Sarah H.R. Bank

REU program-Summer 2004
Univ. of Wisconsin - Madison
Madison, WI 53706

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3/4keV ROSAT

WARNING! This site is under construction, as is the research it will be based on.

X-Ray Production Mechanisms

Thermal Emission

Line Emission

Line Excitation
  • In a hot gas atoms are moving around with the increased kinetic energy inherent in their temperature. This allows collisions between atoms to excite electrons into higher energy levels, farther away from the nucleus. Atoms can also have electrons excited to higher energy levels by absorbing radiation of just the right energy.

  • When the electron decays back to its lowest energy state it releases, or emits an electron of that same energy. The emitted photon is characteristic of the energy difference between the two energy states of the electron. This emission appears as discrete spectral lines of the appropriate energy.

  • When this excitation occurs collisionally, as happens in hot gases, the observed emission lines denote the temperature of the gas, while the intensity of the lines denotes the atomic abundances (what the gas is made up of).


  • Coming from the german words "brems" meaning braking, and "strahlung" meaning radiation, this thermal emission happens in very hot gasses where the atoms have already been ionised.

  • It's known that when a charged particle is accelerated, radiation is emitted. In the case of bremsstrahlung, the emission occurs because a charged particle is accelerated around the nucleus of an ionised atom.

  • The strong electromagnetic attraction alters the course of the charged particle.

Bremsstrahlung Radiation
Synchrotron Radiation from Relativistic Electrons
  • As in the case with bremsstrahlung, synchrotron radiation occurs when electrons (charged particles) are accelerated around magnetic field lines.

  • For the radiation to be in the x-ray range of the spectrum, these electrons must be moving at relativistic velocities (significant precentages of the speed of light), thus requiring greater energy to alter their paths and releasing greater energies in response.

  • If the speeds are relativistic (fermi acceleration gets them to these speeds) then the radiation also undergoes beaming, where relativistic effects cause the radiation to be compressed to a narrow aperture in the direction of the magnetic field line.

Inverse Compton Scattering

  • When a relativistic electron collides with a slow moving photon, say from the Cosmic Microwave Background (CMB) it can impart some of its energy, accelerating the photon to higher energies.

  • This process is the literal inverse of Compton scattering whereby an electron is able to impart energy to a slow moving electron.

  • X-rays from inverse Compton Scattering are commonly seen in supernovae and active galactic nuclei (AGNs).

Inverse Compton Effect Radiation

The Diffuse X-Ray Background

The developement of x-ray astronomy:

In 1962, three years before Penzias and Wilson discovered the 3K Cosmic Microwave Background, a sounding rocket equipped with a geiger counter was launched in an effort to record solar x-rays reflected from the surface of the moon. The solar x-rays weren't seen, but the first x-ray star (Sco X-1) and the first diffuse cosmological background, in x-rays, was.

Soft x-rays


Three Main regimes of soft x-rays
X-ray Features
  1. >1keV - The main component of this radiation comes from the superposition of extragalactic point sources such as active galactic nuclei (AGN) and quasars where the x-rays are created as mass is actively accreted by super- massive black holes at the centers of distant galaxies.
  2. ¾ keV - This is problematic emission in that a number of sources contribute to define a generally isotropic wash of x-ray light aside from some contamination by thermal x-rays toward the galactic center:
    • at high galactic latitudes - superposition of AGN
    • within the plane of the galaxy (low galactic latitudes) - due to the superposition of some galactic point sources (such as x-ray stars) and hot plasma in the plane of the galaxy
    • towards the galactic center - hot gas in and around the galactic bulge as well as Loop I, a feature which also shows up in radio wavelengths, due to the expanding superbubble of a supernova remnant (SNR) (see figure for the main features of the x-ray sky)
  3. ¼ keV - 106 K gas in a cavity within the plane that our sun happens to be near the center of, known as "The Local Bubble"

The X-ray Sky above 1 keV

ROSAT deep survey

The superposition of extragalactic point sources (AGN and quasars) at high latitudes as well as superposition of galactic point sources, such as x-ray stars and stellar mass black holes, at low galactic latitudes creates a fairly isotropic wash of x-rays. Deep x-ray surveys, akin to the Hubble deep fields, made by ROSAT were necessary to distinguish the individual sources.

The Local Bubble: The ¼ keV Sky

Where X-ray Astronomy is Headed: The ¾ keV Sky

Why is X-ray astronomy so fascinating?

How are astrophysical x-rays measured?