A Multi-epoch X-Ray Study of the Spiral Galaxy NGC 7331

Abstract

X-ray point sources in galaxies are dominated by X-ray binaries (XRBs) that are variables or transients and whether their variability would alter the X-ray luminosity functions (XLF) is still in debate.Here we report on NGC 7331 as an example to test this with 7 Chandra observations. Their detection limit is 7×10 37 erg s −1 in the 0.3 -8.0 keV energy range by assuming a power-law spectral model with a photon index of 1.7. We detected 55 X-ray sources. Thirteen of them are variables for which 3 of them are transients and some of the sources possess a bimodal luminosity-hardness ratio feature, which is often observed among X-ray binaries. Nine more ultra-luminous X-ray sources are found comparing to previous studies and 8 are likely to be low-mass or high-mass XRBs. Twenty-one optical counterpart candidates are found based on the Hubble Space Telescope images, but we cannot rule out the possibility of positional coincidence. The spectral analysis of SN 2014C shows a trend of increasing soft photons and decreasing hydrogen column densities as its outer shell expands. We fit the 7 incompleteness-corrected XLFs to both a power-law (PL) and a power-law with an exponential cut-off (PLC) model using Bayesian method, which is the first time used in XLF fitting. The hierarchical PLC model can describe the XLF of NGC 7331 best with a slope of ∼ 0.5 and a luminosity cut-off around 8×10 38 erg s −1 . This study proves that multi-epoch observations decrease the deviation due to the variable luminous sources in XLFs.

Seven Years of Coordinated Chandra–NuSTAR Observations of SN 2014C Unfold the Extreme Mass-loss History of Its Stellar Progenitor

“…The inferred best-fitting parameters as well as absorbed and unabsorbed fluxes are reported in Appendix B, Table 3. Our measurements are consistent with the X-ray analysis presented in Jin & Kong (2019).…”

[CII] map of the molecular ring and arms of the spiral galaxy NGC 7331

“…Initially discovered by William Herschel in 1784, NGC 7331 has been widely studied over the past 200 years. Located at a distance of 14.7 Mpc (Freedman et al 2001), NGC 7331 has been included in a large number of galaxy surveys and has been mapped at wavelengths spanning Xrays (Jin & Kong 2019) to millimeter CO lines (Leroy et al 2009). Studies of NGC 7331 have found that it has a large molecular ring and L FIR ∼ 2 × 10 10 L (Thilker et al 2007).…”

X-ray binaries in M51 I: catalog and statistics

“…Fabbiano (1989Fabbiano ( , 2006 present an extensive summary of the X-ray source populations in nearby spiral galaxies. X-ray properties such as X-ray luminosity, hardness ratios, and variability can be utilized to identify and study X-ray binaries (Kaaret et al 2001;Prestwich et al 2003;Luan et al 2018;Jin & Kong 2019;Sell et al 2019).…”