2005 From ScienceWeek
ASTRONOMY: MYSTERIOUS SALVOS FROM THE GALACTIC CENTER
The following points are made by S.R. Kulkarni and E.S. Phinney (Nature 2005 434:28):
1) New work[1] reports a bright bursting radio source near (in projection at least) the center of our Galaxy. The researchers suggest that the object (dubbed GCRT J1745-3009) is a prototype of a new class of particularly bright coherently emitting radio transients. Because the distance and precise position of the source are as yet unknown, more mundane explanations are still possible. But the manner of its discovery, and the potentially exciting interpretation, will inspire more dedicated searches for radio transients.
2) The essential facts are as follows: While observing the central region of our Galaxy at radio wavelengths, Hyman et al[1] discovered five strong bursts, each lasting approximately 10 minutes, and separated by approximately 77 minutes, coming from the same 10-arcsecond region of the sky. No emission, steady or otherwise, is seen in subsequent (and archival) searches or between the bursts.
3) The source is not well enough localized to identify counterparts at other wavelengths, so its distance is unknown. GCRT J1745-3009 could, like most stars in its direction, be near the center of our Galaxy (about 24,000 light years or 8000 parsecs distant), in which case its radio luminosity is around an impressive one-hundredth that of the Sun. But because the center of our Galaxy is so interesting, astronomers tend to stare there more than anywhere else. So it is possible that the source is much nearer (say, 300 light years or 100 parsecs), less luminous, and only coincidentally projected near the Galactic Centre.
4) The duration of the burst limits the size of the source to less than the distance travelled by light over the burst duration. Armed with this knowledge, we can compute the equivalent “black-body” temperature of the emitter. Cosmic radio sources have a natural thermostat[2] that normally restricts this brightness temperature to less than 10^(12) kelvin. But the brightness temperature of GCRT J1745-3009, the radio source observed by Hyman et al[1], exceeds this if it does indeed lie farther from Earth than 100 parsecs. In some cases, temperatures higher than the 10^(12)-kelvin thermostat are seen through a form of trickery[3] involving special relativity: when emitting matter is racing towards Earth at nearly the speed of light, much higher apparent brightness temperatures will be inferred.
5) Galactic examples of such astronomical tricksters are black holes[4] and neutron stars[5] in binary systems accreting mass from a companion star. GCRT J1745-3009 could be one of those objects. However, the known examples have prominent X-ray emission, whereas no X-ray emission from GCRT J1745-3009 has been reported in other studies (the RXTE, ROSAT and ASCA space missions). On the other hand, if the roughly 77-minute interval between the source’s radio bursts is an orbital period in an accreting binary system, only the smallest star or a white dwarf can fit in the tiny orbit. The accretion rate in such a binary system would be low, and the accretion might also be radiatively inefficient, so it could hide well below the X-ray limits. It is possible that the radio source, modulated by absorption of the radio waves in a stellar wind, is an “X-ray quiet, radio-loud” X-ray binary, similar to certain types of active galactic nuclei, but with stellar mass.
References (abridged):
1. Hyman, S. D. et al. Nature 434, 50-52 (2005)
2. Readhead, A. C. S. Astrophys. J. 426, 51-59 (1994)
3. Rees, M. J. Nature 211, 468-470 (1966)
4. Mirabel, I. F. & RodrÃguez, L. F. Annu. Rev. Astron. Astrophys. 37, 409-443 (1999)
5. Fender, R. et al. Nature 427, 222-224 (2004)
Nature http://www.nature.com/nature
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ASTRONOMY: ON THE CENTER OF THE MILKY WAY
The following points are made by T.J. Lazio and T.N. LaRosa (Science 2005 307:686):
1) At a distance of just 25,000 light years (2.5 x 10^(20) m), the center of our Galaxy, the Milky Way, provides the foundation for understanding phenomena in other galaxies. The central black hole (1) and regions of intense star formation in its vicinity can be probed at 100 times the resolution of even the nearest galaxies. Nonetheless, even the basic properties of a key component of the galactic center, its magnetic field, remain poorly understood.
2) Magnetic fields have the potential to transform, store, and explosively release energy, to transport angular momentum, and to confine high-energy plasmas into powerful jet flows. They are therefore central to astrophysical activity from stellar to galactic scales.
3) Magnetic fields are found throughout the Milky Way. Measurements suggest that the magnetic field in the spiral disk of our Galaxy has two components, one globally ordered and the other random, with approximately equal strengths of ~0.3 nT (2); the globally ordered component generally follows the spiral arms of the galaxy. Key questions about the magnetic field in the galactic center are whether it is comparable in strength or much stronger than the field in the disk, and whether it is globally ordered or largely random.
4) approximately 20 years ago, the first high-resolution radio images of the galactic center (3) revealed numerous magnetic structures that are unique to the galactic center. The most striking of these is the galactic center radio arc, a series of parallel linear filaments, each of which is merely a few light years wide yet more than 100 light years long. Also observed were a number of isolated linear features that were variously referred to as streaks, threads, and filaments. The relation between these isolated filaments and the bundled filaments of the radio arc remains unknown.
5) These filamentary structures are distinguished by extreme length-to-width ratios (~10 to 100), nonthermal spectra, and a high intrinsic polarization (~30%, and in some cases approaching the theoretical maximum of 70% for synchrotron radiation). The polarization and nonthermal spectra are consistent with the filaments being produced by synchrotron radiation from relativistic electrons spiraling around a magnetic field. Detailed measurements of individual filaments have shown that the magnetic fields are aligned longitudinally with the filament.(4,5)
References (abridged):
1. G. C. Bower et al., Science 304, [704] (2004)
2. R. Beck, Space Sci. Rev. 99, 243 (2001)
3. F. Yusef-Zadeh et al., Nature 310, 557 (1984)
4. M. Morris, E. Serabyn, Annu. Rev. Astron. Astrophys. 34, 645 (1996)
5. C. C. Lang, K. R. Anantharamaiah, N. E. Kassim, T. J. W. Lazio, Astrophys. J. 521, L41 (1999)
Science http://www.sciencemag.org
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ON THE BLACK HOLE AT THE CENTER OF OUR GALAXY
The following points are made by Alexei V. Filippenko (Proc. Nat. Acad. Sci. 1999 96:9993):
1) Some galaxies are known to have very “active” central regions from which enormous amounts of energy are emitted each second. These “active galactic nuclei” are probably powered by accretion of matter into a supermassive black hole of 10^(6) to 10^(9) solar-masses. The center of our own Galaxy exhibits mild activity, especially at radio wavelengths: so-called “nonthermal radiation” characteristic of high-energy electrons spiraling in magnetic fields is emitted by a compact object at the Galactic center known as *Sagittarius A*. Does the center harbor a supermassive black hole?
2) One approach is to determine whether stars in the central region are moving very rapidly, as would be expected if a large mass were present. During the past 5 years, two teams have obtained high-resolution images of our Galactic center, each team on several occasions, so that temporal changes in the positions of stars could be detected. The observations were conducted at infrared wavelengths, which penetrate the gas and dust between Earth and the Galactic center (a distance of approximately 25,000 light years) much more readily than optical light. In summary, the data are in excellent agreement with the conclusion that the gravitational potential of the central region of our Galaxy is dominated by a single object. The derived mass of this object is (2.6 +- 0.2) x 10^(6) solar-masses, and the mass density within a radius of 0.05 light-years is at least 6 x 10^(9) solar-masses per cubic light-year, effectively eliminating all possibilities other than a black hole.
3) Although our Galaxy provides the most convincing case for the existence of supermassive black holes, observations of the centers of a few other galaxies bolster the conclusion. For example, very precise measurements of the galaxy NGC 4258 reveal a central compact object with a derived mass 3.6 x 10^(7) solar-masses. On somewhat larger scales, spectra obtained with the Hubble Space Telescope show gas and stars rapidly moving in a manner consistent with the presence of a supermassive black hole. The most massive existing case, that of the giant elliptical galaxy M87, is approximately 3 x 10^(9) solar-masses. Moreover, x-ray observations of some active galactic nuclei reveal emission from a hot disk of gas apparently very close to a black hole, since extreme relativistic effects are detected. In general, it now seems that a supermassive black hole is found in nearly every large galaxy amenable to such observations.
4) The author concludes: “In the last decade of the 20th century, black holes have moved firmly from the arena of science fiction to that of science fact. Their existence in some *binary star systems, and at the centers of massive galaxies, is nearly irrefutable. They provide marvelous laboratories in which the strong-field predictions of Einstein’s general theory of relativity can be tested.”
Proc. Nat. Acad. Sci. http://www.pnas.org
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Notes by ScienceWeek:
Sagittarius A*: Sagittarius A is a prominent radio source in the constellation Sagittarius, coincident with or close to the center of our Galaxy. It is a highly complex region consisting of a central core approximately 50 light-years in diameter. Sagittarius A* is a compact component at the heart of the central core of Sagittarius A. Sagittarius A* is an intense source of radio waves, and is apparently unique in our Galaxy: while everything else in our Galaxy is on the move as they follow their orbits, Sagittarius A* is absolutely stationary and must therefore lie exactly at the Galaxy’s center. Many astronomers, in fact, use Sagittarius A* as the “Greenwich Meridian” of the Galaxy.
binary star systems: Binary stars are a pair of stars revolving around a common center of mass under the influence of their mutual gravitational attraction, and apparently the majority of stars in the Universe are binaries and not singlets. In some cases the binary system is resolvable into two components, and in other cases the presence of a second star is inferred by perturbations in the motion or emitted radiation of the first star. If the binaries are close enough, they may share stellar material, and this results in a particular kind of stellar evolution. In the black hole-binary systems mentioned in this report, matter transfers from a relatively normal star (known as the “secondary star”) to a dark compact object (the “primary”). Recent comparisons of x-ray and optical brightness suggest that in many cases the dark primary in such a binary system is a black hole.
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