RET 2013 MIT HAYSTACK OBSERVATORY Natural Sources of Radio.

RET 2013MIT HAYSTACK OBSERVATORY

Natural Sources of Radio

Learning Objectives

NGSS Performance ExpectationsDevelop and use a model of two objects interacting

through electric or magnetic fields to illustrate the forces between objects and the changes in energy of the objects due to the interaction.

Force = ma …acceleration of a charge is the primary mechanism for EM radiation emission

We will investigate the nature of those forces leading to emision.

Evaluate the validity and reliability of claims in published materials of the effects that different frequencies of electromagnetic radiation have when absorbed by matter..

Lecture Outline

Types of emissionThermal emission

Background, Blackbody radiation Emission spectra analysis

Non-thermal emission

Lecture 1: Overview

Thermal EmissionRadiative Transfer process overviewFoundation of Thermal Emission

Kinetic molecular theoryTypes of thermal emission

Blackbody Emission Free-Free emission Spectral Line emission

Atomic Molecular

Radiative transfer process

Radiative Transfer

processes

Blackbody emission

Free-free radiation

Spectral line emission

Cyclotron and Synchrotron radiation

Observed light

Radiative Source Processes

Natural Sources of Information from EM radiation, Radio specifically

Any process that will accelerate a charged particles will produce EM radiation˃ This could be a free electron traveling through the vaccum of space and being

affected by a magnetic field and thus accelerated ˃ It could be a bound electron or proton and the motion associated with

thermal energy is causing quick accelerations associated with that motion.

• The Kinetic molecular theory states all matter is made of tiny particles in constant motiono The constant motion will generate EM radiation o We call this type of emission, thermal emission

Types of EM emmission:

The type of radiation tells us something about the source

Thermal emission

• Blackbody radiation

• Spectral line emission

• Free-free radiationNon-Thermal

• Cyclotron emission

• synchrotron emission

• MASERs

Blackbody radiation

All macroscopic (everyday) objects emit EM radiation at all times!! (if T > 0 K)

explaination: The Kinetic Molecular Theory,KMT» all matter is made up of tiny particles (atoms,

molecules, sub-atomic particles) in constant motion.

Temperature is a direct measure of average kinetic energy of all microscopic particles.

Velocity vector

Atom or molecule

# of

mol

ecul

es

Average Kinetic Energy

T

Distribution of the # of particles at each level of kinetic energy

Radiation Laws

Wein's Law» Wavelength of peak

emmission

˃ Wavelength of peak emission is inversely proportional to the Temperature.

˃ Higher Temp == lower (blue)

˃ Lower Temp == Higher (red)

Radio emitted as blackbody radiation

» Recall that the EM spectrum ranges from frequencies of 1 cycle per second (1 Hz) to

Radiation laws cont.

» Stephan's Law˃ The power output from the

surface of a blackbody radiator is proportial to the Temperature to the 4th power

Blackbody Radiation Summary

» The KMT represents particles as moving at a distribution of Kinetic energy

» Accelerating charges create EM waves, The different accelerations produce different frequencies

» A blackbody spectrum represents the distribution of EM radiation and changes with temperature

» Link to Starter Activity:˃ Imagine each student traveling

randomly and they were carrying a flashlight that changed color depending on their speed. An observer from distance would see a combination of all the different colors represented by the different speeds. If put through a simple spectrometer or prism it would produce a spectrum. That’s the blackbody spectrum.

Lecture 2: Spectral line analysis

Wave nature of lightParticle nature of lightSpectroscopy for absorption and emission

processes

Spectral Line emission (spectroscopy)

Radiation can be examined with a simple spectrometer

Interaction principle

The way that atoms and molecules absorb and emit radiation can tell us something about their nature or identity.

Demo: Hydrogen emission Continuous spectrum

Absorption spectrum

Emission spectrum

Spectral Absorption and emission process

UV Photon

Electron moved from ground state to elevated state.Absorption

Electron falls down to ground state again

A photon is emitted equal in energy to the difference between ground state and excited state. Emission

Generating multiple spectral lines

Each transition from higher to lower state emits a photon of a certain energy and therefore wavelength

Spectral line analysis

» The emission spectra of an element provides a fingerprint that allows scientists to deduce its presence from the observation of the specta˃ Analogy: Bar code

» Detecting composition˃ The composition of an object is determined

by matching its spectral lines with laboratory spectra of known atoms and molecules

» Link to Unit Starter:˃ What if every element and molecule has a specific set of seats available on the

bleachers:+ You would only see a specific # of emission lines as electrons move up and

down into them?˃ That’s exactly how atoms and molecules work. ˃ They have a fingerprint that is their absorption/emision spectrum that is

unique to that element if you look for the transitions that should set it apart from all the others.

˃ The cataloguing of these transition locations and energies in the lab has helped scientists find many atomic and molecular species in the night sky remotely.

Case Study: the 21 cm line

"2nd" Motion:

- Both the proton and the electron are going to have an individual spin

• The spin of both can therefore be in the same direction (aligned) or in opposite directions (anti-aligned)

• Because of quantum mechanics, it turns out when the spins are aligned, the hydrogen is higher in energy

http://upload.wikimedia.org/wikipedia/commons/thumb/e/e1/HydrogenLineParallel.svg/500px-HydrogenLineParallel.svg.png

Aligned vs. Anti-Aligned

- Even though the Aligned version is higher in energy, its electron still exists in the S orbital

• Instead, the aligned version compared to the anti-aligned version has hyperfine structure

Emission

- It is possible for hydrogen to jump from its higher energy aligned state to the lower energy anti-aligned state

• Very unlikely to happen:o probability of 2.9×10−15 s−1

o time it takes for a single isolated H atom to undergo this transition is ~ 10,000,000 yrs

• When it does happen, it releases a specific wavelength of light...

o Care to guess what that wavelength is?

The 21-cm Line:

- The energy gap between the hyperfine structures directly corresponds to the 21-cm wavelength (1420.405... MHz)• This wavelength was predicted by

Jan Oort and Hendrick C. van de Hulst in 1944

• Discovered by Edward Mills Purcell and Harold Irving Ewen in 1951

http://upload.wikimedia.org/wikipedia/commons/thumb/f/f7/Green_Banks_-_Ewen-Purcell_Horn_Antenna.jpg/321px-Green_Banks_-_Ewen-Purcell_Horn_Antenna.jpg

The 21-cm Line:

- So what's the point? What can be done with this information?

• First use of this was in 1952 where the first maps of neutral hydrogen in our galaxy were made

• These maps using the doppler shift of the 1420 MHz spectral line revealed the spiral structure of our galaxy

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http://upload.wikimedia.org/wikipedia/commons/thumb/a/a4/NGC_6384_HST.jpg/320px-NGC_6384_HST.jpg

Summary of thermal emmision

» So we have seen that if matter is moving in any way, charged particles are being accelerated

» If charges are being accelerated EMR photons are being produced

» The power and spectral distribution of those photons depends on The Temperature of the material.

» Therefore: We can detect the temperature of materials in space by analyzing the light coming to us on earth.!

NON-THERMAL EMISSION

AND OTHER WEIRDNESS

Lecture 3

Remember that the temperature of an object can be inferred from the peak wavelength of the blackbody spectrum. λ~1/T

This energy distribution can be modelled very accurately. Everything resembling this shape is called THERMAL radiation.

Click icon to add picture

Comparison of Thermal vs. Non-Thermal radiation

Think of intensity as the number of photons

In thermal radiation, most photons are at the peak frequency, thus you can relate that to the Temperature (average kinetic energy)

In non-thermal you can’t do that ……

Thermal

Non-thermal

Non-thermal

Thermal

Eskridge, Paul. "Active Galactic Nuclei." Notes for Week 14 Astronomy 101 Spring 2014. Minnesota State University, 6 Jan. 2014.

Direct observations leading to new insights

Particle physics studies the properties of the fundamental particles of matter.

Uses very high energyAlows us to discover how particles behave at

these high energies.Non-thermal emission processes were

discovered in this way.

From these types the synchrotron radiation seemed to fit the models for non-thermal sources

The non-thermal emission properties were used to model the spectra of quasars and other radio sources.

The spectra of these could be explained with the models

Synchrotron Radiation

First discovered in a Bell Laboratory particle accelerator called a ‘synchrotron’ (1947)

The power law distribution was very different from the Maxwellian-Planck distribution in that it increased with higher frequency

High energy sources could then be detected by this unusual spectral feature especially at x-ray and gamma-ray bands.

Examples of Astrophysical Synchrotron Radiation

The bluish region in the center of the crab nebula is caused by synchrotron radiation

The bluish jet from M87 is emerging from the AGN core

Case Study: Blazars (yes, that is an actual group of objects in astronomy)

In 1963 Maarten Schmidt discovered quasars using radio wave measurements Quasars – Quasi-star radio sources Quasars are:

Very distant (100s of billion LY) Very bright (about the same amount of light as our entire

galaxy) Highly Variable (changing in periods of days to years)

This was a discovery that confirmed the big bang cosmological model over the static universe model.

Blazars cont.

Blazars are radio “quiet” but have red shifts similar to quasars and are therefor very distant. Blazars are originally named BL Lac objects from

observations of the star BL lacertae

Eskridge, Paul. "Active Galactic Nuclei." Notes for Week 14 Astronomy 101 Spring 2014. Minnesota State University, 6 Jan. 2014.

Other Sources of Non-Thermal (Synchrotron) Radiation: MASERS

Microwave Amplification by Stimulated Emission of Radiation Emissions from a

particular transition are used as a pump for sustained emission from other molecules

Added together the radiation becomes amplified

Fish, Vincent L., and Loránt O. Sjouwerman. "GLOBAL VERY LONG BASELINE INTERFEROMETRY OBSERVATIONS OF THE 6.0 GHz HYDROXYL MASERS IN ONSALA 1." The Astrophysical Journal 716.1 (2010): 106-13. Web.

MASERs cont.

Requirements for interstellar MASERsLow density

Less than 104 cm-3

This is very difficult to achieve in the Lab but is very high density for interstellar media

But high gain Lots of particles in the path along the line of site

Therefore, we need large regions in space to form masers 1014 cm3

Summary of Non-Thermal Sources

Non-Thermal sources have a different energy distribution function. Basically everything that doesn’t look like

this is non-thermal

Synchrotron radiation observed in particle accelerators explains the spectra of distant quasars

Observations of non-thermal radiation has lead to important discoveries of Active Galactic Nuclei (AGN)

References

1. "Astronomy: A Beginner's Guide to the Universe" 7th ed. Chaisson, E.; McMillan, S. Pearson Education inc. 2013 p.503

2. http://physics.nist.gov/cgi-bin/cuu/Value?me|search_for=electron+mass

3. “Outer Space is not Empty: A Teaching Unit in Astrochemistry”. RET 2004 Haystack Observatory MIT. Wesley Johnson and Roy Riegel.

References

4. Course: ASTR 122: Birth, Life and Death of Stars http://jersey.uoregon.edu/~imamura/122/astro.122.html

5. http://www.pbs.org/wgbh/aso/tryit/radio/indext.html6. http://galileo.phys.virginia.edu/classes/241L/emwaves/emwaves.htm7. http://www.astro.utu.fi/~cflynn/astroII/l4.html8. http://scienceworld.wolfram.com/physics/BrightnessTemperature.html9. Eskridge, Paul. "Active Galactic Nuclei." Notes for Week 14 Astronomy 101

Spring 2014. Minnesota State University, 6 Jan. 2014. Web. 24 July 2014. <http://frigg.physastro.mnsu.edu/~eskridge/astr101/week14.html>.

10. Fish, Vincent L., and Loránt O. Sjouwerman. "GLOBAL VERY LONG BASELINE INTERFEROMETRY OBSERVATIONS OF THE 6.0 GHz HYDROXYL MASERS IN ONSALA 1." The Astrophysical Journal 716.1 (2010): 106-13. Web.