ar X iv : a st ro - p h / 01 02 49 2 v 1 2 8 Fe b 20 01 Evidence for a Supernova in the Ic Band Light Curve of the Optical Transient of GRB 970508

Unique data on BV RI brightness curves of the OT of GRB 970508 obtained with the 6-m telescope have been interpreted in the framework of the GRB-SN (supernovae) connection. The effect must be maximal in the Ic band as OT GRB 970228 [5]. The peak absolute (MB) magnitude of the suggested SN must be around -19.5 for the OT of GRB 970508. If all or the main part of long GRBs are associated with SNe, the GRB host galaxies (for ground-based observations, at least) must be dimmer than the peak magnitude of the SN [9]). The Ic band light curve of GRB 970508 afterglow exhibits a rebrightening phase, or a ”shoulder” [7]. This spectral property lasts for not less than 51 days [8]. Over this period, the color index V-I increases by 1.6 mag, while Ic = 23.04+/-0.14 for the OT. The deviation from the ”average” power-law exceeds 1 magnitude. Recent papers show that GRB optical afterglows have unusual temporal and V RI-spectral properties: GRB 970228 [5], GRB 980326 [2] , and possibly GRB 990712, and GRB 991208 (Castro-Tirado A., in these proceedings). All these OTs have temporal and spectral characteristics similar to those of GRB 970508, for which the Ic band light curve is consistent with the assumption that the SN emission overtakes the OT flux at late times, i.e. nearly 31 days after the GRB. It should be noted that the emission of all type-I SNe shows a strong UV deficit owing to absorption lines below 3900Å. Thus, a weak flux is expected from the suggested SN in the Rc band for a redshift z of 0.835 corresponding to the Urest (3652Å) in the rest frame. In contrast, the SN brightness must be maximal at larger wavelengths (Fig1), i.e. in the Ic band as observed for GRB 970508 (and for GRB 970228 with z = 0.695 [5]), plus a time profile stretching for the light curve of the SN by a factor of 1+ z. As z = 0.835, the Ic band in the observer frame corresponds to the Brest (4448Å) band in the rest frame, the observed Ic flux is thus used to derive the peak absolute magnitude (MB) of the assumed SN. At the ”shoulder” the brightness of GRB 970508 OT corresponds to the maximum luminosity of a peculiar SN with MB around -19.5 if for GRB990708 host we have MBrest around -18.5 [8] (Fig.2). We can consider the type Ib/c SNe as a preliminary model of GRB/SN gamma-ray bursters, but the peak magnitudes of Type Ib/c (or core collapse) SNe are not constant: MB is from -16 to -19.5. Thus, the GRB host galaxies (for ground-based observations, at least) must be dimmer than the peak magnitude of the SN (see Table 1 in [9]). SN1997ef has been recognized as a peculiar SN Ic from its light curve (Mazzali P., et al., in these proceedings). It shows a very