Telescopes must respond quickly to pinpoint transient gamma-ray bursts and pick up their afterglow. Mysterious 'dark' bursts seemed to produce no light at optical wavelengths, but a faint signal has now been detected.
Gamma-ray bursts (GRBs) are intense flashes of radiation that originate in the furthest reaches of the Universe. The bursts are expected to be followed by an afterglow of radiation at optical wavelengths. But for many GRBs (around 60% of the total sample of less than 40 bursts that have now been followed up), this afterglow seems to be missing. Hence, they have become known as 'dark' bursts. In a paper to appear in the Astrophysical Journal, Berger et al.1 report data from an armada of telescopes that reveal a faint optical afterglow from a GRB that would otherwise have been classed as 'dark'. Their observations suggest that the seeming lack of optical emission from dark bursts is in fact a result of insufficiently prompt and sensitive observations.
Astronomers had thought that GRBs occurred within our Galaxy, but in the early 1990s it was realized that these explosive events actually occur at cosmological (extra-galactic) distances, of the order of 15 billion light years away. The distance issue was settled by an observational breakthrough — the accurate and rapid location of GRBs by the Italian–Dutch spacecraft Beppo–SAX. Following the initial detection and accurate location of gamma-rays by Beppo–SAX, the optical-radiation counterpart could be determined by other telescopes. This also makes it possible to measure the distance of the burst from the Earth. More importantly, detailed follow-up observations could then be made of the burst's afterglow and its host galaxy at other wavelengths by some of the most powerful telescopes in the world, both ground-based and space-based (in particular, the Hubble Space Telescope and the Chandra X-ray Observatory).
The field of prompt GRB follow-up observations has burgeoned in the past four years, in both the observational and the theoretical areas. Afterglow observations are being made at X-ray, optical, microwave, infrared and radio wavelengths. In each observational band, and by combining data from different bands, astronomers can make clever use of the data, such as providing a measure of the angular size of the emitting region by observing radio variability2.
For gamma-ray bursts, the observed optical (and X-ray) emission falls off rapidly with time. Perhaps in no other area of astronomy are observations needed so rapidly — ideally, minutes to a few hours following the burst. Any given telescope on the Earth has only about a 10% chance of making a quick observation, considering its location and the variety of limitations with which it must contend. The result described by Berger et al.1 was made possible by a rapid response from the HETE spacecraft, in concert with an interplanetary network of spacecraft3 in orbit around Earth and Mars, and beyond. But such opportunities are rare.
The optical emission from the burst studied by Berger et al. (GRB 020124) decayed even more rapidly than most gamma-ray bursts, and it resided in the faintest host galaxy seen so far. It seems that the characteristic faintness and rapid decay of dark bursts is an intrinsic property of the burst, rather than the result of some other possible, external condition such as dust in the host galaxy. Berger et al. conclude that GRB 020124 is intrinsically faint at optical wavelengths and that it would have been classified as a dark GRB were it not for the prompt observations. Although the authors are cautious about drawing strong conclusions from this single burst, their result provides a new constraint on the models of bursts and afterglows that may render some of them untenable.
Theorists are thrilled to be guided and constrained by the new observational data. Most of them agree that a GRB is generated by very high-energy electrons and positrons produced in an initial explosion, and collimated into a jet perhaps only a few degrees wide. But the origin of the explosion and the type of star or stellar system involved are still shrouded in mystery. In fact, there are indications that more than one type of explosion is at work in the GRB phenomenon.
The future looks promising for obtaining accurate positions for more GRBs, with accompanying follow-up observations of their afterglow. In August, HETE recorded another triumph, by pinpointing a GRB less than four hours after the burst, leading to a flurry of observations from major telescopes, and from amateur astronomers as well. This provided a wealth of data on the GRB afterglow, including radio observations, an optical polarization measurement and fast Hubble and Chandra follow-up observations. The distance of the GRB from the Earth was also determined within a day of its detection by HETE.
On a more negative note, however, Beppo–SAX ceased operation earlier this year and the HETE spacecraft, although still operating, has had limited success, producing only a few GRB locations in the past couple of years because of operational and programmatic problems. The interplanetary network of spacecraft, exploited by Berger et al.1 in conjunction with HETE, can itself provide accurate locations of GRBs, although usually only many hours after the event, because communication with the distant spacecraft is limited.
But in late 2003, NASA will launch the Swift spacecraft4, which is expected to locate around 150 GRBs per year. In 2006, the GLAST spacecraft5 will likewise detect numerous GRBs and will cover an unprecedented broad energy range in the X-ray and gamma-ray regions. Beyond that, a large, next-generation GRB mission6 is under study, which would cover almost the entire sky with a sensitivity perhaps ten times that of Swift. In addition, about a dozen robotic and automated telescopes are operational or being developed to catch the early optical emission from GRBs. And last but not least, as has been proved on at least four occasions so far, a well-prepared and knowledgeable amateur astronomer with a modest-sized telescope can make a substantial scientific contribution to the field of GRB research.
Berger, E. et al. Astrophys. J. (in the press); Preprint astro-ph/ 0207320, http://arXiv.org
Waxman, E., Kulkarni, S. & Frail, D. Astrophys. J. 497, 288–293 (1998).
Hurley, K. et al. Astrophys. J. Suppl. 122, 497–501 (1999).
Grindlay, J. et al. in Gamma-Ray Burst and Afterglow Astronomy 2001: A Workshop Celebrating the First Year of the HETE Mission (ed. Ricker, G.) (Am. Inst. Phys., in the press).