Small glitches and other rotational irregularities of the Vela pulsar

Abstract

Context. Glitches are sudden increases in the rotation rate ν of neutron stars, which are thought to be driven by the neutron superfluid inside the star. The Vela pulsar presents a comparatively high rate of glitches, with 21 events reported since observations began in 1968. These are amongst the largest known glitches (17 of them have sizes Δν/ν ≥ 10−6) and exhibit very similar characteristics. This similarity, combined with the regularity with which large glitches occur, has turned Vela into an archetype of this type of glitching behaviour. The properties of its smallest glitches, on the other hand, are not clearly established. Aims. We explore the population of small-amplitude, rapid rotational changes in the Vela pulsar and determine the rate of occurrence and sizes of its smallest glitches. This will help advance our understanding of the actual distribution of glitch sizes and inter-glitch waiting times in this pulsar, which has implications for theoretical models of the glitch mechanism. Methods. High-cadence observations of the Vela pulsar were taken between 1981 and 2005 at the Mount Pleasant Radio Observatory. An automated systematic search was carried out that investigated whether a significant change of spin frequency ν and/or the spin-down rate ν̇ takes place at any given time. Results. We find two glitches that have not been reported before, with respective sizes Δν/ν of (5.55 ± 0.03) × 10−9 and (38 ± 4) × 10−9. The latter is followed by an exponential-like recovery with a characteristic timescale of 31 d. In addition to these two glitch events, our study reveals numerous events of all possible signatures (i.e. combinations of Δν and Δν̇ signs), all of them small with |Δν|/ν < 10−9, which contribute to the Vela timing noise. Conclusions. The Vela pulsar presents an under-abundance of small glitches compared to many other glitching pulsars, which appears genuine and not a result of observational biases. In addition to typical glitches, the smooth spin-down of the pulsar is also affected by an almost continuous activity that can be partially characterised by small step-like changes in ν, ν̇ or both. Simulations indicate that a continuous wandering of the rotational phase, following a red spectrum, could mimic such step-like changes in the timing residuals.

The impact of glitches on young pulsar rotational evolution

“…Δ𝜈 𝑔 /𝜈 10 −6 ) interspersed between them. Glitches of similar amplitudes are seldom seen among the Vela-like pulsars, and indeed Vela itself appears to have a real under-abundance of small amplitude glitches (Howitt et al 2018;Espinoza et al 2021). A potential clue as to why these pulsars do not possess detectable large 𝑛, despite being situated near the Vela-like population in 𝑃-𝑃 space, comes from the lack of consistently large Δ 𝜈 𝑔 components associated with our glitch measurements in Table 2.…”

Systematic upper limits on the size of missing pulsar glitches in the first UTMOST open data release

“…Espinoza et al (2014) employed an automated glitch detection algorithm to search for glitches in the Crab pulsar, and reported a minimum glitch size in that case, which is intrinsic to the pulsar and not an artifact of the detector performance. This technique 6 http://www.jb.man.ac.uk/pulsar/glitches/gTable.html has also been applied to the Vela pulsar, and a lack of small glitches was reported in that case also (Espinoza et al 2021). However, the detector returns a large number of (anti-)glitch candidates which must be classified after the fact as timing noise or glitches.…”

“…Espinoza et al (2014) and Yu & Liu (2017) employed automated glitch detection methods to evaluate detectability limits. The technique presented by Espinoza et al (2014) has been applied to timing data from the Crab and Vela pulsars (Espinoza et al 2021), while the technique presented by Yu & Liu (2017) was tested on simulated data but not used to search for glitches in real datasets. Most recently, Singha et al (2021) have developed an real-time glitch detection pipeline which operates with minimal human intervention and incorporated it into the timing programme at the Ooty Radio Telescope.…”

Gravitational waves from small spin-up and spin-down events of neutron stars

“…These deviations consist of relatively large steps in spin frequency, known as glitches, and smaller deviations often known as timing noise, not normally resolved into individual events. Espinoza et al (2014Espinoza et al ( , 2021 have added to this picture by identifying a population of small timing events, that could be interpreted as spin-ups, smaller than previously resolved glitches, which clearly form a separate population of events from glitches. They also found evidence for similar small spin-down events.…”

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Recently, Espinoza et al (2014Espinoza et al ( , 2021 reported the existence of "glitch candidates" and "anti-glitch candidates" which are effectively small spin-ups and spin-downs of a neutron star with magnitudes smaller than those seen in typical glitches. The physical origin of these small events is not yet understood.

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