# Felix Aharonian's research while affiliated with The Cambridge Institute For Advanced Studies and other places

## Publications (115)

Article
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The Fermi bubbles are giant, γ-ray-emitting lobes emanating from the nucleus of the Milky Way discovered in ~1–100 GeV data collected by the Large Area Telescope on board the Fermi Gamma-Ray Space Telescope. Previous work has revealed substructure within the Fermi bubbles that has been interpreted as a signature of collimated outflows from the Gala...
Article
Full-text available
Galaxy clusters can be sources of high-energy (HE) γ-ray radiation due to the efficient acceleration of particles exceeding EeV energies. At present, though, the only candidate for emitting HE γ-rays is the Coma cluster, towards which an excess of γ-ray emission has been detected by the Fermi Large Area Telescope (LAT). Using ∼12.3 years of Fermi-L...
Article
We critically assess limits on the maximum energy of protons accelerated within superbubbles around massive stellar clusters, considering a number of different scenarios. In particular, we derive under which circumstances acceleration of protons above peta-electronvolt (PeV) energies can be expected. While the external forward shock of the superbub...
Preprint
Full-text available
We critically assess limits on the maximum energy of protons accelerated within superbubbles around massive stellar clusters, considering a number of different scenarios. In particular, we derive under which circumstances acceleration of protons above peta-electronvolt (PeV) energies can be expected. While the external forward shock of the superbub...
Article
The recent discovery of a new population of ultrahigh-energy gamma-ray sources with spectra extending beyond 100 TeV revealed the presence of Galactic PeVatrons—cosmic-ray factories accelerating particles to PeV energies. These sources, except for the one associated with the Crab Nebula, are not yet identified. With an extension of 1° or more, most...
Preprint
Full-text available
The recent discovery of a new population of ultra-high-energy gamma-ray sources with spectra extending beyond 100 TeV revealed the presence of Galactic PeVatrons - cosmic-ray factories accelerating particles to PeV energies. These sources, except for the one associated with the Crab Nebula, are not yet identified. With an extension of 1 degree or m...
Article
Pulsars are factories of relativistic electrons and positrons that propagate away from the pulsar, eventually permeating our Galaxy. The acceleration and propagation of these cosmic particles are a matter of intense debate. In the last few years, we have had the opportunity to directly observe the injection of these particles into the interstellar...
Article
The HAWC Collaboration has reported the detection of an extended γ-ray emission around the Geminga and Monogem pulsars of a few degree extension. Very recently, the LHAASO Collaboration released also the data for an extended γ-ray emission around the pulsar PSR J0622+3749. This flux can be explained with electrons and positrons injected from these...
Article
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The active galactic nucleus 4C +28.07 is a flat spectrum radio quasar, one of the brightest at γ-ray energies. We study its multi-wavelength emission by analysing ∼12.3 years of Fermi-LAT data in the γ-ray band and Swift-XRT/UVOT available data in X-ray and Optical-to-Ultraviolet bands. In the γ-ray band, five flaring periods have been detected, an...
Preprint
Full-text available
The Fermi Bubbles are giant, γ-ray emitting lobes emanating from the nucleus of the Milky Way [1, 2] discovered in ∼1-100 GeV data collected by the Large Area Telescope on board the Fermi Gamma-Ray Space Telescope [3]. Previous work [4] has revealed substructure within the Fermi Bubbles that has been interpreted as a signature of collimated outflow...
Preprint
Cosmic Rays (CRs) propagating through dense molecular clouds (MCs) produce gamma rays which carry direct information about the CR distribution throughout the Galaxy. Observations of gamma rays in different energy bands allow exploration of the average CR density in the Galactic Disk, the so-called level of the "CR Sea". Fermi-LAT observations have...
Preprint
Full-text available
Galaxy clusters can be sources of high-energy (HE) $\gamma$-ray radiation due to the efficient acceleration of particles exceeding EeV energies. At present, though, the only candidate for emitting HE $\gamma$-rays is the Coma cluster, towards which an excess of $\gamma$-ray emission has been detected by the Fermi Large Area Telescope (LAT). Using $... Preprint The active galactic nucleus 4C +28.07 is a flat spectrum radio quasar, one of the brightest at$\gamma$-ray energies. We study its multi-wavelength emission by analysing$\sim12.3$years of \textit{Fermi-LAT} data in the$\gamma$-ray band and \textit{Swift-XRT/UVOT} available data in X-ray and Optical-to-Ultraviolet bands. In the$\gamma$-ray band,... Preprint Full-text available Star-forming regions have been proposed as potential Galactic cosmic-ray accelerators for decades. Cosmic ray acceleration can be probed through observations of gamma-rays produced in inelastic proton-proton collisions, at GeV and TeV energies. We analyze more than 11 years of Fermi-LAT data from the direction of Westerlund 2, one of the most massi... Conference Paper Full-text available Conference Paper Full-text available Preprint The propagation of cosmic-ray electrons and positrons in the proximity of the Geminga pulsar is examined considering the transition from the quasi-ballistic, valid for the most recently injected particles, to the diffusive transport regime. For typical interstellar values of the diffusion coefficient, the quasi-ballistic regime dominates the lepton... Article Full-text available The extension of the cosmic-ray spectrum beyond 1 petaelectronvolt (PeV; 10¹⁵ electronvolts) indicates the existence of the so-called PeVatrons—cosmic-ray factories that accelerate particles to PeV energies. We need to locate and identify such objects to find the origin of Galactic cosmic rays¹. The principal signature of both electron and proton P... Article X-ray observations of kilo-parsec scale jets indicate that a synchrotron origin of the sustained non-thermal emission is likely. This requires distributed acceleration of electrons up to near PeV energies along the jet. The underlying acceleration mechanism is still unclear. Shear acceleration is a promising candidate, as velocity-shear stratificat... Article Star-forming regions have been proposed as potential Galactic cosmic ray accelerators for decades. Cosmic ray acceleration can be probed through observations of gamma-rays produced in inelastic proton-proton collisions, at GeV and TeV energies. In this paper, we analyse more than 11 years of Fermi-LAT data from the direction of Westerlund 2, one of... Preprint Star-forming regions have been proposed as potential Galactic cosmic-ray accelerators for decades. Cosmic-ray acceleration can be probed through observations of gamma-rays produced in inelastic proton-proton collisions, at GeV and TeV energies. In this paper, we analyze more than 11 years of Fermi-LAT data from the direction of Westerlund 2, one of... Preprint Full-text available X-ray observations of kilo-parsec scale jets indicate that a synchrotron origin of the sustained non-thermal emission is likely. This requires distributed acceleration of electrons up to near PeV energies along the jet. The underlying acceleration mechanism is still unclear. Shear acceleration is a promising candidate, as velocity-shear stratificat... Preprint The galactic diffuse$\gamma$-ray emission, as seen by Fermi Large Area Telescope (LAT), shows a sharp peak in the region around 4 kpc from the Galactic center, which can be interpreted either as due to an enhanced density of cosmic-ray accelerators or to a modification of the particle diffusion in that region. Observations of$\gamma$-rays origina... Article The galactic diffuse γ -ray emission, as seen by Fermi Large Area Telescope (LAT), shows a sharp peak in the region around 4 kpc from the Galactic center, which can be interpreted either as due to an enhanced density of cosmic-ray accelerators or to a modification of the particle diffusion in that region. Observations of γ -rays originating in mole... Preprint Full-text available Recently, a diffuse emission of 1-100 GeV$\gamma$-rays has been detected from the direction of Andromeda. The emission is centered on the galaxy, and extends for$\sim 100-200$kpc away from its center. Explaining the extended$\gamma-$ray emission within the framework of standard scenarios for the escape of cosmic rays injected in the galactic di... Article Low-energy cosmic rays (LECRs) contribute substantially to the energy balance of the interstellar medium. They play also significant role in the heating and chemistry of gas, and, consequently, on the star formation process. Because of the slow propagation coupled with enhanced energy losses of subrelativistic particles, LECRs are concentrated arou... Preprint Low-energy cosmic rays (LECRs) contribute substantially to the energy balance of the interstellar medium. They play also significant role in the heating and chemistry of gas, and, consequently, on the star formation process. Because of the slow propagation coupled with enhanced energy losses of subrelativistic particles, LECRs are concentrated arou... Preprint Full-text available Cutoff energy in a synchrotron radiation spectrum of a supernova remnant (SNR) contains a key parameter of ongoing particle acceleration. We systematically analyze 11 young SNRs, including all historical SNRs, to measure the cutoff energy, thus shedding light on the nature of particle acceleration at the early stage of SNR evolution. The nonthermal... Article We analyze the energy distributions of final (stable) products—gamma rays, neutrinos, and electrons—produced in inelastic proton–proton collisions in the PeV energy regime. We also calculate the energy spectrum of synchrotron radiation from secondary electrons, assuming that these are promptly cooled in the surrounding magnetic field. We show that... Article Full-text available We report the analysis of the Fermi-Large Area Telescope data from six nearby giant molecular clouds (MCs) belonging to the Gould Belt and the Aquila Rift regions. The high statistical γ-ray spectra above 3 GeV well described by power laws make it possible to derive precise estimates of the cosmic-ray (CR) distribution in the MCs. The comparison of... Preprint Full-text available We report the analysis of the Fermi-Large Area Telescope data from six nearby giant molecular clouds (MCs) belonging to the Gould Belt and the Aquila Rift regions. The high statistical {\gamma}-ray spectra above 3 GeV well described by power laws make it possible to derive precise estimates of the cosmic-ray (CR) distribution in the MCs. The compar... Preprint We analyze the energy distributions of final (stable) products - gamma rays, neutrinos, and electrons - produced in inelastic proton-proton collisions in the PeV energy regime. We also calculate the energy spectrum of synchrotron radiation from secondary electrons, assuming that these are promptly cooled in the surrounding magnetic field. We show t... Preprint We present the analysis of 9.7 years Fermi-LAT data of the middle-aged supernova remnant W44 and the massive molecular gas complex that surrounds it. We derived a high-quality spectral energy distribution of gamma-radiation of the shell over three decades. The very hard spectrum below 1 GeV supports the earlier claims regarding the hadronic origin... Preprint We report the detection of high-energy gamma-ray signal towards the young star-forming region, W40. Using 10-year Pass 8 data from the Fermi Large Area Telescope (Fermi-LAT), we extracted an extended gamma-ray excess region with a significance of about 18sigma. The radiation has a spectrum with a photon index of 2.49 +/- 0.01. The spatial correlati... Article We report the detection of high-energy γ -ray signal towards the young star-forming region, W40. Using 10-yr Pass 8 data from the Fermi Large Area Telescope ( Fermi -LAT), we extracted an extended γ -ray excess region with a significance of ~18 σ . The radiation has a spectrum with a photon index of 2.49 ± 0.01. The spatial correlation with the ion... Article In nuclei of starburst galaxies (SBGs), the combination of an enhanced rate of supernova explosions and a high gas density suggests that cosmic rays (CRs) can be efficiently produced, and that most of them lose their energy before escaping these regions, resulting in a large flux of secondary products, including neutrinos. Although the flux inferre... Article Full-text available High energy γ-rays from giant molecular clouds (GMCs) carry direct information about the spatial and energy distributions of galactic cosmic rays (CRs). The recently released catalogs of GMCs contain sufficiently massive clouds to be used as barometers for probing, through their γ-ray emission, the density of CRs throughout the galactic disk. Based... Preprint Analyzing Chandra data of Tycho's supernova remnant (SNR) taken in 2000, 2003, 2007, 2009, and 2015, we search for time variable features of synchrotron X-rays in the southwestern part of the SNR, where stripe structures of hard X-ray emission were previous found. By comparing X-ray images obtained at each epoch, we discover a knot-like structure i... Preprint Full-text available The synchrotron cooling of relativistic electrons is one of the most effective radiation mechanisms in astrophysics. It not only accompanies the process of particle acceleration but also has feedback on the formation of the energy distribution of the parent electrons. The radiative cooling time of electrons decreases with energy as$t_{\rm syn} \pr...
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We report on the detection of very high energy (VHE; E > 100 GeV) gamma-ray emission from the BL Lac objects KUV 00311-1938 and PKS 1440-389 with the High Energy Stereoscopic System (H.E.S.S.). H.E.S.S. observations were accompanied or preceded by multiwavelength observations with Fermi/LAT, XRT and UVOT onboard the Swift satellite, and ATOM. Based...
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We present results of spectrometric studies based on the observations of very strong 3C 279 flares in high and very-high-energy bands and discuss their implications regarding the origin of radiation mechanisms. The FSRQ 3C 279 (z=0.536) is one of the most luminous gamma-ray emitting AGN. It shows variability on time scales down to minutes during st...
Article
The Crab Nebula is an extreme particle accelerator that boosts the energy of electrons up to a few PeV ($10^{15} \ \rm eV$), close to the maximum energy allowed theoretically. The physical conditions in the acceleration site and the nature of the acceleration process itself remain highly uncertain. The key information about the highest-energy accel...
Article
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...
Preprint
Full-text available
The Crab Nebula is an extreme particle accelerator boosting the energy of electrons up to a few PeV ($10^{15} \ \rm eV$), close to the maximum energy allowed by theory. The physical conditions in the acceleration site and the nature of the acceleration process itself remain highly uncertain. The key information about the highest energy accelerated...
Preprint
Full-text available
We present a numerical study of the properties of the flow produced by the collision of a magnetized anisotropic pulsar wind with the circumbinary environment. We focus on studying the impact of the high wind magnetization on the geometrical structure of the shocked flow. This work is an extension of our earlier studies that focused on a purely hyd...
Preprint
Full-text available
In nuclei of starburst galaxies, the combination of an enhanced rate of supernova explosions and a high gas density suggests that cosmic rays can be efficiently produced, and that most of them lose their energy before escaping these regions, resulting in a large flux of secondary products, including neutrinos. Although the flux inferred from an ind...
Article
We argue that the anomalously high fluxes of positrons and antiprotons found in cosmic rays (CR) can be satisfactorily explained by introducing two additional elements to the current “standard” paradigm of Galactic CRs. First, we propose that the antiparticles are effectively produced in interactions of primary CRs with the surrounding gas not only...
Article
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...