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Title: New flaring of an ultraluminous X-ray source in NGC 1365
Authors: Soria, R.
Baldi, A.
Risaliti, G.
Fabbiano, G.
Zezas, A.
Soria, R.
Risaliti, G.
King, A.
La Parola V.
First Published: 21-Aug-2007
Publisher: Royal Astronomical Society (RAS)
Citation: Monthly Notices of the Royal Astronomical Society, 2007, 379 (4), pp. 1313-1324
Abstract: We have studied a highly variable ultraluminous X-ray source (ULX) in the Fornax galaxy NGC 1365, with a series of 12 Chandra and XMM–Newton observations between 2002 and 2006. In 2006 April, the source peaked at a luminosity ≈3 × 1040 erg s−1 in the 0.3–10 keV band (similar to the maximum luminosity found by ASCA in 1995), and declined on an e-folding time-scale ≈3 d. The X-ray spectrum is always dominated by a broad power-law-like component. When the source is seen at X-ray luminosities ≈1040 erg s−1, an additional soft thermal component (which we interpret as emission from the accretion disc) contributes ≈1/4 of the X-ray flux; when the luminosity is higher, ≈3 × 1040 erg s−1, the thermal component is not detected and must contribute <10 per cent of the flux. At the beginning of the decline, ionized absorption is detected around ∼0.5–2 keV; it is a possible signature of a massive outflow. The power law is always hard, with a photon index Γ≈ 1.7 (and even flatter at times), as is generally the case with bright ULXs. We speculate that this source and perhaps most other bright ULXs are in a high/hard state: as the accretion rate increases well above the Eddington limit, more and more power is extracted from the inner region of the inflow through non-radiative channels, and is used to power a Comptonizing corona, jet or wind. The observed thermal component comes from the standard outer disc; the transition radius between outer standard disc and Comptonizing inner region moves further out and to lower disc temperatures as the accretion rate increases. This produces the observed appearance of a large, cool disc. Based on X-ray luminosity and spectral arguments, we suggest that this accreting black hole has a likely mass ∼50–150 M⊙ (even without accounting for possible beaming).
DOI Link: 10.1111/j.1365-2966.2007.12031.x
ISSN: 0035-8711
eISSN: 1365-2966
Version: Publisher Version
Status: Peer-reviewed
Type: Journal Article
Rights: This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2007 the authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved. Deposited with reference to the publisher’s archiving policy available on the SHERPA/RoMEO website.
Appears in Collections:Published Articles, Dept. of Physics and Astronomy

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