GCN Circular 41595
Sean MacBride (University of Zurich), Shreya Anand (Stanford University), Erin Howard, Ian Sullivan, David Wang, Lynne Jones, Peter Yoachim, Eric Bellm (University of Washington), Bob Armstrong (Lawrence Livermore National Laboratory), Michael Wood-Vasey (University of Pittsburgh), Alex Drlica-Wagner, Ken Herner (Fermi National Accelerator Laboratory), Gautham Narayan (University of Illinois), Tatiana Acero Cuellar, Federica Bianco (University of Delaware), Bruno Sánchez (Aix Marseille Université), John Banovetz (Lawrence Berkeley National Laboratory), Kris Mortensen, Paulina Venegas, Yijung Kang, Yousuke Utsumi, and Tiago Ribeiro (NSF-DOE Vera C Rubin Observatory), reporting on behalf of NSF-DOE Vera C Rubin Observatory:
We observed the localization region of the binary black hole merger S250725j, reported by LIGO/Virgo/KAGRA (GCN 41154, GCN 41168) 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. This is the first Target-of-Opportunity observation by Rubin Observatory.
Rubin Observatory is conducting a Science Validation (SV) survey as part of its commissioning activities [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 as a part of commissioning the Rubin Target-of-Opportunity (ToO) system. We used the SCiMMA Hopskotch infrastructure to send the alert data from LVK to Rubin Observatory infrastructure [3]. The observing strategy for this gravitational wave event follows the community recommendation for a binary black-hole merger candidate of this distance and sky condition [4]. The observing strategy calls for multiple epochs of observation, but weather conditions have prevented further observations.
The localization region of S250725j is not inside the SV survey footprint, so no prior Rubin images of this region were available. We obtained two visits in each of the u, r, and i bands with 30s exposures. Observations were taken on the night of July 29, 2025, beginning at 02:28:07.875 UTC and concluding at 03:20:18.800 UTC. We observed with nine 9.6 square degree pointings centered on the following ICRS coordinates:
| RA | DEC |
|---|---|
| 230.604 | -38.402 |
| 232.198 | -35.510 |
| 232.568 | -41.024 |
| 233.614 | -32.596 |
| 234.198 | -38.132 |
| 235.633 | -35.209 |
| 236.925 | -32.268 |
| 237.769 | -37.776 |
We covered 94% of the total localization area in all three bands, reaching a median depth of 24.1 mag in r band, 23.8 mag in the i band, and 23.6 in u band.
Due to the lack of prior Rubin observations of the region, we performed image differencing against DECam templates from DES DR2 [5]. To support the usage of DECam templates, we devised a custom image processing task to create DECam templates from DES templates for use with LSSTCam observations. 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 [6].
With only a single epoch of observations at Rubin observatory, we are unable to characterize viable candidates from Rubin observations alone. Here, we report on candidates reported in GCN 41206 and GCN 41177.
We detect four candidates reported by the DESGW collaboration (GCN 41206), and two candidates reported by the GW-MMADS collaboration (GCN 41177, two candidates recovered by both collaborations). AT2025sib, AT2025srk, and AT2025srm are detected in both r and i band difference images. At the position of AT2025srm and AT2025sib there is a galaxy in the DES template images, and a point source present in the difference images. AT2025srk shows a point source present in the LSSTCam images, but not the DES templates. We caution that AT2025shz may be a variable star, as there is a point source at the location in the DES templates.
| AT name | Observation Date | RA | Dec | r | unc_r | i | unc_i |
|---|---|---|---|---|---|---|---|
| AT2025sib | 2025-07-29 | 235.548892 | -35.002876 | 20.95 | 0.03 | 20.76 | 0.03 |
| AT2025srk | 2025-07-29 | 234.755524 | -36.425993 | 21.17 | 0.04 | 21.31 | 0.06 |
| AT2025srm | 2025-07-29 | 232.987656 | -36.816564 | 21.90 | 0.05 | 22.99 | 0.21 |
| AT2025shz | 2025-07-29 | 231.072434 | -37.622634 | 22.25 | 0.07 | 21.97 | 0.11 |
Analysis of these candidates is ongoing.
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.
[1] https://sitcomtn-005.lsst.io, §6
[3] www.scimma.org
[4] www.inspirehep.net/literature/2846480