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 18-40keV, while it was found being in a constant low/hard spectral state. The broad-band (0.3-100keV) averaged spectrum obtained by combining INTEGRAL/IBIS and Swift/XRT data has been fitted with a thermal Comptonization model and an electron temperature >~24keV 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 J1753.5-2349, 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.

Based on observations with INTEGRAL, an ESA project with instruments and science data centre funded by ESA member states (especially the PI countries: Denmark, France, Germany, Italy, Switzerland, Spain), Czech Republic and Poland, and with participation of Russia and the USA.

E-mail: melania.delsanto@iasf-roma.inaf.it