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

TL;DR

This study analyzes five years of MAXI X-ray data of Cygnus X-1, revealing state-dependent long-term variability patterns across different energy bands and providing insights into the nature of its spectral states.

Contribution

It presents the first detailed analysis of long-term low-frequency variability of Cygnus X-1 in different spectral states using MAXI data, highlighting state-dependent fractional RMS variations.

Findings

Fractional RMS variation decreases with energy in the low/hard state.

Fractional RMS variation increases with energy in the high/soft state.

In the 10-20 keV band, RMS variation is three times higher in the high/soft state.

Abstract

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.