By David Green

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Filed under : Scitizen >> Exoplanets >> News >> The Youngest Galactic Supernova Remnant

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The Youngest Galactic Supernova Remnant

26 May, 2008 09:18 am

The remains of the most recent supernova in our Milky Way galaxy, about 150 years ago at most, have been identified from radio and X-ray observations.

Supernovae, exploding stars, produce heavy elements both before and during their explosions, which they eject into the surroundings to become part of future generations of stars. Supernovae stir up and heat interstellar gas, and they generate `cosmic rays', ultra-high-energy subatomic particles that fill our Galaxy and that are constantly raining down on the Earth. In some cases, the explosions of more massive stars, may also leave behind ultradense objects like neutron stars or pulsar. The energy and material released in a supernova explosion expands into the surrounding tenuous gas and dust between the stars, and form an extended `supernova remnant'.

G1.9+0.3 was first identified as a supernova remnant in 1984 by Dave Green (University of Cambridge) and colleagues, and studied in more detail 1985 with NRAO's Very Large Array radio telescope. Given its small angular size, it was thought to be young, i.e. probably less than about a thousand years old. However, in 2007, X-ray observations made with NASA's Chandra X-ray Observatory by Stephen Reynolds (North Carolina State University) and colleagues revealed emission that was larger, by about 16%, than an earlier radio image observed in 1985, which imply is it must be very young. Since some of the differences between the images *may* be due to the fact that the comparison was between images made at very different frequencies, further radio observations of G1.9+0.3 were made in March this year with the VLA. Comparison of images made from these new observations and those from 1985 clearly confirm that G1.9+0.3 has expanded considerably in the intervening 23 years. Extrapolating backwards implies an age of about 150 years for the remnant, assuming no deceleration. In practice it is expected that the expanding debris from the supernova explosion will sweep up surrounding gas and dust, so will be decelerated to some extent, and hence this the true age will be somewhat less than 150 years.

We do know of other young supernova remnants in our Galaxy, less than about two thousand years old. Several supernovae explosions were seen in the last two millenia as new stars If we look now at the positions of these recorded new stars we see the supernova remnants that these explosions have produced, including the well known Crab Nebula, which was produced by a supernova seen in AD 1054. In addition, there is a well called Cas A, which although it was *probably* not seen historically, we know is about 340 years old from optical expansion studies. But G1.9+0.3 is less than have the age of Cas A, and fits nicely into the large gap in the known ages of supernova remnants in our galaxy.

Although the distance to G1.9+0.3 is not well known, from its X-ray spectrum it is thought to be near the centre of our Galaxy. This means that its expansion speed is very large, about 15,000 km/s. Its position near the Galactic centre also means that the supernova that produced it would not be visible optically, due to the relatively large distance to it, and the fact that will also be a large amount of gas and dust obscuration towards it. But, we can now observe the remnant at radio and energetic X-ray wavelengths, as these can penetrate gas and dust.

G1.9+0.3 is a very unusual and interesting supernova remnant, it is the youngest known supernova remnant in the Galaxy, and it has the highest expansion speed, and continued studies of it will provide further information on supernovae, their remnants, and the physics of fast shock waves.

CAPTION: Separate images showing the 1985 (above) and 2008 (below) radio images.

Reference:

"The Youngest Galactic Supernova Remnant: G1.9+0.3", Reynolds S. P., Borkowski K. J., Green D. A., Hwang, U., Harrus I., Petre R., 2008, ApJL, in press. Available at: http://uk.arxiv.org/abs/0803.1487.

"The radio expansion and brightening of the very young supernova remnant G1.9+0.3", Green D. A., Reynolds S. P., Borkowski K. J., Hwang, U., Harrus I., Petre R., 2008, MNRAS (Letters), 387, L54. Available at: http://uk.arxiv.org/abs/0804.2317.

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