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[vsnet-grb-info 39270] LIGO/Virgo/KAGRA S250727dc: DECam GW-MMADS candidates
by GCN Circulars 31 Jul '25

31 Jul '25

TITLE: GCN CIRCULAR NUMBER: 41214 SUBJECT: LIGO/Virgo/KAGRA S250727dc: DECam GW-MMADS candidates DATE: 25/07/31 20:21:13 GMT FROM: Tomas Cabrera at Carnegie Mellon University <tcabrera(a)andrew.cmu.edu> Tomás Cabrera (CMU), Xander J. Hall (CMU), Lei Hu (CMU), Antonella Palmese (CMU), Igor Andreoni (UNC), Keerthi Kunnumkai (CMU), Brendan O’Connor (CMU), on behalf of the Gravitational Wave MultiMessenger Astronomy DECam Survey (GW-MMADS) team DECam observed the southern high probability area of the LVK gravitational wave candidate S250727dc (GCN 41179) using the wide-field Dark Energy Camera (DECam) on the 4m Blanco telescope, as part of the Gravitational Wave Multi-Messenger Astronomy DECam Survey (GW-MMADS; PI: Andreoni & Palmese). Observations started at 2025-07-29 09:39 UTC and cover the 90% probability area (GCN 41188). We run the SFFT difference imaging (Hu et al. 2022) on the available images, filter out likely stars and moving objects, and visually inspect the remaining transients. All candidates included in this GCN have been reported to the TNS. We report the novel candidates that pass our cuts at this stage in the first table; in the following sections we present genuine transients we reject as gravitational wave counterparts, and the confirmation of some previously reported candidates. We report the discovery of the following 3 nuclear transients that we do not exclude from our search. For AT2025stf, the nucleus of the host is ambiguous, and so it is possible this transient is non-nuclear, which would eliminate it as a favored candidate. | internal_id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i | | -------------- | ------- | -- | --- | ------------------- | ----- | --------- | ----------- | | T202507290634429m374411 | AT 2025ssz | 98.678750 | -37.736389 | 2025-07-29 09:49:00.192 | 21.476 | 0.140 | 0.487 | | T202507290636497m363725 | AT 2025ste | 99.207083 | -36.623611 | 2025-07-29 09:49:00.192 | 22.437 | 0.408 | 1.627 | | T202507290626025m372843 | AT 2025stf | 96.510373 | -37.478478 | 2025-07-29 09:47:11.817 | 21.193 | 0.073 | -0.211 | The following transients are coincident with objects in the quaia and Milliquas catalogs, and have redshifts that place them outside of the 90% localization volume for the gravitational wave event. | internal_id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i | possible_host | host_z | | -------------- | ------- | -- | --- | ------------------- | ----- | --------- | ----------- | ----------- | --------- | | T202507290621597m351731 | AT 2025ssx | 95.498727 | -35.291956 | 2025-07-29 10:26:17.504 | 20.945 | 0.061 | 0.661 | quaia 2891332298170436480 | 0.47 | | T202507290634416m350638 | AT 2025sta | 98.673141 | -35.110434 | 2025-07-29 09:52:36.137 | 19.783 | 0.047 | 0.288 | quaia 5581689916779289344 | 0.53 | | T202507290629125m373923 | AT 2025std | 97.302216 | -37.656276 | 2025-07-29 09:49:00.617 | 19.945 | 0.035 | 0.819 | milliquas 6dF J062912.5-373922 | 0.06 | The following transients are nuclear, but are hosted by nearby galaxies (as determined by their large angular sizes), and so are unlikely to be associated with the gravitational wave event. | internal_id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i | | -------------- | ------- | -- | --- | ------------------- | ----- | --------- | ----------- | | T202507290623386m353830 | AT 2025ssy | 95.910743 | -35.641755 | 2025-07-29 09:45:22.413 | 19.100 | 0.013 | 0.454 | | T202507290627447m354045 | AT 2025stg | 96.936244 | -35.679174 | 2025-07-29 09:58:01.892 | 20.724 | 0.047 | 0.916 | | T202507290618452m351815 | AT 2025sth | 94.688504 | -35.304203 | 2025-07-29 09:45:22.413 | 19.080 | 0.014 | 0.441 | | T202507290627067m352915 | AT 2025sti | 96.778033 | -35.487632 | 2025-07-29 09:58:01.892 | 17.192 | 0.004 | 0.622 | The following transients are non-nuclear, and appear to be supernovae. | internal_id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i | | -------------- | ------- | -- | --- | ------------------- | ----- | --------- | ----------- | | T202507290626444m373046 | AT 2025stb | 96.684801 | -37.512899 | 2025-07-29 09:47:11.817 | 21.174 | 0.072 | 0.028 | | T202507290625106m332028 | AT 2025stc | 96.294042 | -33.341084 | 2025-07-29 09:56:12.517 | 20.252 | 0.056 | -0.166 | We additionally confirm the following candidates initially reported in GCN 41211, including the nuclear candidate with a Quaia host at a favorable redshift (AT 2025ssi). | internal_id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i | | -------------- | ------- | -- | --- | ------------------- | ----- | --------- | ----------- | | T202507290636054m360136 | AT 2025ssf | 99.022404 | -36.026624 | 2025-07-29 10:23:00.741 | 20.624 | 0.040 | [no mag_i] | | T202507290631153m360431 | AT 2025ssg | 97.813758 | -36.075317 | 2025-07-29 10:23:00.741 | 20.412 | 0.028 | [no mag_i] | | T202507290636352m372956 | AT 2025ssi | 99.146615 | -37.499005 | 2025-07-29 09:49:00.617 | 20.112 | 0.047 | 0.391 | Further analysis is underway. We thank the CTIO and NOIRLab staff for supporting these observations and the data calibration. View this GCN Circular online at https://gcn.nasa.gov/circulars/41214. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39269] LIGO/Virgo/KAGRA S250725j: GOTO pre-trigger detection of AT2025sib
by GCN Circulars 31 Jul '25

31 Jul '25

TITLE: GCN CIRCULAR NUMBER: 41213 SUBJECT: LIGO/Virgo/KAGRA S250725j: GOTO pre-trigger detection of AT2025sib DATE: 25/07/31 14:27:23 GMT FROM: Nusrin Habeeb at University of Leicester <nh312(a)leicester.ac.uk> B. Godson, N. Habeeb, K. Ackley, D. O’Neill, A. Kumar, G. Ramsay, M. J. Dyer, J. Lyman, K. Ulaczyk, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, and J. Casares report on behalf of the GOTO collaboration: We report on historical observations of counterpart candidate AT 2025sib (Hall et al. GCN 41177, McMahon et al. GCN 41206) with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024). We find evidence of emission of the source starting at 2025-07-13 08:51 with a 3-sigma observation of L = 20.2 +/- 0.3 AB mag, with our final observation on 2025-07-21 10:11 with L = 20.8 +/- 0.3 AB mag at 3-sigma. We cannot rule out the variable nature of this source and find no evidence of the decay reported in Hall et al. GCN 41177 at these early times. We note that the source appears coincident with an object in the 2MASS Point Source Catalog, 2MASS15421160-3500115 (Cutri et al 2003). GOTO last covered the field of S250725j serendipitously during survey mode on 2025-07-21. We did not perform follow-up for S250725j due to poor weather conditions at GOTO-South. In addition, we find no evidence of pre-detection emission for the other sources reported in GCN 41177, GCN 41206. Continued analysis of additional candidates is ongoing. Difference imaging was performed using deeper template observations. Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and are not corrected for Galactic extinction. GOTO (https://goto-observatory.org) is a network of telescopes that is principally funded by the STFC and operated at the Roque de los Muchachos Observatory on La Palma, Spain, and Siding Spring Observatory in NSW, Australia, on behalf of a consortium including the University of Warwick, Monash University, Armagh Observatory & Planetarium, the University of Leicester, the University of Sheffield, the National Astronomical Research Institute of Thailand (NARIT), the University of Turku, the University of Portsmouth, the University of Manchester and the Instituto de Astrofisica de Canarias (IAC). View this GCN Circular online at https://gcn.nasa.gov/circulars/41213. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39268] EP250711a: CrAO and Assy optical observations
by GCN Circulars 31 Jul '25

31 Jul '25

TITLE: GCN CIRCULAR NUMBER: 41212 SUBJECT: EP250711a: CrAO and Assy optical observations DATE: 25/07/31 14:09:22 GMT FROM: Nicolai Pankov at HSE, IKI RAS <colinsergesen(a)gmail.com> N. Pankov (HSE, IKI), V. Rumyantsev (CrAO), V. Kim (FAI), A. Pozanenko (IKI), A. Volnova (IKI) report on behalf of IKI-GRB-FuN: We observed the field of a fast X-ray transient EP250711a detected by EP (Wang et. al, GCN 41078) using the 2.6-meter Shajn telescope (ZTSh) of the Crimean Astrophysical Observatory (CrAO) and the 1.5-meter AZT-20 telescope of the Assy-Turgen Astrophysical Observatory (Assy). The observations at Assy started on 2025-07-21 at 19:53:57 UT, i.e. ~10.14 days since trigger. A series of 101x60 s images in the r'-band was obtained. The observations at CrAO started on 2025-07-24 at 20:28:51 UT, i.e. ~13.17 days since trigger. A series of 61x120 s images was obtained in the R-band. In the co-add image taken by ZTSh we tentatively detect a source at the position of the optical source initially detected by NOT (Fu et. al, GCN 41071) that was also observed by COLIBRÍ (Becerra et. al, GCN 41073; Becerra, GCN 41084). The preliminary photometry is provided below: Date UT start t-T0 Exp. Filter OT S/N UL Telescope (mid, days) (n*s) (3sigma) 2025-07-21 19:53:57 10.14828 101x60 r' n/d n/d 22.5 Assy/AZT-20 2025-07-24 20:28:51 13.17980 61x120 R 23.5 2.0 23.1 CrAO/ZTSh To calibrate both r' and R bands we used common stars of the USNO-B1.0 (R2) and the PS1 DR2 (rmag) catalogs. No correction was made for the Galactic extinction. We note that our result is fainter in comparison with the last COLIBRÍ observation (Becerra, GCN 41084) taken between 1.4192 and 1.725 days since trigger. We may suggest the source might be the host galaxy. Ref. stars USNO-B1.0 / PS1 RA Dec R2 rmag 292.1601 4.6919 16.86 17.44 292.1451 4.7120 17.17 17.76 292.1579 4.6937 17.98 18.76 View this GCN Circular online at https://gcn.nasa.gov/circulars/41212. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39267] LIGO/Virgo/KAGRA S250727dc: DECam DESGW Epoch 1 Candidates
by GCN Circulars 31 Jul '25

31 Jul '25

TITLE: GCN CIRCULAR NUMBER: 41211 SUBJECT: LIGO/Virgo/KAGRA S250727dc: DECam DESGW Epoch 1 Candidates DATE: 25/07/31 13:19:11 GMT FROM: Isaac McMahon at University of Zürich <isaac.mcmahon(a)ligo.org> Isaac McMahon, Sean MacBride, Marcelle Soares-Santos (UZH), Nora Sherman (Boston U.), Simran Kaur (U. of Michigan/UZH), Lillian Joseph (Benedictine U.), Ken Herner, H. Thomas Diehl (Fermilab), reporting on behalf of the Dark Energy Survey Gravitational Wave (DESGW) Team: At 2025-07-29 09:39:10 UTC, the Dark Energy Camera (DECam) began the first epoch of observations in response to the LVK alert issued for the candidate gravitational-wave event S250727dc (GCN 41179). Observations covered the 90% localization region of the event (Prop ID: 2023B-851374; PI Andreoni/Palmese). We process the images with our difference imaging pipeline (Herner et al. 2020) using DES and public DECam images as templates. We employ the autoscan machine learning code (Goldstein et al 2015) to reject subtraction artifacts. We require an autoscan score of at least 0.7 on at least one of those detections. We also match our candidates against the ALLWISE, Milliquas, Quaia, and LQAC-6 AGN catalogs (Secrest et al. 2015, Flesch 2023, Storey-Fisher et al. 2024, Souchay et al. 2024) within the LVK localization volume to determine if any correspond to known active galactic nuclei. After candidate selection we report the 4 high confidence candidates listed below, including 2 nuclear candidates (likely AGN), and 1 likely supernova. Additionally, the transient AT2025ssi is the probable AGN Quaia_1070591 with a measured redshift of z~0.24 +- 0.08, which falls within the reported LVK volume. Further observations are ongoing, and we encourage followup of the 4 candidates identified herein. | TYPE | ID | ATNAME | RA | DEC | MAG_G | MAG_G_ERR | MAG_R | MAG_R_ERR | MAG_I | MAG_I_ERR | MAG_Z | MAG_Z_ERR | |----------|---------|--------|------------|-----------|-------|-----------|-------|-----------|-------|-----------|-------|-----------| | SN_LIKE | 3334740 | AT2025ssf | 99.02238 | -36.026584 | 20.65 | 0.05 | 20.32 | 0.03 | N/A | N/A | 21.21 | 0.12 | | AGN_LIKE | 3334614 | AT2025ssg | 97.813775 | -36.075323 | 21.06 | 0.1 | 20.58 | 0.07 | N/A | N/A | 19.99 | 0.04 | | AGN_LIKE | 3334106 | AT2025ssh | 97.339644 | -36.600508 | 21.79 | 0.21 | N/A | N/A | 21.08 | 0.09 | 19.5 | 0.04 | | AGN_LIKE | 3334650 | AT2025ssi | 99.146548 | -37.499065 | 20.09 | 0.05 | N/A | N/A | 19.68 | 0.03 | N/A | N/A | Additionally, we also recover two of the candidates reported by SWIFT-XRT (GCN 41201). One is the known blazar BL Lacertae (S250727dc_X1), which has been mentioned to be highly variable. The other (S250727dc_X7) is the galaxy ESO 365-5. Both of these galaxies have been spectroscopically measured to have redshifts z=0.055 (García-Pérez et al. 2024) and z=0.045 (Jones et al. 2009) respectively, which both lie outside of the LVK localization volume. | TYPE | ID | ATNAME | RA | DEC | MAG_G | MAG_G_ERR | MAG_R | MAG_R_ERR | MAG_I | MAG_I_ERR | MAG_Z | MAG_Z_ERR | |----------|---------|--------|------------|-----------|-------|-----------|-------|-----------|-------|-----------|-------|-----------| | AGN_LIKE | 3334338 | S250727dc_X1 | 96.778098 | -35.487631 | 17.32 | 0.01 | 16.98 | 0.01 | N/A | N/A | 16.5 | 0.0 | | AGN_LIKE | 3334206 | S250727dc_X7 | 94.688512 | -35.304098 | 18.79 | 0.02 | 18.32 | 0.02 | 18.34 | 0.01 | 18.51 | 0.02 | We do not recover any of the other candidates reported by SWIFT-XRT, nor the transient AT2025smm reported by SWIFT-UVOT (GCN 41187). 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. View this GCN Circular online at https://gcn.nasa.gov/circulars/41211. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39266] EP250730a: Global MASTER-Net observations report
by GCN Circulars 31 Jul '25

31 Jul '25

TITLE: GCN CIRCULAR NUMBER: 41210 SUBJECT: EP250730a: Global MASTER-Net observations report DATE: 25/07/31 08:45:26 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov(a)xray.sai.msu.ru> V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, G.Antipov, A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU), O.Gress, N.Budnev (ISU), C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A.Sosnovskij (CrAO), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity), D.Buckley (SAAO), R.Rebolo (The Instituto de Astrofisica de Canarias), L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) MASTER-OAFA robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Argentina (OAFA observatory of San Juan National University) was pointed to the EP250730a ( EP Team et al., GCN 41207) errorbox 762 sec after notice time and 79097 sec after trigger time at 2025-07-31 04:04:26 UT, with upper limit up to 20.0 mag. The observations began at zenith distance = 28 deg. The sun altitude is -74.8 deg. The galactic latitude b = -30 deg., longitude l = 44 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=2953990 We obtain a following upper limits. Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment _________|_____________________|_____________________|____________________________________|_____|_______|_______|________ 79128 | 2025-07-31 04:04:26 | MASTER-OAFA | (20h 48m 25.83s , -04d 35m 50.0s) | C | 60 | 19.5 | 79204 | 2025-07-31 04:05:42 | MASTER-OAFA | (20h 56m 28.38s , -04d 37m 17.4s) | C | 60 | 19.9 | 79264 | 2025-07-31 04:05:42 | MASTER-OAFA | (20h 56m 28.38s , -04d 37m 17.3s) | C | 180 | 20.0 | Coadd 79380 | 2025-07-31 04:08:39 | MASTER-OAFA | (20h 48m 21.81s , -04d 36m 50.2s) | C | 60 | 19.6 | 80221 | 2025-07-31 04:22:39 | MASTER-OAFA | (20h 56m 21.95s , -04d 36m 17.7s) | C | 60 | 19.4 | 80384 | 2025-07-31 04:25:22 | MASTER-OAFA | (20h 48m 19.67s , -04d 38m 26.9s) | C | 60 | 19.3 | 80459 | 2025-07-31 04:26:37 | MASTER-OAFA | (20h 56m 28.18s , -04d 38m 08.1s) | C | 60 | 19.0 | 82546 | 2025-07-31 05:01:24 | MASTER-OAFA | (20h 48m 14.53s , -04d 39m 59.5s) | C | 60 | 19.6 | 82622 | 2025-07-31 05:02:41 | MASTER-OAFA | (20h 56m 20.98s , -04d 38m 21.8s) | C | 60 | 19.7 | 82682 | 2025-07-31 05:02:41 | MASTER-OAFA | (20h 56m 20.99s , -04d 38m 21.9s) | C | 180 | 19.8 | Coadd 82805 | 2025-07-31 05:05:44 | MASTER-OAFA | (20h 48m 09.69s , -04d 38m 26.0s) | C | 60 | 19.0 | 82880 | 2025-07-31 05:06:59 | MASTER-OAFA | (20h 56m 18.72s , -04d 39m 15.7s) | C | 60 | 19.2 | 83049 | 2025-07-31 05:09:47 | MASTER-OAFA | (20h 48m 15.74s , -04d 40m 36.9s) | C | 60 | 18.0 | 83139 | 2025-07-31 05:11:17 | MASTER-OAFA | (20h 56m 15.54s , -04d 38m 36.4s) | C | 60 | 18.4 | 85345 | 2025-07-31 05:48:03 | MASTER-OAFA | (20h 48m 01.89s , -04d 41m 11.2s) | C | 60 | 16.5 | 85423 | 2025-07-31 05:49:21 | MASTER-OAFA | (20h 56m 09.03s , -04d 41m 53.1s) | C | 60 | 14.9 | 85586 | 2025-07-31 05:52:05 | MASTER-OAFA | (20h 48m 05.61s , -04d 43m 03.1s) | C | 60 | 14.1 | 85696 | 2025-07-31 05:53:54 | MASTER-OAFA | (20h 56m 05.97s , -04d 41m 44.0s) | C | 60 | 14.0 | 85861 | 2025-07-31 05:56:40 | MASTER-OAFA | (20h 48m 04.15s , -04d 42m 01.5s) | C | 60 | 15.2 | 85937 | 2025-07-31 05:57:56 | MASTER-OAFA | (20h 56m 01.37s , -04d 42m 30.3s) | C | 60 | 15.1 | 94065 | 2025-07-31 08:13:23 | MASTER-OAFA | (20h 52m 50.00s , -04d 44m 13.9s) | C | 60 | 15.7 | 94143 | 2025-07-31 08:14:41 | MASTER-OAFA | (20h 52m 44.60s , -04d 45m 13.2s) | C | 60 | 16.9 | 94224 | 2025-07-31 08:16:02 | MASTER-OAFA | (20h 52m 50.40s , -04d 45m 05.9s) | C | 60 | 18.6 | 94299 | 2025-07-31 08:17:17 | MASTER-OAFA | (20h 52m 46.84s , -04d 43m 27.0s) | C | 60 | 19.1 | 94359 | 2025-07-31 08:17:17 | MASTER-OAFA | (20h 52m 46.84s , -04d 43m 27.0s) | C | 180 | 19.5 | Coadd 94374 | 2025-07-31 08:18:33 | MASTER-OAFA | (20h 52m 46.83s , -04d 44m 55.5s) | C | 60 | 18.0 | 94452 | 2025-07-31 08:19:50 | MASTER-OAFA | (20h 52m 49.75s , -04d 43m 42.7s) | C | 60 | 19.5 | 94559 | 2025-07-31 08:21:07 | MASTER-OAFA | (20h 52m 44.72s , -04d 44m 13.2s) | C | 120 | 19.7 | 95484 | 2025-07-31 08:36:32 | MASTER-OAFA | (20h 52m 44.73s , -04d 43m 16.8s) | C | 120 | 19.2 | 95621 | 2025-07-31 08:38:50 | MASTER-OAFA | (20h 52m 50.14s , -04d 44m 17.2s) | C | 120 | 19.6 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited. View this GCN Circular online at https://gcn.nasa.gov/circulars/41210. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39265] EP250730a: COLIBRÍ optical upper limit
by GCN Circulars 31 Jul '25

31 Jul '25

TITLE: GCN CIRCULAR NUMBER: 41209 SUBJECT: EP250730a: COLIBRÍ optical upper limit DATE: 25/07/31 07:03:35 GMT FROM: Francesco Magnani at Aix-Marseille Université, CPPM/CNRS <francesco.magnani.work(a)gmail.com> Francesco Magnani (CPPM), Alan M. Watson (UNAM), Rosa L. Becerra (U Roma), Antonio de Ugarte Postigo (LAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Sarah Antier (OCA), Jean-Luc Atteia (IRAP), Fredd Alvarez (UNAM), Camila Angulo (UNAM), Dalya Akl (AUS), Nathaniel R. Butler (ASU) , Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Benjamin Schneider (LAM), and Ny Avo Rakotondrainibe (LAM): We imaged the field the EP250730a (Wang et al., GCN Circ. 41207) using the DDRAGO wide-field imager on the COLIBRÍ telescope. We observed from 2025-07-31 04:13 to 05:35 UTC (from 22.1 to 23.5 hours after the trigger, starting 23 min after the report by Wang et al., GCN Circ. 41207) and obtained 64 minutes of exposure in the i filter. The data were reduced and coadded with the COLIBRÍ pipeline and analysed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction. In our stacked image, we do not detect any new source within the EP/WXT uncertainty region (Wang et al., GCN Circ. 41207) to a 5-sigma upper limit of: i > 23.1 We thank the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir and the COLIBRÍ and DDRAGO engineering teams. COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico. View this GCN Circular online at https://gcn.nasa.gov/circulars/41209. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39264] GRB 250727A: Swift-XRT afterglow confirmation and refined analysis
by GCN Circulars 31 Jul '25

31 Jul '25

TITLE: GCN CIRCULAR NUMBER: 41208 SUBJECT: GRB 250727A: Swift-XRT afterglow confirmation and refined analysis DATE: 25/07/31 05:57:17 GMT FROM: Phil Evans at U of Leicester <pae9(a)star.le.ac.uk> K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), T. Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB), M.G. Bernardini (INAF-OAB), S. Dichiara (PSU), J.A. Kennea (PSU), D.N. Burrows (PSU) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team: We have analysed 5.9 ks of XRT data for GRB 250727A, from 2.4 ks to 300.7 ks after the SVOM/ECLAIRs trigger. The data are entirely in Photon Counting (PC) mode. During this second observation, source 1 (SWIFT J234539.1-811248) given in GCN Circ. 41180 is found to have faded at the 3.7-sigma level, so we believe this to be the X-ray afterglow. We also note that EP-FXT found this source to have faded (GCN Circ. 41199). Using 1353 s of PC mode data and 1 UVOT image, we find an enhanced XRT position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 356.41475, -81.21390 which is equivalent to: RA (J2000): 23h 45m 39.54s Dec(J2000): -81d 12' 50.1" with an uncertainty of 4.3 arcsec (radius, 90% confidence). The source is fading with alpha >0.3. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00019965. This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41208. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39263] EP250730a: Einstein Probe detection of an X-ray transient
by GCN Circulars 31 Jul '25

31 Jul '25

TITLE: GCN CIRCULAR NUMBER: 41207 SUBJECT: EP250730a: Einstein Probe detection of an X-ray transient DATE: 25/07/31 03:50:45 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> B.-T. Wang (YNAO, CAS), Y. L. Wang (NAO, CAS; ICE, CSIC-IEEC), X.P. Xu, W. Yuan (NAO, CAS) on behalf of the Einstein Probe (EP) team: We report on the detection of an X-ray transient by EP-WXT through ground data analysis, designated EP250730a (ObsID:11900327297). The transient was detected at 2025-07-30T06:06:09 (UTC). The WXT position of the source is R.A. = 313.264 deg, DEC = -4.648 deg (J2000) with an uncertainty of 2.1 arcmin (radius, 90% C.L. statistical and systematic). The transient event lasted for about 250 seconds. The average 0.5-4 keV spectrum can be fitted with an absorbed powerlaw model, with nH fixed at the Galactic value of 6.0 x 10^20 cm^-2 and a photon index of 1.84 (-0.38, +0.41). The unabsorbed average 0.5-4 keV flux is estimated to be 1.93 (-0.39, +0.48)e-10 erg/s/cm2. The peak flux is about 2e-9 erg/s/cm2. The uncertainties are at the 90% confidence level for the above parameters. Further FXT observations are scheduled. Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). View this GCN Circular online at https://gcn.nasa.gov/circulars/41207. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39262] LIGO/Virgo/KAGRA S250725j: DECam DESGW Epoch 1-2 Candidates
by GCN Circulars 31 Jul '25

31 Jul '25

TITLE: GCN CIRCULAR NUMBER: 41206 SUBJECT: LIGO/Virgo/KAGRA S250725j: DECam DESGW Epoch 1-2 Candidates DATE: 25/07/31 00:52:54 GMT FROM: Isaac McMahon at University of Zürich <isaac.mcmahon(a)ligo.org> Isaac McMahon, Sean MacBride, Marcelle Soares-Santos (UZH), Nora Sherman (Boston U.), Simran Kaur (U. of Michigan/UZH), Lillian Joseph (Benedictine U.), Ken Herner, Tom Diehl (Fermilab), reporting on behalf of the Dark Energy Survey Gravitational Wave (DESGW) Team: At 2025-07-25 23:13:43 UTC and 2025-07-28 23:13:54 UTC, the Dark Energy Camera (DECam) began the first and second epoch of observations in response to the LVK alert issued for the candidate gravitational-wave event S250725j (GCN 41154). We observed nine fields centered on the following ICRS coordinates: | RA | DEC | |----------|----------| | 231.833836 | -37.23415| |234.585764 | -35.2584| |232.520774 | -35.64614| |233.935432 | -36.84845| |236.026051 | -36.42542| |230.4476 | -35.99735| |233.180871 | -34.0649| |236.640727 | -34.83446| |229.723284 | -37.58219| These pointings cover the 90% localization region of candidate gravitational-wave event S250725j. All fields were observed in DECam g, r, i, z filters with 90-second exposures. The limiting magnitudes achieved for epoch 1 were 23.2 in g, 22.9 in r, and 22.6 in i, and 21.6 in z. The limiting magnitudes achieved for epoch 2 were 22.5 in g, 22.6 in r, and 22.3 in i, and 21.6 in z. We process the images with our difference imaging pipeline (Herner et al. 2020) using DES and public DECam images as templates. We employ the autoscan machine learning code (Goldstein et al 2015) to reject subtraction artifacts. Candidates were initially selected by requiring at least two high signal to noise detections, which were separated in time in order to reject moving objects. We also require an autoscan score of at least 0.7 on at least one of those detections. We also match our candidates against the ALLWISE, Milliquas, and Quaia AGN catalogs (Secrest et al 2015, Flesch 2023, Storey-Fisher et al 2024) within the LVK localization volume to determine if any correspond to known active galactic nuclei. After candidate selection we report the 3 high confidence candidates listed below, including 2 nuclear candidates (likely active galactic nuclei) and 1 likely supernova. Further observations are ongoing, and we encourage followup of the 3 candidates identified herein. | TYPE | ID | ATNAME | RA | DEC | MAG_G | MAG_G_ERR | MAG_R | MAG_R_ERR | MAG_I | MAG_I_ERR | MAG_Z | MAG_Z_ERR | |---------- |--------- |-------- |------------ |----------- |------- |----------- |------- |----------- |------- |----------- |------- |----------- | | SN_LIKE | 3141695 | AT2025srk | 234.755524 | -36.425993 | 21.51 | 0.02 | 21.0 | 0.02 | 21.27 | 0.03 | 22.06 | 0.27 | | AGN_LIKE | 3237805 | AT2025srl | 234.90473 | -36.726032 | 23.55 | 0.19 | N/A | N/A | 19.74 | 0.01 | N/A | N/A | | AGN_LIKE | 3141486 | AT2025srm | 232.987656 | -36.816564 | 22.43 | 0.06 | 22.07 | 0.04 | 22.74 | 0.17 | 21.21 | 0.05 | Additionally, we also recover two of the candidates reported by GW-MMADS (GCN 41177). The candidate AT2025sib is of particular note, as its photometric redshift reported by 2MASS (Bilicki et al 2014) lies within the LVK 90% volume. | TYPE | ID | ATNAME | RA | DEC | MAG_G | MAG_G_ERR | MAG_R | MAG_R_ERR | MAG_I | MAG_I_ERR | MAG_Z | MAG_Z_ERR | |---------- |--------- |-------- |------------ |----------- |------- |----------- |------- |----------- |------- |----------- |------- |----------- | | AGN_LIKE | 3141729 | AT2025shz | 231.072453 | -37.6226 | 22.77 | 0.08 | 22.36 | 0.08 | N/A | N/A | N/A | N/A | | SN_LIKE | 3141898 | AT2025sib | 235.548953 | -35.002893 | 21.72 | 0.04 | 20.7 | 0.02 | N/A | N/A | 20.72 | 0.07 | 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. View this GCN Circular online at https://gcn.nasa.gov/circulars/41206. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 39261] LIGO/Virgo/KAGRA S250727dc: Swift-UVOT follow up of AT 2025smm
by GCN Circulars 30 Jul '25

30 Jul '25

TITLE: GCN CIRCULAR NUMBER: 41205 SUBJECT: LIGO/Virgo/KAGRA S250727dc: Swift-UVOT follow up of AT 2025smm DATE: 25/07/30 20:28:25 GMT FROM: Paul Kuin at MSSL <npkuin(a)gmail.com> N.P.M. Kuin (UCL-MSSL), M.H. Siegel (PSU), A.A. Breeveld (UCL-MSSL), C. Gronwall (PSU), N.J. Klingler (NASA/GSFC UMBC CRESST II),F.E. Marshall (NASA/GSFC), S.R. Oates (Lancaster U.), M. De Pasquale (University of Messina), M.J. Page (UCL-MSSL), S. Shilling (Lancaster U.), A. D’Aì (INAF-IAFPA), P. D’Avanzo (INAF-OAB), A.P. Beardmore (U. Leicester), M.G. Bernardini (INAF-OAB),S. Campana (INAF-OAB), S.B. Cenko (NASA/GSFC), J.J. Delaunay (PSU), S. Dichiara (PSU), V. D’Elia (ASI-SSDC & INAF-OAR), R.A.J.Eyles-Ferris (U. Leicester), P.A. Evans (U. Leicester), R. Gayathri (PSU), D. Hartmann (Clemson University), J.A. Kennea (PSU), S. Laha (NASA/GSFC),H.A. Krimm (NSF), D. B. Malesani (DTU Space), P. O’Brien (U. Leicester), J.P. Osborne (U. Leicester), T. Parsotan (NASA/GSFC), M. Perri (ASDC), K.L. Page (U. Leicester), D.M. Palmer (LANL), S. Ronchini (PSU), T. Sbarrato (INAF-OAB), B. Sbarufatti (INAF-OAB), G. Tagliaferri (INAF-OAB), E. Troja (U. Tor Vergata, INAF), and A. Tohuvavohu (U. Toronto)report: Swift re-observed the location of AT 2025smm in the high-probability region of LIGO/Virgo/KAGRA S250725dc from 1.56 to 1.63 d after the trigger. The source is no longer detected. we obtained the following 3-sigma upper limits: Filter Exposure(s) 3-sigma UL AB mag) UVW2 606 22.3 UVM2 466 21.8 UVW1 303 21.5 U 151 20.8 B 151 20.1 V 151 19.3 No corrections were made for interstellar extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/41205. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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