A non-glitch speed-up event in the Crab Pulsar

Context: The rotation history of the Crab Pulsar is well described by (1) a rotation frequency $\nu$ and a slowdown model that is specified by its first two time derivatives $\dot \nu$ and $\ddot \nu$, known as the secular slowdown model, (2) occasional (once in $\approx 2$ years) significant and abrupt increases in the magnitude of $\nu$ and $\dot \nu$ (occurring on timescales of minutes), known as glitches, and (3) much slower increases and decreases in $\nu$ and $\dot \nu$ (occurring over months and years) that are an order of magnitude smaller, known as timing noise. Aims: This work reports a speed-up event in the Crab Pulsar that occurred around $2015$ February that is distinct from glitches and timing noise. Methods: Monthly $\nu$s and $\dot \nu$s of the Crab Pulsar, obtained at radio frequencies and published by Jodrell Bank Observatory (JBO), are used to demonstrate the speed-up event. Monthly arrival times of the Crab Pulsar's pulse, also published by JBO, combined with X-ray data from the RXTE, SWIFT, and NUSTAR observatories are used to verify the result. Results: The speed-up event is caused by a persistent increase in $\dot \nu$, which results in a monotonic increase in $\nu$. Over the last $\approx 550$ days, $\nu$ has increased monotonically by an amount that is $\approx 10$ times larger than the timing noise level. Conclusions: This is a unique event in the Crab Pulsar. This is probably due to a small increase in the Crab Pulsar's internal temperature. In its absence, the next large glitch in the Crab Pulsar is expected to occur around $2019$ March. However, this event could have an important bearing on its occurrence.