arXiv subjects

F. A. Aharonian

Publications and source records attributed to F. A. Aharonian.

At least 73 records · Page 4Linked to original sources

arXiv2007

First ground based measurement of atmospheric Cherenkov light from cosmic rays

A recently proposed novel technique for the detection of cosmic rays with arrays of Imaging Atmospheric Cherenkov Telescopes is applied to data from the High Energy Stereoscopic System (H.E.S.S.). The method relies on the ground based detection of Cherenkov light emitted from the primary particle prior to its first interaction in the atmosphere. The charge of the primary particle (Z) can be estimated from the intensity of this light, since it is proportional to Z$^2$. Using H.E.S.S. data, an energy spectrum for cosmic-ray iron nuclei in the energy range 13--200 TeV is derived. The reconstructed spectrum is consistent with previous direct measurements and is one of the most precise so far in this energy range.

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arXiv2007

Inverse Compton Scenarios for the TeV Gamma-Ray Emission of the Galactic Centre

The intense Compton cooling of ultra-relativistic electrons in the Klein-Nishina regime in radiation dominated environments, such as that found in the Galactic Centre, may result in radically different electron spectra than those produced by Synchrotron cooling. We explore these effects and their impact on the X-ray and gamma-ray spectra produced in electron accelerators in this region in comparison to elsewhere in our galaxy. We discuss the broad-band emission expected from the newly discovered pulsar wind nebula G 359.95-0.04 and the possible relationship of this X-ray source to the central TeV gamma-ray source HESS J1745-290. Finally we discuss the possible relationship of the Galactic Centre INTEGRAL source IGR J1745.6-2901 to the TeV emission.

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arXiv2007

XMM-Newton observations reveal the X-ray counterpart of the very-high-energy gamma-ray source HESSJ1640-465

We present X-ray observations of the as of yet unidentified very high-energy (VHE) gamma-ray source HESSJ1640-465 with the aim of establishing a counterpart of this source in the keV energy range, and identifying the mechanism responsible for the VHE emission. The 21.8 ksec XMM-Newton observation of HESSJ1640-465 in September 2005 represents a significant improvement in sensitivity and angular resolution over previous ASCA studies in this region. These new data show a hard-spectrum X-ray emitting object at the centroid of the H.E.S.S. source, within the shell of the radio Supernova Remnant (SNR) G338.3-0.0. This object is consistent with the position and flux previously measured by both ASCA and Swift-XRT but is now shown to be significantly extended. We argue that this object is very likely the counterpart to HESSJ1640-465 and that both objects may represent the Pulsar Wind Nebula of an as of yet undiscovered pulsar associated with G338.3-0.0.

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arXiv2006

H.E.S.S. observations of the Galactic Center region and their possible dark matter interpretation

The detection of gamma-rays from the source HESS J1745-290 in the Galactic Center (GC) region with the H.E.S.S. array of Cherenkov telescopes in 2004 is presented. After subtraction of the diffuse gamma-ray emission from the GC ridge, the source is compatible with a point-source with spatial extent less than 1.2'(stat.) (95% CL). The measured energy spectrum above 160 GeV is compatible with a power-law with photon index of 2.25 +/- 0.04(stat.) +/- 0.10 (syst.) and no significant flux variation is detected. These measurements are discussed in the framework of dark matter annihilation. It is found that the bulk of the VHE emission must have non-dark-matter origin. Loose constraints on the velocity-weighted annihilation cross section are derived assuming the presence of an astrophysical non-dark-matter gamma-ray contribution.

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arXiv2006

Fast variability of TeV gamma-rays from the radio galaxy M 87

The detection of fast variations of the TeV (10^12 eV) gamma-ray flux, on time-scales of days, from the nearby radio galaxy M 87 is reported. These variations are ~10 times faster than that observed in any other waveband and imply a very compact emission region with a dimension similar to the Schwarzschild radius of the central black hole. We thus can exclude several other sites and processes of the gamma-ray production. The observations confirm that TeV gamma-rays are emitted by extragalactic sources other than blazars, where jets are not relativistically beamed towards the observer.

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arXiv2006

XMM-Newton observations of HESS J1813-178 reveal a composite Supernova remnant

We present X-ray and 12CO(J=1-0) observations of the very-high-energy (VHE) gamma-ray source HESS J1813-178 with the aim of understanding the origin of the gamma-ray emission. Using this dataset we are able to undertake spectral and morphological studies of the X-ray emission from this object with greater precision than previous studies. NANTEN 12CO(J=1-0) data are used to search for correlations of the gamma-ray emission with molecular clouds which could act as target material for gamma-ray production in a hadronic scenario. The NANTEN 12CO(J=1-0) observations show a giant molecular cloud of mass 2.5 10^5 M$_{\sun}$ at a distance of 4 kpc in the vicinity of HESS J1813-178. Even though there is no direct positional coincidence, this giant cloud might have influenced the evolution of the gamma-ray source and its surroundings. The X-ray data show a highly absorbed non-thermal X-ray emitting object coincident with the previously known ASCA source AX J1813-178 showing a compact core and an extended tail towards the north-east, located in the centre of the radio shell-type Supernova remnant (SNR) G12.82-0.2. This central object shows morphological and spectral resemblance to a Pulsar Wind Nebula (PWN) and we therefore consider that the object is very likely to be a composite SNR. We discuss the scenario in which the gamma-rays originate in the shell of the SNR and the one in which they originate in the central object. We demonstrate, that in order to connect the core X-ray emission to the VHE gamma-ray emission electrons have to be accelerated to energies of at least 1 PeV.

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arXiv2006

3.9 day orbital modulation in the TeV gamma-ray flux and spectrum from the X-ray binary LS 5039

New observations of LS 5039, a High Mass X-ray Binary comprising a massive star and compact object, were carried out with the High Energy Stereoscopic System of Cherenkov Telescopes (H.E.S.S.) in 2005 at very high energy (VHE) gamma-ray energies. These observations reveal that its flux and energy spectrum are modulated with the 3.9 day orbital period of the binary system. This is the first time in gamma-ray astronomy that orbital modulation has been observed, and periodicity clearly established using ground-based gamma-ray detectors. The VHE gamma-ray emission is largely confined to half of the orbit, peaking around the inferior conjunction epoch of the compact object. For this epoch, there is also a hardening of the energy spectrum in the energy range between 0.2 TeV and a few TeV. The flux vs. orbital phase profile provides the first clear indication of gamma-ray absorption via pair production within an astrophysical source, a process which is expected to occur if the gamma-ray production site is situated within ~1 AU of the compact object. Moreover the production region size must be not significantly greater than the binary separation (~0.15 AU). Notably, these constraints are also considerably smaller than the collimated outflows or jets (extending out to ~1000 AU) observed in LS 5039. The spectral hardening could arise from variations with phase in the maximum electron energies, and/or the dominant VHE gamma-ray production mechanism.

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arXiv2006

Discovery of the two wings of the Kookaburra complex in VHE gamma -rays with H.E.S.S

Aims. Search for Very High Energy gamma-ray emission in the Kookaburra complex through observations with the H.E.S.S. array. Methods. Stereoscopic imaging of Cherenkov light emission of the gamma-ray showers in the atmosphere is used for the reconstruction and selection of the events to search for gamma-ray signals. Their spectrum is derived by a forward-folding maximum likelihood fit. Results. Two extended gamma-ray sources with an angular (68%) radius of 3.3-3.4' are discovered at high (>13sigma) statistical significance: HESS J1420-607 and HESS J1418-609. They exhibit a flux above 1 TeV of (2.97+/-0.18stat +/-0.60sys)x10-12 and (2.17+/-0.17stat +/-0.43sys)x10-12 cm-2 s-1, respectively, and similar hard photon indices ~2.2. Multi-wavelength comparisons show spatial coincidence with the wings of the Kookaburra. Two pulsar wind nebulae candidates, K3/PSR J1420-6048 and the Rabbit, lie on the edge of the H.E.S.S. sources. Conclusions. The two new sources confirm the non-thermal nature of at least parts of the two radio wings which overlap with the gamma-ray emission and establish their connection with the two X-ray pulsar wind nebulae candidates. Given the large point spread function of EGRET, the unidentified source(s) 3EG J1420-6038/GeV J1417-6100 could possibly be related to either or both H.E.S.S. sources. The most likely explanation for the Very High Energy gamma-rays discovered by H.E.S.S. is inverse Compton emission of accelerated electrons on the Cosmic Microwave Background near the two candidate pulsar wind nebulae, K3/PSR J1420-6048 and the Rabbit. Two scenarios which could lead to the observed large (~10 pc) offset-nebula type morphologies are briefly discussed.

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arXiv2006

Energy spectra of gamma-rays, electrons and neutrinos produced at proton-proton interactions in the very high energy regime

We present new parametrisations of energy spectra of secondary particles, $π$-mesons, gamma-rays, electrons and neutrinos, produced in inelastic proton-proton collisions. The simple analytical approximations based on imulations of proton-proton interactions using the public available SIBYLL code, provide very good accuracy for energy distributions of secondary products in the energy range above 100 GeV. Generally, the recommended analytical formulae deviate from the simulated distributions within a few percent over a large range of $x=E_i/E_p$ - the fraction of energy of the incident proton transferred to the secondaries. Finally, we describe an approximate procedure of continuation of calculations towards low energies, down to the threshold of $π$-meson production.

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arXiv2006

Radio to TeV radiation initiated by termination of hadronic jets from microquasars in the ISM

Microquasars (MQs) are potential candidates to produce a non-negligible fraction of the observed galactic cosmic rays. The protons accelerated at the jet termination shock interact with the interstellar medium and may produce extended emission detectable at different energy bands through several processes: neutral pion-decay produce high-energy and very high-energy gamma-rays, secondary electrons produced by charged pion-decay generate synchrotron and bremsstrahlung emission. In addition, the jets of MQs themselves are likely sources of gamma-rays. We discuss about the association between the intrinsic and the indirect emission coming from these objects.

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arXiv2006

Microquasar hadronic jets at very high-energy gamma-rays

Microquasars (MQs) present emission over the whole spectrum, from radio wavelengths to gamma-rays. The microquasar spectral energy distribution is very complex, being a signature of the different physical processes that generate the radiation emitted by these objects. In this work, we estimate the amount of broad-band emission produced by relativistic protons, released from the jet of a MQ, interacting with high density regions of the ISM. We show that a two components source, the microquasar itself and the region of interaction between the jets and the ISM, could be unveiled by the new instruments at high-energy and very high-energy gamma-rays.

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arXiv2006

Off-axis emission from relativistic plasma flows

We show that there is no universal law describing how the spectra and luminosity of synchrotron and inverse Compton radiation from relativistic jets change with increasing observation angle. Instead, the physics of particle acceleration leaves pronounced imprints in the observed spectra and allows for a freedom in numerous modifications of them. The impact of these effects is the largest for high-energy radiation and depends on the details of particle acceleration mechanism(s), what can be used to discriminate between different models. Generally, the beam patterns of relativistic jets in GeV-TeV spectral domain are much wider than the inverse Lorentz factor. The off-axis emission in this energy range appear to be brighter, have much harder spectra and a much higher cut-off frequency compared to the values derived from Doppler boosting considerations alone. The implications include the possibility to explain high-latitude unidentified EGRET sources as off-axis but otherwise typical relativistic-jet sources, such as blazars, and the prediction of GeV-TeV afterglow from transient jet sources, such as Gamma-Ray Bursts. We also discuss the phenomenon of beam-pattern broadening in application to neutrino emission.

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arXiv2006

Discovery of Very-High-Energy Gamma-Rays from the Galactic Centre Ridge

The origin of Galactic cosmic rays (with energies up to 10^15 eV) remains unclear, though it is widely believed that they originate in the shock waves of expanding supernova remnants. Currently the best way to investigate their acceleration and propagation is by observing the gamma-rays produced when cosmic rays interact with interstellar gas. Here we report observations of an extended region of very high energy (VHE, >100 GeV) gamma-ray emission correlated spatially with a complex of giant molecular clouds in the central 200 pc of the Milky Way. The hardness of the gamma-ray spectrum and the conditions in those molecular clouds indicate that the cosmic rays giving rise to the gamma-rays are likely to be protons and nuclei rather than electrons. The energy associated with the cosmic rays could have come from a single supernova explosion around 10,000 years ago.

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arXiv2005

Point-like gamma ray sources as signatures of distant accelerators of ultra high energy cosmic rays

We discuss the possibility of observing distant accelerators of ultra high energy cosmic rays in synchrotron gamma rays. Protons propagating away from their acceleration sites produce extremely energetic electrons during photo-pion interactions with cosmic microwave background photons. If the accelerator is embedded in a magnetized region, these electrons will emit high energy synchrotron radiation. The resulting synchrotron source is expected to be point-like and detectable in the GeV-TeV energy range if the magnetic field is at the nanoGauss level.

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arXiv2005

A possible association of the new VHE gamma-ray source HESS J1825--137 with the pulsar wind nebula G18.0--0.7

We report on a possible association of the recently discovered very high-energy $γ$-ray source HESS J1825--137 with the pulsar wind nebula (commonly referred to as G 18.0--0.7) of the $2.1\times 10^{4}$ year old Vela-like pulsar PSR B1823--13. HESS J1825--137 was detected with a significance of 8.1 $σ$ in the Galactic Plane survey conducted with the H.E.S.S. instrument in 2004. The centroid position of HESS J1825--137 is offset by 11\arcmin south of the pulsar position. \emph{XMM-Newton} observations have revealed X-ray synchrotron emission of an asymmetric pulsar wind nebula extending to the south of the pulsar. We argue that the observed morphology and TeV spectral index suggest that HESS J1825--137 and G 18.0--0.7 may be associated: the lifetime of TeV emitting electrons is expected to be longer compared to the {\it XMM-Newton} X-ray emitting electrons, resulting in electrons from earlier epochs (when the spin-down power was larger) contributing to the present TeV flux. These electrons are expected to be synchrotron cooled, which explains the observed photon index of $\sim 2.4$, and the longer lifetime of TeV emitting electrons naturally explains why the TeV nebula is larger than the X-ray size. Finally, supernova remnant expansion into an inhomogeneous medium is expected to create reverse shocks interacting at different times with the pulsar wind nebula, resulting in the offset X-ray and TeV $γ$-ray morphology.

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arXiv2005

Broad-band electromagnetic radiation from microquasars interacting with ISM

Microquasars (MQs) are galactic objects with relativistic jets that constitute a source population which can be responsible for production of a non-negligible fraction of the observed galactic cosmic rays. These relativistic protons, associated with the termination of the jet, interact with the interstellar medium and, at certain surrounding conditions, may lead to production of detectable fluxes of high-energy and very high-energy gamma-rays. This radiation is accompanied by the broad-band emission of secondary electrons from decays of $π^\pm$-mesons produced through synchrotron, bremstrahlung and inverse Compton process. The features of broad-band emission initiated by proton-proton (pp) interactions in such a scenario is discussed in the context of the strategy of search for counterparts of high-energy and very high-energy gamma-ray sources in the galactic plane.

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arXiv2005

A new population of very high energy gamma-ray sources in the Milky Way

Very high energy gamma-rays probe the long-standing mystery of the origin of cosmic rays. Produced in the interactions of accelerated particles in astrophysical objects, they can be used to image cosmic particle accelerators. A first sensitive survey of the inner part of the Milky Way with the High Energy Stereoscopic System (H.E.S.S.) reveals a population of eight previously unknown firmly detected sources of very high energy gamma-rays. At least two have no known radio or X-ray counterpart and may be representative of a new class of `dark' nucleonic cosmic ray sources.

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arXiv2005

Nonthermal X-radiation of SNR RX J1713.7-3946: The Relations to a Nearby Molecular Cloud

The recent X-ray and CO observations of RX J1713.7-3946 show that a significant fraction of the nonthermal X-ray emission of this unique supernova remnant associates, in one way or another, with a molecular cloud interacting with the west part of the shell. This adds a new puzzle in the origin of X-ray emission which cannot be easily explained within the standard model in accordance of which X-rays are result of synchrotron radiation of multi-TeV electrons accelerated by supernova shock waves. We explore an alternative origin of the X-ray emission assuming that it is produced by secondary electrons resulting from high energy hadronic interactions in the molecular gas. Such a scenario could explain in a quite natural way the apparent correlation between the X-ray and CO morphologies. However, the TeV gamma-ray emission recently reported by H.E.S.S. significantly constrains the parameter space of this model. Namely, this mechanism cannot reproduce the bulk of the observed X-ray flux unless one postulates existence of a PeV cosmic-ray component penetrating with an unusually hard spectrum into the dense cloud.

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