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

TL;DR

This study uses NuSTAR X-ray observations to analyze the spectral properties and accretion disc geometry of two neutron star low-mass X-ray binaries, employing advanced reflection models to constrain physical parameters.

Contribution

First application of self-consistent reflection models ({ t relxill} and { t relxillNS}) to NuSTAR spectra of these sources, providing precise measurements of disc radius and inclination.

Findings

Inner disc radius constrained to ~2 R_{ISCO} for Swift J17480

Inner disc radius less than 1.3 R_{ISCO} for IGR J17511-3057

Both sources require low inclination angles

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 ({\tt relxill} and {\tt 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 $30\pm 1\degree$ for Swift~J17480. We determine an inner disc radius of $\lesssim 1.3\;R_{ISCO}$ and inclination of $44\pm 3\degree$ 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 $\sim 42$ km ($3.6\:R_{ISCO})$, 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.