The dwarf planet (1) Ceres, the largest object in the main asteroid belt with a mean diameter of about 950 kilometres, is located at a mean distance from the Sun of about 2.8 astronomical units (one ...astronomical unit is the Earth-Sun distance). Thermal evolution models suggest that it is a differentiated body with potential geological activity. Unlike on the icy satellites of Jupiter and Saturn, where tidal forces are responsible for spewing briny water into space, no tidal forces are acting on Ceres. In the absence of such forces, most objects in the main asteroid belt are expected to be geologically inert. The recent discovery of water vapour absorption near Ceres and previous detection of bound water and OH near and on Ceres (refs 5-7) have raised interest in the possible presence of surface ice. Here we report the presence of localized bright areas on Ceres from an orbiting imager. These unusual areas are consistent with hydrated magnesium sulfates mixed with dark background material, although other compositions are possible. Of particular interest is a bright pit on the floor of crater Occator that exhibits probable sublimation of water ice, producing haze clouds inside the crater that appear and disappear with a diurnal rhythm. Slow-moving condensed-ice or dust particles may explain this haze. We conclude that Ceres must have accreted material from beyond the 'snow line', which is the distance from the Sun at which water molecules condense.
The dwarf planet Ceres (equatorial diameter 963km) is the largest object that has remained in the main asteroid belt (Russell and Raymond, 2012), while most large bodies have been destroyed or ...removed by dynamical processes (Petit et al. 2001; Minton and Malhotra, 2009). Pre-Dawn investigations (McCord and Sotin, 2005; Castillo-Rogez and McCord, 2010; Castillo-Rogez et al., 2011) suggest that Ceres is a thermally evolved, but still volatile-rich body with potential geological activity, that was never completely molten, but possibly differentiated into a rocky core, an ice-rich mantle, and may contain remnant internal liquid water. Thermal alteration should contribute to producing a (dark) carbonaceous chondritic-like surface (McCord and Sotin, 2005; Castillo-Rogez and McCord, 2010; Castillo-Rogez et al., 2011; Nathues et al., 2015) containing ammoniated phyllosilicates (King et al., 1992; De Sanctis et al., 2015 and 2016). Here we show and analyse global contrast-rich colour mosaics, derived from a camera on-board Dawn at Ceres (Russell et al., 2016). Colours are unexpectedly more diverse on global scale than anticipated by Hubble Space Telescope (Li et al., 2006) and ground-based observations (Reddy et al. 2015). Dawn data led to the identification of five major colour units. The youngest units identified by crater counting, termed bright and bluish units, are exclusively found at equatorial and intermediate latitudes. We identified correlations between the distribution of the colour units, crater size, and formation age, inferring a crustal stratigraphy. Surface brightness and spectral properties are not correlated. The youngest surface features are the bright spots at crater Occator (~Ø 92km). Their colour spectra are highly consistent with the presence of carbonates while most of the remaining surface resembles modifications of various types of ordinary carbonaceous chondrites.
► Prospects for DM searches with CTA at various targets. ► Prospects for ALP searches with CTA. ► Prospects for LIV searches with CTA. ► Prospects for neutrinos, magnetic monopoles and gravitational ...waves searches with CTA.
The Cherenkov Telescope Array (CTA) is a project for a next-generation observatory for very high energy (GeV–TeV) ground-based gamma-ray astronomy, currently in its design phase, and foreseen to be operative a few years from now. Several tens of telescopes of 2–3 different sizes, distributed over a large area, will allow for a sensitivity about a factor 10 better than current instruments such as H.E.S.S, MAGIC and VERITAS, an energy coverage from a few tens of GeV to several tens of TeV, and a field of view of up to 10°. In the following study, we investigate the prospects for CTA to study several science questions that can profoundly influence our current knowledge of fundamental physics. Based on conservative assumptions for the performance of the different CTA telescope configurations currently under discussion, we employ a Monte Carlo based approach to evaluate the prospects for detection and characterisation of new physics with the array.
First, we discuss CTA prospects for cold dark matter searches, following different observational strategies: in dwarf satellite galaxies of the Milky Way, which are virtually void of astrophysical background and have a relatively well known dark matter density; in the region close to the Galactic Centre, where the dark matter density is expected to be large while the astrophysical background due to the Galactic Centre can be excluded; and in clusters of galaxies, where the intrinsic flux may be boosted significantly by the large number of halo substructures. The possible search for spatial signatures, facilitated by the larger field of view of CTA, is also discussed. Next we consider searches for axion-like particles which, besides being possible candidates for dark matter may also explain the unexpectedly low absorption by extragalactic background light of gamma-rays from very distant blazars. We establish the axion mass range CTA could probe through observation of long-lasting flares in distant sources. Simulated light-curves of flaring sources are also used to determine the sensitivity to violations of Lorentz invariance by detection of the possible delay between the arrival times of photons at different energies. Finally, we mention searches for other exotic physics with CTA.
The measurement of an excess in the cosmic-ray electron spectrum between 300 and 800 GeV by the ATIC experiment has – together with the PAMELA detection of a rise in the positron fraction up to ...≈100 GeV – motivated many interpretations in terms of dark matter scenarios; alternative explanations assume a nearby electron source like a pulsar or supernova remnant. Here we present a measurement of the cosmic-ray electron spectrum with H.E.S.S. starting at 340 GeV. While the overall electron flux measured by H.E.S.S. is consistent with the ATIC data within statistical and systematic errors, the H.E.S.S. data exclude a pronounced peak in the electron spectrum as suggested for interpretation by ATIC. The H.E.S.S. data follow a power-law spectrum with spectral index of 3.0±0.1(stat.)± 0.3(syst.), which steepens at about 1 TeV.
The extragalactic background light (EBL) is the diffuse radiation with the second highest energy density in the Universe after the cosmic microwave background. The aim of this study is the ...measurement of the imprint of the EBL opacity to γ-rays on the spectra of the brightest extragalactic sources detected with the High Energy Stereoscopic System (H.E.S.S.). The originality of the method lies in the joint fit of the EBL optical depth and of the intrinsic spectra of the sources, assuming intrinsic smoothness. Analysis of a total of ~105γ-ray events enables the detection of an EBL signature at the 8.8σ level and constitutes the first measurement of the EBL optical depth using very-high energy (E > 100 GeV) γ-rays. The EBL flux density is constrained over almost two decades of wavelengths 0.30 μm, 17 μm and the peak value at 1.4 μm is derived as λFλ = 15 ± 2stat ± 3sys nW m-2 sr-1.
Observations of the Crab nebula with HESS Aharonian, F.; Akhperjanian, A. G.; Bazer-Bachi, A. R. ...
Astronomy and astrophysics (Berlin),
10/2006, Letnik:
457, Številka:
3
Journal Article
Recenzirano
Odprti dostop
Context.The Crab nebula was observed with the HESS stereoscopic Cherenkov-telescope array between October 2003 and January 2005 for a total of 22.9 h (after data quality selection). This period of ...time partly overlapped with the commissioning phase of the experiment; observations were made with three operational telescopes in late 2003 and with the complete 4 telescope array in January–February 2004 and October 2004–January 2005. Aims.Observations of the Crab nebula are discussed and used as an example to detail the flux and spectral analysis procedures of HESS. The results are used to evaluate the systematic uncertainties in HESS flux measurements. Methods.The Crab nebula data are analysed using standard HESS analysis procedures, which are described in detail. The flux and spectrum of γ-rays from the source are calculated on run-by-run and monthly time-scales, and a correction is applied for long-term variations in the detector sensitivity. Comparisons of the measured flux and spectrum over the observation period, along with the results from a number of different analysis procedures are used to estimate systematic uncertainties in the measurements. Results.The data, taken at a range of zenith angles between $45^{\circ}$ and $65^{\circ}$, show a clear signal with over 7500 excess events. The energy spectrum is found to follow a power law with an exponential cutoff, with photon index $\Gamma = 2.39$ ± $0.03_{{\rm {stat}}}$ and cutoff energy $E_{\rm c} = (14.3 \pm 2.1_{{\rm {stat}}})~{\rm TeV}$ between 440 GeV and 40 TeV. The observed integral flux above 1 TeV is $(2.26 \pm 0.08_{{\rm {stat}}}) \times 10^{-11}~{{\rm cm}^{-2}~{\rm s}^{-1}}$. The estimated systematic error on the flux measurement is estimated to be 20%, while the estimated systematic error on the spectral slope is 0.1.
The diffuse extragalactic background light consists of the sum of the starlight emitted by galaxies through the history of the Universe, and it could also have an important contribution from the ...‘first stars’, which may have formed before galaxy formation began. Direct measurements are difficult and not yet conclusive, owing to the large uncertainties caused by the bright foreground emission associated with zodiacal light. An alternative approach is to study the absorption features imprinted on the γ-ray spectra of distant extragalactic objects by interactions of those photons with the background light photons. Here we report the discovery of γ-ray emission from the blazars H 2356 - 309 and 1ES 1101 - 232, at redshifts z = 0.165 and z = 0.186, respectively. Their unexpectedly hard spectra provide an upper limit on the background light at optical/near-infrared wavelengths that appears to be very close to the lower limit given by the integrated light of resolved galaxies. The background flux at these wavelengths accordingly seems to be strongly dominated by the direct starlight from galaxies, thus excluding a large contribution from other sources—in particular from the first stars formed. This result also indicates that intergalactic space is more transparent to γ-rays than previously thought.
Aims. Observations of shell-type supernova remnants (SNRs) in the GeV to multi-TeV γ-ray band, coupled with those at millimetre radio wavelengths, are motivated by the search for cosmic-ray ...accelerators in our Galaxy. The old-age mixed-morphology SNR W 28 (distance ~2 kpc) is a prime target due to its interaction with molecular clouds along its northeastern boundary and other clouds situated nearby. Methods. We observed the W 28 field (for ~40 h) at very high energy (VHE) γ-ray energies ($E > 0.1$ TeV) with the HESS. Cherenkov telescopes. A reanalysis of EGRET $E > 100$ MeV data was also undertaken. Results from the NANTEN 4 m telescope Galactic plane survey and other CO observations were used to study molecular clouds. Results. We have discovered VHE γ-ray emission (HESS J1801-233) coincident with the northeastern boundary of W 28 and a complex of sources (HESS J1800-240A, B and C) ~0.5° south of W 28 in the Galactic disc. The EGRET source (GRO J1801-2320) is centred on HESS J1801-233 but may also be related to HESS J1800-240 given the large EGRET point spread function. The VHE differential photon spectra are well fit by pure power laws with indices Γ ~2.3 to 2.7. The spectral indices of HESS J1800-240A, B, and C are consistent within statistical errors. All VHE sources are ~10′ in intrinsic radius except for HESS J1800-240C, which appears pointlike. The NANTEN 12CO($J = 1{-}0$) data reveal molecular clouds positionally associating with the VHE emission, spanning a ~15 km s-1 range in local standard of rest velocity. Conclusions. The VHE/molecular cloud association could indicate a hadronic origin for HESS J1801-233 and HESS J1800-240, 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 W 28 (~2 kpc) and extending up to ~4 kpc. Assuming 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 SNRs like W 28. Additionally and/or alternatively, particle acceleration may come from several catalogued SNRs and SNR candidates, the energetic ultra compact HII region W 28A2, and the HII regions M 8 and M 20, 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 W 28.
We report on a survey of the inner part of the Galactic plane in very high energy gamma rays with the H.E.S.S. Cerenkov telescope system. The Galactic plane between +/-30° in longitude and +/-3° in ...latitude relative to the Galactic center was observed in 500 pointings for a total of 230 hr, reaching an average flux sensitivity of 2% of the Crab Nebula at energies above 200 GeV. Fourteen previously unknown sources were detected at a significance level greater than 4 σ after accounting for all trials involved in the search. Initial results on the eight most significant of these sources were already reported elsewhere (Aharonian and coworkers). Here we present detailed spectral and morphological information for all the new sources, along with a discussion on possible counterparts in other wavelength bands. The distribution in Galactic latitude of the detected sources appears to be consistent with a scale height in the Galactic disk for the parent population smaller than 100 pc, consistent with expectations for supernova remnants and/or pulsar wind nebulae.
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