Hard X-ray Tail Discovered in the Clocked Burster GS 1826-238

Abstract

The LMXB NS GS 1826-238 was discovered by Ginga in 1988 September. Due to the presence of quasi-periodicity in the type I X-ray burst rate, the source has been a frequent target of X-ray observations for almost 30 years. Though the bursts were too soft to be detected by INTEGRAL/SPI, the persistent emission from GS 1826-238 was detected over 150 keV during the ~10 years of observations. Spectral analysis found a significant high-energy excess above a Comptonization model that is well fit by a power law, indicating an additional spectral component. Most previously reported spectra with hard tails in LMXB NS have had an electron temperature of a few keV and a hard tail dominating above ~50 keV with an index of \Gamma ~ 2-3. GS 1826-238 was found to have a markedly different spectrum with $ kT_e \sim 20 $ keV and a hard tail dominating above ~150 keV with an index of \Gamma ~ 1.8, more similar to BHXRB. We report on our search for long-term spectral variability over the 25-370 keV energy range and on a comparison of the GS 1826-238 average spectrum to the spectra of other LMXB NS with hard tails.