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

Abstract

We present a broadband spectral analysis of the atoll source 4U~1636-536 observed for $\sim 92$ 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 $L_{3-79keV}=(1.03\pm 0.01)\times 10^{37}$ 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 $\Gamma \sim 1.9$, $kT_{e}\sim28 \kev{}$, 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 $R_{in}=(3.2-4.7)R_{ISCO}$ ($\simeq 16-24R_g\text{or}36-54$ km). This inner disc radius suggests that the neutron star magnetic field strength is $B\le 2.0\times 10^9$ G.

11 pages, 11 figures, 2 Tables, Accepted for publication in MNRAS

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Cited by in corpus (4)

• NICER observations of the evidence of Poynting-Robertson drag and disk reflection during type I X-ray bursts from 4U 1636$-$536
• Burst-Disk Interaction in 4U 1636-536 as observed by NICER
• A Spectroscopic Angle on Central Engine Size Scales in Accreting Neutron Stars
• NuSTAR and AstroSat observations of thermonuclear X-ray bursts with short-recurrence times in 4U 1636$-$536