Long-Term Monitoring of the Black Hole Candidate Swift J1753.5-0127 with INTEGRAL/SPI

The black hole candidate Swift J1753.5-0127 went into outburst in 2005 June. Rather than fade into quiescence as most black-hole-candidate transients do, it has remained in a low hard spectral state for most of the $\sim$ 9 years after outburst. The persistent emission while in a hard state is reminiscent of the black hole Cyg X-1 and the black hole candidates 1E 1740.7-2942 and GRS 1758-258. Thus far hard X-ray/soft gamma-ray results have focused mainly on the 2005 flare, with a few additional observations during 2007. Here, we present results from INTEGRAL/SPI observations from 2005 - 2010 spanning the 22 - 650 keV energy range. Spectral analysis shows a weak high-energy excess ( $\sim$ 2.9 $\sigma$) above a cutoff powerlaw model that is well fit by a powerlaw suggesting an additional spectral component. Observations of Cyg X-1, 1E 1740.7-2942, and GRS 1758-258 have shown similar spectra requiring an additional high-energy component The SPI results are compared to previously reported results for Swift J1753.5-0127 as well as observations of other sources.
Comment: 11 pages, 4 figues