TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  4998
SUBJECT: OT 060420: Likely Not a New Cosmic Transient
DATE:    06/04/23 20:04:08 GMT
FROM:    Robert Nemiroff at Michigan Tech.  <nemiroff@mtu.edu>

R. J. Nemiroff and L. Shamir report on 
behalf of the Night Sky Live / CONCAM 
project

We verify the report in GCN 4997 that OT 
060420 does not appear on MASCOT
frames.  Because the OT was near 
MASCOT's limiting brightness and to be 
conservative, we then co-added MASCOT 
frames taken around the time of 
candidate OT 060420 in an attempt to 
rival the sensitivity of the Cerro 
Pachon CONCAM images.  Given an 
assumption of equal bandpass, detection 
of OT 060420 was expected but not found. 
  Inspection of the Cerro Pachon CONCAM 
images also show somewhat unusual OT 
PSFs.  More detailed analyses of the 
Canary Island CONCAM images showed them 
to be generally less reliable than we 
originally expected.  Therefore, it is 
now our best guess that the cause for 
candidate OT 060420 is a rare series of 
similar, extended, precisely-placed, 
time-consecutive cosmic-ray hits.  We no 
longer encourage follow-up observations. 
  For interest, here is a short movie of 
the candidate transient as it appeared 
on the Cerro Pachon CONCAM frames: 
http://nightskylive.net/temp/movie2_cp060420.gif

We are grateful to A. Smette and the 
MASCOT team for making their images 
available.