A new publication 'A deep spectromorphological study of the gamma-ray emission surrounding the young massive stellar cluster Westerlund 1' has been accepted by 'Astronomy and Astrophysics' on July 21, 2022. It is available on arXiv:2207.10921 . In a study using 165h of observations, collected over more than a decade, corresponding authors Lars Mohrmann and Andreas Specovius orchestrated the analysis and intergretation of VHE gamma ray emission of the most massive cluster in the Local Group. The results are featured in the August 2022 edition of the H.E.S.S. source of the month series.

Cosmic magnetic fields are extremely effective in shaping the universe but are very difficult to track and measure. Manuel Meyer has completed a Marie-Curie Project to study cosmic magnetic fields using gamma-rays observed with the HESS array and the Fermi-LAT satellite, as reported by the Cordis office of the European Comission .

A new publication 'Search for dark matter annihilation signals in the H.E.S.S. Inner Galaxy Survey' has been accepted by Physical Review Letters on July 13, 2022 and is available on arXiv:2207.10471 . The corresponding authors, Alessandro Montanari, Emmanuel Moulin and Denys Malyshev have been the driving force in the analysis of a data set that has been compiled throughout 2014-2020. 546h of observations covering 60 square degrees provide the most constraining limits on annihilating DM in the mass range 500 GeV - 50 TeV. The limits obtained in the Tau+Tau- and e+e− channels challenge natural velocity-weighted annihilation cross section values expected for the thermal-relic WIMPs.

While semi-annual collaboration meetings have been much appreciated gatherings of members of the H.E.S.S. collaboration since 2003 the series of these events had to be paused during the Covid pandemic when travel and meeting restictions did not allow in-person meetings. After the last meeting in pre-Covid times, in September/October 2019 - coinciding with the commencement of the first HESS extension phase - all collaboration meetings had to arranged as virtual events. The collaboration very much appreciated the first opportunity for an in-person meeting again for the 1st semester 2022 collaboration meeting in Paris. 90 members of the collaboration welcomed the opporunity to physically meet colleagues (again) and discuss the interesting new science results and progress in operations and data analysis.

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.

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.

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.

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.

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'