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NOAO Newsletter - NOAO Highlights! - September 1997 - Number 51


An Optical Counterpart of a Gamma-Ray Burst

Howard Bond (STScI) used the KPNO 0.9-m to identify an optical counterpart to a gamma-ray burst (GRB), contributing to the apparent resolution of the three-decade-old debate on the distance scale of GRB sources. Bond was observing post-AGB stars in globular clusters on the evening of 8 May 1997, when Jules Halpern called from Berkeley with the news that a gamma-ray burst (GRB 970508) had been detected with the BeppoSAX satellite a few hours earlier, and that unusually accurate coordinates were already known (diameter of error box about 20'). Bond obtained the first CCD frames as soon as the sky was dark--only 5.5 hours after the burst had occurred. Because of the wide field of view afforded by the 0.9-m telescope's 2048 X 2048 chip (23' X 23'), the entire error circle could be covered in one exposure. However, comparison of these frames with images from STScI's Digitized Sky Survey disclosed no newly brightened objects down to the limit of the DSS.

On the following night, Bond obtained additional frames; in the meantime, the gamma-ray error box had moved by 7' and shrunk to a diameter of 10'. Now, by blinking the frames from the two nights, Bond immediately found a 20.5-mag star-like source near the center of the error circle which had brightened by about one magnitude since the previous night (see the figure). The discovery was communicated immediately to the Central Bureau for Astronomical Telegrams at the Center for Astrophysics.

image
Caption: Discovery images for the optical counterpart of GRB 970508, obtained with the KPNO 0.9-m telescope. These are V-band frames obtained on 10 May 1997 UT (top, 1800s exposure) and 9 May (bottom, 600s). Each frame is 138" high. The GRB counterpart is the variable source marked in the top frame, which brightened by 1 mag between 9 and 10 May.

First discovered serendipitously in the late 1960's, GRB sources are distributed isotropically across the sky. Two schools of thought have arisen on their nature, with one camp proposing that they are located in the halo of the Milky Way, and the other placing them at cosmological distances. (For summaries of the two views, see the "Great Debate" papers in the December 1995 issue of PASP.)

With an apparent optical counterpart of GRB 970508 now blazing in the sky, the race was on to obtain an optical spectrum. The following night was completely cloudy at Kitt Peak, preventing the 4-m and MDM 2.5-m observers from obtaining spectra, and the large hour angle of the object and equipment problems prevented observations at Lick Observatory. Finally, Charles Steidel and collaborators obtained a spectrum with the Keck II telescope, showing a continuum with a superposed absorption-line spectrum with a redshift z = 0.835. Assuming that the optical variable is indeed the counterpart of the GRB, this observation seems to settle the debate (setting aside the unlikely possibility that there are two classes of GRBs, one local and one cosmological).

The high-z absorption lines are attributable to an intervening galaxy or possibly to a galaxy hosting the GRB source. The 0.9-m observations show that the optical fireballs from GRBs, at maximum light, outshine supernovae by several magnitudes, making them the most luminous astrophysical objects other than QSOs. Followup observations of GRB 970508 (now fading as about t-1) have now been obtained with the Hubble Space Telescope (which so far have not revealed a host galaxy, suggesting either that the host is intrinsically rather faint, or that the GRB source lies in the extreme halo of one of several neighboring faint galaxies). Theoretical explanations mostly involve the merger of two neutron stars.


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