Hadronic gamma-rays from RX J1713.7−3946? Gabici, S; Aharonian, F. A
Monthly notices of the Royal Astronomical Society. Letters,
11/2014, Letnik:
445, Številka:
1
Journal Article
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RX J1713.7−3946 is a key object to check the supernova remnant paradigm of the origin of Galactic cosmic rays. While the origin of its gamma-ray emission (hadronic versus leptonic) is still debated, ...the hard spectrum at GeV energies reported by the Fermi collaboration is generally interpreted as a strong argument in favour of a leptonic scenario. On the contrary, we show that hadronic interactions can naturally explain the gamma-ray spectrum if gas clumps are present in the supernova remnant shell. The absence of thermal X-rays from the remnant fits well within this scenario.
The cosmic ray electron spectrum exhibits a break at a particle energy of ∼1 TeV and extends without any attenuation up to ∼20 TeV. Synchrotron and inverse Compton energy losses strongly constrain ...the time of emission of ∼20 TeV electrons to ≈2×104 yr and the distance of the potential source(s) to ≈100–500 pc, depending on the cosmic ray diffusion coefficient. This suggests that maybe one nearby discrete source may explain the observed spectrum of high energy electrons. Given the strong energy dependence (∝1/E) of the cooling time of TeV electrons, the spectral shape of the electron spectrum above the ∼1 TeV break strongly depends on the history of injection of these electrons from the source. In this paper we show that a local, continuous (on timescales of ∼105 yr) but fading electron accelerator, with a characteristic decay time of ∼104 yr, can naturally account for the entire spectrum of cosmic ray electrons in the TeV domain. Although the standard "nearby pulsar" scenario naturally meets this time condition, it is (almost) excluded by recent measurements of the positron fraction, which above ∼100 GeV saturates at a level well below 0.5 and drops above ∼400–500 GeV. The second potential source population, the supernova remnants, accelerate mostly electrons, rather than positrons. However, they hardly can provide an effective production of multi-TeV electrons via the standard diffusive shock acceleration scenario for ∼105 yr. A third possibility are stellar wind shocks, which however are likely to be continuous with nearly constant luminosity on timescales ≫10 kyr and probably cannot match the time requirement of our potential source. Therefore, we face a real challenge in the identification of the origin of the source of multi-TeV electrons. Thus, the link of this source with known particle accelerators would require a dramatic revision of the standard paradigms of acceleration and escape in such objects.
Molecular clouds are expected to emit non–thermal radiation due to cosmic ray interactions in the dense magnetized gas. Such emission is amplified if a cloud is located close to an accelerator of ...cosmic rays and if energetic particles can leave the accelerator site and diffusively reach the cloud. We consider here a situation in which a molecular cloud is located in the proximity of a supernova remnant which is efficiently accelerating cosmic rays and gradually releasing them in the interstellar medium. We calculate the multiwavelength spectrum from radio to gamma rays which is emerging from the cloud as the result of cosmic ray interactions. The total energy output is dominated by the gamma–ray emission, which can exceed the emission in other bands by an order of magnitude or more. This suggests that some of the unidentified TeV sources detected so far, with no obvious or very weak counterparts in other wavelengths, might be in fact associated with clouds illuminated by cosmic rays coming from a nearby source. Moreover, under certain conditions, the gamma–ray spectrum exhibits a concave shape, being steep at low energies and hard at high energies. This fact might have important implications for the studies of the spectral compatibility of GeV and TeV gamma–ray sources.
ABSTRACT
The escape process of particles accelerated at supernova remnant (SNR) shocks is one of the poorly understood aspects of the shock acceleration theory. Here we adopt a phenomenological ...approach to study the particle escape and its impact on the gamma-ray spectrum resulting from hadronic collisions both inside and outside of a middle-aged SNR. Under the assumption that in the spatial region immediately outside of the remnant the diffusion coefficient is suppressed with respect to the average Galactic one, we show that a significant fraction of particles are still located inside the SNR long time after their nominal release from the acceleration region. This fact results into a gamma-ray spectrum that resembles a broken power law, similar to those observed in several middle-aged SNRs. Above the break, the spectral steepening is determined by the diffusion coefficient outside of the SNR and by the time dependence of maximum energy. Consequently, the comparison between the model prediction and actual data will contribute to determining these two quantities, the former being particularly relevant within the predictions of the gamma-ray emission from the halo of escaping particles around SNRs, which could be detected with future Cherenkov telescope facilities. We also calculate the spectrum of runaway particles injected into the Galaxy by an individual remnant. Assuming that the acceleration stops before the SNR enters the snowplow phase, we show that the released spectrum can be a featureless power law only if the accelerated spectrum is ∝ p−α with α > 4.
Abstract
We present a solution for the ultraviolet – submillimetre (submm) interstellar radiation fields (ISRFs) of the Milky Way (MW), derived from modelling COBE, IRAS and Planck maps of the ...all-sky emission in the near-, mid-, far-infrared and submm. The analysis uses the axisymmetric radiative transfer model that we have previously implemented to model the panchromatic spectral energy distributions (SEDs) of star-forming galaxies in the nearby universe, but with a new methodology allowing for optimization of the radial and vertical geometry of stellar emissivity and dust opacity, as deduced from the highly resolved emission seen from the vantage point of the Sun. As such, this is the first self-consistent model of the broad-band continuum emission from the MW. In this paper, we present model predictions for the spatially integrated SED of the MW as seen from the Sun, showing good agreement with the data, and give a detailed description of the solutions for the distribution of ISRFs, as well as their physical origin, throughout the volume of the galaxy. We explore how the spatial and spectral distributions of our new predictions for the ISRF in the MW affects the amplitude and spectral distributions of the gamma rays produced via inverse Compton scattering for cosmic ray (CR) electrons situated at different positions in the galaxy, as well as the attenuation of the gamma rays due to interactions of the gamma-ray photons with photons of the ISRF. We also compare and contrast our solutions for the ISRF with those incorporated in the galprop package used for modelling the high-energy emission from CR in the MW.
The inverse Compton (IC) scattering of relativistic electrons is one of the major gamma-ray production mechanisms in different environments. Often, the target photons for IC scattering are dominated ...by blackbody (or graybody) radiation. In this case, the precise treatment of the characteristics of IC radiation requires numerical integrations over the Planckian distribution. Formally, analytical integrations are also possible but they result in series of several special functions; this limits the efficiency of usage of these expressions. The aim of this work is the derivation of approximate analytical presentations that would provide adequate accuracy for the calculations of the energy spectra of upscattered radiation, the rate of electron energy losses, and the mean energy of emitted photons. Such formulae have been obtained by merging the analytical asymptotic limits. The coefficients in these expressions are calculated via the least-squares fitting of the results of numerical integrations. The simple analytical presentations, obtained for both the isotropic and anisotropic target radiation fields, provide adequate (as good as 1%) accuracy for broad astrophysical applications.
We analyze three scenarios to address the challenge of ultrafast gamma-ray variability reported from active galactic nuclei. We focus on the energy requirements imposed by these scenarios: (i) ...external cloud in the jet, (ii) relativistic blob propagating through the jet material, and (iii) production of high-energy gamma-rays in the magnetosphere gaps. We show that while the first two scenarios are not constrained by the flare luminosity, there is a robust upper limit on the luminosity of flares generated in the black hole magnetosphere. This limit depends weakly on the mass of the central black hole and is determined by the accretion disk magnetization, viewing angle, and the pair multiplicity. For the most favorable values of these parameters, the luminosity for 5-minute flares is limited by , which excludes a black hole magnetosphere origin of the flare detected from IC 310. In the scopes of scenarios (i) and (ii), the jet power, which is required to explain the IC 310 flare, exceeds the jet power estimated based on the radio data. To resolve this discrepancy in the framework of scenario (ii), it is sufficient to assume that the relativistic blobs are not distributed isotropically in the jet reference frame. A realization of scenario (i) demands that the jet power during the flare exceeds by a factor 102 the power of the radio jet relevant to a timescale of 108 years.
A new numerical code, designed for the detailed numerical treatment of nonlinear diffusive shock acceleration, is used for the modeling of particle acceleration and radiation in young supernova ...remnants (SNRs). The model is based on spherically symmetric hydrodynamic equations complemented with transport equations for relativistic particles. For the first time, the acceleration of electrons and protons by both forward and reverse shocks is studied through detailed numerical calculations. We model the energy spectra and spatial distributions of nonthermal emission of the young SNR RX J1713.7-3946 and compare the calculations with the spectral and morphological properties of this object obtained in broad energy band from radio to very high-energy gamma rays. We discuss the advantages and shortcomings of the so-called hadronic and leptonic models, which assume that the observed TeV gamma-ray emission is produced by accelerated protons and electrons, respectively. We discuss also a 'composite'scenario when the gamma-ray flux from the main parts of the shell has inverse Compton origin, but with a non-negligible contribution of hadronic origin from dense clouds interacting with the shell.
Context.
Cosmic rays (CRs) propagating through dense molecular clouds (MCs) produce
γ
-rays, which carry direct information about the CR distribution throughout the Galaxy. Observations of
γ
-rays in ...different energy bands allow for the exploration of the average CR density in the Galactic disk, the so-called level of the “CR sea”. Observations with the
Fermi
-Large Area Telescope (LAT) demonstrated the method’s feasibility based on two dozen MCs in our Galaxy. However, the potential of
Fermi
-LAT is limited to the exploration of the most massive and relatively nearby MCs; thus, the current observations cover only a tiny fraction of the Milky Way.
Aims.
In this work, we aim to study the prospects of expanding the CR measurements to very and ultra-high energies and remote parts of the Galaxy with the current and next-generation detectors.
Methods.
Based on calculations of fluxes expected from MCs, we formulated the requirements to the sensitivity of the post-
Fermi
-LAT detectors in order to map GeV-TeV CRs in the Galactic disk. We also explored the potential of the current and future air-shower and atmospheric Cherenkov telescope arrays for the extension of CR studies to multi-TeV and PeV energy bands.
Results.
We demonstrated that the improvement of the
Fermi
-LAT sensitivity by a factor of a few would allow a dramatic increase in the number of detectable MCs, covering almost the entire Galaxy. The recently completed Large High altitude air Shower Observatory should be able to take the first CR probes at PeV energies in the coming five years or so.
ABSTRACT
Observations from the radio to the gamma-ray wavelengths indicate that supernova remnant (SNR) shocks are sites of effective particle acceleration. It has been proposed that the presence of ...dense clumps in the environment where supernovae explode might have a strong impact on the shape of the hadronic gamma-ray spectrum. Here we present a detailed numerical study of the penetration of relativistic protons into clumps that are engulfed by a SNR shock, taking into account the magneto-hydrodynamical properties of the background plasma. We show that the spectrum of protons inside clumps is much harder than that in the diffuse inter-clump medium, and we discuss the implications for the formation of the spectrum of hadronic gamma-rays, which no longer reflects the acceleration spectrum of protons, resulting substantially modified inside the clumps owing to propagation effects. For the Galactic SNR RX J1713.7 – 3946, we show that a hadronic scenario including dense clumps inside the remnant shell is able to reproduce the broadband gamma-ray spectrum from GeV to TeV energies. Moreover, we argue that small clumps crossed by the shock could provide a natural explanation for the non-thermal X-ray variability observed in some hotspots of RX J1713.7 – 3946. Finally, we discuss the detectability of gamma-ray emission from clumps with the upcoming Cherenkov Telescope Array, and the possible detection of the clumps themselves through molecular lines.
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