The Birth of a Supernova Seen in Real Time

Supernovae represent the explosive death of the most massive stars in
the Universe, that live fast and die young, often within just a few
million years.  These explosions give birth to neutron stars and black
holes, and fuel galaxy evolution through the injection of heavy
elements and mechanical energy.  Indeed, much of the material from
which the Solar system and life on Earth were forged, processed and
dispersed by supernovae and their progenitor stars.

The basic picture of a supernova explosion involves the collapse of
the iron core at the end of the star's life.  The collapsing core
reaches nuclear densities within tens of milliseconds, and then
rebounds, thereby generating a shock wave that eventually blows up the
star.  The details of the mechanism, however, remain highly uncertain,
and even the most sophisticated numerical simulations fail to generate
a clear explosion.

The explosion also leads to the synthesis of new elements.  Of
particular importance is Nickel-56 whose radioactive decay gives rise
to bright optical emission -- the hallmark of supernova explosions.
The optical light becomes progressively brighter, reaching a peak
level about two weeks after the explosion.  Since supernova are
invariably discovered through their optical emission, they are
generally found days to weeks after the actual explosion when most of the
fireworks are already over.  

However, it has long been predicted that the most extraordinary and
luminous component of a supernova explosion is actually an intense and
short-lived X-ray outburst at the moment that the shock wave rips the
star apart, the so-called "break out" emission.  Unfortunately, since
such a transient signal is incredibly hard to detect, this X-ray
signature has never been observed.  As a result, no supernova has ever
been seen at the time of explosion.  We therefore lack an
understanding of the first minutes, hours, and days in the evolution
of supernovae.

All of this changed dramatically on January 9 of this year when we
essentially won the astronomers' lottery.  During our brief
observation with Swift's X-ray Telescope (XRT) of a supernova in the
nearby galaxy, NGC 2770 (d=27 Mpc), another star in the same galaxy
exploded.  The explosion was marked by an extremely
luminous X-ray outburst, $L_X\approx 10^{44}~\rm erg~s^{1}$, lasting
just a few minutes.  Thanks to a stoke of good luck, we
serendipitously recorded the whole event from start-to-finish with the
XRT. 

Swift's co-aligned Ultra-Violet Optical Telescope (UVOT) did not
detect an optical counterpart contemporaneous with the X-ray outburst,
but just two hours later it spotted a new optical source rising
quickly in brightness.  Through optical spectroscopy we quickly
established that the nature of the transient was a very young
supernova.  In particular, the lack of hydrogen and silicon lines in
the spectrum marked the event as a Type Ibc supernova (SN Ibc) which
was thereafter designated SN 2008D.  The phenomenal X-ray outburst
can therefore be interpreted as the long-awaited confirmation and first ever
detection of shock break out emission from a new-born supernova explosion.

However, it is also interesting to note that about 1 in every 1000 SNe
Ibc gives rise to a gamma-ray burst (GRB) or an X-ray Flash (XRF).
The hallmark of GRBs and XRFs are their ultra-relativistic outflows
(bulk Lorentz factor, Gamma=100), collimated into extremely narrow
jets, and to which about 10^51 erg is coupled.  Given the
extraordinary X-ray outburst we saw from Type Ibc 2008D, it is natural
to question whether the high energy emission could be attributed to a
relativistic GRB or XRF outflow.

Radio observations are the only way to address this question since
they uniquely measure the energetics and geometry of the fastest
material ejected in supernova explosions.  These fast ejecta race out
ahead of the slower bulk stellar debris and shock-accelerate electrons in the
supernova's immediate environment.  Synchrotron emission is produced
as the electrons gyrate in amplified magnetic fields. By monitoring
the evolution of the synchrotron emission, the properties of the
fastest ejecta and the density of the environment are directly
revealed.
 
To determine the nature of the X-ray outburst of SN 2008D -- shock
break out emission or relativistic outflow -- we initiated a dedicated
monitoring campaign with the VLA and CARMA beginning just hours after
the X-ray outburst.  Shortly thereafter, we detected a new radio source that
grew in intensity over the next few weeks.  Our densely sampled
observations spanning 1.4 to 95 GHz are shown in Figure~2.

Twenty days after the outburst, the radio emission peaked at 8.46 GHz with 
radio spectral luminosity of 10^27 erg/s/Hz, a factor of
10^4 lower than those of typical GRB afterglows, but comparable to
the radio luminosities observed for nearby core-collapse SNe.  Based on our
modeling of the temporal and spectral evolution of the radio emission,
we firmly constrain the velocity of the fastest ejecta to be just v=0.25 c 
with a kinetic energy  10^48 erg, or 0.1% of the total energy kinetic
energy of the explosion.  As a result, this explosion bares no resemblance to
the extraordinary energetics and velocities inferred for GRBs and
XRFs.  The radio emission thus points to an ordinary core-collapse
supernova in which only a tiny fraction of the explosion energy is coupled
to fast ejecta.

Of course, the ultimate clincher in constraining the expansion speed
of the ejecta is through direct measurements with VLBI since it
may enables us to actually resolve the ejecta.  Along this line, we
obtained VLBA observations of SN 2008D at 1 month after the explosion
(Figure 2).  While the relativistic ejecta from a GRB or XRF would be
easily resolved in this observation, the non-relativistic material
form an ordinary supernova would be far too small.  We show that the
ejecta are not resolved in the VLBA map, imposing a conservative upper
limit on the speed of the fastest ejecta to be Gamma=3.  We further
note that these observations rule out the possibility of a
relativistic jet pointed away from our line-of-sight.

In conclusion, our radio observations have enabled us to distinguish
the nature of the SN 2008D X-ray outburst.  As we report in a recent
paper to Nature, every aspect of the supernova was fully in line with
expectations for an ordinary core-collapse explosion.  The X-ray outburst
is well described by the long-predicted, but never yet seen, shock break
out emission, and we clearly rule out any contribution from an 
associated GRB or XRF.  

The most exciting conclusion of this study is that SN 2008D was
perfectly ordinary explosion, caught in the act of exploding for the
very first time.  We can therefore expect that all ordinary supernova
produce similarly spectacular X-ray outburst marking the time of
explosion which may be used as an early warning beacon to enable
follow-up from ground based facilities enabling searches for neutrino
and gravitation wave signatures, which may ultimately unravel the
mystery of supernova explosions.  Looking forward, the field of supernova
discovery and study is on the brink of a paradigm shift -- with more
powerful X-ray satellites being designed now (e.g. EXIST), we predict 
that hundreds of supernovae will be discovered each year 
in the act of exploding, thanks to their powerful X-ray outbursts.