SDSS J144850.08+160803.1 g + Ha light curves

We carried out XMM-Newton X-ray observations to confirm its AGN status, along with narrowband H{alpha} and broadband SDSS g' monitoring over five months (March-July 2024) using a 60-cm telescope at the Caucasus Mountain Observatory. These time series allowed us both to probe the short-timescale variability and extract the time lag between the BLR and accretion disk (AD) continuum. XMM-Newton detected J1448+16 as a bright X-ray point source with a photon index of {GAMMA}=2.32^+0.15^_-0.13_ and X-ray luminosity of L_2-10_keV=(3.3^+0.5^_-0.4_)x10^41^erg/s, confirming its AGN activity.

From the SDSS optical spectrum and X-ray properties, we estimated a BH mass of ~(0.9-2.4)x10^5^M_{sun}_ and Eddington rate of ~37-112%. We report high-amplitude ~55% intra-night (~1.7h) Ha variability for this highly accreting IMBH and extract a tentative BLR RM radius estimate of ~1-8 days. This work offers a proof of concept for further high-Eddington-rate IMBH variability studies and BLR RM campaigns, which will be essential for an efficient calibration of the R-L relation at the faint end.