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ScienceWeek
ASTROPHYSICS: ON GAMMA RAY BURSTS
The following points are made by Stan Woosley (Nature 2004 430:623):
1) It is seven years since astronomers were first able to pinpoint the origin of a gamma-ray burst (GRB) in a distant galaxy[1-3]. Since then, estimates of the energy associated with these titanic explosions have waxed and waned. Using only the brightness and distance of these objects, calculations had suggested that an amount of matter almost equal to the mass of the Sun was being transformed into gamma-rays with near-perfect efficiency. But then evidence emerged for "beaming" -- the compression of the burst energy into narrow beams, or jets -- and the energy estimates went down by a factor of about a hundred, to an amount equivalent to merely a few supernovae[4]. New evidence[5] suggests that the bursts observed so far are but the bright tip of the iceberg. It seems likely that many fainter explosions have gone unseen simply because our detectors were not sensitive enough.
2) The first indication that GRBs might have a broad range of energies came in 1998 with the discovery of a nearby event, "only" 130 million light years away (equivalent to a redshift, z, of 0.0085). Despite its proximity, GRB 980425 -- named for the day on which it was observed -- was not particularly bright. Its inferred total energy, emitted over 20 seconds, was only 7 x 10^(47) erg -- about 10,000 times less than a typical GRB, but still 10 trillion times more than the Sun emits in the same time. The burst was also "soft" compared with other GRBs: no prompt emission was seen above an energy of 300 kiloelectronvolts (keV); and it was accompanied by a very bright supernova, SN 1998bw, whose extreme luminosity, velocity, and radio emission placed it in a class by itself. Because of its low energy, however, astronomers argued as to whether GRB 980425 really was associated with the nearby supernova (from which the redshift was calculated), or whether it just happened to be in the same general direction but much farther away. Others argued that, even if the GRB and supernova were connected, the GRB was an unusual one, in a class by itself, with little relation to the others.
3) Time and further observation have diminished those arguments. X-ray observations of SN 1998bw spanning several years demonstrate a smooth evolution in brightness that matches that of the GRB, from just one day after its appearance. Moreover, as inferred from radio and X-ray emission, the energy in the relativistic ejecta (the matter thrown out at more than 90% of the speed of light) was about 100 times more than that of the burst itself, and hence comparable to that of other GRBs. For some reason, GRB 980425 put a huge amount of energy into making a supernova (10^(52) erg), a lot of energy into relativistic ejecta (10^(50) erg), but relatively little into gamma-rays moving towards Earth (10^(48) erg). Why was its gamma-ray emission so low? And, given its faintness, might there be many other similar events that have escaped detection?
4) Then came GRB 031203. This burst was discovered from space last December by the gamma-ray telescope INTEGRAL; the details of its gamma-ray emission were reported by Sazonov et al(5) and Soderberg et al (2004). Like GRB 980425, the new burst lasted approximately 20 seconds and was close to our Galaxy. Most GRBs have a redshift that is greater than one, but the host galaxy of GRB 031203 was at z = 0.1055 -- making it, at 1.6 billion light years, the next-closest event after GRB 980425. From the distance and observed flux, the inferred energy of GRB 031203 was 0.6-1.4 x 10^(50) erg, intermediate between GRB 980425 and more typical bursts. As the burst faded, a supernova was revealed, SN 2003lw, with properties similar to SN 1998bw. It looked as if a twin to GRB 980425 had been found.
5) But there were differences. Although faint compared with most bursts, GRB 031203 was a hundred times more luminous than GRB 980425; and its spectrum is "hard", more like that of a typical GRB. A spectrum is described as hard if the typical energy of its gamma-rays is high. Many researchers had come to accept (if not fully understand) an empirical relation, called the "Amati relation", between hardness and a burst's total energy. But the hard spectrum of GRB 031203 promises to upset this apple-cart: the Amati relation would suggest a gamma-ray energy of about 10 keV for GRB 031203; instead, INTEGRAL measured[5] values in excess of 190 keV.
References (abridged):
1. Costa, E. et al. Nature 387, 783-785 (1997)
2. van Paradijs, J. et al. Nature 386, 686-689 (1997)
3. Metzger, M. R. et al. Nature 387, 878-880 (1997)
4. Frail, D. A. et al. Astrophys. J. 562, L55-L58 (2001)
5. Sazonov, S. Yu., Lutovinov, A. A. & Sunyaev, R. A. Nature 430, 646-648 (2004)
Nature http://www.nature.com/nature
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ASTROPHYSICS: ON LONG GAMMA-RAY BURSTS
The following points are made by Andrew MacFadyen (Science 2004 303:45):
1) Approximately once a day, explosions from the depths of the Cosmos bathe Earth in blasts of low-energy gamma rays. These gamma-ray bursts (GRBs), discovered by military satellites in the late 1960s, are the most luminous explosions in the Universe. The common long-duration variety is believed to mark the death of a star many times more massive than our Sun and the birth of a black hole (or exotic neutron star). Supernova SN2003dh, discovered on 29 March 2003 beneath the fading glare of a long GRB, confirms this picture (1,2).
2) GRBs are short nonthermal flashes of ~100-keV gamma rays. About two-thirds of all GRBs have mean durations of 35 s. These "long" GRBs have softer spectra than their short-duration cousins (3). They may vary on millisecond time scales, shut off for a few seconds and then turn back on, or last longer than 2000 s. Given their rapid variability (on a microsecond time scale) and large energy (~10^(52) ergs), it is likely that a compact object of stellar mass powers the GRB explosion.
3) In 1966, Colgate predicted that GRBs were caused by the emergence of a shock wave from a star during a supernova explosion (4). The details of his model turned out to be incorrect, but long GRBs are now believed to be indeed caused by the death of massive stars. However, these stars do not explode in the way that ordinary stars do. They produce asymmetric outflows traveling almost at the speed of light. Such ultrarelativistic outflows are required to produce the observed nonthermal spectrum and rapid variability. The kinetic energy of the explosive outflow from a long GRB is thought to be ~10 times that of a supernova. A key question is why much of this energy is concentrated in a small mass (10^(-5) solar masses) in a GRB instead of in several solar masses in a supernova. In both GRBs and supernovae, much more energy (10^(53) ergs) may be released as neutrinos and gravitational waves. Detailed knowledge of the partitioning of energy among photons (from gamma rays to radio wavelengths), neutrinos, and gravitational waves is critical to fully understanding long GRBs.
4) Observable long GRBs occur about once every 10 million years per galaxy. However, x-ray observations indicate that GRBs are beamed like a flashlight, so that we only see the one in several hundred pointed in our direction. Hence the true rate, including the GRBs pointed away from us, is one per 10,000 years per galaxy. Given that supernovae occur about once every 100 years per galaxy, roughly 1 in 100 supernovae results in a long GRB. The GRB might, for example, be caused by the formation of a black hole (collapsar) (5), a highly magnetized neutron star (magnetar), or a supermassive spinning neutron star.
References (abridged):
1. K. Z. Stanek et al., Astrophys. J. 591, L17 (2003)
2. J. Hjorth et al., Nature 423, 847 (2003)
3. S. Rosswog, Science 303, 46 (2004)
4. S. A. Colgate, Can. J. Phys. 46, S476 (1968)
5. S. E. Woosley, Astrophys. J. 405, 273 (1993)
Science http://www.sciencemag.org
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ASTRONOMY: ON SHORT GAMMA-RAY BURSTS
The following points are made by Stephan Rosswog (Science 2004 303:46):
1) More than 35 years after their initial discovery, gamma-ray bursts (GRBs) remain one the most exciting and mysterious events in our Universe. GRBs are flashes of energetic photons that outshine, for a short moment, the whole rest of the gamma-ray sky. They are classified as "short" or "long" depending on whether their duration is shorter or longer than 2 s. A wealth of information about long GRBs has been gathered, including afterglows at different wavelengths, redshifts, host galaxies, and indications that the bursts occur close to star-forming regions (1). In contrast, information about short GRBs is so far restricted to gamma rays alone.
2) In principle, the two classes of GRBs could be caused by two different types of progenitors, or they could have the same kind of progenitor, which acts in two different ways depending on its initial conditions (such as its initial rotation). It is more likely that each class of GRB has a different kind of progenitor, because there are substantial differences apart from the duration of the burst. For example, the number of subpulses that constitute the light curve of a GRB is different for short and for long bursts (2). The spectra of short GRBs are harder (3); that is, they have a larger fraction of high-energy photons, their peak energy is larger (4), and they evolve differently in time (2). The peak energies of the spectra are influenced by the cosmological expansion -- that is, for more remote bursts, one would expect lower peak energies -- and by the speed with which the radiation-producing matter is ejected from the central engine. Therefore, a higher peak energy of the short bursts could mean either that their central engines produce higher outflow velocities or that they occur on average closer to Earth. Statistical arguments (5) also point to a relatively local origin of the short GRB class.
3) Compact binary mergers -- the coalescence of either two neutron stars or of a neutron star and a low-mass black hole --have long been the standard model for the central engine of GRBs. Such mergers provide huge energy reservoirs of several 10^(53) ergs and would yield a natural explanation for the shortest time scales observed in GRBs. More recently, the association of long GRBs with supernovae has been substantiated. Compact binary mergers are now thought to power only short GRBs.
4) If the gamma-ray emission were beamed into a narrow jet, we could detect a short GRB only if it is by chance directed toward Earth, and we would miss most bursts that occur in the Universe. This would mean that the "true" event rate is much higher than our observed one. The estimated compact binary merger rates are high enough to explain all short GRBs, even if the gamma-ray emission is strongly beamed. Because of the long time from the birth of the binary system to its death in the final coalescence, GRBs resulting from compact binary mergers are expected to occur relatively late in the life of the Universe. It is estimated that they occur at 0.5 to 0.8 times the cosmological redshift of long GRBs, providing further support for a relatively local origin of short GRBs.
References (abridged):
1. A. MacFadyen, Science 303, 45 (2004)
2. J. P. Norris, G. F. Marani, J. T. Bonnel, Astrophys. J. 534, 248 (2000)
3. C. Kouveliotou et al., Astrophys. J. 413, L101 (1993)
4. W. S. Paciesas, R. D. Preece, R. S. Mallozzi, in Gamma-Ray Bursts in the Afterglow Era, E. Costa, F. Frontera, J. Hjorth, Eds. (Springer, Berlin/Heidelberg, 2001), pp. 248-251
5. S. Mao, R. Narayan, T. Piran, Astrophys. J. 420, 171 (1994)
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