A Tidal Disruption Event from an Intermediate-mass Black Hole Revealed by Comprehensive Multi-wavelength Observations

TL;DR

This study presents comprehensive multi-wavelength observations of AT 2018cqh, a tidal disruption event in the nucleus of a dwarf galaxy, arguing for an IMBH with $M_{ m BH}\sim(1-6)\times10^{5}\,M_\odot$. The analysis reveals a rare high-state X-ray plateau lasting $>500$ days and an unusually long X-ray rise ($\geq550$ days), accompanied by soft thermal X-ray spectra and a potential Comptonization component, all pointing to near-Eddington accretion onto an IMBH with disk formation and circularization on multi-year timescales. Host-galaxy properties, including a post-starburst dwarf environment and a transient [Fe X] coronal line, support a nuclear origin and IMBH interpretation, with scaling relations placing $M_{\rm BH}$ in the IMBH regime. The work provides a unique, well-sampled view of IMBH TDE evolution across optical, X-ray, and radio bands, constraining disk physics, accretion states, and the demographic of IMBHs in low-mass galaxies, and it reinforces TDEs as valuable probes of dormant IMBHs.

Abstract

Tidal disruption events (TDEs) occur when a star crosses the tidal radius of a black hole (BH) and is ripped apart, providing a novel and powerful way to probe dormant BHs over a wide mass range. In this study, we present our late-time observations and comprehensive multi-wavelength analyses of an extraordinary TDE at the center of a dwarf galaxy, which exhibited successive flares in the optical, X-ray, and radio bands. Notably, we discovered an unexpected high-state X-ray plateau phase following the peak until the present time. Along with its reported prolonged rise lasting at least 550 days, these unique characteristics are consistent with the scenario of a TDE caused by an intermediate-mass black hole (IMBH) with a mass of approximately $(1-6) \times 10^5$ solar masses. Furthermore, scaling relations derived from the host-galaxy properties indicated a similar BH mass in concert. This discovery highlights the invaluable role of TDEs in the search for elusive IMBHs.

Figures (10)

• Figure 1: Multi-wavelength behavior of AT 2018cqh. a: radio light curves from the VLA (octagons) and ASKAP surveys (pentagons, stars, hexagons) across different frequencies. The colors transition from red to orange, indicating an increase in frequency. The downward arrows show the flux upper limits at 3$\sigma$ level. A comparison of the radio light curves from other events is presented in Supplementary Fig. \ref{['radiolc']}. b: unabsorbed X-ray light curve in the 0.3--2.0 keV band, sourced from SRG/eROSITA (squares), Swift/XRT (diamonds), EP/FXT (thin diamonds), and XMM/EPIC (circles). All eROSITA data are taken from Figure 10 in Bykov et al. (2024) Bykov+2024. The violet dotted line represents a fit to the X-ray data collected after July 2021, based on the assumption that the flux follows a $t^{-5/3}$ decay trend. The dark violet dashed line indicates the plateau phase. The X-ray light curves of other optically-selected TDEs with late-time X-ray detections are compared in Supplementary Fig. \ref{['xraylc']}. c: optical host-subtracted light curves in the Gaia$G$-band (cyan hexagons), ZTF $g$-band (blue circles), ZTF $r$-band (green circles) and ZTF $i$-band (yellow circles). The gold vertical lines denote the observation periods for optical spectra obtained by DESI and P200/DBSP. The flare part of the optical light curves is zoomed in and shown in Supplementary Fig. \ref{['optlc']}, and the color evolution compared to other transient events is displayed in Supplementary Fig. \ref{['optcolor']}. The error bars represent 1$\sigma$ uncertainties. Source data are provided as a Source Data file.
• Figure 1: Optical behavior of AT 2018cqh. a: optical host-subtracted light curves in the Gaia$G$-band (cyan hexagons), ZTF $g$-band (blue circles), ZTF $r$-band (green circles). For comparison, the black crosses and dashed black line represent the $g$-band light curve of the TDE PS1-10jh Gezari+2012, while the gray crosses and dotted gray line show the $g$-band light curve of the Bowen fluorescence flare (BFF) AT 2017bgt Trakhtenbrot+2019a. Both events are matched in peak absolute magnitude. b: host-subtracted optical color evolution in the ZTF $g-r$ color, shown by gray circles. The error bars represent 1$\sigma$ uncertainties. Source data are provided as a Source Data file.
• Figure 2: X-ray spectra of AT 2018cqh. a--f includes the best-fit model shown as a solid line. Residuals for each spectrum are presented below to assess the fit quality. From a to f, the feature data includes Swift/XRT on 9 August 2023, EP/FXT on 10 August 2024, Swift/XRT from 19 September 2024 to 7 October 2024, Swift/XRT from 21 October 2024 to 30 November 2024, Swift/XRT from 9 December 2024 to 29 December 2024, and XMM/EPIC on 22 Jan 2025. For EP/FXT, the lighter spectrum represents FXTA, while the darker one corresponds to FXTB. For XMM/EPIC, the spectra are ordered from light to dark as follows: MOS1, MOS2, and EPIC-pn, which were fitted simultaneously. The dotted lines indicate the background level for each spectrum. The plotted data for each spectrum are rebinned to facilitate clearer display. The error bars represent 1$\sigma$ uncertainties. Source data are provided as a Source Data file.
• Figure 2: Mean $g-r$ color and color change ($\Delta(g-r)/t$) of AT 2018cqh. The mean $g-r$ color is measured from all good detections in the ZTF light curve. The color change is calculated using a 3-day binned ZTF light curve to reduce the impact of false variations caused by detection uncertainties. The background is adapted from Figure 1 in van Velzen et al. (2021a) vanVelzen+2021a. All background events are nuclear transients from the first half of ZTF survey observations, with contours enclosing two-thirds of spectroscopically classified supernovae (dark gray circles and squares) and AGNs (light gray circles) in the sample vanVelzen+2021a. AT 2018cqh (orange star) and other TDEs (black thin diamonds) show an almost constant optical color, whereas most supernovae show a color increase in post-peak observations. The error bars represent 1$\sigma$ uncertainties. Source data are provided as a Source Data file.
• Figure 3: Pre-outburst optical image of AT 2018cqh alongside multi-wavelength observations during its outburst. The optical flare was first detected on 16 June 2018, the X-ray outburst on 21 January 2020, and the radio flare on 25 June 2021. a: pseudo-colored pre-outburst image captured by the Hyper Suprime-Cam (HSC) on the 8.2m Subaru Telescope, utilizing $g$, $r$, and $z$ bands. The yellow square highlights the host galaxy SDSS J023346.93$-$010128.3, while the red dot indicates the location of the outburst. From b to d, the images of AT 2018cqh are presented, with observation dates indicated: ZTF observation in the $g$ band, EP/FXT observation in the 0.3--10 keV band, and ASKAP observation at 887.5 MHz.