Adv. Space Res. Vol. 13, No. 12, pp. (12)715—(12)718, 1993 0273—1177i93 $6.00 + 0.00 Printed in Great Britain. Mi rights reserved. Copyright © 1993 COSPAR COMPTEL OBSERVATIONS OF GAMMA- RAY BURSTS A. Connors,*** W. Collmar,* L. Hanlon,t W. Hermsen,** R. M. Kippen,*** L. Kuiper,** M. McConnell,*** J. Ryan,’~” V. Schönfe1der,~ M. Varendorff,* 0. R. Wihiainst and C. Winklert * Max-Planck Institut für Extraterrestrische Physik~ D-8046 Garching, Germany ** SRON-Leiden, P. O.B. 9504, 2300 RA Leiden, The Netherlands “~‘~ ISEOS, University of New Hampshire, Durham, NH 03824, USA t Astrophysics Division, ESA/ESTEC, 22(X) AG NoordwUk, The Netherlands ABSTRACT The COMPTEL experiment on GRO images 0.7 — 30 MeV celestial gamma—radiation that falls within its 1 steradian field of view. During the first fifteen months in orbit, preliminary localizations from BATSE triggers indicated that about 1 in 6 cosmic events could have fallen within COMPTEL’s field of view. We summarize work on the brightest of these gamma—ray bursts and present new position constraints for GRB 911118 and GRB 920622. INTRODUCTION The imaging Compton Telescope, COMPTEL, is one of four instruments on board the Compton Gamma Ray Observatory (CGRO). Since shortly after its launch in April 1991, COMPTEL has been accumulat- ing measurements of the positions, time profiles, and spectra of gamma—ray bursts in the MeV range. In COMPTEL’s imaging telescope or ‘double scatter’ mode (0.7 — 30 MeV), it measures both positions and spectra of cosmic 7-ray bursts that fall within its 1 steradian field of view. In addition, in burst or ‘single detector’ mode COMPTEL accumulates independent 0.1 — 1.1 MeV and 1 — 10 MeV spectra in two of its lower Nat detectors /1/. Upon receipt of a BATSE burst or solar flare trigger, ‘burst’ and ‘tail’ spec- tra are read out at higher time resolutions (typically 0.1 — 0.5 and 6 — 12 seconds, respectively) before returning to background mode (typically 100 s). Initial spectral analysis of data from the first five bright 7-ray bursts detected in the field of view (GRB 910503, S(> 1MeV) -~ 1 x lO 4erg-cm2 GRB 910425, S(> 1MeV) .— 4 x lO5erg-cm2 GRB 910601, S(> 1MeV) .— 2x lO5erg-cm2 and GRB 910814, S(> 1MeV) ~- 1 x lO4erg-cm2), showed the 0.1 — 10 MeV spectra to be roughly characterized by power— laws with indices in the range —2.2 to —2.75 /2,3/. Positions for these four, plus GRB 910627, have al- ready been published elsewhere /4,5/. In this paper we add to this sample positions for GRB 911118 and GRB 920622, two more bright bursts observed in the field of view during the first fifteen months in orbit. An overview of spectral results during the first year will be given in /6/. DATA AND ANALYSIS Roughly half of the BATSE burst trigger messages contain preliminary positions, with typical error radii of 10°~ 15°. Of these, about 16% had been given BATSE positions within 450 of COMPTEL’s telescope zenith, and are potential candidates for imaging. For each burst, one produces a light—curve using the BATSE trigger time to select an appropriate time window. Only telescope events which satisfy the opti- mum event selection criteria are used. In Figures 1 and 2 we display these time profiles for GRB 911118 and GRB 920622. COMPTEL uses several imaging methods. One is a maximum entropy technique, which directly estimates the count—rate per angular bin on the sky. This general method is detailed by /7/; and its application to imaging 7-ray bursts by /8/. In this paper we present quantitative constraints on source positions, using a maximum—likelihood fit to a model of a point source plus a flat background, convolved with the COMP- TEL instrument response /9,10/. In COMPTEL’s imaging or ‘double scatter’ mode, a photon which Compton—scatters in one of the seven upper Dl detectors, is then detected in one of the lower fourteen high—Z D2. detectors /1/. In the simplest case of a single Compton scatter in Dl and complete photo—absorption in D2, the possible 7-ray source positions lie on a circle of radius ~ around the direction of the scattered photon, with 1 , (1) C2 JASR 13~12-UU (12)715