Kavli Affiliate: Erin Kara
| First 5 Authors: Arkadip Basak, Arkadip Basak, , ,
| Summary:
Cygnus X-1 is a persistent, high-mass black hole X-ray binary (BHXRB) which
in the hard state shows many similar properties to transient BHXRBs, along with
intriguing differences, such as the lack of quasi-periodic oscillations. Here,
we compare for the first time the detailed spectral-timing properties of Cyg
X-1 with a transient BHXRB, MAXI J1820+070, combining data from XMM-Newton and
NICER with contemporaneous INTEGRAL data to study the power spectra, rms
spectra and time-lags over a broad 0.5 – 200 keV range. We select bright hard
state MAXI J1820+070 data with similar power-spectral shapes to the Cyg X-1
data, to compare the source behaviours while accounting for the evolution of
spectral-timing properties, notably the lags, through the hard state. Cyg X-1
shows no evidence for soft lags in the 1 – 10 Hz frequency range where they are
clearly detected for MAXI J1820+070. Furthermore the low-frequency hard lags
and rms-spectra evolve much more strongly during the hard state of Cyg X-1 than
for MAXI J1820+070. We argue that these differences cannot be explained by the
different black hole masses of these systems, but may be related to their
different accretion rates and corresponding locations on the hardness-intensity
diagram. We conjecture that there is a significant luminosity-dependence of
coronal geometry in the hard state of BHXRBs, rather than an intrinsic
difference between Cyg X-1 and transient BHXRBs. This possibility has also been
suggested to explain a common time-lag feature that appears in the hard
intermediate states of Cyg X-1 and transient BHXRBs.
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