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Probing the hard and intermediate states of X-ray binaries using short time-scale variability
- Publication Year :
- 2016
-
Abstract
- Below an accretion rate of approximately a few per cent of the Eddington accretion rate, X-ray binary systems are not usually found in the soft spectral state. However, at accretion rates a factor of a few lower still, in the hard state, there is another spectral transition which is well observed but not well understood. Below ~0.5-1 per cent of the Eddington accretion rate (m_crit), the spectral index hardens with increasing accretion rate, but above m_crit, although still in the hard state, the spectral index softens with increasing accretion rate. Here we use a combination of X-ray spectral fitting and a study of short time-scale spectral variability to examine the behaviour of three well-known X-ray binaries: Cygnus X-1, GX 339-4 and XTE J1118+480. In Cygnus X-1 we find separate hard and soft continuum components, and show using root-mean-square (rms) spectra that the soft component dominates the variability. The spectral transition at m_crit is clearly present in the hard-state hardness-intensity diagrams of Cygnus X-1. Above m_crit, GX 339-4 shows similar softer-when-brighter behaviour at both long and short time-scales. Similarly, XTE J1118+480, which remains well below m_crit, is harder-when-brighter behaviour on all time-scales. We interpret these results in terms of two continuum components: a hard power-law which dominates the spectra when the accretion rate is low, probably arising from Comptonisation of cyclo-synchrotron photons from the corona, and a soft power-law which dominates at higher accretion rates, arising from Comptonisation of seed photons from the accretion disc.<br />18 pages, 13 figures; accepted for publication in Monthly Notices of the Royal Astronomical Society, 2016 February 22
- Subjects :
- Physics
High Energy Astrophysical Phenomena (astro-ph.HE)
Spectral index
Photon
010308 nuclear & particles physics
Astrophysics::High Energy Astrophysical Phenomena
X-ray
Astronomy
Binary number
FOS: Physical sciences
Astronomy and Astrophysics
Astrophysics
Astrophysics::Cosmology and Extragalactic Astrophysics
01 natural sciences
Accretion (astrophysics)
Spectral line
Accretion rate
Space and Planetary Science
0103 physical sciences
Spectral fitting
Astrophysics - High Energy Astrophysical Phenomena
010303 astronomy & astrophysics
Astrophysics::Galaxy Astrophysics
Subjects
Details
- Language :
- English
- Database :
- OpenAIRE
- Accession number :
- edsair.doi.dedup.....b7226b64ea8b9b2eb2cc0fbdbfcdb303