Probing the nature of AX J0043-737: Not an 87 ms pulsar in the SMC

TL;DR

This study re-evaluates the nature of AX J0043-737, initially thought to be an 87 ms pulsar, and concludes it is an active galactic nucleus (AGN) behind the SMC based on X-ray observations and variability analysis.

Contribution

The paper provides a detailed multi-epoch X-ray analysis that identifies the source as an AGN, challenging previous pulsar classification and demonstrating the importance of follow-up observations.

Findings

Source is an AGN at redshift 0.95

X-ray spectrum fits an absorbed power law with photon index 1.7

Long-term variability consistent with AGN behavior

Abstract

AX J0043-737 is a source in the ASCA catalogue, the nature of which is uncertain. It is most commonly classified as a Crab-like pulsar in the Small Magellanic Cloud (SMC) following apparent detection of pulsations at ~ 87 ms from a single ASCA observation. A follow-up ASCA observation was not able to confirm this, and the X-ray detection of the source has not been reported since. With a dedicated XMM-Newton, observation, we studied the nature of the source. We ascertained the source position, searched for the most probable counterpart and studied the X-ray spectrum. We also analysed other archival observations with the source in the field of view to study its long-term variability. With the good position localisation capability of XMM-Newton, we identify the counterpart of the source as MQS J004241.66--734041.3, an AGN behind the SMC at a redshift of 0.95. The X-ray spectrum can be fitted with an absorbed power law with a photon-index of $\Gamma=1.7$ which is consistent with that expected from AGNs. By comparing the current XMM-Newton, observation with an archival XMM-Newton and two other ASCA observations of the source, we find signatures of long-term variability which is also a common phenomenon in AGNs. All of the above are consistent with AX J0043-737 being an AGN behind the SMC.