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Discovery of a Featureless Tidal Disruption Event at z~1 with the Wide Field Survey Telescope

The Wide Field Survey Telescope has discovered WFST250820mmsw/AT2025wet, the highest-redshift non-jetted tidal disruption event known to date at z~1.037, which exhibits a featureless blue continuum and provides a unique opportunity to investigate the intrinsic extreme-UV emission and energy budget of TDEs.

Original authors: Jiazheng Zhu, Zelin Xu, Ning Jiang, Ji-an Jiang, Tinggui Wang, Yuhan Yao, Ryan Chornock, Erica Hammerstein, Yibo Wang, Min-Xuan Cai, Shifeng Huang, Wenkai Li, Mingxin Wu, Chichuan Jin, Jie Lin, Jianwe

Published 2026-05-22

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Jiazheng Zhu, Zelin Xu, Ning Jiang, Ji-an Jiang, Tinggui Wang, Yuhan Yao, Ryan Chornock, Erica Hammerstein, Yibo Wang, Min-Xuan Cai, Shifeng Huang, Wenkai Li, Mingxin Wu, Chichuan Jin, Jie Lin, Jianwei Lyu, Dezheng Meng, Weiyu Wu, Zhengyan Liu, Junhan Zhao, Ziqing Jia, Chengyi Wang, Lulu Fan, Xu Kong, Feng Li, Ming Liang, Jinling Tang, Hairen Wang, Jian Wang, Yongquan Xue, Ji Yang, Hongfei Zhang, Wen Zhao, Qingfeng Zhu

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: Catching a Cosmic "Star Crash"

Imagine a supermassive black hole as a giant, invisible vacuum cleaner sitting at the center of a galaxy. Usually, it sits quietly. But sometimes, a star wanders too close and gets ripped apart by the black hole's gravity. This violent event is called a Tidal Disruption Event (TDE). It's like a cosmic car crash that creates a massive, bright flash of light.

For years, astronomers have been finding these crashes, but mostly in our "backyard" of the universe (relatively close by). This paper reports the discovery of a brand new TDE that is much, much farther away than any non-jetted one we've seen before. It's so far away that the light took over 8 billion years to reach us.

The Discovery: A Flash in the Dark

A new telescope called the Wide Field Survey Telescope (WFST), which acts like a giant, high-speed camera scanning the sky, spotted a strange, blue flash of light in a distant galaxy.

The Name: The team calls it AT 2025wet (or WFST250820mmsw).
The Location: It happened in a galaxy about 10 billion light-years away (redshift $z \approx 1.037$ ).
The Look: The flash was incredibly blue and bright—about 1,000 times brighter than the galaxy it lived in. It faded slowly over a month, which is a classic sign of a star being eaten.

The Mystery: Why Is It "Featureless"?

When astronomers look at light from space, they usually see "fingerprints" called spectral lines. These are like barcodes that tell us what elements (like hydrogen, helium, or iron) are present.

The Surprise: When the team used powerful telescopes (Keck and MMT) to look at the light from AT 2025wet, they saw nothing. No barcodes, no fingerprints, just a smooth, blue curve.
The Analogy: Imagine listening to a song. Usually, you can hear the specific instruments (drums, guitar, vocals). This event was like hearing a pure, smooth tone with no instruments at all. In astronomy, this "featureless" look is a specific signature of a certain type of star-crash event.

Solving the Puzzle: Is It a Black Hole or a Monster Star?

The team had to make sure this wasn't something else, like a dying star (supernova) or a galaxy waking up (Active Galactic Nucleus).

The Host Galaxy: They looked at the galaxy hosting the event. It's a massive, old galaxy with a huge central black hole (about 100 million times the mass of our Sun).
The Smoking Gun: The galaxy was quiet before the flash. There were no signs of a "monster star" explosion, and the light didn't behave like a typical active galaxy. The combination of the location, the speed of the flash, and the "featureless" light confirmed it is a Tidal Disruption Event.

The "Missing Energy" Mystery

Here is the most exciting part of the paper.

The Problem: When we look at these flashes in visible light (what our eyes see), they seem to have a certain amount of energy. But physics models suggest they should be releasing way more energy than we can see. It's like seeing a campfire that feels warm but doesn't seem to be producing enough heat to explain the smoke. Scientists call this the "Missing Energy Puzzle."
The Theory: The missing energy is likely being released as Extreme Ultraviolet (EUV) light. This is a type of light that is invisible to our eyes and standard telescopes because Earth's atmosphere blocks it, and it's usually absorbed by gas in space before it reaches us.
The Breakthrough: Because AT 2025wet is so far away, the expansion of the universe has stretched the light. The invisible EUV light from the event has been "stretched" (redshifted) into the visible blue light that our telescopes can see.
The Result: By studying this distant event, the team found that the light fits a "power-law" pattern (a specific mathematical shape) rather than a simple "blackbody" curve. This suggests that the event is indeed dumping a massive amount of energy into the UV range, solving the missing energy puzzle for this specific case.

Why This Matters

This discovery is a milestone for two reasons:

It's the Record Holder: It is the highest-redshift (farthest) non-jetted TDE ever found. It proves that our new telescopes can find these rare events in the deep universe.
It Opens a New Window: Because the universe stretched the invisible UV light into visible light, we got a direct look at the "true" energy output of a TDE. This helps scientists understand how black holes eat stars and where all that missing energy goes.

In short: Astronomers used a new, powerful camera to spot a star getting eaten by a black hole 10 billion light-years away. The event was so far away that the invisible "missing energy" was stretched into visible light, allowing scientists to finally see the full picture of how these cosmic crashes work.

Technical Summary: Discovery of a Featureless Tidal Disruption Event at z ∼1

Problem and Motivation
Optical tidal disruption events (TDEs) have historically been confined to low redshifts ( $z \lesssim 0.5$ ) and the bright end of the luminosity function, limiting the census of dormant supermassive black holes (SMBHs) and hindering the study of intrinsic spectral energy distributions (SEDs). A central unresolved issue in TDE physics is the "missing energy" puzzle: optical TDEs are often modeled as blackbodies with temperatures of $(1-4) \times 10^4$ K, yet theoretical models (reprocessing of X-rays or stream-stream collisions) predict that the SED should peak in the extreme-ultraviolet (EUV) regime. Because this peak is redshifted out of observable windows for local events, the total radiative output remains uncertain. High-redshift TDEs offer a unique opportunity to shift the rest-frame far-UV emission into observable optical/UV bands, allowing for direct tests of SED models and the origin of optical emission.

Methodology
The authors report the discovery of the transient WFST250820mmsw/AT 2025wet using the 2.5-meter Wide Field Survey Telescope (WFST) as part of the Deep High-cadence ugr-band Survey (DHugr). The event was detected in the $g$ -band with a peak magnitude of $\sim 22$ , approximately 3 magnitudes brighter than its host galaxy.

Observations: Multi-wavelength follow-up included:
- WFST: High-cadence $u, g, r$ imaging to construct light curves.
- Spectroscopy: Three optical spectra obtained with Keck/LRIS (2025 Oct, Dec) and MMT/Binospec (2025 Nov), alongside archival DESI data.
- Swift/UVOT: UV photometry in $UVW2$ and $UVW1$ bands.
- Swift/XRT & Einstein Probe (FXT): X-ray observations to constrain high-energy emission.
- Archival Data: Host galaxy photometry from ZTF, HSC-SSP, SDSS, and UKIDSS to characterize the pre-outburst environment.
Analysis:
- Redshift Determination: Identified via host galaxy absorption lines (Ca II H&K) in the Keck spectra and archival DESI data.
- SED Modeling: Multi-band photometry was fitted with both blackbody and power-law models using MCMC sampling.
- Host Characterization: The host galaxy SED was modeled using CIGALE to derive stellar mass and star formation rate, which were then used to estimate the central black hole mass via empirical $M_{BH}-M_{\star}$ relations.

Key Results

Discovery and Classification: AT 2025wet is located at a redshift of $z = 1.037$ , making it the highest-redshift non-jetted TDE known to date. The transient is spatially coincident with the host nucleus ( $\Delta \text{RA} \approx 0.06''$ ) with a chance-coincidence probability of $\sim 5 \times 10^{-5}$ .
Spectroscopic Properties: The Keck/LRIS spectrum taken near the optical peak reveals a featureless blue continuum with no discernible emission lines (e.g., no C IV, C III], Mg II, or H $\beta$ ). No spectral features indicative of a foreground or background source were detected.
SED and Luminosity:
- Blackbody fits to the multi-band SED yield a constant temperature of $\sim 19,000$ K and a peak bolometric luminosity of $(8.27^{+0.92}_{-0.71}) \times 10^{44}$ erg s $^{-1}$ .
- However, the authors note that a single-temperature blackbody provides a poor fit ( $\chi^2_{\text{red}} = 7.41$ ) when including UV data. A power-law model ( $F_\nu \propto \nu^{-1.4}$ ) provides a significantly better fit ( $\chi^2_{\text{red}} = 0.63$ ), suggesting the SED peaks at wavelengths shorter than the optical band (likely in the EUV).
Host and Black Hole Properties: The host galaxy has a stellar mass of $\sim 10^{11.2} M_\odot$ and shows no evidence of prior AGN activity. Based on the stellar mass, the central black hole mass is estimated at $\sim 10^8 M_\odot$ .
X-ray Constraints: No significant X-ray emission was detected by Swift/XRT or Einstein Probe. The 3 $\sigma$ upper limit on X-ray luminosity is $1.5 \times 10^{44}$ erg s $^{-1}$ , resulting in a low ratio of $L_X/L_{\text{opt,bb}} < 0.19$ , which disfavors a jetted TDE scenario.

Significance and Claims
The paper claims that AT 2025wet represents a robust, featureless TDE at $z > 1$ , providing the first direct observational window into the rest-frame UV SED of such events. The authors argue that the featureless nature of the spectrum, combined with the high luminosity and lack of X-rays, distinguishes this event from turn-on AGNs and weak-emission-line quasars.

The primary significance lies in the ability of high-redshift TDEs to test the "missing energy" hypothesis. The observation that the SED is better described by a power law extending into the UV, rather than a simple blackbody, supports models where the intrinsic SED peaks in the EUV. This suggests that a substantial fraction of the radiative energy in TDEs is emitted in the EUV, which is typically unobservable for local events due to the lack of a redshifted window. The authors conclude that ongoing and future deep optical surveys (WFST, LSST) are essential for expanding the census of high-redshift TDEs, which will be critical for unraveling the physics of optical TDE emission and the demographics of SMBHs at cosmic noon.