PSR B1259 - 63/SS 2883 is a \gamma -ray binary system composed of a radio pulsar in a long ( 1236.7 days ) and elliptical ( e \sim 0.87 ) orbit around a Be star .
In its 2010 periastron passage , multiwavelength emission from radio to TeV was observed , and an unexpected GeV flare was detected by the Fermi Large Area Telescope ( LAT ) .
Here we present the results of the LAT monitoring of PSR B1259 - 63 during its most recent 2014 periastron passage .
We confirm that the GeV flare is recurrent within the orbit .
The comparison of the 2014 and 2010 periastron passages shows overall similarities of flare durations , average flux levels , and spectra .
In contrast , the detailed time evolutions of the two flares present interesting differences .
Indeed , the light curves of the two flares show both a different structure and peak energy flux ( 9.6 \pm 1.8 \times 10 ^ { -10 } erg cm ^ { -2 } s ^ { -1 } and 7.1 \pm 1.3 \times 10 ^ { -10 } erg cm ^ { -2 } s ^ { -1 } , respectively in 2010 and 2014 ) .
While the tail of the 2010 flare the flux decayed exponentially , in 2014 it persisted at a high level .
The interpretation of these differences as well as of the flare themselves is subject of debate .