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        The Variability Behavior of NGC 925 ULX-3

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        Author
        Earnshaw, Hannah P.
        Brightman, Murray
        Harrison, Fiona A.
        Heida, Marianne
        Jaodand, Amruta
        Middleton, Matthew J.
        Roberts, Timothy P.
        Walton, Dominic J.
        Attention
        2299/25657
        Abstract
        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.
        Publication date
        2022-07-25
        Published in
        The Astrophysical Journal
        Published version
        https://doi.org/10.3847/1538-4357/ac79b0
        Other links
        http://hdl.handle.net/2299/25657
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