The largest glitch observed in the Crab pulsar

We have observed a large glitch in the Crab pulsar (PSR B0531+21). The glitch occurred around MJD 58064 (2017 November 8) when {the pulsar underwent an increase in the rotation rate of $\Delta \nu = 1.530 \times 10^{-5}$ Hz, corresponding to a fractional increase of $\Delta \nu / \nu = 0.516 \times 10^{-6}$} making this event the largest glitch ever observed in this source. Due to our high-cadence and long-dwell time observations of the Crab pulsar we are able to partially resolve a fraction of the total spin-up of the star. This delayed spin-up occurred over a timescale of $\sim$1.7 days and is similar to the behaviour seen in the 1989 and 1996 large Crab pulsar glitches. The spin-down rate also increased at the glitch epoch by $\Delta \dot{\nu} / \dot{\nu} = 7 \times 10^{-3}$. In addition to being the largest such event observed in the Crab, the glitch occurred after the longest period of glitch inactivity since at least 1984 and we discuss a possible relationship between glitch size and waiting time. No changes to the shape of the pulse profile were observed near the glitch epoch at 610 MHz or 1520 MHz, nor did we identify any changes in the X-ray flux from the pulsar. The long-term recovery from the glitch continues to progress as $\dot{\nu}$ slowly rises towards pre-glitch values. In line with other large Crab glitches, we expect there to be a persistent change to $\dot{\nu}$. We continue to monitor the long-term recovery with frequent, high quality observations.