The Fermi blazar sequence Ghisellini, G; Righi, C; Costamante, L ...
Monthly notices of the Royal Astronomical Society,
07/2017, Letnik:
469, Številka:
1
Journal Article
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Abstract
We revisit the blazar sequence exploiting the complete, flux-limited sample of blazars with known redshift detected by the Fermi satellite after 4 yr of operations (the 3LAC sample). We ...divide the sources into γ-ray luminosity bins, collect all the archival data for all blazars, and construct their spectral energy distribution (SED). We describe the average SED of blazars in the same luminosity bin through a simple phenomenological function consisting of two broken power laws connecting with a power law describing the radio emission. We do that separately for BL Lacs and for flat spectrum radio quasars (FSRQs) and also for all blazars together. The main results are: (i) FSRQs display approximately the same SED as the luminosity increases, but the relative importance of the high-energy peak increases; (ii) as a consequence, the X-ray spectra of FSRQs become harder for larger luminosities; (iii) BL Lacs indeed form a sequence: they become redder (i.e. smaller peak frequencies) with increasing luminosities, with a softer γ-ray slope and a larger dominance of the high-energy peak; (iv) for all blazars (BL Lacs+FSRQs), these properties become more prominent, as the highest luminosity bin is populated mostly by FSRQs and the lowest luminosity bin mostly by BL Lacs. This agrees with the original blazar sequence, although BL Lacs never have an average γ-ray slope as hard as found in the original sequence. (v) At high luminosities, a large fraction of FSRQs show signs of thermal emission from the accretion disc, contributing to the optical–UV (ultraviolet).
Abstract The origin of the astrophysical high-energy neutrinos discovered by IceCube is currently a major mystery. The recent detection of IceCube-170922A, an ∼300 TeV neutrino potentially correlated ...with the flaring γ-ray source TXS 0506+056, directs attention towards BL Lac objects (BL Lacs), the subclass of blazars with weak emission lines. While high-energy neutrinos can be produced via photohadronic interactions between protons accelerated in their jets and ambient low-energy photons, the density of the latter in such objects had generally been thought to be too low for efficient neutrino emission. Here we consider the role of radiatively inefficient accretion flows (RIAFs), which can plausibly exist in the nuclei of BL Lacs, as the source of target photons for neutrino production. Based on simple model prescriptions for the spectra of RIAFs at different accretion rates, we find that they can be sufficiently intense to allow appreciable neutrino emission for the class of low-synchrotron-peak BL Lacs such as TXS 0506+056. In contrast, for high-synchrotron-peak BL Lacs, including Mkn 421 and Mkn 501, the contribution of RIAFs is subdominant and their neutrino production efficiency can remain low, consistent with their non-detection by IceCube to date.
Context.
Flat-spectrum radio quasars (FSRQs) are the most powerful blazars in the
γ
-ray band. Although they are supposed to be good candidates in producing high-energy neutrinos, no secure detection ...of FSRQs has been obtained to date, except for a possible case of PKS B1424-418.
Aims.
In this work, our aim was to compute the expected flux of high-energy neutrinos from FSRQs using standard assumptions for the properties of the radiation fields filling the regions surrounding the central supermassive black hole.
Methods.
Starting from the FSRQ spectral sequence, we computed the neutrino spectrum assuming interaction of relativistic protons with internal and external radiation fields. We studied the neutrino spectra resulting from different values of free parameters
Results.
The result we obtained is that high-energy neutrinos are naturally expected from FSRQs in the sub-EeV–EeV energy range and not at PeV energies. This justifies the non-observation of neutrinos from FSRQs with the present technology, since only neutrinos below 10 PeV have been observed. We found that for a non-negligible range of the parameters, the cumulative flux from FSRQs is comparable to or even exceeds the expected cosmogenic neutrino flux. This result is intriguing and highlights the importance of disentangling these point-source emissions from the diffuse cosmogenic background.
High-energy neutrinos from FR0 radio galaxies? Tavecchio, F; Righi, C; Capetti, A ...
Monthly notices of the Royal Astronomical Society,
04/2018, Letnik:
475, Številka:
4
Journal Article
Recenzirano
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Abstract
The sources responsible for the emission of high-energy (≳100 TeV) neutrinos detected by IceCube are still unknown. Among the possible candidates, active galactic nuclei with relativistic ...jets are often examined, since the outflowing plasma seems to offer the ideal environment to accelerate the required parent high-energy cosmic rays. The non-detection of single-point sources or – almost equivalently – the absence, in the IceCube events, of multiplets originating from the same sky position – constrains the cosmic density and the neutrino output of these sources, pointing to a numerous population of faint sources. Here we explore the possibility that FR0 radio galaxies, the population of compact sources recently identified in large radio and optical surveys and representing the bulk of radio-loud AGN population, can represent suitable candidates for neutrino emission. Modelling the spectral energy distribution of an FR0 radio galaxy recently associated with a γ-ray source detected by the Large Area Telescope onboard Fermi, we derive the physical parameters of its jet, in particular the power carried by it. We consider the possible mechanisms of neutrino production, concluding that pγ reactions in the jet between protons and ambient radiation is too inefficient to sustain the required output. We propose an alternative scenario, in which protons, accelerated in the jet, escape from it and diffuse in the host galaxy, producing neutrinos as a result of pp scattering with the interstellar gas, in strict analogy with the processes taking place in star-forming galaxies.
ABSTRACT
We examine the potential multimessenger role of BL Lac objects emitting at high energy (so-called HBL) focusing on the limits on the energy density of soft radiation in the jet frame, a ...critical parameter that regulates the proton cooling and the fragmentation of ultrahigh-energy cosmic ray nuclei possibly accelerated in the jet. We show that (under the assumption that the high-energy emission bump is dominated by inverse Compton emission) the energy density of any external soft radiation field (e.g. produced by a layer surrounding the jet or in the accretion flow) cannot be larger than few times that associated with the observed synchrotron radiation produced in the emission region. Quite interestingly, the constraint that we derive is generally stronger than the limit obtained from the condition that the source is transparent to very high-energy γ-rays. Using this constraint, we can derive a robust upper limit for the efficiency of the photopion reaction leading to the emission of PeV neutrinos, fπ ≲ 10−5, which makes HBL quite inefficient neutrino sources. For the photodisintegration of nuclei, the results are more dependent on the spectral properties of the radiation field. The photodisintegration efficiency is safely below 1 (and nuclei can escape intact) for a ‘canonical’ spectrum of the soft radiation field ∝ ν−0.5. For radiation fields characterized by a softer spectrum and extended over a large portion of the jet, the efficiency increases and for an appreciable fraction of the sources nuclei with energies above 1019 eV might suffer significant photodisintegration.
The functionality of graphene as lubricant material is affected by extrinsic factors, such as the film thickness and the environmental conditions. Graphite lubricating capability depends as well on ...air humidity. To accurately describe the tribochemistry mechanisms underlying these behaviours we adopt a Quantum Mechanics/Molecular Mechanics approach. We show that reactive edges are able to cause a huge friction increase, which is quantified for graphene flakes between sliding diamond surfaces. Moreover, folds spontaneously formed in single layer graphene under tribological conditions are shown to be highly reactive due to carbon re-hybridization. This observation offers a new hint for interpreting the dependence of graphene friction on the number of layers. Both water and oxygen molecules are found to be effective in quenching the reactivity of defects by dissociative chemisorption. However, peculiar mechanisms of water molecules makes humidity more effective than oxygen for enabling the lubricity of graphitic media. They include collective processes as Grotthus-like proton diffusion enhanced by confinement, and the strong change in hydrophilic character of the passivated media. This comprehensive study sheds a new light on debated issues of graphene and graphite tribology, and highlights the potentiality of these materials for metal-free catalysis, e.g., for H production by water splitting.
To date, no clear conclusion has been reached on the atomistic mechanisms that govern the observed decrease of lubricating capabilities of MoS2 in humid environments. Based on abinitio molecular ...dynamic calculations, we show that intercalated water molecules hinder the sliding motion of both regular and defective layers considerably, with the velocities decaying exponentially with time. However, in the presence of an applied load and exposed edge terminations, water is rapidly removed from the interface and is adsorbed on the edges either in undissociated form or as OH/H fragments. These outcomes suggest that the interlayer slipperiness can be reduced by the presence of water even in the absence of any chemical oxidation. Our work could help to set up more dedicated experiments to further tackle a technologically relevant issue for the applications of MoS2-based lubricants.
To date, no clear conclusion has been reached on the atomistic mechanisms that govern the observed decrease of lubricating capabilities of MoS2 in humid environments. Based on ab initio molecular ...dynamic calculations, we show that intercalated water molecules hinder the sliding motion of both regular and defective layers considerably, with the velocities decaying exponentially with time. However, in the presence of an applied load and exposed edge terminations, water is rapidly removed from the interface and is adsorbed on the edges either in undissociated form or as OH/H fragments. These outcomes suggest that the interlayer slipperiness can be reduced by the presence of water even in the absence of any chemical oxidation. Our work could help to set up more dedicated experiments to further tackle a technologically relevant issue for the applications of MoS2‐based lubricants.
Sliding away: The atomistic mechanisms that govern the observed decrease of lubricating capabilities of MoS2 in humid environments is examined by means of ab initio molecular dynamic calculations.
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