Discovery of 146.8 s pulsations from EP J005146.9-730930, a new transient Be/X-ray binary in the SMC

TL;DR

This paper reports the discovery of a new transient Be/X-ray binary pulsar in the SMC, exhibiting 146.8 s pulsations, spectral analysis, and flux evolution during an outburst, with implications for magnetic field estimates.

Contribution

The study identifies and characterizes a new Be/X-ray binary pulsar in the SMC, providing detailed spectral and timing analysis and confirming its optical counterpart.

Findings

Detected 146.8 s pulsations in X-ray flux.

Observed flux decrease by a factor of 10 over one month.

Estimated magnetic field strength lower limit of 3.3 x 10^12 G.

Abstract

Recent observations of the Small Magellanic Cloud (SMC) with Einstein Probe (EP) revealed a new transient X-ray source, most likely identified as Be/X-ray binary. To characterise the X-ray properties of EP J005146.9-730930 and in particular to look for pulsations in the X-ray flux, we triggered an XMM-Newton anticipated target of opportunity observation. To follow the flux evolution during the outburst we monitored the source for about three months with the Follow-up X-ray Telescope of EP. The XMM-Newton observation was performed on 2024 September 15 and we used the data from the European Photon Imaging Camera (EPIC) for detailed spectral and timing analyses. The EPIC X-ray spectrum is well described by an absorbed power law with photon index of 1.25 +/- 0.04 and the timing analysis revealed pulsations with 146.79 +\- 0.03 s. The source flux had decreased by a factor of about 10 since the observed maximum about one month before the XMM-Newton observation. EP J005146.9-730930 was never detected significantly during serendipitous observations before September 2024. The characteristics of the X-ray brightening suggest the source was discovered during a type II outburst reaching an X-ray peak luminosity of ~2 x 10^37 erg/s. The trend of spectral hardening towards higher luminosities observed by EP suggests that the source is accreting below the critical luminosity, yielding an estimated lower limit for the pulsar magnetic field strength of 3.3 x 10^12 G. The improved X-ray position confirms the candidate Be star OGLE J005147.58-730924.7 as optical counterpart. We conclude that EP J005146.9-730930 = SXP 146.8 is a new Be/X-ray binary pulsar in the SMC.