After the first trials in the 1980’s for idfentifying optical transients in plate archives, the end of the century meant a breaktrough in the GRB puzzle.
With the advent of the X-ray satellites BeppoSAX and RossiXTE, it has been possible to carry out deep multi-wavelength observations of the counterparts associated with the long GRBs class just within a few hours of occurence, thanks to the observation of the fading X-ray emission that follows the more energetic gamma-ray photons once the GRB event has ended. The fact that this emission (the afterglow) extends at longer wavelengths, has led to the discovery optical/IR/radio counterparts in 1997-2000, greatly improving our understanding of these sources.
The R-band light curve of the GRB 970508 optical counterpart
BeppoSAX made possible to detect the first X-ray afterglow following GRB 970228 whose precise localization led to the discovery of the first optical transient (or optical afterglow, OA) associated to a GRB.
The light curve exhibited a power-law decay with the flux proportional to F t^(-alpha} with alpha = 1.1. PL declines have been measured for 26 OAs in 1997-2000 yielding values in the range 0.8 < alpha < 2.3 with = 1.35.
GRB 970508 was the clue to the distance: optical spectroscopy obtained during the OA maximum brightness allowed a direct determination of a lower limit for the redshift (z > 0.835), implying D > 4 Gpc and E >7 x 10^51 erg.
It was the first proof that GRB sources lie at cosmological distances. The flattening of the decay at T_0 + 100 d revealed the contribution of a constant brightness source -the host galaxy- seen in late-time imaging at T_0 + 1 yr.
The 15 GRB redshifts measured so far are in the range 0.430 < z < 4.50 with = 1.5 and they were derived either from absorption lines in the OA spectrum, from the Lyman-alpha line break, or from emission lines arising in the host galaxy.
Significant early optical emission may arise from the reverse shock, i.e. strong optical flashes accompanying gamma-ray emission should be a generic characteristic (at least for typical GRBs, with E = 10^53 erg and the density rho = 1 cm^(-3)).
Such observations will allow: i) to derive the Lorentz factor by the relative timing of optical and gamma-ray emission, ii) to pinpoint the process by which the shells responsible for the external shock arise, and iii) to constraint the environment.
The ROTSE experiment achieved the detection simultaneously to the GRB of the bright optical emission from GRB 990123: the most luminous object ever recorded, with M_V = -36 (peaking at m_V = 8.9), implying that at least some subsets of GRBs do exhibit variable optical emission as violent as the gamma-ray variations.
The first optical/near-IR counterparts have been found for 30 precisely localized GRBs in 1997-2000.
In any case, only the population of GRBs with durations of few seconds has been explored. Short bursts lasting less than 1 s, that follow the -3/2 slope in the log N-log S diagram (in contrast to the longer bursts) remain to be detected at longer wavelengths.
Future missions should be able to address some of the issues still to be solved, i.e. prompt optical observations should be persued !
Contrapartida Discovery images of the GRB 980703 optical counterpart at the spanish 0.8-m IAC telescope