arXiv subjects

Felix Aharonian

Publications and source records attributed to Felix Aharonian.

At least 127 records · Page 7Linked to original sources

arXiv2011

The Milky Way Heart: Investigating molecular gas and gamma-ray morphologies in the Central Molecular Zone

Since the discovery of a broad distribution of very high energy (VHE; >0.1 TeV) gamma-rays in the Central Molecular Zone (CMZ) of the Galaxy in 2006 by the HESS collaboration, the correlation of this emission with the integrated intensity of the CS(1-0) molecular line emission has inferred a hadronic origin for the gamma-rays. Here we describe the beginning of our investigation into the strength of this correlation utilising new multi-line millimeter data from the Mopra CMZ and HOP surveys and multi-wavelength GBT radio continuum observations towards the CMZ and compare these in detail with the diffuse TeV gamma-ray emission from HESS. The benefit of these new data is that they allow us to simultaneously observe and analyse correlations using a large number (>10) of molecular species, some of which contain their isotopologue pairs. The use of isotopologue pairs is especially powerful, since it allows one to analyse the optical depth of a number of different molecular species, thus investigating the nature of the correlation over a range of different physical conditions. Here we begin by comparing the integrated line emission and continuum radio emission with the diffuse gamma-ray emission, and, by using isotopologue pairs such as HCN/H$^{13}$CN, obtain optical depths throughout the CMZ corresponding to regions of both strong and weak gamma-ray emission. We find that the radio continuum better matches the peak of the gamma-ray emission, which corresponds to the more compact -- compared to the relatively coarse resolution of the gamma-ray images -- sources in the CMZ. Using the isotopologue pairs, we find that the optical depth at all positions and velocities within the CMZ are about 2--4. This is similar to that found for the CS(1--0) line and would underestimate the mass of the CMZ, potentially explaining why molecular line emission peaks appear offset from the gamma-ray peaks.

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arXiv2011

Gamma Ray Signal from the Pulsar Wind in the Binary Pulsar system PSR B1259-63/LS2883

Binary pulsar systems emit potentially detectable components of gamma ray emission due to Comptonization of the optical radiation of the companion star by relativistic electrons of the pulsar wind, both before and after termination of the wind. The recent optical observations of binary pulsar system PSR B1259-63/LS 2883 revealed radiation properties of the companion star which differ significantly from previous measurements. In this paper we study the implications of these observations for the interaction rate of the unshocked pulsar wind with the stellar photons and the related consequences for fluxes of high energy (HE) and very high energy (VHE) gamma rays. We show that the signal should be strong enough to be detected with Fermi close to the periastron passage, unless the pulsar wind is strongly anisotropic or the Lorentz factor of the wind is smaller than $10^3$ or larger that $10^5$. The higher luminosity of the optical star also has two important implications: (i) attenuation of gamma rays due to photon-photon pair production, and (ii) Compton drag of the unshocked wind. While the first effect has an impact on the lightcurve of VHE gamma rays, the second effect may significantly decrease the energy available for particle acceleration after termination of the wind.

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arXiv2011

The Fermi Bubbles: Giant, Multi-Billion-Year-Old Reservoirs of Galactic Center Cosmic Rays

Recently evidence has emerged for enormous features in the gamma-ray sky observed by the Fermi-LAT instrument: bilateral `bubbles' of emission centered on the core of the Galaxy and extending to around 10 kpc above and below the Galactic plane. These structures are coincident with a non-thermal microwave `haze' found in WMAP data and an extended region of X-ray emission detected by ROSAT. The bubbles' gamma-ray emission is characterised by a hard and relatively uniform spectrum, relatively uniform intensity, and an overall luminosity ~4 x 10^37 erg/s, around one order of magnitude larger than their microwave luminosity while more than order of magnitude less than their X-ray luminosity. Here we show that the bubbles are naturally explained as due to a population of relic cosmic ray protons and heavier ions injected by processes associated with extremely long timescale (>~8 Gyr) and high areal density star-formation in the Galactic center.

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arXiv2011

On the Detectability of High-Energy Galactic Neutrino Sources

With the arrival of km**3 volume scale neutrino detectors the chances to detect the first astronomical sources of TeV neutrinos will be dramatically increased. While the theoretical estimates of the neutrino fluxes contain large uncertainties, we can formulate the conditions for the detectability of certain neutrino sources phenomenologically. In fact, since most galactic neutrino sources are transparent for TeV gamma-rays, their detectability implies a minimum flux of the accompanying gamma-rays. For a typical energy-dependence of detection areas of km**3 volume neutrino detectors, we obtain the quantitative condition I_gamma(20 TeV)>2*10^-15 ph/(cm**2 s), that thanks to the normalization of the gamma-ray spectrum at 20 TeV appears to be quite robust, i.e. almost independent of the shape of energy spectrum of neutrinos. We remark that this condition is satisfied by the young supernova remnants RX J1713.7-3946 and RX J0852.0-4622 (Vela Jr) - two of the strongest galactic gamma-ray sources. The preliminary condition for the detectability of high energy neutrinos is that the bulk of gamma-rays has a hadronic origin: A new way to test this hypothesis for RX J1713.7-3946 is proposed. Finally, we assess the relevance of a neutrino detector located in the Northern Hemisphere for the search for galactic neutrino sources. In particular, we argue that if the TeV neutrino sources correlate with the galactic mass distribution, the probability that some of them will be observed by a detector in the Mediterranean Sea is larger by a factor of 1.4-2.9 compared to the one of IceCube.

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arXiv2010

Wild at Heart:-The Particle Astrophysics of the Galactic Centre

We treat of the high-energy astrophysics of the inner ~200 pc of the Galaxy. Our modelling of this region shows that the supernovae exploding here every few thousand years inject enough power to i) sustain the steady-state, in situ population of cosmic rays (CRs) required to generate the region's non-thermal radio and TeV γ-ray emis-sion; ii) drive a powerful wind that advects non-thermal particles out of the inner GC; iii) supply the low-energy CRs whose Coulombic collisions sustain the temperature and ionization rate of the anomalously warm, envelope H2 detected throughout the Cen-tral Molecular Zone; iv) accelerate the primary electrons which provide the extended, non-thermal radio emission seen over ~150 pc scales above and below the plane (the Galactic centre lobe); and v) accelerate the primary protons and heavier ions which, advected to very large scales (up to ~10 kpc), generate the recently-identified WMAP haze and corresponding Fermi haze/bubbles. Our modelling bounds the average magnetic field amplitude in the inner few degrees of the Galaxy to the range 60 < B/microG < 400 (at 2 sigma confidence) and shows that even TeV CRs likely do not have time to penetrate into the cores of the region's dense molecular clouds before the wind removes them from the region. This latter finding apparently disfavours scenarios in which CRs - in this star-burst-like environment - act to substantially modify the conditions of star-formation. We speculate that the wind we identify plays a crucial role in advecting low-energy positrons from the Galactic nucleus into the bulge, thereby explaining the extended morphology of the 511 keV line emission. (abridged)

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arXiv2010

Gamma-Rays and the Far-Infrared-Radio Continuum Correlation Reveal a Powerful Galactic Centre Wind

We consider the thermal and non-thermal emission from the inner 200 pc of the Galaxy. The radiation from this almost star-burst-like region is ultimately driven dominantly by on-going massive star formation. We show that this region's radio continuum (RC) emission is in relative deficit with respect to the expectation afforded by the Far- infrared-Radio Continuum Correlation (FRC). Likewise we show that the region's gamma-ray emission falls short of that expected given its star formation and resultant supernova rates. These facts are compellingly explained by positing that a powerful (400-1200 km/s) wind is launched from the region. This wind probably plays a number of important roles including advecting positrons into the Galactic bulge thus explaining the observed ~kpc extension of the 511 keV positron annihilation signal around the GC. We also show that the large-scale GC magnetic field falls in the range ~100-300 microG and that - in the time they remain in the region - GC cosmic rays do not penetrate into the region's densest molecular material.

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arXiv2010

The ASTRO-H Mission

The joint JAXA/NASA ASTRO-H mission is the sixth in a series of highly successful X-ray missions initiated by the Institute of Space and Astronautical Science (ISAS). ASTRO-H will investigate the physics of the high-energy universe by performing high-resolution, high-throughput spectroscopy with moderate angular resolution. ASTRO-H covers very wide energy range from 0.3 keV to 600 keV. ASTRO-H allows a combination of wide band X-ray spectroscopy (5-80 keV) provided by multilayer coating, focusing hard X-ray mirrors and hard X-ray imaging detectors, and high energy-resolution soft X-ray spectroscopy (0.3-12 keV) provided by thin-foil X-ray optics and a micro-calorimeter array. The mission will also carry an X-ray CCD camera as a focal plane detector for a soft X-ray telescope (0.4-12 keV) and a non-focusing soft gamma-ray detector (40-600 keV) . The micro-calorimeter system is developed by an international collaboration led by ISAS/JAXA and NASA. The simultaneous broad bandpass, coupled with high spectral resolution of Delta E ~7 eV provided by the micro-calorimeter will enable a wide variety of important science themes to be pursued.

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arXiv2010

Limitations on the Photo-disintegration Process as a Source of VHE Photons

We consider whether photo-disintegration is ever able to provide an effective mechanism for the production of VHE $γ$-ray emission from astrophysical sources. We find that the efficiency of this process is always smaller by a factor $A/Z^{2}$ ($\sim 4/A$) than that of nuclei cooling through Bethe-Heitler pair-production. Furthermore, for sources optically thin to TeV emission, we find that the efficiency of this process can be no more than $3\times 10^{-5}(R_{\rm source}/R_{\rm Larmor})$, where $R_{\rm source}$ is the source size and $R_{\rm Larmor}$ is the CR nuclei Larmor radius. We conclude that this process is unable to provide an effective mechanism for VHE $γ$-ray emission from astrophysical sources.

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arXiv2010

Active Galactic Nuclei and gamma rays

The supermassive black holes harboured in active galactic nuclei are at the origin of powerful jets which can emit copious amounts of gamma-rays. The exact interplay between the infalling matter, the black hole and the relativistic outflow is still poorly known, and this parallel session of the 12th Marcel Grossman meeting intended to offer the most up to date status of observational results with the latest generation of ground and space-based instruments, as well as the theoretical developments relevant for the field.

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arXiv2009

A peculiar jet and arc of molecular gas toward the rich and young stellar cluster Westerlund 2 and a TeV gamma ray source

We have discovered remarkable jet- and arc-like molecular features toward the rich and young stellar cluster Westerlund2. The jet has a length of ~100 pc and a width of ~10 pc, while the arc shows a crescent shape with a radius of ~30 pc. These molecular features each have masses of ~10000 solar mass and show spatial correlations with the surrounding lower density HI gas. The jet also shows an intriguing positional alignment with the core of the TeV gamma ray source HESS J1023-575 and with the MeV/GeV gamma-ray source recently reported by the Fermi collaboration. We argue that the jet and arc are caused by an energetic event in Westerlund 2, presumably due to an anisotropic supernova explosion of one of the most massive member stars. While the origin of the TeV and GeV gamma-ray sources is uncertain, one may speculate that they are related to the same event via relativistic particle acceleration by strong shock waves produced at the explosion or by remnant objects such as a pulsar wind nebula or microquasar.

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arXiv2008

The TeV emitter structure in LS 5039

LS 5039 is an X-ray binary detected at very high energies. Along the orbit, there is a significant detection even during the superior conjunction of the compact object, when very large gamma-ray opacities are expected. Electromagnetic cascades, which may make the system more transparent to gamma-rays, are hardly efficient for reasonable magnetic fields in the massive star surroundings. A jet-like flow could transport energy to regions where the photon-photon absorption is much lower and the TeV radiation is not so severely absorbed. Otherwise, in the standard pulsar scenario for LS 5039, the emitter would be located between the star and the compact object, which would imply the violation of the observational constraints at X-rays.

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arXiv2008

The puzzling MILAGRO hot spots

We discuss the reported detection by the MILAGRO experiment of localised hot spots in the cosmic ray arrival distribution and the difficulty of interpreting these observations. A model based on secondary neutron production in the heliotail is shown to fail. An alternative model based on loss-cone leakage through a magnetic trap from a local source region is proposed.

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arXiv2008

Fast Variability of Nonthermal X-Ray Emission in Cassiopeia A: Probing Electron Acceleration in Reverse-Shocked Ejecta

Recent discovery of the year-scale variability in the synchrotron X-ray emission of the supernova remnant (SNR) RX J1713.7-3946 has initiated our study of multi-epoch X-ray images and spectra of the young SNR Cassiopeia A based on the Chandra archive data taken in 2000, 2002, and 2004. We have found year-scale time variations in the X-ray intensity for a number of X-ray filaments or knots associated with the reverse-shocked regions. The X-ray spectra of the variable filaments are characterized by a featureless continuum, and described by a power law with a photon index within 1.9-2.3. The upper limits on the iron K-line equivalent width are 110 eV, which favors a synchrotron origin of the X-ray emission. The characteristic variability timescale of 4 yr can be explained by the effects of fast synchrotron cooling and diffusive shock acceleration with a plausible magnetic field of 1 mG. The X-ray variability provides a new effective way of studying particle acceleration at supernova shocks.

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arXiv2007

On the formation of TeV radiation in LS 5039

The recent detections of TeV gamma-rays from compact binary systems show that relativistic outflows (jets or winds) are sites of effective acceleration of particles up to multi-TeV energies. In this paper, we discuss the conditions of acceleration and radiation of ultra-relativistic electrons in LS 5039, the gamma-ray emitting binary system for which the highest quality TeV data are available. Assuming that the gamma-ray emitter is a jet-like structure, we performed detailed numerical calculations of the energy spectrum and lightcurves accounting for the acceleration efficiency, the location of the accelerator, the speed of the emitting flow, the inclination angle of the system, as well as specific features related to anisotropic inverse Compton scattering and pair production. We conclude that the accelerator should not be deep inside the binary system unless we assume a very efficient acceleration rate. We show that within the IC scenario both the gamma-ray spectrum and flux are strongly orbital phase dependent. Formally, our model can reproduce, for specific sets of parameter values, the energy spectrum of gamma-rays reported by HESS for wide orbital phase intervals. However, the physical properties of the source can be constrained only by observations capable of providing detailed energy spectra for narrow orbital phase intervals ($Δϕ\ll 0.1$).

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arXiv2007

TeV lightcurve of PSR B1259-63/SS2883

The inverse Compton scattering of ultrarelativistic electrons accelerated at the pulsar wind termination shock is believed to be responsible for TeV gamma-ray signal recently reported from the binary system PSR B1259-63/SS2883. While this process can explain the energy spectrum of the observed TeV emission, the detected gamma-ray fluxes do not agree with the published theoretical predictions of the TeV lightcurve. The main objective of this paper is to show that the HESS results can be explained, under certain reasonable assumptions, by inverse Compton scenarios of gamma-ray production in the system. In this paper we study evolution of the energy spectra of relativistic electrons under different assumptions about the acceleration and energy-loss rates of electrons, and the impact of these processes on the lightcurve of IC gamma-rays. We demonstrate that the observed TeV lightcurve can be explained (i) by adiabatic losses which dominate over the entire trajectory of the pulsar with a significant increase towards the periastron, or (ii) by the sub-TeV cutoffs in the energy spectra of electrons due to the enhanced rate of Compton losses close to the periastron. The Compton deceleration of the pulsar wind contributes to the decrease of the nonthermal power released in the accelerated electrons, and thus to the reduction of the IC and synchrotron components of radiation close to the periastron. Although this effect alone cannot explain the observed lightcurves, the Comptonization of the pulsar wind leads to the formation of gamma-radiation with a line-type energy spectrum. While the HESS data already constrain the Lorentz factor of the wind, $Γ\le 10^6$, future observations of this object with GLAST should allow a deep probe of the wind Lorentz factor in the range between $10^4$ and $10^6$.

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arXiv2007

Galactic Sources of High Energy Neutrinos

The undisputed galactic origin of cosmic rays at energies below the so-called knee implies an existence of a nonthemal population of galactic objects which effectively accelerate protons and nuclei to TeV-PeV energies. The distinct signatures of these cosmic accelerators are high energy neutrinos and gamma-rays produced through hadronic interactions. While gamma-rays can be produced also by directly accelerated electrons, high energy neutrinos provide the most straightforward and unambiguous information about the nucleonic component of accelerated particles. The planned km-cube class high energy neutrino detectors are expected to be sensitive enough to provide the first astrophysically meaningful probes of potential VHE neutrino sources. This optimistic prediction is based on the recent discovery of high energy gamma-ray sources with hard energy spectra extending to 10 TeV and beyond. Amongst the best-bet candidates are two young shell-type supernova remnants - RXJ1713.7-4946 and RXJ0852.0-4622, and perhaps also two prominent plerions - the Crab Nebula and Vela X. Because of strong absorption of TeV gamma-rays, one may expect detectable neutrino fluxes also from (somewhat fainter) compact TeV gamma-ray emitters like the binary systems LS5039 and LS I+61~303, and, hopefully, from hypothetical "hidden" or "orphan" neutrino sources.

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arXiv2006

Gamma rays from molecular clouds

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.

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arXiv2005

The X-ray Jet in Centaurus A: Clues on the Jet Structure and Particle Acceleration

We report detailed studies of the X-ray emission from the kpc scale jet in the nearest active galaxy, Cen A. 41 compact sources were found within the jet, 13 of which were newly identified. We construct the luminosity function for the detected jet-knots and argue that the remaining emission is most likely to be truly diffuse, rather than resulting from the pile-up of unresolved faint knots. The transverse jet profile reveals that the extended emission has the intensity peak at the jet boundaries. We note that limb-brightened jet morphologies have been observed previously at radio frequencies in some jet sources, but never so clearly at higher photon energies. Our result therefore supports a stratified jet model, consisting of a relativistic outflow including a boundary layer with a velocity shear. In addition, we found that the X-ray spectrum of the diffuse component is almost uniform across and along the jet. We discuss this spectral behavior within a framework of shock and stochastic particle acceleration processes. We note some evidence for a possible spectral hardening at the outer sheath of the jet. Due to the limited photon statistics of the present data, further deep observations of Cen A are required to determine the reality of this finding, however we note that the existence of the hard X-ray features at outer jet boundaries would provide an important challenge to theories for the evolution of ultra-relativistic particles within the jets.

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