Searching for propeller-phase ULXs in the XMM-Newton Serendipitous Source Catalogue

TL;DR

This study searches for transient ultraluminous X-ray sources (ULXs) in the XMM-Newton catalog to identify potential neutron star ULXs exhibiting propeller regime behavior, revealing five candidates with significant variability and one with bimodal flux possibly indicating propeller effects.

Contribution

It introduces a method to identify candidate propeller-phase ULXs by analyzing long-term variability and flux behavior in existing X-ray catalog data, highlighting potential new neutron star ULXs.

Findings

Identified five ULXs with over an order of magnitude variability.

Detected one ULX, M51 ULX-4, with bimodal flux behavior suggestive of the propeller regime.

Predicted approximately 200 similar bimodal ULXs could exist but remain undetected.

Abstract

We search for transient sources in a sample of ULXs from the 3XMM-DR4 release of the XMM-Newton Serendipitous Source Catalogue in order to find candidate neutron star ULXs alternating between an accreting state and the propeller regime, in which the luminosity drops dramatically. By examining their fluxes and flux upper limits, we identify five ULXs that demonstrate long-term variability of over an order of magnitude. Using Chandra and Swift data to further characterise their light curves, we find that two of these sources are detected only once and could be X-ray binaries in outburst that only briefly reach ULX luminosities. Two others are consistent with being super-Eddington accreting sources with high levels of inter-observation variability. One source, M51 ULX-4, demonstrates apparent bimodal flux behaviour that could indicate the propeller regime. It has a hard X-ray spectrum, but no significant pulsations in its timing data, although with an upper limit of 10% of the signal pulsed at ~1.5 Hz a pulsating ULX cannot be excluded, particularly if the pulsations are transient. By simulating XMM-Newton observations of a population of pulsating ULXs, we predict that there could be approximately 200 other bimodal ULXs that have not been observed sufficiently well by XMM-Newton to be identified as transient.