Optical Afterglows

For 50% of the bursts positioned with high accuracy it has been possible to discover an optical afterglow; its typical magnitude ranges between 19-21, about one day after the gamma-ray burst. For more than 50% of the bursts, whose X-ray afterglow has been detected, the search for an optical afterglow was unfruitful, despite several bursts have been observed at early epochs with sensitive telescopes. So far, GRB990123 is the only burst for which a simultaneous optical emission was observed by the robotic telescope ROTSE, 22 seconds after the gamma-ray onset, triggered by both BeppoSAX and BATSE, reaching $V \sim 9$ ([Akerlof et al., 1999,Akerlof et al., 2000]); in this case, the power emitted in the optical band was $\sim$ 1% of the gamma band. This optical emission has been interpreted as the signature of a reverse shock moving into the ejecta ([Sari & Piran, 1999]), according to the fireball model (see next section). In particular, the ROTSE points in the light curve are fitted by a steeper power law than the afterglow points, thus suggesting two different radation mechanisms ([Pian, 2001]).

In its early phase, an optical afterglow can outshine its host galaxy, that becomes visible only weeks or months later, when the afterglow has faded below the telescope sensitivity and the measured light curve seems to flatten; this happened for the first time in the case of GRB970508, also famous for being the first burst whose radio afterglow was detected (see below).

Optical non-detection might be connected with an extinction along the line of sight or within the source; moreover, low Galactic latitude bursts may be obscured. However, in some cases non-detection is probably due to the extinction by dust in the host galaxy and/or to the absorption by the intergalactic medium: see the cases of GRB970828 ([Djorgovski et al., 2001b]), and GRB980329.

GRB990510 was the first burst, whose optical afterglow showed a linear polarization ( $\Pi = 1.7 \pm 0.2$%), as measured $\sim 18.5$ hours after the burst ([Covino et al., 1999]). This property seems to confirm the synchrotron nature of the afterglow spectrum and seems to support the jet-like geometry of the outflow (see section about GRB models).