Friday, March 2, 2012

Blazars and Axions

Ruth and Katie's discussion based around this paper.

Picture of a blazar
The small black circle at the centre is the black hole, the torus shows the accretion area.  The blobs are clumps of hot gas and you can see the main jets coming from the black hole out of the centre.

Blazar problem:
We observe fluctuations in gamma rays coming from the blazar at a high rate.  Rate of fluctuation tells us about the size of the object emitting.  Causality tells us that bigger objects take longer to fluctuate.  Because we observe a high rate of fluctuations we infer that the object emitting must be small, perhaps something small and dense close to the black hole.

We also know that the higher the energy of a particle, the more it gets scattered.  (cf GZK cut-off for cosmic rays)  Higher energy particle =higher cross section.  The energy of the gamma rays we observe the fluctuation in is too high to be coming from near the black hole, since we woud expect the gamma rays to be scattered by other things in the region and therefore be less energetic by the time we observe them.

Suggestion:  Maybe the fluctuation is coming from a piece of hot gas further away from the black hole which just happens to be in the jet?  (see picture)  Perhaps not convincing...

Axions to the rescue!
In the presence of a magnetic field (like near a black hole), photons can turn into axions.  If the gamma rays coming from near the black hole turn to axions, since axions don't interact much, they would travel through the accretion region without scattering.  Once through and in the presence of another magnetic filed (maybe once they reach the Milky Way), the axions can turn back into photons and we observe them as high energy gamma rays.

Observational Axion stuff
When we observe high energy sources of gamma rays, we expect a change in luminosity and their spectrum to be softened by interactions with the extragalactic background.  The extragalactic background is eg the photons from stars, the CMB etc.  We expect this since we know something about their energy and hence the cross section.  However, we observe a more transparent extragalactic background than we expect.  It is theorised that this could be due to the photons from the high energy source converting to axions, travelling un-scattered through most of the extragalactic medium as axions, before changing back into photons which we observe.

Strong CP Problem in QCD - Why did we invent axions in the first place?
The QCD Lagrangian has a term which proportional to theta F F^{dual} which is some kind of quark/gluon field.  Theta is a dimensionless constant, so for no fine tuning we expect it to be ~1.  The only measurable thing we can measure which depends on theta is the Neutron Dipole Moment, which is tiny.  So maybe theta is not a constant but a dynamical field corresponding to the axion.  (Dynamical means value changes, field = particle).

Since axions don't interact much, they are good candidates for dark matter.  But it turns out that when you put axions into a theory of dark matter you solve one fine tuning problem by creating many more.  So they're not such a good candidate after all.  String theory produces lots of axions (Lagrangian terms proportional to theta F F^{dual}), whose properties depend on how you compactify a manifold.


  1. Great post! Also, check out the Sky & Tel article: :-)