Projects of the Sherpas team

The sherpas group is involved in theoretical and observational studies of accretion and ejection around young stars, compact objects as well as high energy processes : relativistic jets, high energy emission, gamma ray bursts. Our main activities are : theoretical studies and numerical simulations of magnetised plasmas, studies of acceleration and emission processes of high and very high energy particles (X rays, gamma rays, cosmic rays), and participation in instrumental collaborations : satellites observing in X (XMM) and gamma (Fermi), Cherenkov telescopes (H.E.S.S, CTA). Our activities fall within the themes selected by the ASTRONET network, designed to direct the 5-25 years prospective of research in astrophysics of the European Union : do we understand the extremes of the Universe ?

High energy emission processes

Motivations
The most spectacular examples of high energy emission in astrophysics are certainly observed in compact systems where the dominant source of gravitation can be a black hole (from a few solar masses, like in black hole X-ray binaries (BHXrB), to millions of solar masses like in active galactic nuclei (AGN)), a neutron star or a white dwarf. In these systems, the gravitational energy released by the accreting gas is generally thought to be dissipated partly in optical/UV/soft X-rays as thermal heating in an optically thick ”cold” plasma, the accretion disc, and partly in X-rays in a hot and optically thin plasma (the so-called corona). The accretion disc feeds the central compact object, while outflows of various kinds (wind and jets) may stream matter and energy away. This complex phenomenology is seen in all compact objects (an artist view of an AGN and a BHXrB is shown Fig. 1). The observation of these astrophysical sources allow us to probe the physical conditions close to black holes over a wide range of size and time scales, both being directly proportional to the black hole mass. For instance, the light crossing time of the inner regions of X-ray binaries is a fraction of a millisecond while it is of the order of a few thousands of seconds in the most massive AGNs.

Figure 1 : On the Left is shown the radio-view of an AGN, the radio galaxy Cygnus A as observed by the Very Large Array/NRAO instrument at a wavelength λ = 6 cm. The Right panels show the sketch of the inner part of the AGN (bottom) and of a BHBXrB (upper). Both kinds of system exhibit the same components, however, the mass/size difference of these systems ranges typically over factors of 106-109.

Objectives

While compact objects have been studied since many decades, the details of the radiative processes in the BH environment, as well as the physics of the accretion-ejection phenomena, are still far from being entirely understood.

  • what are the geometry, dynamics and energetics of the accretion flow onto the compact object ?
  • What are the processes at the origin of the high energy emission liberated in their close environment and what is the origin of its variability ?
  • What is the interplay between the accretion and ejection phenomena ?

Methodology

We probe the physical conditions close to compact object thanks to multiwavelength observations from radio to gamma-rays. Our group is more specifically involved, as PI and co-I, in high energy observations with high energy satellites like XMM-Newton, Chandra and NuSTAR

Kinetic processes and particle acceleration

Overview
There is overwhelming observational evidence that compact objects (white dwarfs, neutron stars, black holes) are involved in some of the most extreme particle acceleration phenomena in the Universe. Particle spectra escaping from these cosmic accelerators are broad steep power laws extending to tremendous energies, many orders of magnitude above the particles rest mass energies (i.e., they are ultra-relativistic). There are currently two main acceleration mechanisms under scrutiny by the community : diffusive shock acceleration and magnetic reconnection.

Global 3D PIC simulation of an inclined pulsar magnetosphere (Cerutti et al. 2016)
Full general relativistic PIC simulation of a rotating (Kerr) black hole magnetosphere (Parfrey et al. 2018)

Objectives
The work of the team in this field tries to answer the following fundamental questions :

  • How are particles accelerated within relativistic magnetic reconnection sites ?
  • How and where are particle accelerated within neutron star and black hole magnetospheres ?
  • How are particle accelerated at relativistic, magnetized collisionless shocks ?
  • What are the observable signatures ?

Astrophysical objects of interest

  • Pulsars, winds and pulsar wind nebulae
  • Stellar mass and supermassive black holes (X-ray binaries, AGN, Sgr A*)
  • Relativistic jets, blazars
  • Gamma-ray bursts
  • Nova outbursts

Methods
Studying particle acceleration requires a complete description of plasma phenomena at the kinetic scale, i.e., below the particle Larmor radius scale. The particle-in-cell (PIC) technique is a numerical method well-suited for this purpose : it solves the motion of a large collection of charged particles along with the evolution of the electromagnetic fields and their coupling between them. The team uses and develops the Zeltron code, a state-of-the-art PIC code optimized for studying astrophysical relativistic plasmas.

Contact
Benoit Cerutti, Benjamin Crinquand, Guillaume Dubus, Guy Pelletier

Astrocladistics and astrostatistics

Unsupervised classification (called clustering) aims at looking for structures in a data space made of n objects and p variables. These structures reflect either similarities (groups or classes), or relationships such as evolutionary paths. Statistical methods (machine learning, data mining) deal with the first category, while graph and phylogenetic approaches excel in the second one (path minimization).

Initiated in 2011 by Didier Fraix-Burnet, astrocladistics concerns the use of phylogenetic methods in astrophysics, cladistics, also called Maximum Parsimony, being one of them but certainly the most general. Initially thought for the case of galaxy diversification, astrocladistics has been successfully applied on all kinds of astrophysical entities, such as stars, stellar clusters (globular and open), and even Jupiter and Saturn satellites ! To read more about astrocladistics, visit my web page and my dedicated web-blog.

Cladogram of 14 Dwarf galaxies of the Local Group obtained with 24 characters (observables and derived quantities). Bootstrap values (above) and Decay indices (below) are indicated for each node. The outgroup (SagDig) has been chosen because it contains the lowest amount of metallic material, suggesting that it is made up of more primordial material (see Fraix-Burnet et al 2006 Astronomy & Astrophysics 455, 845-851 https://hal.archives-ouvertes.fr/hal-00067781v2).
Author D. Fraix-Burnet. Licence CC-BY. Image credits : NGC 55 : David Malin ; LGS 3 and Pegasus DIG : Deidre A. Hunter ; Antlia : Mike Irwin ; NGC 185 : David M. Delgado ; NGC 147 : Walter Nowotny ; Sag Dig : Hubble Heritage Team (AURA / STScI), Y. Momany (U. Padua) et al., ESA, NASA ; Leo A, Sextans B, IC 1613 and IC 10 : Corradi, R.L.M. et al., 2003, ING Newsletter No. 7, p. 11 ; NGC 3109 : NASA/ STScI ; Phoenix : Knut Olsen (CTIO) \& Phillip Massey (Lowell Observatory), (NOAO / CTIO / KPNO) ; NGC 205 : Atlas Image [or Atlas Image mosaic] courtesy of 2MASS/UMass/IPAC-Caltech/NASA/NSF ; background image : Canada-France-Hawaii Telescope / Coelum.