CTA

From HESS …

I coordinated the work of the H.E.S.S. team at LAPP between 2010 and 2016, including the physics analyses as well as the commissioning and operation of the camera of the fifth telescope.

Technical projects: mechanics and H.E.S.S.-2 camera security

The fifth 36 m telescope (H.E.S.S.-2) came into operation in 2012. The camera of this large telescope consists of over 2000 photomultipliers with their associated readout electronics. It weighs about 2.75 tonnes. The solution proposed by the LAPP team and adopted by the collaboration is an automatic mechatronic system, closely combining mechanics, automation and IT, capable of positioning and removing the camera from the telescope nose and replacing it with a clone. This system is complemented by a device called « autofocus » which allows the camera’s focusing distance (mirror-to-camera distance) to be adjusted. The camera control part includes the monitoring of the camera environment parameters as well as the control of a number of elements such as doors, calibration LEDs, photodiodes, fans…

Technical projects: multi-variable data analysis methods and performance improvements

In the physics analysis we focused on the details of the reconstruction of the Chernenkov data, and then investigated all possibilities for further optimisation. This approach has been developed with two goals: to prepare future analyses with phase 2 of the experiment (notably the operation of the fifth telescope and the exploration of the threshold energies of the few tens of GeV); to continue the combination of energy and morphological information of the extended galactic sources to better understand the different evolutionary phases of the latter.
At LAPP, multi-variable statistical analysis methods derived from particle physics have been developed and validated: an analytical multi-variable method based on the application of probability density estimators (Xeff); then we have optimized the multi-variable approach through the alternative application of computational methods based on learning processes, e.g. BDT algorithms (« boosted decision tree »).

Physics analysis: Pulsar Wind Nebula (PWN)

At LAPP we have considered PWN as a class of objects of interest for the application of our analytical methods, as well as for their role as electron and positron flux « generators », fundamental for the spectral study of cosmic radiation confined into our galaxy. PWN are responsible for the majority of gamma emissions in our galaxy. They present varied morphologies, linked to their history, to the characteristics of the associated pulsar, but also to the interactions with the interstellar medium and with the supernova ejecta. The morphological and spectral study of a middle-aged prototype, Vela X, has allowed us to understand the physics of these objects: their evolution in an inhomogeneous environment; the inverse Compton origin of the gamma emission; the cooling times of the electrons; the speed of electron diffusion as well as their energy budget. A multi-wavelength comparison work has been carried out.

Physics analysis: Supernova remnants and molecular clouds

Shell supernova remnants are prime candidates for accelerating cosmic rays to energies of the order of 10¹⁵eV. The detection of these objects in gamma rays, although it confirms the acceleration of particles, does not confirm that they are indeed hadronic accelerators. A concentration of dense matter, such as a molecular cloud, is an ideal probe to reveal the expected cosmic ray overdensity in the immediate vicinity of the accelerator. In particular, the analysis of two supernova remnants interacting with a dense cloud: the HESS J1923+141 source located in the region of W51 and G349.7+0.2 then made it possible to highlight the hadronic origin of the gamma rays detected.

Physics analysis: search for dark matter

The gamma channel at TeV for indirect dark matter search (especially in the hypothesis of a massive WIMP) has been explored since 2003 by H.E.S.S. Dark matter particles can annihilate each other by emitting photons in the wavelength of gamma rays. This phenomenon is all the more likely as the density of dark matter is high.
We have been pursuing with H.E.S.S. the observations of dwarf galaxies characterized by a large mass-to-light ratio and more particularly the dwarf galaxy of Sagittarius, then by combining the data from four more targets (Fornax, Coma Berenices, Carina and Sculptor) obtaining one of the most constraining exclusion curve of gamma signal of all dwarf galaxies observed to date at TeV energies.

Physics analysis: star forming regions

We explored a new type of object: star clusters; more specifically, very young star clusters still integrated into the giant molecular cloud that gave birth to them.
The stellar winds of these objects are in fact capable of forming a shock wave, potentially a site of particle acceleration and therefore of gamma-ray emission. Their contribution to the flow of cosmic rays detected on Earth could also be significant.
Our study aimed at estimating the stellar wind acceleration efficiency of the cluster by constraining the model developed with data from the H.E.S.S. telescopes and the Fermi-LAT satellite.

Physics analysis: the Large Magellanic Cloud (LMC)

The Large Magellanic Cloud (LMC) is a satellite galaxy of the Milky Way, located at 48 kpc. Observed since the commissioning of the four telescopes in 2004, the LMC is one of the most observed sources by H.E.S.S. and the first discovery of an extragalactic pulsar nebula in gamma-ray astronomy. Thanks to multi-wavelength modelling we have estimated the energy supplied at electron acceleration and the rotation period of the pulsar. In 2014 H.E.S.S. demonstrated that the LMC emission comes from three objects of stellar origin: a pulsar wind nebula, a supernova remnant, and a « superbubble » .

Physics analysis: active galactic nuclei

In 2013 at LAPP we opened up a field of research about extragalactic observations in order to study in particular active galactic nuclei (AGN) and gamma-ray bursts (GRB). This new orientation was consistent with our commitments to the construction of the large CTA telescopes at the North site in La Palma, which are better adapted to this physics.

This project has produced important results:

– Study of the connection of AGN population and diffuse gamma-ray background with Fermi and H.E.S.S. data.
– Follow up of GRB alerts with the H.E.S.S.2 telescope, whose energy threshold goes down to about 50 GeV;
– Study of the physics of Blazars using a multi-messenger approach (in particular the variability of the blazar PKS 2155-304)

… to CTA

The Cherenkov Telescope Array, CTA is the next-generation gamma-ray instrument. CTA will use more than 100 telescopes located in both the northern and southern hemispheres giving it a collection area that exceeds one million square metres.

Four LSTs will be arranged at the centre of both the northern and the southern hemisphere arrays to cover the unique low energy sensitivity of CTA between 20 and 200 GeV.

I led the CTA project at the LAPP before I took over the management of the laboratory; various ideas were taken up and implemented.
LST is an original scientific vision that has been transformed into reality thanks to the scientific and technical members of the LAPP team with whom I had the pleasure of working.

Only a small fraction of the total mass of the Universe is formed by known particles (baryonic matter and neutrinos), while the rest is composed of dark matter – 25 percent of the total energy density of the Universe. CTA will detect very high-energy gamma rays, as indirect traces of annihilation products of weak interacting massive particle (WIMP), promising candidates of dark matter or axion-like particle. In particular, CTA will use its unprecedented sensitivity and energy resolution to capture gamma rays exactly over the energy range that corresponds to the heavier end of the WIMP masses.

The “Extreme Universe” is the most violent side of the Cosmos that hosts a diverse population of astrophysical objects experiencing extreme physical conditions, collectively designated “transients”, exploding, flaring up or dramatically intensifying their emission activity. Such transient are prominent emitters of high-energy gamma rays and likely of other messengers (neutrinos, gravitational waves). They are objects such as gamma-ray bursts, neutrino alerts, gravitational wave alerts, selected optical/radio transients and serendipitous very high energy (VHE) transients.

CTA will elucidate the physics of time-variable gamma-ray phenomena with unprecedented precision since able to guarantee a relatively large field of view, sensitivity to energies as low as 20 GeV, fast repositioning by following up alerts of transient events issued by monitoring instruments, as well as discovering transients on its own.

During the last 10 years my scientific interest for the investigation of “dark matter” as well as for the multiwavelength investigation of “transients” has induced me to lead an in-depth experimental investigation of solutions to implement some of CTA main requirements devoted to such challenges: re-positioning speed, low energy threshold, high sensitivity to fainter sources with soft energy spectra.

I have been heavily involved with my team in the R&D, design and implementation of three main subsystems/solutions:

• the high mechanical structures of the telescope, the “LST telescope arch” and the mechanical interface with the camera frame, the CSS (Camera Support Structure) system;
• the mechatronic drive system;
• the high-sensitivity and fast algorithms for data analysis in quasi-real time.

Our studies aimed at fulfilling the specifications of the LST, a giant Cherenkov telescope standing 45 m tall and weighing around 100 tonnes, which is required to be extremely nimble, with the goal to be able to re-position within 20 seconds. We succeeded finally to build the CSS arch in tubular structure made of reinforced carbon fibre in cooperation with an industrial partner (Lorima); a full consistent mechatronic drive system to pilot and optimize the trajectory while minimizing the lasting time for repositioning of the telescope.

Building (Large Size Telescope) LST-1:

The press communication of the LST-1 inauguration (French).

The CNRS “Crystal Collectif” 2019 distinguished LAPP-CTA team

The LAPP team is composed of scientists and engineers from various technical fields (mechanics, computer science, mechatronics and electronics). They cooperate with researchers and colleagues from other international institutes. Together, they have made complementarity and mutual emulation the keys to their success and the pride of our unit and of the CNRS more widely. They have ensured the continuous progress of their LST-1 project, in all its phases, they have taken up major technical challenges for a search for excellence. They were able to apply working methods based on iterative, incremental and adaptive development and operation cycles, both within the multidisciplinary LAPP team and more widely in cooperation with their international collaborators.
At LAPP we have designed and realised together diverse and mutually coherent solutions in the fields of mechanics, mechatronic instrumentation, electronics and computer science to build a technologically advanced instrument for the observation of transient cosmic phenomena at extreme energies with unprecedented sensitivity.

Our video on Youtube:

And … between January and February 2020, in the midst of technical analyses to verify the telescope’s performance and adjust its operating parameters, LST-1 unveils all its scientific power …

LST-1 observes the pulsar, the neutron star, at the centre of the Crab Nebula

Artificial intelligence boosts high-energy astronomy

The CTA Observatory Data Management was one of my main leading actions within the CTA international consortium and during the pre-construction phase. Among different aspects at LAPP we led important developements to capture the scientific value of the future CTA-LST’s observations, in almost real time and for the follow-up of astrophysical alerts of transients such as GRBs or Active Galactic Nuclei.

Highly innovative IT developments have been initiated:

• the optimisation, through the parallelisation of data calibration and reconstruction software;
• the combination of new digital architectures to accelerate data processing;
• the exploration of new software programming frameworks and I/O optimisations;
• the design and research of innovative algorithms and workflow. Rapidly detecting events among the hundreds of gigabytes that will be generated every second by the future CTA observatory is the challenge that at LAPP we met using Artificial Intelligence. It is a project initiated within the framework of ASTERICS/OBELICS then ESCAPE.

GammaLearn from the OBELICS repository