  GeV flares from PSR B1259-63/LS 2883 were seen starting around 30 days after
the two periastron passages in 2010 and 2014. The flares are clearly delayed
compared to the occurrence of the X-ray and TeV flux peaks during the
post-periastron disk crossing. While several attempts have been put forward to
explain this phenomenon, the origin of these GeV flares remains a puzzle. Here
we present a detailed analysis of the observational data taken by the Fermi and
Swift observatories over the 2017 September periastron passage. For the first
time, we find short-lived but powerful GeV flares on time scales of down to
three hours. The onset of the GeV flaring period in 2017 is also delayed
compared to those seen in 2011 and 2014. Supplemented by a re-analysis of
previous data, we compare the Fermi/LAT, Swift/XRT and Swift/UVOT light curves
in 2017 with those taken over the 2010 and 2014 periastrons, and difference in
UVOT light curves are noted.
