arXiv subjects
Publications and source records attributed to Stefano Gabici.
We will here discuss how the gamma-ray emission from molecular clouds can be used to probe the cosmic ray flux in distant regions of the Galaxy and to constrain the highly unknown cosmic ray diffusion coefficient. In particular we will discuss the GeV to TeV emission from runaway cosmic rays penetrating molecular clouds close to young and old supernova remnants and in molecular clouds illuminated by the background cosmic ray flux.
Observations of molecular clouds in the gamma ray domain provide us with a tool to study the distribution of cosmic rays in the Galaxy. This is because cosmic rays can penetrate molecular clouds, undergo hadronic interactions in the dense gas, and produce neutral pions that in turn decay into gamma rays. The detection of this radiation allows us to estimate the spectrum and intensity of cosmic rays at the cloud's position. Remarkably, this fact can be used to constrain the cosmic ray diffusion coefficient at specific locations in the Galaxy.
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 the 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 exhibit 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.
Diffusive shock acceleration operating at expanding supernova remnant shells is by far the most popular model for the origin of galactic cosmic rays. Despite the general consensus received by this model, an unambiguous and conclusive proof of the supernova remnant hypothesis is still missing. In this context, the recent developments in gamma ray astronomy provide us with precious insights into the problem of the origin of galactic cosmic rays, since production of gamma rays is expected both during the acceleration of cosmic rays at supernova remnant shocks and during their subsequent propagation in the interstellar medium. In particular, the recent detection of a number of supernova remnants at TeV energies nicely fits with the model, but it still does not constitute a conclusive proof of it, mainly due to the difficulty of disentangling the hadronic and leptonic contributions to the observed gamma ray emission. In this paper, the most relevant cosmic-ray-related results of gamma ray astronomy are briefly summarized, and the foreseeable contribution of future gamma ray observations to the final solution of the problem of cosmic ray origin is discussed.
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 cosmic rays can leave the accelerator and diffusively reach the cloud. We consider the situation in which a molecular cloud is located in the proximity of a supernova remnant which is accelerating cosmic rays and gradually releasing them into the interstellar medium. We calculate the multiwavelength spectrum from radio to gamma rays which emerges 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 from 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 counterpart in other wavelengths, might be associated with clouds illuminated by cosmic rays coming from a nearby source.
Kinetic approaches provide an effective description of the process of particle acceleration at shock fronts and allow to take into account the dynamical reaction of the accelerated particles as well as the amplification of the turbulent magnetic field as due to streaming instability. The latter does in turn affect the maximum achievable momentum and thereby the acceleration process itself, in a chain of causality which is typical of non-linear systems. Here we provide a technical description of two of these kinetic approaches and show that they basically lead to the same conclusions. In particular we discuss the effects of shock modification on the spectral shape of the accelerated particles, on the maximum momentum, on the thermodynamic properties of the background fluid and on the escaping and advected fluxes of accelerated particles.
The recent HESS detections of supernova remnant shells in TeV gamma-rays confirm the theoretical predictions that supernova remnants can operate as powerful cosmic ray accelerators. If these objects are responsible for the bulk of galactic cosmic rays, then they should accelerate protons and nuclei to 10^15 eV and beyond, i.e. act as cosmic PeVatrons. The model of diffusive shock acceleration allows, under certain conditions, acceleration of particles to such high energies and their gradual injection into the interstellar medium, mainly during the Sedov phase of the remnant evolution. The most energetic particles are released first, while particles of lower energies are more effectively confined in the shell, and are released at later epochs. Thus the spectrum of nonthermal paticles inside the shell extends to PeV energies only during a relatively short period of the evolution of the remnant. For this reason one may expect spectra of secondary gamma-rays and neutrinos extending to energies beyond 10 TeV only from T \lesssim 1000 yr old supernova remnants. On the other hand, if by a chance a massive gas cloud appears in the \lesssim 100 pc vicinity of the supernova remnant, ``delayed'' multi-TeV signals of gamma-rays and neutrinos arise when the most energetic partices emerged from the supernova shell reach the cloud. The detection of such delayed emission of multi-TeV gamma-rays and neutrinos allows indirect identification of the supernova remnant as a particle PeVatron.
If highly efficient, cosmic ray production can have a significant effect on the X-ray emission from SNRs as well as their dynamical evolution. Using hydrodynamical simulations including diffusive shock acceleration, we produce spectra for both the thermal and nonthermal forward shock emission. For a given ambient density and explosion energy, we find that the position of the forward shock at a given age is a strong function of the acceleration efficiency, providing a signature of cosmic-ray production. Using an approximate treatment for the ionization state of the plasma, we investigate the effects of slow vs. rapid heating of the postshock electrons on the ratio of thermal to nonthermal X-ray emission at the forward shock. We also investigate the effects of magnetic field strength on the observed spectrum for efficient cosmic-ray acceleration. The primary effect of a large field is a considerable flattening of the nonthermal spectrum in the soft X-ray band. Spectral index measurements from X-ray observations may thus be indicators of the postshock magnetic field strength. The predicted gamma-ray flux from inverse-Compton (IC) scattering and neutral pion decay is strongly affected by the ambient conditions and, for the particular parameters used in our examples, the IC emission at E ~ 1 TeV exceeds that from pion decay, although at both lower and higher energies this trend is reversed for cases of high ambient density. More importantly, high magnetic fields produce a steepening of the electron spectrum over a wide energy range which may make it more difficult to differentiate between IC and pion-decay emission solely by spectral shape.
Clusters of galaxies and the large scale filaments that connect neighboring clusters are expected to be sites of acceleration of charged particles and sources of non-thermal radiation from radio frequencies to gamma rays. Gamma rays are particularly interesting targets of investigation, since they may provide precious information on the nature and efficiency of the processes of acceleration and magnetic confinement of hadrons within clusters of galaxies. Here we review the status of viable scenarios that lead to the production of gamma rays from large scale structures and are compatible with the multifrequency observations that are already available. We also discuss the possibility of detection of gamma rays with space-borne telescopes such as GLAST and ground based Cherenkov telescopes, and the physical information that may be gathered from such observations.
We discuss the possibility of observing ultra high energy cosmic ray sources inhigh energy gamma rays. Protons propagating away from their accelerators produce secondary electrons during interactions with cosmic microwave background photons. These electrons start an electromagnetic cascade that results in a broad band gamma ray emission. We show that in a magnetized Universe ($B \gtrsim 10^{-12}$ G) such emission is likely to be too extended to be detected above the diffusebackground. A more promising possibility comes from the detection of synchrotron photons from the extremely energetic secondary electrons. Although this emission is produced in a rather extended region of size $\sim 10Mpc$, it is expected to be point-like and detectable at GeV energies if the intergalactic magnetic field is at the nanogauss level.
It is believed that the observed diffuse gamma ray emission from the galactic plane is the result of interactions between cosmic rays and the interstellar gas. Such emission can be amplified if cosmic rays penetrate into dense molecular clouds. The propagation of cosmic rays inside a molecular cloud has been studied assuming an arbitrary energy and space dependent diffusion coefficient. If the diffusion coefficient inside the cloud is significantly smaller compared to the average one derived for the galactic disk, the observed gamma ray spectrum appears harder than the cosmic ray spectrum, mainly due to the slower penetration of the low energy particles towards the core of the cloud. This may produce a great variety of gamma ray spectra.
The dynamical reaction of the particles accelerated at a shock front by the first order Fermi process can be determined within kinetic models that account for both the hydrodynamics of the shocked fluid and the transport of the accelerated particles. These models predict the appearance of multiple solutions, all physically allowed. We discuss here the role of injection in selecting the {\it real} solution, in the framework of a simple phenomenological recipe, which is a variation of what is sometimes referred to as {\it thermal leakage}. In this context we show that multiple solutions basically disappear and when they are present they are limited to rather peculiar values of the parameters.
Particle acceleration in collisionless astrophysical shocks, i.e., diffusive shock acceleration (DSA), is the most likely mechanism for producing cosmic rays, at least below 10^{15} eV. Despite the success of this theory, several key elements, including the injection of thermal particles, remains poorly understood. We investigate injection in strongly nonlinear shocks by comparing a semi-analytic model of DSA with a Monte Carlo model. These two models treat injection quite differently and we show, for a particular set of parameters, how these differences influence the overall acceleration efficiency and the shape of the broad-band distribution function.
Merger and accretion shocks in clusters of galaxies can accelerate particles via first order Fermi process. Since this mechanism is believed to be intrinsically efficient, shocks are expected to be modified by the backreaction of the accelerated particles. Such a modification might induce appreciable effects on the non--thermal emission from clusters and a suppression of the heating of the gas at strong shocks. Here we consider in particular the gamma ray emission and we discuss the capability of Cherenkov telescopes such as HESS to detect clusters at TeV energies.
There is a growing evidence that extended radio halos are most likely generated by electrons reaccelerated via some kind of turbulence generated in the cluster volume during major mergers. It is well known that Alfvén waves channel most of their energy flux in the acceleration of relativistic particles. Much work has been done recently to study this phenomenon and its consequences for the explanation of the observed non-thermal phenomena in clusters of galaxies. We investigate here the problem of particle-wave interactions in the most general situation in which relativistic electrons, thermal protons and relativistic protons exist within the cluster volume. The interaction of all these components with the waves, as well as the turbulent cascading and damping processes of Alfvén waves, are treated in a fully time-dependent way. This allows us to calculate the spectra of electrons, protons and waves at any fixed time. The {\it Lighthill} mechanism is invoked to couple the fluid turbulence, supposedly injected during cluster mergers, to MHD turbulence. We find that present observations of non-thermal radiation from clusters of galaxies are well described within this approach, provided the fraction of relativistic hadrons in the intracluster medium (ICM) is smaller than 5-10 %.
Particle acceleration is expected to take place at shocks that form during the process of large scale structure formation. Electrons accelerated at such shocks can upscatter a small fraction of the photons in the cosmic microwave background up to the gamma ray band. Here we make predictions about the detectability of the $γ$--ray emission from forming clusters of galaxies with future GeV and TeV gamma ray telescopes. We also estimate the contribution of these sources to the extragalactic diffuse gamma ray background.
The nonthermal radiation observed from a handfull of clusters of galaxies (CG) is the proof that particle acceleration occurs in the intracluster medium (ICM). It is often believed that shock surfaces associated with either mergers of CG, or with the cosmological inflow of matter onto clusters during structure formation may be the sites for acceleration. We discuss here the effectiveness of shock acceleration in the ICM, stressing that merger related shocks are typically weak, at least for the so-called major mergers. We investigate the implications of shock strenghts for gamma ray emission from single CG and for their detectability with future gamma ray satellites (such as GLAST and AGILE) and ground based Cherenkov telescopes. We also discuss the contribution of clusters to the extragalactic diffuse gamma ray background (EDGRB).
Particle acceleration in clusters of galaxies is expected to take place at both merger and accretion shocks. The electron component may be energized to energies of several TeV, and upscatter a small fraction of the photons in the cosmic microwave background up to gamma ray energies. We address here the issue of the detectability of the gamma radiation generated either in merger events or during the accretion of matter onto cluster potential wells. The predictions are specialized to the cases of GLAST and AGILE, for which a few tens of clusters are expected to be detected, and to Cherenkov telescopes, for which however the perspectives do not appear to be optimistic.