Analysis of the IFI+Iqueye observations of the Crab pulsar taken around the epoch of the major November 2017 glitch

After a catastrophic supernova explosion, the core of a star collapses inwards until the contraction is halted and a new equilibrium configuration is reached. One of the possible stable remnants is the neutron star, where gravity is balanced by nuclear forces and the degenerate pressure provided by neutrons. A peculiar type of neutron star is the pulsar: a highly magnetised, fast rotating compact object which embodies an ideal astrophysical laboratory to study Physics under extreme conditions. The pulsar emission is characterised by a repeating signal, which usually has a well defined shape and occurs with an accuracy that could compete with that of the atomic clocks. Since this pulsing signal is due to the passage of the beamed emission from the poles of the star across the line of sight of the observer, the rotational period of the neutron star coincides with the time interval between two pulses, which correspond the same pole. Therefore, the analysis of the time of arrival of the pulses is a fundamental tool to constrain the timing behaviour, the rotational evolution and the physical properties of the pulsar. Indeed, albeit the signal coming from a pulsar is remarkably stable, sometimes the neutron star undergoes an unpredictable and sudden spin up event, called glitch, which is usually followed by an exponential decay of the rotational frequency ν towards the pre-glitch values. In this work we analyzed the time behaviour of the Crab pulsar (PSR B0531+21), which is the young neutron star at the centre of the Crab Nebula, with rotational frequency ν ∼ 30 Hz. Until 2017, the Crab pulsar was known to produce glitches with a spin frequency increase of 10 −9 ≤ ∆ν/ν ≤ 10 −7 ; however, around MJD 58064 (8 November 2017), the Crab experienced what is now confirmed to be its largest glitch ever detected: the magnitude of the step increase was ∆ν/ν = 0.516×10 −6 in the radio band. We parsed the observations of the Crab pulsar made with the very fast optical photon counter Iqueye, mounted at the 122 cm Galileo Telescope in Asiago through a dedicated optical fiber interface (IFI). The observations were carried out on 2017 October 4 and 7, November 17 and 18, December 30. The timing accuracy and the amount of data available allowed us to pinpoint the major November 2017 glitch in the optical domain and to compare it with the results provided by the Jodrell Bank radio ephemeris. Furthermore, we searched for evidence of changes in the geometry of the emission regions and/or in the magnetosphere of the pulsar possibly induced by the glitch.