Gamma-Ray Spectral State Transitions of GRO J1719-24

Abstract

We report BATSE earth-occultation observations of the long-term soft gamma-ray (30 keV - 1.7 MeV) flux and spectral variability of the transient source GRO J1719-24 covering a 1000-day period between 13 January 1993 and 10 October 1995. During this period, the source underwent a major outburst in the fall of 1993 when the 35-100 keV flux rose from a quiescent state of less than 16 mCrab before 17 September 1993 to a level of 1.5 Crab on 3 October and remained approximately at this level over the next ~70 days. It then decreased sharply to the pre-transition quiescent level of ~44 mCrab on 21 December where it remained until 5 September 1994. During a 400-day period between 5 September 1994 and 10 October 1995, the source again underwent a series of five transitions when the 35-100 keV flux increased to low-intensity levels of \~200-400 mCrab. The low-state spectra were generally characterized by a power law with spectral index of ~2. The high-state spectra on the other hand have two components: a thermal Comptonized shape below ~200 keV with electron temperature kTe of ~37 keV and optical depth of ~2.8, and a soft power-law tail with photon index of ~3.4 above 200 keV that extends to ~500 keV. The softer high-intensity spectrum and the harder low-intensity spectrum intercept at ~400 keV. The non-thermal power-law gamma-ray component seen in both the high and low-intensity spectra suggests that the persistent non-thermal emission source is coupled to the hot and variable thermal emission source in the system. This is similar to that seen in two other gamma-ray emitting black-hole candidates GRO J0422+32 and Cygnus X-1, suggesting that perhaps similar system configurations and processes are occurring in these systems.