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Swift Monitoring of M51: A 38 day Superorbital Period for the Pulsar ULX7 and a New Transient Ultraluminous X-Ray Source

Brightman, Murray and Earnshaw, Hannah and Fürst, Felix and Harrison, Fiona A. and Heida, Marianne and Israel, Gianluca and Pike, Sean and Stern, Daniel and Walton, Dominic J. (2020) Swift Monitoring of M51: A 38 day Superorbital Period for the Pulsar ULX7 and a New Transient Ultraluminous X-Ray Source. Astrophysical Journal, 895 (2). Art. No. 127. ISSN 1538-4357. https://resolver.caltech.edu/CaltechAUTHORS:20200414-070245589

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Abstract

We present the results from a monitoring campaign made with the Neil Gehrels Swift Observatory of the M51 galaxies, which contain several variable ultraluminous X-ray sources (ULXs). The ongoing campaign started in 2018 May, and we report here on ~1.5 yr of observations. The campaign, which consists of 106 observations, has a typical cadence of 3–6 days, and has the goal of determining the long-term X-ray variability of the ULXs. Two of the most variable sources were ULX7 and ULX8, both of which are known to be powered by neutron stars that are exceeding their isotropic Eddington luminosities by factors of up to 100. This is further evidence that neutron-star-powered ULXs are the most variable. Our two main results are, first, that ULX7 exhibits a periodic flux modulation with a period of 38 days varying over a magnitude and a half in flux from peak to trough. Since the orbital period of the system is known to be 2 days, the modulation is superorbital, which is a near-ubiquitous property of ULX pulsars. Second, we identify a new transient ULX, M51 XT-1, the onset of which occurred during our campaign, reaching a peak luminosity of ~10⁴⁰ erg s⁻¹, before gradually fading over the next ~200 days until it slipped below the detection limit of our observations. Combined with the high-quality Swift/X-ray Telescope lightcurve of the transient, serendipitous observations made with Chandra and XMM-Newton provide insights into the onset and evolution of a likely super-Eddington event.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.3847/1538-4357/ab7e2aDOIArticle
https://arxiv.org/abs/1912.04431arXivDiscussion Paper
ORCID:
AuthorORCID
Brightman, Murray0000-0002-8147-2602
Earnshaw, Hannah0000-0001-5857-5622
Fürst, Felix0000-0003-0388-0560
Harrison, Fiona A.0000-0003-2992-8024
Heida, Marianne0000-0002-1082-7496
Israel, Gianluca0000-0001-5480-6438
Pike, Sean0000-0002-8403-0041
Stern, Daniel0000-0003-2686-9241
Walton, Dominic J.0000-0001-5819-3552
Alternate Title:Swift monitoring of M51: A 38-day super-orbital period for the pulsar ULX7 and a new transient ULX
Additional Information:© 2020 The American Astronomical Society. Received 2019 December 9; revised 2020 February 19; accepted 2020 March 4; published 2020 June 4. We wish to thank the Swift PI, Brad Cenko for approving the target of opportunity requests we made to observe M51, as well as the rest of the Swift team for carrying them out. We also acknowledge the use of public data from the Swift data archive. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester. The work of D.S. was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. D.J.W. acknowledges support from an STFC Ernest Rutherford Fellowship.
Group:Astronomy Department, Space Radiation Laboratory
Funders:
Funding AgencyGrant Number
NASA/JPL/CaltechUNSPECIFIED
Science and Technology Facilities Council (STFC)UNSPECIFIED
Subject Keywords:X-ray point sources ; X-ray transient sources ; X-ray photometry ; X-ray bright point ; X-ray active galactic nuclei ; Transient sources ; Light curves ; Galaxy nuclei ; Active galactic nuclei
Issue or Number:2
Classification Code:Unified Astronomy Thesaurus concepts: X-ray point sources (1270); X-ray transient sources (1852); X-ray photometry (1820); X-ray bright point (1812); X-ray active galactic nuclei (2035); Transient sources (1851); Light curves (918); Galaxy nuclei (609)
Record Number:CaltechAUTHORS:20200414-070245589
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20200414-070245589
Official Citation:Murray Brightman et al 2020 ApJ 895 127
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:102514
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:14 Apr 2020 17:06
Last Modified:08 Jun 2020 17:09

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