GRB221009A is the brightest gamma-ray burst ever detected. To probe the very-high-energy (VHE, $>$100 GeV) emission, the High Energy Stereoscopic System (H.E.S.S.) began observations 53 hours after ...the triggering event, when the brightness of the moonlight no longer precluded observations. We derive differential and integral upper limits using H.E.S.S. data from the third, fourth, and ninth nights after the initial GRB detection, after applying atmospheric corrections. The combined observations yield an integral energy flux upper limit of $\Phi_\mathrm{UL}^{95\%} = 9.7 \times 10^{-12}~\mathrm{erg\,cm^{-2}\,s^{-1}}$ above $E_\mathrm{thr} = 650$ GeV. The constraints derived from the H.E.S.S. observations complement the available multiwavelength data. The radio to X-ray data are consistent with synchrotron emission from a single electron population, with the peak in the SED occurring above the X-ray band. Compared to the VHE-bright GRB190829A, the upper limits for GRB221009A imply a smaller gamma-ray to X-ray flux ratio in the afterglow. Even in the absence of a detection, the H.E.S.S. upper limits thus contribute to the multiwavelength picture of GRB221009A, effectively ruling out an IC dominated scenario.
Most $\gamma$-ray detected active galactic nuclei are blazars with one of
their relativistic jets pointing towards the Earth. Only a few objects belong
to the class of radio galaxies or misaligned ...blazars. Here, we investigate the
nature of the object PKS 0625-354, its $\gamma$-ray flux and spectral
variability and its broad-band spectral emission with observations from
H.E.S.S., Fermi-LAT, Swift-XRT, and UVOT taken in November 2018. The H.E.S.S.
light curve above 200 GeV shows an outburst in the first night of observations
followed by a declining flux with a halving time scale of 5.9h. The
$\gamma\gamma$-opacity constrains the upper limit of the angle between the jet
and the line of sight to $\sim10^\circ$. The broad-band spectral energy
distribution shows two humps and can be well fitted with a single-zone
synchrotron self Compton emission model. We conclude that PKS 0625-354, as an
object showing clear features of both blazars and radio galaxies, can be
classified as an intermediate active galactic nuclei. Multi-wavelength studies
of such intermediate objects exhibiting features of both blazars and radio
galaxies are sparse but crucial for the understanding of the broad-band
emission of $\gamma$-ray detected active galactic nuclei in general.
In July 2021, PKS 1510-089 exhibited a significant flux drop in the high-energy gamma-ray (by a factor 10) and optical (by a factor 5) bands and remained in this low state throughout 2022. Similarly, ...the optical polarization in the source vanished, resulting in the optical spectrum being fully explained through the steady flux of the accretion disk and the broad-line region. Unlike the aforementioned bands, the very-high-energy gamma-ray and X-ray fluxes did not exhibit a significant flux drop from year to year. This suggests that the steady-state very-high-energy gamma-ray and X-ray fluxes originate from a different emission region than the vanished parts of the high-energy gamma-ray and optical jet fluxes. The latter component has disappeared through either a swing of the jet away from the line-of-sight or a significant drop in the photon production efficiency of the jet close to the black hole. Either change could become visible in high-resolution radio images.
Magnetic fields in galaxies and galaxy clusters are believed to be the result of the amplification of intergalactic seed fields during the formation of large-scale structures in the universe. ...However, the origin, strength, and morphology of this intergalactic magnetic field (IGMF) remain unknown. Lower limits on (or indirect detection of) the IGMF can be obtained from observations of high-energy gamma rays from distant blazars. Gamma rays interact with the extragalactic background light to produce electron-positron pairs, which can subsequently initiate electromagnetic cascades. The \(\gamma\)-ray signature of the cascade depends on the IGMF since it deflects the pairs. Here we report on a new search for this cascade emission using a combined data set from the Fermi Large Area Telescope and the High Energy Stereoscopic System. Using state-of-the-art Monte Carlo predictions for the cascade signal, our results place a lower limit on the IGMF of \(B > 7.1\times10^{-16}\) G for a coherence length of 1 Mpc even when blazar duty cycles as short as 10 yr are assumed. This improves on previous lower limits by a factor of 2. For longer duty cycles of \(10^4\) (\(10^7\)) yr, IGMF strengths below \(1.8\times10^{-14}\) G (\(3.9\times10^{-14}\) G) are excluded, which rules out specific models for IGMF generation in the early universe.
A&A 666, A124 (2022) Young massive stellar clusters are extreme environments and potentially
provide the means for efficient particle acceleration. Indeed, they are
increasingly considered as being ...responsible for a significant fraction of
cosmic rays (CRs) accelerated within the Milky Way. Westerlund 1, the most
massive known young stellar cluster in our Galaxy is a prime candidate for
studying this hypothesis. While the very-high-energy $\gamma$-ray source HESS
J1646-458 has been detected in the vicinity of Westerlund 1 in the past, its
association could not be firmly identified. We aim to identify the physical
processes responsible for the $\gamma$-ray emission around Westerlund 1 and
thus to better understand the role of massive stellar clusters in the
acceleration of Galactic CRs. Using 164 hours of data recorded with the High
Energy Stereoscopic System (H.E.S.S.), we carried out a deep
spectromorphological study of the $\gamma$-ray emission of HESS J1646-458. We
furthermore employed H I and CO observations of the region to infer the
presence of gas that could serve as target material for interactions of
accelerated CRs. We detected large-scale ($\sim 2^\circ$ diameter) $\gamma$-ray
emission with a complex morphology, exhibiting a shell-like structure and
showing no significant variation with $\gamma$-ray energy. The combined energy
spectrum of the emission extends to several tens of TeV, and is uniform across
the entire source region. We did not find a clear correlation of the
$\gamma$-ray emission with gas clouds as identified through H I and CO
observations. We conclude that, of the known objects within the region, only
Westerlund 1 can explain the bulk of the $\gamma$-ray emission. Several CR
acceleration sites and mechanisms are conceivable, and discussed in detail.
(abridged)
Young massive stellar clusters are extreme environments and potentially provide the means for efficient particle acceleration. Indeed, they are increasingly considered as being responsible for a ...significant fraction of cosmic rays (CRs) accelerated within the Milky Way. Westerlund 1, the most massive known young stellar cluster in our Galaxy is a prime candidate for studying this hypothesis. While the very-high-energy \(\gamma\)-ray source HESS J1646-458 has been detected in the vicinity of Westerlund 1 in the past, its association could not be firmly identified. We aim to identify the physical processes responsible for the \(\gamma\)-ray emission around Westerlund 1 and thus to better understand the role of massive stellar clusters in the acceleration of Galactic CRs. Using 164 hours of data recorded with the High Energy Stereoscopic System (H.E.S.S.), we carried out a deep spectromorphological study of the \(\gamma\)-ray emission of HESS J1646-458. We furthermore employed H I and CO observations of the region to infer the presence of gas that could serve as target material for interactions of accelerated CRs. We detected large-scale (\(\sim 2^\circ\) diameter) \(\gamma\)-ray emission with a complex morphology, exhibiting a shell-like structure and showing no significant variation with \(\gamma\)-ray energy. The combined energy spectrum of the emission extends to several tens of TeV, and is uniform across the entire source region. We did not find a clear correlation of the \(\gamma\)-ray emission with gas clouds as identified through H I and CO observations. We conclude that, of the known objects within the region, only Westerlund 1 can explain the bulk of the \(\gamma\)-ray emission. Several CR acceleration sites and mechanisms are conceivable, and discussed in detail. (abridged)
Achieving more accurate reacting flow numerical solutions apparently demand employing higher-order schemes, utilizing finer grids, and benefiting from more advanced chemistry models. One major ...objective of this work is to extend an inclusive low-order upwind-biased scheme in the context of finite-volume-element method to predict turbulent reacting flows on coarse grid resolutions very reliably. In this regard, a low-order upwind-biased scheme is suitably extended to approximate the mixture fraction variances at the cell-faces. This scheme implements the reacting flow physics explicitly in deriving the proposed mixture fraction variance expressions. These physical implementations enhance the derived expressions to result in superior turbulent reacting flow solutions even on coarse grid resolutions. To assess the accuracy of new expressions, we simulate a sample turbulent non-premixed flame with strong non-equilibrium effects of turbulence on chemistry. The comparisons show that the current low-order scheme is robust enough to predict the complex structure of non-premixed flames very reliably even on coarse grids.
We extend a hybrid finite-volume-element (FVE) method to treat the laminar reacting flow in cylindrical coordinates considering the collocation of all chosen primitive variables. To approximate the ...advection fluxes at the cell faces, we use the upwind-biased physical influence scheme PIS and derive a few new extended expressions applicable in the cylindrical frame. These expressions are derived for both the Navier-Stokes and reactive flow governing equations, of which the latter expressions are considered novel in the finite-volume formulation. To validate our derived expressions, the current results are compared with the experimental data and other available numerical solutions. The results show that the accuracy of the current expressions is better than those derived by the past similar numerical investigators.
In this work, we derive a few new advective flux approximation expressions, apply them in a hybrid finite-volume-element (FVE) formulation, and solve the turbulent reacting flow governing equations ...in the cylindrical frame. To derive these advective-kinetic-based expressions, we benefit from the advantages of a physical influence scheme (PIS) basically, extend it to the cylindrical frame suitably, and approximate the required advective flux terms at the cell faces more accurately. The present numerical scheme not only respects the physics of flow correctly but also resolves the pressure–velocity coupling problem automatically. We also suggest a bi-implicit algorithm to solve the set of coupled turbulent reacting flow governing equations, in which the turbulence and chemistry governing equations are solved simultaneously. To evaluate the accuracy of new derived FVE–PIS expressions, we compare the current solutions with other available numerical solutions and experimental data. The comparisons show that the new derived expressions provide some more advantages over the past numerical approaches in solving turbulent diffusion flame in the cylindrical frame. Indeed, the current method and formulations can be used to solve and analyze the turbulent diffusion flames in the cylindrical coordinates very reliably.
