MAXI observations of long-term variations of Cygnus X-1 in the low/hard   and the high/soft states
  Long-term X-ray variability of the black hole binary, Cygnus X-1, was studied with five years of MAXI data from 2009 to 2014, which include substantial periods of the high/soft state, as well as the low/hard state. In each state, Normalized Power Spectrum densities (NPSDs) were calculated in three energy bands of 2-4 keV, 4-10 keV and 10-20 keV. The NPSDs in a frequency from 1e-7 Hz to 1e-4 Hz are all approximated by a power-law function with an index -1.35 ~ -1.29. The fractional RMS variation ($\eta$), calculated in the above frequency range, was found to show the following three properties; (1) $\eta$ slightly decreases with energy in the low/hard state; (2) $\eta$ increases towards higher energies in the high/soft state; and (3) in the 10-20 keV band, $\eta$ is 3 times higher in the high/soft state than in the low/hard state. These properties were confirmed through studies of intensity-correlated changes of the MAXI spectra. Of these three findings, the first one is consistent with that seen in the short-term variability during the LHS. The latter two can be understood as a result of high variability of the hard-tail component seen in the high/soft state with the above very low frequency range, although the origin of the variability remains inconclusive. 
Juri Sugimoto, Tatehiro Mihara, Shunji Kitamoto, Masaru Matsuoka,   Mutsumi Sugizaki, Hitoshi Negoro, Satoshi Nakahira and K...
astro-ph.HE