The spiral galaxy NGC 4559: X-ray point sources and diffuse emission

The inclined nearby (d = 9.7 Mpc) spiral galaxy NGC 4559 was observed with the ROSAT PSPC. Seventeen point sources were detected in a 15' x 15' field, seven of which fall within the D-25 ellipse of the galaxy. The most luminous source is associated with the nucleus of the galaxy, which has a luminosity of L(x) = 7 x 10(39) erg s(-1) and is slightly extended, suggesting a superposition of several compact sources or diffuse emission surrounding a compact central source. The source appears absorbed by a column of N-H = 10(21) cm(-2) above the Galactic value, so that the intrinsic luminosity of the source exceeds the quoted value by about a factor two. No time variation of the X-ray emission from the central source or any other source in NGC 4559 could be established. The central X-ray source appears slightly offset from the optical center of the galaxy. The larger offset in the soft band may be due to partial shielding through the HI disk of the emission from an extended distribution of X-ray binaries, or diffuse emission surrounding the center. Another bright source (L(x) = 6 x 10(39) erg s(-1), N-H similar or equal to 1.7 x 10(21) cm(-2), intrinsic L(x) greater than or similar to 1.5 x 10(40) erg s(-1)) was found in an outer spiral arm, coincident with an apparent group of HII regions. Since little radio or infrared emission emerges from this location, we conject that the X-rays originate from a single, several 100 year old buried supernova remnant that expands into gas with density of order 10(4) cm(-3). Diffuse, unresolved emission within the galaxy's bulge and inner disk was detected with a total X-ray luminosity of L(x) = 1.5 x 10(39) erg s(-1). Star formation activity at a rate of similar to 0.2 M. yr(-1) would produce sufficient X-ray emitting sources to account for this emission. However, the X-rays could also originate from globular cluster X-ray binaries without in voking recent star formation activity. No emission was detected from a possible halo. In the outer disk we detect a depression of soft X-rays that is likely caused by absorption of soft X-ray background radiation originating beyond the galaxy; the background surface brightness is estimated at S-x greater than or similar to 3 x 10(-4) cts s(-1) arcmin(-2). An upper limit of L(x) = 1.9 x 10(38) erg s(-1) was derived for the X-ray luminosity of the Type II SN 1941A.