November 13, 1998
X-ray astronomers and g-ray astronomers of the Max Planck Institute for Extraterrestrial Physics in Garching/Germany, have discovered a young supernova remnant which is exceptionally close to Earth (Nature, Vol. 396, 12 November 1998). The remnant is just 700 light years away and it was created about 700 years ago when a star exploded in the southern sky in the constellation Vela ("sail").
"Our analysis shows that this is the nearest supernova remnant to have occurred during recent human history; other similarly close remnants in the Milky Way are of age of at least 10.000 years and more", Dr. Bernd Aschenbach from the Garching Max Planck Institute explains. "Now, it is up to optical astronomers and radio astronomers to confirm and extend our results."
The discoveries are being reported in two separate contributions in "Nature's" issue of November 12, 1998, volume 396. The data on which these discoveries rest have been taken with the German X-ray astronomy satellite ROSAT, which has been developed and built under the direction of the Garching Max Planck Institute, and by the COMPTEL instrument, built by an international collaboration under the leadership of the Max Planck Institute as well, on board of the U.S. American g-ray astronomy Observatory "COMPTON".
During the first all-sky survey with imaging X-ray telescopes also the Vela constellation has been mapped by ROSAT. This is a region in the sky well known to astronomers. In soft X-rays the Vela region is dominated by a huge and bright supernova remnant, the Vela supernova remnant, with a diameter of almost 200 light years, which still continues to expand at supersonic speed. More than 10.000 years ago a star exploded as a gigantic supernova and it gave rise to the clouds of hot gas which we see today. (Outside of the boundary of the explosion cloud Dr. Bernd Aschenbach has discovered numerous fragments of the progenitor star, a result which has been published in "Nature" in March 1995.)
When Dr. Aschenbach was analysing the Vela supernova remnant in a way differing from standard software routines developed for ROSAT, in particular by extracting only the highest energy photons accessible with ROSAT, the image of the Vela supernova remnant changed drastically. For photon energies greater than 1300 electron volts the soft X-rays of the Vela supernova remnant had disappeared almost completely and a previously unknown, fairly circular emission region of about 2 degrees diameter, which is about four times the size of the full moon, emerged at the south-east corner of the Vela remnant (c.f. the pictures attached).
"We were stunned; there is no way around, this is a new supernova remnant", Dr. Aschenbach says enthusiastically. "There are no other X-ray sources in the sky we know of which show this sort of shape and brightness distribution, except supernova remnants". The previously unknown object was named "RX J0852.0-4622" according to the position in the sky.
Further analysis showed: "RX J0852.0-4622" is extremely hot at a temperature of about 30,000,000 Kelvin. This means: "RX J0852.0-4622" is a very young object, otherwise it would have cooled down to much lower temperatures already. But because "RX J0852.0-4622" is young it could have reached the angular extent of 2 degrees only if it is relatively close to Earth, otherwise just a small patch of X-ray emission would have been visible. "Detailed analysis and comparison with the well-studied remnant of the supernova which occurred in the year 1006 demonstrate that the new supernova remnant can not be significantly older than 1500 years and it can not be located at distances greater than 1000 parsec or 3300 light years", Dr. Aschenbach explains. "And the low X-ray surface brightness of "RX J0852.0-4622" can be attributed to a low matter density of just 0.04 gas and dust particles per cubic-centimeter, surrounding the star before it exploded. This is indeed low compared with standard values being about 20 times higher."
The case that "RX J0852.0-4622" is a supernova remnant was finally settled by the g-ray astronomers. They specialize in studies of the g-ray emission from radioactive decay of atomic nuclei. During the sudden death of a star in a supernova, which takes a fraction of a second, matter density and temperature in the star reach levels at which atomic nuclei change and reformat. Most of the chemical elements including their isotopes are being released from the star to the world at supernova explosions, without these stellar ashes no life would have been possible. The matter expelled in the supernova process is further being used, it is the "raw material" for formation of the next generation of stars and planets.
Many isotopes are not stable; they decay at a variety of times scales, which can be measured as "life-time" and which is a charateristic of each individual isotope. Eventually only the known "natural" chemical elements remain. Often the decay of an isotope is accompanied by the emission of g-rays of very specific energies. These g-ray lines are as unique as a finger-print for each radioactive isotope. Among other elements titanium-44 forms in a supernova explosion. It is produced exclusively during "silicon burning" and it decays over scandium to calcium by emitting a g-ray line of 1.156 million electron volt. The same group of g-ray astronomers had discovered this line for the first time from the well known young galactic supernova remnant "Cassiopeia A" already back some years ago.With the discovery of the titanium-44 g-ray line now from "RX J0852.0-4622" it was clear: "RX J0852.0-4622" is a young closeby supernova remnant.
Despite the fact that the production yield of titanium-44 in "RX J0852.0-4622" is not known -- it is being produced in every type of supernova but at different rates -- the g-ray measurements could be used to further constrain the age and distance of "RX J0852.0-4622" making use of the "life-time" of titanium-44. It is concluded that the supernova occurred in the 13th century at a distance of about 700 light years from Earth. "This is the first time that a previously unknown supernova remnant has been found by means of the titanium-44 g-ray line", Dr. Anatoli Iyudin from the Max Planck Institute for Extraterrestrial Physics comments about this success.
Coming years are very likely to show further discoveries of supernova remnants. In our Milky Way two to three stars in every 100 years are expected to explode; this follows from a comparison with external galaxies. But for the last 1000 years only seven remnants have been found so far. The remaining supernovae and their remnants might have escaped detection because the optical light might have gone lost in intervening interstellar gas and dust clouds. Hard X-rays and g-rays are not blocked by these clouds, so that the missing supernova remnants might be found soon by the instruments on board of the next X-ray astronomy and g-ray astronomy satellites, which are already being built and which are close to launch.
IMAGE CAPTION: Figure 1: At X-ray photon energies of more than 1300 electron volts a new X-ray source of circular shape emerges at the lower-left corner of the "Vela" supernova remnant; picture taken by the ROSAT X-ray astronomy satellite. X-ray astronomers and g-ray astronomers of the Max Planck Institute for Extraterrestrial Physics (MPE) have demonstrated that this new source is a young, previously unknown remnant, which has been borne in a supernova event at a distance of about 700 light years just 700 years ago. The white patch in the upper right shows the bright supernova remnant "Puppis-A", which is far behind the "Vela" supernova remnant and is unrelated to it. Photo by Max Planck Society/MPE.
Figure 2: Two ROSAT X-ray images of the same region of the sky: The left hand image shows that the soft X-ray emission of the "Vela" supernova remnant taken for energies between 100 and 2400 electron volts dominates the region and outshines everything else except the very bright supernova remnant "Puppis-A" towards the upper right corner, which however lies behind and is not related to "Vela". Picking just the X-ray photons with energies greater than 1300 electron volts shows that "Vela" is soft, i.e. no "hard" X-ray photons are left; and the new supernova remnant with a diameter of two degrees, which the X-ray astronomers and g-ray astronomers of the Max Planck Institute have discovered, can clearly be seen in the lower left part. Photo by Max Planck Society/MPE.
Figure 3: The image taken in the light of the 1.156 million electron volts g-ray line shows a clear concentration of titanium-44 in the area, which includes the X-ray object "RX J0852.0-4622" discovered by ROSAT. The g-ray line image has been produced from data taken with the COMPTEL instrument which was built by an international collaboration under the leadership of the Max Planck Institute for Extraterrestrial Physics. Because titanium-44 is exclusively produced in supernovae explosions there was no doubt for the X-ray astronomers and the g-ray astronomers the new object is a previously unknown supernova remnant. Photo by Max Planck Society/MPE.