🔭 astrophysics

AT2024lhc and AT2024kmq in the landscape of featureless tidal disruption events

This paper characterizes two high-mass black hole tidal disruption events, AT2024kmq and AT2024lhc, as luminous, long-lasting, and hard X-ray sources, providing evidence that the soft-to-hard spectral state transition in TDEs occurs at a critical accretion rate threshold and scales inversely with black hole mass.

Original authors: Yuhan Yao, Ryan Chornock, Andrew Mummery, Raffaella Margutti, Marat Gilfanov, Muryel Guolo, Eric R. Coughlin, Wenbin Lu, Joheen Chakraborty, Dheeraj R. Pasham, Kate D. Alexander, Olivia Aspegren, Char

Published 2026-06-03

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Yuhan Yao, Ryan Chornock, Andrew Mummery, Raffaella Margutti, Marat Gilfanov, Muryel Guolo, Eric R. Coughlin, Wenbin Lu, Joheen Chakraborty, Dheeraj R. Pasham, Kate D. Alexander, Olivia Aspegren, Charlotte R. Angus, Xinze Guo, Xander J. Hall, Erica Hammerstein, K. -Ryan Hinds, Anna Y. Q. Ho, Xiaoshan Huang, Elias Kammoun, Natalie LeBaron, Matteo Lucchini, Zoë McGrath, Matt Nicholl, Daniel A. Perley, R. Michael Rich, Genevieve Schroeder, Xinyue Sheng, Jesper Sollerman, Jean Somalwar, Jacob R. Wise, Michael W. Coughlin, Andrew Drake, Matthew J. Graham, George Helou, Joahan C. Jaimes, Mansi M. Kasliwal, Ashish A. Mahabal, Pavel Medvedev, Josiah Purdum, Ben Rusholme, Rashid Sunyaev

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: Cosmic "Star-Snacking"

Imagine a massive black hole as a giant, invisible vacuum cleaner floating in space. Usually, it just sits there, quietly. But sometimes, a star wanders too close. The black hole's gravity is so strong that it rips the star apart, stretching it like a piece of taffy. This event is called a Tidal Disruption Event (TDE).

When the star gets torn apart, its debris swirls around the black hole, heats up, and glows brightly. This is the "snack" the black hole is eating. Most of the time, we can see this snack glowing in different colors, like a rainbow of gas clouds.

However, the two events studied in this paper—AT2024kmq and AT2024lhc—are special. They are "featureless." Instead of a colorful rainbow with distinct lines (like a barcode), they look like a smooth, bright blue light with no patterns. It's like looking at a perfectly smooth, glowing blue marble instead of a messy, colorful firework.

The Mystery: Why Are They So Blue and Bright?

The researchers found that these two events are happening around super-massive black holes (about 100 million times heavier than our Sun). Because the black holes are so huge, the physics of the "snack" is different.

The "Blue Marble" Effect: The light from these events is incredibly hot and blue. The paper suggests that the gas is so hot and ionized (charged up) that it wipes out the usual colorful patterns we see in other TDEs. It's like turning up the heat on a stove until the metal glows so bright white that you can't see the texture anymore.
Two Types of Blue: The researchers noticed something interesting. If you look at all the "featureless" TDEs we know of, they seem to fall into two groups:
- Group A: Very bright and huge (like AT2024kmq and AT2024lhc).
- Group B: Dimmer and smaller.
  This suggests there might be two different ways to make a "featureless" blue light, rather than just one continuous range.

The X-Ray Surprise: The "Hot Corona"

Usually, when a black hole eats a star, it starts off glowing softly in X-rays (like a warm glow) and only gets "hard" and energetic later on. But these two events were different.

The Early Spark: They started glowing with hard, energetic X-rays almost immediately, right when the optical light peaked.
The Tiny Engine: The X-rays changed brightness very quickly (in just a few hours). This tells us the source of the X-rays must be very small and compact, sitting very close to the black hole.
The Analogy: Think of a normal TDE as a campfire that slowly builds up heat. These two events were like a sudden, intense flare from a tiny, super-hot spotlight right next to the fire. The paper suggests this is caused by a "corona" (a hot, energetic cloud of particles) forming very quickly around the black hole.

The "Precursor": The Appetizer

Before the main event (the big flash of light), both of these TDEs had a small "appetizer" phase.

What happened: A few weeks before the main explosion, there was a smaller, reddish flash of light that faded away quickly.
The Cause: The researchers believe this was caused by the torn-apart star's debris crashing into itself. Imagine two streams of water colliding in mid-air; the splash creates a burst of energy. This "self-intersection" happened early because the black holes were so massive, pulling the debris together faster than usual.

The "No Radio" Rule

Usually, when black holes eat stars, they shoot out powerful jets of particles (like a garden hose spraying water). These jets create radio waves.

The Finding: The researchers looked for these radio waves but found nothing.
The Conclusion: This means there were no powerful jets shooting out at us. The energy was contained in the hot gas and the corona, not blasted away into space.

The "State Change" Theory

The paper connects these findings to a bigger idea about how black holes eat.

The Analogy: Think of a black hole's eating habit like a car shifting gears.
- Low Mass Black Holes: They eat slowly. They stay in "Soft Gear" (cool, steady X-rays) for a long time before shifting to "Hard Gear" (hot, energetic X-rays).
- High Mass Black Holes (like these two): Because they are so huge, they shift into "Hard Gear" almost instantly.
The Threshold: The paper argues that this switch happens when the black hole is eating at a specific speed (about 3% of its maximum possible speed). Once it drops below that speed, the "gear" shifts. Because massive black holes eat so fast initially, they hit this "switch" point very quickly, explaining why we see hard X-rays right away.

Summary

In short, this paper studies two rare cosmic events where a star was eaten by a giant black hole. These events are unique because:

They glow a smooth, featureless blue.
They shoot out hard X-rays immediately.
They have a small "appetizer" flash caused by debris crashing into itself.
They don't shoot out radio jets.

The researchers conclude that these events happen around the biggest black holes, which eat so fast that they switch their "eating mode" almost instantly, creating a hot, compact engine that glows brightly without the usual colorful patterns.

Technical Summary: AT2024lhc and AT2024kmq in the landscape of featureless tidal disruption events

Problem and Context
Tidal Disruption Events (TDEs) serve as unique laboratories for studying accretion physics around massive black holes (MBHs). While many TDEs exhibit broad emission lines (H, He, Bowen fluorescence), a distinct class of "featureless" TDEs displays blue continua without discernible spectral features. These events are predominantly associated with high-mass black holes ( $M_{BH} \sim 10^8 M_\odot$ ), where general relativistic effects significantly influence debris dynamics. A critical open question in TDE physics is whether these events undergo accretion state transitions analogous to stellar-mass X-ray binaries (XRBs)—specifically, transitions from a soft, thermal-disk dominated state to a hard, Comptonized corona state—and how the timing of these transitions depends on black hole mass and accretion rate. Furthermore, the physical origin of the UV/optical emission in these high-mass, featureless events remains debated, with existing models often underestimating black hole masses or failing to explain the lack of spectral lines.

Methodology
The authors present comprehensive multi-wavelength observations of two luminous, spectroscopically featureless TDEs, AT2024kmq and AT2024lhc, discovered by the Zwicky Transient Facility (ZTF).

Observations: The study utilizes data from ZTF, ATLAS, the Liverpool Telescope, and the Keck, Lick, and NOT telescopes for optical photometry and spectroscopy. X-ray coverage includes Swift/XRT, NICER, NuSTAR, Chandra, and XMM-Newton. Radio constraints were obtained using the Very Large Array (VLA).
Analysis:
- Spectroscopy: Optical spectra were analyzed to confirm the absence of broad emission lines. Host galaxy properties (stellar mass, extinction, velocity dispersion $\sigma_*$ ) were derived via population synthesis and spectral fitting. Black hole masses were estimated using $M_{BH}-\sigma_*$ relations and refined using the Bayesian tidalspin framework to account for Hills mass constraints.
- Photometry: UV/optical light curves were modeled using blackbody fits to derive temperature ( $T_{bb}$ ) and radius ( $R_{bb}$ ). X-ray light curves and spectra were analyzed to determine flux variability timescales and photon indices ( $\Gamma$ ).
- Population Synthesis: To contextualize these events, the authors assembled a sample of 49 ZTF TDEs across four spectral subclasses. They performed a bootstrap $K$ -means clustering analysis to test for bimodality in peak blackbody luminosity and radius.
- Theoretical Modeling: The authors compared observed X-ray spectral evolution against time-dependent accretion disk theory. They utilized numerical population synthesis (using FitTeD) to calculate the transition timescales ( $t_{tr}$ ) required for TDE disks to reach a critical Eddington ratio ( $\dot{m} \approx 0.03$ ) across a range of black hole masses.

Key Results

Early Hard X-ray Emission: Both AT2024kmq and AT2024lhc exhibit luminous ( $L_X \sim 10^{44}$ erg s $^{-1}$ ), rapidly variable ( $\sim 1.3$ –4.8 hr timescales), and hard X-ray spectra ( $1.7 < \Gamma < 3$ ) emerging no later than the optical peak. This contrasts with lower-mass TDEs, which typically show soft X-rays initially and transition to hard states only after hundreds of days.
Compact Corona: The rapid X-ray variability constrains the emitting region to $\lesssim 10 r_g$ . The lack of radio detection disfavors relativistic jets, supporting a model where the hard X-rays arise from thermal Comptonization in a compact corona ( $\lesssim 10 r_g$ ).
Optical Precursors: Both events display optical precursors $\sim 30$ days before the main peak, consistent with shock dissipation from the self-intersection of stellar debris streams. AT2024kmq's precursor is cooler ( $T \sim 6600$ K) and larger, while AT2024lhc's is hotter ( $T \sim 2 \times 10^4$ K).
Population Bimodality: Analysis of the featureless TDE population reveals a statistically significant bimodality in peak blackbody luminosity and radius. AT2024kmq and AT2024lhc belong to a high-luminosity, large-radius subclass, distinct from a subluminous, compact subclass. This bimodality may reflect different physical regimes (high luminosity vs. compact photosphere) required to maintain the high ionization necessary to suppress spectral lines.
Mass-Dependent State Transitions: By comparing the X-ray evolution of 11 TDEs with early eROSITA data, the authors find a mass segregation in spectral states: low-mass TDEs ( $M_{BH} \sim 10^6 M_\odot$ ) remain soft for $\sim 2$ years; intermediate masses ( $\sim 10^7 M_\odot$ ) transition at $\sim 1$ year; and high masses ( $\sim 10^8 M_\odot$ ) are hard from the outset.
Critical Accretion Rate: The observed mass segregation is quantitatively reproduced by disk theory assuming the soft-to-hard transition occurs when the accretion rate drops to a critical threshold of $\dot{m} \approx 0.03 \dot{M}_{Edd}$ . The transition timescale scales as $t_{tr} \propto M_{BH}^{-3/4}$ , explaining why high-mass black holes enter the hard state much earlier in their evolution.

Significance and Claims
The paper claims that AT2024kmq and AT2024lhc provide the first clear evidence of luminous, hard X-ray emission contemporaneous with the optical peak in optically selected TDEs. The authors interpret this as evidence that TDEs undergo soft-to-hard accretion state transitions at a critical Eddington ratio of $\dot{m} \sim 0.03$ , phenomenologically similar to XRBs but scaled to much longer absolute timescales due to the black hole mass dependence ( $t_{tr} \propto M_{BH}^{-3/4}$ ).

The work suggests that the "featureless" spectral class is not monolithic but bimodal, likely driven by the physical conditions (luminosity and photospheric size) required to sustain high ionization. The authors note that current models for TDE UV/optical emission struggle in the high-mass regime, as standard accretion disk theory successfully explains the X-ray state transitions but may require additional physics (such as stream-disk collision shocks) to fully account for the UV/optical luminosity, particularly for AT2024kmq which exceeds the expected Eddington limit at peak. Finally, the paper posits that the rarity of observed optical precursors may be a selection effect, as they are likely more prominent in high-mass systems where self-intersection occurs closer to the black hole, yet these systems are less frequent in the overall TDE rate.