# The reflection spectrum of the low-mass X-ray binary 4U 1636-53

## TL;DR

This study analyzes NuSTAR X-ray observations of the low-mass X-ray binary 4U 1636-53, modeling its reflection spectrum to understand accretion disc properties and the source of hard X-ray emission.

## Contribution

It compares multiple spectral models, including relativistic reflection models, to determine the most plausible inclination and characteristics of the accretion disc in 4U 1636-53.

## Key findings

- RELXILL and RELXILLLP models provide more realistic disc inclination estimates.
- The inclination derived from KYRLINE and RELXILL models is inconsistent with observational constraints.
- The study discusses the primary source of hard X-rays in the system.

## Abstract

We present 3-79 keV NuSTAR observations of the neutron star low-mass X-ray binary 4U 1636-53 in the soft, transitional and hard state. The spectra display a broad emission line at 5-10 keV. We applied several models to fit this line: A GAUSSIAN line, a relativistically broadened emission line model, KYRLINE, and two models including relativistically smeared and ionized reflection off the accretion disc with different coronal heights, RELXILL and RELXILLLP. All models fit the spectra well, however, the KYRLINE and RELXILL models yield an inclination of the accretion disc of $\sim88\degree$ with respect to the line of sight, which is at odds with the fact that this source shows no dips or eclipses. The RELXILLLP model, on the other hand, gives a reasonable inclination of $\sim56\degree$. We discuss our results for these models in this source and the possible primary source of the hard X-rays.