X-ray flashes from the low-mass X-ray binary IGR J17407−2808

Abstract

IGR J17407−2808 is an enigmatic and poorly studied X-ray binary that was recently observed quasi-simultaneously with NuSTAR and XMM-Newton. In this paper we report the results of this observational campaign. During the first 60 ks of observation, the source was caught in a relatively low emission state characterised by a modest variability and an average flux of ∼8.3 × 10−13 erg cm−2 s−1 (4−60 keV). Subsequently, IGR J17407−2808 entered a significantly more active emission state that persisted for the remaining ∼40 ks of the NuSTAR observation. During this state, IGR J17407−2808 displayed several fast X-ray flares of ∼1 − 100 s in duration and featuring profiles with either single or multiple peaks. The source flux in the flaring state reached values as high as ∼2 × 10−9 erg cm−2 s−1 (4−60 keV), leading to a measured dynamic range during the NuSTAR and XMM-Newton campaign of ≳103. We also analysed available archival photometric near-infrared data for IGR J17407−2808 in order to improve the constraints available so far on the nature of the donor star hosted in this system. Our analysis shows that the donor star can be either a rare K or M-type sub-subgiant, a K-type main sequence or subgiant star. Our findings support the classification of IGR J17407−2808 as a low-mass X-ray binary. We discuss the X-ray behaviour of the source as recorded by NuSTAR and XMM-Newton in view of this revised classification.