galaxies. As a consequence of these two observations, a merger between two galaxies should bring two supermassive black holes to the new, more massive galaxy formed from the merger. The two black holes gradually in-spiral toward the center of this galaxy, engaging in a gravitational tug-of-war with the surrounding stars. The result is a black hole dance. Such a dance is expected to occur in our own Milky Way Galaxy in about 3 billion years, when it collides with the Andromeda Galaxy.

The team of astronomers used two new techniques to discover the waltzing black holes. First, they identified waltzing black holes and their velocities by the disco ball of the red-shift or blue-shift.

The second technique for identifying waltzing black holes through a chance discovery of a curious-looking galaxy. While visually inspecting images of galaxies taken with the Advanced Camera for Surveys on the Hubble Space Telescope, the team noticed a galaxy with a tidal tail of stars, gas, and dust, an unmistakable sign that the galaxy had recently merged with another galaxy, and the galaxy also featured two bright nuclei near its center. The team recognized that the two bright nuclei might be the AGNs of two waltzing black holes, a hypothesis seemingly supported by the recent galaxy merger activity evinced by the tidal tail. To test this hypothesis, the very next night the team obtained a spectrum of the galaxy with the DEIMOS spectrograph on the 10-meter (400-inch) Keck II Telescope on Mauna Kea, Hawaii.

The spectrum showed that the two central nuclei in the galaxy were indeed both AGNs, supporting the team’s hypothesis that the galaxy has two supermassive black holes. The black holes may be waltzing within the host galaxy, or the galaxy may have a recoiling black hole kicked out of the galaxy by gravity wave emission; additional observations are necessary to distinguish between these explanations.

Comerford said these new techniques can be used to find many more waltzing pairs in the future.

Source: AAS, Dr. Julia Comerford’s website

January 4, 2010December 24, 2015 by Nancy Atkinson

Stellar Destruction Could Be from Intermediate Black Hole

NGC 1399, an elliptical galaxy about 65 million light years from Earth. Credit: NASA, Chandra

A dense stellar remnant has been ripped apart by a black hole a thousand times as massive as the Sun. If confirmed, this discovery would be a cosmic double play: it would be strong evidence for an intermediate mass black hole — which has been a hotly debated topic — and would mark the first time such a black hole has been caught tearing a star apart. Scientists believe a mysterious intense X-ray emission, called an “ultraluminous X-ray source” or ULX is responsible for the destruction. “Astronomers have made cases for stars being torn apart by supermassive black holes in the centers of galaxies before, but this is the first good evidence for such an event in a globular cluster,” said Jimmy Irwin of the University of Alabama, who led the study.

The new results come from the Chandra X-ray Observatory and the Magellan telescope, and were announced at the 215th American Astronomical Society meeting today.

The scenario is based on Chandra observations, which revealed the ULX in a dense cluster of old stars, and optical observations that showed a peculiar mix of elements associated with the X-ray emission. Taken together, a case can be made that the X-ray emission is produced by debris from a disrupted white dwarf star that is heated as it falls towards a massive black hole. The optical emission comes from debris further out that is illuminated by these X-rays.

The intensity of the X-ray emission places the source in the category, meaning that it is more luminous than any known stellar X-ray source, but less luminous than the bright X-ray sources (active galactic nuclei) associated with supermassive black holes in the nuclei of galaxies. The nature of ULXs is a mystery, but one suggestion is that some ULXs are black holes with masses between about a hundred and several thousand times that of the Sun, a range intermediate between stellar-mass black holes and supermassive black holes located in the nuclei of galaxies.

Evidence from NASA's Chandra X-ray Observatory and the Magellan telescopes suggest a star has been torn apart by an intermediate-mass black hole in a globular cluster. Credit: NASA, Chandra

This ULX is in a globular cluster, NGC 1399, an elliptical galaxy about 65 million light-years from Earth that is a very old and crowded conglomeration of stars. Astronomers have suspected that globular clusters could contain intermediate-mass black holes, but conclusive evidence for this has been elusive.

Irwin and his colleagues obtained optical spectra of the object using the Magellan I and II telescopes in Las Campanas, Chile. These data reveal emission from gas rich in oxygen and nitrogen but no hydrogen, a rare set of signals from globular clusters. The physical conditions deduced from the spectra suggest that the gas is orbiting a black hole of at least 1,000 solar masses. The abundant amount of oxygen and absence of hydrogen indicate that the destroyed star was a white dwarf, the end phase of a solar-type star that has burned its hydrogen leaving a high concentration of oxygen. The nitrogen seen in the optical spectrum remains an enigma.

“We think these unusual signatures can be explained by a white dwarf that strayed too close to a black hole and was torn apart by the extreme tidal forces,” said coauthor Joel Bregman of the University of Michigan.

Theoretical work suggests that the tidal disruption-induced X-ray emission could stay bright for more than a century, but it should fade with time. So far, the team has observed there has been a 35% decline in X-ray emission from 2000 to 2008.

Irwin said at today’s press conference that a new survey just getting started will look for more globular clusters with x-ray sources.

Sources: Chandra, AAS Meeting

December 28, 2009December 24, 2015 by Nicholos Wethington

An Exotic Source for Cosmic Rays: ‘Baby’ Black Holes

Cosmic rays – particles that have been accelerated to near the speed of light – stream out from our Sun all of the time, though they are positively sluggish compared to what are called Ultra-High-Energy Cosmic Rays (UHECRs). These types of cosmic rays originate from sources outside of the Solar System, and are much more energetic than those from our Sun, though also much rarer. The merger between a white dwarf and neutron star or black hole may be one source of these rays, and such mergers may occur often enough to be the most significant source of these energetic particles.

The Sloan White dwArf Radial velocity data Mining Survey (SWARMS) – which is part of the Sloan Digital Sky Survey – recently uncovered a binary system of exotic objects only 50 parsecs away from the Solar System. This system, named SDSS 1257+5428, appears to be a white dwarf star that is orbiting a neutron star or low-mass black hole. Details about the system and its initial discovery can be found in a paper by Carles Badenes, et al. here.

Co-author Todd Thompson, assistant professor in the Department of Astronomy at Ohio State University, argues in a recent letter to The Astrophysical Journal Letters that this type of system, and subsequent merger of these exotic remnants of stars, may be commonplace, and could account for the amount of UHECRs that are currently observed. The merger between the white dwarf and neutron star or black hole may also create a black hole of low mass, a so-called “baby” black hole.

Thompson wrote in an email interview:

“White dwarf/neutron star or black hole binaries are thought to be quite rare, although there is a huge range in the number per Milky Way-like galaxy in the literature. SWARMS was the first to detect such a system using the “radial velocity” technique, and the first to find such an object so nearby, only 50 parsecs away (about 170 light years). For this reason, it was very surprising, and its relative proximity is what allowed us to make the argument that these systems must be quite common compared to most previous expectations. SWARMS would have had to be very lucky to see something so rare so near by.”

Thompson, et al. argue that this type of merger may be the most significant source of UHECRs in the Milky Way galaxy, and that one should merge in the galaxy about every 2,000 years. These types of mergers may be slightly less common than Type Ia supernovae, which originate in binary systems of white dwarfs.

A white dwarf merging with a neutron star would also create a low-mass black hole of about 3 times the mass of the Sun. Thompson said, “In fact, this scenario is likely since we think that neutron stars cannot exist above 2-3 times the mass of the Sun. The idea is that the WD would be disrupted and accrete onto the neutron star and then the neutron star would collapse to a black hole. In this case, we might see the signal of BH formation in gravity waves.”

The gravity waves produced in such a merger would be above the detectable range by the Laser Interferometer Gravitational-Wave Observatory (LIGO), an instrument that uses lasers to detect gravity waves (of which none have been detected…yet), and even possibly a spaced base gravitational wave observatory, NASA’s Laser Interferometer Space Antenna, LISA.

Common cosmic rays that come from our Sun have an energy on the scale of 10^7 to 10^10 electron-volts. Ultra-high-energy cosmic rays are a rare phenomenon, but they exceed 10^20 electron-volts. How do systems like SDSS 1257+5428 produce cosmic rays of such high energy? Thompson explained that there are two equally fascinating possibilities.

In the first, the formation of a black hole and subsequent accretion disk from the merger would generate a jet somewhat like those seen at the center of galaxies, the telltale sign of a quasar. Though these jets would be much, much smaller, the shockwaves at the front of the jet would accelerate particles to the necessary energies to create UHECRs, Thompson said.

In the second scenario, the neutron star steals matter off of the white dwarf companion, and this accretion starts it rotating rapidly. The magnetic stresses that build at the surface of the neutron star, or “magnetar”, would be able to accelerate any particles that interact with the intense magnetic field to ultra-high energies.

The creation of these ultra-high-energy cosmic rays by such systems is highly theoretical, and just how common they may be in our galaxy is only an estimate. It remains unclear so soon after the discovery of SDSS 1257+5428 whether the companion object of the white dwarf is a black hole or neutron star. But the fact that SWARMS made such a discovery so early in the survey is encouraging for the discovery of further exotic binary systems.

“It is not likely that SWARMS will see 10 or 100 more such systems. If it did, the rate of such mergers would be very (implausibly) high. That said, we’ve been surprised many times before. However, given the total area of the sky surveyed, if our estimate of the rate of such mergers is correct, SWARMS should see only about 1 more such system, and they may see none. A similar survey in the southern sky (there is nothing at present comparable to the Sloan Digital Sky Survey, on which SWARMS is based) should turn up approximately 1 such system,” Thompson said.

Observations of SDSS 1257+5428 have already been made using the Swift X-ray observatory, and some measurements have been taken in the radio spectrum. No source of gamma-rays was to be found in the location of the system using the Fermi telescope.

Thompson said, “Probably the most important forthcoming observation of the system is to get a true distance via parallax. Right now, the distance is based on the properties of the observed white dwarf. In principle,
it should be relatively easy to watch the system over the next year and get a parallax distance, which will alleviate many of the uncertainties surrounding the physical properties of the white dwarf.”

Source: Arxiv, email interview with Todd Thompson

December 11, 2009December 24, 2015 by Nicholos Wethington

Fermi Spies Energetic Blazar Flare

A comparison of the Fermi images from November 2nd and December 3rd of this year, showing the brightening of 3C 454.3. Image Credit: NASA

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The blazar 3C 454.3, a bright source of gamma rays from a galaxy 7 billion light-years away just got a whole lot brighter. Observations from the Fermi gamma-ray telescope confirm that since September 15th the blazar has flared up considerably, increasing in gamma-ray brightness by about ten times in the from earlier this past summer, making it currently the brightest gamma-ray source in the sky.

3C 454.3 is a blazar, a jet of energetic particles that is caused by the supermassive black hole at the center of a galaxy. Most galaxies are thought to house a supermassive black hole at their center, and as it chomps down matter from the accretion disk that surrounds it, the supermassive black hole can form large jets that stream out light and energy in fantastic proportions. In the case of 3C 454.3, one of these jets is aimed at the Earth, which allows for us to see and study it.

This blazar has started to outshine the Vela pulsar, which because it is only 1,000 light-years away from the Earth is generally the brightest gamma-ray source in the sky. 3C 454.3 is almost twice as bright as Vela in the gamma-ray part of the spectrum, even though it lies 7 million times further away from the Earth. 3c 454.3 has also brightened significantly in the infrared, X-ray, radio and visible light.

This is not the first time the blazar has shown an increase in brightness. Over the course of observations of the blazar, it flared-up in brightness in May 2005, and again in July and August of 2007.

Dr. Erin Wells Bonning, Postdoctoral Associate at the Yale Center for Astronomy and Astrophysics, said of the recent flare in comparison with previous brightening events:

“In 2005, it reached a R-band magnitude of 12. Our peak observed R-band magnitude was 13.83, so we’re still not at the brightness of the 2005 outburst (about a factor of 5 below). On July 19, 2007, it reached a R-band magnitude of 13, not as bright as the 2005 event, but still brighter than we see it now. In 2005, there were no gamma-ray instruments to observe 3C 454.3, but the 2007 flare was observed by AGILE with a flux above 100 MeV of 3 +- 1 * 10^-6 cts/s/cm^s. The Fermi and AGILE count rates for Dec 2-3, 2009 are 6-9 times as high. So, interestingly, although it is not currently as bright optically as it was in 2007, it is a good deal brighter in gamma-rays.”

The Fermi gamma-ray space telescope (formerly GLAST) keeps tabs on the gamma-ray emissions from many sources in the sky. 3C 454.3 is just one of the top ten brightest sources of gamma-rays visible to the satellite, a list of which can be found in an article Nancy wrote in March, The Top Ten Gamma-Ray Sources from the Fermi Telescope.

Of course, the blazar 3C 454.3 is not as intrinsically bright as many of the Gamma-Ray Bursts observed by telescopes like Swift and Fermi, but it is the consistently brightest source of gamma-rays in the sky right now. Bonning said that, “While both GRBs and blazars are highly beamed toward us, the Lorentz factors (speed of particles in the jet) associated with GRBs are much higher than in blazars, causing them to appear brighter due to special relativistic effects.”

Observations 3C 454.3 are continuing in all wavelengths to capture the light curve of the event, and better understand these periodic flares. Bonning said, “The source has been relatively quiescent since it emerged from behind the Sun, and began to increase in brightness around the end of July. It then entered a bright period of fairly rapid variability, peaking every 20 days or so. The most recent, very intense, flare began around the end of November. Per our [Astronomer’s Telegram], since Nov 21, 3C 454 has increased about a factor of 3 in brightness in both optical and infrared. (B, V, and R filters are in optical wavelengths, and J and K are near-infrared). Similarly, the gamma-ray flux has increased also by a factor of 3 in the 0.1-300 GeV band over the same period.”

The cause of the intermittent flare-ups in 3C 454.3 and other blazars is still a mystery, but this current brightening will give astronomers better data as to what the possible cause could be. There seem to be no periodic events associated with the flares in blazars (with the exception of the possible “supermassive black hole binary” OJ 287).

Bonning said of a potential cause, “This is actually a very active field of research – there are numerous existing models, but no one hypothesis is clearly preferred. Perhaps particles have been shocked at some location in the blazar jet, or the jet may be precessing so that is closer to our line of sight, or there may be some other explanation.”

There will be numerous telescopes around the world zooming in on the current flare-up. According to Bonning:

“Blazars are multi-wavelength objects — their spectral energy distribution covers radio through gamma-rays, so a diverse collection of facilities will be observing 3C 454.3 during this outburst. Besides Fermi, the Italian AGILE satellite has been observing in gamma rays. The Swift X-ray telescope began monitoring in early December. The blazar monitoring group at Boston University headed by Alan Marscher is observing it with VLBA (radio; 13GHz). There is also a radio astronomy group at Michigan also observing with VLBA, as well one headed by Yuri Kovalev at Max Planck institute in Germany. There is an optical program with the ATOM telescope associated with the HESS TeV instrument in Namibia. (3C 454.3 is not bright at TeV energies, by the way.) This is not an exhaustive list by any means, but at any rate numerous facilities across the globe and operating at a wide range of energies will be taking a very close look at 3C 454.3 as it goes through this flare.”

Source: NASA press release, email interview with Erin Wells Bonning

December 11, 2009December 24, 2015 by Nicholos Wethington

The Shrinking Doughnut Around a Black Hole

GX 339-4, illustrated here, is a binary system of a black hole and a star. Astronomers were able to measure how the disk around the black hole shrinks for the first time. Image Credit: Credit: ESO/L. Calcada

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Homer Simpson would be sad: recent observations of the binary system of a black hole and its companion star have shown the retreat of the doughnut-shaped accretion disk around the black hole. This shrinking ‘doughnut’ was seen in observations of the binary system GX 339-4, a system composed of a star similar in mass to the Sun, and a black hole of ten solar masses.

As the black hole feeds on gas flowing out from the orbiting star, the change in flow of the gas produces a varying size in the disk of matter that piles up around the black hole in a torus shape. For the first time, the changes in the size of this disk have been measured, showing just how much smaller the doughnut becomes.

GX-339-4 lies 26,000 light-years away in the constellation Ara. Every 1.7 days in the system, a star orbits around the more massive black hole. This system, and others like it, show periodic flares of X-ray activity when gas that is being stolen from the star by the black hole gets heated up in the accretion disk that piles up around the black hole. Over the last seven years, the system has had four energetic outbursts in the last seven years, making it a quite active black hole/stellar binary system.

The material falling into the hole forms jets of highly energized photons and gas, one of which is pointed in the direction of the Earth. It is these jets that a team of international astronomers observed using the Suzaku X-ray observatory, operated jointly by the Japan Aerospace Exploration Agency and NASA, and NASA’s X-ray Timing Explorer satellite. The results of their observations were published in the Dec. 10 issue of The Astrophysical Journal Letters.

Though the system was faint when they took their measurements with the telescopes, it was producing steady jets of X-rays. The team was looking for the signature of X-ray spectral lines produced by the fluorescence of iron atoms in the disk. The strong gravity of the black hole shifts the energy of the X-rays produced by the iron, leaving a characteristic spectral line. By measuring these spectral lines, they were able to determine with rather high confidence the size of the shrinking disk.

Here’s how the shrinking occurs: the part of the disk that is closer to the black hole is denser when there is more gas flowing out from the star that accompanies it. But when this flow is reduced, the inner part of the disk heats up and evaporates. During the brightest periods of the black hole’s output, the disk was calculated to be within about 30 km (20 miles) of the black hole’s event horizon, while during lower periods of luminosity the disk retreats to greater than 27 times further, or to 1,000 km (600 miles) from the edge of the black hole.

This has an important implication in the study of how black holes form their jets; even though the accretion disk evaporates close to the black hole, these jets remain at a steady output.

John Tomsick of the Space Sciences Laboratory at the University of California, Berkeley said in a NASA press-release, “This doesn’t tell us how jets form, but it does tell us that jets can be launched even when the high-density accretion flow is far from the black hole. This means that the low-density accretion flow is the most essential ingredient for the formation of a steady jet in a black hole system.”

Read the pre-print version of the teams’ letter. If you want more information on how the X-rays from the disks around black holes can help determine their shape and spin, check out an article from Universe Today from 2003, Iron Can Help Determine if a Black Hole is Spinning.

Source: NASA/Suzaku press release

December 1, 2009December 24, 2015 by Nicholos Wethington

Quasar Caught Building Future Home Galaxy

An artist's impression of how quasars may be able to construct their own galaxies. Image Credit: ESO/L. Calcada

The birth of galaxies is quite a complicated affair, and little is known about whether the supermassive black holes at the center of most galaxies formed first, or if the matter in the galaxy accreted first, and formed the black hole later. Observations of the quasar HE0450-2958, which is situated outside of a galaxy, show the quasar aiding a nearby galaxy in the formation of stars. This provides evidence for the idea that supermassive black holes can ‘build’ their own galaxies.

The quasar HE0450-2958 is an odd entity: normally, supermassive black holes – also known as quasars – form at the center of galaxies. But HE0450-2958 doesn’t appear to have any host galaxy out of which it formed. This was a novel discovery in its own right when it was made back in 2005. Here’s our original story on the quasar, Rogue Supermassive Black Hole Has No Galaxy.

The formation of the quasar still remains a mystery, but current theories suggest that it formed out of cold interstellar gas filaments that accreted over time, or was somehow ejected from its host galaxy by a strong gravitational interaction with another galaxy.

The other oddity about the object is its proximity to a companion galaxy, which it may be aiding to form stars. The companion galaxy lies directly in the sights of one of the quasar’s jets, and is forming stars at a frantic rate. A team of astronomers from France, Germany and Belgium studied the quasar and companion galaxy using the Very Large Telescope at the European Southern Observatory. The astronomers were initially looking to find an elusive host galaxy for the quasar.

The phenomenon of ‘naked quasars’ has been reported before, but each time further observations are made, a host galaxy is found for the object. Energy streaming from the quasars can obscure a faint galaxy that is hidden behind dust, so the astronomers used the VLT spectrometer and imager for the mid-infrared (VISIR). Mid-infrared observations readily detect dust clouds. They combined these observations with new images obtained from the Hubble Space Telescope in the near-infrared.

Observations of HE0450-2958, which lies 5 billion light years from Earth, confirmed that the quasar is indeed without a host galaxy, and that the energy and matter streaming out of the jets is pointed right at the companion galaxy. This scenario is ramping up star formation in that galaxy: 340 solar masses of stars a year are formed in the galaxy, one-hundred times more than for a typical galaxy in the Universe. The quasar and the galaxy are close enough that they will eventually merge, finally giving the quasar a home.

David Elbaz of the Service d’Astrophysique, who is the lead author of the paper which appeared in Astronomy & Astrophysics, said “The ‘chicken and egg’ question of whether a galaxy or its black hole comes first is one of the most debated subjects in astrophysics today. Our study suggests that supermassive black holes can trigger the formation of stars, thus ‘building’ their own host galaxies. This link could also explain why galaxies hosting larger black holes have more stars.”

‘Quasar feedback’ could be a potential explanation for how some galaxies form, and naturally the study of other systems is needed to confirm whether this scenario is unique, or a common feature in the Universe.

Source: ESO, Astronomy & Astrophysics