XMM–Newton and Swift spectroscopy of the newly discovered very faint X-ray transient IGR J17494–3030

Abstract

Abstract A growing group of low-mass X-ray binaries are found to be accreting at very faint X-ray luminosities of <1036 erg s−1 (2–10 keV). One such system is the new X-ray transient IGR J17494–3030. We present Swift and XMM–Newton observations obtained during its 2012 discovery outburst. The Swift observations trace the peak of the outburst, which reached a luminosity of ∼7 × 1035 (D/8 kpc)2 erg s−1 (2–10 keV). The XMM–Newton data were obtained when the outburst had decayed to an intensity of ∼8 × 1034 (D/8 kpc)2 erg s−1. The spectrum can be described by a power law with an index of Γ ∼ 1.7 and requires an additional soft component with a blackbody temperature of ∼0.37 keV (contributing ∼20 per cent to the total unabsorbed flux in the 0.5–10 keV band). Given the similarities with high-quality spectra of very faint neutron-star low-mass X-ray binaries, we suggest that the compact primary in IGR J17494–3030 is a neutron star. Interestingly, the source intensity decreased rapidly during the ∼12 h XMM–Newton observation, which was accompanied by a decrease in inferred temperature. We interpret the soft spectral component as arising from the neutron-star surface due to low-level accretion, and propose that the observed decline in intensity was the result of a decrease in the mass-accretion rate on to the neutron star.