GRB 970111 and GRB 970228

A. J. Castro-Tirado and J. Gorosabel, Laboratorio de Astrofisica Espacial y Fisica Fundamental, Madrid, report, on behalf also of J. Heidt, T. Seitz, E. Thommes, C. Wolf, N. Lund, H. Pedersen, E. Costa, F. Frontera, J. Heise, J. in't Zand, C. Bartolini, A. Guarnieri, A. Masetti, A. Piccioni and E. Palazzi: "Following the detection of GRB 970111 (IAUC 6533), deep frames (25 min in B and 10 min in R) were obtained on Jan. 12.2, Feb. 10.2 and 11.2 UT (only R on the last occasion) with the Calar Alto 2.2-m telescope (+ CAFOS). No fading object was detected, within the entire IPN/SAX error box (IAUC 6578), between the first set of images (taken 19 hr after the burst) and the later data. Any fading was < 0.2 mag for objects with B < 21, R < 20.8 and < 0.5 mag for those down to B = 23, R = 22.6. Taking into account the fact that GRB 970111 has a higher gamma-ray fluence than GRB 970228 does, and unless the flux from x-ray to optical wavelengths was absorbed by intervening matter close to the source, the nondetection of either an x-ray counterpart (by the SAX narrow-field instruments) or a fading optical counterpart in the GRB 970111 field indicates a significant difference in the behavior of GRB 970111 in comparison to that of GRB 970228, for which a presumably-related x-ray/optical counterpart has been reported (IAUC 6576, 6584)."