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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.

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


Hannah P. Earnshaw

Marianne Heida

Murray Brightman

Felix Fürst

Fiona A. Harrison

Amruta Jaodand

Matthew J. Middleton

Rajath Sathyaprakash

Daniel Stern

Dominic J. Walton


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.


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.

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
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Published Journal Article (1.7 Mb)

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© 2020. The American Astronomical Society. All rights reserved.

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