Title:              COMPTEL observations of the strong gamma-ray burst
                    GRB 940217.
Authors:            WINKLER, C.; KIPPEN, R.M.; BENNETT, K.; COLLMAR, W.;
                    HANLON, L.O.; HERMSEN, W.; O'FLAHERTY, K.S.; RYAN,
                    J.; SCHOENFELDER, V.; STEINLE, H.; WILLIAMS, O.R.
Journal:            Astronomy and Astrophysics, v.302, p.765
Publication Date:   10/1995
Origin:             A&A via CDS
A&A Keywords:       GAMMA-RAY BURSTS, RADIATION MECHANISMS: NON-THERMAL
Abstract Copyright: (c) 1995: Astronomy & Astrophysics
Bibliographic Code: 1995A&A...302..765W

Abstract

The imaging MeV gamma-ray telescope COMPTEL on board the Compton Gamma-Ray
Observatory has observed a very strong (S[>0.3MeV]=2.03x10^-4^erg/cm2),
complex, and long lasting (162s) gamma-ray burst on February 17, 1994 (GRB
940217). The burst consists of several peaks. Temporal fluctuations occur on
timescales as short as 100 ms. Hard-to-soft spectral evolution has been
observed during the burst emission and also within individual peaks. The
photon spectra obtained within the peaks can be modelled by single power law
spectra and by broken power laws with break energies at around 1MeV. The
best-fit power law slopes vary between 1.1 and 3.5 throughout the event. The
spectra are consistent within COMPTEL's different observing modes and also
agree very well with contemporaneous EGRET and BATSE measurements. No
significant line emission is detected in any of the recorded spectra.
Precise location ([{alpha}_2000_, {delta}_2000_]=[29.5deg, 3.8deg], with a
3{sigma} error radius of about 1deg) has been obtained through direct
COMPTEL MeV imaging in combination with timing analysis by the
Interplanetary Network. Four hours after the event, the position derived by
COMPTEL was distributed to a network of multi-wavelength observers. No
detection of a fading or quiescent counterpart has so far been reported at
any wavelength. During a 20 minute time interval starting ~90 minutes after
the burst event, the EGRET instrument on board the Compton Gamma-Ray
Observatory reported "post-burst" emission at high energies (up to GeV).
COMPTEL does not detect any significant "post-burst" emission at low
energies (0.3-30MeV), and our upper limits are marginally consistent with
the EGRET detections. Using high energy spectral and temporal information,
distance limits to GRB 940217 between 145 AU and 11.7 kpc have been derived
assuming homogenous and isotropic emission. If GRB 940217 is at cosmological
distances (e.g. at 1Gpc) the lower limit on the relativistic bulk Lorentz
beam factor is >18. This implies that our observed breaks at around 1MeV are
either too low in energy to be produced by {gamma}-{gamma} interaction (via
e+e- - pair creation), or GRB 940217 is closer than 1Gpc.

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