Saturday, August 22, 2009

Weird things in the night

The weird transient event SCP 06F6
Image Credit: NASA / ESA / K. Barbary

In spite of everything we know about the Universe, there are still some things out there that we cannot yet even provide a basic explanation for. At the supernova conference I attended this last week, we heard a talk about a new type of object that is not yet explained.

In 2006, the Hubble Telescope was watching a patch of sky for supernovae in very distant galaxies. Hubble finds supernovae by watching for "new" sources of light that get brighter and then fade away; different types of supernovae brighten and fade in specific ways, and so the most interesting ones could be flagged for additional observations.

One such event with the boring name of SCP 06F6 (pictured above) was very curious. It got brighter over a period of 100 days, and then faded away over a similar length of time. That time scale is very long for supernovae. Also, SCP 06F6 did not take place in a detectable galaxy, so if it is not in our own Milky Way galaxy, it took place either on the far edge of the Universe, or it took place in a very wimpy galaxy. But we don't know.

Astronomers took a spectrum of SCP 06F6. Spectra, or the splitting of light into component colors, are a very powerful tool for studying astronomical objects. We can learn the composition of stars and the distances of other galaxies by studying spectra. Different types of supernovae all have different types of spectra. But the spectrum of SCP 06F6 is like no other supernova spectrum. It shows what looks like carbon molecules in a galaxy at a distance of 1.8 billion light years (z=0.14, for those of you who speak redshift).

1.8 billion light years is a big distance, but it's not that big for today's large telescopes and the Hubble Telescope. We should be able to see most small galaxies at that distance, and, again, we don't see a galaxy here. Also, supernovae are very powerful explosions, and carbon molecules are easily broken apart in such extreme conditions. This explanation doesn't add up. It could also be possible to explain the lines as calcium at a distance of 6 billion light years (z=0.5), but even that isn't all that far for supernovae.

Since the discovery of SCP 06F6, supernova searches have started to find many similar events, with the same long rise times, the same weird spectra, and the same lack of a host galaxy. Odd. Very odd.

So, what else could these events be? Astronomer Andy Howell showed the spectrum of a couple of these things, and my first thought was that they look like a rare type of white dwarf called "peculiar DQ" white dwarfs. These white dwarfs show what looks like carbon in their spectra, but at slightly the wrong wavelengths. This could be due to magnetic fields on the white dwarfs, or maybe due to hydrogen adding itself to the carbon molecules (making hydrocarbon white dwarfs!). White dwarfs would be in our own galaxy, so you wouldn't expect to see a host galaxy in Hubble pictures (we're in it!). These peculiar white dwarfs are also very faint, so if they are thousands of light-years away in our galaxy, we would not see the star before or after the event.

But the white dwarf explanation doesn't explain how these things get brighter and fainter. Gravitational lensing is one possibility. If another star passes between us and the distant white dwarf, the gravity of the interloping star can focus the light and make a star get much brighter than it was, even over a timescale of weeks and months. This would seem like a natural explanation to me, with one major problem. Gravitational lensing does not change the color of a star, only its brightness. But there is pretty good evidence that SCP 06F6 and its kin are changing color. If true, that would seem to rule out a gravitational lens. Maybe. (I can think of ad-hoc ways to save it, but then why should these ad-hoc ways happen more than once in different parts of the sky?).

There are many other ideas astronomers have proposed, from Texas-sized asteroids running into white dwarfs in our galaxy to carbon-rich stars halfway across the universe being shredded by distant black holes to exotic types of supernovae. We know so little about these events, it is hard to rule any weird things out yet. As we discover more of these, though, hopefully we can build up enough information to explain these rare and mysterious events.

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