Very high energy (VHE; E > or =, slanted E 100 GeV) and high-energy (HE; 100 MeV < or =, slanted E < or =, slanted 100 GeV) data from gamma-ray observations performed with the H.E.S.S. telescope ...array and the Fermi-LAT instrument, respectively, are analyzed in order to investigate the non-thermal processes in the starburst galaxy NGC 253. A power-law fit to the differential HE gamma-ray spectrum reveals a photon index of Gamma = 2.24 + or - 0.14 sub(stat) + or - 0.03 sub(sys) and an integral flux between 200 MeV and 200 GeV of F(0.2-200 GeV) = (4.9 + or - 1.0 sub(stat) + or - 0.3 sub(sys)) x 10 super(-9) cm super(-2) s super(-1). No evidence for a spectral break or turnover is found over the dynamic range of both the LAT instrument and the H.E.S.S. experiment: a combined fit of a power law to the HE and VHE gamma-ray data results in a differential photon index Gamma = 2.34 + or - 0.03 with a p-value of 30%..
Very high energy (VHE) γ-rays (E > 100 GeV) have been detected from the direction of the Galactic Centre up to energies E > 10 TeV. Up to now, the origin of this emission is unknown due to the ...limited positional accuracy of the observing instruments. One of the counterpart candidates is the supermassive black hole (SMBH) Sgr A*. If the VHE emission is produced within ≈1015 cm ≈1000 rG(rG=G M/c2 is the Schwarzschild radius) of the SMBH, a decrease of the VHE photon flux in the energy range 100–300 GeV is expected whenever an early-type or giant star approaches the line of sight within ≈ milliarcseconds (mas). The dimming of the flux is due to absorption by pair-production of the VHE photons in the soft photon field of the star, an effect we refer to as pair-production eclipse (PPE). Based upon the currently known orbits of stars in the inner arcsecond of the Galaxy, we find that PPEs lead to a systematic dimming in the 100–300 GeV band at the level of a few per cent and lasts for several weeks. Since the PPE affects only a narrow energy band and is well correlated with the passage of the star, it can be clearly discriminated against other systematic or even source-intrinsic effects. While the effect is too small to be observable with the current generation of VHE detectors, upcoming high count-rate experiments like the Cherenkov telescope array (CTA) will be sufficiently sensitive. Measuring the temporal signature of the PPE bears the potential to locate the position and size of the VHE-emitting region within the inner 1000 rG or in the case of a non-detection exclude the immediate environment of the SMBH as the site of γ-ray production altogether.
Very high energy (VHE, E > 100 GeV) γ-ray flaring activity of the high-frequency peaked BL Lac object PG 1553+113 has been detected by the H.E.S.S. telescopes. The flux of the source increased by a ...factor of 3 during the nights of 2012 April 26 and 27 with respect to the archival measurements with a hint of intra-night variability. No counterpart of this event has been detected in the Fermi-Large Area Telescope data. This pattern is consistent with VHE γ-ray flaring being caused by the injection of ultrarelativistic particles, emitting γ-rays at the highest energies. The dataset offers a unique opportunity to constrain the redshift of this source at z = 0.49 ± 0.04 using a novel method based on Bayesian statistics. The indication of intra-night variability is used to introduce a novel method to probe for a possible Lorentz invariance violation (LIV), and to set limits on the energy scale at which Quantum Gravity (QG) effects causing LIV may arise. For the subluminal case, the derived limits are E{sub QG,1} > 4.10 × 10{sup 17} GeV and E{sub QG,2} > 2.10 × 10{sup 10} GeV for linear and quadratic LIV effects, respectively.
This Letter reports the discovery of a remarkably hard spectrum source, HESS J1641-463, by the High Energy Stereoscopic System (H.E.S.S.) in the very high energy (VHE) domain. HESS J1641-463 remained ...unnoticed by the usual analysis techniques due to confusion with the bright nearby source HESS J1640-465. It emerged at a significance level of 8.5 standard deviations after restricting the analysis to events with energies above 4 TeV. It shows a moderate flux level of varphi(E > 1 TeV) = (3.64 0.44 sub(stat) 0.73 sub(sys)) x 10 super(-13) cm super(-2) s super(-1), corresponding to 1.8% of the Crab Nebula flux above the same energy, and a hard spectrum with a photon index of Gamma = 2.07 0.1l sub(stat) 0.20 sub(sys). It is a point-like source, although an extension up to a Gaussian width of sigma = 3 arcmin cannot be discounted due to uncertainties in the H.E.S.S. point-spread function. The VHE gamma-ray flux of HESS J1641-463 is found to be constant over the observed period when checking time binnings from the year-by-year to the 28 minute exposure timescales. HESS J1641-463 is positionally coincident with the radio supernova remnant SNR G338.5+0.1. No X-ray candidate stands out as a clear association; however, Chandra and XMM-Newton data reveal some potential weak counterparts. Various VHE gamma-ray production scenarios are discussed. If the emission from HESS J1641-463 is produced by cosmic ray protons colliding with the ambient gas, then their spectrum must extend close to 1 PeV. This object may represent a source population contributing significantly to the galactic cosmic ray flux around the knee.
The giant radio galaxy M 87 with its proximity (16 Mpc), famous jet, and very massive black hole ((3 - 6) x 10 super(9) M sub(middot in circle)) provides a unique opportunity to investigate the ...origin of very high energy (VHE; E > 100 GeV) gamma-ray emission generated in relativistic outflows and the surroundings of supermassive black holes. M 87 has been established as a VHE gamma-ray emitter since 2006. The VHE gamma-ray emission displays strong variability on timescales as short as a day. In this paper, results from a joint VHE monitoring campaign on M 87 by the MAGIC and VERITAS instruments in 2010 are reported. During the campaign, a flare at VHE was detected triggering further observations at VHE (H.E.S.S.), X-rays (Chandra), and radio (43 GHz Very Long Baseline Array, VLBA). The excellent sampling of the VHE gamma-ray light curve enables one to derive a precise temporal characterization of the flare: the single, isolated flare is well described by a two-sided exponential function with significantly different flux rise and decay times of tau sub(d) super(rise) = (1.69 + or - 0.30) days and tau sub(d) super(decay) = (0.611 + or - 0.080) days, respectively. While the overall variability pattern of the 2010 flare appears somewhat different from that of previous VHE flares in 2005 and 2008, they share very similar timescales (~day), peak fluxes (Phi sub(>0.35TeV) Asymptotically = to (1-3) x 10 super(-11) photons cm super(-2) s super(-1)), and VHE spectra. VLBA radio observations of 43 GHz of the inner jet regions indicate no enhanced flux in 2010 in contrast to observations in 2008, where an increase of the radio flux of the innermost core regions coincided with a VHE flare. On the other hand, Chandra X-ray observations taken ~3 days after the peak of the VHE gamma-ray emission reveal an enhanced flux from the core (flux increased by factor ~2; variability timescale <2 days). The long-term (2001-2010) multi-wavelength (MWL) light curve of M 87, spanning from radio to VHE and including data from Hubble Space Telescope, Liverpool Telescope, Very Large Array, and European VLBI Network, is used to further investigate the origin of the VHE gamma-ray emission. No unique, common MWL signature of the three VHE flares has been identified. In the outer kiloparsec jet region, in particular in HST-1, no enhanced MWL activity was detected in 2008 and 2010, disfavoring it as the origin of the VHE flares during these years. Shortly after two of the three flares (2008 and 2010), the X-ray core was observed to be at a higher flux level than its characteristic range (determined from more than 60 monitoring observations: 2002-2009). In 2005, the strong flux dominance of HST-1 could have suppressed the detection of such a feature. Published models for VHE gamma-ray emission from M 87 are reviewed in the light of the new data.
Vela X is a region of extended radio emission in the western part of the Vela constellation: one of the nearest pulsar wind nebulae, and associated with the energetic Vela pulsar. A deeper ...observation of the entire Vela X nebula region, also including larger offsets from the cocoon, has been performed with HESS. This re-observation was carried out in order to probe the extent of the non-thermal emission from the Vela X region at TeV energies and to investigate its spectral properties. To increase the sensitivity to the faint gamma -ray emission from the very extended Vela X region, a multivariate analysis method combining three complementary reconstruction techniques of Cherenkov-shower images is applied for the selection of gamma -ray events. The Vela X region exhibits almost uniform gamma -ray spectra over its full extent: the differential energy spectrum can be described by a power-law function with a hard spectral index ... and an exponential cutoff at an energy of TeV.(ProQuest: ... denotes formulae/symbols omitted.)
The previously unidentified very high-energy (VHE) gamma -ray source HESS J1303-631 is re-examined including new data from the HESS Cherenkov telescope array in order to identify this object. ...Detailed morphological and spectral studies of VHE gamma -ray emission as well as of the XMM-Newton X-ray data are performed. Significant energy-dependent morphology of this source, as well as the identification of an associated X-ray PWN from XMM-Newton observations enable identification of the VHE source as an evolved PWN associated to the pulsar PSR J1301-6305. This identification is supported by the one zone leptonic model, which suggests that the energetics of the gamma -ray and X-ray radiation are such that they may have a similar origin in the pulsar nebula. This would explain the low level of synchrotron radiation and the difficulty in detecting counterparts at lower energies, the reason this source was originally classified as a "dark' VHE gamma -ray source.
In some galaxy clusters, powerful active galactic nuclei (AGN) have blown bubbles with cluster scale extent into the ambient medium. The main pressure support of these bubbles is not known to date, ...but cosmic rays are a viable possibility. Hydra A, the closest galaxy cluster hosting a cluster scale AGN outburst, located at a redshift of 0.0538, is investigated for being a gamma-ray emitter with the High Energy Stereoscopic System (H.E.S.S.) array and the Fermi Large Area Telescope (Fermi-LAT). Data obtained in 20.2 h of dedicated H.E.S.S. observations and 38 months of Fermi-LAT data, gathered by its usual all-sky scanning mode, have been analyzed to search for a gamma-ray signal. No signal has been found in either data set. Upper limits on the gamma-ray flux are derived and are compared to models. The non-detection of Hydra A in gamma-rays has important implications on the particle populations and physical conditions inside the bubbles in this system.
1RXS J101015.9-311909 is a galaxy located at a redshift of z = 0.14 hosting an active nucleus belonging to the class of bright BL Lac objects. The authors report on results from VHE observations ...performed between 2006 and 2010 with the H.E.S.S. instrument, an array of four imaging atmospheric Cherenkov telescopes. H.E.S.S. data have been analysed with enhanced analysis methods, making the detection of faint sources more significant. VHE emission at a position coincident with 1RXS J101015.9-311909 is detected with H.E.S.S. for the first time. In a total good-quality livetime of about 49 h, the authors measure 263 excess counts, corresponding to a significance of 7.1 standard deviations. The photon spectrum above 0.2 TeV can be described by a power-law with a photon index of ... . The integral flux above 0.2 TeV is about 0.8% of the flux of the Crab nebula and shows no significant variability over the time reported. (ProQuest: ... denotes formulae/symbols omitted.)
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