LIGO/Virgo/KAGRA S250830bp
GCN Circular 42033
Hemanth Bommireddy (U de Chile), Regis Cartier (UA), Felipe Olivares (U Hawaii), reporting on behalf of the Dark Energy Survey Gravitational Wave (DESGW) spectroscopy team:
We report on AT2025wzx and AT2025wpo, which were observed by SOAR 2025-09-10T03:10:05 UTC (PI Bommireddy). We clearly see broad Halpha and Hbeta in AT2025wzx, securing its classification as an AGN at z~0.23, thus, we therefore conclude this candidate to be far too distant to be associated with S250830bp (GCN 41607). In contrast, AT2025wpo shows a noisy featureless spectrum, preventing a firm classification.
We thank SOAR Staff for supporting these observations.
GCN Circular 41741
Xander J. Hall (CMU), Lei Hu (CMU), Tomás Cabrera (CMU), James Freeburn (UNC), Antonella Palmese (CMU), Brendan O’Connor (CMU), Igor Andreoni (UNC), Keerthi Kunnumkai (CMU), on behalf of the Gravitational Wave MultiMessenger Astronomy DECam Survey (GW-MMADS) team report:
DECam observed the high probability area of the LVK gravitational wave candidate S250830bp (GCN 41606) using the wide-field Dark Energy Camera (DECam) on the 4m Blanco telescope. Observations started at 2025-09-05T04:12:03 UTC (Prop ID: 2025B-485252; PI Soares-Santos) and covered the highest 90% probability region of the event (GCN 41607).
We run the SFFT difference imaging (Hu et al. 2022) on the available images, filter out likely stars and moving objects, and then visually inspect the remaining transients. We report on TNS 13 newly identified transients within the LVK 99% CI area. We report the new transient AT 2025xab as a likely supernova and note that none of our nuclear transients show any compelling evolution.
A crossmatch with Gaia reveals AT 2025wzv and AT 2025wzw (McMahon et al. GCN 41739) as galactic sources, thus we conclude these are not associated with S250830bp (GCN 41607).
We also provide updates on previously announced candidates:
We report that AT 2025wpk has shown little to no color evolution or brightness change since its last observation and that AT 2025wpv has shown a slow decline in brightness, dropping ~0.1 mag/day.
Finally, we report on AT 2025wpq (GCN 41643), which was observed by SOAR at 2025-09-04T05:32:07 UTC (PI Andreoni). We clearly see broad Halpha and Hbeta securing its classification as a quasar at a z ~ 0.4, thus, we therefore conclude this candidate is far too distant to be associated with S250830bp (GCN 41607).
Further analysis is underway.
We thank the CTIO and NOIRLab staff for supporting these observations and the data calibration.
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.
GCN Circular 41643
Xander J. Hall (CMU), Lei Hu (CMU), Tomás Cabrera (CMU), Brendan O’Connor (CMU), Antonella Palmese (CMU), Igor Andreoni (UNC), James Freeburn (UNC), Keerthi Kunnumkai (CMU), on behalf of the Gravitational Wave MultiMessenger Astronomy DECam Survey (GW-MMADS) team report:
DECam observed the high probability area of the LVK gravitational wave candidate S250830bp (GCN 41606) using the wide-field Dark Energy Camera (DECam) on the 4m Blanco telescope. Observations started at 2025-09-01T03:39 UTC (Prop ID: 2025B-485252; PI Soares-Santos) and covered the highest 90% probability region of the event (GCN 41607).
We run the SFFT difference imaging (Hu et al. 2022) on the available images, filter out likely stars and moving objects, and then visually inspect the remaining transients. We report on TNS new transients within the LVK 99% CI area, and we report here select transients of high quality and interest:
First, one matched to a NED galaxy within the 90% volume (GCN 41608):
| id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i | NED separation |
|---|---|---|---|---|---|---|---|---|
| T202509012137497m775946 | AT 2025wpv | 324.457179 | -77.996131 | 2025-09-01 03:57:43.776 | 22.6 | 0.3 | 1.16 | 1.6” |
Next, a transient of a known Gaia quasar (Gavras et al. 2023):
| id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i | Gaia separation |
|---|---|---|---|---|---|---|---|---|
| A202509012159035m780432 | AT 2025wpq | 329.764660 | -78.075678 | 2025-09-01 03:41:20.544 | 20.3 | 0.3 | 0.6 | 0.165” |
Finally, we confirm the transient reported by McMahon et al. (GCN 41640):
| id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i |
|---|---|---|---|---|---|---|---|
| T202509012157542m772445 | AT 2025wpk | 329.475736 | -77.412522 | 2025-09-01 03:49:33.024 | 22.2 | 0.2 | 1.2 |
Further analysis is underway.
We thank the CTIO and NOIRLab staff for supporting these observations and the data calibration.
GCN Circular 41640
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 2025-09-01 03:39:18 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 S250830bp (GCN 41606). We observed four fields centered on the following ICRS coordinates:
| RA | DEC |
|---|---|
| 324.165777 | -77.51123 |
| 327.955221 | -77.54517 |
| 323.965777 | -77.61123 |
| 327.755221 | -77.64517 |
These pointings cover the 90% localization region of candidate gravitational-wave event S250830bp.
All fields were observed in DECam g, r, i, z filters with 90-second exposures. The limiting magnitudes achieved for epoch 1 were 21.8 in g, 21.8 in r, and 21.5 in i, and 20.7 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.
Poor observing conditions during observing complicated positive identification of transient candidates. However, we report one candidate below with good detections in multiple bands. The candidate is separated by 5.58 arcseconds from the center of the apparent host galaxy LEDA 236249, which does not have a reported redshift.
| 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 | -77.41256 | 22.41 | 0.24 | 21.39 | 0.09 | 21.1 | 0.06 | 20.93 | 0.14 |
Further observations are ongoing and we encourage followup of the event region and the above candidate.
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.
GCN Circular 41610
T. Mihara, N. Kawai (RIKEN),
M. Nakajima, H. Negoro, K. Takagi (Nihon U.),
S. Sugita, M. Serino, Y. Kawakubo, H. Hiramatsu, Y. Kondo (AGU)
report on behalf of the MAXI team:
We examined MAXI/GSC all-sky X-ray images (2-20 keV)
after compact binary merger candidate S250830bp at 2025-08-30 10:24:18.852 UTC (GCN 41606, 41607).
At the trigger time of S250830bp, the high-voltage of MAXI/GSC was on,
but the FOV was out of the 90% credible region of the Bilby skymap.
The credible region was observed about 3 minutes before the GW trigger time with GSC, and
the first one-orbit (92 min) scan observation after the event covered 100%
of the 90% credible region from 11:52:34 to 11:53:19 UTC (T0+5296 to T0+5341 sec).
No significant new source was found in the region in those observations.
A typical 1-sigma averaged upper limit obtained in one scan observation
is 20 mCrab at 2-20 keV.
If you require information about X-ray flux by MAXI/GSC at specific coordinates,
please contact the submitter of this circular by email.
GCN Circular 41609
M. L. Brozzetti (UniPG/INFN), G. Dálya (L2IT/EotvosU), G. Greco (INFN), M. Bawaj (UniPG/INFN), T. Matcovich (UniPG/INFN), S. Cutini (INFN) , R. De Pietri (UniPR/INFN), Marica Branchesi (GSSI)
On behalf of the GLADEnet Team.
We analyzed the completeness of the GLADE+ [1] catalog within the 90% credible localization volume of the S250830bp event from the 4-Update alert from the GCN Circular 41606.
The completeness value is 6.6e-1 in the B-band using the last released skymap : Bilby.multiorder.fits,0, which means that the catalogue contains 66% of the total light in the B-band expected from galaxies in the localization volume.
A total of 517 galaxies are identified within the 90% gravitational volume. The complete list of galaxies can be downloaded from the GLADEnet webpage [2] : https://virgo.pg.infn.it/gladenet/catalogs/
GLADEnet allows for the interactive visualization of the 90% localization area and its intersection with regions of high extinction as defined in GLADE+. Furthermore, the first 1000 galaxies can be explored interactively, enabling users to filter galaxies based on their 3D probability density or their absolute B magnitude. The ligo.skymap cross-match method [3,4] is used to obtain the list of galaxies.
References:
[1]GLADE+: An Extended Galaxy Catalogue for Multimessenger Searches with Advanced Gravitational-wave Detectors G. Dálya et al. MNRAS, 514,1, pp.1403-1411, 2022
[2] GLADEnet: A progressive web app for multi-messenger cosmology and electromagnetic follow-ups of gravitational-wave sources M. L. Brozzetti, G. Dálya, G. Greco, M. Bawaj, T. Matcovich, M. Branchesi, T. Boch, M. Baumann, S. Cutini, R. De Pietri et al. (4 more) A&A, 684, A44 (2024)
[3] Singer, L. P., Chen, H.-Y., Holz, D. E., et al. 2016, Astropys. J. Lett., 829, L15. doi:10.3847/2041-8205/829/1/L15
[4] Singer, L. P., Chen, H.-Y., Holz, D. E., et al. 2016, Astropys. J. Supp., 226, 10. doi:10.3847/0067-0049/226/1/10
GCN Circular 41608
David O. Cook (Caltech/IPAC), Rick Ebert (Caltech/IPAC), George Helou (Caltech/IPAC), Joseph M. Mazzarella (Caltech/IPAC), Marion Schmitz (Caltech/IPAC), and Leo Singer (NASA/GSFC)
On behalf of the NASA/IPAC Extragalactic Database (NED) Team.
We spatially cross-matched the LVK S250830bp-4-Update sky localization with the NED Local Volume Sample (NED-LVS; Cook et al. 2023), which is a subset of NED with a redshift or redshift-independent distance less than 1000 Mpc. We find 72 galaxies within the 90% containment volume, and we list here the top 20 galaxies sorted by the joint probability of the 3D localization and the WISE W1 luminosity (an observable proxy for stellar mass). For the full or top 20 list of galaxies in the 90% volume go either to the NED Gravitational Wave Followup service at https://ned.ipac.caltech.edu/uri/NED::GWF/ or click on the following links:
Full List Download: https://ned.ipac.caltech.edu/uri/NED::GWFglist/fits/S250830bp/4
Top 20 List Download: https://ned.ipac.caltech.edu/uri/NED::GWFglist/fits/S250830bp/4/20
The NED-GWF service provides downloadable galaxy lists and visualizations for candidate host galaxies. For each GW alert, these products are automatically generated and made available within minutes to expedite efficient electromagnetic follow-up observations. The NED top 20 list is sorted by the joint probability of the 3D localization and the WISE W1 luminosity, but users can sort on additional pre-computed prioritization metrics (star formation rate, P_3D * P_SFR; and specific star formation rate, P_3D * P_sSFR; etc.) which are available via downloading the entire galaxy list inside the event's probability volume.
| objname | ra | dec | objtype | DistMpc | DistMpc_unc | m_NUV | m_NUV_unc | m_Ks | m_Ks_unc | m_W1 | m_W1_unc | P_3D | P_3D_LumW1 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| WISEA J215127.50-771123.7 | 327.86462 | -77.18992 | G | 452.47 | 0.65 | null | null | 12.307 | 0.126 | 10.624 | 0.006 | 1.78e-05 | 2.49e-06 |
| WISEA J214848.24-774454.9 | 327.20101 | -77.74860 | G | 463.54 | null | null | null | 13.779 | 0.215 | 12.525 | 0.010 | 5.02e-05 | 1.28e-06 |
| WISEA J214657.45-773648.1 | 326.73941 | -77.61338 | G | 462.69 | null | null | null | 13.264 | 0.192 | 12.822 | 0.014 | 5.09e-05 | 9.83e-07 |
| WISEA J215919.74-775235.8 | 329.83229 | -77.87662 | G | 451.54 | 0.65 | null | null | 12.506 | 0.114 | 12.064 | 0.008 | 1.84e-05 | 6.79e-07 |
| WISEA J213625.95-773427.9 | 324.10814 | -77.57443 | G | 382.59 | null | null | null | 13.364 | 0.175 | 12.873 | 0.009 | 5.20e-05 | 6.48e-07 |
| WISEA J214445.54-771938.0 | 326.18978 | -77.32723 | G | 452.13 | null | 21.430 | 0.312 | 13.324 | 0.160 | 13.240 | 0.012 | 4.93e-05 | 6.16e-07 |
| WISEA J213608.67-772811.0 | 324.03613 | -77.46974 | G | 356.09 | null | null | null | 13.524 | 0.185 | 12.576 | 0.014 | 4.08e-05 | 5.79e-07 |
| WISEA J215658.48-774037.0 | 329.24368 | -77.67696 | G | 403.25 | null | null | null | 13.090 | 0.145 | 12.789 | 0.009 | 3.68e-05 | 5.55e-07 |
| WISEA J213700.75-773426.0 | 324.25313 | -77.57391 | G | 325.21 | 0.65 | 20.310 | 0.217 | 12.016 | 0.075 | 11.838 | 0.006 | 2.20e-05 | 5.14e-07 |
| WISEA J213454.05-772745.8 | 323.72522 | -77.46275 | G | 372.81 | null | null | null | 13.059 | 0.143 | 13.016 | 0.016 | 4.75e-05 | 4.92e-07 |
| WISEA J215146.52-780557.3 | 327.94386 | -78.09927 | G | 368.25 | 0.65 | null | null | 12.882 | 0.135 | 12.228 | 0.010 | 2.20e-05 | 4.63e-07 |
| WISEA J213733.97-780150.6 | 324.39156 | -78.03074 | G | 345.10 | null | 20.443 | 0.108 | 13.417 | 0.204 | 11.831 | 0.007 | 1.74e-05 | 4.63e-07 |
| WISEA J212943.06-775542.2 | 322.42945 | -77.92839 | G | 372.02 | 0.65 | null | null | 12.273 | 0.114 | 12.187 | 0.009 | 1.85e-05 | 4.12e-07 |
| WISEA J213608.13-772733.1 | 324.03389 | -77.45922 | G | 319.73 | 0.65 | 21.351 | 0.351 | 12.228 | 0.076 | 11.985 | 0.007 | 2.01e-05 | 3.96e-07 |
| WISEA J212913.11-773431.7 | 322.30466 | -77.57550 | G | 378.32 | 0.65 | 20.656 | 0.234 | 13.354 | 0.165 | 12.886 | 0.013 | 2.92e-05 | 3.50e-07 |
| WISEA J213641.68-775334.3 | 324.17371 | -77.89287 | G | 411.08 | null | 21.445 | 0.179 | 13.521 | 0.179 | 13.546 | 0.029 | 4.09e-05 | 3.18e-07 |
| WISEA J213632.95-765253.9 | 324.13731 | -76.88165 | G | 460.55 | 0.65 | null | null | 12.341 | 0.134 | 12.450 | 0.009 | 1.07e-05 | 2.86e-07 |
| WISEA J213724.56-773623.7 | 324.35235 | -77.60660 | G | 536.58 | null | null | null | 13.729 | 0.212 | 13.039 | 0.022 | 1.35e-05 | 2.84e-07 |
| WISEA J215903.81-773020.5 | 329.76589 | -77.50571 | G | 426.05 | null | null | null | 13.020 | 0.144 | 12.826 | 0.010 | 1.74e-05 | 2.83e-07 |
| WISEA J212501.23-771251.5 | 321.25514 | -77.21431 | G | 436.36 | null | null | null | 13.307 | 0.143 | 12.903 | 0.015 | 1.69e-05 | 2.66e-07 |
Table 1: Top 20 galaxies in NED-LVS that fall in the 90% probability volume for S250830bp sorted by the joint probability of 3D position and WISE W1 luminosity (P_3D * P_LumW1). Galaxy is the NED preferred name. RA and Dec are the Equatorial coordinates in degrees (J2000). Objtype is the object type of the galaxy candidate. Distance is the distance to the galaxy in Mpc. m_NUV and mErr_NUV are the apparent magnitude and error from GALEX. m_Ks and mErr_Ks are the apparent magnitude and error from 2MASS. m_W1 and mErr_W1 are the apparent magnitude and error from AllWISE. P_3D is the probability that the galaxy is in the volume given the distance of GW event. P_3D_LumW1 is the joint probability within the volume weighted by the WISE1 luminosity of the galaxy (P_3D * P_LumW1).
GCN Circular 41607
The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:
We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250830bp (GCN Circular 41606). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page:
https://gracedb.ligo.org/superevents/S250830bp
Assuming the candidate is astrophysical, the updated classification of the GW signal, in order of descending probability,
is BBH (>99%), Terrestrial (<1%), BNS (<1%), or NSBH (<1%).
Based on posterior support from parameter estimation [1], under the assumption that the candidate S250830bp is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass above one solar mass (HasNS) is <1%. [2] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [2] HasRemnant is assumed to be zero when the heavier component mass is below 1 solar mass. Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%. The probability that the lighter compact object is below 1 solar mass (HasSSM) is <1%.
For the Bilby.multiorder.fits,0 sky map, the 90% credible region is well fit by an ellipse with an area of 4 deg2 described by the following DS9 region (right ascension, declination, semi-major axis, semi-minor axis, position angle of the semi-minor axis):
icrs; ellipse(21h43m, -77d36m, 1.36d, 0.88d, 15.49d)
Marginalized over the whole sky, the a posteriori luminosity distance estimate is 427 +/- 69 Mpc (a posteriori mean +/- standard deviation).
For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.
[1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040
[2] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
GCN Circular 41606
The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:
We identified the compact binary merger candidate S250830bp during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-08-30 10:24:18.852 UTC (GPS time: 1440584676.852). The candidate was found by the Aframe [1], cWB [2], cWB BBH [3], GstLAL [4], MBTA [5], MLy [6], PyCBC Live [7], and SPIIR [8] analysis pipelines.
S250830bp is an event of interest because its false alarm rate, as estimated by the online analysis, is 3.2e-10 Hz, or about one in 1e2 years. The event's properties can be found at this URL:
https://gracedb.ligo.org/superevents/S250830bp
The classification of the GW signal, in order of descending probability, is BBH (84%), NSBH (16%), Terrestrial (<1%), or BNS (<1%).
Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [9] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [9] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is 16%.
The source chirp mass falls with highest probability in the bin (5.5, 11.0) solar masses, assuming the candidate is astrophysical in origin.
Two sky maps are available at this time and can be retrieved from the GraceDB event page:
* amplfi.multiorder.fits,0, an initial localization generated by AMPLFI [10], distributed via GCN and SCiMMA notices about 24 seconds after the candidate event time.
* bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [11], distributed via GCN and SCiMMA notices about 5 minutes after the candidate event time.
The preferred sky map at this time is bayestar.multiorder.fits,0. For the bayestar.multiorder.fits,0 sky map, the 90% credible region is 11 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 429 +/- 100 Mpc (a posteriori mean +/- standard deviation).
For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.
[1] Marx et al. PRD 111, 042010 (2025) doi:10.1103/PhysRevD.111.042010
[2] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004
[3] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018
[4] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008
[5] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234
[6] Skliris et al. PRD 110, 104034 (2024) doi:10.1103/PhysRevD.110.104034
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