SIGMA observation of hard X-ray emission from the ultrasoft X-ray transient Triangulum Australe X-1 (A1524 - 62)

Abstract

While monitoring the region of the sky containing Cir X-1, the SIGMA telescope detected a new hard X-ray source whose position is compatible with that of TrA X-1 (A1524 - 62), an ultrasoft X-ray transient discovered 15 years ago by Ariel 5. The SIGMA result supports the idea that TrA X-1 may be powered by accretion onto a black hole, as previously proposed by White and coworkers on the basis of its soft X-ray spectral properties. Since none of the X-ray all-sky monitors operating at the time of the SIGMA observation has reported a bright state for TrA X-1, the high-energy emission detected by SIGMA seems more likely to be related to a low-intensity outburst, further characterized by a hard spectrum with a weak soft X-ray component.