MAXI observations of long-term variations of Cygnus X-1 in the low/hard and the high/soft states

Abstract

The 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 for frequencies from 10-7 Hz to 10-4 Hz are all approximated by a power-law function with an index -1.35-1.29. The fractional RMS variation η, calculated in the above frequency range, was found to show the following three properties: (1) η slightly decreases with energy in the low/hard state; (2) η increases towards higher energies in the high/soft state; and (3) in the 10-20 keV band, η is three 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 low/hard state. 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.