TITLE:   GCN CIRCULAR
NUMBER:  25197
SUBJECT: LIGO/Virgo S190728q: Second update on neutrino search with IceCube
DATE:    19/07/28 14:35:47 GMT
FROM:    Stefan Countryman at ICECUBE/Columbia U  <stc2117@columbia.edu>

IceCube Collaboration (http://icecube.wisc.edu/) reports:

This is an update of GCN 25192 including the p-value from the Bayesian search.

Searches for track-like muon neutrino events detected by IceCube consistent with the sky localization of gravitational-wave candidate S190728q in a time range of 1000 seconds [1] centered on the alert event time (2019-07-28 06:36:50.529 UTC to 2019-07-28 06:53:30.529 UTC) have been performed. During this time period IceCube was collecting good quality data. Two hypothesis tests were conducted. The first search is a maximum likelihood analysis which searches for a generic point-like neutrino source coincident with the given GW skymap [2]. The second uses a Bayesian approach to quantify the joint GW + neutrino event significance, which assumes a binary merger scenario and accounts for known astrophysical priors in the significance estimate, such as GW source distance [3].

One track-like event is found in spatial and temporal coincidence with the gravitational-wave candidate S190728q calculated from the map circulated in the 4-Initial notice. This represents an overall p-value of 0.03 (1.84 sigma) from the generic transient search and an overall p-value of 0.013 (2.22 sigma) for the Bayesian search. These p-values measure the consistency of the observed track-like events with the known atmospheric backgrounds.

An earlier search (GCN 25185) based on preliminary information of S190728q yielded no significant p-values for the worse GW localization.

The reported p-values can differ due to the estimated distance of the GW candidate. The distance is used as a prior in Bayesian binary merger search, while it is not taken into account in the generic transient point-like source search.

Properties of the coincident events are shown below.

   dt|ra (deg)|dec (deg)|Angular Uncertainty(deg)|p-value (bayesian)|p-value(generic transient)
-----+--------+---------+------------------------+------------------+--------------------------
 -360|312.87  |5.85     |4.81                    |0.013             |0.039


where:
dt = Time offset (sec) of track event with respect to GW trigger. Angular uncertainty = Angular uncertainty of track event: the radius of a circle representing 90% CL containment by area.
p-value = the p-value for this specific track event from each search.
RA & Dec = Right ascension and declination in degrees quoted in J2000 epoch

The IceCube Neutrino Observatory is a cubic-kilometer neutrino detector operating at the geographic South Pole, Antarctica. The IceCube realtime alert point of contact can be reached at roc@icecube.wisc.edu


[1] Baret et al., Astroparticle Physics 35, 1 (2011)
[2] Braun et al., Astroparticle Physics 29, 299 (2008)
[3] Bartos et al. arXiv:1810.11467 (2018) and Countryman et al.arXiv:1901.05486 (2019)