AstroNote 2025-204

We explored on-sky data acquired by the Vera C. Rubin Observatory, which were released as part of the Data Preview 1 (DP1), searching for extragalactic transients. 

In order to identify reliable transients, we queried difference image analysis (DIA) data products, irrespective of whether the image subtraction residual was “positive” or “negative” with respect to the templates because these may include transient flux.

Our analysis focused on the four extragalactic fields included in DP1, namely ECDFS, EDFS, Fornax, and LELF. To maximize the likelihood of the discovered transients to be genuinely extragalactic, we crossmatched the Rubin DIA Objects with Legacy Survey DR10. Sources that were co-located (separation < 1 arcsec) with catalogued sources well modeled by a point-spread-function were discarded, along with those co-located with bright galactic nuclei. Moreover, we selected only those Rubin transient candidates with a possible host galaxy cataloged within a 30 arcsecond radius.

After visual inspection of the DIA light curves and the Rubin image cutouts, we report the following newly-identified extragalactic transient candidates:

Rubin ID, IAU ID, Rubin_RA (deg), Rubin_Dec (deg)

611255759837069401, AT2024aigg, 53.124768, -27.739815 

592913706862510093, AT 2024aigl, 59.85066258, -48.78070146

592915356129951945, AT 2024aigk, 58.88258339, -48.4621422

Additional candidates are currently being vetted and require further analysis to establish if they are bona-fide extragalactic transients. 

We then cross-matched the Rubin DIA Objects with known sources from the IAU Transient Name Server (TNS), similarly to what Lee et al. (AstroNote 2025-194) did during their visual inspection of the Rubin First Look images released on June 23, 2025. In the 7 fields observed as part of DP1, we found 17 TNS-reported sources which are located within 1 arcsec from Rubin DIA Objects. We checked that these were not flagged as possible Solar System Objects in the Rubin DP1 catalog or co-located within 1 arcsec with PSF-like sources in LS DR10 data. We visually inspected the remaining candidates and deemed 8 to show a likely astrophysical flux change. These are presented in the table below, where coordinates (RA, Dec) are expressed in degrees and separations (Sep) in arcseconds. 

Rubin ID, IAU ID, Rubin_RA, Rubin_Dec, Rubin_TNS_Sep, Nearest_Source_Sep

592914119179370575, 2024ahzi, 58.334940, -48.750276, 0.94, 0.29

609782208097419314, 2024ahzc, 52.838269, -28.279918, 0.38, 0.081

609781520902651937, 2024ahyy, 52.892627, -28.412625, 1.58, 0.15

648365101908230164, 2021bjp, 37.086759, 6.615278, 0.05, 0.35

611253973130674268, 2024ahwk, 52.462466, -28.217948, 0.85, 0.64

628768506166383034, 2023yft, 106.382527, -10.760905, --, 0.11

648374722634973207, 2024aaux, 38.594794, 7.214630, 1.37, 0.40

604064060438217244, 2024ackk, 40.168098, -34.296983, --, 0.31

This work was carried out largely via the Rubin Science Platform (O'Mullane et al. 2024; Jurić et al., 2019; Dubois-Felsmann et al., 2019) and made use of Data Preview 1 data products (NSF-DOE Vera C. Rubin Observatory, 2025; Legacy Survey of Space and Time Data Preview 1  https://doi.org/10.71929/rubin/2570308). We thank the Rubin Observatory project team and the Rubin Science Collaborations for the excellent software resources and tutorials. The Andreoni Transient Astronomy Lab is supported by NSF Award AST 2505775 and NASA grant 24-ADAP24-0159.