C. A. Young

Observations of GRB 990123 by the Compton gamma ray observatory

GRB 990123 was the first burst from which simultaneous optical, X-ray, and gamma-ray emission was detected; its afterglow has been followed by an extensive set of radio, optical, and X-ray observations. We have studied the gamma-ray burst itself as observed by the Compton Gamma Ray Observatory detectors. We find that gamma-ray fluxes are not correlated with the simultaneous optical observations and that the gamma-ray spectra cannot be extrapolated simply to the optical fluxes. The burst is well fitted by the standard four-parameter GRB function, with the exception that excess emission compared with this function is observed below ~15 keV during some time intervals. The burst is characterized by the typical hard-to-soft and hardness-intensity correlation spectral evolution patterns. The energy of the peak of the νfν spectrum, Ep, reaches an unusually high value during the first intensity spike, 1470 ± 110 keV, and then falls to ~300 keV during the tail of the burst. The high-energy spectrum above ~1 MeV is consistent with a power law with a photon index of about -3. By fluence, GRB 990123 is brighter than all but 0.4% of the GRBs observed with BATSE, clearly placing it on the - power-law portion of the intensity distribution. However, the redshift measured for the afterglow is inconsistent with the Euclidean interpretation of the - power law. Using the redshift value of ≥1.61 and assuming isotropic emission, the gamma-ray energy exceeds 1054 ergs.

A search for micro cosmic gamma-ray bursts in BATSE one second continuous data

Based on a previous successful search for untriggered solar microflares we have conducted a search for untriggered cosmic gamma-ray bursts (GRBs). The large number of untriggered hard x-ray solar flares suggests that there might exist a class of GRBs with temporal and spectral properties similar to that of hard x-ray solar flares, possibly with similar physics as well. We have scanned a subset of BATSE’s continuous 1 second data from four channels independently. This search can identify GRBs with characteristic rise times longer than 1 second, while the electronic identification system only triggers on rise times of 64-1024 ms. We can also search for softer events, not necessarily SGRs, by including the lowest energy channel. A search of continuous unbiased data can lower the search sensitivity to a minimum determined by the intrinsic and orbit-modulated background. We present the results of our progress.

COMPTEL gamma-ray observations of the C4 solar flare on 20 January 2000

The “Pre-SMM” (Vestrand and Miller 1998) picture of gamma-ray line (GRL) flares was that they are relatively rare events. This picture was quickly put in question with the launch of the Solar Maximum Mission (SMM). Over 100 GRL flares were seen with sizes ranging from very large GOES class events (X12) down to moderately small events (M2). It was argued by some (Bai 1986) that this was still consistent with the idea that GRL events are rare. Others, however, argued the opposite (Vestrand 1988; Cliver, Crosby and Dennis 1994), stating that the lower end of this distribution was just a function of SMM’s sensitivity. They stated that the launch of the Compton Gamma-ray Observatory (CGRO) would in fact continue this distribution to show even smaller GRL flares. In response to a BACODINE cosmic gamma-ray burst alert, COMPtonTELescope on the CGRO recorded gamma rays above 1 MeV from the C4 flare at 0221 UT 20 January 2000. This event, though at the limits of COMPTEL’s sensitivity, clearly shows a nuclear line e...

X- and gamma -ray observations of the 15 November 1991 solar flare

This work expands the current understanding of the 15 November 1991 Solar Flare. The flare was a well observed event in radio to gamma-rays and is the first flare to be extensively studied with the benefit of detailed soft and hard X-ray images. In this work, we add data from all four instruments on the Compton Gamma Ray Observatory. Using these data we determined that the accelerated electron spectrum above 170 keV is best fit with a power law with a spectral index of −4.6, while the accelerated proton spectrum above 0.6 MeV is fit with a power law of spectral index −4.5. From this we computed lower limits for the energy content of these particles of ∼1023u2009ergs (electrons) and ∼1027u2009ergs (ions above 0.6 MeV). These particles do not have enough energy to produce the white-light emission observed from this event. We computed a time constant of 26−15+20u2009s for the 2.223 MeV neutron capture line, which is consistent at the 2σ level with the lowest values of ∼70 s found for other flares. The mechanism for this...

High-energy measurements of the 1991 november 15 solar flare

Abstract We report λ-ray observations of the 1991 November 15 solar flare. Although the event was not as well measured as some with the Compton Gamma Ray Observatory (CGRO) because the axis of the Compton spacecraft was far from the solar direction, the data are comprehensive and span a wide range of energies with good sensitivity and resolution. Of particular interest are the observations that the energetic proton spectrum was harder during the impulsive phase, the transport of protons in the corona was diffusive rather than adiabatic, the abundance of 3 He may be higher than is normally believed and there is not enough energy in the energetic electrons to power the white light emission, although proton heating is still possible.

MeV measurements of gamma-ray bursts by CGRO-COMPTEL

Since the launch of the Compton Gamma-Ray Observatory in April 1991, the imaging COMPTEL telescope has accumulated positions and 0.75–30 MeV spectra of more than thirty gamma-ray bursts within its ∼π sr field of view. In an ongoing collaboration with BACODINE/GCN, COMPTEL positions are relayed to a global network of multiwavelength observers in near real time (∼10 minutes). Here we summarize the MeV properties, and present spatial, spectral, and temporal data for the latest of these events, GRB 970807. In concurrence with earlier SMM and current BATSE, OSSE, and EGRET measurements, COMPTEL data add to the accumulating evidence that GRB spectra do seem to have a characteristic shape: a peak (in E2F(E)) around several hundred keV; and a power law above (spectral index 1.5–3.5) extending beyond the COMPTEL energy range.