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Title:
A Study on the Long-term Spectral Variability of the Black-hole Candidate X-ray Binary Cyg X-1
Authors:
Dong, Ai-Jun; Wang, Jian-Cheng; Xue, Li
Affiliation:
AA(Chuzhou College, Chuzhou 239000), AB(National Astronomical Observatories/Yunnan Observatory, Chinese Academy of Sciences, Kunming 650011), AC(Xiamen University, Xiamen 361005)
Publication:
Chinese Astronomy and Astrophysics, Volume 33, Issue 3, p. 265-278. (ChA&A Homepage)
Publication Date:
07/2009
Origin:
ELSEVIER
DOI:
10.1016/j.chinastron.2009.07.007
Bibliographic Code:
2009ChA&A..33..265D

Abstract

Using the All-Sky Monitor (ASM, 1.5-12-kev) data of Rossi X-ray Timing Explorer (RXTE) from January 1996 to May 2005, we have made a detailed analysis of the correlation between photon-count rate and spectral hardness ratio HR2 (5-12 keV/3-5 keV) of the black-hole candidate X-ray binary Cyg X-1 in 3 energy bands, namely the A-band (1.5-3 keV), B-band (3-5 keV) and Cband (5-12 keV). By the study on the ASM data of 1-day time scale, we find: (1) When Cyg X-1 is in the soft state, the A-band photon-count rate and hardness ratio HR2 exhibit an anticorrelation, but in B-band and C-band there appears the positive correlation. When Cyg X-1 is in hard state, the photon-count rates in the A,B,C bands are all inversely correlated with the hardness ratio HR2; (2) No matter whether Cyg X-1 is in the soft state or the hard state, the hardness ratios HR2 and HR1 are always positively correlated. In addition, we have analyzed the “dwell by dwell” data of the ASM, and obtained the following interesting results: (1) In the period of MJD = 52600-52760 (while Cyg X-1 is in the hard state), the photon-count rates in the A-band and B-band are inversely correlated with HR2, but in the C-band there appears a relatively strong positive correlation; (2) During the hard state, a clear anticorrelation exists between the hardness ratios HR2 and HR1.
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