Studying Faint Ultra Hard X-ray Emission from AGN in GOALS LIRGs with Swift BAT

TL;DR

This study analyzes Swift BAT ultra hard X-ray data for LIRGs, revealing higher AGN detection rates in luminous infrared galaxies and confirming the survey's effectiveness in identifying faint and obscured AGN populations.

Contribution

First analysis of all-sky Swift BAT data for LIRGs, demonstrating enhanced detection of faint and obscured AGN and validating the survey's utility for studying ultra hard X-ray sources.

Findings

LIRGs have higher AGN detection frequency than matched galaxies.

Half of high IR luminosity LIRGs are detected as AGN.

High column density AGN are more common in LIRGs.

Abstract

We present the first analysis of the all-sky Swift BAT ultra hard X-ray (14-195 keV) data for a targeted list of objects. We find the BAT data can be studied at 3x fainter limits than in previous blind detection catalogs based on prior knowledge of source positions and using smaller energy ranges for source detection. We determine the AGN fraction in 134 nearby (z<0.05) luminous infrared galaxies (LIRGS) from the GOALS sample. We find that LIRGs have a higher detection frequency than galaxies matched in stellar mass and redshift at 14-195 keV and 24-35 keV. In agreement with work at other wavelengths, the AGN detection fraction increases strongly at high IR luminosity with half of high luminosity LIRGs (50%, 6/12, log L_IR/L_sun>11.8) detected. The BAT AGN classification shows 97% (37/38) agreement with Chandra and XMM AGN classification using hardness ratios or detection of a iron K-alpha line. This confirms our statistical analysis and supports the use of the Swift BAT all-sky survey to study fainter populations of any category of sources in the ultra hard X-ray band. BAT AGN in LIRGs tend to show higher column densities with 40\pm9% showing 14-195 keV/2-10 keV hardness flux ratios suggestive of high or Compton-thick column densities (log N_H>24 cm^-2), compared to only 12\pm5% of non-LIRG BAT AGN. We also find that using specific energy ranges of the BAT detector can yield additional sources over total band detections with 24% (5/21) of detections in LIRGs at 24-35 keV not detected at 14-195 keV.