The long-term X-ray flux distribution of Cygnus X-1 using RXTE-ASM and MAXI observations

TL;DR

This study analyzes long-term X-ray flux distributions of Cygnus X-1 using RXTE-ASM and MAXI data, revealing that the flux states follow a log-normal distribution consistent with known spectral states, and discusses the nature of intermediate states.

Contribution

It demonstrates that the long-term flux distribution of Cygnus X-1's states is log-normal, and clarifies the nature of intermediate states as transitions rather than separate states.

Findings

MAXI data components fit by log-normal distributions matching spectral states.

RXTE-ASM data shows mixed distributions, possibly due to intermediate state prevalence.

Long-term flux distributions are consistent with shorter timescale findings.

Abstract

We studied the long term flux distribution of Cygnus X-1 using RXTE-ASM lightcurves in two energy bands B (3-5 keV) & C (5-12.1 keV) as well as MAXI lightcurves in energy bands B (4-10 keV) & C (10-20 keV). The flux histograms were fitted using a two component model. For MAXI data, each of the components is better fitted by a log-normal distribution, rather than a Gaussian one. Their best fit centroids and fraction of time the source spends being in that component are consistent with those of the Hard and Soft spectral states Thus, the long term flux distribution of the states of Cygnus X-1 have a log-normal nature which is the same as that found earlier for much shorter time-scales. For RXTE-ASM data, one component corresponding approximately to the Hard state is better represented by a log-normal but for the other one a Gaussian is preferred and whose centroid is not consistent with the Soft state.This discrepancy, could be due to the larger fraction of the Intermediate state (~11.25%) in the RXTE-ASM data as compared to the MAXI one (~4%). Fitting the flux distribution with three components did not provide an improvement for either RXTE-ASM or MAXI data, suggesting that the data corresponding to the Intermediate state may not represent a separate spectral state, but rather represent transitions between the two states.