The (re)appearance of NGC 925 ULX-3, a new transient ULX

Earnshaw, Hannah P.; Heida, Marianne; Brightman, Murray; Fürst, Felix; Harrison, Fiona A.; Jaodand, Amruta; Middleton, Matthew J.; Roberts, Timothy P.; Sathyaprakash, Rajath; Stern, Daniel; Walton, Dominic J.

Abstract

We report the discovery of a third ULX in NGC 925 (ULX-3), detected in 2017 November by Chandra at a luminosity of L X = (7.8 ± 0.8) × 1039 erg s−1. Examination of archival data for NGC 925 reveals that ULX-3 was detected by Swift at a similarly high luminosity in 2011, as well as by XMM-Newton in 2017 January at a much lower luminosity of L X = (3.8 ± 0.5) × 1038 erg s−1. With an additional Chandra nondetection in 2005, this object demonstrates a high dynamic range of flux of factor gsim26. In its high-luminosity detections, ULX-3 exhibits a hard power-law spectrum with Γ = 1.6 ± 0.1, whereas the XMM-Newton detection is slightly softer, with ${\rm{\Gamma }}={1.8}_{-0.1}^{+0.2}$, and is also well-fitted with a broadened disk model. The long-term light curve is sparsely covered and could be consistent either with the propeller effect or with a large-amplitude superorbital period, both of which are seen in ULXs, in particular those with neutron star accretors. Further systematic monitoring of ULX-3 will allow us to determine the mechanism by which ULX-3 undergoes its extreme variability and to better understand the accretion processes of ULXs.

Citation

Earnshaw, H. P., Heida, M., Brightman, M., Fürst, F., Harrison, F. A., Jaodand, A., …Walton, D. J. (2020). The (re)appearance of NGC 925 ULX-3, a new transient ULX. Astrophysical Journal, 891(2), Article 153. https://doi.org/10.3847/1538-4357/ab77b8

• Journal Article Type: Article
• Acceptance Date: Feb 17, 2020
• Online Publication Date: Mar 16, 2020
• Publication Date: Mar 10, 2020
• Deposit Date: Feb 28, 2020
• Publicly Available Date: Mar 24, 2020
• Journal: Astrophysical Journal
• Print ISSN: 0004-637X
• Publisher: American Astronomical Society
• Peer Reviewed: Peer Reviewed
• Volume: 891
• Issue: 2
• Article Number: 153
• DOI: https://doi.org/10.3847/1538-4357/ab77b8
• Related Public URLs: https://arxiv.org/pdf/2001.00642.pdf