A Systematic Chandra study of Sgr A$^{\star}$: I. X-ray flare detection
  Daily X-ray flaring represents an enigmatic phenomenon of Sgr A$^{\star}$ ---
the supermassive black hole at the center of our Galaxy. We report initial
results from a systematic X-ray study of this phenomenon, based on extensive
{\it Chandra} observations obtained from 1999 to 2012, totaling about 4.5 Ms.
We detect flares, using a combination of the maximum likelihood and Markov
Chain Monte Carlo methods, which allow for a direct accounting for the pile-up
effect in the modeling of the flare lightcurves and an optimal use of the data,
as well as the measurements of flare parameters, including their uncertainties.
A total of 82 flares are detected. About one third of them are relatively
faint, which were not detected previously. The observation-to-observation
variation of the quiescent emission has an average root-mean-square of
$6\%-14\%$, including the Poisson statistical fluctuation of faint flares below
our detection limits. We find no significant long-term variation in the
quiescent emission and the flare rate over the 14 years. In particular, we see
no evidence of changing quiescent emission and flare rate around the pericenter
passage of the S2 star around 2002. We show clear evidence of a short-term
clustering for the ACIS-S/HETG 0th-order flares on time scale of $20-70$ ks. We
further conduct detailed simulations to characterize the detection
incompleteness and bias, which is critical to a comprehensive follow-up
statistical analysis of flare properties. These studies together will help to
establish Sgr A$^{\star}$ as a unique laboratory to understand the astrophysics
of prevailing low-luminosity black holes in the Universe.
