A New Type of Supernova?

The gamma-ray burst GRB 080503 occurred at about 0830 EDT on
Saturday, 3 May 2008, and woke me up. It turned out to be a
short-hard burst with no optical afterglow. Unfortunately the burst
was not too far from the Sun, so it is not easy to get follow-up
observations. Initially the burst behaved as a normal short-hard
burst does. It had an X-ray afterglow which faded quite
quickly, but no optical afterglow. However, on 4 May things got
interesting.

About one day after the burst observers
at Gemini-North detected a new source at the location of
the X-ray afterglow that was not there immediately after the
burst went off. This new source got brighter and then started fading
after about one day after the burst. This was unusual. In general
the optical afterglows fade after the first few minutes, and do not
get brighter with time. There have been a handful of exceptions, but
usually an optical afterglow that gets brighter a day or more after
the burst is an indication that there is a supernova associated with
the gamma-ray burst. It is generally believed that the so-called
long-soft gamma-ray bursts are caused by Type Ib/c sueprnovae, but no
one has seen a supernova associated with a short-hard gamma-ray burst.

Some people have interpreted GRB 080503 as being a
mini-supernova. A mini-supernova, also known as
a macronova, has been predicted by at least two research
groups over the past decade. The idea is that when two neutron stars
merge there is an explosion and neutron-rich material is ejected. The
initial explosion produces the gamma-ray burst, and then neutron decay
in the ejecta produces an optical transient that looks like a
supernova. The optical light gets brighter over a few days, and then
fades away. The details of the decay of the ejecta are not well
known, and there seem to be at least three different decay channels
that can produce the optical light. As far as I can tell no-one knows
much about this. The idea is that GRB 080503 is one of these
mini-supernovae.

This is an extreme suggestion. The late brightening may be due
to something mundane, like a the gamma-ray burst expanding into clumpy
dust and gas, or material from an accretion disc spiralling into
whatever is left after the two neutron stars merged. We do not know
enough about the physics of merging neutron starts be be able to make
confident predictions here. However, it is possible that astronomers
at the Gemini-North telescope discovered a new type of
supernova last Sunday.

The source marked "s2" in the cyan circle is the
suspected optical afterglow of GRB 080503. The blue circle marks the
location of the X-ray afterglow that was detected
by Swift. This circle shows the
accuracy that the X-ray source can be located to. The source
marked "s1" may be the host galaxy of GRB 080503, but this
is uncertain. The distance to this gamma-ray burst is not known.
This image was taken with the Gemini-North telescope.