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

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 less than or similar to 6.1 rest-frame days in the observed-frame 0.5-5 keV band. The event lasted for approximate to 5.0-7.3 days, and then XID 403 dimmed by a factor of >6.0 in less than or similar to 6.1 days. After approximate to 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 Gamma = 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) x 10(43) erg 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 approximate to 2500 angstrom) 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.

Automated summary

We investigated the extreme X-ray variability of an active galactic nucleus in Chandra Deep Field-South and found two significant X-ray brightening events. The first event lasted for approximately 5-7 days, while the second event lasted for 251 days. The X-ray absorber in this nucleus is likely a powerful accretion disk wind.