LIGO/Virgo/KAGRA S230802aq

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

We identified the compact binary merger candidate S230802aq during
real-time processing of data from LIGO Hanford Observatory (H1) at
2023-08-02 11:33:59.961 UTC (GPS time: 1375011257.961). The candidate
was found by the GstLAL [1] analysis pipeline.

S230802aq is an event of interest because its false alarm rate, as
estimated by the online analysis, is 2.2e-08 Hz, or about one in 1
year, 5 months. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S230802aq

The classification of the GW signal, in order of descending
probability, is BBH (90%), NSBH (6%), Terrestrial (3%), 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 57%. [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 68%.

Two sky maps are available at this time and can be retrieved from the
GraceDB event page:
 * bayestar.multiorder.fits,0, an initial localization generated by
BAYESTAR [3], distributed via GCN notice about 26 seconds after the
candidate event time.
 * bayestar.multiorder.fits,1, an initial localization generated by
BAYESTAR [3], distributed via GCN notice about 5 minutes after the
candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,1. For
the bayestar.multiorder.fits,1 sky map, the 90% credible region is
24221 deg2. Marginalized over the whole sky, the a posteriori
luminosity distance estimate is 444 +/- 156 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. arXiv:2305.06286
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  (2023) and Ewing et al.
arXiv:2305.05625
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  (2023)
 [2] Chatterjee et al. ApJ 896, 54 (2020)
 [3] Singer & Price PRD 93, 024013 (2016)

T. Mihara, N. Kawai (RIKEN),
H. Negoro, M. Nakajima (Nihon U.),
S. Sugita, M. Serino, H. Hiramatsu, H. Nishikawa, Y. Kondo (AGU),
Y. Kawakubo (LSU)
report on behalf of the MAXI team: 

We examined MAXI/GSC all-sky X-ray images (2-20 keV)
after compact binary merger candidate S230802aq at 2023-08-02 11:33:59 UTC (GCN #34314
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 ).

At the trigger time of S230802aq, the high-voltage of MAXI/GSC was on.
The instantaneous field of view of GSC at the GW trigger time covered 1% of the 90% credible region
of the bayestar sky map, in which we found no significant new X-ray source.
The first one-orbit (92 min) scan observation with GSC after the event covered 62%
of the 90% credible region of the bayestar skymap from 11:33:59 to 13:05:58 UTC (T0+0 to T0+5519 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. 

J. Wood (NASA/MSFC) and J. Mangan (CNRS/IJCLab) report on behalf of the Fermi-GBM Team:

For S230802aq (GCN 34314
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 ) and using the initial bayestar skymap, Fermi-GBM was observing 59.4% 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 S230802aq. 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=335.9, Dec=25.3 with a radius of 67.6 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:     1.2       2.1       4.6
1.024 s:     0.3       0.6       1.3
8.192 s:     0.1       0.1       0.2

Assuming the a posteriori mean luminosity distance of 444 Mpc from the GW detection, we estimate the following intrinsic luminosity upper limits over the 1 keV-10 MeV energy range (in units of 10^48 erg/s):

Timescale  Soft    Normal   Hard 
------------------------------------
0.128s:       4.3       7.0       25.
1.024s:       1.0       2.1       7.1
8.192s:       0.3       0.4       1.3

G. Waratkar (IITB), V. Bhalerao (IITB), M. Dixit (IITB), D. Bhattacharya (Ashoka University/IUCAA), A. R. Rao (TIFR), S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration:

We have carried out a search for X-ray candidates in Astrosat CZTI data in a 100-sec window around the trigger time of the event S230802aq (UTC 2023-08-02 11:33:59, GraceDB event). We use the bayestar.multiorder.fits,0 map (https://gracedb.ligo.org/api/superevents/S230802aq/files/bayestar.multiorder.fits,0) for our analysis. CZTI is a coded aperture mask instrument that has a considerable effective area for about 29% of the entire sky but is also sensitive to brighter transients from the entire sky. At the time of the merger, Astrosat's nominal pointing is RA, DEC = 17:22:28.7, -41:35:57.7 (260.6196,-41.5994), which is ~128 deg away from the maximum probability location. At the time of the merger event, the Earth-satellite-transient angle corresponding to the maximum probability location is ~53 deg and is occulted by Earth in the satellite's frame. In a time interval of 100 sec around the event, the region of the localization map not occulted by Earth in the satellite's frame has a total probability of 0.70 (70%) of containing the source.

CZTI data were de-trended to remove orbit-wise background variation. We then searched data from the four independent, identical quadrants for coincident spikes in the count rates. Searches were undertaken by binning the data in 0.1s, 1s, and 10s respectively. Statistical fluctuations in background count rates were estimated by using data from 5 preceding orbits. We selected confidence levels such that the probability of a false trigger in a 100-sec window is 10^-4. We do not find any evidence for any hard X-ray transient in this window, in the CZTI energy range of 20-200 keV.

We use a detailed mass model of the satellite to calculate the direction-dependent instrument response for points in the visible sky. We then assume the source is modeled as a power law with photon index alpha = -1, and convert our count rate upper limits to direction-dependent flux limits. We obtain the following upper limits for source flux in the 20-200 keV band by taking a probability-weighted mean over the visible sky:

0.1 s: flux limit= 1.68e-05 ergs/cm^2/s; fluence limit = 1.68e-06 ergs/cm^2
1.0 s: flux limit= 3.31e-06 ergs/cm^2/s; fluence limit = 3.31e-06 ergs/cm^2
10.0 s: flux limit= 3.84e-07 ergs/cm^2/s; fluence limit = 3.84e-06 ergs/cm^2

CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, URSC, IUCAA, SAC, and PRL. The Indian Space Research Organisation funded, managed, and facilitated the project.

CZTI GRB detections are reported regularly on the payload site at:
http://astrosat.iucaa.in/czti/?q=emgw

We have conducted further analysis of the LIGO Hanford Observatory (H1) data around the time of the compact binary merger (CBC) candidate S230802aq (GCN Circular 34314
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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/S230802aq

Based on posterior support from parameter estimation [1], under the assumption that the candidate S230802aq is astrophysical in origin, the probability that the lighter compact object is consistent with a neutron star mass (HasNS) is 9%. [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 62%.

For the Bilby.offline0.multiorder.fits,0 sky map, the 90% credible region is 25885 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 576 +/- 246 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/.