arXiv subjects

F. Aharonian

Publications and source records attributed to F. Aharonian.

At least 289 records · Page 16Linked to original sources

arXiv2007

An Exceptional VHE Gamma-Ray Flare of PKS 2155-304

The high-frequency peaked BL Lac PKS 2155-304 at redshift z=0.116 is a well-known VHE (>100 GeV) gamma-ray emitter. Since 2002 its VHE flux has been monitored using the H.E.S.S. stereoscopic array of imaging atmospheric-Cherenkov telescopes in Namibia. During the July 2006 dark period, the average VHE flux was measured to be more than ten times typical values observed from the object. This article focuses solely on an extreme gamma-ray outburst detected in the early hours of July 28, 2006 (MJD 53944). The average flux observed during this outburst is I(>200 GeV) = (1.72$\pm$$0.05_{\rm stat}$$\pm$$0.34_{\rm syst}$) $\times$ 10$^{-9}$ cm$^{-2}$ s$^{-1}$, corresponding to ~7 times the flux, I(>200 GeV), observed from the Crab Nebula. Peak fluxes are measured with one-minute time scale resolution at more than twice this average value. Variability is seen up to ~600 s in the Fourier power spectrum, and well-resolved bursts varying on time scales of ~200 seconds are observed. There are no strong indications for spectral variability within the data. Assuming the emission region has a size comparable to the Schwarzschild radius of a ~10$^9 M_\odot$ black hole, Doppler factors greater than 100 are required to accommodate the observed variability time scales.

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arXiv2007

Detection of VHE gamma-ray emission from the distant blazar 1ES 1101-232 with H.E.S.S. and broadband characterisation

The blazar 1ES 1101-232 was observed with the High Energy Stereoscopic System (H.E.S.S.) of Atmospheric Cherenkov Telescopes (ACT) in 2004 and 2005, for a live time of 43 hours. VHE (E > 10^11 eV) gamma-rays were detected for the first time from this object. VHE observations of blazars are used to investigate the inner parts of the blazar jets, and also to study the extragalactic background light (EBL) in the near-infrared band. Observations in 2005 were conducted in a multiwavelength campaign, together with the RXTE satellite and optical observations. In 2004, simultaneous observations with XMM-Newton were obtained. 1ES 1101-232 was detected with H.E.S.S. with an excess of 649 photons, at a significance of 10 sigma. The measured VHE gamma-ray flux amounts to dN/dE = (5.63 +- 0.89) x 10^-13 (E/TeV)^-(2.94 +- 0.20) cm^-2 s^-1 TeV^-1, above a spectral energy threshold of 225 GeV. No significant variation of the VHE gamma-ray flux on any time scale was found. 1ES 1101-232 exhibits a very hard spectrum, and at a redshift of z=0.186, is the blazar with the highest confirmed redshift detected in VHE gamma-rays so far. The data allow the construction of truly simultaneous spectral energy distributions of the source, from the optical to the VHE band. Using an EBL model with νF_ν= 14 nW m^-2 sr^-1 at 1.5 μm as presented in Aharonian et al. (Nature, 2006, 440, 1018) suggests an intrinsic VHE power output peak of the source at above 3 TeV.

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arXiv2007

Discovery of two candidate pulsar wind nebulae in very-high-energy gamma rays

We present the discovery of two very-high-energy gamma-ray sources in an ongoing systematic search for emission above 100 GeV from pulsar wind nebulae in survey data from the H.E.S.S. telescope array. Imaging Atmospheric Cherenkov Telescopes are ideal tools for searching for extended emission from pulsar wind nebulae in the very-high-energy regime. H.E.S.S., with its large field of view of 5 degrees and high sensitivity, gives new prospects for the search for these objects. An ongoing systematic search for very-high-energy emission from energetic pulsars over the region of the Galactic plane between -60 degrees < l < 30 degrees, -2 degrees < b < 2 degrees is performed. For the resulting candidates, the standard H.E.S.S. analysis was applied and a search for multi-wavelength counterparts was performed. We present the discovery of two new candidate gamma-ray pulsar wind nebulae, HESS J1718-385 and HESS J1809-193. H.E.S.S. has proven to be a suitable instrument for pulsar wind nebula searches.

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arXiv2007

Discovery of a point-like very-high-energy gamma-ray source in Monoceros

The complex Monoceros Loop SNR/Rosette Nebula region contains several potential sources of very-high-energy (VHE) gamma-ray emission and two as yet unidentified high-energy EGRET sources. Sensitive VHE observations are required to probe acceleration processes in this region. The H.E.S.S. telescope array has been used to search for very high-energy gamma-ray sources in this region. CO data from the NANTEN telescope were used to map the molecular clouds in the region, which could act as target material for gamma-ray production via hadronic interactions. We announce the discovery of a new gamma-ray source, HESS J0632+058, located close to the rim of the Monoceros SNR. This source is unresolved by H.E.S.S. and has no clear counterpart at other wavelengths but is possibly associated with the weak X-ray source 1RXS J063258.3+054857, the Be-star MWC 148 and/or the lower energy gamma-ray source 3EG J0634+0521. No evidence for an associated molecular cloud was found in the CO data.

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arXiv2007

Detection of extended very-high-energy gamma-ray emission towards the young stellar cluster Westerlund 2

Results from gamma-ray observations by the H.E.S.S. telescope array in the direction of the young stellar cluster Westerlund 2 are presented. Stereoscopic imaging of Cherenkov light emission of gamma-ray induced showers in the atmosphere is used to study the celestial region around the massive Wolf-Rayet (WR) binary WR 20a. Spectral and positional analysis is performed using standard event reconstruction techniques and parameter cuts. The detection of a new gamma-ray source is reported from H.E.S.S. observations in 2006. HESS J1023-575 is found to be coincident with the young stellar cluster Westerlund 2 in the well-known HII complex RCW 49. The source is detected with a statistical significance of more than 9 sigma, and shows extension beyond a point-like object within the H.E.S.S. point-spread function. The differential gamma-ray spectrum of the emission region is measured over approximately two orders of magnitude in flux. The spatial coincidence between HESS J1023-575 and the young open cluster Westerlund 2, hosting e.g. the massive WR binary WR 20a, requires a look into a variety of potential models to account for the observed very-high-energy (VHE) gamma-ray emission. Considered emission scenarios include emission from the colliding wind zone of WR 20a, collective stellar winds from the extraordinary ensemble of hot and massive stars in the stellar cluster Westerlund 2, diffusive shock acceleration in the wind-blown bubble itself, and supersonic winds breaking out into the interstellar medium (ISM). The observed source extension argues against a single star origin of the observed VHE emission.

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arXiv2007

Search for Pulsed VHE Gamma-Ray Emission from Young Pulsars with H.E.S.S

We present the results of a search for pulsed very-high-energy (VHE) gamma-ray emission from young pulsars using data taken with the H.E.S.S. imaging atmospheric Cherenkov telescope system. Data on eleven pulsars, selected according to their spin-down luminosity relative to distance, are searched for gamma-ray signals with periodicity at the respective pulsar spin period. Special analysis efforts were made to improve the sensitivity in the 100 GeV gamma-ray energy domain in an attempt to reduce the gap between satellite and ground-based gamma-ray instruments. No significant evidence for pulsed emission is found in any data set. Differential upper limits on pulsed energy flux are determined for all selected pulsars in the approximate gamma-ray energy range between 100 GeV and 50 TeV, using different limit determination methods, testing a wide range of possible pulsar light curves and energy spectra. The limits derived here imply that the magnetospheric VHE gamma-ray production efficiency in young pulsars is less than 10^-4 of the pulsar spin-down luminosity, requiring spectral turnovers for the high-energy emission of four established gamma-ray pulsars, and constrain the inverse Compton radiation component predicted by several outer gap models.

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arXiv2006

Primary particle acceleration above 100 TeV in the shell-type Supernova Remnant RX J1713.7-3946 with deep H.E.S.S. observations

We present deep H.E.S.S. observations of the supernova remnant (SNR) RX J1713.7-3946. Combining data of three years - from 2003 to 2005 - we obtain significantly increased statistics and energy coverage as compared to earlier 2003 & 2004 results. The data are analysed separately for the different years. Very good agreement of the gamma-ray morphology and the differential spectra is found when comparing the three years. The combined gamma-ray image of the 2004 and 2005 data reveals the morphology of RX J1713.7-3946 with unprecedented precision. An angular resolution of 0.06 deg is achieved, revealing the detailed structure of the remnant. The combined spectrum of all three years extends over three orders of magnitude, with significant gamma-ray emission approaching 100 TeV. The cumulative significance above 30 TeV is 4.8 sigma, while for energies between 113 and 294 TeV an upper limit on the gamma-ray flux of 1.6 x 10^-16 cm^-2 s^-1 is obtained. The energy coverage of the H.E.S.S. data is presumably at the limit of present generation Cherenkov telescopes. The measurement of significant gamma-ray emission beyond 30 TeV formally implies the existence of primary particles of at least that energy. However, for realistic scenarios of very-high-energy gamma-ray production, the Inverse Compton scattering of very-high-energy electrons and pi^0 decay following inelastic proton-proton interactions, the measured gamma-ray energies imply that efficient acceleration of primary particles to energies exceeding 100 TeV is taking place in the shell of the SNR RX J1713.7-3946.

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arXiv2006

Exploring Broadband GRB Behavior During gamma-ray Emission

The robotic ROTSE-III telescope network detected prompt optical emission contemporaneous with the gamma-ray emission of Swift events GRB051109A and GRB051111. Both datasets have continuous coverage at high signal-to-noise levels from the prompt phase onwards, thus the early observations are readily compared to the Swift XRT and BAT high energy detections. In both cases, the optical afterglow is established, declining steadily during the prompt emission. For GRB051111, there is evidence of an excess optical component during the prompt emission. The component is consistent with the flux spectrally extrapolated from the gamma-rays, using the gamma-ray spectral index. A compilation of spectral information from previous prompt detections shows that such a component is unusual. The existence of two prompt optical components - one connected to the high-energy emission, the other to separate afterglow flux, as indicated in GRB051111 - is not compatible with a simple ``external-external'' shock model for the GRB and its afterglow.

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arXiv2006

Nucleonic gamma-ray production in Pulsar Wind Nebulae

Observations of the inner radian of the Galactic disk at very high energy (VHE) gamma-rays have revealed at least 16 new sources. Besides shell type super-nova remnants, pulsar wind nebulae (PWN) appear to be a dominant source population in the catalogue of VHE gamma-ray sources. Except for the Crab nebula, the newly discovered PWN are resolved at VHE gamma-rays to be spatially extended (5-20 pc). Currently, at least 3 middle aged ($t>10$ kyrs) PWN (Vela X, G18.0-0.7, and G313.3+0.6 in the ``Kookaburra'' region) and 1 young PWN MSH 15-5{\it2} ($t=1.55$ kyrs) have been identified to be VHE emitting PWN (sometimes called ``TeV Plerions''). Two more candidate ``TeV Plerions'' have been identifed and have been reported at this conference [1]. In this contribution, the gamma-ray emission from Vela X is explained by a nucleonic component in the pulsar wind. The measured broad band spectral energy distribution is compared with the expected X-ray emission from primary and secondary electrons. The observed X-ray emission and TeV emission from the three middle aged PWN are compared with each other.

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arXiv2006

Energy dependent gamma-ray morphology in the Pulsar wind nebula HESSJ1825-137

Detailed morphological and spatially resolved spectral studies reveal the very high-energy (VHE) gamma-ray aspects of this object with unprecedented precision. We confirm previous results obtained in a survey of the Galactic Plane in 2004. The gamma-ray emission extends asymmetrically to the south and south-west of the energetic pulsar PSR J1826-1334, that is thought to power the pulsar wind nebula. The differential gamma-ray spectrum of the whole emission region is measured over more than two orders of magnitude, from 270 GeV to 35 TeV, and shows indications for a deviation from a pure power law. Spectra have also been determined for spatially separated regions of HESS J1825-137. The photon indices from a power-law fit in the different regions show a softening of the spectrum with increasing distance from the pulsar and therefore an energy dependent morphology. This is the first time that an energy dependent morphology has been detected in the VHE gamma-ray regime. The VHE gamma-ray emission of HESS J1825-137 is phenomenologically discussed in the scenario where the gamma-rays are produced by VHE electrons via Inverse Compton scattering. The high gamma-ray luminosity of the source cannot be explained on the basis of constant spin-down power of the pulsar and requires higher injection power in past.

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arXiv2006

Absorption of Nuclear Gamma-rays on the Starlight Radiation in FR I Sources: the Case of Centaurus A

Several BL Lac objects are confirmed sources of variable and strongly Doppler-boosted TeV emission produced in the nuclear portions of their relativistic jets. It is more than probable, that also many of the FR I radio galaxies, believed to be the parent population of BL Lacs, are TeV sources, for which Doppler-hidden nuclear gamma-ray radiation may be only too weak to be directly observed. Here we show, however, that about one percent of the total time-averaged TeV radiation produced by the active nuclei of low-power FR I radio sources is inevitably absorbed and re-processed by photon-photon annihilation on the starlight photon field, and the following emission of the created and quickly isotropized electron-positron pairs. In the case of the radio galaxy Centaurus A, we found that the discussed mechanism can give a distinctive observable feature in the form of an isotropic gamma-ray halo. It results from the electron-positron pairs injected to the interstellar medium of the inner parts of the elliptical host by the absorption process, and upscattering starlight radiation via the inverse-Compton process mainly to the GeV-TeV photon energy range. Such a galactic gamma-ray halo is expected to possess a characteristic spectrum peaking at ~0.1 TeV photon energies, and the photon flux strong enough to be detected by modern Cherenkov Telescopes and, in the future, by GLAST. These findings should apply as well to the other nearby FR I sources.

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arXiv2006

Observations of the Crab Nebula with H.E.S.S

The Crab nebula was observed with the H.E.S.S. stereoscopic Cherenkov-telescope array between October 2003 and January 2005 for a total of 22.9 hours (after data quality selection). Observations were made with three operational telescopes in late 2003 and with the complete 4 telescope array in January - February 2004 and October 2004 - January 2005. The observations are discussed and used as an example to detail the flux and spectral analysis procedures of H.E.S.S., and to evaluate the systematic uncertainties in H.E.S.S. flux measurements. The flux and spectrum of gamma-rays from the source are calculated on run-by-run and monthly time-scales, and a correction is applied for long-term variations in the detector sensitivity. Comparisons of the measured flux and spectrum over the observation period, along with the results from a number of different analysis procedures are used to estimate systematic uncertainties in the measurements. The energy spectrum is found to follow a power law with an exponential cutoff, with photon index $Γ= 2.39 \pm 0.03\stat$ and cutoff energy $E_{c} = (14.3 \pm 2.1\stat) \textrm{TeV}$ between 440 GeV and 40 TeV. The observed integral flux above 1 TeV is $(2.26 \pm 0.08\stat) \times 10^{-11} cm^{-2} s^{-1}$. The estimated systematic error on the flux measurement is estimated to be 20%, while the estimated systematic error on the spectral slope is 0.1.

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arXiv2006

Dynamics and High Energy Emission of the Flaring HST-1 Knot in the M 87 Jet

Stimulated by recent observations of a radio-to-X-ray synchrotron flare from HST-1, the innermost knot of the M 87 jet, as well as by a detection of a very high energy gamma-ray emission from M 87, we investigated the dynamics and multiwavelength emission of the HST-1 region. We study thermal pressure of the hot interstellar medium in M 87 and argue for a presence of a gaseous condensation in its central parts. Interaction of the jet with such a feature is likely to result in formation of a converging reconfinement shock in the innermost parts of the M 87 jet. We show that for a realistic set of the outflow parameters, a stationary and a flaring part of the HST-1 knot located \~100 pc away from the active center can be associated with the decelerated portion of the jet matter placed immediately downstream of the point where the reconfinement shock reaches the jet axis. We discuss a possible scenario explaining a broad-band brightening of the HST-1 region related to the variable activity of the central core. We show that assuming a previous epoch of the high central black hole activity resulting in ejection of excess particles and photons down along the jet, one may first expect a high-energy flare of HST-1 due to inverse-Comptonisation of the nuclear radiation, followed after a few years by an increase in the synchrotron continuum of this region. If this is the case, then the recently observed increase in the knot luminosity in all spectral bands could be regarded as an unusual echo of the outburst that had happened previously in the active core of the M 87 radio galaxy.

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arXiv2006

A low level of extragalactic background light as revealed by gamma-rays from blazars

The diffuse extragalactic background light consists of the sum of the starlight emitted by galaxies through the history of the Universe, and it could also have an important contribution from the first stars, which may have formed before galaxy formation began. Direct measurements are difficult and not yet conclusive, owing to the large uncertainties caused by the bright foreground emission associated with zodiacal light. An alternative approach is to study the absorption features imprinted on the gamma-ray spectra of distant extragalactic objects by interactions of those photons with the background light photons. Here we report the discovery of gamma-ray emission from the blazars H 2356-309 and 1ES 1101-232, at redshifts z=0.165 and z=0.186, respectively. Their unexpectedly hard spectra provide an upper limit on the background light at optical/near-infrared wavelengths that appears to be very close to the lower limit given by the integrated light of resolved galaxies. The background flux at these wavelengths accordingly seems to be strongly dominated by the direct starlight from galaxies, thus excluding a large contribution from other sources -- in particular from the first stars formed. This result also indicates that intergalactic space is more transparent to gamma-rays than previously thought.

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arXiv2006

The Anomalous Early Afterglow of GRB 050801

The ROTSE-IIIc telescope at the H.E.S.S. site, Namibia, obtained the earliest detection of optical emission from a Gamma-Ray Burst (GRB), beginning only 21.8 s from the onset of Swift GRB 050801. The optical lightcurve does not fade or brighten significantly over the first ~250 s, after which there is an achromatic break and the lightcurve declines in typical power-law fashion. The Swift/XRT also obtained early observations starting at 69 s after the burst onset. The X-ray lightcurve shows the same features as the optical lightcurve. These correlated variations in the early optical and X-ray emission imply a common origin in space and time. This behavior is difficult to reconcile with the standard models of early afterglow emission.

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arXiv2005

Ground-Based Gamma-Ray Astronomy at Energies Above 10 TeV: Searching for Galactic PeV Cosmic-Ray Accelerators

The origin of Galactic CRs up the knee energy remains unanswered and provides strong motivation for the study of gamma-ray sources at energies above 10 TeV. We discuss recent results from ground-based gamma-ray Cherenkov imaging systems at these energies as well as future observational efforts in this direction. The exciting results of H.E.S.S. give clues as to the nature of Galactic CR accelerators, and suggest that there is a population of Galactic gamma-ray sources with emission extending beyond 10 TeV. A dedicated system of Cherenkov imaging telescopes optimised for higher energies appears to be a promising way to study the multi-TeV gamma-ray sky.

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arXiv2005

Is the Giant Radio Galaxy M87 a TeV Gamma-Ray Emitter?

For the first time an excess of photons above an energy threshold of 730 GeV from the giant radio galaxy M87 has been measured at a significance level above 4 sigma. The data have been taken during the years 1998 and 1999 with the HEGRA stereoscopic system of 5 imaging atmospheric Cherenkov telescopes. The excess of 107.4 +- 26.8 events above 730 GeV corresponds to an integral flux of 3.3% of the Crab flux or N_gamma(E > 730 GeV) = (0.96 +- 0.23) * 10^(-12) phot.cm^(-2)s^(-1). M87 is located at the center of the Virgo cluster of galaxies at a relatively small redshift of z = 0.00436 and is a promising candidate among the class of giant radio galaxies for the emission of TeV gamma-radiation. The detection of TeV gamma-rays from M87 -- if confirmed -- would establish a new class of extragalactic source in this energy regime since all other AGN detected to date at TeV energies are BL Lac type objects.

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arXiv2005

A detailed spectral and morphological study of the gamma-ray supernova remnant RX J1713.7-3946 with H.E.S.S

We present results from deep observations of the Galactic shell-type supernova remnant (SNR) RX J1713.7-3946 (also known as G347.3-0.5) conducted with the complete H.E.S.S. array in 2004. Detailed morphological and spatially resolved spectral studies reveal the very-high-energy (VHE -- Energies E > 100 GeV) gamma-ray aspects of this object with unprecedented precision. Since this is the first in-depth analysis of an extended VHE gamma-ray source, we present a thorough discussion of our methodology and investigations of possible sources of systematic errors. Gamma rays are detected throughout the whole SNR. The emission is found to resemble a shell structure with increased fluxes from the western and northwestern parts. The differential gamma-ray spectrum of the whole SNR is measured over more than two orders of magnitude, from 190 GeV to 40 TeV, and is rather hard with indications for a deviation from a pure power law at high energies. Spectra have also been determined for spatially separated regions of RX J1713.7-3946. The flux values vary by more than a factor of two, but no significant change in spectral shape is found. There is a striking correlation between the X-ray and the gamma-ray image. Radial profiles in both wavelength regimes reveal the same shape almost everywhere in the region of the SNR. The VHE gamma-ray emission of RX J1713.7-3946 is phenomenologically discussed for two scenarios, one where the gamma rays are produced by VHE electrons via Inverse Compton scattering and one where the gamma rays are due to neutral pion decay from proton-proton interactions. In conjunction with multi-wavelength considerations, the latter case is favoured. However, no decisive conclusions can yet be drawn regarding the parent particle population dominantly responsible for the gamma-ray emission from RX J1713.7-3946.

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