GCN Circular 41739
Isaac McMahon, Sean MacBride, Marcelle Soares-Santos (UZH), Lillian Joseph (Benedictine U.), reporting on behalf of the Dark Energy Survey Gravitational Wave (DESGW) Team:
At 2025-09-06 04:12:03 UTC, the Dark Energy Camera (DECam) conducted the second epoch of observations in response to the LVK alert issued for the candidate gravitational-wave event S250830bp (GCN 41606). We observed the same fields as epoch 1 (GCN 41640) and employed the same analysis. The average limiting magnitudes achieved for epoch 2 were 20.7 in g, 21.4 in r, and 21.3 in i, and 21.1 in z.
We report the following seven candidates which have multiple good detections on both days of observation. The candidate reported previously, AT2025wpk, is confirmed to be variable and dimming with good detections in all four bands. The other six candidates are all nuclear-type transients at the centers of previously detected objects.
| ID | ATNAME | RA | DEC | MAG_G | MAG_G_ERR | MAG_R | MAG_R_ERR | MAG_I | MAG_I_ERR | MAG_Z | MAG_Z_ERR |
|---|---|---|---|---|---|---|---|---|---|---|---|
| DESGW_3336579 | AT2025wpk | 329.476163 | -77.412560 | 22.43 | 0.24 | 21.58 | 0.19 | 21.30 | 0.13 | 21.05 | 0.09 |
| DESGW_3336526 | AT2025wzu | 325.368195 | -77.603585 | N/A | N/A | 21.16 | 0.09 | N/A | N/A | 21.76 | 0.17 |
| DESGW_3336621 | AT2025wzw | 323.031060 | -77.708095 | 23.21 | 0.49 | 21.74 | 0.23 | 21.52 | 0.10 | 20.68 | 0.07 |
| DESGW_3336660 | AT2025wzv | 328.307211 | -78.281736 | 15.94 | 0.01 | N/A | N/A | N/A | N/A | N/A | N/A |
| DESGW_3336681 | AT2025wpr | 321.643320 | -77.755092 | N/A | N/A | 21.07 | 0.08 | 21.72 | 0.14 | 20.98 | 0.10 |
| DESGW_3336921 | AT2025wpo | 323.141341 | -78.204743 | 20.22 | 0.06 | 20.27 | 0.06 | 20.32 | 0.06 | 20.38 | 0.06 |
| DESGW_3337296 | AT2025wzx | 321.552602 | -78.046756 | N/A | N/A | 21.77 | 0.16 | 20.95 | 0.07 | 21.65 | 0.19 |
In particular, AT2025wzv seems to be very luminous and actively brightening in g-band but shows little to no transient activity in other bands. This behavior makes it an interesting transient but unlikely to be associated with S250830bp.
We also recover the transient AT2025wpq reported in Hall et al (GCN 41643), although we note that the redshift reported for the host AGN by the Quaia catalog (Storey-Fisher et al 2024) is z = 0.43 +- 0.06, outside of the confidence volume for S250830bp. We do not recover the transient AT2025wpv.
Further observations are ongoing and we encourage followup of the event region and the above candidates.
The DECam Search & Discovery Program for Optical Signatures of Gravitational Wave Events (DESGW) is carried out by the Dark Energy Survey (DES) collaboration in partnership with wide-ranging groups in the community. DESGW uses data obtained with the Dark Energy Camera (DECam), which was constructed by the DES collaboration with support from the Department of Energy and member institutions, and utilizes data as distributed by the Science Data Archive at NOIRLAB. NOIRLAB is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. We thank the Cerro Tololo observatory staff for their support in acquiring these observations.