Joint XMM-Newton and H.E.S.S. view of PSR B1259-63 before stellar disc passage

PSR B125963 is one of only two known GRLBs that hosts a pulsar orbiting a Bestar. Close to periastron the system is detected from radio through Xray up to10 TeV energies. The latest TeV analysis indicates that in the very high energydomain, it exhibits a 3peak light curve around periastron. While two of themaxima roughly coincide with the periods when the pulsar crosses the Bestardisc and can be explained within existing models, the origin of the third peakremains elusive. We ask for joint H.E.S.S./XMMNewton observations of this peakin 2021 to probe its periodic appearance in the TeVs and to search for andinvestigate its Xray counterpart. Due to groundbased visibility constraints,this will be the only opportunity to observe in periastron in TeV and Xrays for next 7 years. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]