The Variability Behavior of NGC 925 ULX-3

Hannah P. Earnshaw, Murray Brightman, Fiona A. Harrison, Marianne Heida, Amruta Jaodand, Matthew J. Middleton, Timothy P. Roberts, Dominic J. Walton

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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.
Original languageEnglish
Number of pages10
JournalThe Astrophysical Journal
Issue number1
Publication statusPublished - 25 Jul 2022


  • 330
  • High-Energy Phenomena and Fundamental Physics


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