Journal
PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF JAPAN
Volume 65, Issue 4, Pages -Publisher
OXFORD UNIV PRESS
DOI: 10.1093/pasj/65.4.80
Keywords
accretion, accretion disk; black hole physics; stars: individual (Cygnus X-1); X-ray: binaries
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Funding
- Special Postdoctoral Researchers Program in RIKEN
- JSPS KAKENHI [24740129]
- STFC [ST/K000861/1] Funding Source: UKRI
- Science and Technology Facilities Council [ST/K000861/1] Funding Source: researchfish
- Grants-in-Aid for Scientific Research [23244024, 24740129] Funding Source: KAKEN
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Unified X-ray spectral and timing studies of Cygnus X-1 in the low/hard and hard intermediate state were conducted in a model-independent manner, using broadband Suzaku data acquired on 25 occasions from 2005 to 2009 with a total exposure of similar to 450 ks. The unabsorbed 0.1-500 keV source luminosity changed over 0.8%-2.8% of the Eddington limit for 14.8 solar masses. Variations on short (1-2 seconds) and long (days to months) time scales require at least three separate components: a constant component localized below similar to 2 keV, a broad soft one dominating in the 2-10 keV range, and a hard one mostly seen in the 10-300 keV range. In view of the truncated disk/hot inner-flow picture, these are respectively interpreted as emission from the truncated cool disk, a soft Compton component, and a hard Compton component. Long-term spectral evolution can be produced by the constant disk increasing in temperature and luminosity as the truncation radius decreases. The soft Compton component likewise increases, but the hard Compton does not, so that the spectrum in the hard intermediate state is dominated by the soft Compton component; on the other hand, the hard Compton component dominates the spectrum in the dim low/hard state, probably associated with a variable soft emission providing seed photons for the Comptonization.
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