D.J. Crary

Hard X-Ray Variability of the Black Hole Candidate GRO J0422+32 during Its 1992 Outburst

We have studied the hard X-ray variability of the soft X-ray transient GRO J0422+32 with BATSE in the 20-100 keV energy band. Our analysis covers 180 days following the first X-ray detection of the source on 1992 August 5, fully covering its primary and secondary X-ray outbursts. We compute power density spectra (PDSs) in the 20-50, 50-100, and 20-100 keV energy bands, in the frequency interval 0.002-0.488 Hz. The PDSs of GRO J0422+32 are approximately flat up to a break frequency, and decay as a power law above it, with index ~1. During the first 70 days of the X-ray outburst, the PDSs of GRO J0422+32 show a significant quasi-periodic oscillation (QPO) peak near ~0.2 Hz, superposed on the power-law tail. The break frequency of the PDSs obtained during the primary X-ray outburst of GRO J0422+32 occurs at 0.041±0.006 Hz; during the secondary outburst, the break is at 0.081±0.015 Hz. The power density at the break ranged between 44% and 89% Hz-1/2 (20-100 keV). The canonical anticorrelation between the break frequency and the power density at the break, observed in Cyg X-1 and other BHCs in the low state, is not observed in the PDSs of GRO J0422+32. We compare our results with those of similar variability studies of Cyg X-1. The relation between the spectral slope and the amplitude of the X-ray variations of GRO J0422+32 is similar to that of Cyg X-1; however, the relation between the hard X-ray flux and the amplitude of its variation is opposite to what has been found in Cyg X-1. Phase lags between the X-ray flux variations of GRO J0422+32 at high and low photon energies could only be derived during the first 30 days of its outburst. During this period, the variations in the 50-100 keV band lag those in the 20-50 keV energy band by an approximately constant phase difference of 0.039(3) rad in the frequency interval 0.02-0.20 Hz. The time lags of GRO J0422+32 during the first 30 days of the outburst decrease with frequency as a power law with index 0.9 for ν>0.01 Hz.

Timescale invariance of rapid-x-ray variability of the black hole candidate GRO J1719-24

We present the results of an analysis of the time variability of the soft X-ray transient GRO J1719-24 (Nova Oph 1993), as observed with BATSE. Our analysis covers the entire ~80 day outburst, beginning with the first detection of this black hole candidate on 1993 September 25. We obtained power density spectra (PDSs) of the data in the 20-100 keV energy band, covering the frequency interval 0.002-0.488 Hz. The PDSs show a significant quasi-periodic oscillation peak, the centroid frequency of which increased from ~0.04 Hz at the onset of the outburst to ~0.3 Hz at the end. Additional noise is present in the PDSs, which we describe in terms of two components. We find that the evolution of the PDSs can be described as a gradual stretching by a factor of ~7.5 in frequency of the power spectrum, accompanied by a decrease of the power level by the same factor, such that the integrated power in a scaled frequency interval remains constant.

Correlation between BATSE Hard X-Ray Spectral and Timing Properties of Cygnus X-1

We have analyzed approximately 1100 days of Cygnus X-1 hard X-ray data obtained with BATSE to study its rapid variability. We find for the first time correlations between the slope of the spectrum and the hard X-ray intensity, and between the spectral slope and the amplitude of the rapid variations of the hard X-ray flux. We compare our results with expectations from current theories of accretion onto black holes.

BATSE Observations and Orbit Determination of the Be/X-Ray Transient EXO 2030+375

The Be/X-ray binary transient pulsar EXO 2030+375 (Ps≈42 s) has been observed with the large-area detectors (LADs) of the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO). Beginning in 1991 May, 22 outbursts were observed over 4 years. Thirteen outbursts between 1992 February and 1993 August occurred consecutively at intervals of ≈46 days, close to the orbital period determined by Parmar and colleagues using EXOSAT data. The pulse profiles from the BATSE data are double peaked and show no significant energy or luminosity dependence, unlike the EXOSAT observations of 1985 May-August. An exponential model was used to fit the observed hard X-ray energy spectra from the 13 consecutive outbursts. When EXOSAT discovered this pulsar during a giant outburst in 1985 May, the X-ray luminosity peaked at LX=1.0×1038 ergs s-1 (1-20 keV), assuming a 5 kpc distance to the source. The BATSE outbursts are found to be weaker, 0.3×1037 ≤ LX(1-20 keV) ≤ 3.0×1037 ergs s-1 after extrapolating the observed flux (20-50 keV) to the EXOSAT energy band. Pulse phases derived from the 13 outbursts were fitted to two different models to determine a binary orbit. The new orbit is used to estimate 95% confidence limits for the mean peak spin frequency change during the outbursts observed with BATSE. This and the mean peak flux are compared to the spin-up rates and fluxes determined by EXOSAT from the 1985 giant outburst, where disk accretion was thought to have occurred. It is unclear whether these normal outbursts were driven by wind or disk accretion.

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.

Hard X-Ray Lags in GRO J1719--24

We have used the Fourier cross spectra of GRO J1719-24, obtained with BATSE, to estimate the phase lags between the X-ray flux variations in the 20-50 and 50-100 keV energy bands as a function of Fourier frequency in the interval 0.002-0.488 Hz. Our analysis covers the entire ~80 day X-ray outburst of this black hole candidate, following the first X-ray detection on 1993 September 25. The X-ray variations in the 50-100 keV band lag those in the 20-50 keV energy band by an approximately constant phase difference of 0.072 ± 0.010 rad in the frequency interval 0.02-0.20 Hz. The peak phase lags in the interval 0.02-0.20 Hz are about twice those of Cyg X-1 and GRO J0422+32. These results are consistent with models for Comptonization regions composed of extended nonuniform clouds around the central source.

Orbit determination for the Be/X-ray transient EXO 2030+375

The Be/X-ray binary transient pulsar EXO 2030+375 has been observed during twenty-two outbursts over four years (1991–1994) using the large area detectors (LADs) of the Burst and Transient Source Experiment (BATSE) on the Compton Gamma-Ray Observatory (CGRO). Thirteen outbursts between 1992 February and 1993 August occurred regularly at ∼46 day intervals, close to the orbital period determined using EXOSAT data (Parmar et al. 1989). EXOSAT discovered this pulsar during a “giant” outburst in 1985 May (Parmar et al. 1985). All BATSE outbursts were “normal” type. Pulse phases derived from the thirteen consecutive outbursts were fit to two different models to determine a binary orbit. A summary of the results are presented here.

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.