Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userElectromagnetic cascade masquerade: a way to mimic $gamma$--axion-like particle mixing effects in blazar spectra
58
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
Context. Most of the studies on extragalactic {gamma}-ray propagation performed up to now only accounted for primary gamma-ray absorption and adiabatic losses (absorption-only model). However, there is growing evidence that this model is oversimplified and must be modified in some way. In particular, it was found that the intensity extrapolated from the optically-thin energy range of some blazar spectra is insufficient to explain the optically-thick part of these spectra. This effect was interpreted as an indication for {gamma}-axion-like particle (ALP) oscillation. On the other hand, there are many hints that a secondary component from electromagnetic cascades initiated by primary {gamma}-rays or nuclei may be observed in the spectra of some blazars. Aims. We study the impact of electromagnetic cascades from primary {gamma}-rays or protons on the physical interpretation of blazar spectra obtained with imaging Cherenkov telescopes. Methods. We use the publicly-available code ELMAG to compute observable spectra of electromagnetic cascades from primary {gamma}-rays. For the case of primary proton, we develop a simple, fast and reasonably accurate hybrid method to calculate the observable spectrum. We perform the fitting of the observed spectral energy distributions (SEDs) with various physical models: the absorption-only model, the electromagnetic cascade model (for the case of primary {gamma}-rays), and sever
rate research
Read More
Recent progress in very high energy (VHE, E >100 GeV) $gamma$-ray observations, together with advances in the extragalactic background light (EBL) modelling, allows to search for new phenomena such as $gamma$-axion-like particle ($gamma rightarrow$ ALP) oscillation and to explore the extragalactic magnetic field (EGMF) strength and structure. These studies are usually performed by searching for some deviation from the so-called absorption-only model, that accounts for only primary photon absorption on the EBL and adiabatic losses. In fact, there exist several indications that the absorption-only model is incomplete. We present and discuss the intergalactic electromagnetic cascade model (IECM) --- the simplest model that allows to coherently explain all known anomalies. This model has a number of robust signatures that could be searched for with present and future instruments. The IECM model may serve as a new background template, allowing to make future searches for $gamma rightarrow$ ALP oscillation more robust. A detailed account of our calculations is available in astro-ph/1609.01013v2 (A&A,in print).
Secondary {gamma}-rays from intergalactic cascades may contribute to observable spectra of blazars, also modifying observable angular and temporal distributions. In this paper we briefly review basic features of intergalactic electromagnetic cascade physics, suggest a new approximation for {gamma}-ray mean free path, consider angular patterns of magnetically broadened cascade emission, and present an example of a fit to the observable blazar spectrum.
The decay of a massive pseudoscalar, scalar and U(1) boson into an electron-positron pair in the presence of strong electromagnetic backgrounds is calculated. Of particular interest is the constant-crossed-field limit, relevant for experiments that aim to measure high-energy axion-like-particle conversion into electron-positron pairs in a magnetic field. The total probability depends on the quantum nonlinearity parameter - a product of field and lightfront momentum invariants. Depending on the seed particle mass, different decay regimes are identified. In the below-threshold case, we find the probability depends on a non-perturbative tunnelling exponent depending on the quantum parameter and the particle mass. In the above-threshold case, we find that when the quantum parameter is varied linearly, the probability oscillates nonlinearly around the spontaneous decay probability. A strong-field limit is identified in which the threshold is found to disappear. In modelling the fall-off of a quasi-constant-crossed magnetic field, we calculate probabilities beyond the constant limit and investigate when the decay probability can be regarded as locally constant.
The growing interest in axion-like particles (ALPs) stems from the fact that they provide successful theoretical explanations of physics phenomena, from the anomaly of the CP-symmetry conservation in strong interactions to the observation of an unexpectedly large TeV photon flux from astrophysical sources, at distances where the strong absorption by the intergalactic medium should make the signal very dim. In this latter condition, which is the focus of this review, a possible explanation is that TeV photons convert to ALPs in the presence of strong and/or extended magnetic fields, such as those in the core of galaxy clusters or around compact objects, or even those in the intergalactic space. This mixing affects the observed ${gamma}$-ray spectrum of distant sources, either by signal recovery or the production of irregularities in the spectrum, called wiggles, according to the specific microscopic realization of the ALP and the ambient magnetic field at the source, and in the Milky Way, where ALPs may be converted back to ${gamma}$ rays. ALPs are also proposed as candidate particles for the Dark Matter. Imaging Atmospheric Cherenkov telescopes (IACTs) have the potential to detect the imprint of ALPs in the TeV spectrum from several classes of sources. In this contribution, we present the ALP case and review the past decade of searches for ALPs with this class of instruments.
Astrophysical gamma-ray sources come in a variety of sizes and magnetizations. We deduce general conditions under which gamma-ray spectra from such sources would be significantly affected by axion-photon mixing. We show that, depending on strength and coherence of the magnetic field, axion couplings down to ~ 1/(10**13 GeV) can give rise to significant axion-photon
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
