B.C. Rubin

Observations of 4U 1700-37 with BATSE

The eclipsing binary X-ray source 4U 1700-37 has been continually monitored by the BATSE experiment on the Compton Gamma Ray Observatory since the spring of 1991. Using source measurements at times of Earth occultation, we observe an average (uneclipsed) flux of 0.23 crab in the 20-120 keV band. The flux is highly variable, with occasional flaring behavior on timescales from hundreds of seconds to several hours and intensities as bright as 1 crab. The uneclipsed spectrum is well represented by an optically thin thermal bremsstrahlung model with a temperature of 25 keV independent of source intensity or orbital phase. An upper limit of 4% on the pulse fraction has been obtained for pulse periods between 2 and 700 s. Average orbital light curves from almost 1000 days of occultation measurements have been constructed. These profiles are used to measure: (1) the eclipse semiangle, Theta(sub E) = 28.6 deg +/- 2.1 deg in the 20-120 keV band, and (2) the decrease in orbital period, P(dot)/P = -(3.3 +/- 0.6) x 10(exp -7) 1/ yr. Estimates of system physical parameters are obtained using Monte Carlo simulations to propagate errors in measured and assumed parameters. For the X-ray source mass we find M(sub x) = 2.6(sub -1.4)(sup +2.3) solar mass, and for the mass and radius of the optical companion, M(sub 0) = 30(sub -7)(sup +11) solar mass and R(sub 0) = 18(sub -2)(sup +2) solar radius.

Observation of GX 339-4 hard state outbursts in 1991 and 1992

Two hard state (X-ray low state) outbursts in the GX 339-4 system in 1991 June-October and 1992 September-1993 January were detected and monitored by the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO). The outbursts were remarkably similar in both temporal and spectral aspects. The spectrum in the 20-300 keV band evolves from a hard, Comptonized shape to a steeper (alpha approximately equals -3) power law during each outburst.

Search for rapid X-ray variability from the black hole candidate GRO J1655-40

We have examined 15 days of CGRO/BATSE data, obtained during the first outburst of the black hole candidate source GRO J1655-40, to search for rapid variability of its X-ray flux. We find no evidence for significant variability of GRO J1655-40 during our observations, with a 2 σ upper limit to the fractional rms amplitude in the frequency range 0.03-0.488 Hz of 6.6%. We cannot, on the basis of our observations, determine the source state (low, high, or very high state) of GRO J1655-40.

BATSE images of the galactic center region

The non‐collimated detectors of BATSE (Burst And Transient Source Experiment) on the Compton Gamma Ray Observatory (CGRO) have been used to produce wide field images of celestial hard X‐ray and gamma‐ray sources in the energy range between 20 keV and 500 keV. The occultation process is modeled as a Radon transform and then available Radon inversion software packages are applied to produce images. BATSE can simultaneously image 70% of the sky in as little as a few days and the whole sky in 3‐4 weeks. A field of view of a single image can be chosen in software to be from 5°×5° to 40°×40°. Multiple images with different FOVs centered on different parts of the sky can be produced simultaneously. Selected images obtained from this method for the galactic center region are presented here.

BATSE discovery of a hard X‐ray pulsar:GRO J1008‐57

BATSE detection of a new hard x‐ray pulsar is reported. The source was first observed on 14 Jul 1993, reached a maximum intensity on 25 Jul 1993, then declined smoothly until it fell below the detection threshold on 16 Aug 1993. The BATSE location was sufficiently well determined to permit OSSE, ASCA, & ROSAT observations, leading to an improved source location. The observed period at the solar system barycenter was 93.548±0.002s, 93.5665±0.0005s, and 93.541±0.004 on 15 Jul 1993, 23 Jul 1993, and 10 Aug 1993, respectively. The source is detected between ∼20 keV and 160 keV, with a spectrum fit by an optically thin thermal bremsstrahlung form, having a kT of 25.4±2.1 keV during the rise to maximum intensity, decreasing monotonically during the remainder of the outburst to 17.1±2.5 keV on 5 Aug 1993. The pulse profile in this energy range has a single broad peak, with the maximum occurring later at higher energies. The pulse profile, intensity history, and spectral behavior observed by BATSE during the outb...

Discovery of the hard x‐ray pulsar GRO J1008‐57 by BATSE

BATSE detection of a new hard x‐ray pulsar is reported. The source was first observed on 14 Jul 1993, reached a maximum intensity on 25 Jul 1993, then declined smoothly until it became undetectable on 16 Aug 1993. The BATSE location was adequate to permit OSSE, ASCA, and ROSAT observations, leading to an improved source location. The observed period at the solar system barycenter was 93.548±0.002s, 93.5665±0.0005s, and 93.541±0.004 on 15 Jul 1993, 23 Jul 1993, and 10 Aug 1993, respectively. The source is detected between ∼20 keV and 160 keV, with a spectrum fit by an optically thin thermal bremsstrahlung form, having a kT of 25.4±2.1 keV during the rise to maximum intensity, decreasing monotonically during the remainder of the outburst to 17.1±2.5 keV on 5 Aug 1993. The pulse profile in this energy range has a single broad peak, with the maximum occurring later at higher energies. The pulse profile, intensity history, and spectral behavior observed by BATSE are reported.

Rapid Hard X-Ray Variability in GRO J0422+32

The soft X-ray transient (SXT) GRO J0422+32 (Nova Persei 1992) was detected with the Burst And Transient Source Experiment (BATSE) on board the CGRO on 1992 August 5 (Paciesas et al. 1992) (Truncated Julian Day [TJD] 8839). The source intensity of GRO J0422+32 increased rapidly, reaching a flux of ∼ 3 Crab (40-230 keV) within days after its first detection (Harmon et al. 1992). Hereafter, the X-ray intensity of the source decreased exponentially with a decay time of ∼ 43 days (Vikhlinin et al. 1995). A secondary maximum of the X-ray intensity was reached at TJD 8978, 139 days after the first detection of the source. The daily averaged flux history of GRO J0422+32 in the 40-150 keV energy band is presented in Figure 1.

Rapid X-ray variability in GRO J0422+32 (Nova Per 1992)

We present the results of an analysis of the time variability of the soft X-ray transient GRO J0422+32 (Nova Per 1992) observed with BATSE. Our analysis covers the entire ∼200 day outburst, beginning with the first detection of the source on 1992 August 5. We obtained power density spectra (PDS) in the 20–100 keV energy band covering the frequency interval 0.002–0.488 Hz. The PDSs of GRO J0422+32 show a peak of quasi-periodic oscillations (QPOs) with a centroid frequency of ∼200 mHz, during the first 120 days of the outburst.

A search for untriggered low‐energy events in the BATSE data base

We have started a search in the BATSE data base for low‐energy (∼25–55 keV) events, which cannot (under the current configuration) cause an onboard trigger. We present here some preliminary results, in particular the detection of several very weak events temporally clustered in a period when the experiment was triggered by emission attributed to the Soft Gamma‐ray Repeater SGR 1900+14 (Mazets et al.). We suggest that these events most likely originate from the same source.