Chandra Discovery of a Binary Active Galactic Nucleus in Mrk 739

TL;DR

This paper reports the discovery of a binary active galactic nucleus in Mrk 739 using high-resolution X-ray observations, revealing a system where one AGN is optically hidden but detectable in X-rays, highlighting the importance of hard X-ray imaging.

Contribution

First detection of a binary AGN in Mrk 739 with high spatial resolution X-ray data, emphasizing the role of X-ray observations in uncovering optically obscured AGN in merging galaxies.

Findings

Two luminous nuclei separated by 3.4 kpc identified as AGN

Western nucleus shows high variability and hard spectrum, indicating AGN activity

Large molecular gas reservoir likely fuels the binary AGN

Abstract

We have discovered a binary AGN in the galaxy Mrk 739 using Chandra and Swift BAT. We find two luminous L_2-10 keV=1.1*10^43 and 1.0*10^42 erg/s, unresolved nuclei with a projected separation of 3.4 kpc (5.8\pm0.1) coincident with two bulge components in the optical image. The western X-ray source (Mrk 739W) is highly variable (2.5x) during the 4-hour Chandra observation and has a very hard spectrum consistent with an AGN. While the eastern component was already known to be an AGN based on the presence of broad optical recombination lines, Mrk 739W shows no evidence of being an AGN in optical, UV, and radio observations, suggesting the critical importance of high spatial resolution hard X-ray observations (>2 keV) in finding these binary AGN. A high level of star formation combined with a very low L_[O III]/L_2-10 keV ratio cause the AGN to be missed in optical observations. CO observations of the (3-2) and (2-1) lines indicate large amounts of molecular gas in the system that could be driven towards the black holes during the violent galaxy collision and be key to fueling the binary AGN. Mrk 739E has a high Eddington ratio of 0.71 and a small black hole (log M_BH=7.05\pm0.3) consistent with an efficiently accreting AGN. Other than NGC 6240, this stands as the nearest case of a binary AGN discovered to date.