The inner 10 pc of our Galaxy contains many counterpart candidates of the very high energy (VHE; >100 GeV) γ-ray point source HESS J1745-290. Within the point spread function of the H.E.S.S. ...measurement, at least three objects are capable of accelerating particles to VHE and beyond and of providing the observed γ-ray flux. Previous attempts to address this source confusion were hampered by the fact that the projected distances between these objects were of the order of the error circle radius of the emission centroid (34 arcsec, dominated by the pointing uncertainty of the H.E.S.S. instrument). Here we present H.E.S.S. data of the Galactic Centre region, recorded with an improved control of the instrument pointing compared to H.E.S.S. standard pointing procedures. Stars observed during γ-ray observations by optical guiding cameras mounted on each H.E.S.S. telescope are used for off-line pointing calibration, thereby decreasing the systematic pointing uncertainties from 20 to 6 arcsec per axis. The position of HESS J1745-290 is obtained by fitting a multi-Gaussian profile to the background-subtracted γ-ray count map. A spatial comparison of the best-fitting position of HESS J1745-290 with the position and morphology of candidate counterparts is performed. The position is, within a total error circle radius of 13 arcsec, coincident with the position of the supermassive black hole Sgr A* and the recently discovered pulsar wind nebula candidate G359.95-0.04. It is significantly displaced from the centroid of the supernova remnant Sgr A East, excluding this object with high probability as the dominant source of the VHE γ-ray emission. PUBLICATION ABSTRACT
The inner 10 pc of our Galaxy contains many counterpart candidates of the very high energy (VHE; > 100 GeV) g-ray point source HESS J1745-290. Within the point spread function of the H.E.S.S. ...measurement, at least three objects are capable of accelerating particles to VHE and beyond and of providing the observed g-ray flux. Previous attempts to address this source confusion were hampered by the fact that the projected distances between these objects were of the order of the error circle radius of the emission centroid (34 arcsec, dominated by the pointing uncertainty of the H.E.S.S. instrument). Here we present H.E.S.S. data of the Galactic Centre region, recorded with an improved control of the instrument pointing compared to H.E.S.S. standard pointing procedures. Stars observed during g-ray observations by optical guiding cameras mounted on each H.E.S.S. telescope are used for off-line pointing calibration, thereby decreasing the systematic pointing uncertainties from 20 to 6 arcsec per axis. The position of HESS J1745-290 is obtained by fitting a multi-Gaussian profile to the background-subtracted g-ray count map. A spatial comparison of the best-fitting position of HESS J1745-290 with the position and morphology of candidate counterparts is performed. The position is, within a total error circle radius of 13 arcsec, coincident with the position of the supermassive black hole Sgr A* and the recently discovered pulsar wind nebula candidate G359.95-0.04. It is significantly displaced from the centroid of the supernova remnant Sgr A East, excluding this object with high probability as the dominant source of the VHE g-ray emission.
Context. The detection of gamma-rays in the very-high-energy (VHE) range (100 GeV−100 TeV) offers the possibility of studying the parent population of ultrarelativistic particles found in ...astrophysical sources, so it is useful for understanding the underlying astrophysical processes in nonthermal sources. Aims. The discovery of the VHE gamma-ray source HESS J1507-622 is reported and possibilities regarding its nature are investigated. Methods. The H.E.S.S. array of imaging atmospheric Cherenkov telescopes (IACTs) has a high sensitivity compared with previous instruments (~1% of the Crab flux in 25 h observation time for a 5σ point-source detection) and has a large field of view (~5° in diameter). HESS J1507-622 was discovered within the ongoing H.E.S.S. survey of the inner Galaxy, and the source was also studied by means of dedicated multiwavelength observations. Results. A Galactic gamma-ray source, HESS J1507-622, located ~3.5° from the Galactic plane was detected with a statistical significance >9σ. Its energy spectrum is well fitted by a power law with spectral index Γ = 2.24 ± 0.16stat ± 0.20sys and a flux above 1 TeV of (1.5 ± 0.4stat ± 0.3sys) × 10-12 cm-2 s-1. Possible interpretations (considering both hadronic and leptonic models) of the VHE gamma-ray emission are discussed in the absence of an obvious counterpart.
The rapidly varying (~10 min timescale) non-thermal X-ray emission observed from Sgr A$^{\star}$ implies that particle acceleration is occuring close to the event horizon of the supermassive black ...hole. The TeV γ-ray source HESS J1745-290 is coincident with Sgr A$^{\star}$ and may be closely related to its X-ray emission. Simultaneous X-ray and TeV observations are required to elucidate the relationship between these objects. We report on joint HESS/Chandra observations performed in July 2005, during which an X-ray flare was detected. Despite a factor of ≈9 increase in the X-ray flux of Sgr A$^{\star}$, no evidence is found for an increase in the TeV γ-ray flux from this region. We find that an increase in the γ-ray flux of a factor of 2 or greater can be excluded at a confidence level of 99%. This finding disfavours scenarios in which the keV and TeV emission are associated with the same population of accelerated particles and in which the bulk of the γ-ray emission is produced within ~1014 cm (~100$\,R_{\rm S}$) of the supermassive black hole.
Aims.Very high energy (VHE; $E>100$ GeV) γ-ray studies were performed for 18 active galactic nuclei (AGN) from a variety of AGN classes. Methods.VHE observations of a sample of 14 AGN, considered ...candidate VHE emitters, were made with the High Energy Stereoscopic System (HESS) between January 2005 and July 2007. Large-zenith-angle observations of three northern AGN (Mkn 421, Mkn 501, 1ES 1218+304), known to emit VHE γ-rays, were also performed in order to sample their spectral energy distributions (SEDs) above 1 TeV. In addition, the VHE flux from 1ES 1101-232, previously detected by HESS in 2004-2005, was monitored during 2006 and 2007. Results.As significant detections from the HESS observation program are reported elsewhere, the results reported here are primarily integral flux upper limits. The average exposure for each of the 14 VHE-candidate AGN is ~7 h live time, and the observations have an average energy threshold between 230 GeV and 590 GeV. Upper limits for these 14 AGN range from <0.9% to <4.9% of the Crab Nebula flux, and eight of these are the most constraining ever reported for the object. The brief (<2.2 h each) large-zenith-angle observations yield upper limits for Mkn 501 (<20% Crab above 2.5 TeV) and 1ES 1218+304 (<17% Crab above 1.0 TeV), and a marginal detection ($3.5\sigma$) of Mkn 421 (50% Crab above 2.1 TeV). 1ES 1101-232 was marginally detected ($3.6\sigma$, 1.7% Crab above 260 GeV) during the 2006 (13.7 h live time) observations, but not in the 2007 (4.6 h live time) data. The upper limit in 2007 (<1.9% Crab above 260 GeV) is below the average flux measured by HESS from 2004-2006.
We report the discovery of faint very high energy (VHE; E > 100 GeV) g-ray emission from the radio galaxy Centaurus A in observations performed with the High Energy Stereoscopic System (H.E.S.S.) ...experiment, an imaging atmospheric Cherenkov telescope array consisting of four telescopes located in Namibia. Centaurus A has been observed for more than 120 hr. A signal with a statistical significance of 5.0s is detected from the region including the radio core and the inner kpc jets. The integral flux above an energy threshold of ~250 GeV is measured to be 0.8% of the flux of the Crab Nebula (apparent luminosity: L(>250 GeV) 2.6 X 1039 erg s-1, adopting a distance of 3.8 Mpc). The spectrum can be described by a power law with a photon index of 2.7 ± 0.5stat ± 0.2sys. No significant flux variability is detected in the data set. However, the low flux only allows detection of variability on the timescale of days to flux increments above a factor of ~15-20 (3s and 4s, respectively). The discovery of VHE g-ray emission from Centaurus A reveals particle acceleration in the source to >TeV energies and, together with M 87, establishes radio galaxies as a class of VHE emitters.
A search for a dark matter (DM) annihilation signal into $\gamma$-rays toward the direction of the Canis Major (CMa) overdensity is presented. The nature of CMa is still controversial and one ...scenario represents it as a dwarf galaxy, making it an interesting candidate for DM annihilation searches. A total of 9.6 hours of high quality data were collected with the H.E.S.S. array of Imaging Atmospheric Cherenkov Telescopes (IACTs) and no evidence for a very high energy $\gamma$-ray signal is found. Upper limits on the CMa dwarf galaxy mass of the order of 10$^{9}$ M$_{\odot}$ are derived at the 95% C.L. assuming neutralino masses in the range 500 GeV - 10 TeV and relatively large annihilation cross-sections. Constraints on the velocity-weighted annihilation cross section $<\sigma v>$, are calculated for specific WIMP scenarios, using a NFW model for the DM halo profile and taking advantage of numerical simulations of hierarchical structure formation. 95% C.L. exclusion limits of the order of 5 $\times$ 10$^{-24}$ cm$^{3}$ s$^{-1}$ are reached in the 500 GeV - 10 TeV DM particle mass interval, assuming a total halo mass of 3 $\times$ 10$^{8}$ M$_{\odot}$.
Aims: The BL Lac object RGB J0152+017 (z=0.080) was predicted to be a very high-energy (VHE; > 100 GeV) gamma-ray source, due to its high X-ray and radio fluxes. Our aim is to understand the ...radiative processes by investigating the observed emission and its production mechanism using the High Energy Stereoscopic System (H.E.S.S.) experiment. Methods: We report recent observations of the BL Lac source RGB J0152+017 made in late October and November 2007 with the H.E.S.S. array consisting of four imaging atmospheric Cherenkov telescopes. Contemporaneous observations were made in X-rays by the Swift and RXTE satellites, in the optical band with the ATOM telescope, and in the radio band with the Nancay Radio Telescope. Results: A signal of 173 gamma-ray photons corresponding to a statistical significance of 6.6 sigma was found in the data. The energy spectrum of the source can be described by a powerlaw with a spectral index of 2.95+/-0.36stat+/-0.20syst. The integral flux above 300 GeV corresponds to ~2% of the flux of the Crab nebula. The source spectral energy distribution (SED) can be described using a two-component non-thermal synchrotron self-Compton (SSC) leptonic model, except in the optical band, which is dominated by a thermal host galaxy component. The parameters that are found are very close to those found in similar SSC studies in TeV blazars. Conclusions: RGB J0152+017 is discovered as a source of VHE gamma-rays by H.E.S.S. The location of its synchrotron peak, as derived from the SED in Swift data, allows clearly classification it as a high-frequency-peaked BL Lac (HBL).
We observed the W28 field (for ~40 h) at Very High Energy (VHE) gamma-ray energies (E>0.1 TeV) with the H.E.S.S. Cherenkov telescopes. A reanalysis of EGRET E>100 MeV data was also undertaken. ...Results from the NANTEN 4m telescope Galactic plane survey and other CO observations have been used to study molecular clouds. We have discovered VHE gamma-ray emission (HESSJ1801-233) coincident with the northeastern boundary of W28, and a complex of sources (HESSJ1800-240A, B and C) ~0.5 deg south of W28, in the Galactic disc. The VHE differential photon spectra are well fit by pure power laws with indices Gamma~2.3 to 2.7. The NANTEN ^{12}CO(J=1-0) data reveal molecular clouds positionally associating with the VHE emission, spanning a ~15 km s^{-1} range in local standard of rest velocity. The VHE/molecular cloud association could indicate a hadronic origin for HESSJ1801-233 and HESSJ1800-240, and several cloud components in projection may contribute to the VHE emission. The clouds have components covering a broad velocity range encompassing the distance estimates for W28 (~2 kpc), and extending up to ~4 kpc. Assuming a hadronic origin, and distances of 2 and 4 kpc for cloud components, the required cosmic ray density enhancement factors (with respect to the solar value) are in the range ~10 to ~30. If situated at 2 kpc distance, such cosmic ray densities may be supplied by a SNR like W28. Additionally and/or alternatively, particle acceleration may come from several catalogued SNRs and SNR candidates, the energetic ultra compact HII region W28A2, and the HII regions M8 and M20 along with their associated open clusters. Further sub-mm observations would be recommended to probe in detail the dynamics of the molecular clouds at velocites >10 km s^{-1}, and their possible connection to W28.
The rapidly varying (~10 minute timescale) non-thermal X-ray emission observed from Sgr A* implies that particle acceleration is occuring close to the event horizon of the supermassive black hole. ...The TeV gamma-ray source HESS J1745-290 is coincident with Sgr A* and may be closely related to its X-ray emission. Simultaneous X-ray and TeV observations are required to elucidate the relationship between these objects. We report on joint H.E.S.S./Chandra observations performed in July 2005, during which an X-ray flare was detected. Despite a factor of 9 increase in the X-ray flux of Sgr A*, no evidence is found for an increase in the TeV gamma-ray flux from this region. We find that an increase in the gamma-ray flux of a factor of 2 or greater can be excluded at a confidence level of 99%. This finding disfavours scenarios in which the keV and TeV emission are associated with the same population of accelerated particles and in which the bulk of the gamma-ray emission is produced within ~10^{14} cm (~100 R_S) of the supermassive black hole.
![Localizing the VHE [gamma]-...](http://api.summon.serialssolutions.com/2.0.0/image/issn/XR4PS5YY4K/0035-8711/small "Localizing the VHE [gamma]-ray source at the Galactic Centre")
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