Cosmic-ray acceleration in a superbubble

Cosmic-rays are very energetic particles that pervade the Universe. Discovered one century ago, their origin is still unclear. In the Galaxy, the most promising sources are the powerful magnetized shock waves triggered by supernovae, which mark the cataclysmic end of a star: the thermonuclear disruption of a white dwarf (as for SN 1006) or the core-collapse of a massive star (as for RX J1713-3946). Massive stars are born in groups, called OB associations. And as they live fast and die young, they are likely to spend their whole life into clusters. Their strong stellar winds, together with their subsequent explosions, profoundly alter their environment: they carve huge, hot and tenous cavities around them, called superbubbles. It is naturally expected that these objects play a key role in the acceleration of cosmic rays in the Galaxy. And measurements of the composition of the cosmic rays indeed point to OB associations and their superbubbles as the likely source of the material of a substantial fraction of them. But the particles that we collect on the Earth don't tell where they come from: being charged particles, they wandered a lot in the Galactic magnetic field before reaching us. We can spot freshly accelerated particles next to their sources thanks to the high-energy radiation they emit. But, until recently, observations of superbubbles were very scarce.

The situation changed dramatically in the past years with the rise of a new generation of instruments. The Fermi satellite is one of them. Launched by NASA in 2008, this international mission observes the gamma-ray sky between the energies of about 100 MeV to 100 GeV with unprecedented sensitivity. It monitors all the sky, and, of particular interest for us today, a massive star forming region in the direction of Cygnus. Analyzing the data from high-energy instruments is a difficult task, especially for extended sources. After modelling and carefully removing a number of backgound diffuse emissions, scientists found a significant excess of high-energy emission from this part of the sky. They were stricken to see that the emission is coincident with the cavities formed around the numerous clusters of massive stars: it nicely follows the ionization fronts eroding the surrounding molecular clouds, as if in a "cocoon". Various objects can produce gamma-ray radiation (individual stars, pulsars and their nebulae), but after checking all possibilities the collaborators concluded that such small sources don't make a good match to the extended emission. The most likely explanation is that the cocoon is filled with cosmic-rays, that slowly diffuse inside the superbubble, and radiate some of their energy in the process (they could be either electrons or protons). The energy distribution of the observed photons is notably hard, meaning that the fraction of particles with the highest energies is higher than usual. Because it's all the more easy for particles to escape their accelerator when they are energetic, this hardness points to freshly accelerated particles. So here we have it: the Fermi observation provides the first strong direct evidence for the long-advocated hypothesis that OB associations host cosmic-ray factories.

The two main clusters in the cocoon are so young (4 to 6 million years for Cygnus OB2, 6 million years for NGC6910) that they might or not have experienced supernovae yet. But the winds of massives stars themselves can produce similar shock waves. And they generate lots of magnetic turbulence in the bubble, which traps particles in the cocoon, where they can get re-accelerated by other shocks and/or by magnetic waves. Existing models of acceleration into superbubbles predict spectra with a shape similar to that infered, which gives credance to the scenario. However it is too early for a detailed comparison of models with the cocoon spectrum: more work is needed on both fronts. Theoreticians will refine their models, and observers will have a closer look a other regions. The Arches, Quintuplet, and Sgr B2 clusters are good targets towards the Galactic centre, as are Westerlund 1 and Westerlund 2. These objects have already been detected at even higher energies, by one of the other prominent new instruments, the H.E.S.S. telescope.

AREPO vs. Gadget-2

Remarkably the image posted here is not of actual galaxies but rather those simulated by two codes.  The top is from a code called AREPO while the bottom is from the code Gadget-2.  Even more remarkable is that this simulation was for a code test of AREPO to see how it compared to a well established code like Gadget-2.

Both codes were developed by Volker Springel at Heidelberg University.  Both codes solve hydrodynamics and gravitational collapse.  However, the two codes could not be more different in terms of how they solve the hydrodynamics. 

Gadget-2 solves hydrodynamics via smoothed particle hydrodynamics (SPH).  SPH codes solve hydrodynamics in a similar way to N-body simulations.  Fluids after all are just a collection of particles whose collective evolution can be explained by the Navier-Stokes equations.  SPH just assumes that the particles in question are very large.  Since fluid mechanics a scale free, so long as your particles are not too large they will act the same as a fluid with much smaller particles.

SPH codes are popular for cosmology simulations because they allow you to solve gravity in a similar way to normal N-body codes.  This is convenient because the most computationally expensive part is usually solving the gravity.  There are a number of tricks one can use with N-body simulations to reduce the expense which can be applied to SPH as well.

The downside is that SPH does a poor job of capturing shocks and other fluid discontinuities.  These are important to track as these control fluid mixing as well as heating of the gas.  Thus SPH usually gets the general structure correct but it looks clumpy, due to the particles, and shocks are much more diffusive than one would like.

In order to capture shocks and discontinuities properly one typically falls back on grid based codes.  These codes treat the fluid in its continuum limit rather than treating it as a collection of particles.  Having a grid allows one to solve the equations of fluid dynamics directly at each grid interface, leading to much sharper discontinuities.  The problem with these types of codes is that gravity is difficult to and the resolution does not adjust with the flow, unlike in SPH where the resolution naturally follows where the gas is.

One can alleviate this problem by doing adaptive mesh refinement (AMR), which adapts the structure of the grid to the flow pattern.  However, you still have a problem that the code is Eulerian rather than Lagrangian.  In an Eulerian calculation one fixes a frame with respect to the gas and then calculates the flow with respect to that frame.  This can lead to large errors if the flow is moving fast through the frame in question.  On the other hand Lagrangian calculations follow a gas element of a specified mass.  The fluid equations are solved in the frame of that gas, reducing errors due to its motion.  This is another advantage SPH has.

Ideally one would like to have both the advantages of SPH and grid based codes.  Well AREPO was developed to do just that.  AREPO has what is known as a unstructured mesh, which is common in engineering circles.  However, AREPO takes it one step further by allowing the mesh to move with the flow in a psuedo-Lagrangian way.  This allows the grid to resize and shape itself as the fluid evolves, putting higher resolution in denser areas and reducing the errors due to fluid motion.

This leads to the much sharper and better looking galaxies in the AREPO image.  One can see movies showing individual galaxies, here.  It's uncanny how realistic these simulations look compared to real observations.

All is not roses for AREPO though as it is an expensive code to run, taking 50% longer to execute than Gadget-2.  Plus the memory overhead is large for these type of codes.  Nonetheless, the pay off one gets in terms of resulting simulation is quite impressive.

One more thing to wrap up this ridiculously long post (congrats to making it this far).  Fermi released their 2 year all sky map.  It shows even more sources than the 1 year map and with greater detail.  Very cool stuff.

The Crab's Gamma-Ray Flare

As noted on this blog last month, the Crab Nebula was seen by the Fermi telescope to flare in the gamma ray band. The flare itself is pictured to the right and was the subject of yesterday's APOD. The flare itself lasted for 3 days and saw a 30 fold increase in emission from the Crab.

Immediately after this occurred astronomers from all over the world looked with different instruments to see if there was increased emission from the Crab Nebula in other bands. No increase was detected though.

This is not the first time the Crab has done this, but it is confusing to astronomers trying to explain the flare. Since the flare lasts for only 3 days we know that the region of emission can't be that large. Since it was seen only in the gamma-rays seen by Fermi, we know that it is probably due to fluctuations in the synchrotron spectrum of the nebula. At these high energies the synchrotron light that is being seen is coming from the highest energy electrons, so fluctuations at these energies indicates fluctuations either in the magnetic field or the population of the highest energy electrons. No one is quite sure which, and while several theories have been tossed about, there is no favored model.

The variability in the gamma-ray is also a little concerning for observers in those bands as the Crab Nebula is used as a standard calibration source since it's emission is thought to be relatively steady. This is still likely true in the normal state, so astronomers just have to be aware if they are observing during a flaring state or not. Also most of the experiments using the Crab as a standard work in the TeV gamma-ray which is much higher energy than Fermi and do not see any fluctuations in the Crab's emission.