Glitches and flares

Recently on the arxiv: Searching for X-ray Variability in the Glitching Anomalous X-ray Pulsar 1E 1841-045 in Kes 73, by Zhu and Kaspi.

Pulsars normally spin down very regularly — like clockwork, as the saying goes, and many pulsars spin down as regularly as a good atomic clock. But some pulsars, once in a while, will suddenly start spinning more quickly. This sudden (instantaneous as far as we can measure) spin-up is called a "glitch", and its full explanation remains mysterious. Generally, all we see is that the pulsar is suddenly spinning faster: no heating of the crust, no sudden X-ray emission, no radiative changes at all, just a suddenly-faster pulsar.

Anomalous X-ray pulsars (AXPs) are one kind of "magnetar", pulsars whose magnetic field is so enormous that its decay powers the X-ray emission of the star. They exhibit many peculiar behaviours, and are a major field of study in pulsar research. AXPs will occasionally become much more active for a while: they become much brighter, they emit random blasts of X-rays, and they do other peculiar things. It seems as if they may glitch every time they become active like this. If we want to find some sort of relationship between glitches and these active periods, it would be valuable to know whether an active period happens every time AXPs glitch, or whether AXPs sometimes have "quiet" glitches, like normal pulsars. That's what this paper tries to answer.

First of all: why don't ordinary pulsar glitches show radiative effects? After all, if something gives the pulsar a kick hard enough to start it spinning faster, shouldn't it get hot or something? Well, perhaps, but we think glitches happen well below the surface of the star. We think neutron stars have a superfluid interior. Superfluids are somewhat like superconductors in that, in certain settings, they flow with exactly zero viscosity. So when a pulsar is slowing down due to external drag on its magnetic field, there's no (or very little) viscosity to slow the superfluid as well. It's a bit like spinning a raw egg: you can stop it briefly with your fingers and then let go, and it will resume spinning, because the liquid inside never stopped. Glitches, we think, happen when the superfluid suddenly transfers some angular momentum to the crust, suddenly spinning the crust more rapidly. While there's presumably some energy dissipation involved, this happens down at the bottom of the crust, so that energy is not readily observed. So in normal pulsars, we think glitches happen due to sudden internal events and have no external radiative consequences; the pulsar just spins faster.

So why do we think glitches and radiative events might be related in AXPs? Well, from a theoretical point of view, the magnetic field decay that powers AXPs isn't the simple gradual process that "decay" suggests. Since neutron star crusts are excellent conductors and very rigid, it is nearly impossible for magnetic field lines to move around in them. (Any movement of field lines in a conductor would set up a current in the surface that opposed that movement, and with such a good conductor, for practical purposes the lines are fixed in place.) But underneath the surface, not only is the field slowly decaying, it's bound to a superfluid superconductor that's roiling turbulently away. (We think.) So the field lines bound to the crust are continually being pulled around from underneath. If the forces become strong enough, the crust can crack, and a plate can twist in place. Beyond the massive energy release that such an earthquake-like event involves directly, the field lines above the surface, which were straight before, are now twisted. Imagine a bundle of threads passing through a piece of fabric, while someone twists them from underneath: suddenly the fabric tears and the underneath untwists — but now the threads above the surface are twisted. And when you have magnetic field lines in open space twisted like that, they can accelerate particles vigorously. In a magnetar, this produces a fireball of extremely hot particles, which we see because they emit X-rays. To bring us back to glitches, this "crust cracking" is also likely to transfer some angular momentum from the superfluid to the crust.

If that sounds a bit vague and hand-wavy, I have to say I agree with you; neutron star interiors are very complicated systems, and we have no good terrestrial analogues and very few ways to study them in situ. But the observational fact remains: every AXP active event we've monitored, there has also been a glitch, or at least something that looks like a glitch but our error bars are too big to be sure. So it looks like there may be a relationship.

But what about the other direction: does every glitch have a radiative event, or are there some "quiet" glitches? The former would simplify the theory; there'd be only one kind of glitch and it would always come with a radiative event. But it looks like there really may be "quiet" glitches, which is what this paper argues.

There has been some debate over this point in the literature before, complicated by the limited set of X-ray instruments that are available. On the one hand we have XMM-Newton and Chandra, which are wonderful telescopes, producing images with arcsecond resolution and high-quality spectra. But it's difficult to get any time resolution out of them (you usually just get an average brightness, no indication of pulsations) and it's difficult to get any time on them: the fact that they're great telescopes means there are thousands of projects clamoring for time on them. So if you get time to observe an object, it's a single observation here or there. On the other hand there is the Rossi X-ray Timing Explorer, a satellite that can't produce any sort of image at all — it has one "pixel" that's a degree across. But it is very sensitive and it does have great time resolution. Best of all, for our purposes, there's not that much demand for it. So there has been a project for years now to take a look at each of the AXPs every few weeks with RXTE. You'd never get that kind of time on XMM-Newton or Chandra unless the AXP was flying through our solar system dancing a jig.

Unfortunately, the poor spatial resolution of RXTE means that anything within a degree of the source you're interested in shows up in the data. For AXPs, though, this isn't so bad: even though they're not the only source in the field of view, we can predict their rotation exactly, so we can (say) compare the brightness when they're "on" with the brightness when they're "off". That way everything else averages out and we measure only the brightness of the AXP. The tricky bit with this is that what we measure is the pulsed brightness of the AXP: it's, if you like, the difference between the light side and the dark side. If you brightened both sides exactly equally, we might not see any difference.

The reason this is a problem is that there are several AXPs that have glitched during the period that they were monitored with RXTE, for which the pulsed brightness has not changed at all: not the brightness, not the profile, just the timing. So does this mean these were "quiet" glitches? Well, maybe. It's hard to imagine radiative changes brightening the whole star without changing the difference between the hot parts and the cold parts, but it's just barely possible that that's what happened. So what this paper does is to go back and look at one particular AXP that had three glitches, and which the monitoring tells us had no effect on the pulsed flux. For this pulsar there are observations with imaging instruments before and after all the glitches, so all together it should be possible to check for variations in absolute, phase-averaged flux as well.

(Plot to right and above shows pulsed and total flux; the vertical lines mark the glitches.)

The paper is greatly complicated by the fact that those observations are with four different telescopes (ASCA, Chandra, XMM, and Suzaku), and the AXP is in the middle of an X-ray nebula, which contaminates the flux measurement to different degrees with different instruments. However, by dint of heroic effort, Weiwei manages to calibrate out these effects and show that indeed, there really is no change in flux associated with these glitches: they're genuinely quiet.

What do quiet glitches mean for the theory of AXPs? Well, for one thing, they suggest that there are (at least) two genuinely different kinds of glitches, the radiative ones and the quiet ones. The theorists will have to come up with models that can predict two different kinds of glitches. Worse, there are pulsars that appear to have had both quiet and loud glitches, so they can't just blame differences in the pulsars themselves: there must be two genuinely different ways glitches unfold in the same object. Why that should be is a puzzle, but it's what we see happening. Let's hope that further data on these objects can pin down what's going on.

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