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The first multi-messenger hint for a cosmic ray accelerator?
July 11, 2018

On September 19, 2017, a high-energy neutrino of about 290 TeV (IceCube-170922A) was detected by the IceCube neutrino telescope. An alert was distributed to observatories around the globe within 1 minute. The direction of the neutrino (reconstructed to an area of about 1sq deg and consistent with the location of a known gamma-ray blazar TXS 0506+056) became visible at the location of the H.E.S.S. observatory in Namibia around 4 hours later and follow-up observations were started (ATEL #10787). Observations of gamma-rays in the GeV domain obtained with the LAT instrument onboard the Fermi satellite showed the blazar TXS 0506+056 to be in a flaring state since April 2017. The coincidence of the neutrino correlation with a blazar which was undergoing heightened non-thermal emission caught the interest of the wider astronomical community and triggered a comprehensive multi-wavelength follow-up campaign. During this campaign, high-energy gamma rays with energies up to 400 GeV were detected by the MAGIC instrument on La Palma twelve days after the neutrino event detection. The emission in X-rays showed clear evidence for spectral variability, the flux in the optical V band was the highest observed in recent years. A summary of these observations can be found in IceCube Collaboration et al., Multimessenger observations of a flaring blazar coincident with high-energy neutrino IceCube-170922A, Science 361.

There remains a 0.1% probability that the coincidence of the neutrino event with the flare of TXS 0506+056 is purely a random chance coincidence. However, the observed association of a high-energy neutrino event with a blazar during a period of enhanced gamma-ray emission is suggestive that blazars may indeed be one of the long-sought sources of very high-energy cosmic rays, and hence might be responsible for a fraction of the cosmic neutrino flux observed by IceCube.

The image shows an ASAS-SN optical V-band widefield image of the sky centered on the known position (+) of the TXS 0506+056 blazar. Two known objects from catalogues of gamma-ray sources generated Fermi Large Area Telescope are shown as blue circles, one being TXS 0506+056, with diameter representing the 95% position uncertainty. Also shown are the 50% and 90% containment areas (solid-grey and dashed-red contours, respectively) for the best-fit directional reconstruction of a high-energy neutrino detected on 22 September 2017 that indicate positional coincidence with the blazar TXS 0506+056. Subsequent very-high-energy gamma-ray observations from the MAGIC imaging air Cherenkov telescopes also detected this source, with 68% positional uncertainties of those observations shown as the yellow circle. Inset is a zoomed view of the region of interest surrounding TXS 0506+056. From IceCube Collaboration et al., Multimessenger observations of a flaring blazar coincident with high-energy neutrino IceCube-170922A, Science 361.

Galactic science with 15 years of H.E.S.S. data
April 9, 2018

The H.E.S.S. telescopes have surveyed the Milky Way in gamma-ray light for the last 15 years. To celebrate this anniversary, the H.E.S.S. collaboration has published its largest set of science results to date in a series of papers in a special issue of the journal Astronomy & Astrophysics. The more than a dozen publications include the H.E.S.S. Galactic Plane Survey, studies of the populations of pulsar wind nebulae and supernova remnants, as well as the search for new object classes unseen before in very-high-energy gamma rays such as microquasars or shocks around fast-moving stars. These studies are\ complemented by precision measurements of shell-type supernova remnants such as for example RX J1713-3946 and diffuse emission at the centre of our Galaxy. This legacy data set will serve as a benchmark for the community for the coming years and until the next-generation Cherenkov Telescope Array comes online in the 2020s.

For more information, see the press release.

Agnieszka Jacholkowska, our colleague and friend, passed away during the night of March 26th.
Mar 28, 2018

Dr. Agnieszka Jacholkowska, our colleague and friend, passed away during the night of march 26th, 2018, after a battle against cancer.

Agnieszka has been a very enthusiastic scientist, involved first in particle physics and, in a second stage, in gamma-ray astronomy. Her areas of predilection were the QCD physics and standard model and beyond, the search for dark matter or the test of the violation of Lorentz invariance, domains in which she counts an impressive number of contributions.

After a PhD at the Laboratory of Linear Accelerator (LAL), on the search of charmed quark in the BEBC bubbles chamber experiment at CERN, she returned to the Institut of Physics of Warsaw University to study K-proton interaction and then the Compton diffusion in NA14 at CERN. In 1983, at the LAL again, she investigated the deep inelastic diffusions of muons on nucleons in the EMC collaboration. She was hired, in 1986, at CNRS to work on electron-nucleon interaction in the H1 collaboration and later in ALEPH experiment on the Large Electron Position Collider where she studied heavy quarks physics and bosonic coupling. Her last major contribution in particle physique was on CP violation in the b sector before she switched to astroparticle physics at the LUPM laboratory in Montpelier. She started there to investigate cosmic-ray physics with AMS, CELESTE and the H.E.S.S. experiment and finally joined the LPNHE in Paris, in 2008, to pursue her studies in astroparticle physics in the H.E.S.S. experiment and contribute to developments for the CTA project.

We will thoroughly miss her great intellect and kindness, and we would like to express our deepest condolences to her family.

The birth of multi-messenger astrophysics
Oct 16, 2017

On August 17, 2017, the gravitational wave interferometers Advanced Ligo and Advanced Virgo recorded a signal from the merger of a binary neutron star system, a type of signal that had never been seen before. Complementing this exciting discovery, a large variety of electromagnetic observations were able to record signals from the same event. They range from the detection of a gamma-ray burst about 2 seconds after the gravitational wave event, over near-infrared, optical and UV emission from decay of radioactive nuclei created in the resulting kilonova to X-ray and radio emissions detected several days and weeks after the event. This first and extremely successful observation campaign is marking the beginning of truly multi-messenger astrophysics.

The gravitational wave event was localized within a 3 by 10 degree region, well beyond the H.E.S.S. field of view and requiring multiple pointings to cover the area. The H.E.S.S. target selection identified regions of high probability to find a counterpart of the gravitational wave event. These regions already contained the counterpart SSS17a that has later been identified in the optical domain, several hours after our observations. As a result, H.E.S.S. was the first ground-based pointing instrument to obtain data on this object. A subsequent monitoring campaign with the H.E.S.S. telescopes extended over several days, covering timescales from 0.22 to 5.2 days and energy ranges between 270 GeV to 8.55 TeV. No significant gamma-ray emission has been found within this time interval. The derived upper limits on the very-high-energy gamma-ray flux for the first time constrain non-thermal, high-energy emission following the merger of a confirmed binary neutron star system, and further observations of this source will allow to check whether TeV energies are reached on a larger time scale.

For more information, see the H.E.S.S. paper

Patrick Fleury left us on 14th September 2017.
Sep 17, 2017

Patrick Fleury left us on 14th September 2017.

He acted as director of the PNHE laboratory of Ecole Polytechnique from 1973 to 1984, and oversaw its establishment at Palaiseau.

Throughout his career, Patrick Fleury played a major, and often pioneering rĂ´le in several domains of the research in physics at the CNRS/IN2P3, but also for the massive data processing and very large scale integration (VLSI).

In particle physics, after having been a key person in the bubble chamber era, he steered his laboratory towards electronic detectors and pushed their use at CERN, already from the end of the '60s.

He made fundamental contributions to the emergence of ground-based gamma-ray astronomy and its establishment as a scientific discipline in France and abroad, with the ARTEMIS, CAT, CELESTE, and HESS projects, as well as building-up the participation of the CNRS/IN2P3 in NASA's Fermi gamma-ray satellite.

As president of the scientific evaluation committee of VIRGO, he was an important player in the IN2P3's engagement in the gravitational wave domain.

Patrick was an exceptional scientist. He was a visionary, passionate and clear-thinking, a builder of projects, with great intellectual and moral force, and profound humanity.

H.E.S.S. measures the extension of the Crab nebula in TeV gamma rays
Jul 28, 2017

Resolving source extensions that are smaller than the angular resolution requires a good understanding of the instrument point spread function. Using a novel, more realistic approach to simulate the instrument response, H.E.S.S. has now pushed the point-source resolvability in very-high-energy gamma-ray astronomy to a new level. These so-called run-wise simulations reduce the systematic uncertainties on the gamma-ray direction reconstruction, and allow us to resolve, for the first time, the extension of the Crab Nebula in TeV gamma rays. More information can be found in M. Holler et al., Proc. ICRC 2017 (Busan), arXiv:1707.04196.
First gamma-ray images from the upgraded H.E.S.S. cameras
Mar 1, 2017

The newly refurbished H.E.S.S. cameras in Namibia have detected its first gamma-ray signals: Markarian 421, a well-known blazar in the constellation of Ursa Major, has been observed during an active state and was detected at high significance. After four years of development, testing, production and deployment, this is the last big milestone of the H.E.S.S. I camera upgrade project. See our Source of the Month 03/2017 for details.
The Population of TeV Pulsar Wind Nebulae in the H.E.S.S. Galactic Plane Survey
February 28, 2017

The sky maps and source catalogue of the H.E.S.S. Galactic Plane Survey allow for a detailed study of TeV pulsar wind nebulae found throughout the last decade with H.E.S.S. Besides a correlation between the TeV surface brightness and the pulsar spin-down power, the study hints at a correlation between the offset between pulsar and nebula and the apparent TeV efficiency of the object. The paper can be downloaded from the preprint server: arXiv:1702.08280
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