A Galaxy’s Last Meal: Unveiling a Mid-Sized Black Hole

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Author: Denis Avetisyan

Detailed observations of the AT 2018cqh event confirm it as a rare tidal disruption event powered by an intermediate-mass black hole, offering a unique window into the behavior of these elusive cosmic objects.

The analysis of AT 2018cqh’s multi-epoch optical spectra-comparing observations from 2000, early 2021, and late 2023-reveals evolving spectral features, including prominent coronal lines like $6374 \text{Å}$ [Fex] and the Balmer series (${\rm H}\alpha$, ${\rm H}\beta$, ${\rm H}\gamma$, ${\rm H}\delta$, and ${\rm H}\varepsilon$), alongside forbidden lines of [Oiii], [Nii], and [Sii], demonstrating the transient nature of accretion events and the ephemeral quality of any model attempting to fully capture such phenomena.
The analysis of AT 2018cqh’s multi-epoch optical spectra-comparing observations from 2000, early 2021, and late 2023-reveals evolving spectral features, including prominent coronal lines like $6374 \text{Å}$ [Fex] and the Balmer series (${\rm H}\alpha$, ${\rm H}\beta$, ${\rm H}\gamma$, ${\rm H}\delta$, and ${\rm H}\varepsilon$), alongside forbidden lines of [Oiii], [Nii], and [Sii], demonstrating the transient nature of accretion events and the ephemeral quality of any model attempting to fully capture such phenomena.

Comprehensive multi-wavelength analysis reveals a long-lasting high-state X-ray plateau, providing crucial insights into accretion physics and the properties of intermediate-mass black holes.

Despite decades of searching, intermediate-mass black holes (IMBHs) remain elusive gravitational anchors in the cosmic landscape. Here, we present a comprehensive multi-wavelength analysis detailed in ‘A Tidal Disruption Event from an Intermediate-mass Black Hole Revealed by Comprehensive Multi-wavelength Observations’, reporting on the exceptionally bright tidal disruption event AT 2018cqh. Our observations reveal a prolonged, high-state X-ray plateau following the initial disruption, indicative of an IMBH with a mass of approximately $(1-6) \times 10^5$ solar masses. Does this event represent a previously hidden population of IMBHs lurking in dwarf galaxies, and what further insights can TDEs provide into their formation and accretion physics?

A Stellar Echo: Unveiling the Anomaly

The astronomical event AT 2018cqh initially appeared as a typical tidal disruption event, wherein a star ventures too close to a supermassive black hole and is torn apart. However, observations revealed a perplexing anomaly: following the initial burst of energy, X-ray emissions didn’t fade as predicted by conventional models. Instead, they settled into an extended plateau, persisting for over 500 days – a duration drastically exceeding that of most observed TDEs. This prolonged, steady emission challenges existing theories of accretion and black hole feeding, suggesting either a previously unconsidered mechanism at play or a fundamentally different environment surrounding the black hole. The unusual behavior of AT 2018cqh has prompted a reassessment of how material interacts with black holes following a tidal disruption, opening new avenues for investigating the extreme physics governing these energetic phenomena.

Current theoretical models of tidal disruption events posit a swift accretion phase following the stellar disintegration, resulting in a comparatively brief burst of electromagnetic radiation. However, the observation of AT 2018cqh challenges this framework; the event exhibited a sustained X-ray plateau extending beyond 500 days, a duration vastly exceeding that of typical TDE afterglows. This prolonged emission cannot be readily explained by standard accretion disk physics, suggesting either a fundamentally different accretion process is at play or the existence of a continuous source of energy replenishing the emission. The discrepancy highlights a gap in current understanding, potentially indicating the presence of a previously unrecognized mechanism governing the late-time behavior of matter falling into supermassive black holes, and prompting a reevaluation of TDE dynamics.

The extended X-ray plateau observed in AT 2018cqh represents a critical puzzle piece in the study of tidal disruption events and the behavior of supermassive black holes. Traditional models predict a swift decline in emissions as stellar debris is consumed, yet this event showcased sustained luminosity for over 500 days, demanding a reevaluation of existing frameworks. Investigating the origin of this prolonged emission – potentially linked to the formation of a prolonged accretion disk, outflowing material, or even interactions with a pre-existing disk – offers a unique opportunity to test the limits of general relativity in extreme gravitational environments. Resolving this enigma promises to refine estimations of black hole spin, accretion rates, and the complex interplay between black holes and their surrounding galactic ecosystems, ultimately deepening understanding of these powerful cosmic engines and the fate of stars that venture too close.

Multi-wavelength observations of AT 2018cqh reveal correlated radio, X-ray, and optical light curves exhibiting a decaying trend with a potential plateau phase, providing insights into the dynamics of this tidal disruption event.
Multi-wavelength observations of AT 2018cqh reveal correlated radio, X-ray, and optical light curves exhibiting a decaying trend with a potential plateau phase, providing insights into the dynamics of this tidal disruption event.

The Black Hole’s Mass: An Intermediate Player

AT 2018cqh is attributed to an intermediate-mass black hole (IMBH) with a calculated mass of $1.5 \times 10^5$ solar masses. This places it in a relatively poorly characterized black hole population, distinct from both stellar-mass black holes (typically less than 100 solar masses) and supermassive black holes (exceeding $10^6$ solar masses). The existence and properties of IMBHs are currently an area of active research, as they represent a potential link in the evolutionary chain between the two more commonly observed black hole types and may contribute to the formation of supermassive black holes.

The X-ray emission observed in AT 2018cqh originates from an accretion disk formed during a tidal disruption event. This process occurs when a star approaches the intermediate-mass black hole and is torn apart by its gravitational forces. The disrupted stellar material does not fall directly into the black hole; instead, it enters a swirling disk around the black hole, known as an accretion disk. As material within the disk spirals inward due to viscous forces and angular momentum conservation, gravitational potential energy is converted into kinetic energy, ultimately resulting in the emission of high-energy photons in the X-ray spectrum. The temperature of the inner regions of the accretion disk can reach $10^7$ K or higher, contributing to the observed X-ray luminosity.

Standard Eddington-limited accretion posits a maximum luminosity for a given black hole mass, determined by the balance between outward radiation pressure and inward gravitational force. The observed luminosity plateau in the AT 2018cqh event deviates from this expectation; the sustained, high luminosity for an extended period indicates the accretion disk is not solely governed by Eddington-limited processes. This suggests alternative mechanisms are contributing to the energy output, potentially involving super-Eddington accretion rates or variations in the accretion disk structure and radiative efficiency. Deviations from Eddington-limited accretion can occur when magnetic fields effectively transport angular momentum, allowing material to accrete at higher rates than predicted by radiation pressure alone, or when the disk geometry differs from the standard thin-disk model, affecting radiative transfer.

A multi-wavelength analysis of AT 2018cqh reveals a delayed outburst sequence-optical flare first detected June 2018, followed by X-ray emission in January 2020, and a radio flare in June 2021-originating from host galaxy SDSS J023346.93−-010128.3.
A multi-wavelength analysis of AT 2018cqh reveals a delayed outburst sequence-optical flare first detected June 2018, followed by X-ray emission in January 2020, and a radio flare in June 2021-originating from host galaxy SDSS J023346.93−-010128.3.

A Slow Feast: The Slim Disk Scenario

The accretion disk surrounding the intermediate-mass black hole (IMBH) likely entered a ‘slim disk’ state, a configuration distinct from standard geometrically thin, optically thick disks. This transition is characterized by significant radiation pressure exceeding gas pressure, and a substantial contribution of advective cooling to energy transport. In advective cooling, energy is transported inward with the accreting material rather than by radiative diffusion. This results in a lower effective temperature and allows the disk to maintain a high luminosity, approximately $2.0 \times 10^{43}$ erg/s, at accretion rates exceeding the Eddington limit, while avoiding the instabilities that would typically occur in a standard disk.

Observations from eROSITA, Swift XRT, EP FXT, and XMM-Newton detected sustained X-ray emission from the intermediate-mass black hole accretion disk, measuring a peak luminosity of approximately $2.0 \times 10^{43}$ erg/s. These multi-wavelength data provided crucial constraints on disk parameters such as temperature, density, and size, indicating a departure from standard thin-disk models. Specifically, the prolonged emission timescale and luminosity level suggest a geometrically thick accretion flow where radiation pressure significantly influences the disk structure and dynamics, allowing for a sub-Eddington accretion rate over an extended period.

The sustained X-ray luminosity observed from the intermediate-mass black hole is attributable to a slim disk accretion geometry which permits a reduced accretion rate. Traditional Eddington-limited accretion models predict a rapid decline in luminosity as the accretion rate decreases; however, the slim disk configuration, characterized by increased vertical disk thickness and radiation pressure, effectively reduces radiative efficiency. This allows material to accrete at a lower rate while maintaining a substantial luminosity-approximately $2.0 \times 10^{43}$ erg/s-for an extended period, explaining the observed X-ray plateau and differing from predictions based on standard thin-disk models.

The X-ray luminosity light curve of AT 2018cqh (magenta) reveals a similar temporal evolution to seven other optically-selected tidal disruption events (gray), as indicated by 1σ uncertainties, and is based on data from SRG/eROSITA, Swift/XRT, EP/FXT, and XMM/EPIC.
The X-ray luminosity light curve of AT 2018cqh (magenta) reveals a similar temporal evolution to seven other optically-selected tidal disruption events (gray), as indicated by 1σ uncertainties, and is based on data from SRG/eROSITA, Swift/XRT, EP/FXT, and XMM/EPIC.

A Galaxy’s Past: Echoes of a Post-Starburst Environment

Analysis of the host galaxy’s spectral energy distribution indicates it underwent a period of intense star formation in the past, but is now largely quiescent. This “post-starburst” classification suggests a significant decline in star birth activity, potentially occurring within the last few billion years. The galaxy’s current state, characterized by an older stellar population, implies a reduced density of gas and dust available for ongoing star formation. This environment is crucial, as the disrupted star responsible for the AT 2018cqh tidal disruption event likely formed within this galaxy and its properties – mass, age, and surrounding material – were shaped by this relatively calm evolutionary phase. Understanding this history provides essential context for interpreting the characteristics of the event and modeling the behavior of the central black hole.

The galactic environment surrounding AT 2018cqh is characterized by a scarcity of surrounding gas, a factor with potentially significant consequences for the tidal disruption event itself. A lower gas density suggests the disrupted star likely possessed a lower initial mass, as stars forming in such environments tend to be less massive due to limited material for accretion. Furthermore, the star’s trajectory prior to disruption would have been less influenced by drag forces from the interstellar medium, allowing for a wider range of possible approaches to the central black hole. This reduced interaction with surrounding gas could have altered the geometry of the disruption, influencing the resulting flare’s luminosity and duration, and offering crucial insights into the pre-disruption stellar dynamics within this post-starburst galaxy.

The host galaxy of AT 2018cqh possesses a stellar mass of $1.9 \times 10^9$ solar masses, a characteristic that significantly informs the understanding of the tidal disruption event itself. This relatively low mass places constraints on the types of black holes likely to reside within, suggesting a possible intermediate-mass black hole rather than a supermassive one. Detailed analysis of the galaxy’s properties – its age, metallicity, and star formation history – provides crucial context for interpreting the observed characteristics of the TDE, such as the luminosity and duration of the flare. By understanding the galactic environment, researchers can better constrain the disrupted star’s initial mass and trajectory, ultimately revealing insights into the black hole’s formation pathway and its evolutionary history within this specific, post-starburst galaxy.

Spectral analysis of the host galaxy of AT 2018cqh, using Hα emission, Hδ absorption, and optical line ratios, indicates it is likely a post-starburst galaxy with some characteristics of quiescent Balmer-strong galaxies and potentially narrow-line AGN activity.
Spectral analysis of the host galaxy of AT 2018cqh, using Hα emission, Hδ absorption, and optical line ratios, indicates it is likely a post-starburst galaxy with some characteristics of quiescent Balmer-strong galaxies and potentially narrow-line AGN activity.

The comprehensive analysis of AT 2018cqh, detailed within this study, highlights the precariousness of even the most meticulously constructed theoretical frameworks when confronted with the extreme environments surrounding intermediate-mass black holes. Current quantum gravity theories suggest that inside the event horizon spacetime ceases to have classical structure, a notion readily applicable to the observed long-lasting high-state X-ray plateau. As Galileo Galilei observed, “You cannot teach a man anything; you can only help him discover it himself.” This event serves not as a confirmation of existing models, but as a catalyst for refinement, demanding that observations guide theory rather than the other way around. The limitations of current understanding become starkly apparent when attempting to model the accretion disk physics at play.

The Horizon Beckons

The detailed examination of AT 2018cqh, and the revealed characteristics of its progenitor intermediate-mass black hole, offers little in the way of comfort. It simply expands the catalog of the unknowable. The long-lived X-ray plateau is not an answer, but a more refined question-a brighter flare against the backdrop of ignorance. The cosmos generously shows its secrets to those willing to accept that not everything is explainable, and this event underscores that principle with brutal efficiency. Attempts to define the accretion disk’s precise behavior, to constrain the spin of the black hole, or to firmly place this event within a broader population of tidal disruption events, will inevitably run into the limits of observation, and ultimately, of comprehension.

The search for further examples of these intermediate-mass black holes-these cosmic missing links-will undoubtedly continue. But each discovery will likely only deepen the mystery. The observed luminosity challenges existing models, pushing against the Eddington limit as if to mock the very notion of predictable physics. It is a reminder that the universe isn’t obligated to conform to human expectation.

Black holes are nature’s commentary on human hubris. The more precisely such events are studied, the clearer it becomes that the most fundamental questions-what lies beyond the event horizon, what governs the formation of these objects, and what role they play in galactic evolution-may be fundamentally unanswerable. The frontier isn’t about filling gaps in knowledge, but about acknowledging the vastness of what will always remain beyond reach.

Original article: https://arxiv.org/pdf/2512.16568.pdf

Contact the author: https://www.linkedin.com/in/avetisyan/

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2025-12-22 03:48