We present a detailed study of a transient in the center of SDSS1115+0544 based on the extensive UV , optical , mid-IR light curves ( LC ) and spectra over 1200 days .
The host galaxy is a quiescent early type galaxy at z = 0.0899 with a blackhole mass of 2 \times 10 ^ { 7 } M _ { \odot } .
The transient underwent a 2.5 magnitude brightening over \sim 120 days , reaching a peak V -band luminosity ( extinction corrected ) of -20.9 magnitude , then fading 0.5 magnitude 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 \mu m , with a peak time delay of \sim 180 days .
The mid-IR LCs are explained as the echo of UV photons by a dust medium with a radius of 5 \times 10 ^ { 17 } cm , consistent with E ( B - V ) of 0.58 inferred from the spectra .
This event is very energetic with an extinction corrected L _ { bol } \sim 4 \times 10 ^ { 44 } erg s ^ { -1 } .
Optical spectra over 400 days in the plateau phase revealed newly formed broad H \alpha, \beta emission with a FWHM of \sim 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 TDE 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 _ { \odot } /yr .
This change occurred on a very short time scale of \sim 120 - 200 days .
The discovery of such a rapid “ turn-on ” AGN poses challenges to accretion disk theories and may indicate such event is not extremely rare .