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

TL;DR

This study targets intermediate-mass black holes by applying reverberation mapping to a highly accreting candidate, J1448+16, to calibrate the BLR radius–luminosity relation at the faint end. It combines XMM-Newton X-ray data with five months of narrow Hα and g' photometry from a 0.6 m telescope to probe short-timescale variability and search for BLR–continuum time lags using ICCF and Gaussian process cross-correlation. The authors infer a BH mass in the IMBH regime (approximately $M_{ m BH}\, ext{in the range}\,(0.9{-}2.4) imes10^{5}\,M_\odot$) and high accretion rates ($L_{ m bol}/L_{ m Edd}$ up to around 1), along with a tentative BLR RM lag of ~1–8 days and strong intra-night Hα variability. This work demonstrates the feasibility of photometric RM campaigns for rapidly accreting IMBHs and highlights the need for high-cadence, high-sensitivity campaigns to robustly calibrate the faint end of the $R-L$ relation and improve IMBH mass estimators.

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$α$-selected spectroscopic sample from SDSS. We carried out XMM-Newton X-ray observations to confirm its AGN status, along with narrowband H$α$ 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 $Γ= 2.32^{+0.15}_{-0.13}$ and X-ray luminosity of $L_{2-10\,\rm{keV}}=(3.3^{+0.5}_{-0.4})\times10^{41}$ erg s$^{-1}$, confirming its AGN activity. From the SDSS optical spectrum and X-ray properties, we estimated a BH mass of $\sim(0.9-2.4)~\times10^{5}M_{\odot}$ and Eddington rate of $\sim37-112\%$. We report high-amplitude $\sim55\%$ intra-night ($\sim1.7$~h) H$α$ variability for this IMBH and extract a tentative BLR RM radius estimate of $\sim1-8~\mathrm{days}$. 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.

Figures (12)

• Figure 1: X-ray images of J1448+16 from EPIC-PN, EPIC-MOS1, and EPIC-MOS2 detectors in total XMM-Newton energy band (0.2--12 keV). The red cross marks the galaxy center from optical observations, and the blue circle marks the XMM detection center, with the circle size representing the coordinate estimation $3\sigma$ error.
• Figure 2: X-ray spectrum of J1448+16 from combined EPIC-PN, EPIC-MOS1, and EPIC-MOS2 exposures. The solid red line represents the best-fit model, which includes a power law with photoelectric absorption with photon index of $\Gamma = 2.32^{+0.15}_{-0.13}$ and hydrogen column density of $\mathrm{nH} = 9.7^{+11.4}_{-9.2}\times10^{19}\mathrm{\;atoms/cm}^2$.
• Figure 3: NBursts fitting results for SDSS spectrum J1448+16 using X-Shooter SSP models and Gaussian BLR profiles. Upper ticks correspond to rest-frame wavelengths, while the middle and bottom ticks correspond to observed wavelengths. Top panel: Full wavelength range of the SDSS spectrum; Bottom panels: Zoom-in on the surrounding ranges of two emission lines $\mathrm{H\beta}$ and $\mathrm{H\alpha}$. In all panels: the black line corresponds to spectrum fluxes; the red line to the best-fit model; the purple line to the stellar population model including multiplicative continuum (stellar component); the light pink line to the additive continuum component describing the AGN continuum; the light blue and green lines to emission lines templates for NLR and BLR, respectively; the dark gray line to the data-model residuals; the colored shaded regions to the filter transmission curves of SDSS g ́in light orange and narrow $\mathrm{H\alpha}$ in light purple; and, finally, the light gray shaded bands to masked and excluded regions from the fit.
• Figure 4: Edge colors for markers in the right graphs correspond to the object classes: blue is AGN, magenta is calibration stars, and dark cyan is validation stars. Left: RC600 light-curve of J1448+16 binned by individual nights. The green points correspond to SDSS g' flux, the red points correspond to H$\alpha$ flux. The pale cyan line represents the weighted mean validation star. Right: Relative variability amplitude in g' and $H\alpha$ bands. The large circles show the variability of the AGN, the magenta stars demonstrate the variability of 33 calibrating stars (due to measurement uncertainties) and the crosses represent the validation sample of 54 faint stars. The vertical cyan line denotes the separating flux (in $g'$-band) between calibration and validation samples.
• Figure 5: Left: Corrected AGN luminosity in individual g-band frames during the night of 2024-06-13. Right: Same as left panel, but for H$\alpha$. The observations span a time range of $\sim2$ hours, the timestamps are relative to the start of the first g-band exposure. The X-axis error bars represent individual exposure times.