A Systematic Chandra study of Sgr A$^{\star}$: II. X-ray flare statistics

TL;DR

This study analyzes X-ray flares from Sgr A* observed by Chandra over 13 years, revealing their statistical properties and challenging existing models of flare mechanisms.

Contribution

It provides detailed distributions of flare fluence and duration, accounts for detection biases, and examines spectral and temporal characteristics, offering new insights into flare physics.

Findings

Fluence follows a power-law distribution with slope ~1.73.

Durations are log-normally distributed with mean log(τ)=3.39.

No significant correlation between fluence and duration.

Abstract

The routinely flaring events from \sgras\ trace dynamic, high-energy processes in the immediate vicinity of the supermassive black hole. We statistically study temporal and spectral properties, as well as fluence and duration distributions, of the flares detected by the \chandra\ X-ray Observatory from 1999 to 2012. The detection incompleteness and bias are carefully accounted for in determining these distributions. We find that the fluence distribution can be well characterized by a power-law with a slope of $1.73^{+0.20}_{-0.19}$, while the durations ($\tau$ in seconds) by a log-normal function with a mean $\log(\tau)=3.39^{+0.27}_{-0.24}$ and an intrinsic dispersion $\sigma=0.28^{+0.08}_{-0.06}$. No significant correlation between the fluence and duration is detected. The apparent positive correlation, as reported previously, is mainly due to the detection bias (i.e., weak flares can be detected only when their durations are short). These results indicate that the simple self-organized criticality model has difficulties in explaining these flares. We further find that bright flares usually have asymmetric lightcurves with no statistically evident difference/preference between the rising and decaying phases in terms of their spectral/timing properties. Our spectral analysis shows that although a power-law model with a photon index of $2.0\pm0.4$ gives a satisfactory fit to the joint spectra of strong and weak flares, there is weak evidence for a softer spectrum of weaker flares. This work demonstrates the potential to use statistical properties of X-ray flares to probe their trigger and emission mechanisms, as well as the radiation propagation around the black hole.