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H.E.S.S. discovers the first VHE gamma-ray Nova.
March 10, 2022

In 2021 the H.E.S.S collaboration discovered the first Nova emitting very high energy (VHE) gamma-ray light. The recurrent Nova RS Ophiuchi exploded on August 8, 2021. The H.E.S.S. telescopes were turned to the constellation Ophiuchus in the night after the first sightings and promptly detected the explosion. Gamma-ray emission from Novae was completely unexpected until the Fermi-LAT experiment serendipitously detected low energy gamma radiation in 2010. While further Novae were observed with LAT, the soft spectra detected in this energy range suggested that such explosions would not generate radiation that was 1000 times more energetic and hence be detectable in the VHE gamma-ray band. The H.E.S.S. discovery is a surprise. Following a prompt alert of the astronomical community to the H.E.S.S. discovery (ATEL 14844) the H.E.S.S. collaboration continued observations for more than one month, resulting in an exquisitely covered light-curve of the event. Further information to our colleagues (ATEL 14857) motivated additional observations in other waveband regimes and resulted in a very rich data set.

Today, the H.E.S.S. gamma-ray studies of RS Oph were published in Science (DOI: 10.1126/science.abn0567). The paper 'Time-resolved hadronic particle acceleration in the recurrent nova RS Ophiuchi' describes the spectral and temporal properties of the gamma-ray emission. They reveal that it takes longer to reach maximum brightness at the highest energies compared to less energetic light and offers insights that allow a more profound understanding of such explosions. The paper discusses the interpretation deduced from the observations in the framework of particle acceleration by shocks expanding from the nova explosion.

In addition to being the first detection of a Galactic transient up to TeV energies, the publication also includes the first report of data obtained with the new camera that had been installed at the largest of the H.E.S.S. telescopes. Nova RS Oph 2021 also was the first VHE gamma-ray source that was visible with the naked eye. The discovery of RS Oph in VHE gamma-ray light illustrates that much is still to be unveiled with the H.E.S.S. array.

H.E.S.S. obtains record amount of high quality observations in 2021
January 14, 2022

A main goal of the first extension of H.E.S.S. operations after October 1, 2019 was an increase of telescope and instrumental reliability and of total observing time. Despite the outbreak of the Covid-19 pandemic a significant increase in operational efficiency resulted in 2020 offering the largest amount of HESS data on record. While the average telescope efficiency of 98% could hardly be exceeded, the collaboration now further increased the amount of data obtained in a single year to a new record of more than 1500h in 2021.

The significant increase was achieved by maintaining the very high reliability of all elements of operations and by extending routine observations into periods of moderate amounts of moonlight. A further increase resulted from a widening of the observing window with an earlier start and a later end of observations each night.

Observations with imaging atmospheric Cerenkov telescopes are conducted with very sensitive detectors that were traditionally only operated under dark sky conditions. Through careful modification of camera settings and adaptation of analysis procedures, observations with the HESS array can now be conducted for about 300h each year without any loss in sensitivity or performance during periods of moderate moonlight or low-level twilight that have previously not been available.

H.E.S.S. operations extended until 2024
October 1, 2021

Major step for the HESS experiment and ground-based gamma-ray astronomy in general: In the current context of emerging multi-messenger, multi-wavelength astronomy, the HESS Steering Committee evaluated the possible options for the future of HESS beyond the currently ongoing first extension phase. After careful analysis of the proposed strategy for the upcoming years, the Steering Committee unanimously voted in favour of a second extension phase of HESS operations for an additional duration of 2 years, ending October 1, 2024. The corresponding extension agreement is currently being finalized between the relevant parties.

The second extension was approved in recognition of the very succeful measures to increase the operational efficiency, further increase of the amount of observing hours by including moonlight observations, significant progress in data analysis, important scientific results emerging from operations in recent years and an attractive science program for the forthcoming three-years. This way, HESS will be preparing for the future CTA era while continuing to deliver exciting and excellent science.

H.E.S.S. collaboration member Alison Mitchell wins Emmy Noether grant
August 14, 2021

Congratulations to Alison Mitchell for winning an Emmy Noether grant from the Deutsche Forschungsgemeinschaft. Her six-year project "Unveiling the Origin of Galactic Cosmic Rays: Exploring Pulsar Environments at the Highest Energies" will use H.E.S.S. and ultimately CTA to tackle this important question in two steps: After focusing on technical improvements to data analysis and towards detailed and precision studies of individual PWNe systems, the second aim will be to increase the number of known TeV PWNe and halos, conducting multi- wavelength and population studies. The grant will enable Alison to start an Emmy Noether team within the H.E.S.S. group of Erlangen University in October 2021.

The project focus is on the acceleration and transport of Cosmic Rays in Pulsar environments. Within the last year, evidence has emerged that Pulsar Wind Nebulae (PWNe) accelerate electrons and positrons to PeV energies, but the origin of other PeV particles remains unconfirmed. Recent measurements of two nearby pulsars showed surprisingly slow diffusion of Cosmic Rays, causing some tension with current theoretical models.

Alison has started her scientific career as a PhD student in the H.E.S.S. group at MPIK in Heidelberg and subsequently worked for the CTAO, the University of Zurich and ETH. During this time Alison remained a member or an associate member, respectively, of the H.E.S.S. collaboration. For her numerous contributions within the H.E.S.S. collaboration, Alison was awarded the H.E.S.S. Prize in 2020.

New insights into gamma ray burst: H.E.S.S. publishes observations of the third GRB detected at very high photon energies
June 04, 2021

Today, June 04, 2021, the H.E.S.S. collaboration published results of observations of the gamma ray burst GRB190829A. The study appeared in Science Vol. 372, Issue 6546, pp. 1081-1085 (2021) and is available on arXiv 2106.02510 as well. Only the third gamma ray burst to have been detected at very high energies (photon energy > 100 GeV), GRB190829A has a very small distance to Earth compared to most GRBs observed in all energy bands. With a redshift of only z=0.0785, the gamma rays emitted from this explosion suffer only little absorption on their way to Earth and its properties can hence be explored even up to TeV energies, significantly exceeding the range offered by the first two GRBs detected in this energy band.

Unlike GRB180720B, detected with the H.E.S.S. telescopes in 2018 and GRB190114C, observed with the MAGIC telescopes earlier in 2019, GRB190829A has been monitored during three subsequent nights. This extended coverage of the decay in flux, together with high-quality spectral studies offered new insights into the physical processes in these most luminous cosmic explosions. Further information may be obtained in the June 2021 edition of the 'HESS - Source of the Month'

Gammapy selected as open-source software of choice for analysis of H.E.S.S. data
May 10, 2021

Data obtained with the H.E.S.S. telescopes are recorded and stored in "root" format, which is widely used in the particle physics community. From the early phases the collaboration developed several proprietary analysis methods. Processed data are stored in proprietary file structures. In order to facilitate exchange and common treatment of data obtained with other facilities and to allow efficient adaptation of open-source software, the H.E.S.S. collaboration explored data analysis with different open-source tools using the open FITS data format for some time.

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Based on extensive tests and development and adaptation, the collaboration now chose Gammapy as its open-source analysis framework. Gammapy v1.0rc Gammapy will be released in the next weeks.

Gammapy-based studies will complement the well-established proprietary analysis methods used by the H.E.S.S. collaboration and facilitate further development of the tools by challenges posed in exploring real observations obtained with H.E.S.S..

H.E.S.S. collaboration member Manuel Meyer wins ERC grant
April 10, 2021

Congratulations to Manuel Meyer for winning a Starting Grant from the European Research Commission on "Searching for axion and axion-like-particle dark matter in the laboratory and with high-energy astrophysical observations". Manuel and his group will join the H.E.S.S. group at the University of Hamburg starting June 2021.

The grant will enable Manuel to start a dedicated research group which will search for hypothetical axions and axion-like particles. Such particles are predicted in certain theories of particle physics and could explain several unresolved questions in particle physics and cosmology. For example, they are candidates to explain the mysterious dark matter. Manuel Meyer’s team will search for axions by contributing to dedicated laboratory experiments and by searching for their signatures with astrophysical observations — including observations of distant galaxies with the H.E.S.S.

Manuel has started his scientific career as a PhD student in the H.E.S.S. collaboration, has worked as a H.E.S.S. member in several groups of the collaboration thererafter and kept his association with the H.E.S.S. collaboration also while working as a Feodor Lynen Research Fellow at Stanford University.