Soft X-ray emission from the spiral galaxy NGC 1313

The nearby barred spiral galaxy NGC 1313 has been observed with the PSPC instrument on board the ROSAT X-ray satellite. Ten individual sources are found. Three sources (X-1, X-2, and X-3 [SN 1978K]) are very bright (∼10⁴⁰ ergs s^-1) and are unusual in that analogous objects do not exist in our Galaxy. We present an X-ray image of NGC 1313 and X-ray spectra for the three bright sources. The emission from the nuclear region (R ≲ 2 kpc) is dominated by source X-1, which is located ∼1 kpc north of the photometric (and dynamical) center of NGC 1313. Optical, far-infrared, and radio images do not indicate the presence of an active galactic nucleus at that position; however, the compact nature of the X-ray source (X-1) suggests that it is an accretion-powered object with central mass M ≳ 10³ M_⊙. Additional emission (L_x ∼ 10³⁹ ergs s^-1) in the nuclear region extends out to ∼2.6 kpc and roughly follows the spiral arms. This emission is from four sources with luminosity of several × 10³⁸ ergs s^-1, two of which are consistent with emission from Population I sources (e.g., supernova remnants, and hot interstellar gas which has been heated by supernova remnants). The other two sources could be emission from Population II sources (e.g., low-mass X-ray binaries). The bright sources X-2 and SN 1978K are positioned in the southern disk of NGC 1313. X-2 is variable and has no optical counterpart brighter than 20.8 mag (V-band). It is likely that it is an accretion-powered object in NGC 1313. The Type II supernova SN 1978K (Ryder et al. 1993) has become extraordinarily luminous in X-rays ∼13 yr after optical maximum.