Refine
Has Fulltext
- no (37)
Language
- English (37)
Is part of the Bibliography
- yes (37)
Keywords
- gamma rays: general (15)
- ISM: supernova remnants (10)
- radiation mechanisms: non-thermal (10)
- cosmic rays (8)
- galaxies: active (5)
- gamma rays: galaxies (5)
- acceleration of particles (4)
- astroparticle physics (4)
- gamma rays: ISM (4)
- ISM: clouds (3)
Institute
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 phi(E > 1TeV) = (3.64 +/- 0.44(stat)+/- 0.73(sys)) x 10(-13) cm(-2) s(-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 +/- 0.11(stat)+/- 0.20(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.
Search for dark matter annihilation signatures in HESS observations of
dwarf spheroidal galaxies
(2014)
Dwarf spheroidal galaxies of the Local Group are close satellites of the Milky Way characterized by a large mass-to-light ratio and are not expected to be the site of nonthermal high-energy gamma-ray emission or intense star formation. Therefore they are among the most promising candidates for indirect dark matter searches. During the last years the High Energy Stereoscopic System (H.E.S.S.) of imaging atmospheric Cherenkov telescopes observed five of these dwarf galaxies for more than 140 hours in total, searching for TeV gamma-ray emission from annihilation of dark matter particles. The new results of the deep exposure of the Sagittarius dwarf spheroidal galaxy, the first observations of the Coma Berenices and Fornax dwarves and the reanalysis of two more dwarf spheroidal galaxies already published by the H.E.S.S. Collaboration, Carina and Sculptor, are presented. In the absence of a significant signal new constraints on the annihilation cross section applicable to weakly interacting massive particles (WIMPs) are derived by combining the observations of the five dwarf galaxies. The combined exclusion limit depends on the WIMP mass and the best constraint is reached at 1-2 TeV masses with a cross-section upper bound of similar to 3.9 x 10(-24) cm(3) s(-1) at a 95% confidence level.
An annihilation signal of dark matter is searched for from the central region of the Milky Way. Data acquired in dedicated on-off observations of the Galactic center region with H.E.S.S. are analyzed for this purpose. No significant signal is found in a total of similar to 9 h of on-off observations. Upper limits on the velocity averaged cross section, <sigma upsilon >, for the annihilation of dark matter particles with masses in the range of similar to 300 GeV to similar to 10 TeV are derived. In contrast to previous constraints derived from observations of the Galactic center region, the constraints that are derived here apply also under the assumption of a central core of constant dark matter density around the center of the Galaxy. Values of <sigma upsilon > that are larger than 3 x 10(-24) cm(3)/s are excluded for dark matter particles with masses between similar to 1 and similar to 4 TeV at 95% C.L. if the radius of the central dark matter density core does not exceed 500 pc. This is the strongest constraint that is derived on <sigma upsilon > for annihilating TeV mass dark matter without the assumption of a centrally cusped dark matter density distribution in the search region.
The Large Magellanic Cloud, a satellite galaxy of the Milky Way, has been observed with the High Energy Stereoscopic System (H.E.S.S.) above an energy of 100 billion electron volts for a deep exposure of 210 hours. Three sources of different types were detected: the pulsar wind nebula of the most energetic pulsar known, N 157B; the radio-loud supernova remnant N 132D; and the largest nonthermal x-ray shell, the superbubble 30 Dor C. The unique object SN 1987A is, unexpectedly, not detected, which constrains the theoretical framework of particle acceleration in very young supernova remnants. These detections reveal the most energetic tip of a g-ray source population in an external galaxy and provide via 30 Dor C the unambiguous detection of g-ray emission from a superbubble.
Re-observations with the HESS telescope array of the very high-energy (VHE) source HESS J1018-589A that is coincident with the Fermi-LAT gamma-ray binary 1FGL J1018.6-5856 have resulted in a source detection significance of more than 9 sigma and the detection of variability (chi(2)/nu of 238.3/155) in the emitted gamma-ray flux. This variability confirms the association of HESS J1018-589A with the high-energy gamma-ray binary detected Fermi-LAT and also confirms the point-like source as a new VHE binary system. The spectrum of HESS J1018-589A is best fit with a power-law function with photon index Gamma = 2.20 +/- 0.14(stat) +/- 0.2(sys). Emission is detected up to similar to 20 TeV. The mean differential flux level is (2.9 +/- 0.4) x 10(-13) TeV-1 cm(-2) s(-1) at 1 TeV, equivalent to similar to 1% of the flux from the Crab Nebula at the same energy. Variability is clearly detected the night-by-night light curve. When folded on the orbital period of 16.58 days, the rebinned light curve peaks in phase with the observed X-ray high-energy phaseograms. The fit of the HESS phaseogram to a constant flux provides evidence of periodicity at the level of N-sigma > 3 sigma. The of the VHE phaseogram and measured spectrum suggest a low-inclination, low-eccentricity system with a modest impact from VHE gamma-ray due to pair production (tau less than or similar to 1 at 300 GeV).
Very high energy (VHE, E > 100 GeV)gamma-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 gamma(-)ray flaring being caused by the injection of ultrarelativistic particles, emitting.-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-QG,E- 1 > 4.10 x 10(17) GeV and E-QG,E- 2 > 2.10 x 10(10) GeV for linear and quadratic LIV effects, respectively.
Spectral lines are among the most powerful signatures for dark matter (DM) annihilation searches in very-high-energy gamma rays. The central region of the Milky Way halo is one of the most promising targets given its large amount of DM and proximity to Earth. We report on a search for a monoenergetic spectral line from self-annihilations of DM particles in the energy range from 300 GeV to 70 TeV using a two-dimensional maximum likelihood method taking advantage of both the spectral and spatial features of the signal versus background. The analysis makes use of Galactic center observations accumulated over ten years (2004-2014) with the H.E.S.S. array of ground-based Cherenkov telescopes. No significant gamma-ray excess above the background is found. We derive upper limits on the annihilation cross section (sigma v) for monoenergetic DM lines at the level of 4 x 10(-28) cm(3) s(-1) at 1 TeV, assuming an Einasto DM profile for the Milky Way halo. For a DM mass of 1 TeV, they improve over the previous ones by a factor of 6. The present constraints are the strongest obtained so far for DM particles in the mass range 300 GeV-70 TeV. Ground-based gamma-ray observations have reached sufficient sensitivity to explore relevant velocity-averaged cross sections for DM annihilation into two gamma-ray photons at the level expected from the thermal relic density for TeV DM particles.
Context. NGC 253 is one of only two starburst galaxies found to emit gamma-rays from hundreds of MeV to multi-TeV energies. Accurate measurements of the very-high-energy (VHE; E> 100 GeV) and high-energy (HE; E > 60 MeV) spectra are crucial to study the underlying particle accelerators, probe the dominant emission mechanism(s) and to study cosmic-ray interaction and transport. Aims. The measurement of the VHE gamma-ray emission of NGC 253 published in 2012 by H.E.S.S. was limited by large systematic uncertainties. Here, the most up to date measurement of the gamma-ray spectrum of NGC 253 is investigated in both HE and VHE gamma-rays. Assuming a hadronic origin of the gamma-ray emission, the measurement uncertainties are propagated into the interpretation of the accelerated particle population. Methods. The data of H.E.S.S. observations are reanalysed using an updated calibration and analysis chain. The improved Fermi-LAT analysis employs more than 8 yr of data processed using pass 8. The cosmic-ray particle population is evaluated from the combined HE-VHE gamma-ray spectrum using NAIMA in the optically thin case. Results. The VHE gamma-ray energy spectrum is best fit by a power-law distribution with a flux normalisation of (1.34 +/- 0.14(stat) +/- 0.27(sys)) x 10(-13) cm(-2) s(-1) TeV-1 at 1 TeV - about 40% above, but compatible with the value obtained in Abramowski et al. (2012). The spectral index Gamma = 2.39 +/- 0.14(stat) +/- 0.25(sys) is slightly softer than but consistent with the previous measurement within systematic errors. In the Fermi energy range an integral flux of F(E > 60 MeV) = (1.56 +/- 0.28(stat) +/- 0.15(sys)) x 10(-8) cm(-2) s(-1) is obtained. At energies above similar to 3 GeV the HE spectrum is consistent with a power-law ranging into the VHE part of the spectrum measured by H.E.S.S. with an overall spectral index Gamma = 2.22 +/- 0.06(stat). Conclusions. Two scenarios for the starburst nucleus are tested, in which the gas in the starburst nucleus acts as either a thin or a thick target for hadronic cosmic rays accelerated by the individual sources in the nucleus. In these two models, the level to which NGC 253 acts as a calorimeter is estimated to a range of f(cal) = 0.1 to 1 while accounting for the measurement uncertainties. The presented spectrum is likely to remain the most accurate measurements until the Cherenkov Telescope Array (CTA) has collected a substantial set of data towards NGC 253.
HESS J1640-465 - an exceptionally luminous TeV gamma-ray supernova remnant (vol 439, pg 2828, 2014)
(2014)
G349.7+0.2 is a young Galactic supernova remnant (SNR) located at the distance of 11.5 kpc and observed across the entire electromagnetic spectrum from radio to high energy (HE; 0.1 GeV < E < 100 GeV) gamma-rays. Radio and infrared observations indicate that the remnant is interacting with a molecular cloud. In this paper, the detection of very high energy (VHE, E > 100 GeV) gamma-ray emission coincident with this SNR with the High Energy Stereoscopic System (HESS.) is reported. This makes it one of the farthest Galactic SNR ever detected in this domain. An integral flux F(E > 400 GeV) = (6.5 +/- 1.1(stat) +/- 1.3(syst)) x 10-11 ph cm(-2) s(-1) corresponding to similar to 0.7% of that of the Crab Nebula and to a luminosity of similar to 10(34) erg s(-1) above the same energy threshold, and a steep photon index Gamma(VHE) = 2.8 +/- 0.27(stat) +/- 0.20(syst) are measured. The analysis of more than 5 yr of Fermi-LAT data towards this source shows a power-law like spectrum with a best-fit photon index Gamma(HE) = 2.2 +/- 0.04.2(stat-0.31sys)(+0.13), The combined gamma-ray spectrum of 0349.7+0.2 can be described by either a broken power law (I3PL) or a power law with exponential (or sub exponential) cutoff (PLC). In the former case, the photon break energy is found at E-br,E-gamma = 551(-30)(+70) GeV, slightly higher than what is usually observed in the HE/VHE gamma-ray emitting middle-aged SNRs known to be interacting with molecular clouds. In the latter case. the exponential (respectively sub-exponential) cutoff energy is measured at E-cat,E-gamma = 1.4(-0.55)(+1.6) (respectively 0.35(-0.21)(+0.75)) TeV. A pion decay process resulting from the interaction of the accelerated protons and nuclei with the dense surrounding medium is clearly the preferred scenario to explain the gamma-ray emission. The BPL with a spectral steepening of 0.5-1 and the PLC provide equally good fits to the data. The product or the average gas density and the total energy content of accelerated protons and nuclei amounts to nu W-p similar to 5 x 10(51) erg cm(-3)
The results of follow-up observations of the TeV gamma-ray source HESS J1640-465 from 2004 to 2011 with the High Energy Stereoscopic System (HESS) are reported in this work. The spectrum is well described by an exponential cut-off power law with photon index Gamma = 2.11 +/- 0.09(stat) +/- 0.10(sys), and a cut-off energy of E-2 = 6.0(-1.2)(+2.0) TeV. The TeV emission is significantly extended and overlaps with the northwestern part of the shell of the SNR G338.3-0.0. The new HESS results, a re-analysis of archival XMM-Newton data and multiwavelength observations suggest that a significant part of the gamma-ray emission from HESS J1640-465 originates in the supernova remnant shell. In a hadronic scenario, as suggested by the smooth connection of the GeV and TeV spectra, the product of total proton energy and mean target density could be as high as W(p)n(H) similar to 4 x 10(52)(d/10kpc)(2) erg cm(-3).
Centaurus A (Cen A) is the nearest radio galaxy discovered as a very-high-energy (VHE; 100 GeV-100 TeV) gamma-ray source by the High Energy Stereoscopic System (H.E.S.S.). It is a faint VHE gamma-ray emitter, though its VHE flux exceeds both the extrapolation from early Fermi-LAT observations as well as expectations from a (misaligned) single-zone synchrotron-self Compton (SSC) description. The latter satisfactorily reproduces the emission from Cen A at lower energies up to a few GeV. New observations with H.E.S.S., comparable in exposure time to those previously reported, were performed and eight years of Fermi-LAT data were accumulated to clarify the spectral characteristics of the gamma-ray emission from the core of Cen A. The results allow us for the first time to achieve the goal of constructing a representative, contemporaneous gamma-ray core spectrum of Cen A over almost five orders of magnitude in energy. Advanced analysis methods, including the template fitting method, allow detection in the VHE range of the core with a statistical significance of 12 sigma on the basis of 213 hours of total exposure time. The spectrum in the energy range of 250 GeV-6 TeV is compatible with a power-law function with a photon index Gamma = 2.52 +/- 0.13(stat) +/- 0.20(sys). An updated Fermi-LAT analysis provides evidence for spectral hardening by Delta Gamma similar or equal to 0.4 +/- 0.1 at gamma-ray energies above 2.8(-0.6)(+1.0) GeV at a level of 4.0 sigma. The fact that the spectrum hardens at GeV energies and extends into the VHE regime disfavour a single-zone SSC interpretation for the overall spectral energy distribution (SED) of the core and is suggestive of a new gamma-ray emitting component connecting the high-energy emission above the break energy to the one observed at VHE energies. The absence of significant variability at both GeV and TeV energies does not yet allow disentanglement of the physical nature of this component, though a jet-related origin is possible and a simple two-zone SED model fit is provided to this end.