X-rays and mode changes in PSR J2021+4026 comma the first variable gamma-ray pulsar

Among the radioquiet pulsars discovered by Fermi, an unique source is PSRJ2021+4026 in the Gamma Cygni supernova remnant. This pulsar has shocked thehighenergy astrophysics community by featuring a simultaneous change in fluxand frequency derivative, breaking the longstanding paradigm of pulsars asstable gammaray sources. Monitoring the gammaray mode changes at differentenergies is key to understand if such events are due to global or local changesin the magnetosphere geometry and/or configurations. We propose a ToO with XMMto study the Xray spectral and timing behaviour of PSR J2021+4026 following amode change. This will provide the first contemporary X and gammaray study ofpulsar mode changes, key to decrypt this intriguing, but still obscure, aspect of pulsar physics. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]