tokens: This work is based on the observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programme 095.D-0343(A). We thank the anonymous referee for his/her very constructive comments to our manuscript. RPM acknowledges financial support from the project TElescopi CHErenkov made in Italy (TECHE).it. CRA 1.05.06.04.01 cap 1.05.08 for the project ‘Studio multi-lunghezze d'onda da stelle di neutroni con particolare riguardo alla emissione di altissima energia’. DGC acknowledges the financial support by CONICYT through a ‘Becas Chile’ fellowship no. 72150555.
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