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https://twitter.com/antiqueDB5/status/1388098205691219970
https://twitter.com/antiqueDB5/status/1388098205691219970
To start things off, J0537-6910 is a very young neutron star that lives in the Large Magellanic
Cloud. It’s not seen at radio wavelengths, but is detectable using X-ray telescopes in space.
Cloud. It’s not seen at radio wavelengths, but is detectable using X-ray telescopes in space.
J0537 is also the most prolific glitching pulsar that we know of. It’s undergone over 50 glitches since it was discovered back in 1998!
To make things even weirder, it’s braking index, a number that tells you how it’s rotation should evolve over long timescales, is -1.25 +/- 0.05.
This is odd because it seems to imply the neutron star’s magnetic field is getting stronger over time. Where would that extra magnetic field strength comes from? Nobody can say for sure...
Aside from the long-term braking index that’s measured across many glitches, J0537 also has a second “inter-glitch” braking index that is much higher than its long-term one (n = 7.4 +/- 0 .8).
The inter-glitch braking index shows up as the positive slope in the spin-down rate between glitches, while the long-term one is responsible for the slow but continuous decrease over many years (
https://arxiv.org/abs/1708.08876 )
https://arxiv.org/abs/1708.08876 )
Similar behaviour is seen in many pulsars, including Vela, the brightest pulsar in the sky!
No one knows exactly what causes the inter-glitch braking indices, but it’s probably related to glitches.
No one knows exactly what causes the inter-glitch braking indices, but it’s probably related to glitches.
But what’s special about a braking index of 7? It could mean J0537 has slightly non-spherical shape. If that’s the case then it would radiate a significant amount of its spin-down energy as gravitational waves.
So what’s wrong with this paper then? Most of it boils down to this figure showing the “inter-glitch braking index versus time since glitch”.
The paper (and the one it too is based on) suggests the inter-glitch braking index starts out very large following a glitch and then decays toward a constant value of 7.
This picture of a decaying braking index is incorrect. What’s actually being shown in this figure is the exponential recovery of J0537’s spin following the glitch back towards its pre-glitch state.
Recovery is a phenomenon seen in dozens of other glitching pulsars. It’s (probably) related to the dynamics of vortices within the superfluid found inside neutron stars.
How can I say for sure the exponential behaviour is just recovery? Because they didn’t bother to fit for it in their timing model! There are no glitch recovery terms to be found in the paper’s glitch table!!
Same with their inter-glitch fits. Not a single decay term in sight! The claimed inter-glitch braking indices of 16, 56 & 34 are all horribly contaminated by unmodelled recoveries.
Large variations in the inter glitch braking indices of pulsars is normal. But had they done the modelling correctly (like https://arxiv.org/abs/1708.08876 ), they likely would’ve measured values consistently close to 7.
Hence another argument they make about glitches with longer wait times between them potentially resulting in a better chance of detecting gravitational waves is wrong. The inter-glitch braking index will still be close to 7 after the glitch!
So does this mean they shouldn’t have searched for gravitational waves from J0537? Absolutely not! You should always try to find gravitational waves in unexpected places. Might end up making an incredible discovery one day!
I do however strongly disagree with the inter-glitch braking index being the main justification to perform the gravitational-wave search.
Inter-glitch braking indices can be anywhere between -70 to 300 after all. Saying that n ~ 7, and therefore it must be gravitational waves is reading too much into a number that could’ve been anything. End
If any of this has piqued an interest in inter-glitch pulsar braking indices, then stay tuned for some VERY cool results from Parkes in a couple months!
