Multi-Epoch Observations of the Nearby Spiral Galaxy NGC 3938 with the Chandra X-ray Observatory

TL;DR

This study analyzes two Chandra X-ray observations of galaxy NGC 3938, detecting 47 sources, identifying variability, and examining the galaxy's luminosity function, star formation, and metallicity, while discussing distance uncertainties.

Contribution

First multi-epoch X-ray analysis of NGC 3938, revealing source variability and insights into galaxy properties with implications for distance estimates.

Findings

47 X-ray sources detected within galaxy

Identification of 3 variable sources

Star formation rate highly dependent on distance

Abstract

We present an analysis of two epochs of ACIS observations of the SA(s)c spiral galaxy NGC 3938 with the Chandra X-ray Observatory. The total exposure time of the observations was 95 ksec with a limiting unabsorbed luminosity of approximately 10^{38}$ ergs/sec assuming a distance of 22 Mpc. A total of 47 discrete merged sources from both epochs were detected at the 3sigma level or greater with the D25 radius. We demonstrate that at the time of the Chandra observations, the nucleus was not detected. We connect the detected sources to counterparts in other wavebands to the degree possible. Based on the two epochs, we identify three variable sources and an additional two that may have varied between the two observations. We do not formally detect any of the five historical supernovae that have occurred in NGC 3938. The luminosity function of NGC 3938 is compared to a recent compilation of 38 galaxies and we identify a potentially significant problem with the `known' distance to NGC 3938. Star formation rate and metallicity values are also computed; the star formation rate is highly dependent upon the adopted distance. The metallicity appears to lie in the range of 8.2-9.2, consistent with values from other work. We include in an appendix a short discussion of the sources that lie in Chandra's field-of-view but lie outside of NGC 3938.