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 ([α_2000_, δ_2000_]=[29.5deg, 3.8deg], with a 3σ 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 γ-γ interaction (via e+e- - pair creation), or GRB 940217 is closer than 1Gpc.
Astronomy and Astrophysics
- Pub Date:
- October 1995
- GAMMA-RAY BURSTS;
- RADIATION MECHANISMS: NON-THERMAL