Swift Sees the Birth of a Black Hole

The NASA-led Swift mission has detected and imaged its first gamma-ray burst, likely the birth cry of a brand new black hole.

The bright and long burst occurred on January 17. It was in the midst of exploding, as Swift autonomously turned to focus in less than 200 seconds. The satellite was fast enough to capture an image of the event with its X-Ray Telescope (XRT), while gamma rays were still being detected with the Burst Alert Telescope (BAT).

“This is the first time an X-ray telescope has imaged a gamma-ray burst, while it was bursting,” said Dr. Neil Gehrels, Swift’s Principal Investigator at NASA’s Goddard Space Flight Center, Greenbelt, Md. “Most bursts are gone in about 10 seconds, and few last upwards of a minute. Previous X-ray images have captured the burst afterglow, not the burst itself.”

“This is the one that didn’t get away,” said Prof. John Nousek, Swift’s Mission Operations Director at Penn State University, State College, Pa. “And this is what Swift was built to do: to detect these fleeting gamma-ray bursts and focus its telescopes on them autonomously within about a minute. The most exciting thing is this mission is just revving up.”

Swift has three main instruments. The BAT detects bursts and initiates the autonomous slewing to bring the XRT and the Ultraviolet/Optical Telescope (UVOT) within focus of the burst. In December the BAT started detecting bursts, including a remarkable triple detection on December 19. Today’s announcement marks the first BAT detection autonomously followed by XRT detection, demonstrating the satellite is swiftly slewing as planned. The UVOT is still being tested, and it was not collecting data when the burst was detected.

Scientists will need several weeks to fully understand this burst, GRB050117, so named for the date of detection. Telescopes in orbit and on Earth will turn to the precise burst location provided by Swift to observe the burst afterglow and the region surrounding the burst.

“We are frantically analyzing the XRT data to understand the X-ray emission seen during the initial explosion and the very early afterglow,” said Dr. David Burrows, the XRT lead at Penn State. “This is a whole new ballgame. No one has ever imaged X-rays during the transition of a gamma-ray burst from the brilliant flash to the fading embers.”

When the UVOT is fully operational, both the XRT and UVOT will provide an in-depth observation of the gamma-ray burst and its afterglow. The burst is gone in a flash, but scientists can study the afterglow to learn about what caused the burst, much like a detective hunts for clues at a crime scene.

The origin of gamma-ray bursts remains a mystery. At least some appear to originate in massive star explosions. Others might be the result of merging black holes or neutron stars. Any of these scenarios likely will result in the formation of a new black hole.

Several of these bursts occur daily somewhere in the visible universe. No prompt X-ray emission (coincident with the gamma-ray burst) has been previously imaged, because it usually takes hours to turn an X-ray telescope towards a burst. Scientists expect Swift to be fully operational by February 1.

Swift, still in its checkout phase, is an international collaboration launched on November 20, 2004. It is a NASA mission in partnership with the Italian Space Agency and the Particle Physics and Astronomy Research Council, United Kingdom.

The spacecraft was built in collaboration with national laboratories, universities and international partners, including Penn State University; Los Alamos National Laboratory, New Mexico; Sonoma State University, Rohnert Park, Calif.; Mullard Space Science Laboratory in Dorking, Surrey, England; the University of Leicester, England; Brera Observatory in Milan; and ASI Science Data Center in Frascati, Italy.

For more information about Swift on the Web, visit:

http://www.nasa.gov/swift

Original Source: NASA News Release

Supermassive Black Holes Early On

NASA’s Chandra X-ray Observatory has obtained definitive evidence that a distant quasar formed less than a billion years after the Big Bang contains a fully-grown supermassive black hole generating energy at the rate of twenty trillion Suns. The existence of such massive black holes at this early epoch of the Universe challenges theories of the formation of galaxies and supermassive black holes.

Astronomers Daniel Schwartz and Shanil Virani of the Harvard-Smithsonian Center for Astrophysics in Cambridge, MA observed the quasar, known as SDSSp J1306, which is 12.7 billion light years away. Since the Universe is estimated to be 13.7 billion years old, we see the quasar as it was a billion years after the Big Bang. They found that the distribution of X-rays with energy, or X-ray spectrum, is indistinguishable from that of nearby, older quasars. Likewise, the relative brightness at optical and X-ray wavelengths of SDSSp J1306 was similar to that of the nearby group of quasars. Optical observations suggest that the mass of the black hole is about a billion solar masses.

Evidence of another early-epoch supermassive black hole was published previously by a team of scientists from the California Institute of Technology and the United Kingdom using the XMM-Newton X-ray satellite. They observed the quasar SDSSp J1030 at a distance of 12.8 billion light years and found essentially the same result for the X-ray spectrum as the Smithsonian scientists found for SDSSp J1306. Chandra’s precise location and spectrum for SDSSp J1306 with nearly the same properties eliminate any lingering uncertainty that precocious supermassive black holes exist.

“These two results seem to indicate that the way supermassive black holes produce X-rays has remained essentially the same from a very early date in the Universe,” said Schwartz. “This implies that the central black hole engine in a massive galaxy was formed very soon after the Big Bang.”

There is general agreement among astronomers that X-radiation from the vicinity of supermassive black holes is produced as gas is pulled toward a black hole, and heated to temperatures ranging from millions to billions of degrees. Most of the infalling gas is concentrated in a rapidly rotating disk, the inner part of which has a hot atmosphere or corona where temperatures can climb to billions of degrees.

Although the precise geometry and details of the X-ray production are not known, observations of numerous quasars, or supermassive black holes, have shown that many of them have very similar X-ray spectra, especially at high X-ray energies. This suggests that the basic geometry and mechanism are the same for these objects.

The remarkable similarity of the X-ray spectra of the young supermassive black holes to those of much older ones means that the supermassive black holes and their accretion disks, were already in place less than a billion years after the Big Bang. One possibility is that millions of 100 solar mass black holes formed from the collapse of massive stars in the young galaxy, and subsequently built up a billion-solar mass black hole in the center of the galaxy through mergers and accretion of gas.

To answer the question of how and when supermassive black holes were formed, astronomers plan to use the very deep Chandra exposures and other surveys to identify and study quasars at even earlier ages.

The paper by Schwartz and Virani on SDSSp J1306 was published in the November 1, 2004 issue of The Astrophysical Journal. The paper by Duncan Farrah and colleagues on SDSS J1030 was published in the August 10, 2004 issue of The Astrophysical Journal.

Chandra observed J1306 with its Advanced CCD Imaging Spectrometer (ACIS) instrument for approximately 33 hours in November 2003. NASA’s Marshall Space Flight Center, Huntsville, Ala., manages the Chandra program for NASA’s Office of Space Science, Washington. Northrop Grumman of Redondo Beach, Calif., formerly TRW, Inc., was the prime development contractor for the observatory. The Smithsonian Astrophysical Observatory controls science and flight operations from the Chandra X-ray Center in Cambridge, Mass.

Additional information and images are available at:
http://chandra.harvard.edu and http://chandra.nasa.gov

Original Source: Chandra News Release

Second Black Hole at the Heart of the Milky Way

Using archived science verification data from the Hokupa?a/QUIRC Adaptive Optics system on Gemini North, a French/US team of astronomers led by Jean-Pierre Maillard of the Institut d?Astrophysique de Paris has confirmed the physical association of a cluster of massive stars in the infrared source IRS 13 near the center of the Milky Way galaxy.

The team also used data from Hubble Space Telescope, the Chandra X-Ray Observatory, the Canada-France-Hawai?i Telescope (CFHT), and the Very Large Array to provide broad spectral coverage to complement the Gemini data. The Gemini observations consisted of deconvolved H and Kp band images that identified the existence of two formerly undetected sources within IRS 13E. In all, seven individual massive stars appear to be associated with what the team believes was once a larger cluster of massive stars held together by a central intermediate-mass black hole of about 1,300 solar masses. (This black hole is distinct from the black hole at the galactic center which has a mass of about four million solar masses.) The seven individual stars of IRS 13E seen within a diameter of about 0.5″ (or projected 0.6 light-year across) are co-moving westward with a similar velocity of about 280 kilometers per second in the plane of the sky.

The compactness of the cluster and the common proper motion of the components suggest that they are kept together by a massive source, a stellar black hole at the center of IRS 13E. The size of the cluster allow to infer a mean orbit radius. The radial velocities (+/- 30 kilometers per second) of the individual stars derived from the BEAR Fourier Transform Spectrometer (CFHT) measurements can be used to estimate the average orbital velocity. The authors then explored a range of orbital assumptions and were able to constraint the mass of the holding black hole to about 1,300 solar masses rather robustly.

The team also speculates that this cluster was once located farther from the galactic center, where the stars could form away from the extreme gravitational influence of the central supermassive black hole. IRS 13E seems to be the wreckage or remnant core of a once larger cluster of stars that is now spiraling towards Sgr A* at the galactic center.

This theory also explains the existence of other massive stars around the galactic center, which are thought to be stars stripped from the cluster due to the gravitational environment around the galaxy?s central black hole.

The Gemini data for this work were obtained by a team led by Francois Rigaut (Gemini Observatory) as part of an adaptive optics demonstration run in July 2000. The results are published in Astronomy and Astrophysics, Volume 423, pgs 155-167 (2004)

Original Source: Gemini News Release

Black Hole at the Heart of a Nebula

Image credit: Harvard CfA
Most galaxies, including the Milky Way, are filled with giant clouds of gas and dust called nebulae that appear as dark silhouettes against the starry background. Nebulae shine only when illuminated or excited by nearby energy sources.

Usually, the energy source is one or more stars. But today at the 204th meeting of the American Astronomical Society in Denver, Colorado, Smithsonian astrophysicist Philip Kaaret (Harvard-Smithsonian Center for Astrophysics) announced that one nebula is illuminated by X-rays from a black hole. Moreover, the brightness of the nebula suggests that the X-ray source may be an intermediate-mass black hole many times larger than most stellar black holes.

This surprising find offers only the second known example of a black hole-illuminated nebula, after LMC X-1 in the Large Magellanic Cloud, and the first example of a nebula powered by an intermediate-mass black hole.

“Astronomers always get excited about new things, and this nebula is certainly something new. Finding it is like getting a royal flush the first time you play poker – it’s that rare,” said Kaaret.

Initially discovered by Manfred Pakull and Laurent Mirioni (University of Strasbourg), the nebula is located 10 million light-years away in the dwarf irregular galaxy Holmberg II. Two years ago, Pakull and Mirioni noted that it seemed to be associated with an ultraluminous X-ray source.

By combining observations from NASA’s Hubble Space Telescope and Chandra X-ray Observatory with those from ESA’s XMM-Newton spacecraft, Kaaret and his colleagues, Martin Ward (University of Leicester) and Andreas Zezas (CfA), pinpointed the X-ray source at the center of the nebula. Moreover, the mystery source is pouring out X-rays at a tremendous rate, shining one million times brighter in X-rays than the Sun shines at all wavelengths of light combined.

The observations by Kaaret and his associates indicate that those X-rays are generated by a black hole gobbling matter from a young, massive companion star at a rate of about one Earth mass every four years. That modest accretion rate is sufficient to ionize and light up a huge 100-light-year-wide swath of the surrounding nebula.

The X-ray emissions provide an important clue to the nature of the black hole. Some astronomers have suggested that X-rays from the source in Holmberg II and similar bright sources are beamed in the Earth’s direction like a searchlight. Such beaming would make the X-ray source appear brighter than it really is, thereby making the black hole appear more massive than it really is.

Kaaret’s data contradict that view, showing instead that the black hole in Holmberg II sends out X-rays evenly in all directions. Therefore, its brightness suggests that it must be more massive than any stellar black hole in our own Galaxy, weighing in at more than 25 times the mass of the Sun and likely more than 40 solar masses. That would rank it as an “intermediate-mass” black hole.

“It’s not easy to explain how intermediate-mass black holes form. Since we only have a few examples to study, every new find is important,” said Kaaret.

This research will be published in a paper co-authored by Kaaret, Ward and Zezas in an upcoming issue of the Monthly Notices of the Royal Astronomical Society.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

Original Source: Harvard CfA News Release

New Black Holes Found in a Virtual Observatory

Image credit: ESA
A European team has used the Astrophysical Virtual Observatory (AVO) to find 30 supermassive black holes that had previously escaped detection behind masking dust clouds. The identification of this large population of long-sought ?hidden? black holes is the first scientific discovery to emerge from a Virtual Observatory. The result suggests that astronomers may have underestimated the number of powerful supermassive black holes by as much as a factor of five.

Black holes collect dust. They lurk at the centres of active galaxies in environments not unlike those found in violent tornadoes on Earth. Just as in a tornado, where debris is often found spinning about the vortex, so in a black hole, a dust torus surrounds its waist. In some cases astronomers can look along the axis of the dust torus from above or from below and have a clear view of the black hole. Technically these objects are then called ?type 1 sources?. ?Type 2 sources? lie with the dust torus edge-on as viewed from Earth so our view of the black hole is totally blocked by the dust over a range of wavelengths from the near-infrared to soft X-rays.

While many dust-obscured low-power black holes (called ?Seyfert 2s?) have been identified, until recently few of their high-power counterparts were known. The identification of a population of high-power obscured black holes and the active galaxies in which they live has been a key goal for astronomers and will lead to greater understanding and a refinement of the cosmological models describing our Universe.

The European AVO science team led by Paolo Padovani from Space Telescope-European Coordinating Facility and the European Southern Observatory in Munich, Germany, now announces the discovery of a whole population of the obscured, powerful supermassive black holes. Thirty of these objects were found in the so-called GOODS (Great Observatories Origins Deep Survey) fields. The GOODS survey consists of two areas that include some of the deepest observations from space- and ground-based telescopes, including the NASA/ESA Hubble Space Telescope, and have become the best studied patches in the sky.

Padovani and the team used an innovative technique. Using a Virtual Observatory (VO) they combined information from multiple wavelengths from Hubble, ESO?s Very Large Telescope and NASA’s Chandra. This unprecedented team effort by the largest telescopes in the world made this discovery possible. The majority of the sources are so faint that it is currently not possible to take spectra of them and the VO techniques made it possible for the researchers to work seamlessly with images and catalogues from many different sources.

According to Paolo Padovani: ?This discovery means that surveys of powerful supermassive black holes have so far underestimated their numbers by at least a factor of two, and possibly by up to a factor of five.?

The paper describing these results has just been accepted by the European journal Astro?nomy & Astrophysics and will be published in an upcoming issue. This is the first refereed scientific paper based on end-to-end use of Virtual Observatory tools. The results in the paper show that the VO has evolved beyond the demonstration level to become a real research tool.

The European Astrophysical Virtual Observatory (AVO), funded partly by the European Commission, is the specific VO used for this project. With this work AVO demonstrates cutting-edge science by giving astronomers easy access to manipulation of image and catalogue data on remote computer networks. Until now objects were normally identified by taking a spectrum with a telescope, but now science is moving into an era where objects are pinpointed efficiently by using easily accessible multiwavelength information.

“These discoveries highlight the kind of scientific impact that Virtual Observatory technologies and standards will have on astronomy world-wide”, said Peter Quinn (European Southern Observatory), director of the AVO. “The Astrophysical Virtual Observatory wants to continue to work with astronomers in Europe to enable more discoveries like this, using combined data from ground- and space-based observatories”.

The team already has plans to investigate the new population of dusty black holes by using even more telescopes: the European Southern Observatory?s Very Large Telescope (near-infrared), NASA?s Spitzer Space Telescope (far-infrared) using emerging new VO tools. This will give further insight into the nature of these sources.

Original Source: ESA News Release

Binary Black Holes Modeled on Computer

Image credit: Penn State
Scientists at Penn State have reached a new milestone in the effort to model two orbiting black holes, an event expected to spawn strong gravitational waves. “We have discovered a way to model numerically, for the first time, one orbit of two inspiraling black holes,” says Bernd Bruegmann, Associate Professor of Physics and a researcher at Penn State’s Institute for Gravitational Physics and Geometry. Bruegmann’s research is part of a world-wide endeavor to catch the first gravity wave in the act of rolling over the Earth.

A paper describing these simulations will be published in the 28 May 2004 issue of the journal Physical Review Letters. The paper is authored by Bruegmann and two postdoctoral scholars in his group at Penn State, Nina Jansen and Wolfgang Tichy.

Black holes are described by Einstein’s theory of general relativity, which gives a highly accurate description of the gravitational interaction. However, Einstein’s equations are complicated and notoriously hard to solve even numerically. Furthermore, black holes pose their very own problems. Inside each black hole lurks what is known as a space-time singularity. Any object coming too close will be pulled to the center of the black hole without any chance to escape again, and it will experience enormous gravitational forces that rip it apart.

“When we model these extreme conditions on the computer, we find that the black holes want to devour and to tear apart the numerical grid of points that we use to approximate the black holes,” Bruegmann says. “A single black hole is already difficult to model, but two black holes in the final stages of their inspiral are vastly more difficult because of the highly non-linear dynamics of Einstein’s theory.” Computer simulations of black hole binaries tend to go unstable and crash after a finite time, which used to be significantly shorter than the time required for one orbit.

“The technique we have developed is based on a grid that moves along with the black holes, minimizing their motion and distortion, and buying us enough time for them to complete one spiraling orbit around each other before the computer simulation crashes,” Bruegmann says. He offers an analogy to illustrate the “co-moving grid” strategy: “If you are standing outside a carousel and you want to watch one person, you have to keep moving your head to keep watching him as he circles. But if you are standing on the carousel, you have to look in only one direction because that person no longer moves in relation to you, although you both are going around in circles.”

The construction of a co-moving grid is an important innovation of Bruegmann’s work. While not a new idea to physicists, it is a challenge to make it work with two black holes. The researchers also added a feedback mechanism to make adjustments dynamically as the black holes evolve. The result is an elaborate scheme that actually works for two black holes for about one orbit of the spiraling motion.

“While modelling black hole interactions and gravitational waves is a very difficult project, Professor Bruegmann’s result gives a good view of how we may finally succeed in this simulation effort,” says Richard Matzner, Professor at the University of Texas at Austin and principal investigator of the National Science Foundation’s former Binary Black Hole Grand Challenge Alliance that laid much of the groundwork for numerical relativity in the 90’s.

Abhay Ashtekar, Eberly Professor of Physics and Director of the Institute for Gravitational Physics and Geometry, adds, “The recent simulation of Professor Bruegmann’s group is a landmark because it opens the door to performing numerical analysis of a variety of black hole collisions which are among the most interesting events for gravitational wave astronomy.”

This research was funded by grants from the National Science Foundation including one to the Frontier Center for Gravitational Wave Physics established by the National Science Foundation in the Penn State Institute for Gravitational Physics and Geometry.

Original Source: Penn State News Release

Black Holes Can Be Ejected From Galaxies

Image credit: Hubble
When black holes collide, look out! An enormous burst of gravitational radiation results as they violently merge into one massive black hole. The ?kick? that occurs during the collision could knock the black hole clear out of its galaxy.

A new study describes the consequences of such an intergalactic collision.

Astrophysicist David Merritt, professor at Rochester Institute of Technology, and co-authors Milos Milosavljevic (Caltech), Marc Favata (Cornell University), Scott Hughes (Massachusetts Institute of Technology) and Daniel Holz (University of Chicago) explore the consequences of kicks induced by gravitational waves in their article, ?Consequences of Gravitational Radiation Recoil,? recently submitted to the Astrophysical Journal and posted online at http://arXiv.org/abs/astro-ph/0402057.

Virtually all galaxies are believed to contain supermassive black holes at their centers. According to current theory, galaxies grow through mergers with other galaxies. When two galaxies merge, their central black holes form a binary system and revolve around each other, eventually coalescing into a single black hole. The coalescence is driven by the emission of gravitational radiation, as predicted by Einstein?s theory of relativity.

Merritt and his colleagues determined how fast a black hole has to move to completely escape a galaxy?s gravitational field. They found that larger and brighter galaxies have stronger gravitational fields and would require a bigger kick to eject a black hole than the smaller systems. Likewise, less forceful impacts could jar the black hole out of its home at the center of a galaxy, only to later rebound back into position.

The kicks also call into question theories that would grow supermassive black holes from hierarchical mergers of smaller black holes, starting in the early universe. ?The reason is that galaxies were smaller long ago, and the kicks would easily have removed the black holes from them,? Merritt says.

According to Merritt and his co-authors, it is more likely that supermassive black holes attained most of their mass through the accretion of gas and that mergers with other black holes only took place after the galaxies had reached roughly their current sizes.

?We know that supermassive black holes exist at the centers of giant galaxies like our own Milky Way,? says Merritt. ?But as far as we know, the smaller stellar systems do not have any black holes. Perhaps they used to, but they were kicked out.?

The kick?a consequence of Einstein?s relativity equations?occurs because gravitational waves emitted during the final plunge are anisotropic, producing recoil. The effect is maximized when one black hole is appreciably larger than the other one.

While astrophysicists have been aware of this phenomenon since the 1960s, until now no one has had the analytical tools necessary to accurately calculate the size of the effect. The first accurate calculation of the size of the kicks was reported in a companion paper by Favata, Hughes and Holz, which also appears online at http://arXiv.org.

Merritt notes that there is no clear observational evidence that the kicks have taken place. He contends that the best chance of finding direct evidence would be locating a black hole shortly after the kick occurs, perhaps in a galaxy that has recently undergone a merger with another galaxy.

?You would see an off-center black hole that hasn?t quite made its way back to the center yet,? he says. ?Even though the probability of observing this is low, now that astronomers know what to look for, I wouldn?t be surprised if someone finds one eventually.?

Original Source: RIT News Release

Star Seen Very Near Black Hole

Image credit: ESO

A team of astronomers have spotted an otherwise normal star make a close pass with the supermassive black hole that lurks at the centre of our Milky Way Galaxy. At its closest approach, the star was only 17 light-hours away from the black hole (three times the distance of the Sun to Pluto). Images of the region were gathered over 10 years using the adaptive optics system on the European Southern Observatory’s Paranal Observatory.

An international team of astronomers [2], lead by researchers at the Max-Planck Institute for Extraterrestrial Physics (MPE), has directly observed an otherwise normal star orbiting the supermassive black hole at the center of the Milky Way Galaxy.

Ten years of painstaking measurements have been crowned by a series of unique images obtained by the Adaptive Optics (AO) NAOS-CONICA (NACO) instrument [3] on the 8.2-m VLT YEPUN telescope at the ESO Paranal Observatory. It turns out that earlier this year the star approached the central Black Hole to within 17 light-hours – only three times the distance between the Sun and planet Pluto – while travelling at no less than 5000 km/sec.

Previous measurements of the velocities of stars near the center of the Milky Way and variable X-ray emission from this area have provided the strongest evidence so far of the existence of a central Black Hole in our home galaxy and, implicitly, that the dark mass concentrations seen in many nuclei of other galaxies probably are also supermassive black holes. However, it has not yet been possible to exclude several alternative configurations.

In a break-through paper appearing in the research journal Nature on October 17th, 2002, the present team reports their exciting results, including high-resolution images that allow tracing two-thirds of the orbit of a star designated “S2”. It is currently the closest observable star to the compact radio source and massive black hole candidate “SgrA*” (“Sagittarius A”) at the very center of the Milky Way. The orbital period is just over 15 years.

The new measurements exclude with high confidence that the central dark mass consists of a cluster of unusual stars or elementary particles, and leave little doubt of the presence of a supermassive black hole at the centre of the galaxy in which we live.

Quasars and Black Holes
Ever since the discovery of the quasars (quasi-stellar radio sources) in 1963, astrophysicists have searched for an explanation of the energy production in these most luminous objects in the Universe. Quasars reside at the centres of galaxies, and it is believed that the enormous energy emitted by these objects is due to matter falling onto a supermassive Black Hole, releasing gravitational energy through intense radiation before that material disappears forever into the hole (in physics terminology: “passes beyond the event horizon” [4]).

To explain the prodigious energy production of quasars and other active galaxies, one needs to conjecture the presence of black holes with masses of one million to several billion times the mass of the Sun. Much evidence has been accumulating during the past years in support of the above “accreting black hole” model for quasars and other galaxies, including the detection of dark mass concentrations in their central regions.

However, an unambiguous proof requires excluding all possible other, non-black hole configurations of the central mass concentration. For this, it is imperative to determine the shape of the gravitational field very close to the central object – and this is not possible for the distant quasars due to technological limitations of the currently available telescopes.

The centre of the Milky Way
The centre of our Milky Way galaxy is located in the southern constallation Sagittarius (The Archer) and is “only” 26,000 light-years away [5]. On high-resolution images, it is possible to discern thousands of individual stars within the central, one light-year wide region (this corresponds to about one-quarter of the distance to “Proxima Centauri”, the star nearest to the solar system).

Using the motions of these stars to probe the gravitational field, observations with the 3.5-m New Technology Telescope (NTT) at the ESO La Silla Observatory (Chile) (and subsequently at the 10-m Keck telescope, Hawaii, USA) over the last decade have shown that a mass of about 3 million times that of the Sun is concentrated within a radius of only 10 light-days [5] of the compact radio and X-ray source SgrA* (“Sagittarius A”) at the center of the star cluster.

This means that SgrA* is the most likely counterpart of the putative black hole and, at the same time, it makes the Galactic Center the best piece of evidence for the existence of such supermassive black holes. However, those earlier investigations could not exclude several other, non-black hole configurations.

“We then needed even sharper images to settle the issue of whether any configuration other than a black hole is possible and we counted on the ESO VLT telescope to provide those”, explains Reinhard Genzel, Director at the Max-Planck Institute for Extraterrestrial Physics (MPE) in Garching near Munich (Germany) and member of the present team. “The new NAOS-CONICA (NACO) instrument, built in a close collaboration between our institute, the Max-Planck Institute for Astronomy (MPIA: Heidelberg, Germany), ESO and the Paris-Meudon and Grenoble Observatories (France), was just what we needed to take this decisive step forward”.

The NACO observations of the Milky Way centre
The new NACO instrument [3] was installed in late 2001 at the VLT 8.2-m YEPUN telescope. Already during the initial tests, it produced many impressive images, some of which have been the subject of earlier ESO press releases [6].

“The first observations this year with NACO gave us right away the sharpest and ‘deepest’ images of the Milky Way Centre ever taken, showing a large number of stars in that area in great detail”, says Andreas Eckart of the University of Cologne, another member of the international team that is headed by Rainer Sch?del, Thomas Ott and Reinhard Genzel from MPE. “But we were still to be overwhelmed by the wonderful outcome of those data!”

Combining their infrared images with high-resolution radio data, the team was able to determine – during a ten-year period – very accurate positions of about one thousand stars in the central area with respect to the compact radio source SgrA*, see PR Photo 23c/02.

“When we included the latest NACO data in our analysis in May 2002, we could not believe our eyes. The star S2, which is the one currently closest to SgrA*, had just performed a rapid swing-by near the radio source. We suddenly realised that we were actually witnessing the motion of a star in orbit around the central black hole, taking it incredibly close to that mysterious object”, says a very happy Thomas Ott, who is now working in the MPE team on his PhD thesis.

In orbit around the central black hole
No event like this one has ever been recorded. These unique data show unambiguously that S2 is moving along an elliptical orbit with SgrA* at one focus, i.e. S2 orbits SgrA* like the Earth orbits the Sun, cf. the right panel of PR Photo 23c/02.

The superb data also allow a precise determination of the orbital parameters (shape, size, etc.). It turns out that S2 reached its closest distance to SgrA* in the spring of 2002, at which moment it was only 17 light-hours [5] away from the radio source, or just 3 times the Sun-Pluto distance. It was then moving at more than 5000 km/s, or nearly two hundred times the speed of the Earth in its orbit around the Sun. The orbital period is 15.2 years. The orbit is rather elongated – the eccentricity is 0.87 – indicating that S2 is about 10 light-days away from the central mass at the most distant orbital point [7].

“We are now able to demonstrate with certainty that SgrA* is indeed the location of the central dark mass we knew existed. Even more important, our new data have “shrunk” by a factor of several thousand the volume within which those several million solar masses are contained”, says Rainer Sch?del, PhD student at MPE and also first author of the resulting paper.

In fact, model calculations now indicate that the best estimate of the mass of the Black Hole at the centre of the Milky Way is 2.6 ? 0.2 million times the mass of the Sun.

No other possibilities
According to the detailed analysis presented in the Nature article, other previously possible configurations, such as very compact clusters of neutron stars, stellar size black holes or low mass stars, or even a ball of putative heavy neutrinos, can now be definitively excluded.

The only still viable non-black hole configuration is a hypothetical star of heavy elementary particles called bosons, which would look very similar to a black hole. “However”, says Reinhard Genzel, “even if such a boson star is in principle possible, it would rapidly collapse into a supermassive black hole anyhow, so I think we have pretty much clinched the case!”

Next observations
“Most astrophysicists would accept that the new data provide compelling evidence that a supermassive black hole exists in the center of the Milky Way. This makes even more likely the supermassive black hole interpretation for the enormous concentration of dark mass detected at the center of many other galaxies”, says Alvio Renzini, VLT Programme Scientist at ESO.

So what remains to be done? The next big quest now is to understand when and how these supermassive black holes formed and why almost every massive galaxy appears to contain one. The formation of central black holes and that of their host galaxies themselves increasingly appear to be just one problem and the same. Indeed, one of the outstanding challenges for the VLT to solve in the next few years.

There is also little doubt that coming interferometric observations with instruments at the VLT Interferometer (VLTI) and the Large Binocular Telescope (LBT) will also result in another giant leap within this exciting field of research.

Andreas Eckart is optimistic: “Perhaps it will even be possible with X-ray and radio observations in the next few years to directly demonstrate the existence of the event horizon.”

Original Source: ESO News Release