By Yajie Yuan
The Crab Nebula, our old friend, has continued giving us big surprises in the past few years, as we recently saw in this KIPAC blogpost (from April 2015, by Jeff Scargle and Roger Blandford). We have been gaining glimpses into these surprises thanks to the excellent performance of the orbiting gamma-ray telescopes, Fermi and AGILE, which have been able to get glimpses into the hidden secrets kept mum for so long in other wavelengths by this old stalwart.
As we have learned in recent times, the Crab produces powerful, short-duration gamma-ray flares about once per year. In the most dramatic class of events, the gamma-ray luminosity (i.e. the brightness) of the nebula during these phases rises rapidly within 10 hours and outshines its quiescent state by a factor of 30 (Buehler et al. 2012). How can this be possible from a source that was previously thought to be so rock-solidly stable?
To understand the origin of the flares, the first pressing question to be addressed is: where in the nebula does a gamma-ray flare originate?
The fast variability time scale of ~10 hours in these flare events indicates that the radiation should come from a relatively small region, not much larger than a light-day across—this is less than 0.05% of the size of the entire nebula, which is some 11 light years in diameter. Unfortunately, because of resolution limitations, the whole nebula only appears as a point in the gamma-ray telescopes, so we cannot directly pinpoint the location of the gamma-ray emission.
We do know that the emission cannot be altered by the central pulsar or anything causally connected to it otherwise we would have observed changes in the pulsed emission, contrary to what has been recorded so far. Thus, the origin should be somewhere in the body of the nebula—and the hints are pointing to the most likely location being the inner part of the nebula.
Figure 1: Inner part of the nebula captured by Chandra X-ray telescope (left) and Hubble in optical (middle) (from here, where downloadable movie versions of the above still image can be found, and another version can be watched directly here on Youtube). The right panel is the blowup of the central region near the pulsar, in near-infrared (from Melatos et al. 2005). (Credit: NASA.)
Now, another way to try to track down the flare origin is to observe the flaring nebula concurrently with telescopes in other wavelengths that have better resolution. Since we believe that the flares are quite likely produced by a local reconfiguration of magnetic structures that releases electromagnetic energy to accelerate particles, we also would expect that such an event could have an impact on lower-energy particles and show up in longer wavelengths as well. An extensive multiwavelength campaign involving a large collaboration coordinated by Rolf Buehler of DESY and NASA’s Martin Weisskopf has been making good progress in the monitoring and followup observations using Chandra (X-ray), Hubble (optical) and Keck (infrared).
The inner nebula has elaborate structures, most prominently the torus, jets, and a few wisps, as can be seen in both X-ray and optical images (see Figure 1). Several of them change with time on a scale of months to years, changes which are believed to be caused by waves excited when the highly relativistic, magnetized, electron-positron pair wind from the pulsar interacts with the nebula environment. To pick out the flare counterpart from all these constantly changing features, one needs to find variations that have strong correlations with the gamma-ray flares.
Past multiwavelength observations did not show any obvious evidence of such correlation. This time, our team tried to focus on one salient feature, which appears to be very close to the pulsar in sky projection at least, called the “inner knot” (labelled “Knot” in Figure 1). This compact region of emission is usually thought to come from a special, oblique portion of the “termination shock”—the region where the pulsar wind meets the nebular material and is abruptly slowed down. We only see a small knot because at this point the outflow of the shock is still quite relativistic and happens to point towards us, so the emission from the outflow gets boosted due to “relativistic beaming.”
Figure 2: Upper panel shows the gamma-ray light curve of the Crab Nebula (the flux vs. date of observation) and the time of Hubble and Keck observations. The lower panel shows the knot-pulsar separation for these observations, as a function of time. (from Rudy et al 2015)
We find that the knot characteristics have strong variability over time; for example, the knot’s separation from the pulsar changes, and its size correlates with the separation from the pulsar while its flux shows anticorrelation, consistent with the motion of the termination shock. Most interestingly, near the two large flares, the knot happens to be at extremal distances from the pulsar, furthest for the earlier flare and closest for the later one (see Figure 2). However, as we are limited by the number of events and observational cadence (i.e. how often the nebula is being monitored), the evidence of the knot variability being correlated with the flares is not statistically significant yet. Hopefully, with better future observations, we will be able to find some instructive hints to the flares. So stay tuned!
The flares have been challenging our physical understanding of relativistic plasmas because this is among the most dramatic cases of particle acceleration we have seen so far. Electrons and positrons are accelerated to PeV energies. At that limit they radiate so rapidly that they lose most of their energy even before finishing one gyro-orbit in the magnetic field, meaning a large electric field has to be maintained to sustain the acceleration—not easy to achieve in highly conductive plasmas.
Here at KIPAC, we are actively engaged in getting a theoretical understanding of the process. Roger Blandford, William East, Krzysztof Nalewajko, Jonathan Zrake and myself have been advocating a new idea we call “magnetoluminescence” to explain the gamma-ray flares. Basically, the dramatic flares happen in the body of the nebula but the ultimate energy source comes from the central engine—the Crab pulsar. The pulsar has strong magnetic field and rotates rapidly: it winds up the magnetic field into toroidal loops and continuously injects all this magnetic energy into the nebula. The magnetic field, embedded in a relativistic pair plasma, could become highly tangled in the nebula—think of the loops as tightly tangled, highly stressed elastic ropes that can suddenly untangle and whip around upon being released from their tension, and release a large amount of magnetic energy at that point to accelerate the plasma to relativistic speeds.
Large electric fields could also then be induced, eventually reaching the point when the breakdown of ideal conductivity happens throughout a significant volume. During this dramatic process particles would be accelerated until they furiously emit gamma rays.
Our preliminary numerical simulations (East et al. 2015, Nalewajko et al 2016, Yuan et al 2016) have shown promising evidence , and we are trying to put different pieces of the puzzle together to see whether we can get the whole picture right. With continuing observational, theoretical and numerical efforts, we look forward to eventually finding answers to the open questions brought up by the Crab. Or maybe this venerable crustacean has even deeper hidden secrets that will be revealed one day?
------------------------------ To explore further:
Here is a link to Yajie's Youtube channel which has a number of simulation videos made from her work on it showing various aspects of the behavior of electromagnetic fields, radiation, and particle flow that would relate generally to the environment around the Crab pulsar and beyond. The details are not fully explained in the descriptions under each video (though many can be found in the papers referenced in the body of the article above) but -- even without knowing all of them, they can be appreciated on their own for the fascinating and alluring patterns and shapes in each clip.
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