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Maurizio Paolillo » 1.High Energy Astrophysics


Contents

The Course will be divided into 3 parts:

Part I – Introduction to high energy processes: radiative transport equation; photoelectric absorption, Bremsstrahlung, Thomson and Compton scattering, Comptonization, Synchrotron emission;

Part II – Detection techniques in High-Energy astrophysics: X-ray and gamma telescopes, scintillators, proportional counters, solid state devices (CCD), coded-masks; diffraction gratings; grazing incidence optics; spatial and temporal modulators; X and _-ray telescopes;

Part III – Astrophysical phenomena characterized by high-energy emission: X-ray emission in the solar system; stellar coronae; X-ray emission in high-mass stars; supernovae; accreting binary systems; active galactic nuclei; galaxy and cluster gaseous halos.

Reference textbooks:

  • High Energy Astrophysics, M. Longair, Cambridge University Press, 2002, vol. 1 & 2
  • High Energy Spectroscopic Astrophysics, S.M.Kahn, P.von Ballmoos & R.A. Sunayev, Saas-Fee Advanced Course 30, 2000
  • Astrophysical Processes: the physics of astronomical phenomena, H. Bradt, Cambridge Univ. Press, 2008.

What is High Energy Astrophysics

High Energy Astrophysics is the branch of Astrophysics which studies the most energetic phenomena of the Universe, through the study of electromagnetic (EM) radiation and particles that they produce.
This field is closely connected to many branches of Physics and Astrophysics, going from the study of Cosmic Rays, to the detection and analysis of EM radiation in the far-UV, X and γ-ray bands.
This Class will focus mainly on the phenomena observable through their X and γ-ray emission, on the physical processes producing such radiation and on the astronomical sources where it originates. Other aspects of this field, such as the study of Cosmic Rays, are discussed in other specific Courses.

Why study High Energy Astrophysics

High Energy Astrophysics probe many physical processes in extreme conditions which are not easily accessible in Laboratories:

  • It allows to study non-equilibrium conditions in interstellar plasmas;
  • At high energies we can study emission from meta-stable states;
  • K and L shell transitions in elements between C and Fe emit in the X-ray band
  • It allows to study many states at once and thus characterize elemental abundances without assumptions on the state of the plasma.

Furthermore high energy photons reveal many aspects of astronomical sources which are often complementary to other wavelengths and sometimes unique to the X and γ ray bands, e.g.:

  • Virial temperatures of many astronomical systems are in the X-ray regime (e.g. accreting binary systems, galaxies, groups and cluster of galaxies)
  • Shock-heated gas falls in the X-ray regime (e.g. supernova remnants);
  • Relativistic plasma emits at short wavelengths (e.g. jets in AGNs, Gamma Ray bursts).

X and γ rays

The Earth atmosphere is not transparent at wavelengths shorter than a few microns (see Figure).

For such reason High-Energy astrophysics developed quite late compared to traditional Astronomy. The discovery of high-energy radiation from the sky had to wait the availability of rockets and satellites, which could bring detectors above the atmosphere.

Historically X and γ rays were distinguished based on their wavelength, choosing a somewhat arbitrary separation around ~10-11 m. Today this criteria has changed due to the wavelength overlap in the emission produced by phenomena traditionally classified as X or γ ray emitters.

Nowadays X-rays are emitted by electrons outside the nucleus, while gamma rays are emitted by the nucleus itself.

Atmospheric absorption as a function of wavelenght.

Atmospheric absorption as a function of wavelenght.


History bits

Before 1960 several experiments were conducted by the US Naval Laboratories to detect X-rays from the Sun, but were unsuccessful in detecting extrasolar high-energy emission.

The group of Riccardo Giacconi (who had been trained as a particle physicist) in the ‘60 conducted the first rocket experiments to measure extrasolar X-ray emission (see Figure).

The discovery of the first cosmic X-ray source in 1962 came as a surprise. This source is called Scorpius X-1, the first X-ray source found in the constellation Scorpius. Based on discoveries in this new field, Riccardo Giacconi received the Nobel Prize in Physics in 2002. It was found that the X-ray emission of Sco X-1 was 10,000 times greater than its optical emission.

This discovery required a source of energy greater than nuclear fusion, which powers stars. This source is now known to be gravity, due to the infall of hot plasma onto a compact object (neutron star or a black hole).

An historical review on the dawn of X-ray Astronomy can be found in the Giacconi’s Nobel Lecture.

The detector launched on June 12, 1962 AS&E rocket. From Giacconi and Gursky, 1974, p. 9.

The detector launched on June 12, 1962 AS&E rocket. From Giacconi and Gursky, 1974, p. 9.


High Energy radiation

High energy photons behave more like particles than like classical waves, due to their quantized nature.

For such reason the detection and analysis of X and γ has many similarities with particle physics.

There are advantages and disadvantages with respect to traditional optical Astronomy:

  • Typical fluxes in the high energy regimes can be as low as 1 photon/s, thus requiring large collecting areas or very long integration times. The analysis requires an appropriate statistical approach since the working regime is often non-gaussian.
  • The large energy of incoming radiation allows the detection of individual photons and the simultaneous measurement of it’s energy, position, and time of arrival. Thus high energy analysis is intrinsically multi-dimensional.

Traditionally High Energy Astronomers use non-standard measurement units, often referring both to energy and wavelength in terms of electronvolts. Typical conversion factors are shown in the Table on the right.


Celestial sources: Stellar coronae

This image shows the Sun as viewed by the Soft X-Ray Telescope (SXT) onboard the orbiting Yohkoh satellite.

The bright, loop-like structures are hot (millions of degrees) plasma (electrically charged gas) confined by magnetic fields rooted in the solar interior. An image of the sun in visible light would show sunspots at the feet of many of these loops.

The halo of gas extending well beyond the sun is called the corona. The darker regions at the North and South poles of the Sun are coronal holes, where the magnetic field lines are open to space and allow particles to escape.

The Sun as viewed by the Soft X-Ray Telescope (SXT) onboard the orbiting Yohkoh satellite. – Courtesy of NASA.

The Sun as viewed by the Soft X-Ray Telescope (SXT) onboard the orbiting Yohkoh satellite. - Courtesy of NASA.


Celestial sources: Supernova remnants

Tycho’s Supernova Remnant, which exploded in 1572.

This is a false-colour x-ray image in which the energy levels (in keV) of the x-rays have been assigned a colours as follows: Red 0.95-1.26 keV, Green 1.63-2.26 keV, Blue 4.1-6.1 keV.

All x-rays images must use processed colours since x-rays (as are radio waves, infra-red) are invisible to the human eye. But they are not invisible to suitable equipment, such as x-ray telescopes. The red and green bands highlight the expanding cloud of plasma with temperatures in the millions of degrees.

Tycho’s Supernova Remnant, which exploded in 1572. – Courtesy of NASA.

Tycho's Supernova Remnant, which exploded in 1572. - Courtesy of NASA.


Celestial sources: Pulsars

The Crab Nebula is a remnant of an exploded star. This is the Crab Nebula in various energy bands, including a hard X-ray image from the HEFT data taken during its 2005 observation run. In the center of the Nebula lies a fast spinning neutron star: a pulsar. – Courtesy of NASA.

The Crab Nebula is a remnant of an exploded star. This is the Crab Nebula in various energy bands, including a hard X-ray image from the HEFT data taken during its 2005 observation run. In the center of the Nebula lies a fast spinning neutron star: a pulsar. - Courtesy of NASA.


Celestial sources: Galaxies

Galaxies emit in X-rays due to the contribution of hot gas, supernova remnants, binary accreting systems and supermassive Black Holes at their center. This image shows the optical(left), UV (center) and X-ray (right) view of a Spiral galaxy. The small circle marks the position of Supernova Ia 2005ke. 
Courtesy of NASA.

Galaxies emit in X-rays due to the contribution of hot gas, supernova remnants, binary accreting systems and supermassive Black Holes at their center. This image shows the optical(left), UV (center) and X-ray (right) view of a Spiral galaxy. The small circle marks the position of Supernova Ia 2005ke. Courtesy of NASA.


Celestial sources: Clusters of Galaxies

Clusters of galaxies emit in the X-ray band due to the hot plasma trapped into their deep potential wall.

This is a false-colour x-ray image of the so-called ‘Bullet cluster’, produced by the fast encounter of two massive clusters.

The bow-shock clearly seen in the right component reveals the collisional nature of the emitting plasma.

X-ray image of the so-called ‘Bullet cluster’ – Courtesy of NASA

X-ray image of the so-called 'Bullet cluster' - Courtesy of NASA


Exercizes

Calculate the virial temperature for:

  1. A cluster of galaxies of 1013 solar masses;
  2. An Active Galactic Nucleus where matter (hydrogen) is accreting onto a supermassive Black Hole of 108 solar masses.

Use textbooks and the web to obtain the typical sizes of these sources.

Solution: T~107 ºK

Sources and online resources

These notes include several contributions from public online material, including (but not limited to):

Niel Brandt’s “Introduction to High-Energy Astronomy”

Niel Brandt’s “High-Energy Astrophysics”

Liz Puchnarewicz’ “High Energy Astrophysics”

Chris Flynn’s “Astrophysics II”

Astrophysical Radiation Processes, R.-S. Bamberg

Astronomy 313 di Steven R. Majewski

I tried to give credit to the appropriate sources whenever possible. Any feedback about these notes is welcome, including reports of missing credits to appropriate sources or requests to remove proprietary material.

I materiali di supporto della lezione

High Energy Astrophysics, M. Longair, Cambridge University Press, 2002, vol. 1 & 2

High Energy Spectroscopic Astrophysics, S.M.Kahn, P.von Ballmoos & R.A. Sunayev, Saas-Fee Advanced Course 30, 2000

Astrophysical Processes: the physics of astronomical phenomena, H. Bradt, Cambridge Univ. Press, 2008.

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