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.
Among metal β-diketonates, nickel acetylacetonate (Ni(acac)2) has been widely employed as a precursor for many chemical structures, due to its catalytic properties. Here, we investigate, by means of ...density functional theory (DFT) calculations, the adsorption and dissociation of this complex: after an evaluation of the structural and electronic properties of Ni(acac)2, a comparison between different dissociation patterns reveals that the most favorable pattern for the complex adsorbed on iron is different from the one suggested by considering the strength of the bonds in the isolated complex and an attempt to generalize this dissociation model is made in this work. Moreover, the most favorable adsorption configurations turned out to be a long bridge positioning of the nickel atom along with an on top positioning of the oxygen atoms of Ni(acac)2, while a short bridge positioning is the most favorable for the central metallic unit alone.
We propose a method to calculate the ideal shear strength τ of two surfaces in contact by ab initio calculations. This quantity and the work of adhesion γ are the interfacial parameters usually ...derived from tip-based friction force measurements. We consider diamond interfaces and quantitatively evaluate the effects of surface orientation and passivation. We find that in the case of fully passivated interfaces, γ is not affected by the orientation and the alignment of the surfaces in contact. On the contrary, τ does show a dependence on the atomic-scale roughness of the interface. The surface termination has a major impact on the tribological properties of diamond. The presence of dangling bonds, even at concentrations low enough to prevent the formation of interfacial C–C bonds, causes an increase in the resistance to sliding by 2 orders of magnitude with respect to the fully hydrogenated case. We discuss our findings in relation to experimental observations.
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
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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.
Molybdenum dithiocarbamate (MoDTC) is a well-known lubricant additive, which, in tribological conditions, is capable of forming layers of MoS2 with excellent friction reduction properties. Despite ...being widely employed in commercial engine oils, a comprehensive theoretical description of the properties of MoDTC is still lacking. In this work, we employ density functional theory to study the structural, electronic, and vibrational properties of MoDTC. We investigate the relative stability of different isomers, different hydrocarbon terminations, and oxidized complexes. Oxidation was found to be energetically favorable for a wide range of conditions, and the most favorable position for oxygen atoms in MoDTC turned out to be the ligand position. These results, along with the calculated reaction energies for different dissociation paths, can be useful to better identify the elementary steps of the decomposition process of MoDTC.
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.
We present accurate calculations of friction in graphene films in configurations simulating the presence of an anchoring substrate. We find that a slider induces both out-of-plane and shear ...deformations, which increase with the thickness of the supported film. We elucidate the new frictional mechanism connected to shear layer motions, which is minimal for systems with the smallest number of layers.
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.
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