Probing periodic and aperiodic variability of X-ray sources in M31, M81, and Centaurus A with <i>Chandra</i>

ABSTRACT Based on archival Chandra observations, we present a systematic timing survey of several hundred X-ray sources in M31, M81, and Centaurus A, mostly low-mass X-ray binaries (LXMBs), focusing on searching and characterizing aperiodic and periodic variability within single observation. We identify flares in 24 sources in M31, 5 in M81, and 26 in Cen A; several display recurrent events. Flare durations span from tens of seconds to a few $10^{4}$ s, with peak luminosities of $10^{37}$–$10^{40}\ \mathrm{erg\ s^{-1}}$ and low flare duty cycles of $4.9\times 10^{-6}$–$3.6\times 10^{-2}$. Dipping events are found in 8 sources in M31, 1 in M81, and 5 in Cen A, including two repeaters. On multi-epoch baselines, the standard deviation of the source luminosity correlates linearly with the mean luminosity, with a coefficient of 0.49 (M31), 0.30 (M81), and 0.67 (Cen A), indicating galaxy-to-galaxy diversity. No statistically significant periodic signals are detected in M81 or Cen A, which, along with several periodic signals previously found among the M31 sources, can be understood considering a joint effect of our detection sensitivity and intrinsic distributions of the orbital period and X-ray luminosity of LMXBs. The ensemble of short-duty-cycle flares, a mix of recurrent and isolated dips, and galaxy-dependent rms–flux factor, supports a picture in which stochastic accretion-rate fluctuations modulate luminosity on $\sim$10–$10^{4}$ s. Conducted at known distances and across distinct host environments, this extragalactic survey provides uniform flare/dip samples and rms–flux scalings for bulge-dominated fields, offering empirical constraints for accretion physics and illustrating the promise of timing analyses in external galaxies using the Chandra archive.