088142

PSR B1259-63 is one of only two known GRLBs that hosts a pulsar orbiting a Be star. Close to periastron the system is detected from radio through X-ray up to 10 TeV energies. The latest TeV analysis indicates that in the very high energy domain comma it exhibits a 3-peak light curve around periastron. While two of the maxima roughly coincide with the periods when the pulsar crosses the Be-star disc and can be explained within existing models comma the origin of the third peak remains elusive. We ask for joint H.E.S.S.-XMM-Newton observations of this peak in 2021 to probe its periodic appearance in the TeVs and to search for and investigate its X-ray counterpart. Due to ground-based visibility constraints comma this will be the only opportunity to observe in periastron in TeV and X-rays for next 7 years.