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Early Near-Infrared Excess and Rapid Disk-Corona Evolution in the Tidal Disruption Event 2024aepd

This paper presents multi-wavelength observations of the tidal disruption event 2024aepd, revealing an early near-infrared excess likely caused by free-free emission from a reprocessing photospheric envelope and a rapid spectral evolution from a thermal disk to a hard power-law dominated by a strengthening corona within the first 300 days.

Original authors: Yongxin Wu, Yanan Wang, Thomas M. Reynolds, Shuyuan Wei, Shiyan Zhong, Zikun Lin, Megan Newsome, Sebastian Gomez, Iair Arcavi, Panos Charalampopoulos, Chun Chen, Rongfeng Shen, Ning-Chen Sun, David Ag

Published 2026-07-17

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CC BY 4.0

Original authors: Yongxin Wu, Yanan Wang, Thomas M. Reynolds, Shuyuan Wei, Shiyan Zhong, Zikun Lin, Megan Newsome, Sebastian Gomez, Iair Arcavi, Panos Charalampopoulos, Chun Chen, Rongfeng Shen, Ning-Chen Sun, David Aguado, Ismael Pérez-Fournon, Frédérick Poidevin, Zhongnan Dong, Niu Li, Weijian Guo, Hu Zou, Jingbo Sun, Nieves Castro-Rodríguez, Antonio Cabrera-Lavers, Ning Jiang, Hengxiao Guo, J. P. Anderson, Tomás E. Müller-Bravo, Seppo Mattila, Claudia P. Gutiérrez, Amit Kumar, G. Pignata, Xiangkun Liu, R. Dastidar, Brajesh Kumar, Xiaowei Liu, Bin Ma, M. Dennefeld, Francesca Onori, Lydia Makrygianni, Mariusz Gromadzki, Xuan Fang

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

Imagine the universe as a grand, chaotic kitchen where supermassive black holes sit at the center of galaxies, acting like hungry, invisible chefs. Usually, these chefs are quiet, but occasionally, a stray star wanders too close to the stove. When this happens, the black hole's immense gravity rips the star apart in a violent event called a "tidal disruption event" (TDE). Think of it like a cosmic spaghetti monster: the star gets stretched into a long, thin stream of gas, some of which falls into the black hole to be eaten, while the rest swirls around, heating up and glowing brightly across the entire spectrum of light.

Scientists have long studied these cosmic feasts, but they've been puzzled by two specific mysteries. First, when the gas falls in, it usually forms a hot, glowing disk that emits soft, low-energy X-rays. But sometimes, a second, harder, more energetic "tail" of X-rays appears, suggesting a super-hot layer of electrons (a "corona") is forming above the disk, like steam rising from a boiling pot. Second, astronomers often see a burst of near-infrared (NIR) light—a type of heat radiation—appearing very early on. For years, they thought this was just dust in the neighborhood getting heated up and re-radiating the light (a "dust echo"), like a spotlight hitting a dusty attic. But what if that dust isn't the culprit? What if the light is coming from the swirling gas itself?

This is the story of TDE 2024aepd, a stellar meal discovered in late 2024. A team of astronomers from around the globe acted like cosmic detectives, watching this event unfold over about 300 days using telescopes that could see everything from radio waves to X-rays. Their goal was to figure out exactly what was cooking in the kitchen and how the ingredients changed over time.

The investigation revealed a fascinating two-part drama. First, the X-ray show was a tale of two states. At the beginning, the light was dominated by the soft, thermal glow of the accretion disk, but there was already a hint of a hard, energetic excess. As time went on, specifically around day 178, the show changed completely. The soft disk light faded, and the hard, energetic "corona" took over, becoming the main source of X-rays. It was as if the kitchen went from a gentle simmer to a roaring, high-pressure fire. This rapid shift suggests that the corona didn't just appear; it grew stronger and eventually dominated the scene, a behavior that helps astronomers understand how black holes evolve over human timescales rather than millions of years.

The second mystery, the early near-infrared excess, was even more intriguing. The team spotted this extra heat glow as early as 40 days after the star was torn apart. When they analyzed the light, they found it didn't look like the smooth, predictable glow of a hot dust cloud. Instead, it had a flat, steady spectrum that didn't fit the "dust echo" theory. If it were dust, the light would have taken much longer to arrive—hundreds of days—because the dust would be far away. But the light arrived way too fast for that.

Instead, the authors suggest a more dynamic explanation: the near-infrared light is coming from the gas itself. They propose that the swirling debris forms a thick, expanding envelope around the black hole. As light tries to escape this dense fog, the lower-frequency (redder) light gets trapped and reprocessed at the outer edges, while the higher-frequency (bluer) light escapes from deeper inside. This "frequency-dependent" reprocessing creates that flat, extra glow in the near-infrared. It's like shining a flashlight through a thick, foggy window; the light that makes it through the fog looks different than the light that escapes directly.

The paper doesn't claim to have solved every mystery, but it strongly argues against the idea that this early infrared glow is just dust. The timing is simply too fast for a dust echo. Instead, the evidence points toward a complex, evolving cloud of gas that is reprocessing the light as it expands. By comparing this event to a few other similar cosmic meals, the team suggests that this kind of early infrared excess might be a common feature of tidal disruption events, waiting to be discovered in more cases.

In the end, TDE 2024aepd gave astronomers a front-row seat to the birth and evolution of a black hole's corona and the complex dance of gas surrounding it. It showed that these cosmic events aren't just simple flashes of light; they are dynamic, evolving systems where the structure of the gas and the nature of the radiation change rapidly, offering a unique laboratory to study the physics of black holes in action.

Technical Summary: Early Near-Infrared Excess and Rapid Disk-Corona Evolution in the Tidal Disruption Event 2024aepd

Problem and Context
Tidal disruption events (TDEs) provide a unique laboratory for studying the formation and evolution of accretion disks and coronae around supermassive black holes (SMBHs) on observable timescales. While X-ray TDEs often exhibit spectral state transitions from soft (disk-dominated) to hard (corona-dominated) phases, the physical mechanisms driving these transitions and the origin of non-thermal X-ray emission remain under debate. Furthermore, the origin of infrared (IR) excesses in TDEs is contested; while mid-IR excesses are typically attributed to dust echoes, the nature of early-time near-infrared (NIR) excesses—recently identified in events like AT2019azh and TDE 2025abcr—remains unclear. Distinguishing between dust echoes and free-free emission from reprocessing envelopes is critical for constraining the density structure and dynamics of TDE outflows.

Methodology
The authors present multi-wavelength observations of the TDE 2024aepd, discovered by the Zwicky Transient Facility (ZTF) on December 5, 2024, at a redshift of $z=0.0835$ . The study spans approximately 300 days post-discovery, covering radio, NIR, UV, optical, and soft X-ray bands.

Observations: Data were collected from ZTF, ATLAS, Mephisto, LCO, Swift-UVOT, Swift-XRT, NOTCam, GTC (EMIR), SYSU 80 cm telescope, e-MERLIN, and ATCA. Spectroscopic follow-up was conducted using NOT, LT, NTT, GTC, LCO, and DESI.
Data Analysis:
- Photometry: Light curves were constructed and corrected for Galactic extinction. Host galaxy subtraction was performed using archival templates (Pan-STARRS, UKIRT).
- Spectral Energy Distribution (SED) Modeling: UV-optical SEDs were fitted with single-temperature blackbody models. NIR excesses were modeled using both modified dust blackbody models (dust echo scenario) and free-free emission models (reprocessing envelope scenario).
- X-ray Spectral Analysis: XRT spectra were analyzed using XSPEC. Models included absorbed multi-temperature blackbodies (diskbb) and power-law components (powerlaw). The evolution of the photon index ( $\Gamma$ ) and hardness ratio was tracked across five stacked epochs.
- Physical Parameter Estimation: The SMBH mass was estimated via stellar velocity dispersion from DESI spectra and via light curve fitting using MOSFiT. The host galaxy properties were derived using CIGALE.
- Free-Free Modeling: A frequency-dependent thermalization framework was applied to the NIR data, assuming a homogeneous spherical medium with electron scattering dominance and a power-law density profile ( $\rho \propto r^{-s}$ ).

Key Results

Black Hole and Host Properties: The host galaxy is a moderately massive ( $M_* \approx 10^{10.1} M_\odot$ ), quiescent system. The SMBH mass is estimated at $\log_{10}(M_{BH}/M_\odot) \approx 6.43$ (from velocity dispersion) and $6.30$ (from MOSFiT), consistent with a low-mass SMBH.
UV-Optical Evolution: The UV-optical emission is well-described by a cooling blackbody. The temperature decreases from $\sim 3.3 \times 10^4$ K to $\sim 2.4 \times 10^4$ K, while the radius peaks at $\sim 6 \times 10^{14}$ cm and subsequently declines. The luminosity follows a power-law decay ( $L \propto t^{-1.52}$ ).
Early NIR Excess: A prominent NIR excess is detected as early as $\sim 40$ $\sim 40$ days, deviating significantly from the Rayleigh-Jeans tail of the UV-optical blackbody. The NIR spectrum is nearly flat ( $f_\nu \propto \nu^{\alpha}$ $f_{ν} \propto ν^{α}$ with $\alpha \approx 0$ $α \approx 0$ ).
- Origin: A conventional dust-echo origin is disfavored because the inferred sublimation radius implies a light-travel time ( $\sim 743$ days) far exceeding the observed lag ( $< 17$ days).
- Preferred Model: The data are better explained by free-free emission from a reprocessing photospheric envelope. The spectral slope suggests a density profile index $s \approx 1.4-1.8$ . The break frequency ( $\nu_{break}$ ) shifts to higher frequencies over time, while the density profile index remains constant, implying evolving reprocessing conditions within a stable density structure.
X-ray Spectral Evolution: The X-ray spectrum initially shows a thermal disk component with a hard excess. From $\sim 178$ days onward, the spectrum becomes power-law dominated and hardens (photon index $\Gamma$ decreases from $\sim 2.8$ to $\sim 1.6$ ). This indicates a rapid transition from a soft, disk-dominated state to a hard, corona-dominated state.
$\Gamma$ – $L_X$ Correlation: In a compiled sample of X-ray TDEs, the authors identify a positive correlation between the photon index $\Gamma$ and the Eddington ratio ( $L_X/L_{Edd}$ ). Unlike X-ray binaries (XRBs) and AGN, which often show a negative correlation at low accretion rates, no clear anti-correlation branch is identified in the current TDE sample, likely due to a lack of sufficiently low-luminosity data points.

Significance and Claims
The paper claims that TDE 2024aepd is the third reported TDE to exhibit an early-time NIR excess, joining AT2019azh and TDE 2025abcr. The study provides strong evidence that such NIR excesses in TDEs are likely caused by free-free emission from an extended, reprocessing outflowing layer rather than dust echoes, offering a new diagnostic for the density structure of TDE debris.

Furthermore, the rapid spectral state transition observed in 2024aepd (occurring at an Eddington ratio of $\sim 0.5\%$ ) reinforces the analogy between TDEs and XRBs regarding disk-corona evolution. The identification of a positive $\Gamma$ – $L_X$ correlation in TDEs suggests that the physical mechanisms governing accretion flow structure (e.g., disk truncation or evaporation) may operate similarly in TDEs and other accreting black hole systems, though the absence of the low-luminosity anti-correlation branch warrants further investigation with larger, deeper samples.

The authors conclude that systematic early-time NIR follow-up is essential to determine the ubiquity of these excesses and to constrain the physical conditions of the reprocessing layers in TDEs.

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