A New Sample of Transient Ultraluminous X-Ray Sources Serendipitously Discovered by Swift/XRT

Creators: Brightman, Murray; Hameury, Jean-Marie; Lasota, Jean-Pierre; Baldi, Ranieri D.; Bruni, Gabriele; Cann, Jenna M.; Earnshaw, Hannah; Fürst, Felix; Heida, Marianne; Jaodand, Amruta; Lazzarini, Margaret; Middleton, Matthew J.; Walton, Dominic J.; Weaver, Kimberly A.

Abstract

Ultraluminous X-ray sources (ULXs) are our best laboratories for studying extreme super-Eddington accretion. Most studies of these objects are of relatively persistent sources; however, there is growing evidence to suggest a large fraction of these sources are transient. Here we present a sample of five newly reported transient ULXs in the galaxies NGC 4945, NGC 7793, and M81 serendipitously discovered in Swift/XRT observations. Swift monitoring of these sources have provided well-sampled lightcurves, allowing for us to model the lightcurves with the disk-instability model of Hameury & Lasota, which implies durations of 60–400 days and that the mass-accretion rate through the disk is close to or greater than the Eddington rate. Of the three source regions with prior Hubble Space Telescope imaging, color–magnitude diagrams of the potential stellar counterparts show varying ages of the possible stellar counterparts. Our estimation of the rates of these sources in these three galaxies is 0.4–1.3 yr⁻¹. We find that, while persistent ULXs dominate the high end of galaxy luminosity functions, the number of systems that produce ULX luminosities are likely dominated by transient sources.

Additional Information

© 2023. 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. We wish to thank the Swift PI, Brad Cenko, for approving the target of opportunity requests we made to observe these transient sources, 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. We also wish to thank Patrick Slane, Director of the Chandra X-ray Center, for approving the DDT requests to observe Swift J130456.1-493158, Swift J095520.7+690401, and Swift J235749.9-323526, and the Chandra team for carrying out the observations. In addition we wish to thank the NuSTAR PI, Fiona Harrison, for approving the DDT request we made to observe Swift J235749.9-323526 as well as the NuSTAR SOC for carrying out the observation. This work was also supported under NASA Contract No. NNG08FD60C. NuSTAR is a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory and funded by the National Aeronautics and Space Administration. This research has made use of the NuSTAR Data Analysis Software (NuSTARDAS) jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (USA). This research has made use of data obtained from the Chandra Source Catalog, provided by the Chandra X-ray Center (CXC) as part of the Chandra Data Archive. This work was also based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA. This research has made use of data and/or software provided by the High Energy Astrophysics Science Archive Research Center (HEASARC), which is a service of the Astrophysics Science Division at NASA/GSFC. Based on observations made with the NASA/ESA Hubble Space Telescope, and obtained from the Hubble Legacy Archive, which is a collaboration between the Space Telescope Science Institute (STScI/NASA), the Space Telescope European Coordinating Facility (ST-ECF/ESAC/ESA), and the Canadian Astronomy Data Centre (CADC/NRC/CSA). The Hubble Source Catalog can be accessed via doi:10.17909/T97P46, and the specific observations used can be accessed via doi:10.17909/jmva-dc49. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. J.M.C.'s research was supported by an appointment to the NASA Postdoctoral Program at the NASA Goddard Space Flight Center, administered by Oak Ridge Associated Universities under contract with NASA. M.H. acknowledges support from an ESO fellowship and J.-P.L. was supported in part by a grant from the French Space Agency CNES. Facilities: Swift (XRT - , UVOT) - , CXO - , NuSTAR - , XMM - , VLA - Very Large Array. Software: CIAO (Fruscione et al. 2006), XSPEC (Arnaud1996).

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