Evidence of disc reflection in the X-ray spectrum of the neutron star low mass X-ray binary 4U 1636-536

TL;DR

This study provides evidence of disc reflection features in the X-ray spectrum of the neutron star low-mass X-ray binary 4U 1636-536, revealing a truncated inner accretion disc and estimating the neutron star's magnetic field strength.

Contribution

First broadband spectral analysis of 4U 1636-536 showing clear disc reflection signatures and constraining the inner disc radius and magnetic field strength.

Findings

Detection of broad Fe-K emission line and reflection hump.

Inner disc truncated at 3.2-4.7 R_ISCO.

Neutron star magnetic field estimated to be ≤ 2.0×10^9 G.

Abstract

We present a broadband spectral analysis of the atoll source 4U~1636-536 observed for $\sim92$ ks with \nustar{}. The source was found to be in a low-luminosity state during this observation with $3-79 \kev{}$ X-ray luminosity of $L_{3-79 keV}=(1.03\pm0.01)\times 10^{37}$ ergs/s, assuming a distance of 6 kpc. We have identified and removed twelve type-I X-ray bursts during this observation to study the persistent emission. The continuum is well described by a thermal Comptonization model {\tt nthcomp} with $\Gamma\sim1.9$, $kT_{e}\sim28 \kev{}$, and $kT_{s}\sim0.9\kev{}$. The \nustar{} data reveal a clear signature of disc reflection, a significantly broad Fe-K emission line (around $5-8\kev{}$), and the corresponding reflection hump (around $15-30\kev{}$). We have modeled the data with two relativistically blurred reflection models. Both families of reflection models prefer truncated inner disc radii prior to the ISCO. We find that the inner disc is truncated with an inner radius of $R_{in}=(3.2-4.7)\;R_{ISCO}$ ($\simeq16-24\,R_{g}\: \text{or}\: 36-54$ km). This inner disc radius suggests that the neutron star magnetic field strength is $B\leq2.0\times10^{9}$ G.