Abstract
Abstract 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.
Highlights
Cyg X-1 was the first black hole candidate to be identified, so is one of the best studied
We find that a two zone Comptonisation model gives a better fit to the spectrum than a single zone model, and significantly reduces the derived black hole spin
The previous softest spectra seen from Cyg X-1 by Suzaku was from 2010 (Yamada et al 2013; Gou et al 2014; Tomsick et al 2014), which is observed by the XIS0 operated with 1/4 window mode with 0.3-s burst option, where we reduced the data in same way as Obs A and Obs B
Summary
Cyg X-1 was the first black hole candidate to be identified, so is one of the best studied. While the resulting mass accretion rate onto the black hole is rather stable, its value is close to that of dramatic spectral transition, so the system shows strong spectral variability It switches between the Compton dominated low/hard state towards the disk dominated high/soft state. This could mean that Cyg X-1 never quite makes a full transition, so that the accretion disk never quite reaches down to the last stable circular orbit, and/or that the coronal emission is fed directly from the stellar wind rather than via the accretion disk (Sugimoto et al 2016) This latter idea might explain its rather different X-ray variability power spectra compared to high/soft states with strong tails in the transients (Done & Gierlinski 2005). We find that a two zone Comptonisation model gives a better fit to the spectrum than a single zone model, and significantly reduces the derived black hole spin
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