Context.The detection of gamma rays in the very-high-energy (VHE) energy range (100 GeV–100 TeV) provides a direct view of the parent population of ultra-relativistic particles found in astrophysical ...sources. For this reason, VHE gamma rays are useful for understanding the underlying astrophysical processes in non-thermal sources. Aims.We investigate unidentified VHE gamma-ray sources that have been discovered with HESS in the most sensitive blind survey of the Galactic plane at VHE energies conducted so far. Methods.The HESS array of imaging atmospheric Cherenkov telescopes (IACTs) has a high sensitivity compared with previous instruments (~$0.01\:\mathrm{Crab}$ in 25 h observation time for a $5\sigma$ point-source detection), and with its large field of view, is well suited for scan-based observations. The on-going HESS survey of the inner Galaxy has revealed a large number of new VHE sources, and for each we attempt to associate the VHE emission with multi-wavelength data in the radio through X-ray wavebands. Results. For each of the eight unidentified VHE sources considered here, we present the energy spectra and sky maps of the sources and their environment. The VHE morphology is compared with available multi-wavelength data (mainly radio and X-rays). No plausible counterparts are found.
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.
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. Observations of very high-energy gamma -rays from blazars provide information about acceleration mechanisms occurring in their innermost regions. Studies of variability in these objects lead ...to a better understanding of the mechanisms in play. Aims. To investigate the spectral and temporal variability of VHE (>100 GeV) gamma -rays of the well-known high-frequency-peaked BLLac object PKS2155-304 with the HESS imaging atmospheric Cherenkov telescopes over a wide range of flux states. Methods. Data collected from 2005 to 2007 were analyzed. Spectra were derived on time scales ranging from 3 years to 4 min. Light curve variability was studied through doubling timescales and structure functions and compared with red noise process simulations. Results. The source was found to be in a low state from 2005 to 2007, except for a set of exceptional flares that occurred in July 2006. The quiescent state of the source is characterized by an associated mean flux level of (4.32 plus or minus 0.09 sub(stat) plus or minus 0.86 sub(syst)) x 10 super(-11) cm super(-2) s super(-1) above 200 GeV, or approximately 15% of the Crab Nebula, and a power-law photon index of Gamma = 3.53 plus or minus 0.06 sub(stat) plus or minus 0.10 sub(syst). During the flares of July 2006, doubling timescales of ~2 min are found. The spectral index variation is examined over two orders of magnitude in flux, yielding different behavior at low and high fluxes, which is a new phenomenon in VHE gamma -ray emitting blazars. The variability amplitude characterized by the fractional rms F sub(var) is strongly energy-dependent and is alpha E super(0.10 plus or minus 0.01). The light curve rms correlates with the flux. This is the signature of a multiplicative process that can be accounted for as a red noise with a Fourier index of ~2. Conclusions. This unique data set shows evidence of a low-level gamma -ray emission state from PKS2155-304 that possibly has a different origin than the outbursts. The discovery of the light curve lognormal behavior might be an indicator of the origin of aperiodic variability in blazars.
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.
Context. The blazar 1ES 1101-232 was observed with the High Energy Stereoscopic System (HESS) of Atmospheric Cherenkov Telescopes (ACT) in 2004 and 2005, for a live time of 43 h. VHE ( E > 10 ...super(11) eV) \gamma-rays were detected for the first time from this object. Aims. 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. Methods. 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. Results. 1ES 1101-232 was detected with HESS with an excess of 649 photons, at a significance of 10\sigma. The measured VHE \gamma-ray flux amounts to {\rm d}N/{\rm d}E = (5.63 \pm 0.89) \times 10 super(-13) (E/{\rm TeV}) - (2.94\pm0.20) } cm super(-2) s super(-1) TeV super(-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. Conclusions. 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 \nu F_{\nu} = 14 nWm super(-2) sr super(-1) at 1.5 similar to \mum as presented in Aharonian et al. (2006a) suggests an intrinsic VHE power output peak of the source at above 3 TeV.
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.
Aims.The complex Monoceros Loop SNR/Rosette Nebula region contains several potential sources of very-high-energy (VHE) γ-ray emission and two as yet unidentified high-energy EGRET sources. Sensitive ...VHE observations are required to probe acceleration processes in this region. Methods.The HESS telescope array has been used to search for very high-energy γ-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 γ-ray production via hadronic interactions. Results.We announce the discovery of a new γ-ray source, HESS J0632+057, located close to the rim of the Monoceros SNR. This source is unresolved by HESS 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 γ-ray source 3EG J0634+0521. No evidence for an associated molecular cloud was found in the CO data.
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 gamma-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 gamma-ray flux reached a maximum of similar to 11 times the Crab flux (>400 GeV), with rise/decay timescales of similar to 1 h, plus a few smaller-amplitude flares superimposed on the decaying phase. The emission in the X-ray and VHE gamma-ray bands is strongly correlated, with no evidence of lags. The spectra also evolve with similar patterns, and are always soft ( photon index G > 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 gamma-ray peak is inside the observed passband, at similar to 400-600 GeV. The VHE spectrum shows a curvature that is variable with time and stronger at higher fluxes. The huge VHE variations (similar to 22x) 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(C)/L(S) similar to 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.
Aims. Results from γ-ray observations by the HESS telescope array in the direction of the young stellar cluster Westerlund 2 are presented. Methods. Stereoscopic imaging of Cherenkov light emission ...of γ-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. Results. The detection of a new γ-ray source is reported from HESS 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 9σ, and shows extension beyond a point-like object within the HESS point-spread function. The differential γ-ray spectrum of the emission region is measured over approximately two orders of magnitude in flux. Conclusions.The spatial coincidence between HESS J1023–575 and the young open cluster Westerlund 2, hosting e.g. the massive WR binary WR 20a, requires one to look into a variety of potential models to account for the observed very-high-energy (VHE) γ-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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