| Abstract : | Neutron stars, observable as radio pulsars, are known for their very
stable rotation and small spin-down rates. A glitch is a timing
irregularity of radio pulsars marked by a sudden increase in the
spin-frequency, often followed by a relaxation towards the unperturbed
frequency. This is thought to be caused by sudden and irregular
transfer of angular momentum to the slowly rotating solid crust by the
superfluid component, or as a result of crust quakes.
Till now, a total of 666 glitches have been observed in 208 radio
pulsars. In an earlier work (with less than half the number of
glitches) we found an indication of bimodality in glitch magnitudes.
Using a Gaussian Mixture Model (GMM) analysis, we find the current
data to bear this trend out. It is clearly bimodal with a strong
statistical significance.
To understand the nature of the underlying cause behind this
bimodality, we consider the correlation of the glitch magnitude with
the before-time (time since the previous glitch) and the after-time
(time to the next glitch). To do this, 491 glitches are chosen from 94
glitching pulsars. Each of these 94 pulsars exhibits multiple glitch
episodes providing inter-glitch intervals between consecutive
glitches. These 491 glitches are then separated into two sets
corresponding to the two Gaussians obtained using GMM analysis (with
appropriately chosen confidence intervals).
As expected, the correlations for the before-time and the after-time
are not similar. However, the most significant finding is that the
nature of the correlations appears to be completely different for the
two sets of glitches belonging to different Gaussians. Considering the
internal structure of neutron stars, we find that this difference is
likely to be an indication of the fact that the glitches are indeed
being caused by two different physical mechanisms.
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