GCN Circular 18458

Possible Fermi-LAT Gamma-ray counterpart for LIGO/Virgo G194575

G. Vianello (Stanford), N. Omodei (Stanford) and Julie McEnery (GSFC)

report on behalf of the Fermi-LAT team:

We report the detection of a possible gamma-ray counterpart for the 
LIGO/VIRGO candidate G194575 in the data of the Fermi Large Area Telescope.

We performed a search for a gamma-ray counterpart above 200 MeV on 
various timescales for 10,000s after the LIGO trigger. The entire 
G194575 error region came within the LAT FOV for some time during this 
interval. In this search we found a new gamma-ray transient, which was 
within the LAT FoV between 1000 and 3500 s after the trigger (from 
2015-10-22 13:53:19 UTC to 2015-10-22 14:36:39 UTC)

The transient is faint and localized on the border of the LIGO/VIRGO 
confidence region. Its significance is around 4 sigma pre-trials. The 
best localization for the possible LAT counterpart is a circle centered on :

(R.A., Dec.) = (221.69, -3.246) deg (J2000)

with a 90% containment radius of 0.52 deg (statistical only).

The significance of this putative excess is below our standard reporting 
threshold of 5 sigma (pre-trials). We cannot confirm nor exclude at this 
stage that the source is not a background fluctuation and  is related to 
the LIGO/VIRGO candidate. Further analysis is ongoing.

However, follow up is strongly encouraged.

The Fermi-LAT point of contact for this candidate is Giacomo Vianello 
(giacomov@stanford.edu).

The Fermi-LAT is a pair conversion telescope designed to cover the 
energy band from 20 MeV to greater than 300 GeV. It is the product of an 
international collaboration between NASA and DOE in the U.S. and many 
scientific institutions across France, Italy, Japan and Sweden.