Space Telescope Science Institute
European Southern Observatory
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Eckart and Genzel on the Mass of Sgr A
Newsletter for Galactic Center Research
Compton Gamma-Ray Observatory
Gamma Ray Large Area Space Telescope (GLAST)
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).
The images released include the black hole Cygnus X-1 and a gamma-ray burst. See the ESA press release:http://sci.esa.int/content/news/index.cfm?aid=21&cid=44&oid=31201
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
NASA's Chandra X-ray Observatory has made a stunning, high-energy
panorama of the central regions of our Milky Way galaxy. The
observations enabled astronomers to determine the nature of the hot gas
that pervades the region and better understand how it enriches the
galactic suburbs with heavy elements.
In a discovery which may help to solve a long-standing mystery, astronomers from the University of Manchester's Jodrell Bank Observatory and other members of an international team have found gas between the densely packed stars that make up the globular cluster 47 Tucanae.
47 Tucanae is one of about 140 globular clusters associated with our galaxy, the Milky Way. As they contain up to a million stars packed into a relatively small space, the combined starlight near its centre would make night as bright as day! 47 Tucanae is one of the most spectacular globular clusters and can be easily seen with the unaided eye in the Southern Hemisphere. At a distance of 16,000 light years it is roughly the same size in the sky as the full moon. The stars in such clusters are very old, and many will have shed much of their mass into space during cataclysmic explosions at the end of their lives. For over 40 years, astronomers have looked for gas in these clusters without success but now, at last, it has been detected.
Interestingly, it is the death of the giant stars whose ejected gas has proved so elusive that has provided the means of its detection. Nature has offered some of them the chance of a second life as their iron cores have collapsed to form rapidly rotating neutron stars somewhat more massive than our Sun. Rotating several hundred times a second, they emit beams of radio waves which can be detected as regular pulses of energy as the beam sweeps across the Earth. These objects, known as "Pulsars", are thus massive cosmic flywheels and have been shown to be incredibly accurate clocks.
Using the 64-m Parkes radio telescope in Australia, the research team have discovered more that 20 of these exotic objects in 47 Tucanae. They have made very precise observations of the minute changes in the observed rotation rates due to the doppler shift caused by the gravitational pull of the cluster. This has enabled them to determine their positions within the cluster. As one would expect, some are on the far side and some on the near side of the cluster's centre. A further measurement made for each pulsar measures the amount of material and gas in the line of sight to us. To their delight, the team found that those on the far side of the cluster had more gas in front of them than those on the near side, thus proving the presence of gas within the cluster.
The international team is very excited about this discovery. Dr. Paulo Freire explains, "Astronomers have searched for indications of the expected gas at all frequencies across the electromagnetic spectrum. Finally we have now a solid detection using a radio telescope to observe pulsars which act as highly sensitive probes of the cluster environment."
Although it needs high precision measurements to detect the gas in the cluster, Dr. Fernando Camilo stresses, "the measured gas density is about 100 times larger than we would have expected from the interstellar medium surrounding 47 Tucanae."
Dr. Michael Kramer points out that the previously unsuccessful observations at other wavelengths are also important to understand their new result. From the previous measurements it can be inferred that the gas detected by the team must represent almost all the material existing in the cluster medium.
Astronomers had looked for gas because it is expected to be accumulated from the mass loss of the massive stars in the cluster and it has been a mystery where the cluster gas had gone. Several explanations had been put forward to explain the missing gas and amongst these were the winds from pulsars. It is somewhat ironic that the very same objects that may be responsible for the removal of most of the gas, have finally lead to its detection.
These latest results are discussed in a paper just published in the Astrophysical Journal Letters.
The members of the research team are Dr. Paulo Freire (University of Manchester, Jodrell Bank Observatory; now at the Arecibo Observatory), Dr. Michael Kramer and Professor Lyne (both at the University of Manchester, Jodrell Bank Observatory), Dr. Fernando Camilo (Columbia University, USA), Dr. Richard Manchester (Australia Telescope National Facility, Australia) and Dr. Nichi D'Amico (Observatorio di Bologna, Italy).