Brad C. Rubin

A search for untriggered gamma‐ray bursts in the BATSE data

In the BATSE data there exist cosmic gamma‐ray bursts which do not meet the on‐board trigger criteria. We report here the results of an off‐line, expost‐facto; search for bursts which would trigger on 1.024 s or 4.096 s timescale integrations. These bursts did not set the on‐board trigger.

BATSE observations of 4U1538‐52: A 530 second pulsar

The BATSE experiment on CGRO, a nearly continuous all sky monitor, is capable of detecting and monitoring pulsars in the hard x‐ray/gamma‐ray energy range. Using data processed with a global background subtraction method, we are able to construct pulsed intensity and pulse period histories for the accreting binary x‐ray pulsar 4U1538‐52 in the 20–50 keV energy band. This pulsar, which has a pulse period of approximately 530 seconds, is believed to be a wind accretor with a high mass companion. It was observed by previous experiments to be in secular spin down during the 1976–1988 period. Here we report observing, for the first time, a secular spin up trend. We are able to use this data to improve the upper limit on orbital period changes.

Long‐term temporal and spectral variation of Cygnus X‐1 observed by BATSE

Preliminary BATSE results for Cygnus X‐1, analyzed by Earth occultation with the standard Mission Operations System (MOPS), show dramatic temporal varations of the broad‐band (20–2000 keV) emission with time scale of the order of days. We report here preliminary results of a study of the temporal and spectral variability of the source using the newly developed ‘‘Enhanced Earth Occultation Analysis’’ developed at JPL. From 25 October to 27 November 1991, variations in spectral form between a Comptonized and power‐law shape were noted. The average photon flux in the 45–140 keV band is near the γ2 level.

Modeling the gamma‐ray background on BATSE

The gamma‐ray background on the BATSE experiment is a complex, time‐varying combination of several factors. These include diffuse galactic and cosmic background; activation induced by particle fluxes in the SAA; atmospheric gamma‐rays; discrete source contributions; and other factors. We have developed techniques for fitting the Continuous and Discriminator BATSE data with multi‐component models. Some of these components are intended to directly represent factors comprising the background, while others are more empirical in nature. The model is used to ‘‘detrend’’ the data and can be used for on‐ground folding in pulsar analysis and searches for long period pulsations, searches for weak, long duration gamma‐ray bursts and transients, background subtraction from known gamma‐ray bursts and other transients, and construction of the burst sky exposure map. Components of these models can be used to measure the diffuse background and obtain refined measurements of discrete sources.

BATSE observations of the massive X‐ray binary 4U1700−37/HD153919

The Earth occultation technique has been used to monitor the massive x‐ray binary (MXRB) 4U1700−37/HD153919 with BATSE. This source has been important in the study of stellar winds from O‐type stars and of radiation and flaring mechanisms of MXRB. We have detected several large flaring episodes, during which the 20 to 50 keV flux from this binary exceeds the flux from the Crab Nebula in the same energy range.

Gamma-ray monitoring of AGN and galactic black hole candidates by the Gamma-Ray Observatory

The Burst and Transient Spectroscopy Experiment (BATSE) on the Compton Gamma‐Ray Observatory has a powerful capability to provide nearly uninterrupted monitoring in the 25 keV–10 MeV range of both AGN and Galactic black hole candidates such as Cygnus X‐1, using the occultation of cosmic sources by the Earth. Progress in background modeling indicates that the data accept region, or fit window τ, around the occultation step can be substantially increased over that conservatively assumed in earlier estimates of BATSE’s Earth occultation sensitivity. We show samples of large‐τ fits to background and source edges. As a result we expect to be able to perform long‐term monitoring of Cygnus X‐1 and many of the brighter AGN for the duration of the CGRO mission.