Letter to the Editor

IGR J11215–5952: a hard X-ray transient displaying recurrent outbursts

INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica Milano, via E. Bassini 15, 20133 Milano, Italy e-mail: sidoli@iasf-milano.inaf.it

Received: 31 January 2006
Accepted: 2 March 2006

Abstract

Context.The hard X-ray source IGR J11215–5952 has been discovered with INTEGRAL during a short outburst in 2005 and proposed as a new member of the class of supergiant fast X-ray transients.Aims.We analysed INTEGRAL public observations of the source field in order to search for previous outbursts from this transient, not reported in literature. Methods.Our results are based on a systematic re-analysis of INTEGRAL archival observations, using the latest analysis software and instrument calibrations. Results.We report the discovery of two previously unnoticed outbursts, spaced by intervals of ~330 days, that occurred in July 2003 and May 2004. The 5–100 keV spectrum of IGR J11215–5952 is well described by a cut-off power law, with a photon index of ~0.5, and a cut-off energy ~15–20 keV, typical of High Mass X-ray Binaries hosting a neutron star. A 5–100 keV luminosity of  erg s^-1 has been derived (assuming 6.2 kpc, the distance of the likely optical counterpart).Conclusions.The 5–100 keV spectral properties, the recurrent nature of the outbursts, together with the reduced error region containing the blue supergiant star HD 306414, support the hypothesis that IGR J11215–5952 is a member of the class of the Supergiant Fast X-ray Transients.