INTEGRAL high energy detection of the transient IGR J11321-5311
Hill, A. B.; Bird, A. J.; Ubertini, P.; Malizia, A.; Bassani, L.; Bazzano, A.; Dean, A. J.; Sguera, V.
United Kingdom, Italy
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+0.2-0.3 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.