LIGO/Virgo/KAGRA S240615dg
GCN Circular 36972
A. Reguitti (INAF-OAB / INAF-OAPd), L. Tartaglia (INAF-OAAb), L. Tomasella (INAF-OAPd), E. Cappellaro (INAF-OAPd), F. Onori (INAF-OAAb), F. De Luise (INAF-OAAb), E. Bigongiari (Univ. Pisa), R. Brivio (INAF-OAB), M. Ferro (INAF-OAB), Y.-D Hu (INAF-OAB), B. Patricelli (Univ. Pisa), P. D'Avanzo (INAF-OAB), E. Brocato (INAF-OAAb) report on behalf of the GRAWITA collaboration:
We carried out optical follow-up observations of the well-localised LIGO/Virgo/KAGRA GW trigger S240615dg (LVK Collaboration, GCN Circ. 36669) with the Schmidt telescopes sited at the INAF Campo Imperatore and Asiago observatories (Italy). Observations from Campo Imperatore started on 2024-06-15 at 23:30 UT (~12 hours after the GW trigger) with the Sloan-r filter.
We covered 3 square degrees within the 90% localisation region of the S240615dg GW event with 3 pointings of 1 square degree each, centred at J2000 celestial coordinates: (6.886, 45.598); (8.112, 45.597) and (8.117, 46.445). These observations covered ~ 60% of the final 90% credible region (LVK Collaboration, GCN Circ. #36704). The 3sigma limiting AB magnitude is r ~ 19 mag.
Observations from Asiago started on 2024-06-17 at 00:00 UT (~ 1.52 days after the GW trigger) with the Sloan-r filter. A second epoch has been carried out starting on 2024-06-18 at 00:10 UT (~ 2.53 days after the GW trigger), also with the Sloan-r filter. We covered 4 square degrees within the 90% localisation region of the S240615dg GW event with 3 pointings of 1 square degree each, centred at J2000 celestial coordinates: (6.892, 45.564); (6.875, 46.418); (8.116, 46.419) and (8.123, 45.556). These observations covered ~ 80% of the final 90% credible region (LVK Collaboration, GCN Circ. #36704). The typical 3sigma limiting AB magnitudes are r ~ 21.5 mag and r ~ 21.6 mag for the first and second epoch, respectively.
Preliminary analysis, which includes image subtraction with the template images from the PanSTARRS all-sky survey does not show evidence for promising candidate counterparts.
We note that within the localisation region of the Swift/XRT source N.32 from GCN Circ. #36698 (Page et al. 2024), we detect a source (WISEA J002455.22+452006.9) with variability at a 2.5sigma level. However, the source is reported in the GAIA EDR3 catalog with a parallax of 2.8+-0.17 mas and a proper motion of 21 mas/yr which, together with a red r-z colour of 1.9 mag from the PS1 catalog, is consistent with a late-type dwarf Galactic star experiencing a high-energy flare.
GCN Circular 36704
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 S240615dg (GCN Circular 36669). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.offline0.multiorder.fits,0, distributed via GCN Notice, is available for retrieval from the GraceDB event page:
https://gracedb.ligo.org/superevents/S240615dg
For the Bilby.offline0.multiorder.fits,0 sky map, the 90% credible region is well fit by an ellipse with an area of 5 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(00h31m, +45d40m, 1.36d, 1.20d, 111.41d)
Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1420 +/- 236 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. (2023) arXiv:2307.13380
GCN Circular 36687
Samuele Ronchini (PSU), Aaron Tohuvavohu (U Toronto), Jamie A. Kennea (PSU), James DeLaunay (PSU), Gayathri Raman (PSU), Tyler Parsotan (NASA GSFC) report:
Swift/BAT was observing >99% of the GW localization probability (bayestar.multiorder.fits) at merger time. A fraction >99% of the GW localization posterior is contained inside the BAT coded FoV.
The LVK notice, distributed in near real-time, triggered the Swift Mission Operations Center operated Gamma-ray Urgent Archiver for Novel Opportunities (GUANO; Tohuvavohu et al. 2020, ApJ, 900, 1).
Upon trigger by this notice, GUANO sent a command to the Swift Burst Alert Telescope (BAT) to save 200 seconds of BAT event-mode data from [-50,+150] seconds around the time of the burst. All the requested event mode data was delivered to the ground.
Using the NITRATES analysis (DeLaunay + Tohuvavohu 2022, ApJ, 941, 169), we searched for emission on 8 timescales from 0.128s to 16.384s in the interval [-20,+20] seconds around the merger time. We find no evidence for a signal, and derive the following upper limits.
We quote the 5-sigma flux upper limits in the 15-350 keV band, weighted over the GW localization, for four spectral templates (soft, normal, and hard GRB-like templates described in [arXiv:1612.02395], and spectral shape from GRB170817A [arXiv:1710.05446]) and for four time bins. In units of 10^-7 erg/s/cm^2:
| time_bin (s) | soft | normal | hard | GRB170817 |
|---|---|---|---|---|
| 0.256 | 2.37 | 2.66 | 2.51 | 2.78 |
| 1.024 | 1.21 | 1.36 | 1.28 | 1.42 |
| 4.096 | 0.66 | 0.74 | 0.70 | 0.77 |
| 16.384 | 0.42 | 0.47 | 0.44 | 0.49 |
The upper limits as function of sky position are plotted here, alongside the GW localization: https://doi.org/10.5281/zenodo.12008258
The solid and dashed lines indicate the 90% and 50% GW contour levels, respectively. The corresponding fits file is also included.
GUANO is a fully autonomous, extremely low latency, spacecraft commanding pipeline designed for targeted recovery of BAT event mode data around the times of compelling astrophysical events to enable more sensitive GRB searches.
A live reporting of Swift/BAT event data recovered by GUANO can be found at: https://www.swift.psu.edu/guano/
GCN Circular 36669
The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:
We identified the compact binary merger candidate S240615dg during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2024-06-15 11:36:20.724 UTC (GPS time: 1402486598.724). The candidate was found by the CWB [1], GstLAL [2], MBTA [3], and PyCBC Live [4] analysis pipelines.
S240615dg 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/S240615dg
After parameter estimation by RapidPE-RIFT [5], the classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), NSBH (<1%), or BNS (<1%).
Assuming the candidate is astrophysical in origin, the probability that the lighter compact object is consistent with a neutron star mass (HasNS) is <1%. [6] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [6] Both HasNS and HasRemnant consider the support of several neutron star equations of state. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%.
Two sky maps are available at this time and can be retrieved from the GraceDB event page:
* bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [7], distributed via GCN notice about 32 seconds after the candidate event time.
* bayestar.multiorder.fits,2, an initial localization generated by BAYESTAR [7], distributed via GCN notice about 5 minutes after the candidate event time.
The preferred sky map at this time is bayestar.multiorder.fits,2. For the bayestar.multiorder.fits,2 sky map, the 90% credible region is well fit by an ellipse with an area of 8 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(00h30m, +45d50m, 1.73d, 1.48d, 127.09d)
Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1321 +/- 238 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] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004
[2] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. (2023) arXiv:2305.05625
[3] Aubin et al. CQG 38, 095004 (2021) doi:10.1088/1361-6382/abe913
[4] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a
[5] Rose et al. (2022) arXiv:2201.05263 and Pankow et al. PRD 92, 023002 (2015) doi:10.1103/PhysRevD.92.023002
[6] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
[7] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013