M. Delaney 1, L. Hanlon 1, B. McBreen 1, F. Quilligan 1, L. Metcalfe 2, R. Laureijs 2, B. Altieri 2, L. Piro 3, M. Feroci 3, E. Costa 3, F. Frontera 3, E. Palazzi 4, J. Heise 5, J. van Paradijs 6, P. Groot 6, T. Galama 6, A. Castro-Tirado 7, J. Gorosabel 7, & C. Kouveliotou 8
1 University College, Dublin, Ireland
2 ISO SOC, VILSPA, Spain
3 Univ. di Ferrara
4 CNR Bologna
5 SRON Utrecht
6 UVA
7 LAEFF
8 USRA/MSFC
Gamma-ray bursts (GRBs) were first discovered more than 25 years ago and since then more than 2000 of these transient, energetic events have been detected. Until the recent ground-breaking discoveries of fading multiwavelength afterglows to a number of well-localised GRBs, no convincing flaring or fading counterparts had been detected, despite 20 years of intensive searches covering most of the electromagnetic spectrum. Redshifted spectroscopic lines in the optical afterglows of a handful of GRBs have now confirmed the cosmological nature of this phenomenon. The appearance of the multiwavelength afterglows is consistent with synchrotron radiation from electrons accelerated by a relativistic shock wave. However, the nature of the underlying emission mechanism, which must provide about 1052 ergs of energy to power the GRB and the afterglows, remains unknown.
The error circles of GRB 970402, GRB 970228, GRB 970508 and GRB 980329, obtained by the X-ray instruments aboard the Italian-Dutch `BeppoSAX' satellite, were observed with ISOCAM and ISOPHOT on timescales ranging from days to months after the GRB events. These Target of Opportunity observations, awarded under Discretionary and Director's time, represent the first prompt gamma-ray burst counterpart searches at far-infrared wavelengths from 11.5 microns to 170 microns and were made possible by the rapid and precise localisations achieved by the BeppoSAX instruments. Previous studies at far-infrared wavelengths, using IRAS data, were conducted many years after the GRB event. Details of the observations and analysis methods will be presented. Particular emphasis will be placed on results obtained for GRB 970508, the first GRB for which a direct distance measurement has been obtained.