The Fornax galaxy cluster was observed with the High Energy Stereoscopic System (H.E.S.S.) for a ... more The Fornax galaxy cluster was observed with the High Energy Stereoscopic System (H.E.S.S.) for a total live time of 14.5 hours, searching for very-highenergy (VHE, E > 100 GeV) γ-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 σv 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 γ-ray flux, due to DM substructures of the DM host halo, or from the Sommerfeld effect, are studied. Additional γ-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 σv 95% C.L. ∼ 10 −23 cm 3 s −1 , depending on the DM particle -4model and halo properties. Additional contribution from DM substructures can improve the upper limits on σv 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 σv 95% C.L. ∼10 −26 cm 3 s −1 .
The giant radio galaxy M 87 with its proximity (16 Mpc), famous jet, and very massive black hole ... more The giant radio galaxy M 87 with its proximity (16 Mpc), famous jet, and very massive black hole ((3 − 6) × 10 9 M ) provides a unique opportunity to investigate the origin of very high energy (VHE; E>100 GeV) γ-ray emission generated in relativistic outflows and the surroundings of super-massive black holes. M 87 has been established as a VHE γ-ray emitter since 2006. The VHE γ-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 arXiv:1111.5341v1 [astro-ph.CO]
Several models of Quantum Gravity predict Lorentz Symmetry breaking at energy scales approaching ... more Several models of Quantum Gravity predict Lorentz Symmetry breaking at energy scales approaching the Planck scale (∼ 10 19 GeV). With present photon data from the observations of distant astrophysical sources, it is possible to constrain the Lorentz Symmetry breaking linear term in the standard photon dispersion relations. Gamma-ray Bursts (GRB) and flaring Active Galactic Nuclei (AGN) are complementary to each other for this purpose, since they are observed at different distances in different energy ranges and with different levels of variability. Following a previous publication of the High Energy Stereoscopic System (H.E.S.S.) collaboration [1], a more sensitive event-by-event method consisting of a likelihood fit is applied to PKS 2155-304 flare data of MJD 53944 (July 28, 2006) as used in the previous publication. The previous limit on the linear term is improved by a factor of ∼3 up to M l QG > 2.1 × 10 18 GeV and is currently the best result obtained with blazars. The sensitivity to the quadratic term is lower and provides a limit of M q QG > 6.4×10 10 GeV, which is the best value obtained so far with an AGN and similar to the best limits obtained with GRB.
The Sculptor and Carina Dwarf spheroidal galaxies were observed with the H.E.S.S. Cherenkov teles... more The Sculptor and Carina Dwarf spheroidal galaxies were observed with the H.E.S.S. Cherenkov telescope array between January 2008 and December 2009. The data sets consist of a total of 11.8 and 14.8 hours of high quality data, respectively. No gamma-ray signal was detected at the nominal positions of these galaxies above 220 GeV and 320 GeV, respectively. Upper limits on the gamma-ray fluxes at 95% C.L. assuming two forms for the spectral energy distribution (a power law shape and one derived from dark matter annihilation) are obtained at the level of 10 −13 to 10 −12 cm −2 s −1 in the TeV range. Constraints on the velocity weighted dark matter particle annihilation cross section for both Sculptor and Carina dwarf galaxies range from σv ∼ 10 −21 cm 3 s −1 down to σv ∼ 10 −22 cm 3 s −1 depending on the dark 3 matter halo model used. Possible enhancements of the gamma-ray flux are studied: the Sommerfeld effect, which is found to exclude some dark matter particle masses, the internal Bremsstrahlung and clumps in the dark-matter halo distributions.
Aims. The recent discovery of the radio shell-type supernova remnant (SNR), G353.6-0.7, in spatia... more Aims. The recent discovery of the radio shell-type supernova remnant (SNR), G353.6-0.7, in spatial coincidence with the unidentified TeV source HESS J1731−347 has motivated further observations of the source with the High Energy Stereoscopic System (H.E.S.S.) Cherenkov telescope array to test a possible association of the γ-ray emission with the SNR. Methods. With a total of 59 hours of observation, representing about four times the initial exposure available in the discovery paper of HESS J1731−347, the γ-ray morphology is investigated and compared with the radio morphology. An estimate of the distance is derived by comparing the interstellar absorption derived from X-rays and the one obtained from 12 CO and HI observations. Results. The deeper γ-ray observation of the source has revealed a large shell-type structure with similar position and extension (r∼0.25 • ) as the radio SNR, thus confirming their association. By accounting for the H.E.S.S. angular resolution and projection effects within a simple shell model, the radial profile is compatible with a thin, spatially unresolved, rim. Together with RX J1713.7−3946, RX J0852.0−4622 and SN 1006, HESS J1731−347 is now the fourth SNR with a significant shell morphology at TeV energies. The derived lower limit on the distance of the SNR of 3.2 kpc is used together with radio and X-ray data to discuss the possible origin of the γ-ray emission, either via inverse Compton scattering of electrons or the decay of neutral pions resulting from proton-proton interaction.
Context. Observations of very high energy γ-rays from blazars provide information about accelerat... more Context. Observations of very high energy γ-rays from blazars provide information about acceleration mechanisms occurring in their innermost regions. Studies of variability in these objects allow a better understanding of the mechanisms at play. Aims. To investigate the spectral and temporal variability of VHE (> 100 GeV) γ-rays of the well-known high-frequency-peaked BL Lac object PKS 2155−304 with the H.E.S.S. imaging atmospheric Cherenkov telescopes over a wide range of flux states. Methods. Data collected from 2005 to 2007 are analyzed. Spectra are derived on time scales ranging from 3 years to 4 minutes. Light curve variability is studied through doubling timescales and structure functions, and is compared with red noise process simulations. Results. The source is found to be in a low state from 2005 to 2007, except for a set of exceptional flares which occurred in July 2006. The quiescent state of the source is characterized by an associated mean flux level of (4.32 ± 0.09 stat ± 0.86 syst ) × 10 −11 cm −2 s −1 above 200 GeV, or approximately 15% of the Crab Nebula, and a power law photon index of Γ = 3.53 ± 0.06 stat ± 0.10 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 behaviour at low and high fluxes, which is a new phenomenon in VHE γ-ray emitting blazars. The variability amplitude characterized by the fractional r.m.s. F var is strongly energy-dependent and is ∝ E 0.19±0.01 . The light curve r.m.s. correlates with the flux. This is the signature of a multiplicative process which can be accounted for as a red noise with a Fourier index of ∼ 2.
Context. Multiwavelength (MWL) observations of the blazar PKS 2155-304 during two weeks in July a... more Context. Multiwavelength (MWL) observations of the blazar PKS 2155-304 during two weeks in July and August 2006, the period when two exceptional flares at very high energies (VHE, E 100 GeV) occurred, provide a detailed picture of the evolution of its emission. The complete data set from this campaign is presented, including observations in VHE γ-rays (H.E.S.S.), X-rays (RXTE, CHANDRA, SWIFT XRT), optical (SWIFT UVOT, Bronberg, Watcher, ROTSE), and in the radio band (NRT, HartRAO, ATCA). Optical and radio light curves from 2004 to 2008 are compared to the available VHE data from this period, to put the 2006 campaign into the context of the long-term evolution of the source. ¶ Aims. The data set offers a close view of the evolution of the source on different time scales and yields new insights into the properties of the emission process. The predictions of synchrotron self-Compton (SSC) scenarios are compared to the MWL data, with the aim of describing the dominant features in the data down to the hour time scale. Methods. The spectral variability in the X-ray and VHE bands is explored and correlations between the integral fluxes at different wavelengths are evaluated. SSC modelling is used to interpret the general trends of the varying spectral energy distribution. Results. The X-ray and VHE γ-ray emission are correlated during the observed high state of the source, but show no direct connection with longer wavelengths. The long-term flux evolution in the optical and radio bands is found to be correlated and shows that the source reaches a high state at long wavelengths after the occurrence of the VHE flares. Spectral hardening is seen in the SWIFT XRT data.
Context: Puppis A is an interesting ∼4 kyr-old supernova remnant (SNR) that shows strong evidence... more Context: Puppis A is an interesting ∼4 kyr-old supernova remnant (SNR) that shows strong evidence of interaction between the forward shock and a molecular cloud. It has been studied in detail from radio frequencies to high-energy (HE, 0.1-100 GeV) γ-rays. An analysis of the Fermi-LAT data has shown an extended HE γ-ray emission with a 0.2-100 GeV spectrum exhibiting no significant deviation from a power law, unlike most of the GeV-emitting SNRs known to be interacting with molecular clouds. This makes it a promising target for imaging atmospheric Cherenkov telescopes (IACTs) to probe the γ-ray emission above 100 GeV. Aims: Very-high-energy (VHE, E ≥ 0.1 TeV) γ-ray emission from Puppis A is for the first time searched for with the High Energy Stereoscopic System (H.E.S.S.). Methods: Stereoscopic imaging of Cherenkov radiation from extensive air showers is used to reconstruct the direction and energy of the incident γ-rays in order to produce sky images and source spectra. The profile likelihood method is applied to constrain the existence of a potential break or cutoff in the photon spectrum. Results: The analysis of the H.E.S.S. data does not reveal any significant emission towards Puppis A. The derived upper limits on the differential photon flux imply that its broadband γ-ray spectrum must exhibit a spectral break or cutoff. By combining Fermi-LAT and H.E.S.S. measurements, the 99% confidence level upper limits on such a cutoff are found to be 450 and 280 GeV, assuming a power law with a simple exponential and a sub-exponential cutoff, respectively. It is concluded that none of the standard limitations (age, size, radiative losses) on the particle acceleration mechanism, assumed to be still on-going at present, can explain the lack of VHE signal. The scenario in which particle acceleration has ceased some time ago is considered as an alternative explanation. The HE/VHE spectrum of Puppis A could then exhibit a break of non-radiative origin, (as observed in several other interacting SNRs, albeit at somewhat higher energies) owing to the interaction with dense and neutral material in particular towards the northeastern region.
The Fornax galaxy cluster was observed with the High Energy Stereoscopic System (H.E.S.S.) for a ... more The Fornax galaxy cluster was observed with the High Energy Stereoscopic System (H.E.S.S.) for a total live time of 14.5 hours, searching for very-highenergy (VHE, E > 100 GeV) γ-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 σv 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 γ-ray flux, due to DM substructures of the DM host halo, or from the Sommerfeld effect, are studied. Additional γ-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 σv 95% C.L. ∼ 10 −23 cm 3 s −1 , depending on the DM particle -4model and halo properties. Additional contribution from DM substructures can improve the upper limits on σv 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 σv 95% C.L. ∼10 −26 cm 3 s −1 .
The giant radio galaxy M 87 with its proximity (16 Mpc), famous jet, and very massive black hole ... more The giant radio galaxy M 87 with its proximity (16 Mpc), famous jet, and very massive black hole ((3 − 6) × 10 9 M ) provides a unique opportunity to investigate the origin of very high energy (VHE; E>100 GeV) γ-ray emission generated in relativistic outflows and the surroundings of super-massive black holes. M 87 has been established as a VHE γ-ray emitter since 2006. The VHE γ-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 arXiv:1111.5341v1 [astro-ph.CO]
Several models of Quantum Gravity predict Lorentz Symmetry breaking at energy scales approaching ... more Several models of Quantum Gravity predict Lorentz Symmetry breaking at energy scales approaching the Planck scale (∼ 10 19 GeV). With present photon data from the observations of distant astrophysical sources, it is possible to constrain the Lorentz Symmetry breaking linear term in the standard photon dispersion relations. Gamma-ray Bursts (GRB) and flaring Active Galactic Nuclei (AGN) are complementary to each other for this purpose, since they are observed at different distances in different energy ranges and with different levels of variability. Following a previous publication of the High Energy Stereoscopic System (H.E.S.S.) collaboration [1], a more sensitive event-by-event method consisting of a likelihood fit is applied to PKS 2155-304 flare data of MJD 53944 (July 28, 2006) as used in the previous publication. The previous limit on the linear term is improved by a factor of ∼3 up to M l QG > 2.1 × 10 18 GeV and is currently the best result obtained with blazars. The sensitivity to the quadratic term is lower and provides a limit of M q QG > 6.4×10 10 GeV, which is the best value obtained so far with an AGN and similar to the best limits obtained with GRB.
The Sculptor and Carina Dwarf spheroidal galaxies were observed with the H.E.S.S. Cherenkov teles... more The Sculptor and Carina Dwarf spheroidal galaxies were observed with the H.E.S.S. Cherenkov telescope array between January 2008 and December 2009. The data sets consist of a total of 11.8 and 14.8 hours of high quality data, respectively. No gamma-ray signal was detected at the nominal positions of these galaxies above 220 GeV and 320 GeV, respectively. Upper limits on the gamma-ray fluxes at 95% C.L. assuming two forms for the spectral energy distribution (a power law shape and one derived from dark matter annihilation) are obtained at the level of 10 −13 to 10 −12 cm −2 s −1 in the TeV range. Constraints on the velocity weighted dark matter particle annihilation cross section for both Sculptor and Carina dwarf galaxies range from σv ∼ 10 −21 cm 3 s −1 down to σv ∼ 10 −22 cm 3 s −1 depending on the dark 3 matter halo model used. Possible enhancements of the gamma-ray flux are studied: the Sommerfeld effect, which is found to exclude some dark matter particle masses, the internal Bremsstrahlung and clumps in the dark-matter halo distributions.
Aims. The recent discovery of the radio shell-type supernova remnant (SNR), G353.6-0.7, in spatia... more Aims. The recent discovery of the radio shell-type supernova remnant (SNR), G353.6-0.7, in spatial coincidence with the unidentified TeV source HESS J1731−347 has motivated further observations of the source with the High Energy Stereoscopic System (H.E.S.S.) Cherenkov telescope array to test a possible association of the γ-ray emission with the SNR. Methods. With a total of 59 hours of observation, representing about four times the initial exposure available in the discovery paper of HESS J1731−347, the γ-ray morphology is investigated and compared with the radio morphology. An estimate of the distance is derived by comparing the interstellar absorption derived from X-rays and the one obtained from 12 CO and HI observations. Results. The deeper γ-ray observation of the source has revealed a large shell-type structure with similar position and extension (r∼0.25 • ) as the radio SNR, thus confirming their association. By accounting for the H.E.S.S. angular resolution and projection effects within a simple shell model, the radial profile is compatible with a thin, spatially unresolved, rim. Together with RX J1713.7−3946, RX J0852.0−4622 and SN 1006, HESS J1731−347 is now the fourth SNR with a significant shell morphology at TeV energies. The derived lower limit on the distance of the SNR of 3.2 kpc is used together with radio and X-ray data to discuss the possible origin of the γ-ray emission, either via inverse Compton scattering of electrons or the decay of neutral pions resulting from proton-proton interaction.
Context. Observations of very high energy γ-rays from blazars provide information about accelerat... more Context. Observations of very high energy γ-rays from blazars provide information about acceleration mechanisms occurring in their innermost regions. Studies of variability in these objects allow a better understanding of the mechanisms at play. Aims. To investigate the spectral and temporal variability of VHE (> 100 GeV) γ-rays of the well-known high-frequency-peaked BL Lac object PKS 2155−304 with the H.E.S.S. imaging atmospheric Cherenkov telescopes over a wide range of flux states. Methods. Data collected from 2005 to 2007 are analyzed. Spectra are derived on time scales ranging from 3 years to 4 minutes. Light curve variability is studied through doubling timescales and structure functions, and is compared with red noise process simulations. Results. The source is found to be in a low state from 2005 to 2007, except for a set of exceptional flares which occurred in July 2006. The quiescent state of the source is characterized by an associated mean flux level of (4.32 ± 0.09 stat ± 0.86 syst ) × 10 −11 cm −2 s −1 above 200 GeV, or approximately 15% of the Crab Nebula, and a power law photon index of Γ = 3.53 ± 0.06 stat ± 0.10 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 behaviour at low and high fluxes, which is a new phenomenon in VHE γ-ray emitting blazars. The variability amplitude characterized by the fractional r.m.s. F var is strongly energy-dependent and is ∝ E 0.19±0.01 . The light curve r.m.s. correlates with the flux. This is the signature of a multiplicative process which can be accounted for as a red noise with a Fourier index of ∼ 2.
Context. Multiwavelength (MWL) observations of the blazar PKS 2155-304 during two weeks in July a... more Context. Multiwavelength (MWL) observations of the blazar PKS 2155-304 during two weeks in July and August 2006, the period when two exceptional flares at very high energies (VHE, E 100 GeV) occurred, provide a detailed picture of the evolution of its emission. The complete data set from this campaign is presented, including observations in VHE γ-rays (H.E.S.S.), X-rays (RXTE, CHANDRA, SWIFT XRT), optical (SWIFT UVOT, Bronberg, Watcher, ROTSE), and in the radio band (NRT, HartRAO, ATCA). Optical and radio light curves from 2004 to 2008 are compared to the available VHE data from this period, to put the 2006 campaign into the context of the long-term evolution of the source. ¶ Aims. The data set offers a close view of the evolution of the source on different time scales and yields new insights into the properties of the emission process. The predictions of synchrotron self-Compton (SSC) scenarios are compared to the MWL data, with the aim of describing the dominant features in the data down to the hour time scale. Methods. The spectral variability in the X-ray and VHE bands is explored and correlations between the integral fluxes at different wavelengths are evaluated. SSC modelling is used to interpret the general trends of the varying spectral energy distribution. Results. The X-ray and VHE γ-ray emission are correlated during the observed high state of the source, but show no direct connection with longer wavelengths. The long-term flux evolution in the optical and radio bands is found to be correlated and shows that the source reaches a high state at long wavelengths after the occurrence of the VHE flares. Spectral hardening is seen in the SWIFT XRT data.
Context: Puppis A is an interesting ∼4 kyr-old supernova remnant (SNR) that shows strong evidence... more Context: Puppis A is an interesting ∼4 kyr-old supernova remnant (SNR) that shows strong evidence of interaction between the forward shock and a molecular cloud. It has been studied in detail from radio frequencies to high-energy (HE, 0.1-100 GeV) γ-rays. An analysis of the Fermi-LAT data has shown an extended HE γ-ray emission with a 0.2-100 GeV spectrum exhibiting no significant deviation from a power law, unlike most of the GeV-emitting SNRs known to be interacting with molecular clouds. This makes it a promising target for imaging atmospheric Cherenkov telescopes (IACTs) to probe the γ-ray emission above 100 GeV. Aims: Very-high-energy (VHE, E ≥ 0.1 TeV) γ-ray emission from Puppis A is for the first time searched for with the High Energy Stereoscopic System (H.E.S.S.). Methods: Stereoscopic imaging of Cherenkov radiation from extensive air showers is used to reconstruct the direction and energy of the incident γ-rays in order to produce sky images and source spectra. The profile likelihood method is applied to constrain the existence of a potential break or cutoff in the photon spectrum. Results: The analysis of the H.E.S.S. data does not reveal any significant emission towards Puppis A. The derived upper limits on the differential photon flux imply that its broadband γ-ray spectrum must exhibit a spectral break or cutoff. By combining Fermi-LAT and H.E.S.S. measurements, the 99% confidence level upper limits on such a cutoff are found to be 450 and 280 GeV, assuming a power law with a simple exponential and a sub-exponential cutoff, respectively. It is concluded that none of the standard limitations (age, size, radiative losses) on the particle acceleration mechanism, assumed to be still on-going at present, can explain the lack of VHE signal. The scenario in which particle acceleration has ceased some time ago is considered as an alternative explanation. The HE/VHE spectrum of Puppis A could then exhibit a break of non-radiative origin, (as observed in several other interacting SNRs, albeit at somewhat higher energies) owing to the interaction with dense and neutral material in particular towards the northeastern region.
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Papers by A. Abramowski