ABSTRACT
Blazars may accelerate protons and/or nuclei as well as electrons. The hadronic component of accelerated particles in blazars may constitute the bulk of their high-energy budget; ...nevertheless, this component is elusive because of the high value of the energy threshold of proton interactions with photon fields inside the source. However, the broad line regions (BLRs) of some flat spectrum radio quasars (FSRQs) may contain a sufficient amount of matter to render primary protons ‘visible’ in γ-rays via hadronuclear interactions. In this paper, we study the persistent γ-ray emission of the FSRQ PKS 1510−089 in its low state, utilizing the publicly available Fermi-LAT data, as well as using the spectrum measured with the Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescopes. We find an indication that there is an excess of γ-rays at the energy range ≳ 20 GeV with respect to a simple baseline log-parabolic intrinsic spectral model. This excess could be explained in a scenario invoking hadronuclear interactions of primary protons on the BLR material with the subsequent development of electromagnetic cascades in photon fields. We present a Monte Carlo calculation of the spectrum of this cascade component, taking as input the BLR photon field spectrum calculated with the cloudy code. To our knowledge, this is the first calculation of an electromagnetic cascade spectrum inside a blazar based on a direct calculation of the photon field spectrum with a spectral synthesis code.
Context. Most of the studies on extragalactic γ-ray propagation performed up to now only accounted for primary γ-ray absorption and adiabatic losses, known as the “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 γ-axion-like particle (ALP) oscillation. On the other hand, there are many hints that a secondary component from electromagnetic cascades initiated by primary γ-rays or nuclei may be observed in the spectra of some blazars. Aims. We study the impact of electromagnetic cascades from primary γ-rays or protons on the physical interpretation of blazar spectra obtained with imaging Cherenkov telescopes. Methods. We used the publicly-available code ELMAG to compute observable spectra of electromagnetic cascades from primary γ-rays. For the case of primary proton, we developed a simple, fast and reasonably accurate hybrid method to calculate the observable spectrum. We performed 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 γ-rays, and several versions of the hadronic cascade model for the case of primary protons. We distinguish the following species of hadronic cascade models: 1) the “basic hadronic model”, in which it is assumed that the proton beam travels undisturbed by the extragalactic magnetic field and that all observable γ-rays are produced by primary protons through photohadronic processes with subsequent development of electromagnetic cascades; 2) the “intermediate hadronic model”, which is the same as the basic hadronic model, but the primary beam is terminated at some redshift zc; and 3) the “modified hadronic model” that includes the contribution from primary, redshifted and partially-absorbed, γ-rays. Results. Electromagnetic cascades show at least two very distinct regimes labelled by the energy of the primary γ-ray (E0): the one-generation regime for the case of E0 < 10 TeV, and the universal regime for E0 > 100 TeV and redshift to the source zs > 0.02. Spectral signatures of the observable spectrum for the case of the basic hadronic model, zs = 0.186 and low energy (E < 200 GeV), are nearly the same as for a purely electromagnetic cascade, but for E > 200 GeV the spectrum is much harder for the case of the basic hadronic model. In the framework of the intermediate hadronic model, the observable spectrum depends only slightly on the primary proton energy, but it strongly depends on zc at E > 500 GeV. As a rule, both electromagnetic and hadronic cascade models provide acceptable fits to the observed SEDs. We show that the best-fit model intensity in the multi-TeV region of the spectrum in the framework of the electromagnetic cascade model is typically greater than the one for the case of the absorption-only model. Finally, for the case of blazar 1ES 0229+200 we provide strong constraints on the intermediate hadronic model, assuming models for the blazar emission and the magnetic field around the source.
A scheme for modeling tracks of recoil nuclei from elastic scattering of hypothetical dark-matter weakly interacting massive particles (WIMPs) is presented. Constraints on the possibility of ...directional detection of WIMPs in experiments where light hydrogen nuclei and groups of C, N, and O nuclei, as well as fluorine nuclei, play the role of a target in the detector used are set. The number of WIMPs interactions per unit mass of the detector was estimated by means of the micrOMEGAs software package for an inert doublet model. It is concluded that, for the purpose of visualizing tracks of recoil nuclei and, accordingly, accomplishing a directional detection of WIMPs with lowest assumed masses of 4 to 10 GeV, preference should be given to target of lower density.
We briefly review contemporary extragalactic γ-ray propagation models. It is shown that the Extragalactic Magnetic Field (EGMF) strength and structure are poorly known. Strict lower limits on the ...EGMF strength in voids are of order 10−17 − 10−20 G, thus allowing a substantial contribution of a secondary component generated by electromagnetic cascades to the observable spectrum. We show that this "electromagnetic cascade model" is supported by data from imaging Cherenkov telescopes and the Fermi LAT detector.
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.
There are indications that a secondary component of cascade photons is present in the spectra of some bright blazars. The main spectral signatures are considered using the electromagnetic model. The ...simplest version of the hadronic cascade model can be distinguished from the electromagnetic cascade at a significance level of ~3 σ for a typical observation of a blazar at the Cherenkov Telescope Array (CTA).
Methods of searching for oscillations of photons to axion-like particles (γ → ALP) in spectra of extragalactic sources are considered. It is shown that the secondary component from the cascades ...initiated by primary gamma rays or nuclei can produce an appreciable background in searches for ALPs in the optically thick region of the spectrum. Experiments with a high energy threshold (~1 TeV and higher) are insufficient for searching for γ → ALP oscillations; low-energy instruments (
E
< 100 GeV) like CTA should also be used.
Technical and production solutions ensuring a commercial boiler fuel with a hydrogen sulfide content of no more than 2 ppm are developed as a result of the basic design of a unit for stripping ...hydrogen sulfide from visbreaking resid. The potential exists for a further increase in the output of the visbreaking plant to 2 million tons/year, where the actual output achieved is 1.4 million tons/year.
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