Encounters
Ginga ○´｜encounter｜ENC｜20260911124749-01-•
The concept of X-ray states was born when Tananbaum et al. (1972) observed a global spectral change in Cyg X–1 in which the soft X-ray flux (2–6 keV) decreased by a factor of 4, the hard flux (10–20 keV) increased by a factor of 2, and the radio counterpart turned on. Thereafter, a similar X-ray transition was seen in A 0620–00 (Coe et al. 1976) and in many other sources as well. The soft state, which was commonly described as ∼1 keV thermal emission, was usually observed when the source was bright, thereby prompting the name “high/soft state”. The hard state, with a typical photon index , was generally seen when the source was faint, hence the name “low/hard state”. In this state, the disk was either not observed above 2 keV, or it appeared much cooler and withdrawn from the BH. An additional X-ray state of BHBs was identified in the *Ginga* era (Miyamoto & Kitamoto 1991; Miyamoto et al. 1993). It was characterized by the appearance of several-Hz X-ray QPOs, a relatively high luminosity (e.g., ), and a spectrum comprised of both a thermal component and a PL component that was steeper () than the hard PL. This state was named the “very high” state.
Source: Remillard & McClintock 2006 ┃ X-Ray Properties… ⚬𓂃
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