phi (totient) wrote,
phi
totient

pulsar pulsar burning bright



For much of the 25 years since the southern summer night when Oscar Duhalde looked up at the night sky with his bare eyes and discovered supernova 1987a, people have been looking for a pulsar in the remnant. There was briefly evidence of one, and a flurry of excited papers suggesting that, as expected for a very young pulsar, it was spinning faster than any pulsar previously observed. Then, forlornly, the literature shifts to confirmation failures, and then to surveys concluding that the initial observations were incorrect.

That's not to say that SN1987a didn't leave something interesting behind, though. Several (older, obviously) supernova remnants have left behind strangely shaped nebulae looking something like this:



These are thought to be the result of a stellar merger, and there are some nice computer simulations that display these kinds of strange shapes. And sure enough, SN1987a's remnant began to resemble something that might be a much younger version:



Hallelujah, the cause of the supernova is identified. Or is it? Let's take a look at some other funny looking nebulae:



OK, η Carinae gives the basic idea of an ejecta shell that has been "pinched" into an hourglass shape. And the Red Rectangle shows some similar shapes. But how are these things being illuminated? Why is the Red Rectangle quite so rectangular, with those nice straight lines in it? It looks to me like sharp cones of illumination within the nebula.

And consider how a pulsar works. A spinning neutron star with a very strong magnetic field emits energy only (or almost only) from the magnetic poles. These emissions form a cone, and anything they illuminate forms a conic section. So sharp lines of illumination are certainly easier for me to believe than sharp lines of density.

And SN1987A's lines sure are sharp. But if we're not on the cone, we wouldn't see the pulsar. Even if the illumination were fluorescent (rather than phosphorescent) and the nebula flickered, we wouldn't be able to make it out because we're getting signal from the whole ring, and thus always seeing, in any given microsecond, the piece of the nebula being illuminated. Except...

These nebula are pretty far across. SN1987A's nebula has a radius of about a light-year. The near side will appear to us to be illuminated pretty soon after the pulsar "turns on". The far side won't appear illuminated until two years later. The two outer rings of what has come to be called the "triple ring nebula" aren't quite mirror images, suggesting that the magnetic axis is a little off center, and also suggesting that the part of the nebula that appears to us to pass through the very center of the inner ring would be the last to appear illuminated to us.

Until then, the pulsar would be visible in the flickering reflections of a lightyear-diameter ejecta shell. After that, it would not. Unless someone were to look at the thing with something that can simultaneously resolve to submillisecond resolution in time and subarcsecond resolution in space.


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