I. Rubin observatory and LSST survey (cosmology, alert broker, and orphan gamma-ray bursts)

II. Cosmic rays (with AMS-02) and dark matter indirect detection

III. DIRAC project (distributed computing resources)

I. Rubin observatory, LSST survey, and dark energy

Observational cosmology, and in particular observations of distant supernovae, have shown that the Universe is expanding at an accelerated rate. This acceleration can be explained by the addition of an energetic component of negative pressure, similar to a cosmological constant, but of unknown origin. To better understand the nature of this dark energy and the entire cosmological model, the Legacy Survey of Space and Time (LSST), which will be carried out at the Vera C. Rubin Observatory, will make it possible to trace the history of expansion of the Universe, via a number of cosmological probes. This 8 m diameter telescope located in the Atacama Desert, Chile, will observe a third of the sky, two to three times a week, for ten years, and will detect hundreds of thousands of supernovae and billions of galaxies. The Dark Energy Science Collaboration (DESC) coordinates the cosmological exploitation of LSST data.

Photo (April 2022) from Vera Observatory. C. Rubin, under construction at Cerro Pachón, Chile, at an altitude of 2,682 m (credit: Rubin Obs/NSF/AURA).


I.1 Filter loader and camera calibration benches for the Rubin project

With over three billion pixels, the camera at the focus of the Vera C. Rubin Observatory Telescope is the largest CCD camera ever built. In addition to the CCDs of the focal plane, the integrated camera has three lenses allowing the focusing of the light and a complex mechanical system (the auto-changer) allowing the optical filter placed in front of the camera to be changed without human intervention.

IN2P3 was responsible for providing the filter changer system. The services of the LPSC contributed to this effort, by developing the device making it possible to transfer the filters from their storage box to the auto-changer (operation repeated regularly according to the rate of observation). This equipment was delivered to SLAC, in the USA, in 2021. For their contributions, E. Lagorio (electronic service) and F. Vezzu (mechanical service) were recipients of the 2021 CNRS Collective Crystal.

The DARK team is also responsible for the development of two projectors (dedicated to the characterization of the camera) as well as their analysis codes (for their scientific exploitation). The CCOB (camera calibration optical benches) called wide beam (4 cm) measures the relative response of all the focal plane CCDs in each of the optical bands, with an accuracy of the order of 1‰. The CCOB says fine beam (2 mm), meanwhile, to measure the alignment of the optical system of the camera (lenses and filter), as well as the total transmission of the optics in each of the wavelengths of the instrument.

Photograph of the fine beam optical calibration bench of the Rubin LSST camera. The bench is made up of a 3 m by 3 m translation table, a cradle and a pivot at the end of which is placed the optical element for emitting the light beam. The device allows light to be sent to any point of the camera over a wide range of angles of incidence.


I.2 Cosmology with LSST galaxy clusters

Clusters of galaxies represent the ultimate stage of formation of the structures of the Universe. The distribution of clusters as a function of mass and redshift is therefore sensitive both to the content of dark matter and dark energy and to their properties. Using the counting of clusters as a cosmological probe therefore requires knowing their mass. This is not an observable quantity, and in the visible range covered by Rubin, weak gravitational lensing is the method of choice for reconstructing it. This effect consists of a deformation of space-time due to the gravitational potential of the cluster (thus of its mass) and results in a deformation of the image of the background galaxies (the shear) and an increase of their brilliance (magnification).

The DARK team is working on preparing the galaxy cluster analysis within DESC. C. Combet took responsibility for the development of the CLMM (Cluster Lensing Mass Modeling) code allowing mass reconstruction through shear, coordinating the work of about fifteen international members of the collaboration (Aguena et al., 2021).

The data from the DESC Data Challenge 2 (DC2) constitute the reference for the preparation of the analyses. C. Payerne (in thesis supervised by C. Combet) led the project aimed at exploiting gravitational shear in the context of DC2 galaxy clusters and the study of its systematics. This work resulted in a dedicated section of the DC2 data validation article (Kovacs et al., 2022) and two internal referenced notes (Payerne et al. 2021a,b). Still within DESC, the team is also heavily involved in the development of the cluster pipeline allowing the estimation of cosmological parameters through cluster counting (and of which mass reconstruction is only one step).

Magnification is a complementary approach to constrain the mass of clusters. In its standard approach, this analysis is more noisy than that using shear. Using public HSC data (precursor of Rubin), C. Murray (postdoc from 2020 to 2022 in the team, via the LabEx ENIGMASS) worked on the development of an innovative approach allowing to use all available spectral information in the analysis, thus gaining in precision on the reconstructed masses.

Galaxy masses in DC2 reconstructed from weak lensing as a functon of the true mass. Figure from C. Payerne published in Kovacs et al. (2022).


I.3 Gravitational shear with LSST: from images to statistical analysis

With the recent arrival in the team of two new members (M. Kuna in April 2021 and C. Doux in January 2022), the team can take an interest in a broader set of themes, and in particular in study of gravitational shear as a cosmological probe. An important and common problem in the study of clusters is that of galaxy blending, i.e. the superposition of galaxies on the line of sight, due to the unequaled depth of the LSST survey. If not corrected, this effect could impact the entire analysis chain up to the reconstruction of cosmological parameters. A working group on this issue exists within the DESC collaboration, coordinated by C. Doux. At the other end of the analysis chain, the methods for extracting the cosmological information contained in the lensing data are also discussed. These include, for example, advanced statistical tools and machine learning, two aspects implemented by C. Doux (before he joined the team) in the analysis of data from the Dark Energy Survey (DES), precursor to Rubin-LSST.


I.4 Transient Sky and FINK alert broker for Rubin

Deep monitoring of a very wide area of ​​the sky, repeated approximately every 3 days by the Rubin Observatory, will reveal a large number of transient objects. These objects will be identified by characterizing the differences between two images of the same part of the sky taken a few hours or days apart. Each difference will lead to an alert intended for the astronomical community, to signify that a transitory phenomenon, and potentially of interest, is in progress. The FINK alert broker (Möller et al., 2020), developed within IN2P3, has been chosen by the Rubin-LSST community as one of the 7 official brokers that will directly receive the entire flow of alerts of the observatory.

Among the millions of alerts generated each night, the DARK team is interested in those that could reveal the existence of afterglow emissions from orphan gamma-ray bursts. Gamma-ray bursts are among the most violent events in the Universe, and result in very intense bursts of γ photons (a few seconds to a few days, at energies from keV to TeV). This gamma emission is essentially due to an extremely collimated jet of relativistic particles and is only visible when this jet points in the direction of the observer. When this jet is viewed at a wide angle, the models predict that the emission at lower energies (optical and radio domains) is observable in the form of slow, low-luminosity transient phenomena, called orphan gamma-ray bursts.

In 2020, with the arrival of J. Bregeon, the DARK team became involved in this theme, with the development of specific orphan burst modules for FINK. Thesis funding was obtained through a call for tenders at UGA. Identifying even one of these orphan bursts would already be a great discovery. If several tens can be characterized, it will be possible to better constrain the populations of gamma-ray bursts, the structure of the jet, etc. This information will be complementary to observations in the gamma domain on the ground and in space, but also to observations of gravitational waves associated with Ligo/Virgo.


II. Galactic cosmic rays and dark matter


II.1 data analysis from the AMS-02 experiment

The AMS-02 experiment aims to study galactic cosmic rays (GCR) from around a hundred MeV to TeV. It is a spectrometer that was installed on the International Space Station (ISS) in May 2011.

The heart of AMS-02 consists of a permanent magnet (1.114 m diameter and 83 cm high) which bends the trajectories of charged particles. Eight planes of silicon trace detectors make it possible to reconstruct it geometrically. Scintillation detectors (time of flight) surround the magnet and signal the passage of a particle (and its direction of travel). The inner cylinder of the magnet is also lined with veto scintillation counters, signaling that a particle is destroyed by collision on the detector (event to be rejected). Above the higher time-of-flight detector is the transition radiation detector (TRD) to identify electrons and positrons. Finally, below the lower time of flight is a Cherenkov counter with annular imaging (RICH detector, developed in the laboratories in the 2000s), for the isotopic identification of nuclei, and an electromagnetic calorimeter which absorbs and measures the energy of charged particles. The complete instrument, with strong measurement redundancy, has very high performance and allows the measurement of particle fluxes of Z=1-26, electrons, positrons and anti-protons. This allows AMS-02 to be able to provide answers to astrophysical questions, dark matter, and even possibly on the enigma of the matter-antimatter asymmetry of the Universe.

The AMS experiment has already collected more than 200 billion events (Aguilar et al., 2021). The LPSC team led the analysis of some nuclear data from AMS-02. The latter were indeed chosen for the publications of the fluxes of the elements of Z=1-5 (p, He, Li, Be and B). In recent years, efforts have turned to isotopic analysis, relying in part on the RICH subdetector. Preliminary results for the isotopic fluxes of Li, Be, and B have been presented at major international conferences by L. Derome (2019 and 2021).


II.2 Phénoménology of cosmic radiation

The phenomenology of the GCR is interested in understanding the mechanisms of propagation in the Galaxy. It is a question of confronting the models with the data, in order to draw conclusions on the origin of the underlying astrophysical processes, as well as the updating of the sources; these questions can only be resolved by an approach that is both multi-wavelength (radio, X, γ) and multi-messenger (photons, neutrinos, leptons and nuclei, even gravitational waves).

Around D. Maurin and in collaboration with members of LAPTh, LAPP, and LUPM (within the framework of the ENIGMASS LabEx and the IN2P3 PHENOD project), efforts have focused on the interpretation of the precision measurements of AMS-02, to ensure its maximum scientific return. Numerous results have been obtained, leading to around fifteen publications in A-rank journals and proceedings of international conferences. Among these results, we can mention

  • A methodological improvement in the analysis of data from the AMS-02 experiment, necessary to avoid biasing the determination of transport parameters in the Galaxy. The combined analysis of the Li/C, Be/C, B/C, N/O, and 3He/4He data from AMS-02 (constraints on the transport parameters) and also of the Be/B ratio (constraints on the size of the galactic halo), as part of N. Weinrich's "Masterarbeit" (Weinrich et al., 2020).
  • Tools for the community have also been made public: first public version of the USINE propagation code; update of the cosmic ray database (CRDB).


II.3 Dark matter indirect detection

Dark matter, which constitutes 26% of the content of the Universe, remains a major enigma of physics. In the dark halo of our Galaxy, the latter could disintegrate or annihilate (in standard particles), leading to an excess compared to the astrophysical contributions expected in the GCR. This excess is sought in the fluxes of antimatter and γ radiation.
The best constraints on dark matter candidates are given by the study of anti-protons in the RCG and by the non-observation of γ photons in dwarf spheroid galaxies (objects that populate the Milky Way). On this first aspect, several publications (in collaboration with the LAPTh) have made it possible to update the calculation of the astrophysical background of antiprotons, in order to derive new constraints from the AMS-02 data. On the second aspect, C. Combet and D. Maurin continued their efforts to provide the community with a fast simulation code for dark matter signals in γ and neutrinos: a third version of the public code CLUMPY was provided to the community in 2019 This version made it possible to extend the targets calculated by the code to the extragalactic domain. In parallel, a new characterization of the signals resulting from the annihilation of matter in the dark halos of the Galaxy was carried out (in collaboration with the LUPM). This study underlined the strong impact of tidal forces in the destruction of these halos, which weakens the dark matter constraints that can be drawn from these objects.
Simulated map with CLUMPY (developed at LPSC) of the annihilation signal from dark matter substructures in the Milky Way. The distribution of the brightest objects makes it possible to study the prospects for the detection of so-called “dark” halos (no stars in these objects) by gamma detectors such as Fermi-LAT or the future CTA. Figure taken from Hütten et al. (2019).


III. Dirac project: toot for using distributed computing resources

Dirac is free software (under GPL V3 license) of the middleware type for managing distributed computing resources (data processing and storage), available here. It is used by many experiments in which IN2P3 is involved, including CTA, ILC, Belle II, BES and LHCb at CERN. Initiated almost 20 years ago by the LHCb experiment, Dirac has been constituted since 2014 as a Consortium which brings together all the international contributing partners. J. Bregeon is technical manager of the project at IN2P3 level, and contributes to the development and promotion of the software to user communities.