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A ~60-day super-orbital period originating from the ultraluminous X-ray pulsar in M82

Brightman, Murray and Harrison, Fiona A. and Bachetti, Matteo and Xu, Yanjun and Fürst, Felix and Walton, Dominic J. and Ptak, Andrew and Yukita, Mihoko and Zezas, Andreas (2019) A ~60-day super-orbital period originating from the ultraluminous X-ray pulsar in M82. Astrophysical Journal, 873 (2). Art. No. 115. ISSN 1538-4357. doi:10.3847/1538-4357/ab0215.

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Ultraluminous X-ray (ULX) pulsars are a new class of object powered by apparent super-critical accretion onto magnetized neutron stars. Three sources in this class have been identified so far; M82 X-2, NGC 5907 ULX-1, and NGC 7793 P13 have been found to have two properties in common; ~1 s spin periods, and for NGC 5907 ULX-1 and NGC 7793 P13 periodic X-ray flux modulations on timescales of ~60–80 days. M82 X-2 resides in a crowded field that includes the ULX M82 X-1 separated from X-2 by 5'', as well as other bright point sources. A 60 day modulation has been observed from the region, but the origin has been difficult to identify; both M82 X-1 and X-2 have been suggested as the source. In this paper we present the analysis of a systematic monitoring campaign by Chandra, the only X-ray telescope capable of resolving the crowded field. From a simple Lomb–Scargle periodogram analysis and a more sophisticated Gaussian Process analysis we find that only X-2 exhibits a periodic signal around 60 days, supporting previous claims that it is the origin. We also construct a phase-averaged flux profile of the modulations from higher-cadence Swift/XRT data and find that the flux variations in the Chandra data are fully consistent with the flux profile. Since the orbit of the neutron star and its companion is known to be 2.5 days, the ~60 day period must be super-orbital in origin. The flux of the modulations varies by a factor of ~100 from the minimum to the maximum, with no evidence for spectral variations, making the origin difficult to explain.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Brightman, Murray0000-0002-8147-2602
Harrison, Fiona A.0000-0003-2992-8024
Bachetti, Matteo0000-0002-4576-9337
Xu, Yanjun0000-0003-2443-3698
Fürst, Felix0000-0003-0388-0560
Walton, Dominic J.0000-0001-5819-3552
Ptak, Andrew0000-0001-5655-1440
Yukita, Mihoko0000-0001-6366-3459
Zezas, Andreas0000-0001-8952-676X
Additional Information:© 2019 The American Astronomical Society. Received 2017 June 15; revised 2019 January 23; accepted 2019 January 24; published 2019 March 11. We thank the anonymous referees and the statistics referee for their critical reviews and input on this paper, which improved it greatly. We would also like to thank Matthew Graham for valuable insight and discussion. M. Bachetti acknowledges support from the Fulbright Scholar Program and A. Zezas acknowledges funding from the European Research Council under the European Union's Seventh Framework Programme (FP/2007–2013)/ERC grant Agreement No. 617001. This project has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie RISE action, grant agreement No. 691164 (ASTROSTAT). The scientific results reported in this article are based on observations made by the Chandra X-ray Observatory. Support for this work was provided by the National Aeronautics and Space Administration through Chandra Award Number GO6-17080X issued by the Chandra X-ray Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of the National Aeronautics Space Administration under contract NAS8-03060. This research has made use of software provided by the Chandra X-ray Center (CXC) in the application package ciao. We also acknowledge the use of public data from the Swift data archive. Facilities: Chandra (ACIS) - , Swift (XRT). -
Group:Space Radiation Laboratory, Astronomy Department
Funding AgencyGrant Number
Fulbright FoundationUNSPECIFIED
European Research Council (ERC)617001
Marie Curie Fellowship691164
Smithsonian Astrophysical ObservatoryUNSPECIFIED
Subject Keywords:galaxies: individual (M82) – stars: neutron – X-rays: binaries
Issue or Number:2
Record Number:CaltechAUTHORS:20190225-080110226
Persistent URL:
Official Citation:Murray Brightman et al 2019 ApJ 873 115
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:93194
Deposited By: Tony Diaz
Deposited On:25 Feb 2019 16:08
Last Modified:16 Nov 2021 16:56

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