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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.
Dwarf spheroidal galaxies are among the most promising targets for detecting signals of Dark Matter (DM) annihilations. The H.E.S.S. experiment has observed five of these systems for a total of about 130 hours. The data are re-analyzed here, and, in the absence of any detected signals, are interpreted in terms of limits on the DM annihilation cross section. Two scenarios are considered: i) DM annihilation into mono-energetic gamma-rays and ii) DM in the form of pure WIMP multiplets that, annihilating into all electroweak bosons, produce a distinctive gamma-ray spectral shape with a high-energy peak at the DM mass and a lower-energy continuum. For case i), upper limits at 95% confidence level of about <sigma upsilon > less than or similar to 3 x 10(-25) cm(3) s(-1) are obtained in the mass range of 400 GeV to 1TeV. For case ii), the full spectral shape of the models is used and several excluded regions are identified, but the thermal masses of the candidates are not robustly ruled out.
Diffuse gamma-ray emission is the most prominent observable signature of celestial cosmic-ray interactions at high energies. While already being investigated at GeVenergies over several decades, assessments of diffuse gamma-ray emission at TeVenergies remain sparse. After completion of the systematic survey of the inner Galaxy, the H.E.S.S. experiment is in a prime position to observe large-scale diffuse emission at TeVenergies. Data of the H.E.S.S. Galactic Plane Survey are investigated in regions off known gamma-ray sources. Corresponding gamma-ray flux measurements were made over an extensive grid of celestial locations. Longitudinal and latitudinal profiles of the observed gamma-ray fluxes show characteristic excess emission not attributable to known gamma-ray sources. For the first time large-scale gamma-ray emission along the Galactic plane using imaging atmospheric Cherenkov telescopes has been observed. While the background subtraction technique limits the ability to recover modest variation on the scale of the H.E.S.S. field of view or larger, which is characteristic of the inverse Compton scatter-induced Galactic diffuse emission, contributions of neutral pion decay as well as emission from unresolved gamma-ray sources can be recovered in the observed signal to a large fraction. Calculations show that the minimum gamma-ray emission from pi(0) decay represents a significant contribution to the total signal. This detection is interpreted as a mix of diffuse Galactic gamma-ray emission and unresolved sources.
The inner region of the Milky Way halo harbors a large amount of dark matter (DM). Given its proximity, it is one of the most promising targets to look for DM. We report on a search for the annihilations of DM particles using gamma-ray observations towards the inner 300 pc of the Milky Way, with the H.E.S.S. array of ground-based Cherenkov telescopes. The analysis is based on a 2D maximum likelihood method using Galactic Center (GC) data accumulated by H.E.S.S. over the last 10 years (2004-2014), and does not show any significant gamma-ray signal above background. Assuming Einasto and Navarro-Frenk-White DM density profiles at the GC, we derive upper limits on the annihilation cross section <sigma nu >. These constraints are the strongest obtained so far in the TeV DM mass range and improve upon previous limits by a factor 5. For the Einasto profile, the constraints reach <sigma nu > values of 6 x 10(-26) cm(3) s(-1) in the W+W- channel for a DM particle mass of 1.5 TeV, and 2 x 10(-26) cm(3) s(-1) in the tau(+)tau(-) channel for a 1 TeV mass. For the first time, ground-based gamma-ray observations have reached sufficient sensitivity to probe <sigma nu > values expected from the thermal relic density for TeV DM particles.
Unprecedented study of the broadband emission of Mrk 421 during flaring activity in March 2010
(2015)
Context. Because of its proximity, Mrk 421 is one of the best sources on which to study the nature of BL Lac objects. Its proximity allows us to characterize its broadband spectral energy distribution (SED).
Aims. The goal is to better understand the mechanisms responsible for the broadband emission and the temporal evolution of Mrk 421. These mechanisms may also apply to more distant blazars that cannot be studied with the same level of detail.
Methods. A flare occurring in March 2010 was observed for 13 consecutive days (from MJD 55 265 to MJD 55 277) with unprecedented wavelength coverage from radio to very high energy (VHE; E > 100 GeV) gamma-rays with MAGIC, VERITAS, Whipple, Fermi-LAT, MAXI, RXTE, Swift, GASP-WEBT, and several optical and radio telescopes. We modeled the day-scale SEDs with one-zone and two-zone synchrotron self-Compton (SSC) models, investigated the physical parameters, and evaluated whether the observed broadband SED variability can be associated with variations in the relativistic particle population.
Results. The activity of Mrk 421 initially was high and then slowly decreased during the 13-day period. The flux variability was remarkable at the X-ray and VHE bands, but it was minor or not significant at the other bands. The variability in optical polarization was also minor. These observations revealed an almost linear correlation between the X-ray flux at the 2-10 keV band and the VHE gamma-ray flux above 200 GeV, consistent with the gamma-rays being produced by inverse-Compton scattering in the Klein-Nishina regime in the framework of SSC models. The one-zone SSC model can describe the SED of each day for the 13 consecutive days reasonably well, which once more shows the success of this standard theoretical scenario to describe the SEDs of VHE BL Lacs such as Mrk 421. This flaring activity is also very well described by a two-zone SSC model, where one zone is responsible for the quiescent emission, while the other smaller zone, which is spatially separated from the first, contributes to the daily variable emission occurring at X-rays and VHE gamma-rays. The second blob is assumed to have a smaller volume and a narrow electron energy distribution with 3 x 10(4) < gamma < 6 x 10(5), where. is the Lorentz factor of the electrons. Such a two-zone scenario would naturally lead to the correlated variability at the X-ray and VHE bands without variability at the optical/UV band, as well as to shorter timescales for the variability at the X-ray and VHE bands with respect to the variability at the other bands.
Conclusions. Both the one-zone and the two-zone SSC models can describe the daily SEDs via the variation of only four or five model parameters, under the hypothesis that the variability is associated mostly with the underlying particle population. This shows that the particle acceleration and cooling mechanism that produces the radiating particles might be the main mechanism responsible for the broadband SED variations during the flaring episodes in blazars. The two-zone SSC model provides a better agreement with the observed SED at the narrow peaks of the low-and high-energy bumps during the highest activity, although the reported one-zone SSC model could be further improved by varying the parameters related to the emitting region itself (delta, B and R), in addition to the parameters related to the particle population.