Radioactive Decay in Supernova Remnants
The COMPTEL instrument could detect both old and young
-- massive star explosions -- through measuring the decay of
radioactive elements in their
Gamma rays from the decay of titanium-44 and aluminum-26, both present
in SNRs, were prime targets for COMPTEL, one of Compton's four
instruments. 44Ti has a half-life of around 60 years;
26Al has a half-life of 700,000 years. As such, they serve
as tracers for both new and old supernovae, respectively.
In 1998, the Compton team came upon one of the closest supernova
explosions after detecting titanium-44 near the
well-known Vela supernova remnant. The undocumented SNR is 650 light
years away and may have been bright enough to be seen on Earth during
the daytime in the early 14th century.
ROSAT image of the|
Vela Supernova Remnant
Refer to the ROSAT
X-ray image (above) of this
What you are seeing is actually three supernova remnants. The
smaller, bright ball in the upper right is the Puppis A SNR; and the
large fiery ball in the center is the Vela SNR. Overpowered by the
bright Vela emission is the newly identified SNR, hiding in the lower
left corner of Vela.
Can't see it? Neither did astronomers.
Now look at a side-by-side comparison of two other ROSAT X-ray images
and a COMPTEL gamma-ray image (below). The first two panels show
the entire region. The second panel, which shows the region in a
slightly different X-ray energy band, begins to reveal the new SNR --
which is actually much closer to Earth than Vela is.
ROSAT and COMPTEL Images of the Vela Supernova Remnant Region
Left - The Vela SNR (filling most of the image) and the smaller
Puppis A SNR (near the 2 o'clock position at the
edge of Vela) in ROSAT Image at X-ray energies 0.1-2.4 KeV;
Center - ROSAT Image for X-ray energies 1.3-2.4 KeV;
Right - COMPTEL Image of newly discovered Supernova Remnant
The third panel zooms in on the titanium-44 emission coming from that
lower left section of the region, for there were no gamma rays
coming from the other parts of the region. Where could the
titanium decay be coming from, astronomers asked? Puppis A and Vela
are too old; their 44Ti has long since decayed. Because
of this, Puppis A and Vela are visible in the ROSAT image but not in the
Astronomers realized that there had to be another SNR, much younger,
hiding in the region, and turned to the COMPTON data. Sure enough,
further ROSAT observations taken independently of the Compton work
confirmed the findings. Both the ROSAT and the Compton observations
were conducted by MPE in Garching, Germany.
A note about the third panel in figure 2: This image is a likelihood
map. The black regions represent the area least likely to produce
gamma rays. Then blue, then red, then yellow. The white area
represents the most likely position for this gamma-ray source.
The newly discovered supernova is relatively nearby in our
should have been easily seen by medieval astronomers, even during
daylight hours. Yet there is no written record of the event. The
COMPTEL astronomers have a couple of guesses on why this may be. One
reason may be simply that the explosion was in the southern sky, so
folks far up in the northern hemisphere (i.e., Europe, much of the
Islamic world and most of China) couldn't see it since this part of
the Milky Way Galaxy never rises above the horizon. Other reasons
may be that the supernova was optically obscured by dust and other
material in front of it, or that the supernova itself was
"optically subluminous," a technical term for being much
fainter than the average supernova.