Sarah H.R. Bank

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

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

X-Ray Production Mechanisms

Thermal Emission

Line Emission

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).

In hotter gasses, there are more collisions and those electrons in the outer shells are prone to being knocked completely free of the atom. This ionizes the atom and exposes inner electron shells. If the gas is hot enough, further collisions can exite these inner electrons. The closer an electron is to the nucleus however, the harder it is to overcome the electromagnetic forces keeping it in its ground state. More energy is required to exite the electron and subsequently, when these inner electrons are exited and eventually decay, the emitted photon also has more energy.
Heavier elements which have more electron shells before reaching the nucleus are not completely ionized except at the hottest temperatures. They're bigger nucleus also means that the emission lines.

Bremsstrahlung

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.

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).

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

energies between 0.1 and 10 keV - thermal production of these photons requires temperatures on the order of 10⁶-10⁸ K
Photoelectric absorption dominates at <10 keV, resulting in varying mean free paths as ~E^-3

Technology

The switch from Geiger detectors to proportional counters so as to discriminate between actual and non-x-ray events.
Atomic absorption edge filters further discriminate between very soft x-rays with energies between 0.1 and 1 keV.
- Atomic absorption edge filters have a characteristic response curve which acts as a narrow bandpass to separate events by small changes in energy.
- The response curves have a gradual drop at low energies and and a sharp upper energy cut off due to the x-rays being better able to penetrate the filter as their energy increases until it reaches tha required value at which a K shell electron can be knocked loose.
- The Be, B, and C bands of the Wisconsin survey are defined by the transmission of filters from the corresponding elements.
- See How are astrophysical x-rays measured? for more information on detection and my contributions to the Wisconsin team.
Low energy observations (<1 keV) are particularly susceptible to contamination from: cosmic rays, UV-rays, 2-50 keV electrons, non-cosmic x-rays scattered in the upper atmosphere due to thermalization of the solar wind, and from low energy electrons by bremsstrahlung in the atmosphere or within the detector itself.

Three Main regimes of soft x-rays

>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.
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)
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

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

The flux is seen to increase with galactic latitude and anticorrelates with the distribution of HI as seen in dust tracing infrared (IR) maps such as the one below taken by the IRAS (Infrared Astronomy Satellite). This suggests a local cavity of hot (10⁶ K) gas at lower than average densities displacing the HI and emitting thermal soft x-rays.
Shadowing has ruled that the local hot bubble is not the only contributor to the x-ray flux because absorption by molecular clouds located above the plane of the Galaxy confirm incoming x-rays from beyond the plane of the Galaxy.
These shadowing observations also suggest that the halo component of the flux is irregular. Clouds along sight paths with different absorptions but similar distances show the same intensity drops and relative intensities. If the halo component were isotropic though, the intensities would differ with the different lines of sight.
Conclusions: At ¼ keV thermal emission from the local bubble and anisotropic contribution from the Galactic halo dominate the flux. The local bubble is an irregularly shaped cavity of a density lower than the average 1 cm^-3 that has been heated to ~10⁶ K and which the sun happens to be within.

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

At ¾ keV the composition is mainly due to the superposition of AGN and local (within 1 Mpc) hot gas within our galactic cluster, the "Local Group." From these observations an upper limit of 20% of the ¾ keV is still unaccounted for. Reasonable estimates place this mystery contribution at closer to 10% flux coming from "missing matter."
This matter could be modeled by diffuse, hot, intergalactic gas whose faint spectra cannot be currently separated out from the diffuse continuum.
High resolution x-ray spectroscopy could differentiate these faint intergalactic lines from the overpowering lines originating locally by their red-shifts.
Because this gas is so diffuse however, and its contribution so faint, the 10% contribution is lost in the statistical "noise" of current data.
Improvements in detector technology could also find that a significant portion of the radiation interpreted as coming from a local hot bubble are due to radiative transfer from the solar wind.

Why is X-ray astronomy so fascinating?

X-ray astronomy is the study of the first cosmological background, but is still full of unresolved issues.
The missing baryon problem looks for normal (baryonic) dark matter that astronomers know should exist, but haven't yet seen.
To detect X-rays with microcalorimeters requires reaching temperatures a fraction of a Kelvin above absolute zero.

How are astrophysical x-rays measured?

Astrophysical x-rays have been measured using several different techniques since the first rocket flights. Initially Geiger detectors which could only detect that an event had taken place, and not how much energy it contained were used.

Recent advancements have brought microcalorimeter technology to the forefront of research. What is a microcalorimeter? Basically, its a thermometer for VERY small temperature changes, such as an x-ray hitting an absorber. This is why it's necessary to operate x-ray microcalorimeters at temperatures just a fraction of a degree above absolute zero.

Microcalorimeters consist of an absorber with a very sensitive thermometer connected to a heat sink. The absorber converts to energy to heat, the thermometer, known as a thermistor, measures this heat, and the heat sink brings the system back to the optimal temperature for detection.

The thermistor is a resistor whose resistance varies with change in temperature, while the heat sink consists of pressureized liquid helium at 1.4K.

Since the helium only gets us down this far, and temperatures of approximately 60 mK, an adiabatic demagnetization refrigerator (ADR) cools the detector down the rest of the way.