## Wednesday, March 2, 2016

### Pump the Brakes...

There has been a deluge of important papers coming out lately and I feel that it is my duty as a pulsar astronomer to not let this one slip by you. This little gem on the braking index of pulsar J1640-4631 is a bit of a paradigm breaker. The abstract alone is just chocked full of awesome nuggets. To explain why it's so important, I suppose I should first explain what a braking index is.

The rotational frequencies, $\nu$, of pulsars are observed to decrease with time. If the simplest toy model of a pulsar as a misaligned dipole corotating with a neutron star is true, this spin-down is anticipated from dipole radiation and can be quantified as

$\dot{\nu}=-K\nu^n$

where $K$ is some constant that depends on the magnetic field strength/orientation and the moment of inertia of the neutron star, and $n$ is the braking index. For straightforward dipole radiation spin-down, $n=3$. If the second derivative of $\nu$ can be measured, the braking index can be measured (assuming the above parameterization is valid):

$n=\frac{\nu\ddot{\nu}}{\dot{\nu}^2}$.

Now, it is exceedingly difficult to measure $\ddot{\nu}$ because it is typically tiny, like a few times $10^{-22}$ inverse cubic seconds (gotta love that unit). Before this paper, $\ddot{\nu}$ had only been measured for about 8 pulsars and the inferred braking index has always been less than 3. Well known phenomena like pulsar wind nebulae can easily reduce the braking index, so these measurements are not incredibly surprising.

As the above figure shows, these authors have measured a braking index greater than 3, and this requires new phenomenology such as evolving magnetic structure, or, tantalizingly, gravitational wave emission. Okay, it's probably not gravitational waves in this case, but who knows.

This is a big deal and it's a cool measurement of a really cool system. The pulsar is very young and is associated with a supernova remnant. The pulsar powers a wind nebula that is the brightest TeV gamma-ray source in the Galaxy.  As far as we can tell with radio telescopes in the southern hemisphere, this is a radio quiet pulsar. All of these inferences were done using X-rays from the pulsar collected with the space-based X-ray telescope NuSTAR. The timing model for this pulsar is incredibly simple and constrained by only around 20 measurement epochs spread out over just two years. Measuring a $\ddot{\nu}$ in just two years is remarkable and only possible because it seems to be such a large value. Anyways, this is a great and short little paper that I highly recommend you take a look at.

Update: The authors claim to have ruled out "timing noise" as the source of this $\ddot{\nu}$, but an eminent colleague of mine is skeptical. More time will tell, and this is still an exciting source to monitor.

#### 1 comment:

1. Great post! Why is more natural to a have breaking index smaller than 3 than bigger than three? it's only a little bit bigger...