A NuSTAR Survey of Nearby Ultraluminous Infrared Galaxies

TL;DR

This study uses NuSTAR, Chandra, and XMM-Newton to analyze the X-ray properties of nearby ULIRGs, revealing their obscuration levels, intrinsic luminosities, and potential X-ray weakness, with implications for understanding their active nuclei.

Contribution

First detailed broadband X-ray spectral modeling of nearby ULIRGs with NuSTAR, revealing obscuration and luminosity characteristics, and identifying potential X-ray weak sources.

Findings

Six ULIRGs detected with NuSTAR allowing detailed spectral analysis.

Only one ULIRG confirmed as Compton-thick AGN; another possibly highly obscured.

Some ULIRGs show lower high-energy X-ray fluxes and ratios compared to Seyfert 1 galaxies.

Abstract

We present a NuSTAR, Chandra, and XMM--Newton survey of nine of the nearest ultraluminous infrared galaxies (ULIRGs). The unprecedented sensitivity of NuSTAR at energies above 10 keV enables spectral modeling with far better precision than was previously possible. Six of the nine sources observed were detected sufficiently well by NuSTAR to model in detail their broadband X-ray spectra, and recover the levels of obscuration and intrinsic X-ray luminosities. Only one source (IRAS 13120--5453) has a spectrum consistent with a Compton--thick AGN, but we cannot rule out that a second source (Arp 220) harbors an extremely highly obscured AGN as well. Variability in column density (reduction by a factor of a few compared to older observations) is seen in IRAS 05189--2524 and Mrk 273, altering the classification of these border-line sources from Compton-thick to Compton-thin. The ULIRGs in our sample have surprisingly low observed fluxes in high energy (>10 keV) X-rays, especially compared to their bolometric luminosities. They have lower ratios of unabsorbed 2--10 keV to bolometric luminosity, and unabsorbed 2--10 keV to mid-IR [O IV] line luminosity than do Seyfert 1 galaxies. We identify IRAS 08572+3915 as another candidate intrinsically X-ray weak source, similar to Mrk 231. We speculate that the X-ray weakness of IRAS 08572+3915 is related to its powerful outflow observed at other wavelengths.