SXP214, an X-ray Pulsar in the Small Magellanic Cloud, Crossing the Circumstellar Disk of the Companion

TL;DR

This study reports on the recent X-ray observations of the SXP214 pulsar in the SMC, revealing a spin-up event, spectral softening, and evidence of the neutron star crossing its Be-star companion's circumstellar disk.

Contribution

It provides new measurements of the pulsar's period, spectral evolution, and a detailed interpretation of the neutron star crossing the circumstellar disk.

Findings

Pulsar period shortened to 211.49 seconds, indicating recent spin-up.

X-ray luminosity increased and spectrum softened over time.

Evidence suggests the neutron star emerged from the circumstellar disk.

Abstract

Located in the Small Magellanic Cloud (SMC), SXP214 is an X-ray pulsar in a high mass X-ray binary system with a Be-star companion. A recent survey of the SMC under a Chandra X-ray Visionary program found the source in a transition when the X-ray flux was on a steady rise. The Lomb-Scargle periodogram revealed a pulse period of 211.49 +/- 0.42 s, which is significantly (>5sigma) shorter than the previous measurements with XMM-Newton and RXTE. This implies that the system has gone through sudden spin-up episodes recently. The pulse profile shows a sharp eclipse-like feature with a modulation amplitude of >95%. The linear rise of the observed X-ray luminosity from <~2x to 7x10^35 erg s^-1 is correlated with steady softening of the X-ray spectrum, which can be described by the changes in the local absorption from N_H ~ 10^24 to <~10^20 cm^-2 for an absorbed power-law model. The soft X-ray emission below 2 keV was absent in the early part of the observation when only the pulsating hard X-ray component was observed, whereas at later times both soft and hard X-ray components were observed pulsating. A likely explanation is that the neutron star was initially hidden in the circumstellar disk of the companion, and later came out of the disk with the accreted material that continued fueling the observed pulsation.