When a standard candle flickers: What happened when the Crab Nebula had a fit?
I’ve previously blogged about extreme particle acceleration producing gamma-rays in many different astrophysical contexts, including galactic binary systems & blazars, but I haven’t talked in any great depth about another source of extremely high energy particles: supernova remnants.
A supernova remnant is the remains of a supernova, and as well as the aforementioned types of astrophysical objects, they are one of the chief sources of gamma-rays in our galaxy, as can be seen from the following image by our old friend Fermi/LAT:
On the extreme right of this skymap we can see the gamma-ray output from the Crab Nebula, or Messier 1 (M1), the remains of a supernova that exploded in 1054 CE. Driving the nebula (by injecting energetic electrons into the surrounding environment) is a central pulsar, one of the first of these types of objects discovered, and an extremely important class of astrophysical object.
In the specific context of gamma-ray astronomy, the Crab Nebula is even more important: for decades since its discovery, its output at gamma-ray energies has always been, absent a previous flaring event in 2009 (Abdo et al. 2011, ADS/ArXiv), remarkably constant, to the point where, as I previously noted, it has been used as a calibration unit for gamma-ray fluxes from other objects in the sky.
However, as a new letter to the Editor of the Astronomy & Astrophysics journal (from a group of Swiss/German astronomers led by Matteo Balbo of the University of Geneva) (A&A 527, L4 (2011), ADS/arXiv) to be published in the March edition notes, on the 18th September last year, the previously stable Crab Nebula erupted in three massive gamma-ray flares, before returning to normal over the next few weeks:
Observing with a number of instruments, including Fermi/LAT, Balbo et al. report that each flare involved a sudden increase in the flux from the Crab Nebula by a factor of ~3.3 followed by a decay back to previous levels of emission over the course of approximately a day. During this time, they also report that the pulsar flux was constant; i.e. no change in the emission from the pulsar was detected. This discontinuity between the flaring and the pulsar luminosity has important consequences, and I’ll come back to these later in the post.
These flares caused quite a kerfuffle in the high-energy astrophysical community, and it was initially thought that perhaps a previously unknown Active Galactic nuclei positionally coincident with the Crab Nebula (i.e. that happened to lie in the same area of the sky) might have been the cause of the increased emission, and not the Crab Nebula itself. However, no changes in the X-ray flux from the area of the sky including the Crab Nebula was observed, ruling out this scenario (AGN are typically very bright in X-rays).
So, on the reasonable assumption that these flares came from the Crab Nebula, Balbo et al. consider the two obvious questions that present themselves: what are the physical mechanisms responsible for the flares, and given the comparatively large size (~10 light years across) of the Crab Nebula as a whole, where exactly did they originate from?
It turns out that Balbo et al. find that these two questions can be answered without having to introduce new physics or making too many changes to what we know about the Crab Nebula. Since we know that whatever their cause, the pulsar itself wasn’t directly responsible (due to the pulsar flux remaining constant), we can use the observed properties of the flares themselves to constrain the size and location of the emitting region using standard astrophysical techniques:
- The length of the flares themselves limit the size of the region responsible for the emission (this is a standard geometric argument often used in AGN research: the size of an emitting region equals the distance light can travel during the shortest variation of its brightness, for example: a one-day variation means a size of one light-day), and Balbo et al. hence report that the size of the flare-emitting region is approximately < 1015 cm (which is approximately 0.001 ly).
- The flare luminosity can be used to compute upper limits on the location of the emitting region, by comparing its luminosity to a calculated property known as the pulsar spin-down luminosity (which is derived from, amongst other things, the pulsar inertia & spin period) and using another calculated property known as the apparent efficiency for VHE gamma-ray production (i.e. a measure of how efficient the pulsar is at converting spin-down energy into observed gamma-rays) for this object. Balbo et al. thus constrain the position of the emitting region to very close (< 6 × 1016 cm) to the pulsar at the centre of the nebula.
This limitation on the position of the emitting region is important: as at the centre of the nebula, surrounding the pulsar, is a jet and torus of high-energy X-ray emitting particles approximately 0.1 ly across (and hence many times the size of both the emitting region and its distance from the central pulsar):
The emitting region is small enough that an increase in the energy of the particles in a particular part of the torus could be responsible for the emission, or interactions between the jet and the torus, or even changes in the jet itself: that the torus was somehow involved is given additional weight by subsequent observations (on the 2nd October), of the torus having increased in luminosity.
However, Balbo et al. discount this latter scenario (i.e. of the flares arising from the jet itself) as unlikely as since the jet in the Crab Nebula is likely to be relativistic, the emission would be highly-beamed and given the inclination of the jet, it would make the flares very difficult to detect.
Next, they consider the physical mechanisms responsible for the flares. As noted by Abdo et al. (2011), there are two main physical processes that are considered responsible for the flux from the Crab Nebula: synchrotron emission (where charged particles accelerate in a magnetic field) and Inverse Comptonisation (IC) (which I’ve talked about in detail in this post, but in summary involves collisions of low energy photons with relativistic electrons). However, if the flare was due to the IC component, then the duration of the flare would be much longer. As Abdo et al. note, in relation to the 2009 flare that lasted approximately 4 days:
The extrapolation of the the LAT spectrum of low-energy component to lower frequencies suggest that it represents synchrotron emission… The brevity of the gamma-ray ﬂares strengthens this scenario: If the ﬂare were instead produced by IC radiation or Bremsstrahlung, the cooling time of the emitting electrons would greatly exceed the ﬂare duration. The cooling via Bremsstrahlung in particle densities <10 cm−3 happens over ∼106 years. Similarly, electrons cooling via IC emission of 100 MeV gamma rays on the photons of the synchrotron component of the Crab Nebula have cooling times ~ 107 years. The average magnetic ﬁeld inside the Crab Nebula is estimated to be ~200 G as deduced from modeling of the broad-band SED, and might be enhanced locally by up to an order of magnitude in the inner nebula. These ﬁelds imply synchrotron cooling times <15 days, comparable to the ﬂare duration, leaving synchrotron radiation as the only plausible process responsible for the gamma-ray emission during the ﬂares.
This conclusion is also reached by Balbo et al., who also note that in the case of the September 2010 flares, this mechanism requires electrons to be accelerated to extreme energies in the PeV (Peta-Electron Volt) range (1015 eV), i.e. orders of magnitude above the energies reached in blazars and microquasars.
As a measure of how much energy this actually is, and in terms of units of energy we’re more familiar with in our everyday lives, 1 PeV is equal to ~1.6× 10-4 Joules. Now imagine a golf ball with a mass of ~46 grams. Using standard formula for kinetic energy, a golf ball moving at a speed of 0.08 m/s (3 inches/second), or ~300 m/hr, has the same (kinetic) energy as the electron energies that the Crab Nebula is accelerating to. Doesn’t seem scary, except a golf ball is 1028 times as massive as an electron!
If this truely is the case, then this makes the Crab Nebula a natural Pevatron, and an astrophysical wonder.
Balbo, M., Walter, R., Ferrigno, C., & Bordas, P. (2011). Twelve-hour spikes from the Crab Pevatron Astronomy & Astrophysics, 527 DOI: 10.1051/0004-6361/201015980
Abdo, A., Ackermann, M., Ajello, M., Allafort, A., Baldini, L., Ballet, J., Barbiellini, G., Bastieri, D., Bechtol, K., Bellazzini, R., Berenji, B., Blandford, R., Bloom, E., Bonamente, E., Borgland, A., Bouvier, A., Brandt, T., Bregeon, J., Brez, A., Brigida, M., Bruel, P., Buehler, R., Buson, S., Caliandro, G., Cameron, R., Cannon, A., Caraveo, P., Casandjian, J., Celik, O., Charles, E., Chekhtman, A., Cheung, C., Chiang, J., Ciprini, S., Claus, R., Cohen-Tanugi, J., Costamante, L., Cutini, S., D’Ammando, F., Dermer, C., de Angelis, A., de Luca, A., de Palma, F., Digel, S., do Couto e Silva, E., Drell, P., Drlica-Wagner, A., Dubois, R., Dumora, D., Favuzzi, C., Fegan, S., Ferrara, E., Focke, W., Fortin, P., Frailis, M., Fukazawa, Y., Funk, S., Fusco, P., Gargano, F., Gasparrini, D., Gehrels, N., Germani, S., Giglietto, N., Giordano, F., Giroletti, M., Glanzman, T., Godfrey, G., Grenier, I., Grondin, M., Grove, J., Guiriec, S., Hadasch, D., Hanabata, Y., Harding, A., Hayashi, K., Hayashida, M., Hays, E., Horan, D., Itoh, R., Johannesson, G., Johnson, A., Johnson, T., Khangulyan, D., Kamae, T., Katagiri, H., Kataoka, J., Kerr, M., Knodlseder, J., Kuss, M., Lande, J., Latronico, L., Lee, S., Lemoine-Goumard, M., Longo, F., Loparco, F., Lubrano, P., Madejski, G., Makeev, A., Marelli, M., Mazziotta, M., McEnery, J., Michelson, P., Mitthumsiri, W., Mizuno, T., Moiseev, A., Monte, C., Monzani, M., Morselli, A., Moskalenko, I., Murgia, S., Nakamori, T., Naumann-Godo, M., Nolan, P., Norris, J., Nuss, E., Ohsugi, T., Okumura, A., Omodei, N., Ormes, J., Ozaki, M., Paneque, D., Parent, D., Pelassa, V., Pepe, M., Pesce-Rollins, M., Pierbattista, M., Piron, F., Porter, T., Raino, S., Rando, R., Ray, P., Razzano, M., Reimer, A., Reimer, O., Reposeur, T., Ritz, S., Romani, R., Sadrozinski, H., Sanchez, D., Parkinson, P., Scargle, J., Schalk, T., Sgro, C., Siskind, E., Smith, P., Spandre, G., Spinelli, P., Strickman, M., Suson, D., Takahashi, H., Takahashi, T., Tanaka, T., Thayer, J., Thompson, D., Tibaldo, L., Torres, D., Tosti, G., Tramacere, A., Troja, E., Uchiyama, Y., Vandenbroucke, J., Vasileiou, V., Vianello, G., Vitale, V., Wang, P., Wood, K., Yang, Z., & Ziegler, M. (2011). Gamma-Ray Flares from the Crab Nebula Science, 331 (6018), 739-742 DOI: 10.1126/science.1199705