Movie of stars orbiting the Black Hole at the Center
of the Milky Way.
Compton Gamma Ray Observatory
ESA's Integral gamma-ray observatory has resolved the diffuse glow of gamma rays in the centre of our Galaxy and has shown that most of it is produced by a hundred individual sources. Integral's high sensitivity and pointing precision have allowed it to detect these celestial objects where all other telescopes, for more than thirty years, had seen nothing but a mysterious, blurry fog of gamma rays ...
During the spring and autumn of 2003, Integral observed the central regions of our Galaxy, collecting some of the perpetual glow of diffuse low-energy gamma rays that bathe the entire Galaxy.
These gamma rays were first discovered in the mid-1970s by high-flying balloon-borne experiments. Astronomers refer to them as the 'soft' Galactic gamma-ray background, with energies similar to those used in medical X-ray equipment.
Initially, astronomers believed that the glow was caused by interactions involving the atoms of the gas that pervades the Galaxy. Whilst this theory could explain the diffuse nature of the emission, since the gas is ubiquitous, it failed to match the observed power of the gamma rays. The gamma rays produced by the proposed mechanisms would be much weaker than those observed. The mystery has remained unanswered for decades.
Now Integral's superb gamma-ray telescope IBIS, built for ESA by an international consortium led by Principal Investigator Pietro Ubertini (IAS/CNR, Rome, Italy), has seen clearly that, instead of a fog produced by the interstellar medium, most of the gamma-rays are coming from individual celestial objects. In the view of previous, less sensitive instruments, these objects appeared to merge together.
In a paper published today in "Nature", Francois Lebrun (CEA Saclay, Gif sur Yvette, France) and his collaborators report the discovery of 91 gamma-ray sources towards the direction of the Galactic centre. Lebrun's team includes Ubertini and seventeen other European scientists with long-standing experience in high-energy astrophysics. Much to the team's surprise, almost half of these sources do not fall in any class of known gamma-ray objects. They probably represent a new population of gamma-ray emitters.
The first clues about a new class of gamma-ray objects came last October, when Integral discovered an intriguing gamma-ray source, known as IGRJ16318-4848. The data from Integral and ESA's other high-energy observatory XMM-Newton suggested that this object is a binary system, probably including a black hole or neutron star, embedded in a thick cocoon of cold gas and dust. When gas from the companion star is accelerated and swallowed by the black hole, energy is released at all wavelengths, mostly in the gamma rays.
However, Lebrun is cautious to draw premature conclusions about the sources detected in the Galactic centre. Other interpretations are also possible that do not involve black holes. For instance, these objects could be the remains of exploded stars that are being energised by rapidly rotating celestial 'powerhouses', known as pulsars.
Observations with another Integral instrument (SPI, the Spectrometer on Integral) could provide Lebrun and his team with more information on the nature of these sources. SPI measures the energy of incoming gamma rays with extraordinary accuracy and allows scientist to gain a better understanding of the physical mechanisms that generate them.
However, regardless of the precise nature of these gamma-ray sources, Integral's observations have convincingly shown that the energy output from these new objects accounts for almost ninety percent of the soft gamma-ray background coming from the centre of the Galaxy. This result raises the tantalising possibility that objects of this type hide everywhere in the Galaxy, not just in its centre.
Again, Lebrun is cautious, saying, "It is tempting to think that we can simply extrapolate our results to the entire Galaxy. However, we have only looked towards its centre and that is a peculiar place compared to the rest."Next on Integral's list of things to do is to extend this work to the rest of the Galaxy. Christoph Winkler, ESA's Integral Project Scientist, says, "We now have to work on the whole disc region of the Galaxy. This will be a tough and long job for Integral. But at the end, the reward will be an exhaustive inventory of the most energetic celestial objects in the Galaxy."
High resolution version available at:
For information about the related INTEGRAL and XMM-Newton discovery of IGRJ16318-4848, see: http://www.esa.int/esaSC/Pr_21_2003_s_en.html
IBIS, Imager on Board the Integral Satellite - IBIS provides sharper gamma-ray images than any previous gamma-ray instrument. It can locate sources to a precision of 30 arcseconds, the equivalent of measuring the height of a person standing in a crowd, 1.3 kilometres away. The Principal Investigators that built the instrument are P. Ubertini (IAS/CNR, Rome, Italy), F. Lebrun (CEA Saclay, Gif sur Yvette, France), G. Di Cocco (ITESRE, Bologna, Italy). IBIS is equipped with the first un-cooled semiconductor gamma-ray camera, called ISGRI, which is responsible for its outstanding sensitivity. ISGRI was developed and built for ESA by CEA Saclay, France.SPI, Spectrometer on Integral - SPI measures the energy of incoming gamma rays with extraordinary accuracy. It is more sensitive to faint radiation than any previous gamma ray instrument and allows the precise nature of gamma ray sources to be determined. The Principal Investigators that developed SPI are J.-P. Roques, (CESR, Toulouse, France) and V. Schoenfelder (MPE, Garching, Germany).
Integral is the International Gamma Ray Astrophysics Laboratory of the European Space Agency. It is a cooperative mission with Russia and is scheduled for launch on 17 October 2002 from the Baikonur Cosmodrome, Kazakhstan, on a Russian Proton rocket, the Russian contribution to the programme. It is the world's most advanced gamma-ray telescope and will provide first-hand observations of the celestial objects that release some of the most energetic radiation of the Universe. In particular, scientists have designed Integral to simultaneously capture gamma rays, X-rays, and visible light from these objects, allowing astronomers on Earth to fully analyse them.
Read more at: http://sci.esa.int/content/news/index.cfm?aid=21&cid=44&oid=30613
From a NASA Press Release
A new gamma ray burst mission, the High-Energy Transient Explorer (HETE-2), made its entrance into space on October 10, 2000, from the Kwajalein Missile Range in the Marshall Islands.
"Gamma ray bursts are stupendous explosions. They are the most energetic events since the Big Bang, yet one occurs about once a day somewhere in the sky," said Dr. George R. Ricker from the Massachusetts Institute of Technology (MIT) in Cambridge, Mass., HETE-2 principal investigator. "The successful launch of HETE-2 means that for the first time we can locate with pinpoint accuracy hundreds of these bursts. Also, HETE-2's ability to relay the accurate location of each burst in real-time to space- and ground-based optical and radio observatories will surely revolutionize this exciting new area of high energy astrophysics."
Gamma ray bursts are a mystery to scientists, and very little is known about their fundamental origin. HETE-2 will embark upon a gamma ray burst fact-finding mission during the four years it is slated to operate. In addition to detecting hundreds of bursts during its mission, it will provide detailed information on the location and light characteristics of many of these bursts.
Within seconds of a burst, HETE-2 will be able to calculate a precise location for that burst. On the ground, a dedicated network of 12 listen-only burst alert stations will relay the data to the MIT control center. From there, information will be transmitted to the Gamma Ray Burst Coordinate Distribution Network at the Goddard Space Flight Center, which can send the information to other observatories worldwide in 10-20 seconds, significantly faster than previously possible. HETE-2 will allow astronomers to see a burst while it is still occurring and allow scientists to study its development at various wavelengths.
The spacecraft carries three main instruments and is supported by a computer network that transmits data to other observatories. The French Gamma Telescope (FREGATE), built by CESR, will detect gamma ray bursts and very bright (higher energy) X-ray transients. The Wide-Field X-ray Monitor (WXM), built by RIKEN and Los Alamos National Laboratory, detects photons slightly lower in energy than the FREGATE does. The WXM therefore will detect fewer gamma ray bursts than FREGATE, but because of its superior resolution, will be able to locate the FREGATE-detected bursts to within 10 arc minutes (an area of sky about equal to 1/10 the size of the full Moon). The oft X-Ray Camera (SXC), built by MIT, covers the lowest energy band of the three instruments. It also provides the best angular resolution, resulting in a location accuracy of about 10 arc seconds, more than an order of magnitude finer than any previous GRB instrument.
HETE-2 is a collaboration between NASA; MIT; Los Alamos National Laboratory, New Mexico; France's Centre National d'Etudes Spatiales (CNES), Centre d'Etude Spatiale des Rayonnements (CESR), and Ecole Nationale Superieure de l'Aeronautique et de l'Espace (Sup'Aero); and Japan's Institute of Physical and Chemical Research (RIKEN). The science team includes members from the University of California (Berkeley and Santa Cruz) and the University of Chicago.
More information on the HETE-2 mission can be found at:
The Compton Gamma-Ray Observatory was launched in 1991. There had been a hope that all NASA's "Great Observatories" would be up simultaneously to send back data across the spectrum about given objects. But since the Space Infrared Telescope Facility won't be working before 2002, that hope is dashed. Astronomers will be blind to gamma rays for some time. A solar gamma-ray spcecraft, NASA's High Energy Solar Spectroscopic Imager (HESSI), is to observe solar flares in x-rays and gamma rays, something particularly important now, at the peak of the sunspot cycle. But it was damaged in testing in March 2000 and its launch has been delayed until 2001. For non-solar observations, NASA's Gamma-ray Large Area Space Telescope (GLAST) is scheduled for launch in 2005. It will have 30 times CGRO's sensitivity for the highest-energy gamma rays and will have 10 times CGRO's angular resolution. The European Space Agency's International Gamma-Ray Astrophysics Laboratory (INTEGRAL), with spectroscopc capabilities, is scheduled for launch in April 2002.
Perhaps the most significant set of discoveries of Compton GRO dealt with gamma-ray bursts. It detected thousands of them and showed that they were randomly distributed across the sky. Its teamwork with the BeppoSAX spacecraft allowed several gamma-ray bursts to be pinpointed as extremely distant, and therefore extremely powerful, objects. A NASA High Energy Transient Explorer-2 (HETE-2), to be launched in July 2000, will be less sensitive than CGRO for these bursts but will pinpoint them in the sky better. The NASA Swift mission, to be launched in 2003, will also have good spatial resolution while being five times more sensitive than Compton to the bursts. These smaller missions have less general capability than the dear departed Compton Gamma-Ray Observatory.
"Compton has been a workhorse for nine years, far exceeding our expectations for a two- to five-year mission," said Dr. Alan Bunner, director of NASA's Structure and Evolution of the Universe science theme, NASA Headquarters, Washington, DC. "New discoveries made by Compton changed our view of the Universe in fundamental ways."
Compton's lasting legacy will be its impact on gamma ray astronomy. The telescope detected more than 400 gamma ray sources, 10 times more than were previously known. Compton recorded more than 2,500 gamma ray bursts; before Compton, only about 300 had been detected.
NASA and international space agencies plan several upcoming missions to continue where Compton left off. The Compton Gamma Ray Observatory was the second of NASA's Great Observatories and the gamma-ray equivalent to the Hubble Space Telescope and the Chandra X-ray Observatory. Compton was launched aboard the Space Shuttle Atlantis in April 1991, and, at 17 tons, was the largest astrophysical payload ever flown at that time.
More information is available on the Internet at:
JPL Press Release
The first of a pair of new telescopes, funded primarily by NASA, has begun an ambitious three-and-a-half year near-infrared survey of the entire celestial sky, peering through the curtain of interstellar dust in the Milky Way galaxy.
The Two-Micron All-Sky Survey (2MASS), based at the University of Massachusetts, Amherst, MA, features two 1.3-meter (51-inch) telescopes, one at a Smithsonian Astrophysical Observatory site atop Mount Hopkins, near Tucson, AZ, and the other at a National Optical Astronomy Observatories site in Cerro Tololo, Chile.
"The sky survey catalogues produced 100 years ago are still useful to astronomers," said 2MASS Project Manager Rae Stiening. "We expect this new, greatly updated survey will be an invaluable resource for the next 100 years."
"Preliminary observations by 2MASS are already suggesting new infrared sources will be discovered," said Program Manager Dr. Michael Klein at NASA's Jet Propulsion Laboratory, Pasadena, CA. "Some of these will be targets for detailed studies for future space observatories, like the Advanced X-Ray Facility (AXAF), the Space Infrared Telescope Facility and the Next Generation Space Telescope."
The survey is designed to catalogue 300 million stars and one million galaxies in the local universe, along with quasars, which are strong, extremely bright radio sources, and galaxies with black holes, the intriguing entities with gravity so powerful not even light can escape.
2MASS will observe many known asteroids and possibly some comets, and it is uniquely sensitive to exotic objects like brown dwarfs, which lack the mass needed to ignite and become full- fledged stars.
The telescopes are equipped with near-infrared detector arrays that will provide the most complete census to date of cool stars in the Milky Way galaxy and provide new data for detailed studies of the galactic structure. Near-infrared emission is at wavelengths roughly two-to-four times longer than visible light and permits astronomers to "see through" the obscuring effects of interstellar dust in the Milky Way galaxy.
As Stiening explained, "Sunsets on Earth look reddish because only red light makes it through the dust in our atmosphere. Infrared observations enable us to penetrate the dust in our galaxy and other galaxies and, therefore, they provide a much clearer view of interior regions."
The 2MASS survey will measure accurately the positions and infrared brightness of stars and galaxies. Combined with complementary ground-based redshift surveys, the 2MASS extra- galactic data will provide a three-dimensional view of large- scale structures in the local universe. The enabling technology for this survey is the breakthrough in large-format infrared detector arrays. These technologies, funded through the U.S. Department of Defense and NASA, are being adapted for astronomical purposes to increase sensitivity dramatically. It's expected the new survey will be some 25,000 times more sensitive than a precursor survey at the California Institute of Technology, Pasadena, CA, nearly 30 years ago. 2MASS uses the type of detectors developed for the Near Infrared Camera and Multi Object Spectrometer on NASA's Hubble Space Telescope.
"Observing time at most telescopes is divided amongst a variety of scientific programs using a suite of different instruments. 2MASS telescopes will be completely dedicated to mapping the sky using one instrument, a three-color infrared camera," said Principal Investigator Dr. Michael Skrutskie, a University of Massachusetts physics and astronomy professor, who leads the science working group that will evaluate the data products. He also managed the design and fabrication effort for the infrared cameras, which are attached to an identical pair of telescopes.
Data will be processed at JPL's Infrared Processing and Analysis Center (IPAC) at Caltech, with a team led by Dr. Roc Cutri, Dr. Charles Beichman and Dr. Thomas Chester. Every two nights, the center will process 60 gigabytes of data, which is more data than processed during the entire Infrared Astronomy Satellite (IRAS) mission of 1983.
Additional information and images are available at the 2MASS website at the following URL's:
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