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Astron. Astrophys. 339, 719-728 (1998)

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An optical study of the GRB 970111 field beginning 19 hours after the gamma-ray burst * ** ***

J. Gorosabel 1, A.J. Castro-Tirado 1, 2, C. Wolf 3, J. Heidt 4, T. Seitz 4, 5, E. Thommes 6, C. Bartolini 7, A. Guarnieri 7, N. Masetti 7, 8, A. Piccioni 7, S. Larsen 9, E. Costa 10, M. Feroci 10, F. Frontera 11, E. Palazzi 8 and N. Lund 12

1 Laboratorio de Astrofísica Espacial y Física Fundamental (LAEFF-INTA), P.O. Box 50727, E-28080 Madrid, Spain
2 Instituto de Astrofísica de Andalucía (IAA-CSIC), P.O. Box 03004, E-18080 Granada, Spain
3 Max-Planck-Institut für Astronomie, Heidelberg, Germany
4 Landessternwarte Heidelberg, Königstuhl, D-69117 Heidelberg, Germany
5 Lehrstuhl für Ergonomie, Technische Universität München, Boltzmannstrasse 15, D-85747 Garching, Germany
6 Royal Observatory, Blackford Hill, Edinburgh Hill, Edinburgh EH9 3HJ, UK
7 Dipartimento di Astronomia, Università di Bologna, Via Zamboni 33, I-40126 Bologna, Italy
8 Istituto Tecnologie e Studio Radiazioni Extraterrestri, CNR, I-40126 Bologna, Italy
9 Niels Bohr Institute for Astronomy, Physics and Geophysics, Copenhagen University Astronomical Observatory, Julian Maries Vej 30, DK-2100 Copenhagen, Denmark
10 Istituto di Astrofisica Spaziale, CNR, Roma, Italy
11 Dipartamento di Fisica, Università di Ferrara, Ferrara, Italy
12 Danish Space Research Institute, DK-2100 Copenhagen, Denmark

Received 24 July 1998 / Accepted 1 September 1998

Abstract

We present the results of the monitoring of the GRB 970111 field that started 19 hours after the event. This observation represents the fastest ground-based follow-up performed for GRB 970111 in all wavelengths. As soon as the detection of the possible GRB 970111 X-ray afterglow was reported by Feroci et al. (1998) we reanalyzed the optical data collected for the GRB 970111 field. Although we detect small magnitude variability in some objects, no convincing optical counterpart is found inside the WFC error box. Any change in brightness 19 hours after the GRB is less than 0.2 mag for objects with B [FORMULA] 21 and R [FORMULA] 20.8.

The bluest object found in the field is coincident with 1SAX J1528.8+1937. Spectroscopic observations revealed that this object is a Seyfert-1 galaxy with redshift [FORMULA], which we propose as the optical counterpart of the X-ray source.

Further observations allowed to perform multicolour photometry for objects in the GRB 970111 error box. The colour-colour diagrams do not show any object with unusual colours. We applied a photometric classification method to the objects inside the GRB error box, that can distinguish stars from galaxies and estimate redshifts. We were able to estimate photometric redshifts in the range [FORMULA] for several galaxies in this field and we did not find any conspicuous unusual object.

We note that GRB 970111 and GRB 980329 could belong to the same class of GRBs, which may be related to nearby sources ([FORMULA]) in which high intrinsic absorption leads to faint optical afterglows.

Key words: gamma rays: bursts – galaxies: Seyfert

* Based on observations collected at the German-Spanish Astronomical Center, Calar Alto, operated by the Max-Planck-Institut für Astronomie, Heidelberg, jointly with the Spanish National Commission for Astronomy.
** Based on observations carried out at the Danish 1.54-m Telescope on the European Southern Observatory, La Silla, Chile
*** Based on observations at the Osservatorio Astronomico di Loiano, Italy.

Send offprint requests to: J. Gorosabel, (jgu@laeff.esa.es)

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© European Southern Observatory (ESO) 1998

Online publication: October 22, 1998
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