Study of the reflection spectra of SAX J1748.9-2021

Abstract

We report the spectral analysis of accretion powered millisecond X-ray pulsar SAX J1748.9-2021 from the NuSTAR observation made during its 2015 outburst. The spectra displayed a broad emission line at $\sim 6.5$ keV with line width of $\sim 0.5$ keV and excess above $\sim 20$ keV due to Compton hump. The persistent emission of SAX J1748.9-2021 is described by a combination of soft thermal component with $kT=0.62_{-0.05}^{+0.03}$ keV and thermally Comptonized component with $k{T}_e=2.50_{-0.03}^{+0.06}$ keV reflected from the disc with reflection fraction of $0.30_{-0.04}^{+0.08}$. We have modeled the reflection spectrum with self-consistent model relxillCP and have found the inclination of the accretion disc to be $32.3_{-4.7}^{{}^{\circ }+4.8}$ and log $\xi$ is equal to $3.05_{-0.34}^{+0.40}$. We have obtained an upper limit of 57 km for inner disc radius; and the estimated magnetic field strength at the poles is less than $3.4\times 10^9$ G. This kind of detailed investigation of reflection spectrum of SAX J1748.9-2021, especially above 10 keV, has been achieved only because of high statistics NuSTAR data of the source.

6 pages, 6 figures. Accepted for publication in MNRAS