A DeepChandraX‐Ray Observation of NGC 1637

We present a deep (168 ks) Chandra observation of the nearby face-on spiral galaxy NGC 1637 and study the X-ray source population. A total of 32 X-ray sources are detected within the D 25 ellipse of NGC 1637, each of which is found to be variable during the 629 day period covered by the observations. Deep Hubble Space Telescope WFPC2 observations of NGC 1637, nearly simultaneous with the Chandra data, were used to search for optical counterparts to the X-ray sources. An ultraluminous ( L X = 2.2 × 10 39 ergs s -1 ; 0.3-7 keV) and variable (factor of 1.4) X-ray source is found to positionally coincide with an optical faint and extended (~25 pc FWHM) counterpart in the HST images ( m V ~ 22.8 mag, m I ~ 21.1 mag). Given the multiband properties of this source, it likely represents a stellar-mass (~8 M ☉ ) black hole candidate located in an OB association in an outer spiral arm of NGC 1637. Our analysis further reveals large amounts of diffuse emission filling the entire extent of the galaxy. The spectrum of the inner ≲300 pc region is best described by a two-temperature thermal plasma emission, with a low-temperature/low-absorption component ( kT low ~ 0.3 keV) and a high-temperature/high-absorption component ( kT high ~ 0.7-0.8 keV), likely associated with an outflow of hot gas into the halo of NGC 1637. Comparison of the multiwavelength properties of a number of nearby galaxies shows that NGC 1637 has remarkably high L X / L FIR and L X / L B ratios, suggesting a recent epoch with an enhanced star formation rate.