Vth School of Astroparticle Physics
23 - 28 May 2016
OHP, Saint Michel l'Observatoire

Physics of the Universe in X-rays

Rationale

X-ray astronomy (100 eV to 100 keV) was historically the starting point of high-energy astronomy, and many categories of astrophysical sources (from stars to all accretion phenomena and to clusters of galaxies) emit X-rays. The "X-ray view" of an object is therefore required for understanding the physical phenomenon. In particular X-ray astronomy remains a reference for all gamma-ray astronomy, since we have to turn to X-rays to study in details the sources detected at higher energy.

Recent observational developments renew the X-ray astronomy landscape. NASA’s NuSTAR telescope, launched in 2012, extends the domain of imaging telescopes beyond 10 keV into hard X-rays. The Japanese Astro-H telescope, which will be launched in 2016, opens fine spectroscopy to faint and extended sources. The Russian-German SRG satellite will be launched in 2017 and its main instrument eROSITA will conduct a survey of all X-ray sources in the sky 20 times more sensitive than the previous one (ROSAT, twenty years ago). The NICER detector will reach the ISS in October 2016 to study the X-ray emission of neutron stars. Starting from 2021 the French-Chinese SVOM mission will relay Swift to monitor the sky in hard X-rays to search for gamma-ray bursts and other transient sources. In 2014, ESA has selected the next large X-ray observatory ATHENA for launch in 2028. This is the next-generation multi-purpose X-ray observatory, successor to XMM-Newton, launched in 1999. In this evolving context it is the right time to take stock of the prospects in X-ray astronomy.

Scientific Context

High-energy astronomy in general could develop thanks to space. X-ray astronomy (100 eV to 100 keV) has been the starting point in the 60s and 70s, and is the most advanced domain. Several categories of sources emit most of their energy in the X-rays.

The accretion disks of compact objects (white dwarfs, neutron stars, black holes) are the brightest X-ray sources. Their properties depend on the mass of the compact object (the mass of stellar black holes are only a few times larger than that of the Sun, the mass of active nuclei in the center of galaxies can be up to a billion times larger) and on its rotation (rotating magnetic fields in pulsars, relativistic effects around rotating black holes), but also on the environment (companion star for compact stellar objects, central areas of galaxies for active nuclei). The central areas of these objects are small and the accretion is unstable, so these objects are often variable and their temporal properties supplement their spectral properties.  These sources are an excellent laboratory for the physics of accretion and ejection flows and, in some cases, in strong gravitational fields (general relativity). Active nuclei play an important role in the formation of galaxies. Their radiation and jet/wind that heat the surrounding gas, limits (but under certain circumstances also strengthens) star formation in primordial galaxies. The very existence of supermassive black holes at high redshift is not well understood within the general framework of structure formation.

The other major class of X-ray emitters are thermal sources beyond a million degrees (clusters of galaxies, hot interstellar medium, stellar coronae, neutron stars surface). Many of these objects are extended and their morphology gives important information. Furthermore, they are often optically thin and narrow lines dominate their spectrum, which allows spectral analysis of most of the heavy elements (oxygen to nickel, sometimes down to carbon and nitrogen). The population of galaxy clusters (number of clusters at a given mass) is a key observable for understanding the structure formation in the universe because clusters are the largest gravitationally bound structures. Supernova remnants supply the interstellar medium with heavy elements and energy (thermal, mechanical, cosmic rays). Stellar coronae reflect the magnetism of stars. Finally, the X-ray emission of neutron stars measures their cooling over time and thus provides constraints on their (not yet known) internal structure.

X-ray astronomy is also important to complement observations in other wavelength ranges. In particular, we can now explore the sky in the gamma-ray domain, from the MeV range (INTEGRAL satellite launched in 2002), to the GeV (Fermi satellite launched in 2008) and TeV from the ground (Cherenkov telescope HESS since 2004). X-ray astronomy acts as a reference for the whole gamma-ray astronomy, since one turns to X-rays to accurately locate the source of the radiation and possibly associate it with an object that is known at longer wavelength. If the source is extended, one can also make detailed images in X-rays, and of course the source’s X-ray spectrum can be compared to the gamma-ray emission.

X-ray astronomy is also naturally related to gravitational waves (whose sources are compact objects) and neutrino astronomy (for the same reason as gamma rays). These new messengers need wide field instruments to find counterparts.  Detectors of transient sources such as gamma-ray bursts (which are actually detected in hard X-rays in order to locate them) can play this role.

Finally, fundamental physics can find interesting test beds in the X-ray domain. Sterile neutrinos or certain types of axions are looked for in the keV range, and the birefringence of vacuum can be tested in X-rays by looking at how the polarization axis rotates as a function of energy.


Training goals

A survey on the occasion of the recent CNES prospective (Dubus et al 2014) has shown a growing interest in X-rays from communities that are not frequent X-ray observers, but for which X-rays are an important complement. This school should meet their expectations by showing the type of diagnostics that can be obtained in X-rays, how to analyze data to achieve it, and what are the current and future instrumental opportunities. Our aim is that X-ray astronomy can optimize use of observatories at other wavelengths (Herschel, ALMA, VLT) or beyond electromagnetism (VIRGO/LIGO, ICECUBE/KM3Net), insofar as understanding a source requires its broadband spectrum, including X-rays.

Even for X-ray astronomers, the domain has become so rich that each person knows well only a fraction of it. The school is the occasion to revisit the entire field. An X-ray image often contains several types of sources, and in order to exploit an extended source (a galaxy cluster or a supernova remnant, for example) it is useful to know the properties of foreground or background sources (stars, active galactic nuclei).

In this context, it is appropriate to take stock of the recent developments in X-ray astronomy. We propose to review that in this school through several axes:

  • Advances in understanding high-energy sources (compact objects, accreting binary sources and active galactic nuclei, supernova explosions, gamma-ray bursts) with two complementary courses on clusters of galaxies and applications in fundamental physics.

  • State of the art and technological advances in terms of telescopes and detectors. These courses are intended for astrophysicists (particularly those that might later carry instrumental projects) so that they are well aware of the capabilities, but also the limitations of the current techniques and their future developments.

  • Data analysis through tutorials on real data. This is particularly important for non-specialists who may get lost in the jargon and specific procedures. Each attendant will have followed at least once all the stages of the analysis with a specialist, making sure that no critical step is forgotten.

Audience

This school is primarily targeted at scientists, post-doctoral fellows and PhD students in the fields of astroparticle physics, astronomy, theoretical physics and high energy physics, willing to acquire complementary skills and/or to change their research topic. Scientists from other fields are welcome if the topic appeals to them.

Prerequisites

A PhD level either in Astronomy, Astrophysics, Theoretical Physics or Particle Physics.

Main topics

PHYSICS
  Radiative mechanisms, thermal process of transfer and absorption | Accretion/ejection flows associated with stellar black holes | Super massive black holes | Neutron stars, pulsars, PWNe, magnetars | Supernovae, supernova remnants, hot interstellar medium | GRBs | Fundamental Physics | Galaxy clusters, the cosmological aspects | Radiative mechanisms, thermal process, transfer and absorption

DATA ANALYSIS
Catalogs, archives | Imaging | Spectroscopy | Variability

INSTRUMENT ON PRESENT AND FUTURE
X-ray telescopes (general principles and specific solutions for high-resolution, surface area, high power) | Collimators hubs and associated detectors | Coded masks and sky monitoring | Semiconductor detectors (CCD, DePFETs) | Spectroscopy (networks, bolometers) | Polarimetry

Key words

X-ray  Astronomy * Black Holes * Gamma-ray bursts * Galaxies clusters * SupernovŠ * Astrophysical jets * Neutrons stars * Fundamental Physics

Scientific Committee

Jean Ballet (IRFU/CEA), Michel Boer (ARTEMIS), JosŔ Busto(CPPM), Paschal Coyle (CPPM), Bernard Degrange (LLR), Yves Gallant (LUPM), Berrie Giebels (LLR), RenÚ Goosmann (UNISTRA), Stavros Katsanevas (APC), JŘrgen Kn÷dlseder (IRAP), Julien Lavalle (LUPM), Benoit Lott (CENBG), Jean Orloff (LPC CLERMONT), Etienne Parizot (APC), Guy Pelletier (IPAG), Pierre Salati (LAPTH), Roland Triay (CPT)

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