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      Core and crust contributions in pulsar glitches: constraints from the slow rise of the largest glitch observed in the Crab pulsar

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          Abstract

          Pulsar glitches are attributed to the sudden re-coupling of very weakly coupled large scale superfluid components in the neutron star interior. This process leads to rapid exchange of angular momentum and an increase in spin frequency. The transfer of angular momentum is regulated by a dissipative mutual friction, whose strength defines the spin-up timescale of a glitch. Hence, observations of glitch rises can be used to shed light on the dominant microphysical interactions at work in the high density interior of the star. We present a simple analytical model, complemented with more detailed numerical simulations, which produces a fast spin-up followed by a more gradual rise. Such features are observed in some large glitches of the Crab pulsar, including the largest recent glitch of 2017. We also use observations to constrain the mutual friction coefficient of the glitch-driving region for two possible locations: the inner crust and outer core of the star. We find that the features of Crab glitches require smaller values of the mutual friction coefficient than those needed to explain the much faster Vela spin-ups. This suggests a crustal origin for the former but an outer core contribution for the latter.

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          Models of Pulsar Glitches

           ,   (2015)
          Radio pulsars provide us with some of the most stable clocks in the universe. Nevertheless several pulsars exhibit sudden spin-up events, known as glitches. More than forty years after their first discovery, the exact origin of these phenomena is still open to debate. It is generally thought that they an observational manifestation of a superfluid component in the stellar interior and provide an insight into the dynamics of matter at extreme densities. In recent years there have been several advances on both the theoretical and observational side, that have provided significant steps forward in our understanding of neutron star interior dynamics and possible glitch mechanisms. In this article we review the main glitch models that have been proposed and discuss our understanding, in the light of current observations.
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            Alteration of the magnetosphere of the Vela pulsar during a glitch

            As pulsars lose energy, primarily in the form of magnetic dipole radiation, their rotation slows down accordingly. For some pulsars, this spin-down is interrupted by occasional abrupt spin-up events known as glitches 1 . A glitch is hypothesized to be a catastrophic release of pinned vorticity 2 that provides an exchange of angular momentum between the superfluid outer core and the crust. This is manifested by a minute alteration in the rotation rate of the neutron star and its co-rotating magnetosphere, which is revealed by an abrupt change in the timing of observed radio pulses. Measurement of the flux density, polarization and single-pulse arrival times of the glitch with high time resolution may reveal the equation of state of the crustal superfluid, its drag-to-lift ratio and the parameters that describe its friction with the crust 3 . This has not hitherto been possible because glitch events happen unpredictably. Here we report single-pulse radio observations of a glitch in the Vela pulsar, which has a rotation frequency of 11.2 hertz. The glitch was detected on 2016 December 12 at 11:36 universal time, during continuous observations of the pulsar over a period of three years. We detected sudden changes in the pulse shape coincident with the glitch event: one pulse was unusually broad, the next pulse was missing (a 'null') and the following two pulses had unexpectedly low linear polarization. This sequence was followed by a 2.6-second interval during which pulses arrived later than usual, indicating that the glitch affects the magnetosphere.
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              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.
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                Author and article information

                Journal
                26 June 2018
                Article
                1806.10168

                http://arxiv.org/licenses/nonexclusive-distrib/1.0/

                Custom metadata
                astro-ph.HE

                High energy astrophysical phenomena

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