X-Ray Unveiling Events in a z ≈ 1.6 Active Galactic Nucleus in the 7 Ms Chandra Deep Field-South

Abstract

We investigate the extreme X-ray variability of a z = 1.608 active galactic nucleus in the 7 Ms Chandra Deep Field-South (XID 403), which showed two significant X-ray brightening events. In the first event, XID 403 brightened by a factor of >2.5 in ≲6.1 rest-frame days in the observed-frame 0.5-5 keV band. The event lasted for ≈5.0-7.3 days, and then XID 403 dimmed by a factor of >6.0 in ≲6.1 days. After ≈1.1-2.5 yr in the rest frame (including long observational gaps), it brightened again, with the 0.5-5 keV flux increasing by a factor of >12.6. The second event lasted over 251 days, and the source remained bright until the end of the 7 Ms exposure. The spectrum is a steep power law (photon index Γ = 2.8 ± 0.3) without obscuration during the second outburst, and the rest-frame 2-10 keV luminosity reaches ${1.5}_{-0.5}^{+0.8}\times {10}^{43}\,\mathrm{erg}\,{{\rm{s}}}^{-1}$ ; there is no significant spectral evolution within this epoch. The infrared-to-UV spectral energy distribution of XID 403 is dominated by the host galaxy. There is no significant optical/UV variability and R-band (rest-frame ≈2500 Å) brightening contemporaneous with the X-ray brightening. The extreme X-ray variability is likely due to two X-ray unveiling events, where the line of sight to the corona is no longer shielded by high-density gas clumps in a small-scale dust-free absorber. XID 403 is probably a high-redshift analog of local narrow-line Seyfert 1 galaxies, and the X-ray absorber is a powerful accretion disk wind. On the other hand, we cannot exclude the possibility that XID 403 is an unusual candidate for tidal disruption events.