[vsnet-grb-info 25935] GRB 200625A: Fermi-LAT detection is likely a flare from PKS 1127-14

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  28035
SUBJECT: GRB 200625A: Fermi-LAT detection is likely a flare from PKS 1127-14
DATE:    20/06/26 14:53:07 GMT
FROM:    Magnus Axelsson at Stockholm U.  <magaxe at kth.se>

M. Axelsson (KTH & Stockholm Univ.), M. Ohno (Hiroshima Univ.), N. Di Lalla (Stanford Univ.), M. Arimoto (Kanazawa Univ.), S. Cutini (INFN Perugia) and F. Longo (Univ. and INFN Trieste) report on behalf of the Fermi-LAT Collaboration:

On June 25th, 2020, Fermi-LAT detected high-energy emission potentially related to the afterglow of GRB 200625A, detected by Fermi-GBM (trigger 614781954/200625532 Fermi GBM Team GCN 28028).

The best LAT on-ground location is found to be

RA, Dec 172.8, -14.7 (degrees, J2000)

with an error radius of 0.2 deg (90% containment, statistical error only). This was 97 deg from the LAT boresight at the time of the GBM trigger:

T0 =  12:45:49.14 UT.

The position is 8 degrees from the GBM location, and came into the LAT FoV ~3500 s after the trigger. The data from Fermi-LAT show a significant increase in the event rate in the time interval 3500-7000 s after the GBM trigger. The photon flux above 100 MeV in this time interval is 5.9e-07 +/- 2.9 e-07 ph/cm2/s, and the estimated photon index above 100 MeV is -1.8 +/-  0.3.

However, we note that the best localization is close (16 arcmin) from the known high-energy source PKS 1127-14 (4FGL J1129.8-1447). Analysis reveals that the significance of the excess increases in the following observation window, and reaches a maximum in the time interval T0+15ks to T0+18ks (photon flux 1.7e-06 +/- 5.7e-7 ph/cm2/s; spectral index -2.1 +/- 0.24).

The increased significance at later times points to PKS 1127-14 (an FSRQ at z=1.18) as the source of the emission, rather than GRB 200625A. However, we do not find any similar excess in the time windows before the trigger (T0-8ks to T0-5ks) or at later times (>T0+30ks), indicating a short and bright flare.

The initial detection of a transient event near the position of GRB 200625A led to a LAT offline position notice being issued. As a result, a Swift ToO observation has been made for this event (likely an FSRQ flare).

The Fermi-LAT point of contact for this burst is Magnus Axelsson (magaxe at kth.se<mailto:magaxe at kth.se>).

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.