One Shot, One Flash: Euclid Captures Known Transients ✛
Context
Euclid was built for cosmology, but its deep images also catch variable objects. Alongside this work, other studies have shown that Euclid can detect transients via image subtraction, while the James Webb Space Telescope reveals supernovae at high redshifts. In the Q1 quick release, each patch of sky was observed only once, so classical difference imaging is impossible. Yet even single exposures carry information, provided you carefully extract it for known objects.
Methods
The authors took Q1 mosaic images of Euclid's three deep fields: Fornax, North, and South. They selected 164 transients from the TNS database that fell within these areas by coordinates and observation time. For 161 objects, they performed PSF-fitting photometry in four filters: IE (optical) and YE, JE, HE (near-infrared). Since no template images were available for background subtraction, the local background was estimated by interpolation around the source. Detection required a signal-to-noise ratio above 2.5 and positional coincidence within uncertainty. For faint or offset sources, they provided upper limits.
Results
Euclid detected 59 transients in the IE filter, 40 in YE, 36 in JE, and 30 in HE. Upper limits were obtained for 93, 91, 88, and 93 objects, respectively. The mean brightness of detections was 23.0 in IE and 21.7 in JE. The limiting depth for an isolated source in IE reached magnitude 28. The recovery fraction for transients discovered within six months before Euclid's observation and brighter than magnitude 24 was 69%, and for those within 100 days, 82%. Among bright examples are the Type Ia supernova SN 2024pvw, caught 15 days before peak in all four filters, and SN 2023aew, whose light curve was traced to 436 days after maximum. For 11 previously 'homeless' transients, Euclid revealed likely faint host galaxies.
Implications
This work aligns with neighboring studies: all use Euclid's deep images to study transients. The difference here is that the authors applied PSF photometry to single Q1 frames on a large scale, rather than to difference images. This allowed them to extract useful information even without repeated observations. Such an approach is important for Euclid's future wide survey, where each patch of sky will be observed only once.
Future development
With the first major data release DR1, the coverage area will grow from 63 to 1900 square degrees. Under the same selection criteria, this will yield more than 5500 transients for follow-up. In the deep fields, repeated epochs will enable image subtraction and direct discovery of new transients. Joint products with the Vera Rubin Observatory will improve light curves and supernova statistics.
Impact
The results will strengthen Type Ia supernova cosmology and the study of their progenitor systems. They will also aid in finding faint host galaxies and investigating the late stages of explosions.
Next steps
Plans include combining Euclid and LSST data for joint light curves. In the deep fields, image subtraction across epochs will begin to find new transients.
Key open problems
For transient astrophysics, the nature of the companion in Type Ia systems and the explosion mechanisms remain open questions. Early infrared observations, like those of SN 2024pvw, directly constrain models. Late-time light curves, like that of SN 2023aew, help distinguish radioactive decay from other energy sources.
The knowledge machine's view
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The authors used single Q1 frames from three Euclid deep fields for PSF photometry of 161 known transients in four filters from optical to near-infrared. They estimated the local background without template subtraction, obtained detections or brightness upper limits, and reconstructed light curves for several objects, including SN 2024pvw up to its peak and SN 2023aew at 436 days after maximum. They also identified likely faint host galaxies for 11 transients.
Where its strength is
A strong point is the demonstration that even single Euclid exposures can yield useful photometry for known transients, which is important for the upcoming wide survey. The careful background estimation and PSF photometry on Q1 mosaics pave the way for systematic use of single-frame snapshots.
Where it could go next
- It might be interesting to compare this approach with experience from transient searches on single frames in other surveys, for example, the work on X-ray clocks in a binary star, which also used sparse observations to analyze variability.based on: X-ray Clock in a Binary Star arXiv:2601.11784
- Related is the study of a cosmic lens that magnified a stellar explosion: perhaps a similar use of gravitational lensing could reveal even fainter transients in Euclid fields.based on: Cosmic Lens Amplified a Star's Explosion a Hundredfold arXiv:2604.07983
- To expand the sample, one could consider methods from the work on a bright flare from the universe's youth, where deep images were used to search for rare events.based on: A Bright Flash from the Universe's Youth arXiv:2607.18972
Worth trying
- Apply PSF photometry to single Euclid frames for transients discovered after Q1 to test the feasibility of retrospective detection.
- Use upper limits to constrain late-time supernova light curves and estimate their energetics.
- Cross-match the identified host galaxies with deep photometry data to study the relationship between transients and their environments.
Why this may matter
This work shows that even single-epoch Euclid observations can be a valuable resource for studying transients, especially in an era when repeat visits are limited. It expands the mission's scientific return and lays the groundwork for future systematic surveys.
Nearby works in our archive
- X-ray Clock in a Binary Starsimilarity 0.51
- Hunting Invisible Particles by Their Tracks in the Spectrumsimilarity 0.51
- The Noise That Gives Away the Chargesimilarity 0.50
- A Bright Flash from the Universe's Youthsimilarity 0.50
- Glowing Dots in Quantum Technologiessimilarity 0.50
- Cosmic Lens Amplified a Star's Explosion a Hundredfoldsimilarity 0.49
The archive is sparse around this work: the closest review is only 0.51, against a usual 0.64. It looks like few have drilled nearby.
Euclid captured SN 2024pvw 15 days before its peak, one of the earliest near-infrared observations of a Type Ia supernova. For comparison, in the CSP-II cosmological sample, such early points occur 2–3 days later.
Key numbers
- number of transients with photometry: 161
- recovery fraction within 100 days before observation: 82%
- limiting depth in IE filter: magnitude 28
- Q1 coverage area: 63.1 square degrees
- expected number of transients in DR1: more than 5500