Broad line region echo from highly accreting intermediate-mass black hole candidate SDSS J144850.08+160803.1. The first probe of intra-night variability and reverberation mapping

Abstract

Elusive intermediate-mass black holes (IMBHs) can be used as ``time-squeezing'' machines, enabling studies of AGN geometry via reverberation mapping on much shorter timescales than their supermassive siblings. Constraints on the BLR radius for IMBH candidates across a broad range of Eddington ratios help probe the unexplored faint end of the radius-luminosity ($R-L$) relation in AGNs. This opens up the opportunity to build a more robust $M_{BH}$ estimator. The present study is aimed at: (a) confirming a highly accreting IMBH candidate and (b) demonstrating the feasibility of the first photometric BLR RM campaign for IMBHs with high Eddington rates. SDSS J144850.08+160803.1 was identified as an IMBH candidate from a broad H$\hat{I}\pm$-selected spectroscopic sample from SDSS. We carried out XMM-Newton X-ray observations to confirm its AGN status, along with narrowband H$\hat{I}\pm$ 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 AD continuum. XMM-Newton detected J1448+16 as a bright X-ray point source with a photon index of $\hat{I}“=2.32_{-0.13}^{+0.15}$ and X-ray luminosity of $L_{2-10\mathrm{keV}}=(3.3_{-0.4}^{+0.5})\times 10^{41}$ erg s${}^{-1}$, confirming its AGN activity. From the SDSS optical spectrum and X-ray properties, we estimated a BH mass of $\sim (0.9-2.4)\times 10^5{M}_{\odot }$ and Eddington rate of $\sim 37-112\%$. We report high-amplitude $\sim 55\%$ intra-night ($\sim 1.7$~h) H$\hat{I}\pm$ variability for this IMBH and extract a tentative BLR RM radius estimate of $\sim 1-8\mathrm{days}$. This work is 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.

Accepted in A&A on 07.08.2025, 20 pages, 12 figures