On August 23 of this year, a supernova exploded in galaxy M101.
M101 is a grand design spiral a mere 22 million light years or thereabouts away (here’s a summary of literature distance measurements from NED). Cosmologically speaking, that’s in our back yard. The closest galaxies to our own are our satellite galaxies, including the Magellanic Clouds. However, the closest big galaxy to our own is the Andromeda Galaxy, which is about 2 million light-years away. The closest actual cluster of galaxies is the Virgo Cluster, at a distance of 65 million light-years.
Why This Is Cool
SN 2001fe is a thermonuclear supernova. These are the types of supernovae that were used to discover the acceleration of the Universe, so they’re cosmologically important. As such, some have pointed out the importance of this supernova as a calibrator for that observation. However, that’s of secondary importance. Yes, you really want to understand the standard candle you’re using cosmologically, but the repeatability of the peak luminosity of these supernovae has already been empirically demonstrated, and that’s what the cosmological result really resets on. What’s more important for this supernova, however, is understanding the supernovae themselves. Supernovae are where we get the vast majority of the atoms heavier than Helium in our Universe; thermonuclear supernovae in particular are primarily responsible for the heavier elements, such as iron. Thus, these are important events not only for measuring the expansion history of the Universe, but for understanding other aspects of our history.
Beyond it being close, it is very awesome how early this supernova was discovered. It was discovered less than a day after the explosion. (Aside: well, really, 22 million years later. But I will be talking about times relative to when the light reach earth. So, when I say “the date of the explosion was August 23”, what I really mean is that the first photons from the star at the moment it was exploding reached Earth on August 23.) The discovery was announced, and a wide variety of follow-up observations started within just a day or two of the explosion. Not only is this the closest thermonuclear supernova to go off during the era of ubiquitous digital imaging on telescopes, but we have also been observing it from the very beginning.
Below is a lightcurve of SN2011fe, that is, a plot of its brightness versus time:
This lightcurve comes from the American Association of Variable Star Observers. It represents the work of amateur astronomers around the world. They each make observations, and submit them to the AAVSO. Historically, many of the AAVSO observations were made using the Mark 1 Eyeball. People would look at a star, and compare its brightness to other stars of known magnitude in the same field in order to figure out the target star’s magnitude. Nowadays, with many amateur astronomers having digital imaging equipment, they can use that (with other stars in the field for calibration) to measure star magnitudes. The data points on the lightcurve above show the blue (B) and yellow-green (V) magnitudes of the supernova. We can see that people started submitting magnitudes within two days of the explosion of the supernova. It peaked around September 11 or 12. Today, it’s still brighter than 11th magnitude, so if you’ve got clear skies and something like an 8-inch telescope, and if you’re in the Northern hemisphere, head into your backyard after sunset and see if you can see the supernova in M101! (You’ll need to use a finding chart to be able to tell what is the supernova versus what is a foreground star. Here is one from the Society for Popular Astronomy.
What science has come from this supernova? We’re still in early days; the supernova is only a couple of weeks past maximum light, so many observations remain to be made, never mind the processing of the data. However, already a couple of papers have shown up about this supernova.
One of the outstanding questions about thermonuclear supernovae is where they come from. Most people agree that they come from a white dwarf star— a dead (no longer performing fusion) star made out of Carbon and Oxygen that is supported by “electron degeneracy pressure”, which is typically half the mass of the Sun but only the size of the Earth. If one of these stars reaches a critical mass of 1.4 Solar Masses, it starts to collapse, triggering runaway fusion that completely blows it away in a thermonuclear supernova. The question, then, is how to get the white dwarf star up to this critical mass.
Most scenarios are divided into two broad classes. The “Single Degenerate” scenario is where the white dwarf has a companion star, such as a normal “main-sequence” star (like the Sun), or a red giant star. If you’d asked me ten years ago, I would have told you that the red giant companion was most likely. The “Double Degenerate” scenario starts with two smaller white dwarf stars, which merge, yielding a merger that is above the critical mass and that therefore explodes. Until the last few years, I believe that most astronomers preferred the single degenerate scenario. However, some observations in recent years have started to indicate that the double degenerate scenario may be more common.
Two papers have shown up on arXiv.org that address this. The first, Horesh et al., arXiv:1109:2912, reports on radio and X-ray observations of the supernova starting just a day after the explosion. The result: they didn’t see anything. While that may sound uninteresting, in fact it is significant. A supernova has an expanding “photosphere”, that is, the bright thick ball that is glowing. As time goes by, the photosphere expands at a rate slower than the gasses that make it up, because the outer layers become thin enough to see through. Outside the expanding photosphere is the blast wave, which propagates through the interstellar gas. The shock wave does two things. First, it compresses magnetic fields; electrons caught in that magnetic field will spiral around it, emitting “synchrotron radiation” in radio wavelengths. Second, it heats up the post-shock gas, which should glow in X-rays. If the supernova happened in a single-degenerate system, and if the non-degenerate companion was undergoing substantial mass loss, then the interstellar material should be thicker, which would lead to both enhanced radio and X-ray emission.
Because they didn’t see any, it means that if SN2011fe came from a single-degenerate system, the companion can not have been a red giant, for no plausible model of a white dwarf/red giant binary system would have a mass loss rate low enough that the X-rays and radio waves would be undetectable. While this might suggest that SN2011fe came from a double-degenerate system, in fact the observations are still consistent with a single-degenerate system where the companion is a less evolved star: a main sequence star or a subgiant.
The other paper, Li et al., arXiv:1109.1593v1, used the adaptive optics system with an infrared camera on the Keck 10m telescope to obtain an extremely precise position for the supernova. They then went into the archives of the Hubble Space Telescope, and pulled out all pre-explosion images of M101 that were available. They used the very precise position to look to see if they could identify a progenitor star. The result:
Nothing! Or, rather, nothing to the detection limits of the HST. Note that the circle on the image to the right is an 8σ error circle. That is, it’s an extremely conservative error circle; “Star 1” and “Star 2” at the edge of the circle are too far away to have any reasonable chance of being the progenitor system of SN2011fe. The fact that the HST couldn’t see anything also rules out a single-degenerate system consisting of a red giant star, or indeed of a single-degenerate system with a companion star more than about 4 times the mass of the Sun. Again, this might seem to favor a double-degenerate progenitor system, but it’s still possible that a dimmer main-sequence star no more massive than (say) 3 or 4 times the mass of the Sun was present as the mass donor for a white dwarf.
This supernova is still hot. As I mentioned above, with a modest telescope, you may still be able to see it for another week or so. I predict that a lot of supernova science comes out of this event, and that we see a plethora of papers on it. It will probably become a “classic” event much in the same way that SN1987A has.