Unveiling the hard X-ray spectrum from the 'burst-only' source SAX J1753.5-2349 in outburst

Abstract

Discovered in 1996 by BeppoSAX during a single type-I burst event, SAX J1753.5-2349 was classified as 'burst-only' source. Its persistent emission, either in outburst or in quiescence, had never been observed before 2008 October, when SAX J1753.5-2349 was observed for the first time in outburst. Based on INTEGRAL observations, we present here the first high-energy emission study (above 10keV) of a so-called 'burst-only'. During the outburst, the SAX J1753.5-2349 flux decreased from 10 to 4 mCrab in 18-40 keV, while it was found being in a constant low/hard spectral state. The broad-band (0.3-100 keV) averaged spectrum obtained by combining INTEGRAL/IBIS and Swift/XRT data has been fitted with a thermal Comptonization model and an electron temperature ≳24 keV inferred. However, the observed high column density does not allow the detection of the emission from the neutron star surface. Based on the whole set of observations of SAX J 1753.5-2349, we are able to provide a rough estimate of the duty cycle of the system and the time-averaged mass-accretion rate. We conclude that the low to very low luminosity of SAX J1753.5-2349 during outburst may make it a good candidate to harbour a very compact binary system. © 2010 The Authors. Journal compilation © 2010 RAS.