PDA

View Full Version : Origin Of High Energy Gamma Rays



quotation
2012-Nov-02, 02:36 PM
http://arxiv.org/pdf/1211.0008.pdf Turbulence, Magnetic Reconnection in Turbulent Fluids and Energetic Particle Acceleration The authors call this a review article, looking for ways to create cosmic rays; but they are also introducing an argument that that cosmic rays may be created in a single rather than a multi step process.
Interesting, I've only skimmed through this (43 pages), but thought the authors were more inclined to an iterative approach where fast reconnection generates a shock, which creates turbulence, which makes it more likely for other reconnection events, etc. I found one last week that might be thought of as an intro to this much more comprehensive "review". It's only 8 pages, and is written in a bit more accessible style. It looks briefly at hadronic, leptonic and hybrid models, but what now seems obvious is that reconnection plays a key role in very high energy emissions:
http://arxiv.org/abs/1210.6847 On the origin of very high energy gamma-rays from radio galaxies

antoniseb
2012-Nov-02, 03:38 PM
I moved this from the "Fun Papers" thread so discussion won't be lost in the middle of a collection thread.

John Jaksich
2012-Nov-03, 08:46 AM
Interesting, I've only skimmed through this (43 pages), but thought the authors were more inclined to an iterative approach where fast reconnection generates a shock, which creates turbulence, which makes it more likely for other reconnection events, etc. I found one last week that might be thought of as an intro to this much more comprehensive "review". It's only 8 pages, and is written in a bit more accessible style. It looks briefly at hadronic, leptonic and hybrid models, but what now seems obvious is that reconnection plays a key role in very high energy emissions:
http://arxiv.org/abs/1210.6847 On the origin of very high energy gamma-rays from radio galaxies

I read through the papers with interest (although I am no expert : ( . . . .) There did appear a publication a few month ago ---that our Sun did eject high energy gamma rays, but none in the TeV energy part of the spectrum. I am somewhat curious if there is any correlation to GRBs since there seems to be a relation between our Sun's gamma ray *burst* and magnetic reconnection lines in coronal mass ejections (as stated in the publication . . . I will attempt to find the publication and post it if necessary).

quotation
2012-Nov-03, 08:10 PM
I read through the papers with interest (although I am no expert : ( . . . .) There did appear a publication a few month ago ---that our Sun did eject high energy gamma rays, but none in the TeV energy part of the spectrum. I am somewhat curious if there is any correlation to GRBs since there seems to be a relation between our Sun's gamma ray *burst* and magnetic reconnection lines in coronal mass ejections (as stated in the publication . . . I will attempt to find the publication and post it if necessary).
I'm working under the assumption that magnetic reconnection scales, so it's basically the same phenomema, but on a much larger scale, hence the much larger energy values, but I'm no expert either. These folks are though: Magnetic Reconnection, Yamada et. al http://rmp.aps.org/abstract/RMP/v82/i1/p603_1

Magnetic reconnection is seen in the evolution of solar flares, coronal mass ejection, and interaction of solar winds with the Earth’s magnetosphere and is considered to occur in the formation of stars (Parker, 1979; Kulsrud, 1998; Biskamp, 2000; Priest and Forbes, 2000). It occurs as the self-organization process in current carrying fusion plasmas, typically observed in major and minor disruptions of tokamak discharges, and in relaxation processes in reversed field pinch (RFP) and spheromak plasmas (Taylor, 1986; Yamada, 1999b). Magnetic reconnection involves a topology change of a set of field lines, which leads to a new equilibrium configuration of lower magnetic energy. During this process magnetic energy is converted to kinetic energy through acceleration or heating of charged particles. Solar flares exhibit the clearest visual examples of magnetic reconnection and have been investigated for more than a half century.
For my money, and because I have no skin in the game, I can say unabashedly that I think magnetic reconnection may be one of the least appreciated processes going on in plain view, which is why I'm always excited to see more work that includes it. Cheers,

quotation
2012-Nov-05, 05:46 PM
Saw this noted today by Antoniseb in Fun Papers and had to check it out.
http://arxiv.org/abs/1211.0274 The brightest gamma-ray flares of blazars
While it does suggest magnetic reconnection most likely plays a role in the creation of gamma ray flares of blazars, and acknowledges the importance of shocks, it seems to present the two (reconnection and shocks) as an either/or proposition, rather than as a both/and process. The latter seems more likely to me given some previous readings on how magnetic islands are filled by shocks (which I've got to retrieve to be sure), but isn't surprising given that reconnection/shocks, really aren't the main point of the article. As regards this discussion, I've included what I think is the most relevant paragraph, which I hope to include in what might be emerging as an annotated bibliography of relevant material surrounding reconnection and gamma production


Many major blazar flares have been associated with disturbances propagating along the jet that later-on are identified with superluminal radio knots (Marscher et al. 2012). Such distrubances can be interpreted as shock waves if the jet is weakly magnetized (Lind & Blandford 1985), or reconnecting magnetic domain interfaces if the jet is highly magnetized (Lovelace et al. 1997; Nalewajko et al. 2011). As most theoretical models for the formation of relativistic jets predict that they are initially significantly magnetized (Blandford & Znajek 1977; Blandford & Payne 1982), and that it is difficult to efficiently convert the magnetic energy flux into kinetic energy flux (Begelman & Li 1994), scenarios based on magnetic reconnection appear to be more likely. Even if the magnetic domains cannot be easily reversed in AGN jets, reconnection could be triggered by short-wavelength current-driven instabilities (CDI; Begelman 1998; Nalewajko & Begelman 2012). On the other hand, shock waves can be efficient in magnetized
jets if the jet features highly evacuated gaps (Komissarov 2012).

quotation
2012-Nov-07, 04:51 PM
Found this draft article yesterday from July, which makes some pretty interesting claims. Whether or not they withstand review remains to be seen, as I'm not sure where or when this may be published, but seems legit:
http://arxiv.org/abs/1207.6153
EPISODIC JETS AS THE CENTRAL ENGINE OF GAMMA-RAY BURSTS
FENG YUAN AND BING ZHANG
Draft version July 27, 2012

Most Gamma-ray bursts (GRBs) have erratic light curves, which demand that the GRB central engine launches an episodic outflow. Recent Fermi observations of some GRBs indicate a lack of the thermal photosphere component as predicted by the baryonic fireball model, which suggests a magnetic origin of GRBs. In view that powerful episodic jets have been observed along with continuous jets in other astrophysical black hole systems, here we propose an intrinsically episodic, magnetically-dominated jet model for GRB central engine. Accumulation and eruption of free magnetic energy in the corona of a differentially-rotating, turbulent accretion flow around a hyperaccreting black hole lead to ejections of episodic, magnetically dominated plasma blobs. These blobs are accelerated magnetically, collide with each other at large radii, trigger rapid magnetic reconnection and turbulence, efficient particle acceleration and radiation, and power the observed episodic prompt gamma-ray emission from GRBs.

As a reviewer I'd be wary of phrases such as "which demand," and in the paper it becomes "likely suggests", as in:

The standard fireball model predicts a strong thermal emission component from the fireball photosphere (Paczýnski 1986; Mészáros & Rees 2000; Pe’er 2006). The non-detection likely suggests that the GRB outflow is magnetically dominated (Zhang & Pe’er 2009; Fan 2010).
hence

A radiation model that invokes magnetic turbulent reconnection triggered by collisions of magnetically-dominated blobs has been proposed (Zhang & Yan 2011), which can interpret the new observational data.
but,

This radiation model requires a central engine that can eject an episodic, magnetically dominated jet.
A model for just such a jet is also provided and seems close to laboratory simulations of jets:

Yuan et al. (2009) proposed a magnetohydrodynamical model for the formation of episodic jets, by analogy of coronal mass ejection (CME) in the Sun. The basic scenario is the following. Closed magnetic field lines continuously emerge out of the accretion flow into the corona. Because of shear and turbulent motion of the accretion flow, the field line is twisted and deformed, resulting in formation of a flux rope in the corona. The flux rope is initially in force balance between magnetic tension and magnetic compression forces. Energy and helicity are accumulated and stored until a threshold is reached. The system then loses its equilibrium and the flux rope is thrust outward by the magnetic compression force in a catastrophic way, which causes an episodic jet. After a magnetic blob is ejected, the magnetic tension is temporarily relaxed. Later magnetic field emergence and distortion restart, and the above delineated process repeats. Within this scenario, no largescale open magnetic field lines are needed.
The repeating process seems likely to me, but whether or not this is the actual engine of GRBs must obviously await further trial.

antoniseb
2012-Nov-07, 05:22 PM
... A model for just such a jet is also provided and seems close to laboratory simulations of jets:

The repeating process seems likely to me, but whether or not this is the actual engine of GRBs must obviously await further trial.

This is pretty far off the thread topic (not criticizing, just informing you... this isn't widely in the press), since GRBs seem to only blast out low-medium energy Gamma Rays. The paper cited, and quite a few others I note (including one today) are about TeV and higher gammas, which GRBs seem not to produce in any quantity.

quotation
2012-Nov-07, 06:31 PM
...are about TeV and higher gammas, which GRBs seem not to produce in any quantity.
Wonder if that could just be a function of their very short lifetime? But just to make sure we're on the same page allow me if you will to briefly outline what little I think I know about VHE (TeV range) Gamma:
Sources: Supernova remnants (SNRs), galactic centers, blazars
Causes: Inverse compton scattering, Synchrotron Self-Compton Scattering, and Pion decay
I haven't really looked very closely yet at SNRs or Pion decay (except that it's thought to be a "byproduct" of quark hadronization during WIMP annihilation?), so was focussed on the galactic centers and blazars, which seem to require a "jetting" mechanism, hence the (personal) digression to a model of possible formation. Obviously there are other sources, and it would be interesting to learn more about them after I get my mind around this GRB model.

antoniseb
2012-Nov-07, 06:50 PM
...
Sources: Supernova remnants (SNRs), galactic centers, blazars
Causes: Inverse compton scattering, Synchrotron Self-Compton Scattering, and Pion decay...
Pion decay is only meaningful if the thing decaying into Pions is already on a relativistic vector in roughly our direction. Pion mass is about 140 millionths of a TeV.
As to SNRs it is possible that large dimension interactions of highly magnetic cores, or other distributed E/M effects could set up large natural accelerators (as yet poorly described) of charged particles... from there, the Inverse Compton, Bremstrahlung, Self-Compton, etc methods could result in converting particle kinetic energy into photons. Galactic Centers and Black Hole jets of any scale are also poorly understood candidates. Something generates these things, and it is probably something massive. magnetic, spinning, turbulent, or otherwise large. Once we are talking about EeV energies, we are also talking about something fairly nearby (under 200 million light years).

quotation
2012-Nov-07, 07:01 PM
Something generates these things, and it is probably something massive. magnetic, spinning, turbulent, or otherwise large.
Hence my seeming preoccupation with magnetic reconnection, or rather Internal Collision-induced MAgnetic Reconnection and Turbulence (ICMART) which is highly efficient at transforming magnetic energy to kinetic energy...

antoniseb
2012-Nov-07, 07:11 PM
Hence my seeming preoccupation with magnetic reconnection, or rather Internal Collision-induced MAgnetic Reconnection and Turbulence (ICMART) which is highly efficient at transforming magnetic energy to kinetic energy...
Sure, no problem. I'm pretty cautious about anything that sounds like wacky-electric-universe stuff, but some aspects of the universe are heavily influenced by E&M, and there's little doubt that this is one of them.