Let me introduce you to the “Cornetto* relic” just published on MNRAS Letter, https://arxiv.org/abs/2004.10752  by #magcow PhD Nicola Locatelli + @fabiogasta @AnnalisBonafede @HambObs @mcrossetti_twit @IRA_INAF @UniboMagazine (*only sold in bars for breakfast, so much missed now!)

The paper tells about the (serendipitous) @LOFAR discovery of the 3rd most extended "radio relic" in the sky, in the not so noticeable cluster of galaxies Abell 2249 - where previously only a BGC radio galaxy was known...and instead there it is, an extremely peculiar object.

Shocks are known to be often present in galaxy clusters, due to their continuous merger activity, which is followed by the release of supersonic perturbations (travelling at >1000 km/s). (see one of our rendering in ENZO simulations)..

…in presence of astrophysical shocks, collisionless plasmas should undergo diffuse shock acceleration ( https://arxiv.org/abs/astro-ph/9711177), i.e. a Fermi-I mechanism in which particles are scattered multiple times across the front, and a tiny fraction of can become reach relativistic.

In DSA, the shock strength sets the acceleration efficiency - but the details of this are unknown. For strong supernova shocks (Mach>>10), ~1% of the kinetic energy of the shock should be channeled into relativistic electrons. What about the much weaker, M<4 shocks in the ICM?

The consensus is that only a super-tiny accel. efficiency is possible, i.e. <0.01%. This is a big problem for most of known radio relics: given their observed power, the efficiency appears to much larger that this - see recent overview by @and_botteon https://arxiv.org/pdf/1907.00966 

The "Cornetto" is peculiar in this respect: it is one of the lowest surf.brightness relics ever discovered, thus this shock is producing only weak radio emission. By modelling its kinetic power, we discovered that the required accel.efficiency is in this case consistent with DSA.

To measure this, we had to model its spectrum: evidence of the Cornetto was also found in archival @VLArray data (only a few spots), while Nicola also obtained a Discretionary Director’s Time at GMRT (610 MHz). (see GMRT image). The radio spectrum is alfa=-1.15.

From the radio spectrum, we can estimate a Mach number at the injection of M~3.79. Then, we can combine with available @ESA_XMM X-ray observations from the Heritage Cluster Project ( http://xmm-heritage.oas.inaf.it ), analysed by @fabiogasta and @mcrossetti_twit to get the shock energy.

There are too many photons to measure the Mach number from X-ray, but enough to measure the ICM density and temperature - and thus to think about energy efficiency by combining with radio observables.

Very interestingly, we can reproduce the observed power with B~μG and an accel.efficiency of ~1e-3-1e-4, which is at the order of what "standard" DSA can do.

Also, we have lower limits from the lack of detected Inverse Compton in XMM data: B>0.4μG, thanks to @fabiogasta and @mcrossetti_twit 's work

But so why should the Cornetto be the first striking case of DSA at work, after many tens of relics discovered so far? Because it lies in a spot of the Pradio-Largest Linear Scale where only @LOFAR has a chance of detecting objects!

This is way we hope that the Cornetto may be the prototype of a class of low-power radio relics, powered by "simple" DSA, that @LOFAR may be start discovering in a systematic way. This would be extremely cool -also because it will give theorists several accel.modes to study.

I conclude the thread with this press-release format overlay that C.Stuardi ( #dranoel) has produced by combining @LOFAR data with SDSS (optical), a very good one! [EoT]