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
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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.
Cosmic ray penetration in diffuse clouds Morlino, G; Gabici, S
Monthly notices of the Royal Astronomical Society. Letters,
07/2015, Letnik:
451, Številka:
1
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Cosmic rays are a fundamental source of ionization for molecular and diffuse clouds, influencing their chemical, thermal, and dynamical evolution. The amount of cosmic rays inside a cloud also ...determines the gamma-ray flux produced by hadronic collisions between cosmic rays and cloud material. We study the spectrum of cosmic rays inside and outside of a diffuse cloud by solving the stationary transport equation for cosmic rays including diffusion, advection, and energy losses due to ionization of neutral hydrogen atoms. We found that the cosmic ray spectrum inside a diffuse cloud differs from the one in the interstellar medium (ISM) for energies smaller than E
br ≈ 100 MeV, irrespective of the model details. Below E
br, the spectrum is harder (softer) than that in the ISM if the latter is a power law ∝p
−s
with s larger (smaller) than ∼0.42.
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.
Supernova remnants are believed to be the main sources of galactic cosmic rays (CR). Within this framework, particles are accelerated at supernova remnant shocks and then released in the interstellar ...medium. The mechanism through which CRs are released and the way in which they propagate still remain open issues. The main difficulty is the high non-linearity of the problem: CRs themselves excite the magnetic turbulence that confines them close to their sources. We solve numerically the coupled differential equations describing the evolution in space and time of the escaping particles and of the waves generated through the CR streaming instability. The warm ionized and warm neutral phases of the interstellar medium are considered. These phases occupy the largest fraction of the disc volume, where most supernovae explode, and are characterized by the significant presence of neutral particles. The friction between those neutrals and ions results in a very effective wave damping mechanism. It is found that streaming instability affects the propagation of CRs even in the presence of ion-neutral friction. The diffusion coefficient can be suppressed by more than a factor of ∼2 over a region of few tens of pc around the remnant. The suppression increases for smaller distances. The propagation of ≈10 GeV particles is affected for several tens of kiloyears after escape, while ≈1 TeV particles are affected for few kiloyears. This might have a great impact on the interpretation of gamma-ray observations of molecular clouds located in the vicinity of supernova remnants.
Cosmic rays are usually assumed to be the main ionization agent for the interior of molecular clouds, where UV and X-ray photons cannot penetrate. Here, we test this hypothesis by limiting ourselves ...to the case of diffuse clouds and assuming that the average cosmic ray spectrum inside the Galaxy is equal to the one at the position of the Sun as measured by Voyager 1 and AMS-02. To calculate the cosmic ray spectrum inside the clouds, we solve the 1D transport equation taking into account advection, diffusion, and energy losses. While outside the cloud particles diffuse, in its interior they are assumed to gyrate along magnetic field lines because ion-neutral friction is effective in damping all the magnetic turbulence. We show that ionization losses effectively reduce the CR flux in the cloud interior for energies below ≈100 MeV, especially for electrons, in such a way that the ionization rate decreases by roughly two order of magnitude with respect to the case where losses are neglected. As a consequence, the predicted ionization rate is more than 10 times smaller than the one inferred from the detection of molecular lines. We discuss the implication of our finding in terms of spatial fluctuation of the Galactic cosmic ray spectra and possible additional sources of low-energy cosmic rays.
Cosmic rays that escape their acceleration site interact with the ambient medium and produce gamma rays as the result of inelastic proton-proton collisions. The detection of such diffuse emission may ...reveal the presence of an accelerator of cosmic rays, and also constrain the cosmic ray diffusion coefficient in its vicinity. Preliminary results in this direction have been obtained in the last few years from studies of the gamma-ray emission from molecular clouds located in the vicinity of supernova remnants, which are the prime candidates for cosmic ray production. Hints have been found for a significant suppression of the diffusion coefficient with respect to the average one in the Galaxy. However, most of these studies rely on the assumption of isotropic diffusion, which may not be very well justified. Here, we extend this study to the case in which cosmic rays that escape an accelerator diffuse preferentially along the magnetic field lines. As a first approximation, we further assume that particles are strongly magnetized and that their transport perpendicular to the magnetic field is mainly due to the wandering of the field lines. The resulting spatial distribution of runaway cosmic rays around the accelerator is, in this case, strongly anisotropic. An application of the model to the case of the supernova remnant W28 demonstrates how the estimates of the diffusion coefficient from gamma-ray observations strongly depend on the assumptions made on the isotropy (or anisotropy) of diffusion. For higher levels of anisotropy of the diffusion, larger values of the diffusion coefficient are found to provide a good fit to data. Thus, detailed models for the propagation of cosmic rays are needed in order to interpret in a correct way the gamma-ray observations.
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
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.
Cosmic ray production in superbubbles Vieu, T; Gabici, S; Tatischeff, V ...
Monthly notices of the Royal Astronomical Society,
03/2022, Letnik:
512, Številka:
1
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ABSTRACT
We compute the production of cosmic rays (CRs) in the dynamical superbubble (SB) produced by a cluster of massive stars. Stellar winds, supernova remnants, and turbulence are found to ...accelerate particles so efficiently that the non-linear feedback of the particles must be taken into account in order to ensure that the energy balance is not violated. High-energy particles do not scatter efficiently on the turbulence and escape quickly after each supernova explosion, which makes both their intensity inside the bubble and injection in the interstellar medium intermittent. On the other hand, the stochastic acceleration of low-energy particles hardens the spectra at GeV energies. Because CRs damp the turbulence cascade, this hardening is less pronounced when non-linearities are taken into account. Nevertheless, spectra with hard components extending up to 1–10 GeV and normalized to an energy density of 1–100 eV cm−3 are found to be typical signatures of CRs produced in SBs. Efficient shock reacceleration within compact clusters is further shown to produce hard, slightly concave spectra, while the presence of a magnetized shell is shown to enhance the confinement of CRs in the bubble and therefore the collective plasma effects acting on them. We eventually estimate the overall contribution of SBs to the Galactic CR content and show typical gamma-ray spectra expected from hadronic interactions in SB shells. In both cases, a qualitative agreement with observations is obtained.
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.
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