Behavior of axionlike particles in smoothed out domainlike magnetic fields
May 9, 201824 pages
Published in:
- Phys.Rev.D 98 (2018) 4, 043018
- Published: Aug 23, 2018
e-Print:
- 1804.09443 [astro-ph.HE]
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Abstract: (APS)
The existence of axionlike particles (ALPs) is predicted by many extensions of the standard model of elementary particles and in particular by theories of superstrings and superbranes. ALPs are very light, neutral, pseudoscalar bosons which are supposed to interact with two photons. They can play an important role in high-energy astrophysics. Basically, in certain circumstances, ALPs substantially enhance the photon survival probability Pγ→γ(E) of a beam emitted by a far-away source through the mechanism of photon-ALP oscillations (E denotes the energy). But in order for this to work, an external magnetic field B must be present. In several cases, B is modeled as a domainlike network with “sharp edges”: all domains have the same size Ldom (set by the B coherence length) and the same strength B, but the direction of B changes randomly and abruptly from one domain to the next. While this model has repeatedly been used since it greatly simplifies the calculations, it is obviously a highly mathematical idealization wherein the components of B are discontinuous across the edges (whence the name sharp edges). It is therefore highly desirable to go a step further and to find out what happens when the edges are smoothed out, namely when the abrupt change of B is replaced by a smooth one. Moreover, this step becomes compelling when the photon-ALP oscillation length losc turns out to be comparable to—or smaller than—Ldom, because in this case, the photon survival probability Pγ→γ(E) critically depends on the domain shape. Finally, it would be more realistic to have Ldom randomly changing within a given range, since it looks rather unlikely that the coherence length of B should be the same everywhere, especially when its source is sufficiently extended. In the present paper, we propose a smoothed out version of the previous domainlike structure of B which incorporates the above changes, and we work out its implications. Even in the present case, we are able to solve analytically and exactly the propagation equation of a monochromatic photon/ALP beam of energy E inside a single smoothed out domain, thereby ultimately evaluating the corresponding photon survival probability Pγ→γ(E) exactly. This fact has the great advantage to drastically shorten the computation time in the applications involving computer simulations as compared to a numerical solution of the beam propagation equation. Actually, it turns out that the condition losc≲Ldom takes place when a photon/ALP beam of either very low E or very large E—both in the gamma-ray band—crosses a variety of astronomical objects, like radio lobes of flat spectrum radio quasars, spiral galaxies, starburst galaxies, elliptical galaxies, and extragalactic space. Thus, the use of our model becomes compelling in all these instances, since the sharp edges model would yield unphysical results. The case of extragalactic space is of particular importance in view of the new generation of gamma-ray observatories like CTA, HAWC, GAMMA-400, LHAASO, and TAIGA-HiSCORE, since in such a situation losc≲Ldom occurs for E≳O(40 TeV) with a large uncertainty, depending on the choice of the model parameters (we shall come back to this fundamental issue in a subsequent paper).Note:
- 30 pages, 14 figures. This version matches the published paper: Phys. Rev. D 98, 043018 (2018)
- Astrophysics and astroparticle physics
- oscillation: length
- magnetic field: external field
- gamma ray: detector
- coherence: length
- scale: TeV
- photon
- axion-like particles
- propagation
- structure
References(165)
Figures(14)
- [1]
- [2]
- [3]
- [4]
- [5]
- [6]
- [7]
- [8]
- [9]
- [10]
- [11]
- [12]
- [13]
- [14]
- [15]
- [16]
- [17]
- [17]
- [18]
- [19]
- [20]
- [21]
- [22]
- [23]
- [24]