Hard X-ray variability of the black hole candidate GRO J0422+32 during its1992 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, fu lly covering its primary and secondary X-ray outbursts. We compute power de nsity 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 approxi mately flat up to a break frequency, and decay as a power law above it, wit h index similar to 1. During the first 70 days of the X-ray outburst, the P DSs of GRO J0422+32 show a significant quasi-periodic oscillation (QPO) pea k near similar to 0.2 Hz, superposed on the power-law tail. The break frequ ency 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% an d 89% Hz(-1/2) (20-100 keV). The canonical anticorrelation between the brea k frequency and the power density at the break, observed in Cyg X-l and oth er BHCs in the low state, is not observed in the PDSs of GRO J0422+32. We c ompare our results with those of similar variability studies of Cyg X-l. Th e relation between the spectral slope and the amplitude of the X-ray variat ions of GRO J0422+32 is similar to that of Cyg X-l; however, the relation b etween the hard X-ray flux and the amplitude of its variation is opposite t o what has been found in Cyg X-l. Phase lags between the X-ray flux variati ons of GRO J0422+32 at high and low photon energies could only be derived d uring 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 approx imately 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 th e outburst decrease with frequency as a power law with index 0.9 for v > 0. 01 Hz.