Letter to the Editor

INTEGRAL high energy detection of the transient IGR J11321–5311

¹ School of Physics and Astronomy, University of Southampton, Highfield, SO17 1BJ, UK e-mail: sguera@astro.soton.ac.uk
² INAF/IASF Roma, via Fosso del Cavaliere 100, 00133 Roma, Italy
³ INAF/IASF Bologna, via Piero Gobetti 101, 40129 Bologna, Italy

Received: 8 March 2007
Accepted: 14 April 2007

Abstract

Context.The transient hard X-ray source IGR J11321-5311 was discovered by INTEGRAL on June 2005, during observations of the Crux spiral arm. To date, this is the only detection of the source to be reported by any X/γ-ray mission.

Aims.To characterize the behaviour and hence the nature of the source through temporal and spectral IBIS analysis.

Methods.Detailed spectral and temporal analysis has been performed using standard INTEGRAL software OSA v.5.1.

Results.To date, IGR J11321-5311 has been detected only once. It was active for ~3.5 h, a short and bright flare lasting ~1.5 h is evident in the IBIS light curve. It reached a peak flux of ~80 mCrab or 2.2  10^-9 erg cm^-2 s^-1 (20–300 keV), corresponding to a peak luminosity of ~1.1  10³⁷ erg s^-1 (assuming a distance of 6.5 kpc). During the outburst, the source was detected with a significance of ~18σ (20–300 keV) and ~8σ (100–300 keV). The spectrum of the total outburst activity (17–300 keV) is best fitted by the sum of a power law (Γ = 0.55 ± 0.18) plus a black body (kT = 1.0 keV), with no evidence for a break up to 300 keV. A spectral analysis at Science Window level revealed an evident hardening of the spectrum through the outburst. The IBIS data were searched for pulsations with no positive result.

Conclusions.The X-ray spectral shape and the flaring behaviour favour the hypothesis that IGR J11321-5311 is an Anomalous X-ray Pulsar, though a different nature can not be firmly rejected at the present stage.