GCN Circular 43257
Shreya Anand (Stanford), Sean MacBride (University of Zürich), Michael Wood-Vasey (University of Pittsburgh), Erin Howard, Eric Bellm (University of Washington), Bob Armstrong (Lawrence Livermore National Laboratory), Igor Andreoni (UNC Chapel Hill), Antonella Palmese (Carnegie Mellon), Federica Bianco (University of Delaware), Tiago Ribeiro, Lynne Jones, Alysha Shugart, Narayan Khadka, Kshitija Kelkar, Danica Zilkova, Kevin Fanning, Paulina Venegas, Kate Napier, Erik Dennihy, Anastasia Alexov, Bob Blum (NSF/DOE Rubin Observatory), Robert Lupton (Princeton), Keith Bechtol (University of Wisconsin-Madison) reporting on behalf of the NSF-DOE Vera C. Rubin Observatory:
We observed the localization region of the sub-solar mass merger candidate S251112cm, reported by LIGO/Virgo/KAGRA (GCN 42650, GCN 42690) with the 9.6 square degree field of view LSST Camera mounted on the 8.4-m Simonyi Survey Telescope at the Vera C. Rubin Observatory.
Rubin Observatory is in an early operations period of continued system optimization, prior to beginning sustained LSST observations [1]. During this period, data quality and acquisition rate are variable. Not all data products that will be available during the Legacy Survey of Space and Time (LSST) operations are currently available [2].
Observations were conducted through engagement of the new Rubin Target-of-Opportunity advisory board, as a sub-solar mass merger does not meet the typical alert quality criteria for Rubin Target-of-Opportunity (ToO) observation [3]. We conducted observations of the localization area in LSST g and i bands, with one 30 second visit for each band, on nights 0, 2, 4, and 6 after the start of observations. First observations began on the night of November 14, 2025.
We previously reported the efficiency of ToO observations on the night of November 15 2025 in GCN Circular 42707. Subsequent ToO observations continued in the following week in accordance with the recommendation from the Rubin ToO advisory board. No observations were taken on the night of November 20th due to engineering optimization activities. We report the per-band localization coverage in area and probability, along with the median limiting magnitude achieved on each night.
| Date | g-band coverage [deg2] | g-band localization probability [%] | Median g-band depth [mag] | i-band coverage [deg2] | i-band localization probability [%] | Median i-band depth [mag] |
|---|---|---|---|---|---|---|
| November 14, 2025 | 849.17 | 39.23 | 24.76 | 761.27 | 36.70 | 23.72 |
| November 15, 2025 | - | - | - | 96.09 | 2.96 | 23.30 |
| November 16, 2025 | 849.18 | 39.23 | 24.18 | 825.23 | 38.75 | 23.25 |
| November 17, 2025 | - | - | - | 28.33 | 0.66 | 23.23 |
| November 18, 2025 | 777.53 | 36.63 | 24.23 | 557.58 | 21.80 | 23.54 |
| November 19, 2025 | 84.06 | 2.96 | 24.39 | 279.55 | 17.36 | 23.62 |
| November 21, 2025 | 849.18 | 39.23 | 24.54 | 513.98 | 18.90 | 23.66 |
| November 22, 2025 | - | - | - | 340.75 | 21.41 | 23.71 |
Due to the lack of prior Rubin observations in most of the imaging area, we perform image differencing against Dark Energy Camera (DECam) templates. To support the usage of DECam templates, we devised a custom image processing task to create DECam templates for use with LSSTCam observations, using coadded images from the Dark Energy Survey (DES), the DECam Local Volume Exploration (DELVE) Survey, and the Dark Energy Camera All Data Everywhere (DECADE) dataset [4, 5, 6]. Apart from the DECam template task, we utilize the standard LSST data release production pipeline to obtain difference images and photometry on difference image sources [7].
To reduce the large volume of alerts in the localization to a viable set of candidate counterparts, we apply the following candidate filtering criteria: 1) exclude negative and dipole subtractions, 2) require at least 3 detections, 3) require detections in multiple filters, 4) require detections across multiple nights. Furthermore, we crossmatch against the WISE All-sky catalog [8], and exclude nuclear (within 3” of the galaxy nucleus) objects whose hosts have WISE colors consistent with active galactic nuclei (w1-w2 < -0.2 and w1-w2 > 0.65 or w2-w3 > 4.5), as well as the Million Quasars (Milliquas) AGN catalog [9] to reject likely AGN. We crossmatch against the Gaia DR3 catalog [10] and filter out likely stellar objects with reliable Gaia parallax measurements (e_Plx < 1.0). We further downselect for 1) all fading candidates, 2) all crossmatched to within a projected distance of 50 kpc of NED galaxies within the GW volume (GCN 42693), and 3) all fast rising candidates (< -0.3 mag/day), similar to young supernovae. For candidates falling into any of these categories, we visually inspect their cutouts and exclude any object that appears to be consistent with an artifact or star.
We display the coordinates and lightcurve properties of the remaining candidates passing visual checks in the table below. We crossmatched the candidates to Legacy Survey Data Release 9 galaxies [11] with a crossmatch radius of 5 kpc / 12" at the GW distance. In the tables below, we note the photometric redshift of the crossmatched host galaxy for galaxy matches with photoz_err < 0.1 and mag_z < 21 mag.
| tnsName | ra | dec | mjd_g | g | unc_g | mjd_i | i | unc_i | photoz | notes |
|---|---|---|---|---|---|---|---|---|---|---|
| AT2025aill | 38.7231 | -57.9901 | 60994.10 | 23.88 | 0.09 | 60994.16 | 22.12 | 0.05 | - | transient-like (c) |
| AT2025aimp | 35.2965 | -52.4648 | 60994.10 | 22.55 | 0.03 | 60994.16 | 21.58 | 0.03 | - | transient-like (b) |
| AT2025aild | 30.4926 | -49.3871 | 60994.09 | 23.05 | 0.04 | 60994.16 | 21.84 | 0.04 | - | transient-like (c) |
| AT2024aeuy | 4.5991 | -44.9641 | 60994.06 | 22.47 | 0.05 | 60994.14 | 21.05 | 0.02 | 0.163 | transient-like (c,d) |
| AT2025aimx | 6.9966 | -45.6209 | 60994.07 | 22.23 | 0.05 | 60994.14 | 21.59 | 0.04 | 0.294 | transient-like (c) |
| AT2025aimi | 5.0418 | -43.5832 | 60994.06 | 22.03 | 0.03 | 60994.14 | 21.15 | 0.02 | 0.179 | transient-like (c) |
| AT2025adil | 4.9861 | -42.849 | 60994.06 | 20.26 | 0.01 | 61001.14 | 20.51 | 0.02 | 0.145 | transient-like (b,d) |
| AT2025aimj | 13.1101 | -40.5702 | 60994.09 | 22.24 | 0.03 | 60994.15 | 22.18 | 0.05 | 0.130 | transient-like; nuclear (a,b) |
| AT2025aims | 14.8886 | -41.2259 | 60994.09 | 21.11 | 0.01 | 60994.15 | 20.99 | 0.02 | - | transient-like (c) |
| AT2025aile | 6.4582 | -39.459 | 60994.08 | 23.58 | 0.07 | 60994.14 | 21.96 | 0.04 | 0.220 | transient-like (c) |
| AT2025adkm | 7.7313 | -39.6507 | 60994.06 | 22.53 | 0.07 | 60994.14 | 20.52 | 0.01 | 0.090 | transient-like (c,d) |
| AT2025ailh | 11.1327 | -37.4762 | 60996.11 | 21.03 | 0.01 | 60994.15 | 20.78 | 0.01 | 0.251 | transient-like (b) |
| AT2025aili | 352.8751 | -25.2769 | 60994.12 | 22.94 | 0.05 | 60994.18 | 21.88 | 0.06 | - | transient-like (c) |
| AT2025aimo | 350.4845 | -21.584 | 60994.12 | 22.87 | 0.04 | 60999.11 | 22.69 | 0.09 | - | transient-like (c) |
| AT2025ailo | 352.6156 | -21.3053 | 60994.12 | 23.33 | 0.06 | 60994.18 | 22.47 | 0.09 | - | transient-like (c) |
| AT2025aimm | 351.7215 | -21.3579 | 60994.12 | 22.97 | 0.05 | 60994.18 | 22.53 | 0.09 | - | transient-like (b) |
| AT2025xhb | 352.3902 | -20.988 | 60994.12 | 22.51 | 0.04 | 60994.18 | 21.35 | 0.03 | - | transient-like (c,d) |
| AT2025aazc | 351.2951 | -21.0291 | 60994.12 | 20.0 | 0.01 | 60994.18 | 19.45 | 0.01 | - | transient-like (c,d) |
| AT2025aimr | 342.8701 | -14.1804 | 60994.11 | 21.39 | 0.02 | 60994.17 | 21.23 | 0.04 | - | transient-like; variable? (c) |
| AT2025vof | 352.0114 | -13.6978 | 60994.12 | 22.72 | 0.04 | 60994.18 | 22.06 | 0.06 | 0.111 | transient-like (c,d) |
| AT2025admd | 345.8632 | -11.0341 | 60996.09 | 20.18 | 0.01 | 60999.1 | 20.32 | 0.01 | - | transient-like (c,d) |
| AT2025aimn | 349.9407 | -11.7227 | 60994.12 | 21.58 | 0.02 | 60994.17 | 21.07 | 0.03 | 0.098 | transient-like (b) |
| AT2025adqf | 344.8881 | -9.4662 | 60994.11 | 21.87 | 0.02 | 60994.17 | 21.42 | 0.05 | 0.211 | transient-like (b,d) |
| AT2025wjk | 342.422 | -8.891 | 60994.11 | 23.26 | 0.1 | 60994.17 | 21.54 | 0.05 | - | transient-like (a,b,d) |
| AT2025adqe | 342.2308 | -0.6142 | 60994.11 | 22.48 | 0.04 | 60999.1 | 22.16 | 0.06 | 0.273 | transient-like (b,d) |
| AT2025ailp | 36.1916 | -55.4659 | 60998.15 | 22.86 | 0.07 | 60994.16 | 21.44 | 0.03 | 0.166 | nuclear; transient-like (b) |
| AT2025aimq | 33.1928 | -54.7575 | 60994.09 | 21.45 | 0.01 | 60994.16 | 21.26 | 0.02 | 0.216 | nuclear; transient-like (c) |
| AT2025adin | 31.0235 | -51.6365 | 60994.09 | 20.26 | 0.01 | 60996.14 | 20.45 | 0.02 | 0.019* | transient-like (b,d) |
| AT2025aimy | 358.9653 | -29.1393 | 60994.13 | 22.37 | 0.02 | 60994.18 | 21.8 | 0.04 | 0.032* | transient-like (b) |
| AT2025ailr | 354.3794 | -21.466 | 60994.13 | 22.3 | 0.03 | 60996.17 | 21.86 | 0.08 | - | nuclear (c) |
| AT2025ailz | 349.9446 | -18.2149 | 60994.12 | 22.78 | 0.04 | 60996.17 | 21.67 | 0.06 | - | nuclear (c) |
| AT2025aima | 352.0986 | -18.2493 | 60994.12 | 21.57 | 0.02 | 60994.18 | 21.19 | 0.03 | - | nuclear (c) |
| AT2025abwx | 343.111 | -14.6323 | 60994.11 | 19.96 | 0.01 | 60994.17 | 19.66 | 0.01 | - | nuclear (c) |
| AT2025ailt | 342.219 | -8.3411 | 60999.06 | 20.52 | 0.01 | 61001.17 | 20.06 | 0.02 | 0.226 | nuclear (b) |
| AT2025aime | 341.6254 | -7.0032 | 60994.11 | 22.98 | 0.07 | 60999.1 | 21.37 | 0.03 | 0.233 | nuclear (c) |
| AT2025mii | 339.6744 | 0.2168 | 60994.11 | 21.52 | 0.02 | 60996.15 | 20.98 | 0.04 | 0.202 | nuclear (c,d) |
| AT2025aimz | 357.3972 | -28.2219 | 60994.13 | 23.55 | 0.07 | 60994.18 | 22.55 | 0.09 | - | nuclear (a) |
| AT2025aina | 347.8744 | -23.2679 | 60994.11 | 21.02 | 0.01 | 60994.18 | 21.33 | 0.03 | - | nuclear (c) |
| AT2025vyp | 12.8853 | -46.7011 | 60994.07 | 22.94 | 0.09 | 60994.15 | 20.87 | 0.02 | - | transient-like (b,d) |
| AT2025aimh | 13.7904 | -45.917 | 60994.09 | 23.39 | 0.06 | 60994.15 | 22.5 | 0.06 | - | transient-like (a,b) |
| AT2025aimg | 10.5375 | -41.0175 | 60994.07 | 22.54 | 0.07 | 60994.15 | 22.05 | 0.04 | - | transient-like (c) |
| AT2025ailv | 348.1483 | -10.4695 | 60994.12 | 22.65 | 0.04 | 60994.17 | 21.89 | 0.06 | 0.297 | transient-like, nuclear (b) |
| AT2025aimf | 349.1835 | -9.0462 | 60996.10 | 22.5 | 0.05 | 60999.11 | 22.4 | 0.07 | - | transient-like (c) |
| AT2025aimd | 341.7576 | -5.2462 | 60994.11 | 22.26 | 0.03 | 60994.17 | 22.35 | 0.11 | - | transient-like (c) |
| AT2025aily | 1.4555 | -34.251 | 60994.13 | 23.11 | 0.05 | 60994.14 | 22.15 | 0.05 | - | transient-like (c) |
| AT2025aimb | 36.6153 | -52.9123 | 60994.09 | 22.79 | 0.04 | 60994.16 | 22.31 | 0.06 | - | nuclear (a,b) |
| AT2025ailn | 13.1636 | -45.0198 | 60994.09 | 22.84 | 0.04 | 60994.15 | 22.43 | 0.06 | - | nuclear (a,b) |
| AT2025ailk | 13.4689 | -41.8303 | 60994.09 | 21.97 | 0.02 | 60998.19 | 21.57 | 0.05 | - | nuclear (c) |
| AT2025aimt | 13.6167 | -39.7345 | 60994.09 | 23.13 | 0.05 | 60994.15 | 22.39 | 0.06 | - | nuclear (c) |
| AT2025ailq | 4.7717 | -34.3991 | 60994.13 | 21.9 | 0.02 | 60994.14 | 21.98 | 0.05 | - | nuclear (c) |
| AT2025aimc | 350.1702 | -12.5501 | 60994.12 | 23.07 | 0.08 | 60999.11 | 22.44 | 0.08 | - | star offset from galaxy? (c) |
| AT2025aimu | 349.0499 | -11.0721 | 60994.12 | 23.62 | 0.09 | 60994.17 | 21.91 | 0.06 | 0.264 | transient-like (a,b) |
| AT2025aimk | 338.775 | -3.2786 | 60994.11 | 22.33 | 0.03 | 60996.15 | 21.83 | 0.08 | 0.269 | nuclear; transient-like (c) |
*Spectroscopic redshifts from the NED-GWF. These two transients are within 50 kpc of a NED galaxy within the GW localization volume. We note, however, that both of these transients are also closely offset from another galaxy with no crossmatch to the Legacy Survey DR8 catalog.
(a) These candidates exhibit some rising behavior in their lightcurves in at least one band. (b) These candidates are fading by >0.1 mag/day in at least one band. (c) These candidates exhibit very slow or nearly flat evolution in both bands. (d) These candidates were previously reported by another survey.
In the table above, we recover the following previously reported candidates: AT2024aeuy (GOTO), AT2025adil (GOTO, ATLAS), AT2025adkm (GW-MMADS), AT2025aazc (ZTF, GOTO, ATLAS, GW-MMADS), AT2025admd (GW-MMADS, GOTO), AT2025adqf (GW-MMADS), AT2025adqe (GW-MMADS, WFST), AT2025vof (ZTF, ATLAS, GOTO, PS1), AT2025xhb (ALeRCE, ATLAS), AT2025wjk (PS1), AT2025adin (GOTO), AT2025abwx (GOTO, ZTF, ATLAS), AT2025mii (YSE, GOTO, PS1, GW-MMADS), AT2025vyp (ATLAS). AT2024aeuy, AT2025aazc, AT2025vof, AT2025xhb, AT2025wjk, AT2025mii, and AT2025vyp were discovered over a month prior to the GW merger time, and hence are unlikely to be associated with S251112cm.
From our table, we highlight the following candidates that exhibit fast-fading (~0.2 mag/day) behavior in at least one band: AT2025aimp, AT2025aimn and AT2025adqf. We also note that AT2025adin and AT2025aimy (both within 50 kpc of a NED-GWF galaxy) exhibit fading in both g- and i-bands.
We also highlight the following candidates that show rising behavior and could therefore be consistent with young supernovae (though we lack non-detections to constrain age): AT2025aimj, AT2025aimz, AT2025aimb, AT2025ailn, AT2025aimu.
For completeness, we also report the following candidates found in our search that have photometric redshifts from Legacy Survey that are inconsistent with the GW localization volume (photo-z > 0.3):
| tnsName | ra | dec | mjd_g | g | unc_g | mjd_i | i | unc_i | photoz | notes |
|---|---|---|---|---|---|---|---|---|---|---|
| AT2025ailu | 349.9378 | -11.0639 | 60994.12 | 23.2 | 0.07 | 60994.17 | 21.93 | 0.06 | 0.31 | transient-like (c) |
| AT2025ailm | 33.0616 | -54.6792 | 60994.09 | 23.46 | 0.06 | 60994.16 | 22.4 | 0.06 | 0.464 | transient-like (c) |
| AT2025ailc | 33.8628 | -49.8144 | 60994.09 | 22.12 | 0.02 | 60994.16 | 22.01 | 0.04 | 0.367 | transient-like (b) |
| AT2025ails | 1.8871 | -35.6877 | 60994.13 | 22.9 | 0.04 | 60994.14 | 22.36 | 0.07 | 0.413 | transient-like; nuclear (a,b) |
| AT2025ailj | 5.0313 | -35.5348 | 60994.13 | 22.18 | 0.03 | 60994.14 | 21.83 | 0.04 | 0.301 | transient-like (c) |
| AT2025aimv | 349.8256 | -11.5522 | 60994.12 | 23.44 | 0.08 | 60994.17 | 22.24 | 0.07 | 0.422 | transient-like (c) |
| AT2025aimw | 338.0421 | -6.4624 | 60994.11 | 21.82 | 0.02 | 60999.1 | 21.61 | 0.06 | 0.31 | transient-like (c) |
| AT2025ailw | 341.7785 | -5.2592 | 60994.11 | 22.68 | 0.04 | 60994.17 | 22.17 | 0.09 | 0.353 | transient-like, nuclear (b) |
| AT2025adjw | 33.5605 | -54.15 | 60994.09 | 21.82 | 0.02 | 60994.16 | 21.56 | 0.03 | 0.313 | nuclear (a,d) |
| AT2025aiki | 3.9504 | -42.2778 | 60994.06 | 21.98 | 0.03 | 60994.14 | 21.68 | 0.04 | 0.364 | nuclear (c) |
| AT2025aiml | 4.8617 | -35.0092 | 60994.13 | 21.31 | 0.01 | 60994.14 | 21.67 | 0.04 | 1.039 | nuclear (c) |
| AT2025ailg | 340.7488 | -1.6196 | 60994.11 | 21.99 | 0.03 | 60996.15 | 21.78 | 0.08 | 0.921 | stellar/nuclear (c) |
| AT2025ailf | 11.8653 | -44.0097 | 60994.07 | 22.43 | 0.07 | 60994.15 | 22.3 | 0.06 | 0.417 | nuclear (a) |
| AT2025ailx | 345.1107 | -9.1446 | 60994.11 | 22.23 | 0.03 | 60994.17 | 21.47 | 0.05 | 0.481 | nuclear (c) |
In addition to the DECam-Rubin processing, we also perform image subtraction with custom Rubin templates dynamically built during the observing campaign. As the flux of detected transients will be present in the templates, we use this stream mainly as an additional verification of candidates identified from the DECam-Rubin stream. We note that the following candidates are recovered independently by both streams: AT2025aild, AT2024aeuy, AT2025adkm, AT2025aimm, AT2025aimr, AT2025aimn, AT2025ailu, AT2025adqf, AT2025adqe, AT2025aima, AT2025vyp, AT2025aimh, AT2025ailj, AT2025ailv, AT2025aimw, AT2025ailw, AT2025aimb, AT2025ailk, AT2025aiml, AT2025aimc, and AT2025ailx.
We encourage further follow-up to discern the nature of the candidates reported above.
We thank all who contributed to the design and implementation of the Vera C. Rubin Observatory Target-of-Opportunity system, including but not limited to, the LSST Transients and Variable Stars Science Collaboration Multiwavelength Characterization and Counterparts subgroup. We especially thank the Rubin Observatory observers, who are instrumental to the performance and success of the LSST.
This material is based upon work supported in part by the National Science Foundation through Cooperative Agreements AST-1258333 and AST-2241526 and Cooperative Support Agreements AST-1202910 and 2211468 managed by the Association of Universities for Research in Astronomy (AURA), and the Department of Energy under Contract No. DE-AC02-76SF00515 with the SLAC National Accelerator Laboratory managed by Stanford University. Additional Rubin Observatory funding comes from private donations, grants to universities, and in-kind support from LSST-DA Institutional Members.
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