arXiv subjects

Felix Aharonian

Publications and source records attributed to Felix Aharonian.

At least 73 records · Page 4Linked to original sources

arXiv2019

Supernova remnants in clumpy media: particle propagation and gamma-ray emission

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 in shaping the hadronic gamma-ray spectrum. Here we present a detailed numerical study about the penetration of relativistic protons into clumps which 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 does not reflect anymore the acceleration spectrum of protons, resulting substantially modified inside the clumps due 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 to the non-thermal X-ray variability observed in some hot spots 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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arXiv2019

NuSTAR observations of the supernova remnant RX J1713.7$-$3946

The shock waves of supernova remnants (SNRs) are prominent candidates for the acceleration of the Galactic cosmic rays. SNR RX J1713.7$-$3946 is one well-studied particle accelerator in our Galaxy because of its strong non-thermal X-ray and gamma-ray radiation. We have performed NuSTAR (3-79 keV) observations of the northwest rim of RX J1713.7$-$3946, where is the brightest part in X-ray and the shock speed is about 4000 km s$^{-1}$. The spatially resolved X-ray emission from RX J1713.7$-$3946 is detected up to 20 keV for the first time. The hard X-ray image in 10-20 keV is broadly similar to the soft-band image in 3-10 keV. The typical spectrum is described by power-law model with exponential cutoff with the photon index $Γ$=2.15 and the cutoff energy $\varepsilon_c$=18.8 keV. Using a synchrotron radiation model from accelerated electrons in the loss-limited case, the cutoff energy parameter ranges 0.6-1.9 keV, varying from region to region. Combined with the previous measurement of the shock speed, the acceleration of electrons is close to the Bohm-limit regime in the outer edge, while the standard picture of accelerated particles limited by synchrotron radiation in SNR shock is not applicable in the inner edge and the filamentary structure.

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arXiv2019

Cosmic ray transport and radiative processes in nuclei of starburst galaxies

The high rate of star formation and supernova explosions of starburst galaxies make them interesting sources of high energy radiation. Depending upon the level of turbulence present in their interstellar medium, the bulk of cosmic rays produced inside starburst galaxies may lose most of their energy before escaping, thereby making these sources behave as calorimeters, at least up to some maximum energy. Contrary to previous studies, here we investigate in detail the conditions under which cosmic ray confinement may be effective for electrons and nuclei and we study the implications of cosmic ray confinement in terms of multifrequency emission from starburst nuclei and production of high energy neutrinos. The general predictions are then specialized to three cases of active starbursts, namely M82, NGC253 and Arp220. Both primary and secondary electrons, as well as electron-positron pairs produced by gamma ray absorption inside starburst galaxies are taken into account. Electrons and positrons produced as secondary products of hadronic interactions are found to be responsible for most of the emission of leptonic origin. In particular, synchrotron emission of very high energy secondary electrons produces an extended emission of hard X-rays that represent a very interesting signature of hadronic process in starburst galaxies, potentially accessible to current and future observations in the X-ray band. A careful understanding of both the production and absorption of gamma rays in starburst galaxies is instrumental to the assessment of the role of these astrophysical sources as sources of high energy astrophysical neutrinos.

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arXiv2019

Nuclear $γ$-ray emission from very hot accretion flows

Optically thin accretion plasmas can reach ion temperatures $T_{\rm i} \geq 10^{10}$K and thus trigger nuclear reactions. Using a large nuclear interactions network, we studied the radial evolution of the chemical composition of the accretion flow toward the black hole and computed the emissivity in nuclear $γ$-ray lines. In the advection dominated accretion flow (ADAF) regime, CNO and heavier nuclei are destroyed before reaching the last stable orbit. The overall luminosity in the de-excitation lines for a solar composition of plasma can be as high as few times $10^{-5}$ the accretion luminosity ($\dot{M}c^2$) and can be increased for heavier compositions up to $10^{-3}$. The efficiency of transformation of the kinetic energy of the outflow into high energy ($\geq 100$~MeV) $γ$-rays through the production and decay of $π^0$-mesons can be higher, up to $10^{-2}$ of the accretion luminosity. We show that in the ADAF model up to 15 percent of the mass of accretion matter can `evaporate' in the form of neutrons.

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arXiv2019

Gamma-ray emission of hot astrophysical plasmas

Very hot plasmas with ion temperature exceeding $10^{10}$ K can be formed in certain astrophysical environments. The distinct radiation signature of such plasmas is the $γ$-ray emission dominated by the prompt de-excitation nuclear lines and $π^0$-decay $γ$-rays. Using a large nuclear reaction network, we compute the time evolution of the chemical composition of such hot plasmas and their $γ$-ray line emissivity. At higher energies, we provide simple but accurate analytical presentations for the $π^0$-meson production rate and the corresponding $π^0\to2γ$ emissivity derived for the Maxwellian distribution of protons. We discuss the impact of the possible deviation of the high energy tail of the particle distribution function from the "nominal" Maxwellian distribution on the plasma $γ$-ray emissivity.

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arXiv2019

Non-Equilibrium Chemistry and Destruction of CO by X-ray Flares

Sources of X-rays such as active galactic nuclei and X-ray binaries are often variable by orders of magnitude in luminosity over timescales of years. During and after these flares the surrounding gas is out of chemical and thermal equilibrium. We introduce a new implementation of X-ray radiative transfer coupled to a time-dependent chemical network for use in 3D magnetohydrodynamical simulations. A static fractal molecular cloud is irradiated with X-rays of different intensity, and the chemical and thermal evolution of the cloud are studied. For a simulated $10^5$ M$_\odot$ fractal cloud an X-ray flux $<0.01$ erg cm$^{-2}$ s$^{-1}$ allows the cloud to remain molecular, whereas most of the CO and H$_2$ are destroyed for a flux of $>1$ erg cm$^{-2}$ s$^{-1}$. The effects of an X-ray flare, which suddenly increases the X-ray flux by $10^5 \times$ are then studied. A cloud exposed to a bright flare has 99% of its CO destroyed in 10-20 years, whereas it takes $>10^3$ years for 99% of the H$_2$ to be destroyed. CO is primarily destroyed by locally generated far-UV emission from collisions between non-thermal electrons and H$_2$; He$^+$ only becomes an important destruction agent when the CO abundance is already very small. After the flare is over, CO re-forms and approaches its equilibrium abundance after $10^3-10^5$ years. This implies that molecular clouds close to Sgr A$^*$ in the Galactic Centre may still be out of chemical equilibrium, and we predict the existence of clouds near flaring X-ray sources in which CO has been mostly destroyed but H is fully molecular.

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arXiv2019

Massive Stars as Major Factories of Galactic Cosmic Rays

The identification of major contributors to the locally observed fluxes of Cosmic Rays (CRs) is a prime objective towards the resolution of the long-standing enigma of CRs. We report on a compelling similarity of the energy and radial distributions of multi-TeV CRs extracted from observations of very high energy (VHE) $γ$-rays towards the Galactic Center (GC) and two prominent clusters of young massive stars, Cyg~OB2 and Westerlund~1. This resemblance we interpret as a hint that CRs responsible for the diffuse VHE $γ$-ray emission from the GC are accelerated by the ultracompact stellar clusters located in the heart of GC. The derived $1/r$ decrement of the CR density with the distance from a star cluster is a distinct signature of continuous, over a few million years, CR injection into the interstellar medium. The lack of brightening of the $γ$-ray images toward the stellar clusters excludes the leptonic origin of $γ$-radiation. The hard, $\propto E^{-2.3}$ type power-law energy spectra of parent protons continues up to $\sim$ 1 PeV. The efficiency of conversion of kinetic energy of stellar winds to CRs can be as high as 10 percent implying that the young massive stars may operate as proton PeVatrons with a dominant contribution to the flux of highest energy galactic CRs.

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arXiv2018

Detection of polarized gamma-ray emission from the Crab nebula with Hitomi Soft Gamma-ray Detector

We present the results from the Hitomi Soft Gamma-ray Detector (SGD) observation of the Crab nebula. The main part of SGD is a Compton camera, which in addition to being a spectrometer, is capable of measuring polarization of gamma-ray photons. The Crab nebula is one of the brightest X-ray / gamma-ray sources on the sky, and, the only source from which polarized X-ray photons have been detected. SGD observed the Crab nebula during the initial test observation phase of Hitomi. We performed the data analysis of the SGD observation, the SGD background estimation and the SGD Monte Carlo simulations, and, successfully detected polarized gamma-ray emission from the Crab nebula with only about 5 ks exposure time. The obtained polarization fraction of the phase-integrated Crab emission (sum of pulsar and nebula emissions) is (22.1 $\pm$ 10.6)% and, the polarization angle is 110.7$^o$ + 13.2 / $-$13.0$^o$ in the energy range of 60--160 keV (The errors correspond to the 1 sigma deviation). The confidence level of the polarization detection was 99.3%. The polarization angle measured by SGD is about one sigma deviation with the projected spin axis of the pulsar, 124.0$^o$ $\pm$0.1$^o$.

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arXiv2018

On the anomalously large extension of the Pulsar Wind Nebula HESS J1825-137

The very high energy (VHE) gamma-ray emission reported from a number of pulsar wind nebulae (PWNe) is naturally explained by the inverse Compton scattering of multi-TeV electrons. However, the physical dimensions of some gamma-ray-emitting PWNe significantly exceed the scales anticipated by the standard hydrodynamical paradigm of PWN formation. The most "disturbing" case in this regard is HESS J1825-137, which extends to distances $r\approx70\rm\,pc$ from the central pulsar PSR J1826-1334. If the gamma-ray emission is indeed produced inside the PWN, but not by electrons that escaped the nebula and diffuse in the interstellar medium (ISM), the formation of such an anomalously extended plerion could be realized, in a diluted environment with the hydrogen number density $n_{ism}\le10^{-2}\rm\,cm^{-3}$. In this paper, we explore an alternative scenario assuming that the pulsar responsible for the formation of the nebula initially had a very short rotation period. In this case, the sizes of both the PWN and the surrounding supernova remnant depend on the initial pulsar period, the braking index, and the ISM density. To check the feasibility of this scenario, we study the parameter space that would reproduce the size of HESS J1825-137. We show that this demand can be achieved if the braking index is small, $n\leq2$ and the pulsar birth period is short, $P_{\rm b}\simeq1\rm\,ms$. This scenario can reproduce the wind termination position, which is expected at $R_{ts}\simeq0.03\rm\,pc$, only in a dense environment with $n_{ism}\geq\rm1\,cm^{-3}$. The requirement of the dense surrounding gas is supported by the presence of molecular clouds found in the source vicinity.

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arXiv2018

Analytical treatment for the development of electromagnetic cascades in intense magnetic fields

In a strong magnetic field, a high-energy photon can be absorbed and then produce an electron-positron pair. The produced electron/positron will in turn radiate a high-energy photon via synchrotron radiation, which then initiates a cascade. We built a one-dimensional Monte-Carlo code to study the development of the cascade especially after it reaches the saturated status, when almost all the energy of the primary particles transfers to the photons. The photon spectrum in this status has a cut-off due to the absorption by magnetic fields, which is much sharper than the exponential one. Below the cut-off, the spectral energy distribution (SED) manifest itself as a broken power-law with a spectral index of $0.5$ and $0.125$, respectively, below and above the broken energy. The SED can be fitted by a simple analytical function, which is solely determined by the product of the cascade scale $R$ and the magnetic field perpendicular to the motion of the particle B_{\perp}, with an accuracy better than 96\%. The similarity of the spectrum to that from the cascade in an isotropic black-body photon field is also studied.

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arXiv2018

On the shape of the gamma-ray spectrum around the "$π^0$-bump"

The "pion-decay" bump is a distinct signature of the differential energy spectrum of $γ$-rays between 100 MeV and 1 GeV produced in hadronic interactions of accelerated particles (cosmic rays) with the ambient gas. We use the recent parametrisations of relevant cross-sections to study the formation of the "pion-decay" bump. The $γ$-ray spectrum below the maximum of this spectral feature can be distorted because of contributions of additional radiation components, in particular, due to the bremsstrahlung of secondary electrons and positrons, the products of decays of $π^\pm$-mesons, accompanying the $π^0$-production. At energies below 100 MeV, a non-negligible fraction of $γ$-ray flux could originate from interactions of sub-relativistic heavy ions. We study the impact of these radiation channels on the formation of the overall $γ$-ray spectrum based on a time-dependent treatment of evolution of energy distributions of the primary and secondary particles in the $γ$-ray production region.

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arXiv2018

On the potential of Cherenkov Telescope Arrays and KM3 Neutrino Telescopes for the detection of extended sources

We discuss the discovery potential of extended very-high-energy (VHE) neutrino sources by the future KM3 Neutrino Telescope (KM3NeT) in the context of the constraining power of the Cherenkov Telescope Array (CTA), designed for deep surveys of the sky in VHE gamma rays. The study is based on a comparative analysis of sensitivities of KM3NeT and CTA. We show that a minimum gamma-ray energy flux of E^2ϕ_γ(10 TeV) > 1x10^{-12} TeV cm^{-2} s^{-1} is required to identify a possible neutrino counterpart with a 3σ significance and 10 years of KM3NeT observations with upgoing muons, if the source has an angular size of R_{src} = 0.1 deg and emits gamma rays with an E^{-2} energy spectrum through a full hadronic mechanism. This minimum gamma-ray flux is increased to the level of E^2ϕ_γ(10 TeV) > 2x10^{-11} TeV cm^{-2} s^{-1} in case of sources with radial extension of R_{src} = 2.0 deg. The analysis methods are applied to the supernova remnant RX J1713.7-3946 and the Galactic Center Ridge, as well as to the recent HAWC catalog of multi-TeV gamma-ray sources.

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arXiv2018

Energetic gamma-ray emission from solar flares

Recent advances in the $γ$-ray observations of solar flares by the \textit{Fermi} satellite, demand revisions in the hadronic $γ$-ray flux computation below 1 GeV. In this work we utilize recently updated pion production cross sections, along with an accurate description of low energy nuclear interactions. Applying these new interaction descriptions to model the \textit{Fermi} Large Area Telescope (LAT) solar flare data, we infer the primary particle spectral parameters. Application of this new cross section description leads to significantly different spectral parameters compared to those obtained previously. Furthermore, the inclusion of nuclei in these calculations leads to a primary spectrum that is generally harder than that required from proton only considerations. Lastly, the flare data at lower MeV energies, detected by the \textit{Fermi} Gamma-ray Burst Monitor (GBM), is demonstrated to provide additional low-energy spectral information.

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arXiv2018

The energy distribution of relativistic electrons in the kilo-parsec scale jet of M87 with Chandra

The X-ray emission from the jets in Active Galactic Nuclei (AGN) carries important information on the distributions of relativistic electrons and magnetic fields on large scales. We reanalyze archival Chandra observations on the jet of M87 from 2000 to 2016 with a total exposure of 1460 kiloseconds to explore the X-ray emission characteristics along the jet. We investigate the variability behaviours of the nucleus and the inner jet component HST-1, and confirm indications for day-scale X-ray variability in the nucleus contemporaneous to the 2010 high TeV gamma-ray state. HST-1 shows a general decline in X-ray flux over the last few years consistent with its synchrotron interpretation. We extract the X-ray spectra for the nucleus and all knots in the jet, showing that they are compatible with a single power-law within the X-ray band. There are indications of the resultant X-ray photon index to exhibit a trend, with slight but significant index variations ranging from $\simeq 2.2$ (e.g. in knot D) to $\simeq 2.4-2.6$ (in the outer knots F, A, and B). When viewed in a multi-wavelength context, a more complex situation is arising. Fitting the radio to X-ray spectral energy distributions (SEDs) assuming a synchrotron origin, we show that a broken power-law electron spectrum with break energy $E_b$ around $1~(300μG/B)^{1/2}$ TeV allows a satisfactorily description of the multi-band SEDs for most of the knots. However, in the case of knots B, C and D we find indications that an additional high energy component is needed to adequately reproduce the broadband SEDs. We discuss the implications and suggest that a stratified jet model may account for the differences.

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arXiv2018

Hitomi X-ray Observation of the Pulsar Wind Nebula G21.5$-$0.9

We present results from the Hitomi X-ray observation of a young composite-type supernova remnant (SNR) G21.5$-$0.9, whose emission is dominated by the pulsar wind nebula (PWN) contribution. The X-ray spectra in the 0.8-80 keV range obtained with the Soft X-ray Spectrometer (SXS), Soft X-ray Imager (SXI) and Hard X-ray Imager (HXI) show a significant break in the continuum as previously found with the NuSTAR observation. After taking into account all known emissions from the SNR other than the PWN itself, we find that the Hitomi spectra can be fitted with a broken power law with photon indices of $Γ_1=1.74\pm0.02$ and $Γ_2=2.14\pm0.01$ below and above the break at $7.1\pm0.3$ keV, which is significantly lower than the NuSTAR result ($\sim9.0$ keV). The spectral break cannot be reproduced by time-dependent particle injection one-zone spectral energy distribution models, which strongly indicates that a more complex emission model is needed, as suggested by recent theoretical models. We also search for narrow emission or absorption lines with the SXS, and perform a timing analysis of PSR J1833$-$1034 with the HXI and SGD. No significant pulsation is found from the pulsar. However, unexpectedly, narrow absorption line features are detected in the SXS data at 4.2345 keV and 9.296 keV with a significance of 3.65 $σ$. While the origin of these features is not understood, their mere detection opens up a new field of research and was only possible with the high resolution, sensitivity and ability to measure extended sources provided by an X-ray microcalorimeter.

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arXiv2017

Temperature Structure in the Perseus Cluster Core Observed with Hitomi

The present paper investigates the temperature structure of the X-ray emitting plasma in the core of the Perseus cluster using the 1.8--20.0 keV data obtained with the Soft X-ray Spectrometer (SXS) onboard the Hitomi Observatory. A series of four observations were carried out, with a total effective exposure time of 338 ks and covering a central region $\sim7'$ in diameter. The SXS was operated with an energy resolution of $\sim$5 eV (full width at half maximum) at 5.9 keV. Not only fine structures of K-shell lines in He-like ions but also transitions from higher principal quantum numbers are clearly resolved from Si through Fe. This enables us to perform temperature diagnostics using the line ratios of Si, S, Ar, Ca, and Fe, and to provide the first direct measurement of the excitation temperature and ionization temperature in the Perseus cluster. The observed spectrum is roughly reproduced by a single temperature thermal plasma model in collisional ionization equilibrium, but detailed line ratio diagnostics reveal slight deviations from this approximation. In particular, the data exhibit an apparent trend of increasing ionization temperature with increasing atomic mass, as well as small differences between the ionization and excitation temperatures for Fe, the only element for which both temperatures can be measured. The best-fit two-temperature models suggest a combination of 3 and 5 keV gas, which is consistent with the idea that the observed small deviations from a single temperature approximation are due to the effects of projection of the known radial temperature gradient in the cluster core along the line of sight. Comparison with the Chandra/ACIS and the XMM-Newton/RGS results on the other hand suggests that additional lower-temperature components are present in the ICM but not detectable by Hitomi SXS given its 1.8--20 keV energy band.

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arXiv2017

Atomic data and spectral modeling constraints from high-resolution X-ray observations of the Perseus cluster with Hitomi

The Hitomi SXS spectrum of the Perseus cluster, with $\sim$5 eV resolution in the 2-9 keV band, offers an unprecedented benchmark of the atomic modeling and database for hot collisional plasmas. It reveals both successes and challenges of the current atomic codes. The latest versions of AtomDB/APEC (3.0.8), SPEX (3.03.00), and CHIANTI (8.0) all provide reasonable fits to the broad-band spectrum, and are in close agreement on best-fit temperature, emission measure, and abundances of a few elements such as Ni. For the Fe abundance, the APEC and SPEX measurements differ by 16%, which is 17 times higher than the statistical uncertainty. This is mostly attributed to the differences in adopted collisional excitation and dielectronic recombination rates of the strongest emission lines. We further investigate and compare the sensitivity of the derived physical parameters to the astrophysical source modeling and instrumental effects. The Hitomi results show that an accurate atomic code is as important as the astrophysical modeling and instrumental calibration aspects. Substantial updates of atomic databases and targeted laboratory measurements are needed to get the current codes ready for the data from the next Hitomi-level mission.

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arXiv2017

Hitomi Observations of the LMC SNR N132D: Highly Redshifted X-ray Emission from Iron Ejecta

We present Hitomi observations of N132D, a young, X-ray bright, O-rich core-collapse supernova remnant in the Large Magellanic Cloud (LMC). Despite a very short observation of only 3.7 ks, the Soft X-ray Spectrometer (SXS) easily detects the line complexes of highly ionized S K and Fe K with 16-17 counts in each. The Fe feature is measured for the first time at high spectral resolution. Based on the plausible assumption that the Fe K emission is dominated by He-like ions, we find that the material responsible for this Fe emission is highly redshifted at ~800 km/s compared to the local LMC interstellar medium (ISM), with a 90% credible interval of 50-1500 km/s if a weakly informative prior is placed on possible line broadening. This indicates (1) that the Fe emission arises from the supernova ejecta, and (2) that these ejecta are highly asymmetric, since no blue-shifted component is found. The S K velocity is consistent with the local LMC ISM, and is likely from swept-up ISM material. These results are consistent with spatial mapping that shows the He-like Fe concentrated in the interior of the remnant and the S tracing the outer shell. The results also show that even with a very small number of counts, direct velocity measurements from Doppler-shifted lines detected in extended objects like supernova remnants are now possible. Thanks to the very low SXS background of ~1 event per spectral resolution element per 100 ks, such results are obtainable during short pointed or slew observations with similar instruments. This highlights the power of high-spectral-resolution imaging observations, and demonstrates the new window that has been opened with Hitomi and will be greatly widened with future missions such as the X-ray Astronomy Recovery Mission (XARM) and Athena.

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