157 research outputs found
The case of the missing VHE GRBs: A retrospective study of Swift gamma-ray bursts with Imaging Atmospheric Cherenkov Telescopes
Gamma-ray bursts (GRBs) are particle acceleration sites that can emit photons
in the very high-energy (VHE) domain through non-thermal processes. From 2004
until 2018, the current generation of Imaging Atmospheric Cherenkov Telescopes
(IACTs) did not detect any GRB in the VHE domain. However, from 2018 to 2020,
five detections have been reported. In this work, we try to solve the case of
the missing VHE GBRs prior to 2018. We aim to identify GRBs that might have
eluded VHE detection in the past years by the H.E.S.S., MAGIC, and VERITAS
IACTs. To do so, we study GRBs with known redshift detected by \emph{Swift}
from 2004 until June 2022. We first identify all GRBs that could have been
observed by these IACTs since 2004, considering observation conditions and
visibility constraints. We assume a relation between the X-rays and the VHE
gamma rays based on the VHE GRBs detected to date and combine this with the
redshift measurements, instrument response information, and observation
conditions to predict the observed VHE gamma-ray flux from the \emph{Swift}-XRT
measurements. We report findings on 12 bright low-redshift GRBs that we
identify as most likely to have been detected in the VHE domain by current
IACTs. The rate of IACT-detectable GRBs with ideal observation conditions is
1 VHE GRB per year with the current configuration. With its lower energy
threshold and higher sensitivity, this rate increases to 4 VHE GRBs per
year with CTA.Comment: Submitted to ApJ
The Vanishing of the Primary Emission Region in PKS 1510-089
In 2021 July, PKS 1510-089 exhibited a significant flux drop in the high-energy γ-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 γ-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 γ-ray and X-ray fluxes originate from a different emission region than the vanished parts of the high-energy γ-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
H.E.S.S. and MAGIC observations of a sudden cessation of a very-high-energy γ-ray flare in PKS 1510-089 in May 2016
The flat spectrum radio quasar (FSRQ) PKS 1510-089 is known for its complex multiwavelength behaviour and it is one of only a few FSRQs detected in very-high-energy (VHE, E> 100 GeV) γ rays. The VHE γ-ray observations with H.E.S.S. and MAGIC in late May and early June 2016 resulted in the detection of an unprecedented flare, which revealed, for the first time, VHE γ-ray intranight variability for this source. While a common variability timescale of 1.5 h has been found, there is a significant deviation near the end of the flare, with a timescale of ∼20 min marking the cessation of the event. The peak flux is nearly two orders of magnitude above the low-level emission. For the first time, a curvature was detected in the VHE γ-ray spectrum of PKS 1510-089, which can be fully explained by the absorption on the part of the extragalactic background light. Optical R-band observations with ATOM revealed a counterpart of the γ-ray flare, even though the detailed flux evolution differs from the VHE γ-ray light curve. Interestingly, a steep flux decrease was observed at the same time as the cessation of the VHE γ-ray flare. In the high-energy (HE, E> 100 MeV) γ-ray band, only a moderate flux increase was observed with Fermi-LAT, while the HE γ-ray spectrum significantly hardens up to a photon index of 1.6. A search for broad-line region (BLR) absorption features in the γ-ray spectrum indicates that the emission region is located outside of the BLR. Radio very-long-baseline interferometry observations reveal a fast-moving knot interacting with a standing jet feature around the time of the flare. As the standing feature is located ∼50 pc from the black hole, the emission region of the flare may have been located at a significant distance from the black hole. If this is indeed a true correlation, the VHE γ rays must have been produced far down in the jet, where turbulent plasma crosses a standing shock
Search for dark matter annihilation signals in the H.E.S.S. Inner galaxy survey
The central region of the Milky Way is one of the foremost locations to look for dark matter (DM) signatures. We report the first results on a search for DM particle annihilation signals using new observations from an unprecedented γ-ray survey of the Galactic Center (GC) region, i.e., the Inner Galaxy Survey, at very high energies (≳100 GeV) performed with the H.E.S.S. array of five ground-based Cherenkov telescopes. No significant γ-ray excess is found in the search region of the 2014-2020 dataset and a profile likelihood ratio analysis is carried out to set exclusion limits on the annihilation cross section ⟨σv⟩. Assuming Einasto and Navarro-Frenk-White (NFW) DM density profiles at the GC, these constraints are the strongest obtained so far in the TeV DM mass range. For the Einasto profile, the constraints reach ⟨σv⟩ values of 3.7×10^{-26} cm^{3} s^{-1} for 1.5 TeV DM mass in the W^{+}W^{-} annihilation channel, and 1.2×10^{-26} cm^{3} s^{-1} for 0.7 TeV DM mass in the τ^{+}τ^{-} annihilation channel. With the H.E.S.S. Inner Galaxy Survey, ground-based γ-ray observations thus probe ⟨σv⟩ values expected from thermal-relic annihilating TeV DM particles
A deep spectromorphological study of the γ -ray emission surrounding the young massive stellar cluster Westerlund 1
Context. 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) that are 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 γ-ray source HESS J1646-458 has been detected in the vicinity of Westerlund 1 in the past, its association could not be firmly identified. Aims. We aim to identify the physical processes responsible for the γ-ray emission around Westerlund 1 and thus to understand the role of massive stellar clusters in the acceleration of Galactic CRs better. Methods. Using 164 h of data recorded with the High Energy Stereoscopic System (H.E.S.S.), we carried out a deep spectromorphological study of the γ-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. Results. We detected large-scale (~2 diameter) γ-ray emission with a complex morphology, exhibiting a shell-like structure and showing no significant variation with γ-ray energy. The combined energy spectrum of the emission extends to several tens of TeV, and it is uniform across the entire source region. We did not find a clear correlation of the γ-ray emission with gas clouds as identified through H I and CO observations. Conclusions. We conclude that, of the known objects within the region, only Westerlund 1 can explain the majority of the γ-ray emission. Several CR acceleration sites and mechanisms are conceivable and discussed in detail. While it seems clear that Westerlund 1 acts as a powerful particle accelerator, no firm conclusions on the contribution of massive stellar clusters to the flux of Galactic CRs in general can be drawn at this point
TeV flaring activity of the AGN PKS 0625-354 in November 2018
Most -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 -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
-opacity constrains the upper limit of the angle between the jet
and the line of sight to . 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 -ray detected active galactic nuclei in general.Comment: 9 pages, 6 figures, accepted for publication in Astronomy &
Astrophysic
HESS J1809193: a halo of escaped electrons around a pulsar wind nebula?
Context. HESS J1809193 is an unassociated very-high-energy -ray
source located on the Galactic plane. While it has been connected to the nebula
of the energetic pulsar PSR J18091917, supernova remnants and molecular
clouds present in the vicinity also constitute possible associations. Recently,
the detection of -ray emission up to energies of 100 TeV with the
HAWC observatory has led to renewed interest in HESS J1809193.
Aims. We aim to understand the origin of the -ray emission of HESS
J1809193.
Methods. We analysed 93.2 h of data taken on HESS J1809193 above 0.27 TeV
with the High Energy Stereoscopic System (H.E.S.S.), using a multi-component,
three-dimensional likelihood analysis. In addition, we provide a new analysis
of 12.5 yr of Fermi-LAT data above 1 GeV within the region of HESS J1809193.
The obtained results are interpreted in a time-dependent modelling framework.
Results. For the first time, we were able to resolve the emission detected
with H.E.S.S. into two components: an extended component that exhibits a
spectral cut-off at 13 TeV, and a compact component that is located close
to PSR J18091917 and shows no clear spectral cut-off. The Fermi-LAT analysis
also revealed extended -ray emission, on scales similar to that of the
extended H.E.S.S. component.
Conclusions. Our modelling indicates that based on its spectrum and spatial
extent, the extended H.E.S.S. component is likely caused by inverse Compton
emission from old electrons that form a halo around the pulsar wind nebula. The
compact component could be connected to either the pulsar wind nebula or the
supernova remnant and molecular clouds. Due to its comparatively steep
spectrum, modelling the Fermi-LAT emission together with the H.E.S.S.
components is not straightforward. (abridged)Comment: 14 pages, 10 figures. Accepted for publication in A&A. Corresponding
authors: Vikas Joshi, Lars Mohrman
H.E.S.S. follow-up observations of GRB221009A
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 above 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.Comment: 10 pages, 4 figures. Accepted for publication in APJL. Corresponding
authors: J. Damascene Mbarubucyeye, H. Ashkar, S. J. Zhu, B. Reville, F.
Sch\"ussle
Detection of extended gamma-ray emission around the Geminga pulsar with H.E.S.S
Geminga is an enigmatic radio-quiet gamma-ray pulsar located at a mere 250 pc
distance from Earth. Extended very-high-energy gamma-ray emission around the
pulsar was discovered by Milagro and later confirmed by HAWC, which are both
water Cherenkov detector-based experiments. However, evidence for the Geminga
pulsar wind nebula in gamma rays has long evaded detection by imaging
atmospheric Cherenkov telescopes (IACTs) despite targeted observations. The
detection of gamma-ray emission on angular scales > 2 deg poses a considerable
challenge for the background estimation in IACT data analysis. With recent
developments in understanding the complementary background estimation
techniques of water Cherenkov and atmospheric Cherenkov instruments, the
H.E.S.S. IACT array can now confirm the detection of highly extended gamma-ray
emission around the Geminga pulsar with a radius of at least 3 deg in the
energy range 0.5-40 TeV. We find no indications for statistically significant
asymmetries or energy-dependent morphology. A flux normalisation of
cmsTeV at 1 TeV is obtained
within a 1 deg radius region around the pulsar. To investigate the particle
transport within the halo of energetic leptons around the pulsar, we fitted an
electron diffusion model to the data. The normalisation of the diffusion
coefficient obtained of
cms, at an electron energy of 100 TeV, is compatible with values
previously reported for the pulsar halo around Geminga, which is considerably
below the Galactic average.Comment: 16 pages, 15 figures, 7 tables. Accepted for publication in Astronomy
& Astrophysic
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