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H.E.S.S. data resolve extragalactic jet in Centaurus A
June 18, 2020

Today the H.E.S.S. collaboration published a breakthrough result in the magazine Nature (Volume 582, pages 356–359, 2020), revealing the gamma-ray emission from the famous radio galaxy Centaurus A (also known as NGC 5128) to be spatially extended over thousands of light-years. This is the first time that an extragalactic source has been spatially resolved in the regime of very high energy gamma-ray astronomy and permits a direct determination of the size the region responsible for gamma-ray emission at TeV energies in an active galaxy. The publication, based on advanced data analysis of a large amount of excellent data obtained with the H.E.S.S. array is a technical achievement and an important step in our understanding of relativistic jets in the Universe.

During the last decades the universe beyond our Milky Way has been probed in the light of very high energy (VHE) gamma-rays, identifying many different kinds of distant Quasars and other active galaxies. Curiously, those distant and extremely luminous sources of gamma-ray emission are very often found to vary within weeks or days - sometimes even as fast as hours. It was hence concluded, that the highly energetic emission originates from very compact regions - very likely linked to a massive Black Hole in the center of the host galaxy. The new study of Centaurus A observations reveals that the emitting regions of these very energetic sources are actually extended over many thousands of light-years.

Centaurus A, one of the first radio galaxies to be discovered, is a well-known active galaxy on the Southern Sky that displays prominent jets in highly resolved radio and X-ray images. The H.E.S.S. array detected TeV gamma-ray radiation from this object in 2009 and studies its spectral and temporal characteristics. A new analysis method that now allows mprphological studies find the gamma-ray source to be spatially extended in the direction of the radio and X-ray jets. While the different angular resolutions that can be achieved in different enrgy ranges prohibit a directs comparison on all angular scales, the figure clearly illustrates that the best-fitting model of the gamma-ray emission lines up with the synchrotron radiation revealed in the radio band.

H.E.S.S. array continues observations during COVID-19 pandemic
May. 10, 2020

While the COVID-19 pandemic has spread around the globe and still imposes severe restrictions in very many countries, the H.E.S.S. array was able to continue operations throughout March and April 2020 and anticipates continuous operations in the forthcoming months. This is very important for scientific investigations within the HESS collaboration and the scientific community at large and is made possible by several favorable circumstances:

The HESS array is located on the Goellschau farm, 120 km south of Windhoek in Namibia, a sparsely populated region in a sparsely populated country. The small local crew and the observers live and work in a very isolated environment already in normal times. Thanks to early and efficient monitoring, Namibia has been able to limit the influx and spread of Corona in the country and suffers very small numbers of infections. The H.E.S.S. site is a very safe place to work and operate. At the same time, Namibian regulations during the pandemic permit the continuation of Scientific Services and thus enabled operations.

The observers that had started their scheduled shift prior to the onset of restrictions have fortunately been able and eager to continue measurements beyond their regular slot, supported by the on-site local H.E.S.S. crew. Advancing further through these difficult times, the H.E.S.S. observing program continues thanks to members of the Namibian H.E.S.S. partner UNAM (University of Namibia), again supported by the local Namibian H.E.S.S. crew. The situation is continuously monitored as the health and safety of everybody involved is of utmost importance.

Continuous observations ensures new scientific discoveries (such as the detections of one of the brightest outbursts yet measured with HESS) in April 2020 and the completion of the test of new instrumentation that has been installed in the end of 2019. The H.E.S.S. collaboration is grateful to all individuals that have made continuous operations and observations possible and continue to do so in the forthcoming times. This includes the local crew in Namibia, shift-teams and many off-site sub-system experts making extra efforts for the operation of the H.E.S.S. array.

First gamma-ray sky map with the new H.E.S.S. CT5 camera, two days after installation
Dec. 9, 2019

The H.E.S.S. collaboration has upgraded its 600 square metre Cherenkov telescope with a new high-performance camera with fully-digital trigger and readout system, and high quantum efficiency photon detectors.

The main goals of the upgrade are a reduction in the energy threshold of the telescope, improved sensitivity, and better stability of operation. The new camera is based on the FlashCam design, which has been developed for the use in the Cherenkov Telescope Array (CTA) by a consortium of the universities from Zürich, Tübingen, Erlangen and Innsbruck under the lead of the Max Planck Institute for Nuclear Physics in Heidelberg. After extensive tests of a complete prototype with its hardware, firmware and software, the camera was shipped to Namibia, where it arrived at the beginning of October 2019. After adaption of the mechanical telescope interfaces, the camera was installed on October 20th (Fig. 1).

Fig. 1: The installation and commissioning teams together with the local on-site technical crew in Namibia in front of the H.E.S.S. CT5 telescope with its new camera (October 23rd , 2019).
The telescope was ready to make astrophysical observations just two days after mechanical installation. In its first night of operation, the telescope was pointed at the Crab Nebula, among other targets. Fig. 2 shows the gamma-ray sky map around the Crab Nebula obtained with the standard H.E.S.S. real-time analysis during these first observations with the upgraded telescope. A clear detection is visible at the position of the Crab Nebula.
Fig. 2: Gamma-ray sky map obtained in real time during the first 28 min observation of the Crab Nebula with the H.E.S.S. CT5 telescope, equipped with the new camera.

In the meantime, the telescope with its new camera is participating in routine observations with the complete H.E.S.S. array, confirming the expected performance improvements and stability of operation. With its high-quantum-efficiency light sensors and sophisticated trigger and readout scheme, the new camera will further boost the performance of H.E.S.S.’s world’s largest Cherenkov telescope. With this success the H.E.S.S. collaboration, together with the FlashCam consortium, have also demonstrated a highly efficient mode for the installation and commissioning of cameras, as will be required for the deployment of the around 100 telescopes of the Cherenkov Telescope Array (CTA).

First detection of gamma-ray burst afterglow in very-high-energy gamma light
Nov. 20, 2019

After a decade-long search, scientists have for the first time detected a gamma-ray burst in very-high-energy gamma light. This discovery was made in July 2018 by the H.E.S.S. collaboration using the huge 28-m telescope of the H.E.S.S. array in Namibia. Surprisingly, this Gamma-ray burst, an extremely energetic flash following a cosmological cataclysm, was found to emit very-high-energy gamma-rays long after the initial explosion.

Extremely energetic cosmic explosions generate gamma-ray bursts (GRB), typically lasting for only a few tens of seconds. They are the most luminous explosions in the universe. The burst is followed by a longer lasting afterglow mostly in the optical and X-ray spectral regions whose intensity decreases rapidly. The prompt high energy gamma-ray emission is mostly composed of photons several hundred-thousands to millions of times more energetic than visible light, that can only be observed by satellite-based instruments. Whilst these space-borne observatories have detected a few photons with even higher energies, the question if very-high-energy (VHE) gamma radiation (at least 100 billion times more energetic than visible light and only detectable with ground-based telescopes) is emitted, has remained unanswered until now.

Fig. 1: Gamma-Ray bursts are the most luminous explosions in the universe. Within a few seconds they radiate more energy than the sun in billions of years. Understanding the physical processes at work in these monstrous explosions is an important goal of modern astrophysics. Artist’s view of a GRB and the formation of extremely fast jets (Credit: ESO/A. Roquette)

On 20 July 2018, the Fermi Gamma-Ray Burst Monitor and a few seconds later the Swift Burst Alert Telescope notified the world of a gamma-ray burst, GRB 180720B. Immediately after the alert, several observatories turned to look at this position in the sky. For H.E.S.S. (High Energy Stereoscopic System), this location became visible only 10 hours later. Nevertheless, the H.E.S.S. team decided to search for a very-high-energy afterglow of the burst. After having looked for a very-high-energy signature of these events for more than a decade, the efforts by the collaboration now bore fruit.

A signature has now been detected with the large H.E.S.S. telescope that is especially suited for such observations. The data collected during two hours from 10 to 12 hours after the gamma-ray burst showed a new point-like gamma-ray source at the position of the burst. While the detection of GRBs at these very-high-energies had long been anticipated, the discovery many hours after the initial event, deep in the afterglow phase, came as a real surprise. The discovery of the first GRB to be detected at such very-high-photon energies is reported in a publication by the H.E.S.S. collaboration et al., in the journal 'Nature' on November 20, 2019.

Fig. 2: The large central H.E.S.S. telescope with 614 m² mirror area that was used for the first detection of a GRB in VHE gamma-ray light and two of the four smaller telescopes, each with 107 m² mirror area. (Credit: MPIK / Christian Föhr, email: christian.foehr@mpi-hd.mpg.de)

GRB 180720B was very strong and lasted for about 50 seconds – a relatively long duration indicating the death of a massive star. In this process, its core collapses to a rapidly rotating black hole. The surrounding gas forms an accretion disk around the black hole, with gas jets ejected perpendicularly to the disk plane creating the gamma-ray flashes. Elementary particles are accelerated in these jets to velocities nearly as high as the speed of light and interact with the surrounding matter and radiation, leading to the copious production of gamma rays.

The very-high-energy gamma radiation which has now been detected not only demonstrates the presence of extremely accelerated particles in GRBs, but also shows that these particles still exist or are created a long time after the explosion. Most probably, the shock wave of the explosion acts here as the cosmic accelerator. Before this H.E.S.S. observation, it had been assumed that such bursts likely are observable only within the first seconds and minutes at these extreme energies, and not many hours after the explosion.

At the time of the H.E.S.S. measurements the X-ray afterglow had already decayed very considerably. Remarkably, the intensities and spectral shapes are similar in the X-ray and gamma-ray regions. There are several theoretical mechanisms for the generation of very-high-energy gamma light by particles accelerated to very-high-energies. The H.E.S.S results strongly constrain the emission to two potential mechanisms. In both cases, however, the observations raise new questions. "Although energetically one of these mechanisms is preferred, both the shape of the H.E.S.S. spectrum, and the energy range of the emission at such late times presents a challenge to both emission scenarios." - says H.E.S.S. scientist Andrew Taylor.

Fig. 3: GRB 180720B in very-high-energy gamma light, 10 to 12 hours after the burst as seen by the large H.E.S.S. telescope. The red cross indicates the position of GRB 180720B, determined from the optical emission of the GRB. (Credit: H.E.S.S. collaboration, email: contact.hess@hess-experiment.eu)

This breakthrough discovery provides new insights into the nature of gamma-ray bursts. As highlighted by Edna Ruiz Velasco, PhD. student at MPIK in Heidelberg and one of the corresponding authors of the publication: "This detection has already revolutionised the way we search for GRBs with Cherenkov Telescopes. Thanks to this GRB and the lessons learnt, our recently improved observational strategy has already payed off. We can expect a future with a great number of GRBs detections at very-high energies and with this, a deeper understanding of these phenomena".

Original publication:

A new very-high-energy component deep in the Gamma-ray Burst afterglow, Nature, 2019
Authors: H.E.S.S. Collaboration et al. *


Prof. Dr. Stefan Wagner (Director, H.E.S.S.)
Phone: +49 6221 5417121
E-mail: swagner@lsw.uni-heidelberg.de

Prof. Dr. Mathieu de Naurois (Deputy Director, H.E.S.S.)
Phone: +33 1 69 33 55 97
E-mail: denauroi@in2p3.fr

Edna L. Ruiz Velasco
Phone: +49 6221 516-137
E-mail: edna.ruiz@mpi-hd.mpg.de

Dr. Daniel Parsons
E-mail: daniel.parsons@mpi-hd.mpg.de

Dr. Andrew Taylor
Phone: +49 3376277195
E-mail: andrew.taylor@desy.de

*The corresponding authors of the original publication are: Edna Ruiz Velasco, Quentin Piel, Robert Daniel Parsons, Elisabetta Bissaldi, Clemens Hoischen, Andrew Taylor, Felix Aharonian, Dmitry Khangulyan
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