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

TL;DR

This study presents the first high-energy X-ray spectrum of the burst-only source SAX J1753.5-2349 during outburst, revealing a persistent low/hard state and providing insights into its accretion behavior and potential binary nature.

Contribution

It provides the first broad-band (0.3-100 keV) spectral analysis of SAX J1753.5-2349 in outburst, using combined INTEGRAL and Swift data, and estimates its duty cycle and accretion rate.

Findings

The source remained in a low/hard spectral state during outburst.

The broad-band spectrum is well fitted by a thermal Comptonisation model.

The system likely hosts a very compact binary.

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 October 2008, 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 10 keV) 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 Comptonisation 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 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 harbor a very compact binary system.