X‐Raying the Ultraluminous Infrared Starburst Galaxy and Broad Absorption Line QSO Markarian 231 withChandra

With 40 ks of Chandra ACIS-S3 exposure, new information on both the starburstand QSO components of the X-ray emission of Markarian 231, an ultraluminousinfrared galaxy and Broad Absorption Line QSO, has been obtained. The bulk ofthe X-ray luminosity is emitted from an unresolved nuclear point source, andthe spectrum is remarkably hard with the majority of the flux emitted above 2keV. Most notably, significant nuclear variability (a decrease of ~45% inapproximately 6 hours) at energies above 2 keV indicates that Chandra hasprobed within light hours of the central black hole. Though we concur withMaloney & Reynolds that the direct continuum is not observed, this variabilitycoupled with the 188 eV upper limit on the equivalent width of the FeKalphaemission line argues against the reflection-dominated model put forth by theseauthors based on their ASCA data. Instead, we favor a model in which a small,Compton-thick absorber blocks the direct X-rays, and only indirect, scatteredX-rays from multiple lines of sight can reach the observer. Extended soft,thermal emission encompasses the optical extent of the galaxy and exhibitsresolved structure. An off-nuclear X-ray source with a 0.35-8.0 keV luminosityof L_X=7x10^{39} erg s^{-1}, consistent with the ultraluminous X-ray sources inother nearby starbursts, is detected. We also present an unpublished FOSspectrum from the HST archive showing the broad C IV absorption.Comment: 29 pages, 16 figures (13 PS and 3 JPG), uses AASTeX. Contact first author for full resolution images. Accepted for publication in the Astrophysical Journal. Corrected author lis