The aperiodic broad-band X-ray variability of Cygnus X-3

TL;DR

This study analyzes the broad-band X-ray variability of Cygnus X-3 across multiple time-scales, revealing power-law behaviors, state-dependent features, and possible tidal resonance effects in the accretion disc.

Contribution

It provides the first comprehensive comparison of the power density spectrum across all states of Cygnus X-3 over a wide frequency range, including long-term variability analysis.

Findings

Power-law PDS with index ~-2 in soft states

Variability dominated by state transitions below ~10^-7 Hz

Evidence for a tidal resonance at orbital time-scales, constraining mass ratio

Abstract

We study the soft X-ray variability of Cygnus X-3. By combining data from the All-Sky Monitor and Proportional Counter Array instruments on the RXTE satellite with EXOSAT Medium Energy (ME) detector observations, we are able to analyse the power density spectrum (PDS) of the source from 10^(-9) - 0.1 Hz, thus covering time-scales from seconds to years. As the data on the longer time-scales are unevenly sampled, we combine traditional power spectral techniques with simulations to analyse the variability in this range. The PDS at higher frequencies (>10^-3 Hz) are for the first time compared for all states of this source. We find that it is for all states well-described by a power-law, with index ~-2 in the soft states and a tendency for a less steep power-law in the hard state. At longer time-scales, we study the effect of the state transitions on the PDS, and find that the variability below ~10^(-7) Hz is dominated by the transitions. Furthermore, we find no correlation between the length of a high/soft state episode and the time since the previous high/soft state. On intermediate time-scales we find evidence for a break in the PDS at time-scales of the order of the orbital period. This may be interpreted as evidence for the existence of a tidal resonance in the accretion disc around the compact object, and constraining the mass ratio to M2/M1<0.3.