[ITAL]ASCA[/ITAL] Observation of an X-Ray–luminous Active Nucleus in Markarian 231

We have obtained a moderately long (10 5 s) ASCA observation of the Seyfert 1 galaxy Markarian 231, the most luminous of the local ultraluminous infrared galaxy population. In the best-fitting model we do not see the X-ray source directly; the spectrum consists of a scattered power-law component and a reflection component, both of which have been absorbed by a column N H ≈ 3 × 10 22 cm -2 . About three-quarters of the observed hard X-rays arise from the scattered component, reducing the equivalent width of the iron Kα line. The implied ratio of 1-10 keV X-ray luminosity to bolometric luminosity, L X / L bol ~ 2%, is typical of Seyfert 1 galaxies and radio-quiet QSOs of comparable bolometric luminosities and indicates that the bolometric luminosity is dominated by the active galactic nucleus (AGN). Our estimate of the X-ray luminosity also moves Mrk 231 in line with the correlations found for AGNs with extremely strong Fe II emission. A second source separated by about 2' is also clearly detected and contributes about 25% of the total flux.