Cataloging the gamma-ray universe and weighing black holes
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weighing black holes, and a hat trick
New catalog of gamma-ray universe published
How to weigh black holes
Astronomers Score a Hat Trick and Narrow Theory on Cosmic Ray Origin
From University of New Hampshire press releases. Contact: Carmelle Druchniak, UNH News Bureau
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PORTSMOUTH, N.H. - Among the events planned for this week's 5th Compton Symposium on gamma ray astronomy is the long-awaited release of the COMPTEL source catalogue, a list of the 63 gamma ray sources detected by the Imaging Compton Instrument (COMPTEL) on NASA's Compton Gamma Ray Observatory.
The University of New Hampshire's Space Science Center, which hosts the three-day symposium Sept. 15-17 at Portsmouth's Sheraton Harborside Hotel, for the past eight years has overseen daily operations of the COMPTEL instrument from Morse Hall on the Durham campus.
"The catalog represents several years of data analysis," says Mark McConnell, UNH associate research professor and member of the COMPTEL team, "and yet it underscores how little we know about the universe as seen in the gamma ray part of the spectrum."
The COMPTEL instrument, partially built at UNH, observes a bandwidth of light 10 times wider than that covered by optical telescopes and a million times more energetic. The other CGRO instruments have produced several catalogues since the satellite's launching in 1991. The Burst and Transient Source Experiment has issued four catalogs covering 1,637 bursts (more than 2,600 have been detected).
The COMPTEL catalogue was eight years in the making. This was due to the unique physics of COMPTEL's observable energy band and the mechanics of the instrument itself. COMPTEL had greater difficulty discerning between source and background gamma ray radiation. Thus, careful modeling of background energy was needed before the COMPTEL team could isolate distinct gamma-ray sources for the catalogue.
Gamma rays occupy the highest energy range in the electromagnetic spectrum, well beyond visible light, ultraviolet and X-rays. They are produced by extreme forces of energy and by atomic decay. The COMPTEL catalogue comprises 63 gamma-ray sources: 32 are steady sources, such as neutron stars and black hole candidates; the remaining 31 are mysterious gamma-ray bursts, which outshine the entire universe before fading within a few seconds.
A major priority for high-energy astrophysicists has been to isolate and understand gamma-ray bursts, which appear without warning somewhere in the observable universe about three times a day. The COMPTEL team at UNH has since written software to allow COMPTEL to zero-in on the bursts within the few seconds they are going off. The software connects COMPTEL to the Gamma-Ray Burst Coordinates Network, a NASA-operated network that notifies dozens of telescopes about bursts in real-time.
Two unique sources that appear in the COMPTEL energy range are titanium 44 and aluminum 26, both produced in supernova explosions. Ti-44 has a half-life of 60 years; Al-26 has a half-life of 700,000 years. The detection of decaying Ti-44 can lead to the discovery of young supernova remnants; decaying Al-26 can point to ancient supernova remnants. Both metals, in fact, played a role in COMPTEL's discovery of a supernova remnant now called GRO/RX J0852, which was as bright as the moon when it exploded 700 years ago yet somehow remained undocumented until last year. The other Ti-44 source is the Cas A supernova remnant seen by the Chandra X-ray Observatory in its recent "first light" observtions.
Other objects in COMPTEL's range include pulsars and active galactic nuclei (AGN), which are thought to host massive black holes of one million to one billion times the mass of the Sun. The most massive AGN often display their maximum luminosity in the COMPTEL energy range, making gamma-ray observations crucial for understanding these objects.
Unlike optical light and X-rays, gamma rays cannot be captured and reflected in mirrors. The high-energy photons would pass right through such a device. COMPTEL must utilize a process called Compton scattering, where a gamma ray strikes an electron and loses energy, similar to a cue ball striking an eight ball. COMPTEL has two sets of detectors that scatter gamma rays -- that is, the detectors act like billiard balls. The detectors are aligned one below the other. The gamma ray passes through, striking an electron in one detector and then another electron in the second detector. By combining measurements of the loss of energy and the change of trajectory, the COMPTEL scientists can construct a likelihood map of the probable gamma ray source location.
The COMPTEL data are analyzed jointly by Max Planck Institute for Extraterrestrial Physics, which also helped build the instrument; NASA's Goddard Space Flight Center, which controls CGRO Science Department of the Astrophysics Division of ESA/ESTEC in Noordwijk, Netherlands; and the University of New Hampshire.
PORTSMOUTH, N.H. -- Scientists have stumbled upon a simple way to deduce the mass of large black holes through a relationship between their radio and x-ray luminosity. The method is easy to apply and doesn't depend on detailed model calculations.
Dr. Insu Yi of the Korea Institute for Advanced Study and Dr. Stephen Boughn of Haverford College have already weighed 10 massive black holes by measuring radio and x-ray flux ratios. The masses are in reasonable agreement with previous measurements of these black holes using more complicated methods.
"By measuring radio and X-ray luminosities," Yi said, "we deduce temperature and density, which in turn give a unique combination of black hole mass and mass accretion rate for a given set of measured radio and X-ray fluxes. Therefore, we not only get estimates on black hole masses but also mass accretion rate estimates."
Black holes are regions in space where matter is so dense and the force of gravity so great that not even light can escape the pull of gravity. Yi and Boughn are weighing super-massive black holes, which contain the mass of millions to billions of suns compressed to region the size of our solar system. Scientists believe these types of black holes likely formed from the rapid collapse of gas in the early Universe and are present in the centers of most galaxies, including our Milky Way.
Yi and Boughn's calculations rely on radio and x-ray fluxes emitted from small, central regions of x-ray bright galaxies. The radio and x-ray emission comes from a black hole's accretion disk, a stream of gas that spirals into the black hole like water down a drain. The hot, fast-moving gas is the source of both radio and x-ray radiation.
Yi said this new method to calculate mass is only applicable to black holes with advection-dominated accretion flows (ADAF). This is a specific type of gas flow into a black hole that radiates energy less efficiently than black holes associated with powerful active galactic nuclei (AGN), popularly known as quasars, blazars and radio galaxies. It is likely that black holes that Yi and Boughn are studying - the more modest x-ray bright galactic nuclei (XBGN) - are much more common than their more flamboyant AGN cousins.
Yi said he could deduce a black hole mass because the radio and X-ray radiation from ADAF is produced from the same energetic electrons that are moving quickly around black holes. This two-tier emission can determine the temperature, density, and magnetic field strength of ADAFs, which are dependent almost entirely by black hole mass and mass accretion rate.
A common, current way to measure black hole mass is to observe the orbit of stars around the suspected black hole. The more massive the black hole, the greater its gravitational pull and the greater its effect on the star's orbit. This is called the stellar dynamical method, based on Johannes Kepler's centuries-old laws. Recently, scientists have also been able to deduce a mass by determining the orbits of diffuse gas via its "maser" emission. (Maser is an acronym for microwave amplification by stimulated emission of radiation, a close relative of the laser). In addition, a theoretical method of obtaining an upper limit to black hole mass involves the measurement of short-term and long-term fluctuations of x-ray flux.
"The stellar dynamical method is widely used," Yi said, "but there are some serious uncertainties about how to interpret stellar motion and their orbits. Our method simply needs two relatively easy measurements, radio flux and hard X-ray flux. It can be used to independently check the existing mass estimates. One useful application of our method is to find black hole candidates and follow up with the conventional dynamical methods."
The only uncertainties to Yi and Boughn's method, Yi said, is the measurement of magnetic field strength, which is not a large issue, and the obtaining of high angular resolution observations, which ensures the x-ray and radio fluxes are being emitted from the very central regions of galaxies.
According to Boughn, the results are encouraging. "When we applied our method to a few nearby AGN with previously measured black hole masses, we found fairly good agreement. And this is considering the simplicity of the ADAF model and the fact that the x-ray measurements had an angular resolution that was rather poor. If the ADAF model turns out to be justified, then our [weighing method] certainly be useful to expand the knowledge about the masses of black holes that are currently thought to reside in the centers of most galaxies."
Yi said by accumulating mass estimates of many black holes, scientists could then concentrate on the physical origins of massive, non-stellar black holes and their mass distribution function.
Yi and Boughn used data attained from two radio telescope arrays, the VLBI and VLA; and several x-ray telescopes, including ASCA, ROSAT and, in a few cases, Einstein. Yi looks forward to higher resolution and sensitivity of Chandra, NASA's x-ray satellite launched in July, as well as Astro-E and XMM, two x-ray satellites to be launched by Japan and Europe, respectively, early next year. In addition, the routine operation of the National Radio Astronomical Observatories' VLBA will result in many high angular resolution radio observations of galactic nuclei.
It was all in a day's work for a team of astronomers from Argentina and South Africa. First, discover a new supernova with a radio telescope. Next, tag two previously unidentified gamma-ray sources with the Compton Gamma Ray Observatory. Then, tie them all together to provide the first observational evidence of the leading cosmic-ray origin theory.
The results of this hat trick of observations were presented by Drs. Jorge A. Combi, Gustavo E. Romero and P. Benaglia of Instituto Argentino de Radioastronomia; and Dr. Justin L. Jonas of Rhodes University in South Africa.
The origin of cosmic rays remains a long-standing mystery in contemporary astrophysics. Scientists suspect that cosmic rays - essentially atomic particles that bombard the earth at nearly light speed - are hurled through space to such great speeds by the shock waves of exploding stars, called supernovae.
Earlier observations have shown supernovae to produce cosmic ray electrons. Today's results, however, provide the first observational evidence of supernovae producing cosmic ray protons, which are a hundred times more common and a much heavier form of cosmic ray.
"Most of cosmic rays that reach the Earth are protons, not electrons," said Combi. "Are these protons also accelerated in supernova remnants? The evidence has been elusive up to now."
The first step in addressing this cosmic mystery was finding the undiscovered supernova remnant. A supernova remnant is the hot gaseous remains of a star explosion. Using the Hartbeesthoek radio telescope in Krugersdorp, South Africa, the astronomers spotted the yet-unnamed remnant within our galaxy about 1,600 light years from Earth. The remnant, produced from a supernova explosion 15,000 years ago, is large yet weak, according to Combi. Its faint radio waves were buried in the background radio emission from the Milky Way and thus went unnoticed until now. The technique used to "remove" the diffuse radiation that veiled the supernova remnant is a feat in its own right.
The astronomers next noticed that the position of the supernova remnant was close to two - and maybe three - unidentified gamma-ray sources in the Third EGRET Catalog, issued earlier this year. EGRET is one of four instruments aboard the Compton Gamma Ray Observatory and was built at NASA/Goddard. The type of gamma-ray radiation that the astronomers observed with EGRET could only be produced by protons colliding with other protons. This collision produces a short-lived sub-atomic particle called a neutral pion and a subsequent burst of energy in the form of two gamma-ray photons.
"We think that the gamma-rays are produced when the protons locally accelerated in the supernova remnant collide with cloud atoms producing copious pions, which decay and yield [gamma-ray] photons in the EGRET energy range," said Romero.
In other words, the astronomers suggest that protons floating in the interstellar medium were accelerated to high speeds by the supernova shock. These protons collided with other protons in a nearby atomic cloud of hydrogen gas and produced the observed gamma rays.
Such a situation was theoretically predicted some years ago by Felix Aharonian at the Max Planck Institute for Extraterrestrial Physics, as well as by other scientists. The simultaneous detection of a supernova remnant, atomic clouds and EGRET sources has remained evasive until now.
According to Romero, the newly discovered supernova "will surely constitute an outstanding natural laboratory for testing our ideas on the origin of cosmic rays in the years to come. Since we can determine the spectral shape of both the electron and proton cosmic rays in the source, we can now compare the relative efficiency of the acceleration mechanism and infer physical conditions in the supernova remnant."
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