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Very fast X-ray spectral variability in Cygnus X-1: origin of the hard- and soft-state emission components

Very fast X-ray spectral variability in Cygnus X-1: origin of the hard- and soft-state emission components
Very fast X-ray spectral variability in Cygnus X-1: origin of the hard- and soft-state emission components
The way in which the X-ray photon index, ?, varies as a function of count rate is a strong diagnostic of the emission processes and emission geometry around accreting compact objects. Here we present the results from a study using a new, and simple, method designed to improve sensitivity to the measurement of the variability of ? on very short time-scales.

We have measured ? in ?2 million spectra, extracted from observations with a variety of different accretion rates and spectral states, on time-scales as short as 16 ms for the high-mass X-ray binary Cygnus X-1 (and in a smaller number of spectra for the low-mass X-ray binary GX 339-4), and have cross-correlated these measurements with the source count rate. In the soft-state cross-correlation functions (CCFs), we find a positive peak at zero lag, stronger and narrower in the softer observations. Assuming that the X-rays are produced by Compton scattering of soft seed photons by high-energy electrons in a corona, these results are consistent with Compton cooling of the corona by seed photons from the inner edge of the accretion disc, the truncation radius of which increases with increasing hardness ratio.

The CCFs produced from the hard-state observations, however, show an anti-correlation which is most easily explained by variation in the energy of the electrons in the corona rather than in variation of the seed photon flux. The hard-state CCFs can be decomposed into a narrow anti-correlation at zero lag, which we tentatively associate with the effects of self-Comptonization of cyclo-synchrotron seed photons in either a hot, optically thin accretion flow or the base of the jet, and a second, asymmetric component which we suggest is produced as a consequence of a lag between the soft and hard X-ray emission. The lag may be caused by a radial temperature/energy gradient in the Comptonizing electrons combined with the inward propagation of accretion rate perturbations
0035-8711
574-584
Skipper, C.J.
82e4af5e-77de-4828-a52b-b79e90812335
McHardy, I.M.
4f215137-9cc4-4a08-982e-772a0b24c17e
Maccarone, T.J.
5d8ee27f-a174-424c-b3b7-a6739bfeea50
Skipper, C.J.
82e4af5e-77de-4828-a52b-b79e90812335
McHardy, I.M.
4f215137-9cc4-4a08-982e-772a0b24c17e
Maccarone, T.J.
5d8ee27f-a174-424c-b3b7-a6739bfeea50

Skipper, C.J., McHardy, I.M. and Maccarone, T.J. (2013) Very fast X-ray spectral variability in Cygnus X-1: origin of the hard- and soft-state emission components. Monthly Notices of the Royal Astronomical Society, 434 (1), 574-584. (doi:10.1093/mnras/stt1044).

Record type: Article

Abstract

The way in which the X-ray photon index, ?, varies as a function of count rate is a strong diagnostic of the emission processes and emission geometry around accreting compact objects. Here we present the results from a study using a new, and simple, method designed to improve sensitivity to the measurement of the variability of ? on very short time-scales.

We have measured ? in ?2 million spectra, extracted from observations with a variety of different accretion rates and spectral states, on time-scales as short as 16 ms for the high-mass X-ray binary Cygnus X-1 (and in a smaller number of spectra for the low-mass X-ray binary GX 339-4), and have cross-correlated these measurements with the source count rate. In the soft-state cross-correlation functions (CCFs), we find a positive peak at zero lag, stronger and narrower in the softer observations. Assuming that the X-rays are produced by Compton scattering of soft seed photons by high-energy electrons in a corona, these results are consistent with Compton cooling of the corona by seed photons from the inner edge of the accretion disc, the truncation radius of which increases with increasing hardness ratio.

The CCFs produced from the hard-state observations, however, show an anti-correlation which is most easily explained by variation in the energy of the electrons in the corona rather than in variation of the seed photon flux. The hard-state CCFs can be decomposed into a narrow anti-correlation at zero lag, which we tentatively associate with the effects of self-Comptonization of cyclo-synchrotron seed photons in either a hot, optically thin accretion flow or the base of the jet, and a second, asymmetric component which we suggest is produced as a consequence of a lag between the soft and hard X-ray emission. The lag may be caused by a radial temperature/energy gradient in the Comptonizing electrons combined with the inward propagation of accretion rate perturbations

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Published date: 1 September 2013
Organisations: Physics & Astronomy, Electronics & Computer Science

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Local EPrints ID: 369467
URI: http://eprints.soton.ac.uk/id/eprint/369467
ISSN: 0035-8711
PURE UUID: e35de684-a45f-4bfe-a2bf-7d179c380822

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Date deposited: 26 Sep 2014 14:05
Last modified: 25 Nov 2019 20:39

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