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

We present a broad-band spectral analysis of the atoll source 4U 1636-536 observed for ~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_{3-79 keV} = (1.03 ± 0.01) × 10³⁷ erg s^-1, assuming a distance of 6 kpc. We have identified and removed 12 type-I X-ray bursts during this observation to study the persistent emission. The continuum is well described by a thermal Comptonization model nthcomp with Γ ~ 1.9, kT_e ~ 28 keV, and kT_s ~ 0.9 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 modelled 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 ($\simeq 16\!-\!24\, R_{\mathrm{ g}}\: \text{or}\: 36\!-\!54$ km). This inner disc radius suggests that the neutron star magnetic field strength is B ≤ 2.0 × 10⁹ G.