THE MAN BEHIND THE CURTAIN: X-RAYS DRIVE THE UV THROUGH NIR VARIABILITY IN THE 2013 ACTIVE GALACTIC NUCLEUS OUTBURST IN NGC 2617

TLDR

Changing‑look AGN are rare and provide key insights into AGN physics. The study monitored NGC 2617 for ~70 days across X‑ray to NIR wavelengths after its brightening was detected by ASAS‑SN. The authors estimated a black‑hole mass of ~4 × 10⁷ M⊙ from Hβ width and radius–luminosity scaling, and modeled the variability as a thermally emitting thin disk illuminated by variable X‑ray flux. NGC 2617 experienced a dramatic outburst with >10× increases in X‑ray and optical/UV flux, changed from a Seyfert 1.8 to a Seyfert 1, and showed disk emission lagging X‑rays by 2–3 days in the UV to 6–9 days in the NIR, with NIR variability more temporally smoothed than the UV.

Abstract

After the All-Sky Automated Survey for SuperNovae (ASAS-SN) discovered a significant brightening of the inner region of NGC 2617, we began a ~70 day photometric and spectroscopic monitoring campaign from the X-ray through near-infrared (NIR) wavelengths. We report that NGC 2617 went through a dramatic outburst, during which its X-ray flux increased by over an order of magnitude followed by an increase of its optical/ultraviolet (UV) continuum flux by almost an order of magnitude. NGC 2617, classified as a Seyfert 1.8 galaxy in 2003, is now a Seyfert 1 due to the appearance of broad optical emission lines and a continuum blue bump. Such "changing look Active Galactic Nuclei (AGN)" are rare and provide us with important insights about AGN physics. Based on the Hbeta line width and the radius-luminosity relation, we estimate the mass of central black hole to be (4 +/- 1) x 10^7 M_sun. When we cross-correlate the light curves, we find that the disk emission lags the X-rays, with the lag becoming longer as we move from the UV (2-3 days) to the NIR (6-9 days). Also, the NIR is more heavily temporally smoothed than the UV. This can largely be explained by a simple model of a thermally emitting thin disk around a black hole of the estimated mass that is illuminated by the observed, variable X-ray fluxes.