Item

Abstract

PSR B1259-63/LS 2883 is a gamma-ray binary system consisting of a pulsar in
an eccentric orbit around a bright Oe stellar-type companion star that features
a dense circumstellar disc. The high- and very-high-energy (HE, VHE) gamma-ray
emission from PSR B1259-63/LS 2883 around the times of its periastron passage
are characterised, in particular, at the time of the HE gamma-ray flares
reported to have occurred in 2011, 2014, and 2017. Spectra and light curves
were derived from observations conducted with the H.E.S.S.-II array in 2014 and
2017.
A local double-peak profile with asymmetric peaks in the VHE light curve is
measured, with a flux minimum at the time of periastron $t_p$ and two peaks
coinciding with the times at which the neutron star crosses the companion's
circumstellar disc ($\sim t_p \pm 16$ d). A high VHE gamma-ray flux is also
observed at the times of the HE gamma-ray flares ($\sim t_p + 30$ d) and at
phases before the first disc crossing ($\sim t_p - 35$ d). PSR B1259-63/LS 2883
displays periodic flux variability at VHE gamma-rays without clear signatures
of super-orbital modulation in the time span covered by H.E.S.S. observations.
In contrast, the photon index of the measured power-law spectra remains
unchanged within uncertainties for about 200 d around periastron. Lower limits
on exponential cut-off energies up to $\sim 40$ TeV are placed.
At HE gamma-rays, PSR B1259-63/LS 2883 has now been detected also before and
after periastron, close to the disc crossing times. Repetitive flares with
distinct variability patterns are detected in this energy range. Such outbursts
are not observed at VHEs, although a relatively high emission level is
measured. The spectra obtained in both energy regimes displays a similar slope,
although a common physical origin either in terms of a related particle
population, emission mechanism, or emitter location is ruled out.