R. T. Skelton

A BATSE Earth Occultation Catalog of 0.03-1.8 MeV Gamma-Ray Source Spectra and Light Curves for Phases 1-3 (1991-1994)

Using the powerful Earth-occultation technique, long-term, nearly continuous monitoring of the entire low-energy gamma-ray sky is now possible with the advent of BATSE, the Burst and Transient Source Experiment on board the Compton Gamma Ray Observatory (CGRO). In this paper, we present a catalog of 34 moderately strong gamma-ray sources measured by BATSE. It consists of 0.03-1.8 MeV photon spectra averaged over weeks and months, and light curves of the 35-200 keV flux, with 1 day resolution, covering the first three phases of the CGRO mission (1991 May through 1994 October). These results have been obtained using the JPL Enhanced BATSE Occultation Package (EBOP) developed under the CGRO Guest Investigator Program. The EBOP concept and approach are also described in some depth. This paper presents highlights extracted from a large EBOP database which has now been archived at the Compton Observatory Science Support Center (COSSC). This database contains a complete record of ~1200 daily source count rates in 14 energy channels along with the corresponding Poisson and systematic errors for 64 sources, including 30 not described here. An interface to XSPEC is included in the archive allowing conversion from count rates to photon fluxes. This paper therefore serves also as a reference and entree into the archive and provides an index and guide for those investigators using the EBOP system and database for their respective scientific investigations.

BATSE Observations of Cygnus X-1

We report preliminary results of ∼65 days of BATSE Earth‐occultation observations of the low energy gamma‐ray (20 keV–1.8 MeV) emission of Cygnus X‐1 covering the period from 23 April to 27 June 1991. Cygnus X‐1 underwent two transitions in this period. The 45–140 keV flux was first observed between the γ1 and γ2 levels (Ling et al. 1987) from 23 April to 14 May. The source began a transition to γ1 on 14 May and stayed near this level from 19 to 29 May, and then moved toward γ2 and remained at approximately this level from 10 to 27 June. Both γ1 (19–29 May) and γ2 (10–27 June) spectra observed by BATSE can be best characterized by the Comptonized model with temperature of 67 keV and 62 keV, respectively, and optical depth of ∼2. There was no evidence for any broad spectral feature near 1 MeV seen previously by several satellite and balloon experiments. A composite Cygnus X‐1 spectrum observed by BATSE and COMPTEL in Viewing Period 2 (31 May–7 June) suggests that a power‐law with index of ∼3.5 may best characterize the spectrum from 200 keV to several MeV.

Status of the BATSE enhanced earth occulation analysis package for studying point sources

The Compton Gamma‐Ray Observatory’s Burst and Transient Source Experiment (BATSE) has a powerful capability to provide nearly uninterrupted monitoring in the 25 keV—1 Mev range of cosmic point sources using occulation by the Earth. A number of interesting results have been obtained to date using the BATSE Mission Operations (MOPS) Level I analysis system. We have been constructing a more physical model for the background variations which will allow use of more data, enhancing the sensitivity by a factor of several. Our ‘‘enhanced’’ package, written in IDL primarily for a VMS environment, accepts input data in the MOPS format. It fits for the diffuse background (including Earth blockage), prompt cosmic‐ray effects, South Atlantic Anomaly activation, atmospheric cosmic‐ray secondaries, and cosmic sources in 14 Medium Energy Resolution energy bands of the Large Area Detectors. Features include the ability to correlate set and rise of a given source and to handle multiple sources within the fit window. The co...

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.

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.