Aims. A detailed study of the spectrum and variability of the source HESS J1745-290 in the Galactic Center (GC) region using new data from the H.E.S.S. array of Cherenkov telescopes is presented. ...Flaring activity and quasi periodic oscillations (QPO) of HESS J1745-290 are investigated. Methods. The image analysis is performed with a combination of a semi-analytical shower model and the statistical moment-based Hillas technique. The spectrum and lightcurves of HESS J1745-290 are derived with a likelihood method based on a spectral shape hypothesis. Rayleigh tests and Fourier analysis of the H.E.S.S. GC signal are used to study the periodicity of the source. Results. With a three-fold increase in statistics compared to previous work, a deviation from a simple power law spectrum is detected for the first time. The measured energy spectrum over the three years 2004, 2005 and 2006 of data taking is compatible with both a power law spectrum with an exponential cut-off and a broken power law spectrum. The curvature of the energy spectrum is likely to be intrinsic to the photon source, as opposed to effects of interstellar absorption. The power law spectrum with an exponential cut-off is characterized by a photon index of 2.10 ± 0.04$_{\mathrm{stat}}$ ± 0.10$_{\mathrm{syst}}$ and a cut-off energy at 15.7 ± 3.4$_{\mathrm{stat}}$ ± 2.5$_{\mathrm{syst}}$ TeV. The broken power law spectrum exhibits spectral indices of 2.02 ± 0.08$_{\mathrm{stat}}$ ± 0.10$_{\mathrm{syst}}$ and 2.63 ± 0.14$_{\mathrm{stat}}$ ± 0.10$_{\mathrm{syst}}$ with a break energy at 2.57 ± 0.19$_{\mathrm{stat}}$ ± 0.44$_{\mathrm{syst}}$ TeV. No significant flux variation is found. Increases in the γ-ray flux of HESS J1745-290 by at least a factor of two would be required for a 3σ detection of a flare with time scales of an hour. Investigation of possible QPO activity at periods claimed to be detected in X-rays does not show any periodicities in the H.E.S.S. signal.
Context. 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 (PSR ...B0833-45). Extended very-high-energy (VHE) γ-ray emission (HESS J0835−455) was discovered using the H.E.S.S. experiment in 2004. The VHE γ-ray emission was found to be coincident with a region of X-ray emission discovered with ROSAT above 1.5 keV (the so-called Vela X cocoon): a filamentary structure extending southwest from the pulsar to the centre of Vela X. Aims. A deeper observation of the entire Vela X nebula region, also including larger offsets from the cocoon, has been performed with H.E.S.S. 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. Methods. To increase the sensitivity to the faint γ-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 γ-ray events. The analysis is performed with the On/Off background method, which estimates the background from separate observations pointing away from Vela X; towards regions free of γ-ray sources but with comparable observation conditions. Results. The γ-ray surface brightness over the large Vela X region reveals that the detection of non-thermal VHE γ-ray emission from the PWN HESS J0835−455 is statistically significant over a region of radius 1.2° around the position α = 08h35m00s, δ = −45°36′00′′ (J2000). The Vela X region exhibits almost uniform γ-ray spectra over its full extent: the differential energy spectrum can be described by a power-law function with a hard spectral index Γ = 1.32 ± 0.06stat ± 0.12sys and an exponential cutoff at an energy of (14.0 ± 1.6stat ± 2.6sys) TeV. Compared to the previous H.E.S.S. observations of Vela X the new analysis confirms the general spatial overlap of the bulk of the VHE γ-ray emission with the X-ray cocoon, while its extent and morphology appear more consistent with the (more extended) radio emission, contradicting the simple correspondence between VHE γ-ray and X-ray emissions. Morphological and spectral results challenge the interpretation of the origin of γ-ray emission in the GeV and TeV ranges in the framework of current models.
We present a modification of the relativistic phase shift method for determining the radio emission geometry from pulsar magnetospheres proposed by Gangadhara & Gupta. Our modification provides a ...method of determining radio emission altitudes that does not depend on the viewing geometry and does not require polarization measurements. We suggest application of the method to the outer edges of averaged radio pulse profiles to identify magnetic field lines associated with the edges of the pulse and, thereby, to test the geometric method based on the measurement of the pulse width at the lowest intensity level. We show that another relativistic method proposed by Blaskiewicz et al. provides upper limits for emission altitudes associated with the outer edges of pulse profiles. A comparison of these limits with the altitudes determined with the geometric method may be used to probe the importance of rotational distortions of the magnetic field and refraction effects in the pulsar magnetosphere. We provide a comprehensive discussion of the assumptions used in the relativistic methods.
Radio pulsar J0631+1036 presents a remarkably clear example of a rare four-component profile, and with apparently large aberration/retardation indicated by its linear polarization-angle traverse, but ...on closer study its profiles are somewhat difficult to understand and interpret. The pulsar's four components do appear to represent inner and outer conal beam pairs with the expected spacing and spectral evolution with frequency. At metre wavelengths, the leading and trailing component pairs are often conflated into an unresolved double form by what seems to be varying amounts of scattering. We assess whether the core/double-cone geometric model, widely used to describe the profiles of slower pulsars, is appropriate for J0631+1036. We find that it is largely compatible apart from difficulties with the emission height and resolved double form of the inner conal features. An aberration/retardation analysis provides 600-km physical emission height values, which are compatible with geometric estimates for the outer conal emission. We also explore several other models and conclude that none are as successful as the core/double-cone model despite its several difficulties.
Aims. The X-ray–TeV connection and the evolution of the emitting particle population is studied in high-energy peaked BL Lac objects, by obtaining spectral information in both bands on sub-hour ...timescales. Methods. Simultaneous observations with HESS, Chandra and the Bronberg optical observatory were performed on the BL Lac object PKS 2155–304 in the night of July 29–30 2006, when the source underwent a major γ-ray outburst during its high-activity state of Summer 2006. This event took place about 44 h after the other major outburst of the night of July 27–28, which is known for its ultrafast variability. An unprecedented 6 to 8 h of simultaneous, uninterrupted coverage was achieved, with spectra and light curves measured down to 7 and 2 min timescales, respectively. Results. The source exhibited one major flare along the night, at high energies. The γ-ray flux reached a maximum of ~11 times the Crab flux (>400 GeV), with rise/decay timescales of ~1 h, plus a few smaller-amplitude flares superimposed on the decaying phase. The emission in the X-ray and VHE γ-ray bands is strongly correlated, with no evidence of lags. The spectra also evolve with similar patterns, and are always soft (photon index Γ > 2), indicating no strong shift of the peaks in the spectral energy distribution towards higher energies. Only at the flare maximum is there evidence that the γ-ray peak is inside the observed passband, at ~400–600 GeV. The VHE spectrum shows a curvature that is variable with time and stronger at higher fluxes. The huge VHE variations (~22$\times$) are only accompanied by small-amplitude X-ray and optical variations (factor 2 and 15% respectively). The source has shown for the first time in a high-energy peaked BL Lac object a large Compton dominance $(L_{\rm C}/L_{\rm S}\sim 10)$ – rapidly evolving – and a cubic relation between VHE and X-ray flux variations, during a decaying phase. These results challenge the common scenarios for the TeV-blazar emission.
Context. Some radio pulsar profiles (in particular those of millisecond pulsars) contain wide emission structures which cover large intervals of pulse phase. Local distortions of an average curve of ...polarisation angle (PA) can be identified in such profiles, and they are often found to be associated with absorption features or narrow emission components. Aims. The features may be interpreted as a convolution of a lateral profile of an emitter with a microscopic radiation pattern of a non-negligible angular extent. Methods. We study a model which assumes that such an extended microbeam of the X-mode curvature radiation is spreading the radiation polarised at a fixed position angle within an interval of pulse phase. Results. The model is capable of interpreting the strongly dissimilar polarisation of double notches in PSR B1821−24A (for which we present new polarisation data from the Nançay Radio Telescope) and PSR J0437−4715. It also explains a step-like change in PA observed at the bifurcated trailing component in the profile of J0437−4715. A generic form of the modelled PA distortion is a zigzag-shaped wiggle, which in the presence of the second polarisation mode (O-mode) can be magnified or transformed into a W- or U-shaped deflection of a total net PA. Conclusions. The model’s efficiency in interpreting dissimilar polarisation effects provides further credence to the stream-based (fan-beam) geometry of pulsar emission. It also suggests that the microbeam width may not always be assumed negligible in comparison with the angular scale of emissivity gradients in the emission region.
The Fornax galaxy cluster was observed with the Fligh Energy Stereoscopic System for a total live time of 14.5 hr, searching for very high energy (VEIE; E > 100GeV) gamma -rays from dark matter (DM) ...annihilation. No significant signal was found in searches for point-like and extended emissions. Using several models of the DM density distribution, upper limits on the DM velocity-weighted annihilation cross-section left angle bracketsigmaupsilonright angle bracket as a function of the DM particle mass are derived. Constraints are derived for different DM particle models, such as those arising from Kaluza-Klein and supersymmetric models. Various annihilation final states are considered. Possible enhancements of the DM annihilation gamma -ray flux, due to DM substructures of the DM host halo, or from the Sommerfeld effect, are studied. Additional gamma -ray contributions from internal bremsstrahlung and inverse Compton radiation are also discussed. For a DM particle mass of 1 TeV, the exclusion limits at 95% of confidence level reach values of left angle bracketsigmaupsilonright angle bracket super(95%C.L.) ~ 10 super(-23) cm super(3) s super(-1), depending on the DM particle model and halo properties. Additional contribution from DM substructures can improve the upper limits on left angle bracketsigmaupsilonright angle bracket by more than two orders of magnitude. At masses around 4.5 TeV, the enhancement by substructures and the Sommerfeld resonance effect results in a velocity-weighted annihilation cross-section upper limit at the level of left angle bracketsigmaupsilonright angle bracket super(95%CL) ~ 10 super(-26) cm super(3) s super(-1).
Aims. This article presents very-high-energy (VHE; E > 100 GeV) data from the γ-ray binary PSR B1259-63 as taken during the years 2005, 2006 and before as well as shortly after the 2007 periastron ...passage. These data extend the knowledge of the lightcurve of this object to all phases of the 3.4 year binary orbit. The lightcurve constrains physical mechanisms present in this TeV source. Methods. Observations of VHE γ-rays with the HESS telescope array using the Imaging Atmospheric Cherenkov Technique were performed. The HESS instrument features an angular resolution of < 0.1° and an energy resolution of < 20%. Gamma-ray events in an energy range of 0.5-70 TeV were recorded. From these data, energy spectra and lightcurve with a monthly time sampling were extracted. Results. VHE γ-ray emission from PSR B1259-63 was detected with an overall significance of 9.5 standard deviations using 55h of exposure, obtained from April to August 2007. The monthly flux of γ-rays during the observation period was measured, yielding VHE lightcurve data for the early pre-periastron phase of the system for the first time. No spectral variability was found on timescales of months. The spectrum is described by a power law with a photon index of Γ = 2.8 ±$0.2_{\mathrm{stat}}$ ±$0.2_{\mathrm{sys}}$ and flux normalisation $\Phi_{0}$ = (1.1 ±$0.1_{\mathrm{stat}}$ ±$0.2_{\mathrm{sys}}$) $\times$ 10-12 TeV-1 cm-2 s-1. PSR B1259-63 was also monitored in 2005 and 2006, far from periastron passage, comprising 8.9 h and 7.5 h of exposure, respectively. No significant excess of γ-rays is seen in those observations. Conclusions. PSR B1259-63 has been re-confirmed as a variable TeV γ-ray emitter. The firm detection of VHE photons emitted at a true anomaly θ≈-0.35 of the pulsar orbit, i.e. already ~50 days prior to the periastron passage, disfavors the stellar disc target scenario as a primary emission mechanism, based on current knowledge about the companion star's disc inclination, extension, and density profile.
Aims.Our aim is to study the production mechanism for very-high-energy (VHE; >100 GeV) γ-rays in distant active galactic nuclei (AGN) and use the observed VHE spectrum to derive limits on the ...Extragalactic Background Light (EBL). We also want to determine physical quantities through the modeling of the object's broad-band spectral energy distribution (SED). Methods.VHE observations (~25 h live time) of the BL Lac 1ES 0347-121 (redshift $z = 0.188$) were conducted with the High Energy Stereoscopic System (HESS) between August and December 2006. Contemporaneous X-ray and UV/optical observations from the SWIFT satellite are used to interpret the SED of the source in terms of a synchrotron self Compton (SSC) model. Results.An excess of 327 events, corresponding to a statistical significance of 10.1 standard deviations, is detected from 1ES 0347-121. Its photon spectrum, ranging from ~250 GeV to ~3 TeV, is well described by a power law with a photon index of $\Gamma = 3.10 \pm 0.23_{\mathrm{stat}} \pm 0.10_{\mathrm{sys}}$. The integral flux above 250 GeV corresponds to ~2% of the flux of the Crab Nebula above the same threshold. No VHE flux variability is detected within the data set. Conclusions.Constraints on the EBL density at optical to near-infrared wavelengths derived from the photon spectrum of 1ES 0347-121 are close to the strongest limits derived previously. The strong EBL limits confirm earlier findings, that the EBL density in the near-infrared is close to the lower limits from source counts. This implies that the universe is more transparent to VHE γ-rays than previously believed. An SSC model provides a reasonable description of the contemporaneous SED.
We study the rotational distortions of the vacuum dipole magnetic field in the context of geometric models of the radio emission from pulsars. We find that at low altitudes the rotation deflects the ...local direction of the magnetic field by at most an angle of the order of r super(2) sub(n), where r sub(n) = r/R sub(lc), r is the radial distance, and R sub(lc) is the light cylinder radius. To the lowest (i.e., second) order in r sub(n), this distortion is symmetric with respect to the plane containing the dipole axis and the rotation axis (, k) plane. The lowest order distortion that is asymmetric with respect to the (, k) plane is third-order in r sub(n). These results confirm the common assumption that the rotational sweepback has negligible effect on the position angle (P.A.) curve. We show, however, that the influence of the sweepback on the outer boundary of the open field line region (open volume) is a much larger effect, of the order of r super(1) sub(n) super(/2). The open volume is shifted backward with respect to the rotation direction by an angle d sub(ov) 6 0.2 sin ar super(1) sub(n) super(/2), where a is the dipole inclination with respect to the rotation axis. The associated phase shift of the pulse profile phi sub(ov) 6 0.2r super(1) sub(n) super(/2) can easily exceed the shift caused by combined effects of aberration and propagation time delays (-2r sub(n)). This strongly affects the misalignment of the center of the P.A. curve and the center of the pulse profile, thereby modifying the delay-radius relation. Contrary to intuition, the effect of sweepback dominates over other effects when emission occurs at low altitudes. For r sub(n) 3 x 10 super(-3) the shift becomes negative; i.e., the center of the P.A. curve precedes the profile center. With the sweepback effect included, the modified delay-radius relation predicts larger emission radii and is in much better agreement with the other methods of determining r sub(n).
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