No optical emission above mag 5 (1 s duration assumed) or mag 13 (full exposure time) or Lg/Lo > 100... 300 has been detected for a few GRBs. The faintest limit (320) exists for GRB 830313 (Hudec 1993). No optical emission above magnitudes 0... 3 (1 s duration assumed) or 4-11 (full exposure time) or Lg/Lo > 0.1... 10 has been detected for many (95) GRBs.
Optical transient (OT) has been detected on the plate taken
7 h after the GRB 790929 inside its
error box (Borovicka et al. 1992)
17 time-correlated photographic plates with limiting magnitudes around 14.5 have been investigated for the following three COMPTEL GRBs.
For GRB 910709 at UT 11.56, no optical activity except variable star Y Dra mentioned later has been found on patrol plates taken 11 h before and 35 h after. For GRB 940708 at UT 20.70, no optical activity has been revealed on plates taken 21 h before, 3 h after and 27 h after. For GRB 940301 at 20.18 UT, no optical activity has been found on the patrol plates taken 24 h before and 71 h after.
Conclusions: No new and/or variable optical objects with amplitudes more than 0.4 mag have been found except for GRB 910709. In this case, a bright object with maximum close to the GRB date and the fading has been revealed and identified with a Mira type variable star Y Dra (variable between mag 6.24 and 15.0). Although the objects are probably not related, this is an another example of background trigger with analogous appearance to real optical afterglows of GRBs.
From the number of 20 BeppoSAX/RXTE GRBs listed, a subset of 8 GRBs is
covered in location and in time (criterion: days) by the
Sonneberg Sky Patrol (altogether 67 plates, evaluation is still in progress).
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