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The chameleon on the branches: spectral state transition and dips in NGC 247 ULX-1

D'a, ì A.; Pinto, C.; Del Santo, M.; Pintore, F.; Soria, R.; Robba, A.; Ambrosi, E.; Alston, W.; Barret, D.; Fabian, A.C.; Fürst, F.; Kara, E.; Kosec, P.; Middleton, M.; Roberts, T.; Rodriguez-Castillo, G.; Walton, D.J.

The chameleon on the branches: spectral state transition and dips in NGC 247 ULX-1 Thumbnail


ì A. D'a

C. Pinto

M. Del Santo

F. Pintore

R. Soria

A. Robba

E. Ambrosi

W. Alston

D. Barret

A.C. Fabian

F. Fürst

E. Kara

P. Kosec

M. Middleton

G. Rodriguez-Castillo

D.J. Walton


Soft Ultra-Luminous X-ray (ULXs) sources are a subclass of the ULXs that can switch from a supersoft spectral state, where most of the luminosity is emitted below 1 keV, to a soft spectral state with significant emission above 1 keV. In a few systems, dips have been observed. The mechanism behind this state transition and the dips nature are still debated. To investigate these issues, we obtained a long XMM–Newton monitoring campaign of a member of this class, NGC 247 ULX-1. We computed the hardness–intensity diagram for the whole data set and identified two different branches: the normal branch and the dipping branch, which we study with four and three hardness–intensity resolved spectra, respectively. All seven spectra are well described by two thermal components: a colder (kTbb ∼ 0.1–0.2 keV) blackbody, interpreted as emission from the photosphere of a radiatively driven wind, and a hotter (kTdisc ∼ 0.6 keV) multicolour disc blackbody, likely due to reprocessing of radiation emitted from the innermost regions. In addition, a complex pattern of emission and absorption lines has been taken into account based on previous high-resolution spectroscopic results. We studied the evolution of spectral parameters and flux of the two thermal components along the two branches and discuss two scenarios possibly connecting the state transition and the dipping phenomenon. One is based on geometrical occultation of the emitting regions, the other invokes the onset of a propeller effect.


D'a, ì. A., Pinto, C., Del Santo, M., Pintore, F., Soria, R., Robba, A., …Walton, D. (2021). The chameleon on the branches: spectral state transition and dips in NGC 247 ULX-1. Monthly Notices of the Royal Astronomical Society, 507(4), 5567-5579.

Journal Article Type Article
Acceptance Date Aug 16, 2021
Online Publication Date Aug 30, 2021
Publication Date 2021
Deposit Date Oct 12, 2021
Publicly Available Date Nov 24, 2021
Journal Monthly Notices of the Royal Astronomical Society
Print ISSN 0035-8711
Electronic ISSN 1365-2966
Publisher Royal Astronomical Society
Peer Reviewed Peer Reviewed
Volume 507
Issue 4
Pages 5567-5579
Related Public URLs


Published Journal Article (2.5 Mb)

Copyright Statement
This article has been accepted for publication in Monthly notices of the Royal Astronomical Society. ©: 2021 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

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