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

Earnshaw, H.P.; Roberts, T.P.; Sathyaprakash, R.

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.

Citation

Earnshaw, H., Roberts, T., & Sathyaprakash, R. (2018). Searching for propeller-phase ULXs in the XMM-Newton Serendipitous Source Catalogue. Monthly Notices of the Royal Astronomical Society, 476(3), 4272-4277. https://doi.org/10.1093/mnras/sty501

• Journal Article Type: Article
• Acceptance Date: Feb 21, 2018
• Online Publication Date: Feb 23, 2018
• Publication Date: May 21, 2018
• Deposit Date: Feb 23, 2018
• Publicly Available Date: Feb 28, 2018
• Journal: Monthly Notices of the Royal Astronomical Society
• Print ISSN: 0035-8711
• Electronic ISSN: 1365-2966
• Publisher: Royal Astronomical Society
• Peer Reviewed: Peer Reviewed
• Volume: 476
• Issue: 3
• Pages: 4272-4277
• DOI: https://doi.org/10.1093/mnras/sty501
• Related Public URLs: https://arxiv.org/pdf/1802.07753.pdf