Ruolan Jin (NTHU) ; Albert Kong(NTHU)
We present Chandra X-ray observations of the nearby spiral galaxy NGC 7331. Fifty-five X-ray point sources with source significance larger than 3 are found within the optical D25 region according to seven Chandra ACIS-S observations taken between 2001 and 2016. The detection limit of our sample is 3.59x10^37erg s^-1 in the 0.5 - 7.0 keV energy range. Twenty of our detected X-ray sources are variables and 3 of the variables are transients. We also find 9 new ultra-luminous X-ray sources (the maximum detected luminosities large than 1x10^39erg s-1) comparing to previous study. Some of the sources possess a bimodal luminosity-hardness ratio (HR) feature, which is often observed among X-ray binaries. We also performed spectral fits for 8 sources with high signal-to-noise ratios as well as the supernova 2014C. The cumulative X-ray luminosity functions (XLF) of point sources can be well described with a power-law with a slope of around -0.99 by using PyMc3, a Bayesian inference based package, in three different ways: pooling, unpooling and hierarchical. We conclude that neither the variability of the X-ray sources nor the bright outliers has much impact on the XLF of NGC 7331 in fitting with a power-law, especially for a hierarchical method.