Review Questions for the AST309 Final
Flux (i.e., brightness) is equal to (Luminosity / 4
where d is distance from you.
You measure the flux, luminosity is a standard candle, so you can solve for the distance
to determine how far away an object is.
: The more distant a galaxy is, the faster it is moving away from us. The
V = H
where V is velocity in km/sec, H
is Hubble’s Constant (approxi-
mately 71), and D is the distance in Mpc.
1. What are the two main reasons why gamma-ray bursts are so hard to study?
Because GRBs are so short in duration, it was necessary to build faster-responding tele-
scopes in order to determine their redshifts, and thus their distances. This has now been
somewhat accomplished, and improvements are being made all the time.
We now believe that they are not isotropic (although they are isotropic in distribution
they are not isotropic in their outputs
), this means that we must be in a special orientation
in order to see them. This in turn means that (a) we cannot be certain what their actual
flux is, and (b) that we are only seeing about 1% of the GRBs that happen.
Another problem is that because their energy output is so high, what we perceive of them
is subject to Relativistic Beaming, or Doppler Boosting. Since the jet is coming almost
straight towards our eyes and since it is traveling at relativistic speeds, the flux that we
detect is exaggerated relative to what it actually is. This is a result of Einstein's theory of
special relativity, since when objects travel very fast, they appear to increase their mass
and therefore their energy. The relativistic effects cause GRBs to be perceived as about a
factor of 100 more energetic than they really are. This also complicates the difficulty in
determining their intrinsic brightness.
2. Why do we think that gamma-ray bursts are associated with star formation?
Now that we have redshift information about many sources, we can look at their distribu-
tion in time (we already know that they are spatially isotropic). It appears that GRBs hap-
pened more often in the past. The other things that we know are more prevalent in the
past are quasars and newly-formed galaxies. These imply that star-formation was more
common, which possibly suggests that there might be a connection between star-
formation and these processes. However, we need more direct evidence rather than a
temporal connection. We do this by looking at what type of galaxies these GRBs occur in
and where in the galaxy they lie. It turns out that there are two additional bits that suggest
GRBs are associated with star formation. The galaxy type is generally in young, active
galaxies, such as disk galaxies that contain lots of gas and dust. The other bit of evidence
is that the GRBs are NOT preferentially located in the galaxy centers, but are instead dis-
tributed throughout the galaxy with a disk-like distribution. This strongly suggests two
things: that GRBs are probably not associated with supermassive black holes, and that