Sigma detection of hard X-ray emission from the soft transient type I X-ray burster KS 1731 - 260

Abstract

Observations of KS 1731 - 260 by the French hard X-ray/soft gamma-ray telescope Sigma reveal the existence of a hard state for which the spectrum extends to 150 keV, with a power-law shape characterized by a photon spectral index of about 2.9 (+/-0.8). Consequently, KS 1731 - 260 must be added to the list of the hard X-ray sources located within the 100 sq deg around the Galactic center. The result also supports the idea that power-law tails are not an exclusive property of the ultrasoft transients, which are black hole candidates, but may also be a temporary characteristic of weak magnetized neutron stars.