LIGO/Virgo/KAGRA S231020ba

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We identified the compact binary merger candidate S231020ba during real-time processing of data from LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) at 2023-10-20 14:29:47.583 UTC (GPS time: 1381847405.583). The candidate was found by the GstLAL [1], MBTA [2], PyCBC Live [3], and SPIIR [4] analysis pipelines.

S231020ba is an event of interest because its false alarm rate, as estimated by the online analysis, is 1.3e-09 Hz, or about one in 25 years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S231020ba

The classification of the GW signal, in order of descending probability, is BBH (91%), NSBH (8%), Terrestrial (<1%), or BNS (<1%).

There was a high rate of noise transients (glitches) in the LIGO Hanford detector which may affect the parameters or the significance of the candidate.

Assuming the candidate is astrophysical in origin, the probability that the lighter compact object is consistent with a neutron star mass (HasNS) is <1%. [5] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [5] 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 7%.

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 [6], distributed via GCN notice about 27 seconds after the candidate event time.
 * bayestar.multiorder.fits,2, an initial localization generated by BAYESTAR [6], 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 1444 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1111 +/- 404 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/userguide/.

 [1] Tsukada et al. PRD 108, 043004 (2023) and Ewing et al. arXiv:2305.05625
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  (2023)
 [2] Aubin et al. CQG 38, 095004 (2021)
 [3] Dal Canton et al. ApJ 923, 254 (2021)
 [4] Chu et al. PRD 105, 024023 (2022)
 [5] Chatterjee et al. ApJ 896, 54 (2020)
 [6] Singer & Price PRD 93, 024013 (2016)

Y. Kawakubo (LSU),
H. Negoro, M. Nakajima (Nihon U.), 
T. Mihara (RIKEN), N. Kawai (RIKEN), 
S. Sugita, M. Serino, H. Hiramatsu, H. Nishikawa, 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 S231020ba at 2023-10-20 14:29:47.000 UTC (GCN #34852
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 ). 

At the trigger time of S231020ba, the high-voltage of MAXI/GSC was off, 
and it was turned on at T0+1153 sec (+19.2 min). 
The first one-orbit (92 min) scan observation with GSC after the event covered 90% 
of the 90% credible region of the bayestar skymap from 14:51:52 to 16:00:18 UTC (T0+1325 to T0+5431 sec). 

No significant new source was found in the region in the one-orbit scan observation. 
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. 

S. Bala (USRA) reports on behalf of the Fermi-GBM Team:

For S231020ba (GCN 34852
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 ) and using the initial bayestar skymap, Fermi-GBM was observing 25.4% of the localization probability at event time.

There was no Fermi-GBM onboard trigger around the event time of the LIGO/Virgo/KAGRA (LVK) detection of GW trigger S231020ba. An automated, blind search for short gamma-ray bursts below the onboard triggering threshold in Fermi-GBM also identified no counterpart candidates. The GBM targeted search, the most sensitive, coherent search for GRB-like signals, was run from +/-30 s around merger time, and also identified no counterpart candidates.

Part of the LVK localization region is behind the Earth for Fermi, located at an RA=121.7, Dec=23.2 with a radius of 67.4 degrees. We therefore set upper limits on impulsive gamma-ray emission for the GW localization region visible to Fermi at merger time. Using the representative soft, normal, and hard GRB-like templates described in arXiv:1612.02395
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 , we set the following 3 sigma flux upper limits over 10-1000 keV, weighted by GW localization probability (in units of 10^-7 erg/s/cm^2):

Timescale Soft Normal Hard
------------------------------------
0.128 s:  3.3 	  4.0 	6.6
1.024 s:  1.3 	  1.5 	2.1
8.192 s:  0.4 	  0.4 	0.7

Assuming the median luminosity distance of 1110.8 Mpc from the GW detection, we estimate the following intrinsic luminosity upper limits over the 1 keV-10 MeV energy range (in units of 10^50 erg/s):

Timescale Soft  Normal Hard
------------------------------------
0.128s:   0.8    0.8     2.3
1.024s:   0.3    0.3     0.7
8.192s:   0.1    0.1     0.2

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further analysis of the LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) data around the time of the compact binary merger (CBC) candidate S231020ba (GCN Circular 34852
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 ). 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/S231020ba

Based on posterior support from parameter estimation [1], under the assumption that the candidate S231020ba is astrophysical in origin, the probability that the lighter compact object is consistent with a neutron star 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] 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 16%.

For the Bilby.offline0.multiorder.fits,0 sky map, the 90% credible region is 1339 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1168 +/- 361 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/userguide/.

 [1] Ashton et al. ApJS 241, 27 (2019) and Morisaki et al. arXiv:2307.13380
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  (2023)
 [2] Chatterjee et al. ApJ 896, 54 (2020)