COMPTEL observations of gamma-ray bursts

Adv. Space Res. Vol. 13, No. 12, pp. (12)715—(12)718, 1993 Printed in Great Britain. Mi rights reserved.

0273—1177i93 $6.00 + 0.00 Copyright © 1993 COSPAR

COMPTEL OBSERVATIONS OF GAMMARAY 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 accumulating 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’ spectra 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 4erg-cm2 GRB 910425, 7-ray bursts detected in the field of view (GRB 910503, S(> 1MeV) -~ 1 x lO S(> 1MeV) .— 4 x lO5erg-cm2 GRB 910601, S(> 1MeV) .— 2 x 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 already 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 optimum 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 COMPTEL 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

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Figure 1. GRB 911118: On the left is the time profile of gamma—rays detected in COMPTEL telescope mode. (BATSE trigger was at 68260 seconds). To the right are 1, 2, and 3o contours of the maximum likelihood ratio, with ~2Ooo, ö 2ooo grid superposed. where e1 and C2 are the energy deposits measured in the upper (Dl) and lower (D2) detectors, respectively, in units of the electron rest—mass. The intersection of these circles would produce an image. In practice the full instrument response is substantially complicated by multiple scatters and partial energy absorption /11/. To simplify, one first transforms to the spatial coordinates x~ ~‘, ~ and ETOT; where ~ is defined in Eq. 1, ETOT is the total energy deposited in both Dl and D2, and (x, ~) represent convenient telescope coordinates for the scattered photon direction. For a single energy interval, COMPTEL’s complex instrument response is factored into an instrument geometry and exposure, which incorprates all information on the absolute telescope position; and a point spread function, which depends only on the relative coordinates x~ ~b,~ (see /11/ and references therein). This is then integrated over an assumed input energy spectrum. For each gamma—ray burst, a point source is convolved with this instrument response, and added to a simple background. This model is compared to the data—counts in each x~ ~‘, ~—bin,using the standard maximum—likelihood statistic appropriate for Poisson counts per bin /9/. The source flux and background level (consistent with zero) are allowed to vary, as contours of constant probability are mapped out in (x, ~) space, at the equivalent of 1, 2, and 3o significance for the case of two parameters /12,13/. RESULTS We display these maximum—likelihood ratio contours, which incorporate statistical errors only, in Figures 1 and 2, for GRB 911118 and GRB 920622. The varying widths of the contours reflect the number of telescope events available for imaging. For COMPTEL locations of strong 7-ray bursts, since the background is negligible, any systematic uncertainty would come from uncertainties in the instrument response, which for imaging is primarily represented by the point-spread function. Preliminary investigations from imaging high signal—to—noise sources such as solar flares and the Crab indicate any systematic uncertainties to be no larger than —.0.5—1°. We expect additional work will allow us to constrain these systematic uncertainties further. Table 1. Summary of COMPTEL burst positions GRB ID (date) 911118 920622

Telescope Zenith 36.0° 45.5°

Equatorial ~2OOO ± 0 10h58m ± 11.” 10”42”’ ± 97tTh

Coordinates 62000 ± u —22.7° ± 1.6° 47.9° ± 1.5°

We summarize our results in Table 1. To the 1o statistical errors, we have added tainties in quadrature.

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Galactic Longitude Figure 2. GRB 920622: On the left is the time profile of gamma—rays detected in COMPTEL telescope mode. (BATSE trigger was at 25506 seconds). To the right are 1, 2, and 3o contours of the maximum likelihood ratio, with ~2OflO,62000 grid superposed. SUMMARY This paper reports positions of two more gamma—ray bursts observed during the first fifteen months within the COMPTEL field of view, bringing the total number located so far to seven. COMPTEL continues to accumulate measurements of light—curves, positions, and energy spectra of cosmic gamma—ray bursts in its 0.7 — 30 MeV (telescope mode) and 0.1 — 10 MeV (burst mode) energy windows. REFERENCES 1. Schönfelder, V., Aarts, H., Bennett, K., de Boer, H., Clear, J., Collmar, W., Connors, A., Dordrecht, A.v., Diehi, R., den Herder, J.W., Hermsen, W., Kippen, R.M., Kuiper, L., Lichti, G., Lockwood, J., Macri, J., McConnell, M., Morris, D., Much, R., Ryan, J., Snelling, M., Stacy, J.G., Steinle, H., Strong, A., Swanenburg, B.N., Taylor, B., de Vries, C., Winkler, C. 1992, Instrument Description and Performance of the Imaging Gamma—Ray Telescope COMPTEL on Board NASA’s Compton Gamma—Ray Observatory, Ap. J. Suppi., in press (1992). 2. Winkler, C., Bennett, K., Bloemen, H., Collmar, W., Connors, A., Diehl, R., Dordrecht, A.v., den Herder, J.W., Hermsen, W., Kippen, R.M., Kuiper, L., Lichti, G., Lockwood, J., McConnell, M., Morris, D., Ryan, J., Schönfelder, V., Stacy, J.G., Steinle, H., Strong, A., Swanenburg, B., Taylor, B.G., Varendorif, M., de Vries, C. 1992, A&A 225, L9 (1992). 3. Collmar, W., Bennett, K., Bloemen, H., de Boer, H., Busetta, M., Connors, A., Diehl, R., Greiner, J., Hanlon, L., den Herder, J.W., Hermsen, W., Kippen M., Kuiper, L., Lichti, G., Lockwood, J., Macri, J., McConnell, M., Morris, D., Much, R., Ryan, J., Schönfelder, V., Stacy, J.G., Steinle, H., Strong, A.W., Swanenburg, B., Varendorif, M., de Vries, C., Webber, W., Williams, O-R., and C. Winkler, AP3A, in press (1992). 4. Winkler, C., Bennett, K., Bloemen, H., de Boer, H., Busetta, M., Collmar, W., Connors, A., Diehl, R., Hanlon, H., den Herder, W., Hermsen, W., Kippen, R.M., Kuiper, L., Lichti, G., Lockwood, J., McConnell, M., Morris, D., Ryan, J., Schönfelder, V., Stacy, J.G., Steinle, H., Strong, A.W., Swanenburg, B.N., Taylor, B.G., Varendorif, M., de Vries, C., and Williams, O.R., in: Conference Proceedings, Huntsville Burst Workshop, ed. W.S. Paciesas and G.J. Fishman, AlP, NY 1992, p. 22. 5. Connors, A., Aarts, H.J.M., Bennett, K., Bloemen, H., de Boer, H., Busetta, M., Collmar, W., Diehl, R., van Dijk, R., Hanlon, L., Kuiper, L., Klumper, A., Lichti, G., Lockwood, J., Macri, J., McConnell, M., Morris, M., Much, R., Schönfelder, V., Simpson, G., Stacy, J.G., Steinle, H., Strong, A.W., Swanenburg, B., Taylor, B., Varendorif, M., de Vries, C., Webber, W., Williams, O.R., and Winkler, C., A&A, in press (1992). 6. Hanlon, L. ci al. in preparation

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