NuSTAR Resolves the First Dual AGN Above 10 keV in SWIFT J2028.5+2543

Abstract

We have discovered heavy obscuration in the dual active galactic nucleus (AGN)in the Swift/Burst Alert Telescope (BAT) source SWIFT J2028.5+ 2543 using Nuclear Spectroscopic Telescope Array (NuSTAR). While an early XMM-Newton study suggested the emission was mainly from NGC 6921, the superior spatial resolution of NuSTAR above 10 keV resolves the Swift/BAT emission into two sources associated with the nearby galaxies MCG+ 04-48-002 and NGC 6921 (z = 0.014) with a projected separation of 25.3 kpc (91′′). NuSTARʼs sensitivity above 10 keV finds both are heavily obscured to Compton-thick levels (NH≈(1–2)×1024cm −2)and contribute equally to the BAT detection (-L10 50 keV int≈ 6 × 10^(42) erg s^(−1)). The observed luminosity of both sources is severely diminished in the 2–10 keV band(< ́--LL 0.1210 keV obs 210keV int), illustrating the importance of >10 keV surveys like those with NuSTAR and Swift / BAT. Compared to rchival X-ray data, MCG+04-48-002 shows significant variability (>3)between observations. Despite being bright X-ray AGNs, they are difficult to detect using optical emission-line diagnostics because MCG +04-48-002 is identified as a starburst / composite because of the high rates of star formation from a luminous infrared galaxy while NGC 6921 is only classified as a LINER using line detection limits. SWIFT J2028.5 + 2543 is the first dual AGN resolved above 10 keV and is the second most heavily obscured dual AGN discovered to date in the X-rays other than NGC 6240.