NuSTAR view of the X-ray transients Swift J174805.3-244637 and IGR J17511-3057

Abstract

We report on the NuSTAR observations of the neutron star low-mass X-ray binary Swift J174805.3-244637 (hereafter Swift J17480) and the accreting millisecond X-ray pulsar IGR J17511-3057 performed on March 4, 2023, and April 8, 2015, respectively. We describe the continuum emission of Swift J17480 with a combination of two soft thermal components and an additional hard X-ray emission described by a power-law. We suggest that the spectral properties of Swift J17480 are consistent with a soft spectral state. The source IGR J17511-3057 exhibits a hard spectrum characterized by a Comptonized emission from the corona. The X-ray spectrum of both sources shows evidence of disc reflection. For the first time, we employ the self-consistent reflection models (relxill and relxillNS) to fit the reflection features in the NuSTAR spectrum. From the best-fit spectral model, we find an inner disc radius ($R_{in}$) is precisely constrained to $(1.99-2.68)R_{ISCO}$ and inclination to $30\pm 1°$ for Swift J17480. We determine an inner disc radius of $\lesssim 1.3R_{ISCO}$ and inclination of $44\pm 3°$ for IGR J17511-3057. A low inclination angle of the system is required for both sources. For the source IGR J17511-3057, spinning at 4.1 ms, the value of co-rotation radius ($R_{co}$) is estimated to be ∼42 km ($3.6R_{ISCO})$, consistent with the position of inner disc radius as $R_{in}\lesssim R_{co}$. We further place an upper limit on the magnetic field strength of the sources, considering the disc is truncated at the magnetospheric radius.