Variability as a new discovery channel for Intermediate-Mass Black Holes in the Time Domain Era

TL;DR

This paper addresses the long-standing absence of a census of intermediate-mass black holes (IMBHs) by arguing that time-domain variability—specifically dwarf AGN variability and tidal disruption events (TDEs)—offers a direct discovery channel. It surveys traditional snapshot methods (spectroscopic, color/SED, radio/X-ray) and demonstrates their limitations for IMBHs, then highlights time-domain approaches as a complementary path, powered by Rubin Observatory LSST and coordinated multiwavelength follow-up. The authors synthesize evidence from local dwarf AGNs (e.g., NGC 4395) and variability surveys to argue for a high local occupation fraction and the viability of population-level inferences, including wandering IMBHs and high-redshift dwarfs, through AGN timing and TDE statistics. They conclude that a time-domain, multi-messenger strategy will enable a robust census of IMBHs across cosmic time, informing seed formation and growth channels and shedding light on the assembly history of black holes.

Abstract

Between the groundbreaking detections of stellar-mass black holes by LIGO/Virgo/KAGRA and JWST's revelation of a surprisingly abundant population of supermassive black holes, one crucial missing link remains: the elusive intermediate-mass black holes (IMBHs). IMBHs represent a key phase in the hierarchical growth of black holes, yet they have persistently evaded detection. Traditional methods, effective for both actively accreting and quiescent black holes, have largely failed to uncover this hidden population. Here, we argue that novel observational strategies--particularly time-domain variability studies of active galactic nuclei (AGN) and tidal disruption events--provide a promising path forward. Finding IMBHs will resolve critical gaps in our understanding of black hole formation and the various mechanisms driving their subsequent growth. The upcoming Vera C. Rubin Observatory, with its unprecedented capacity to monitor the dynamic sky, stands to revolutionize our ability to detect these long-sought IMBHs, shedding new light on the assembly history of black holes across cosmic time.

Figures (4)

• Figure 1: Schematic showing the primary methods for identifying IMBHs. Left to right: Dynamical signatures, AGN accretion, TDE flares, and gravitational waves.
• Figure 2: The observed black hole mass spectrum and the IMBH "mass desert". Included in the figure are stellar-mass black holes with dynamical mass measurements in X-ray binary (XRB) systems Corral-Santana2016; confidently-detected gravitational wave (GW) events TheLIGOScientificCollaboration2025; dynamical IMBH detections Gerssen2002Gebhardt2005Noyola2010Lutzgendorf2013Lutzgendorf2015denBrok2015Baumgardt2017Kiziltan2017Nguyen2019Haberle2024; supermassive black holes from SDSS quasars with single-epoch black hole mass estimates Wu2022; broad-line dwarf AGNs with single-epoch black hole mass estimates Greene2007Dong2012Reines2013Liu2018Chilingarian2018; and tidal disruption events (TDEs) with masses estimated from the stellar velocity dispersion of their host galaxy bulges Wevers2017. The lower mass gap refers to the scarcity of black holes below $\sim 5 M_\odot$ and above the maximum possible mass of a neutron star (e.g., Kreidberg2012). The upper mass gap results from predictions from stellar evolution that pair instability supernova leave no remnants above $\sim 140 M_\odot$ (e.g., Woosley2017). We mark some well known black holes: GW190521 Abbott2020; RGG 118 Baldassare2015; HLX-1 Farrell2009; NGC 4395 Filippenko2003; Sgr A$^{\ast}$EventHorizonTelescopeCollaboration2022; M87 EventHorizonTelescopeCollaboration2019; and the luminous blazar S5 0014+81 Kuhr1983Ghisellini2009.
• Figure 3: $\sim$ 1 month-long TESS optical light curve of the nucleus of the dwarf AGN NGC 4395. Left: SDSS color image of NGC 4395, that contains one of the lowest mass SMBHs known. Right: The TESS light curve (black points with 1$\sigma$ uncertainties) and fitted with a damped random walk (DRW; blue) or higher-order continuous autoregressive moving average (CARMA; green) process Kelly2014. The blue and green bands mark the 1$\sigma$ uncertainty range.
• Figure 4: AGN variability timescales at optical and X-ray wavelengths as functions of SMBH mass. Red squares show measurements of the break timescale for X-ray variability Gonzalez-Martin2012 and the black circles are optical measurements. The thick gray lines indicate the orbital timescale at ten and three times the Schwarzschild radius. The error bars are 1$\sigma$ uncertainties. The characteristic variability timescales is $\sim$ hours -- days in the optical or $\sim$ seconds -- minutes in the X-ray for an IMBH. Figure from: Burke2021science.