Gamma-Ray Spectral State Transitions of GRO J1719–24

• Published in: The Astrophysical journal 2005-03, Vol.622 (1), p.492-502
• Main Authors: Ling, J. C, Wheaton, Wm. A
• Format: Article
• Language: English
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1086/428081

We report the results of an in-depth study of the long-term soft gamma-ray (30 keV to 1.7 MeV) flux and spectral variability of the transient source GRO J1719-24 that was first discovered by BATSE and SIGMA in the fall of 1993. Our results were obtained from the JPL BATSE-EBOP database covering a 1000 day period between 1993 January 13 and 1995 October 10. 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 1993 September 17 to a level of 1.5 crab on October 3. The source remained in this high-intensity state over the next similar to 70 days, during which the 35-100 keV flux decreased monotonically by similar to 33% to similar to 1 crab on December 12, then decreased sharply to the pre-transition quiescent level of similar to 44 mcrab on December 21, where it remained until 1994 September 5. During a 400 day period between 1994 September 5 and 1995 October 10, the source again underwent a series of five transitions when the 35-100 keV flux increased to low-intensity levels of similar to 200-400 mcrab, a factor of 4-7 times lower than what was observed in 1993. The low- and high-intensity states were characterized by two different spectral shapes. The low-state spectra are described by a power law with a spectral index of similar to 2. The high-state spectra, on the other hand, have two components: a thermal Comptonized shape below similar to 200 keV with electron temperature kT sub(e) of similar to 37 keV and optical depth tau similar to 2.8, and a soft power-law tail with photon index of similar to 3.4 above 200 keV that extends to similar to 500 keV. The softer high-intensity spectrum and the harder low-intensity spectrum intersect at similar to 400 keV. The nonthermal power-law gamma-ray component in both the high- and low-intensity spectra suggests that the persistent nonthermal emission source is coupled to the hot and variable thermal emission source in the system. Furthermore, the correlation of the spectral characteristics with the high- and low-intensity states resembles 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. Possible scenarios for interpreting these behaviors are discussed.