GRB 970508

A gamma-ray burst (GRB) is a highly luminous flash of gamma rays—the most energetic form of electromagnetic radiation. GRBs were first detected in 1967 by the Vela satellites (a series of spacecraft designed to detect nuclear explosions in space). The initial burst is often followed by a longer-lived "afterglow" emitted at longer wavelengths (X-ray, ultraviolet, optical, infrared, and radio). The first GRB afterglow to be discovered was the X-ray afterglow of GRB 970228, which was detected by BeppoSAX, an Italian–Dutch satellite originally designed to study X-rays. On Thursday May 8, 1997, at 21:42 UTC, BeppoSAX's Gamma Ray Burst Monitor registered a gamma-ray burst that lasted approximately 15 seconds. and by the Burst and Transient Source Experiment (BATSE) on board the Compton Gamma Ray Observatory. The burst also occurred within the field of view of one of BeppoSAX's two X-ray Wide Field Cameras. Within a few hours, the BeppoSAX team localized the burst to an error box—a small area around the specific position to account for the error in the position—with a diameter of approximately 10 arcminutes. ==Observations==

in New Mexico After a rough position of the burst had been determined, Enrico Costa of the BeppoSAX team contacted astronomer Dale Frail at the National Radio Astronomy Observatory's Very Large Array. Frail began making observations at a wavelength of 20 centimeters at 01:30 UTC, less than four hours after the discovery. The following evening Djorgovski again observed the region. He compared the images from both nights but the error box contained no objects that had decreased in luminosity between May 8 and May 9. which was later confirmed to be the burst's optical afterglow. On the night between May 10 and May 11, 1997, Metzger's colleague Charles Steidel recorded the spectrum of the variable object at the W. M. Keck Observatory. indicating that light from the burst had been absorbed by matter roughly 6 billion light-years from Earth. Although the redshift of the burst itself had not been determined, the absorbent matter was necessarily located between the burst and the Earth, implying that the burst itself was at least as far away. The absence of Lyman-alpha forest features in the spectra constrained the redshift to z ≤ 2.3, Several optical spectra were also obtained at the Calar Alto Observatory at wavelength ranges of and , but no emission lines were identified. On May 13, five days after the first detection of GRB 970508, Frail resumed his observations with the Very Large Array. Over the next month, Frail observed that the luminosity of the radio source fluctuated significantly from day to day but increased on average. The fluctuations did not occur simultaneously along all of the observed wavelengths, which Jeremy Goodman of Princeton University explained as being the result of the radio waves being bent by interstellar plasma in the Milky Way. ==Characteristics==

BeppoSAX's Gamma-Ray Burst Monitor, operating in the energy range of 40–700 keV, recorded a fluence of (1.85 ± 0.3) × 10−6 erg/cm2 (1.85 ± 0.3 nJ/m2), and the Wide Field Camera (2–26 keV) recorded a fluence of (0.7 ± 0.1) × 10−6 erg/cm2 (0.7 ± 0.1 nJ/m2). BATSE (20–1000 keV) recorded a fluence of (3.1 ± 0.2) × 10−6 erg/cm2 (3.1 ± 0.2 nJ/m2). Further analysis by Frail and his colleagues indicated that the total energy released by the burst was approximately 5×1050 ergs (5×1043 J), and Rhoads determined that the total gamma-ray energy was approximately 3×1050 erg (3×1043 J). ==Distance scale and emission model==

Prior to this burst, astronomers had not reached consensus regarding how far away GRBs occur from Earth. Although the isotropic distribution of bursts suggested that they do not occur within the disk of the Milky Way, some astronomers supported the idea that they occur within the Milky Way's halo, concluding that the bursts are visibly faint because they are not highly energetic. Others concluded that GRBs occur in other galaxies at cosmological distances and that they can be detected because they are extremely energetic. The distance measurement and the calculations of the burst's total energy release unequivocally supported the latter theory, effectively ending the debate. Throughout the month of May the radio scintillations became less noticeable until they ceased altogether. This implies that the radio source significantly expanded in the time that had passed since the burst was detected. Using the known distance to the source and the elapsed time before the scintillation ended, Frail calculated that the radio source had expanded at almost the speed of light. While various existing models already encompassed the notion of a relativistically expanding fireball, this was the first strong evidence to support such a model. ==Host galaxy==

The afterglow of GRB 970508 reached a peak total luminosity 19.82 days after the burst was detected. It then faded with a power law slope over about 100 days. The galaxy was well fitted by an exponential disk with an ellipticity of 0.70 ± 0.07. ==See also==