2019ApJ...874...44Y

We present a detailed study of a transient in the center of SDSS1115+0544 based on the extensive UV, optical, mid-IR light curves (LCs) and spectra over 1200 days. The host galaxy is a quiescent early-type galaxy at z = 0.0899 with a black hole mass of 2 x 10⁷ M_☉. The transient underwent a 2.5 mag brightening over ∼120 days, reaching a peak V-band luminosity (extinction corrected) of -20.9 mag, then fading 0.5 mag over 200 days, settling into a plateau of >600 days. Following the optical brightening are the significant mid-IR flares at 3.4 and 4.5 µm, with a peak time delay of ∼180 days. The mid-IR LCs are explained as the echo of UV photons by a dust medium with a radius of 5 x 10¹⁷ cm, consistent with E(B - V) of 0.58 inferred from the spectra. This event is very energetic with an extinction corrected L_bol ∼ 4 x 10⁴⁴ erg s^–1. Optical spectra over 400 days in the plateau phase revealed newly formed broad Hα, β emission with a FWHM of ∼3750 km s^–1 and narrow coronal lines such as [Fe VII], [Ne V]. This flare also has a steeply rising UV continuum, detected by multi-epoch Swift data at +700 to +900 days post optical peak. The broad Balmer lines and the UV continuum do not show significant temporal variations. The slow evolving LCs over 1200 days, the constant Balmer lines, and UV continuum at late times rule out tidal disruption event and SN IIn as the physical model for this event. We propose that this event is a "turn-on" AGN, transitioning from a quiescent state to a type-1 AGN with a sub-Eddington accretion rate of 0.017 M_☉ yr^–1. This change occurred on a very short timescale of ∼120-200 days. The discovery of such a rapid "turn-on" AGN poses challenges to accretion disk theories and suggests more future detections of similar events.