Lecture 1

A Great Debate: The Mystery of Gamma Ray Bursts

What are Gamma Ray Bursts?

The Electromagnetic Spectrum

Light in the electromagnetic spectrum ranges from long wavelengths and low energy (radio) to short wavelengths and high energy (gamma rays).

The Milky Way Galaxy in Visible Wavelengths

The Milky Way Galaxy in Gamma Rays

To see the Milky Way in a bunch of other wavelengths, click here.

Compton Gamma Ray Observatory

Map of 800 Gamma Ray Bursts over the Sky

• Gamma Ray Bursts (GRB's) posed one of the greatest mysteries of modern astronomy

• discovered (by accident) in late 60's during the space race and cold-war

• about three times a day, the gamma-ray sky lights up with a spectacular explosion

• until 1997, no one knew how far away these explosions were

• last 30 milliseconds to 1000 seconds

• outshine all other sources of gamma rays combined, but are not visible to the naked eye

• astronomers first thought that GRB's came from the Milky Way Galaxy's disk

• but they are all over the sky!

• either in a large spherical halo around the Milky Way or billions of light years away

Aha! An Very Faint, Fading Object is Detected in Visible Light at the GRB Position

• end to the 30-year distance controversy?

• redshift can be measured in visible light

What do we mean by redshift?

• start with a spectrum, a plot of INTENSITY vs. WAVELENGTH for the light coming from an object

  

• as Universe expands, and wavelength of light emitted from an object is stretched, lines in spectrum move toward redder wavelengths

• objects furthest away have their light stretched the most, and therefore have lines shifted most towards redder wavelengths

  

• the amount that the lines are shifted redward from their rest positions is called the redshift

To learn more about the relationship between the redshift and distance of an astronomical object, click here.

What does the redshift of the visible object near the GRB position imply for the distance to the GRB?

• redshift is that of distant galaxies and quasars

• GRB is related to this object, so GRB is many billions of light-years away

• therefore, either GRB's release as much energy in 10 seconds as the Sun emits in its entire 10-billion-year lifetime or somewhat less energy escapes along just one direction...

Still don't understand the origin of Gamma Ray Bursts in detail, but

• likely to be associated with especially energetic supernovae that are known as hypernovae

• gamma rays produced inside star as the core collapses

• jet of material expelled along one axis

• GRB detected only if Earth lies along that axis

  

To see a simulation of a gamma ray burst from a hypernova, click here.

And, there are different kinds of GRB's that may have different origins...

• there are long (> 2 sec, average = 30 sec) and short duration GRB's

  

• hypernova model applies only to long duration GRB's

• short duration GRB's even more poorly understood -- merger of neutron stars?