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Black hole spin of Cygnus X-1 determined from the softest state ever observed.

Kawano, Takafumi and Done, Chris and Yamada, Shin’ya and Takahashi, Hiromitsu and Axelsson, Magnus and Fukazawa, Yasushi (2017) 'Black hole spin of Cygnus X-1 determined from the softest state ever observed.', Publications of the Astronomical Society of Japan., 69 (2). p. 36.


We show the softest ever spectrum from Cyg X-1, detected in 2013 with Suzaku. This has the weakest high-energy Compton tail ever seen from this object, so should give the cleanest view of the underlying disk spectrum, and hence the best determination of black hole spin from disk continuum fitting. Using the standard model of a disk with simple non-thermal Comptonization to produce the weak high-energy tail gives a high-spin black hole. However, we get a significantly better fit by including an additional, low-temperature thermal Comptonization component, which allows a much lower black hole spin. Corroboration of the existence of an additional Compton component comes from the frequency-dependent hard lags seen in the rapid variability in archival high/soft state data. These cannot be explained if the continuum is a single non-thermal Comptonization component, but are instead consistent with a radially stratified, multi-zone Comptonization spectrum, where the spectrum is softer further from the black hole. A complex multi-zone Comptonization continuum is required to explain both spectra and timing together, and this has an impact on the derived black hole spin.

Item Type:Article
Full text:(AM) Accepted Manuscript
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Publisher statement:This is a pre-copyedited, author-produced PDF of an article accepted for publication in Publications of the Astronomical Society of Japan following peer review. The version of record Kawano, Takafumi, Done, Chris, Yamada, Shin’ya, Takahashi, Hiromitsu, Axelsson, Magnus & Fukazawa, Yasushi (2017). Black hole spin of Cygnus X-1 determined from the softest state ever observed. Publications of the Astronomical Society of Japan 69(2): 36 is available online at:
Date accepted:19 January 2017
Date deposited:18 July 2017
Date of first online publication:01 March 2017
Date first made open access:01 March 2018

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