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″). NuSTAR 's sensitivity above 10 keV finds both are heavily obscured to Compton-thick levels ( N H ≈ (1–2) × 10 24 cm −2 ) and contribute equally to the BAT detection ( L 10 − 50 keV int ≈ 6 × 10 42 erg s −1 ). The observed luminosity of both sources is severely diminished in the 2–10 keV band ( L 2 − 10 keV obs < 0.1 × L 2 − 10 keV int ), illustrating the importance of >10 keV surveys like those with NuSTAR and Swift /BAT. Compared to archival 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.