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 ν and a slowdown model that is specified by its first two time derivatives \hbox{$\dot \nu$}ν̇ and \hbox{$\ddot \nu$}¨ν, known as the secular slowdown model; (2) occasional (once in ≈2 yr) significant and abrupt increases in the magnitude of ν and \hbox{$\dot \nu$}ν̇ (occurring on timescales of minutes), known as glitches; and (3) much slower increases and decreases in ν and \hbox{$\dot \nu$}ν̇ (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 νs and \hbox{$\dot \nu$}ν̇s 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 \hbox{$\dot \nu$}ν̇, which results in a monotonic increase in ν. Over the last ≈550 days, ν has increased monotonically by an amount that is ≈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.