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The Variability Behavior of NGC 925 ULX-3

Earnshaw, Hannah P. and Brightman, Murray and Harrison, Fiona A. and Heida, Marianne and Jaodand, Amruta and Middleton, Matthew J. and Roberts, Timothy P. and Walton, Dominic J. (2022) The Variability Behavior of NGC 925 ULX-3. Astrophysical Journal, 934 (1). Art. No. 42. ISSN 0004-637X. doi:10.3847/1538-4357/ac79b0.

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We report the results of a 2019–2021 monitoring campaign with Swift and associated target-of-opportunity observations with XMM-Newton and NuSTAR, examining the spectral and timing behavior of the highly variable ultraluminous X-ray source (ULX) NGC 925 ULX-3. We find that the source exhibits a 127–128-day periodicity, with fluxes typically ranging from 1 × 10⁻¹³ to 8 × 10⁻¹³ erg s⁻¹ cm⁻². We do not find strong evidence for a change in period over the time that NGC 925 ULX-3 has been observed, although the source may have been in a much lower flux state when first observed with Chandra in 2005. We do not detect pulsations, and we place an upper limit on the pulsed fraction of ∼40% in the XMM-Newton band, consistent with some previous pulsation detections at low energies in other ULXs. The source exhibits a typical ULX spectrum that turns over in the NuSTAR band and can be fitted using two thermal components. These components have a high temperature ratio that may indicate the lack of extreme inner disk truncation by a magnetar-level magnetic field. We examine the implications for a number of different models for superorbital periods in ULXs, finding that a neutron star with a magnetic field of ∼10¹² G may be plausible for this source. The future detection of pulsations from this source would allow for the further testing and constraining of such models.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Earnshaw, Hannah P.0000-0001-5857-5622
Brightman, Murray0000-0002-8147-2602
Harrison, Fiona A.0000-0003-2992-8024
Heida, Marianne0000-0002-1082-7496
Jaodand, Amruta0000-0002-3850-6651
Middleton, Matthew J.0000-0002-8183-2970
Roberts, Timothy P.0000-0001-8252-6337
Walton, Dominic J.0000-0001-5819-3552
Additional Information:© 2022. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Received 2022 May 10; revised 2022 June 14; accepted 2022 June 15; published 2022 July 25. We thank our anonymous referee for useful suggestions to improve this paper. We wish to thank Dan Stern for useful comments on this work. This work was supported by NASA grants 80NSSC20K1496 and 80NSSC21K0873, as well as by NASA contract NNG08FD60C. T.P.R. acknowledges funding from STFC consolidated grant ST/000244/1. We wish to thank the Swift PI, Brad Cenko, for approving the target-of-opportunity requests we made to monitor NGC 925. We also wish to thank the NuSTAR PI, Fiona Harrison, for approving the DDT request to observe NGC 925 ULX-3 in 2019 December. The scientific results reported in this article are based on observations made by XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA; observations by the NuSTAR mission, a project led by the California Institute of Technology, managed by JPL, and funded by NASA; and observations by the Swift mission, as well as public data from the Swift data archive provided by the UK Swift Science Data Centre at the University of Leicester. We thank the observing teams for these missions for carrying out these observations. Facilities: XMM - Newton X-Ray Multimirror Mission satellite, CXO - Chandra X-ray Observatory satellite, Swift(XRT) - Swift Gamma-Ray Burst Mission, NuSTAR - The NuSTAR (Nuclear Spectroscopic Telescope Array) mission Software: astropy (Astropy Collaboration et al. 2013), CIAO (Fruscione et al. 2006), FTOOLS (Nasa High Energy Astrophysics Science Archive Research Center (Heasarc), 2014), XMM-Newton SAS, PIMMS, HENDRICS (Bachetti 2018; Huppenkothen et al. 2019).
Group:Space Radiation Laboratory, NuSTAR, Astronomy Department
Funding AgencyGrant Number
Science and Technology Facilities Council (STFC)ST/000244/1
Subject Keywords:Ultraluminous x-ray sources; X-ray sources; Neutron stars; Accretion
Issue or Number:1
Classification Code:Unified Astronomy Thesaurus concepts: Ultraluminous x-ray sources (2164); X-ray sources (1822); Neutron stars (1108); Accretion (14)
Record Number:CaltechAUTHORS:20220726-997967000
Persistent URL:
Official Citation:Hannah P. Earnshaw et al 2022 ApJ 934 42
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:115861
Deposited By: George Porter
Deposited On:27 Jul 2022 20:49
Last Modified:27 Jul 2022 20:49

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