Spin period evolution and X-ray spectral characteristics of the SMC pulsar SXP 46.6

TL;DR

This study analyzes 2017 NuSTAR observations of the Be/X-ray binary SXP 46.6 in the SMC to trace its spin evolution and X-ray emission. It measures a continuous spin-up from $P \approx 46.6$ s to $P=45.984(1)$ s and uses the spin derivative with the Ghosh–Lamb accretion-torque model to estimate the neutron star magnetic field, ultimately favoring $B \approx 2.25\times10^{13}$ G after excluding a lower, unphysical solution. The timing analysis reveals a robust double-peaked pulse profile across energy bands, consistent with two antipodal hotspots, while phase-resolved spectroscopy shows the power-law index $\Gamma$ varies with spin phase, hinting at geometry- and accretion-column–driven spectral changes. Together, these results constrain the accretion geometry, magnetic field, and emission mechanisms in SXP 46.6, with implications for Be/X-ray binaries and expectations of high-energy cyclotron features beyond NuSTAR's band.

Abstract

We characterize the Small Magellanic Cloud (SMC) pulsar SXP 46.6 using NuSTAR observations conducted in 2017. The spin period (P) of this neutron star decreased from its discovered value of 46.6 s to a value of 45.984(1) s, indicating a spin-up at the rate of \dot{P} = -1.13 x 10^{-9} s s^{-1}. This spin-up rate is used to calculate a high pulsar magnetic field value of 2.25 x 10^{13} G. This process also gives a low magnetic field value, which we rule out here by constraining the inner accretion disk radius to be less than the radius of the innermost stable circular orbit. The pulse profile, analyzed in soft, hard, and broad X-ray bands, shows a double-peaked structure, consistent with pencil beam emission from two antipodal hot spots on the neutron star surface. We also perform spin phase-resolved spectroscopy for the first time, revealing spectral variations across different phases of the pulsar's rotation. These results offer new insights into the long-term spin evolution and emission properties of SXP 46.6.

Figures (8)

• Figure 1: This figure shows the evolution of the outburst of SXP 46.6 during its 2017 observation with NuSTAR (see section \ref{['sec:obs_analysis']}). It displays the field of view images obtained from the ObsIDs: (a) 50311003002, (b) 50311003004, and (c) 50311001004. The circular regions mark the position of SXP 46.6 in all the three panels. The pulsar was undetected on $3^{rd}$ May 2017 and became visible on $7^{th}$ August 2017. The outburst faded and again became undetected in the observation after five days.
• Figure 2: The figure shows the spin period peak observed at $\sim$45.98 s for the source SXP 46.6 observed in 2017 with NuSTAR in 3--20 keV energy range (section \ref{['sec:result']}).
• Figure 3: The three panels in this figure show the pulse profile obtained by folding the light curve at $\sim$45.98 s for three different energy ranges for the source SXP 46.6 observed with NuSTAR during its 2017 outburst. The variation in the pulse profile hints towards a double peaked trend in all three panels
• Figure 4: This figure displays the variation of counts (top panel) in the energy range 3--20 keV, flux (middle panel: because of the relatively low value of $n_{\mathrm{H}}$, both the unabsorbed and absorbed flux are similar) and photon index (bottom panel) with spin-phase in the energy range 3--20 keV for the source SXP 46.6 observed with NuSTAR during its 2017 outburst. The varying photon index indicates a positive correlation with the double-peaked pulse profile, (see section \ref{['subsec:phase-resolved']}).
• Figure 5: This figure shows the spectral fitting of the NuSTAR data for the source SXP 46.6 during its 2017 outburst (ObsID: 50311003004). The spectrum is limited to 3--20 keV energy range and is fitted using the XSPEC model tbabs*(bbodyrad+powerlaw) yielding $\chi^{2}$/dof = 45/41. The bottom panel shows the residual plot ($\chi$ = (Data - Model)/error); see section \ref{['sec:result']}).