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From ultraluminous X-ray sources to ultraluminous supersoft sources: NGC 55 ULX, the missing link

From ultraluminous X-ray sources to ultraluminous supersoft sources: NGC 55 ULX, the missing link
From ultraluminous X-ray sources to ultraluminous supersoft sources: NGC 55 ULX, the missing link
In recent work with high-resolution reflection grating spectrometers (RGS) aboard XMM–Newton, Pinto et al. have discovered that two bright and archetypal ultraluminous X-ray sources (ULXs) have strong relativistic winds in agreement with theoretical predictions of high accretion rates. It has been proposed that such winds can become optically thick enough to block and reprocess the disc X-ray photons almost entirely, making the source appear as a soft thermal emitter or ultraluminous supersoft X-ray source (ULS). To test this hypothesis, we have studied a ULX where the wind is strong enough to cause significant absorption of the hard X-ray continuum: NGC 55 ULX. The RGS spectrum of NGC 55 ULX shows a wealth of emission and absorption lines blueshifted by significant fractions of the light speed (0.01–0.20)c indicating the presence of a powerful wind. The wind has a complex dynamical structure with the ionization state increasing with the outflow velocity, which may indicate launching from different regions of the accretion disc. The comparison with other ULXs such as NGC 1313 X-1 and NGC 5408 X-1 suggests that NGC 55 ULX is being observed at higher inclination. The wind partly absorbs the source flux above 1 keV, generating a spectral drop similar to that observed in ULSs. The softening of the spectrum at lower (∼ Eddington) luminosities and the detection of a soft lag agree with the scenario of wind clumps crossing the line of sight, partly absorbing and reprocessing the hard X-rays from the innermost region.
0035-8711
2865-2883
Pinto, C.
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Alston, W.
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Soria, R.
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Middleton, M.J.
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Walton, D.J.
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Sutton, A.D.
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Fabian, A.C.
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Earnshaw, H.
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Urquhart, R.
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Kara, E.
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Roberts, T.P.
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Pinto, C.
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Alston, W.
42b16be8-0f37-45b4-b722-c408586a9448
Soria, R.
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Middleton, M.J.
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Walton, D.J.
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Sutton, A.D.
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Fabian, A.C.
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Earnshaw, H.
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Urquhart, R.
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Kara, E.
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Roberts, T.P.
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Pinto, C., Alston, W., Soria, R., Middleton, M.J., Walton, D.J., Sutton, A.D., Fabian, A.C., Earnshaw, H., Urquhart, R., Kara, E. and Roberts, T.P. (2017) From ultraluminous X-ray sources to ultraluminous supersoft sources: NGC 55 ULX, the missing link. Monthly Notices of the Royal Astronomical Society, 468 (3), 2865-2883.

Record type: Article

Abstract

In recent work with high-resolution reflection grating spectrometers (RGS) aboard XMM–Newton, Pinto et al. have discovered that two bright and archetypal ultraluminous X-ray sources (ULXs) have strong relativistic winds in agreement with theoretical predictions of high accretion rates. It has been proposed that such winds can become optically thick enough to block and reprocess the disc X-ray photons almost entirely, making the source appear as a soft thermal emitter or ultraluminous supersoft X-ray source (ULS). To test this hypothesis, we have studied a ULX where the wind is strong enough to cause significant absorption of the hard X-ray continuum: NGC 55 ULX. The RGS spectrum of NGC 55 ULX shows a wealth of emission and absorption lines blueshifted by significant fractions of the light speed (0.01–0.20)c indicating the presence of a powerful wind. The wind has a complex dynamical structure with the ionization state increasing with the outflow velocity, which may indicate launching from different regions of the accretion disc. The comparison with other ULXs such as NGC 1313 X-1 and NGC 5408 X-1 suggests that NGC 55 ULX is being observed at higher inclination. The wind partly absorbs the source flux above 1 keV, generating a spectral drop similar to that observed in ULSs. The softening of the spectrum at lower (∼ Eddington) luminosities and the detection of a soft lag agree with the scenario of wind clumps crossing the line of sight, partly absorbing and reprocessing the hard X-rays from the innermost region.

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From ultraluminous X-ray sources to ultraluminous - Accepted Manuscript
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Accepted/In Press date: 13 March 2017
e-pub ahead of print date: 13 March 2017
Published date: July 2017
Organisations: Astronomy Group

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Local EPrints ID: 412030
URI: http://eprints.soton.ac.uk/id/eprint/412030
ISSN: 0035-8711
PURE UUID: 67e7f433-f1b4-478f-adaf-a5fd5bc1e023

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Date deposited: 05 Jul 2017 16:31
Last modified: 06 Oct 2020 19:39

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Contributors

Author: C. Pinto
Author: W. Alston
Author: R. Soria
Author: M.J. Middleton
Author: D.J. Walton
Author: A.D. Sutton
Author: A.C. Fabian
Author: H. Earnshaw
Author: R. Urquhart
Author: E. Kara
Author: T.P. Roberts

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