The Variability Behavior of NGC 925 ULX-3
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:
-
The Astrophysical Journal
- Pub Date:
- July 2022
- DOI:
- 10.3847/1538-4357/ac79b0
- arXiv:
- arXiv:2206.08397
- Bibcode:
- 2022ApJ...934...42E
- Keywords:
-
- Ultraluminous x-ray sources;
- X-ray sources;
- Neutron stars;
- Accretion;
- 2164;
- 1822;
- 1108;
- 14;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 13 pages, 6 figures, 2 tables. Accepted for publication in ApJ