GRB 990123 was well-observed from MeV gamma-rays through soft X-rays by all instruments on CGRO and by BeppoSAX. Spectacularly, it was also the first to be detected in the optical during the main burst itself (Akerlof and McKay et al., GCN 205), becoming as bright as 9th magnitude in the middle of the event. It was suficiently bright above one MeV that CGRO-COMPTEL imaged the event and broadcast its position world-wide within 10 minutes of burst onset, despite its being nearly 60 degrees from the telescope's pointing direction. Here we report on its MeV properties as seen by CGRO-COMPTEL. Although much has been made of its gamma-ray/X-ray intensity (Djorgovski et al. GCN 216) in fact its high energy properties do not appear to be all that unusual. Its gamma-ray fluence (1E-4+/-.5E-4 ergs/cm(2) , 0.7-30 MeV) was clearly within the range of other bursts observed over the past 7 years by CGRO-COMPTEL (2E-4 -- 3E-6 ergs/cm(2) , 0.7-30 MeV). The burst was softer than average (consistent with OSSE findings; Matz et al GCN 231), with a photon power-law index of -3.+/-0.5 (compared to our sample average of 2.4 +/- 0.6). Again this is within the range of all bursts in the COMPTEL burst catalog. It appears to show the usual general trend of hard-to-soft spectral evolution: above 3 MeV the first peak is clearly significant, with a weak second peak roughly 10 s later; in 1-3 MeV two peaks are strongly visible with a total duration of about 28 s; in the 0.1-1 MeV one finds, in addition, a softer 44 second tail. A slightly unusual feature is the 18 s delay between the start of the softer gamma-ray emission on which BATSE triggered; and the rising edge of the main burst as seen by COMPTEL (and on which BeppoSAX triggered; Piro et al. GCN 199). In sum, our data are consistent with the hypothesis that it was this delay (plus recently improved instrumentation), rather than unprecedented properties of this gamma-ray burst, that allowed this first detection of a startlingly bright optical flash concurrent with a burst. We will present MeV skymaps, light-curves, and spectra of this benchmark burst in more detail.
AAS/High Energy Astrophysics Division #4
- Pub Date:
- April 1999