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

TL;DR

This paper reports on XMM-Newton and Swift observations of the faint X-ray transient IGR J17494-3030, revealing spectral features indicative of a neutron star and suggesting low-level accretion as the cause of its luminosity decline.

Contribution

First detailed spectral analysis of IGR J17494-3030 during outburst decay, indicating a neutron star primary and low-level accretion processes in very-faint X-ray binaries.

Findings

Spectral analysis shows a power-law with soft component, consistent with neutron star accretion.

Luminosity decreased from ~7E35 to ~8E34 erg/s during observations.

Soft component temperature decreased as the source dimmed.

Abstract

A growing group of low-mass X-ray binaries are found to be accreting at very-faint X-ray luminosities of <1E36 erg/s (2-10 keV). Once 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 E35 (D/8 kpc)^2 erg/s (2-10 keV). The XMM-Newton data were obtained when the outburst had decayed to an intensity of ~ 8 E34 (D/8 kpc)^2 erg/s. The spectrum can be described by a power-law with an index of ~1.7 and requires an additional soft component with a black-body temperature of ~0.37 keV (contributing ~20% 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 hr 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 onto the neutron star.