Context.
Runaway stars form bow shocks by ploughing through the interstellar medium at supersonic speeds and are promising sources of non-thermal emission of photons. One of these objects has been ...found to emit non-thermal radiation in the radio band. This triggered the development of theoretical models predicting non-thermal photons from radio up to very-high-energy (VHE,
E
≥ 0.1 TeV) gamma rays. Subsequently, one bow shock was also detected in X-ray observations. However, the data did not allow discrimination between a hot thermal and a non-thermal origin. Further observations of different candidates at X-ray energies showed no evidence for emission at the position of the bow shocks either. A systematic search in the
Fermi
-LAT energy regime resulted in flux upper limits for 27 candidates listed in the E-BOSS catalogue.
Aim.
Here we perform the first systematic search for VHE gamma-ray emission from bow shocks of runaway stars.
Methods.
Using all available archival H.E.S.S. data we search for very-high-energy gamma-ray emission at the positions of bow shock candidates listed in the second E-BOSS catalogue release. Out of the 73 bow shock candidates in this catalogue, 32 have been observed with H.E.S.S.
Results.
None of the observed 32 bow shock candidates in this population study show significant emission in the H.E.S.S. energy range. Therefore, flux upper limits are calculated in five energy bins and the fraction of the kinetic wind power that is converted into VHE gamma rays is constrained.
Conclusions.
Emission from stellar bow shocks is not detected in the energy range between 0.14 and 18 TeV.The resulting upper limits constrain the level of VHE gamma-ray emission from these objects down to 0.1–1% of the kinetic wind energy.
Abstract
In 2017, the Event Horizon Telescope (EHT) Collaboration succeeded in capturing the first direct image of the center of the M87 galaxy. The asymmetric ring morphology and size are consistent ...with theoretical expectations for a weakly accreting supermassive black hole of mass ∼6.5 × 10
9
M
⊙
. The EHTC also partnered with several international facilities in space and on the ground, to arrange an extensive, quasi-simultaneous multi-wavelength campaign. This Letter presents the results and analysis of this campaign, as well as the multi-wavelength data as a legacy data repository. We captured M87 in a historically low state, and the core flux dominates over HST-1 at high energies, making it possible to combine core flux constraints with the more spatially precise very long baseline interferometry data. We present the most complete simultaneous multi-wavelength spectrum of the active nucleus to date, and discuss the complexity and caveats of combining data from different spatial scales into one broadband spectrum. We apply two heuristic, isotropic leptonic single-zone models to provide insight into the basic source properties, but conclude that a structured jet is necessary to explain M87’s spectrum. We can exclude that the simultaneous
γ
-ray emission is produced via inverse Compton emission in the same region producing the EHT mm-band emission, and further conclude that the
γ
-rays can only be produced in the inner jets (inward of HST-1) if there are strongly particle-dominated regions. Direct synchrotron emission from accelerated protons and secondaries cannot yet be excluded.
Abstract
Cosmological
N
-body simulations show that Milky Way–sized galaxies harbor a population of unmerged dark matter (DM) subhalos. These subhalos could shine in gamma-rays and eventually be ...detected in gamma-ray surveys as unidentified sources. We performed a thorough selection among unidentified Fermi-Large Area Telescope Objects (UFOs) to identify them as possible tera-electron-volt-scale DM subhalo candidates. We search for very-high-energy (E ≳ 100 GeV) gamma-ray emissions using H.E.S.S. observations toward four selected UFOs. Since no significant very-high-energy gamma-ray emission is detected in any data set of the four observed UFOs or in the combined UFO data set, strong constraints are derived on the product of the velocity-weighted annihilation cross section 〈
σ
v
〉 by the
J
factor for the DM models. The 95% confidence level observed upper limits derived from combined H.E.S.S. observations reach 〈
σ
v
〉
J
values of 3.7 × 10
−5
and 8.1 × 10
−6
GeV
2
cm
−2
s
−1
in the
W
+
W
−
and
τ
+
τ
−
channels, respectively, for a 1 TeV DM mass. Focusing on thermal weakly interacting massive particles, the H.E.S.S. constraints restrict the
J
factors to lie in the range 6.1 × 10
19
–2.0 × 10
21
GeV
2
cm
−5
and the masses to lie between 0.2 and 6 TeV in the
W
+
W
−
channel. For the
τ
+
τ
−
channel, the
J
factors lie in the range 7.0 × 10
19
–7.1 × 10
20
GeV
2
cm
−5
and the masses lie between 0.2 and 0.5 TeV. Assuming model-dependent predictions from cosmological
N
-body simulations on the
J
-factor distribution for Milky Way–sized galaxies, the DM models with masses >0.3 TeV for the UFO emissions can be ruled out at high confidence level.
Science with the Cherenkov Telescope Array The Cta Consortium, The Cta Consortium
The Astrophysical journal. Supplement series,
01/2019, Volume:
240, Issue:
2
eBook, Journal Article, Book
Peer reviewed
Open access
This book summarizes the science to be carried out by the upcoming Cherenkov Telescope Array, a major ground-based gamma-ray observatory that will be constructed over the next six to eight years. The ...major scientific themes, as well as core program of key science projects, have been developed by the CTA Consortium, a collaboration of scientists from many institutions worldwide. CTA will be the major facility in high-energy and very high-energy photon astronomy over the next decade and beyond. CTA will have capabilities well beyond past and present observatories. Thus, CTA's science program is expected to be rich and broad and will complement other major multiwavelength and multimessenger facilities. This book is intended to be the primary resource for the science case for CTA and it thus will be of great interest to the broader physics and astronomy communities. The electronic version (e-book) is available in open access.
The data taken with the ANTARES neutrino telescope from 2007 to 2010, a total live time of 863 days, are used to measure the oscillation parameters of atmospheric neutrinos. Muon tracks are ...reconstructed with energies as low as 20 GeV. Neutrino oscillations will cause a suppression of vertical upgoing muon neutrinos of such energies crossing the Earth. The parameters determining the oscillation of atmospheric neutrinos are extracted by fitting the event rate as a function of the ratio of the estimated neutrino energy and reconstructed flight path through the Earth. Measurement contours of the oscillation parameters in a two-flavour approximation are derived. Assuming maximal mixing, a mass difference of Δm322=(3.1±0.9)⋅10−3eV2 is obtained, in good agreement with the world average value.
Aims.
Colliding wind binary systems have long been suspected to be high-energy (HE; 100 MeV <
E
< 100 GeV)
γ
-ray emitters.
η
Car is the most prominent member of this object class and is confirmed to ...emit phase-locked HE
γ
rays from hundreds of MeV to ~100 GeV energies. This work aims to search for and characterise the very-high-energy (VHE;
E
>100 GeV)
γ
-ray emission from
η
Car around the last periastron passage in 2014 with the ground-based High Energy Stereoscopic System (H.E.S.S.).
Methods.
The region around
η
Car was observed with H.E.S.S. between orbital phase
p
= 0.78−1.10, with a closer sampling at
p
≈ 0.95 and
p
≈ 1.10 (assuming a period of 2023 days). Optimised hardware settings as well as adjustments to the data reduction, reconstruction, and signal selection were needed to suppress and take into account the strong, extended, and inhomogeneous night sky background (NSB) in the
η
Car field of view. Tailored run-wise Monte-Carlo simulations (RWS) were required to accurately treat the additional noise from NSB photons in the instrument response functions.
Results.
H.E.S.S. detected VHE
γ
-ray emission from the direction of
η
Car shortly before and after the minimum in the X-ray light-curve close to periastron. Using the point spread function provided by RWS, the reconstructed signal is point-like and the spectrum is best described by a power law. The overall flux and spectral index in VHE
γ
rays agree within statistical and systematic errors before and after periastron. The
γ
-ray spectrum extends up to at least ~400 GeV. This implies a maximum magnetic field in a leptonic scenario in the emission region of 0.5 Gauss. No indication for phase-locked flux variations is detected in the H.E.S.S. data.
Introducing the CTA concept Acharya, B.S.; Aguilar, J.; Aharonian, F. ...
Astroparticle physics,
2013, Volume:
43, Issue:
SI
Journal Article
Peer reviewed
Open access
The Cherenkov Telescope Array (CTA) is a new observatory for very high-energy (VHE) gamma rays. CTA has ambitions science goals, for which it is necessary to achieve full-sky coverage, to improve the ...sensitivity by about an order of magnitude, to span about four decades of energy, from a few tens of GeV to above 100TeV with enhanced angular and energy resolutions over existing VHE gamma-ray observatories. An international collaboration has formed with more than 1000 members from 27 countries in Europe, Asia, Africa and North and South America. In 2010 the CTA Consortium completed a Design Study and started a three-year Preparatory Phase which leads to production readiness of CTA in 2014. In this paper we introduce the science goals and the concept of CTA, and provide an overview of the project.
The unidentified very-high-energy (VHE;
E
> 0.1 TeV)
γ
-ray source, HESS J1826−130, was discovered with the High Energy Stereoscopic System (HESS) in the Galactic plane. The analysis of 215 h of HESS ...data has revealed a steady
γ
-ray flux from HESS J1826−130, which appears extended with a half-width of 0.21° ± 0.02
stat
° ± 0.05
sys
°. The source spectrum is best fit with either a power-law function with a spectral index Γ = 1.78 ± 0.10
stat
± 0.20
sys
and an exponential cut-off at 15.2
−3.2
+5.5
TeV, or a broken power-law with Γ
1
= 1.96 ± 0.06
stat
± 0.20
sys
, Γ
2
= 3.59 ± 0.69
stat
± 0.20
sys
for energies below and above
E
br
= 11.2 ± 2.7 TeV, respectively. The VHE flux from HESS J1826−130 is contaminated by the extended emission of the bright, nearby pulsar wind nebula, HESS J1825−137, particularly at the low end of the energy spectrum. Leptonic scenarios for the origin of HESS J1826−130 VHE emission related to PSR J1826−1256 are confronted by our spectral and morphological analysis. In a hadronic framework, taking into account the properties of dense gas regions surrounding HESS J1826−130, the source spectrum would imply an astrophysical object capable of accelerating the parent particle population up to ≳200 TeV. Our results are also discussed in a multiwavelength context, accounting for both the presence of nearby supernova remnants, molecular clouds, and counterparts detected in radio, X-rays, and TeV energies.
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