Suzaku Observations of Local Ultraluminous Infrared Galaxies

Abstract

We report the results from our analysis of Suzaku X-ray Imaging Spectrometer (XIS) (0.5-10 keV) and Hard X-ray Detector (HXD)/PIN (15-40 keV) observations of five well-known local ultraluminous infrared galaxies: IRAS F05189 - 2524, IRAS F08572 + 3915, Mrk 273, PKS 1345 + 12, and Arp 220. The XIS observations of F05189 - 2524 and Mrk 273 reveal strong iron lines consistent with Fe Kα and changes in spectral shapes with respect to previous Chandra and XMM-Newton observations. Mrk 273 is also detected by the HXD/PIN at ~ 1.8σ. For F05189 - 2524, modeling of the data from the different epochs suggests that the change in spectral shape is likely due to the central source switching off, leaving behind a residual reflection spectrum, or an increase in the absorbing column. An increase in the covering fraction of the absorber can describe the spectral variations seen in Mrk 273, although a reduction in the intrinsic active galactic nucleus (AGN) luminosity cannot be formally ruled out. The Suzaku spectra of Mrk 273 are well fitted by a ~ 94% covering fraction model with a column density of ~ 10²⁴ cm^-2. The absorption-corrected log[L _2-10keV/L _IR] ratio is consistent with those found in PG Quasars. The 0.5-10 keV spectrum of PKS 1345 + 12 and Arp 220 seem unchanged from previous observations and their hard X-ray emission is not convincingly detected by the HXD/PIN. The large column density derived from CO observations and the large equivalent width of an ionized Fe line in Arp 220 can be reconciled by an ionized reflection model. F08572 + 3915 is undetected in both the XIS and HXD/PIN, but the analysis of unpublished Chandra data provides a new measurement at low energies.