Blue Collar Scientist

This big hype up to yesterdays huge NASA announcement doesn’t appear to be justified to me. The announcement doesn’t strike me as that big a deal.

Basically, the big NASA story that we’ve been waiting breathlessly for is that astronomers have found a supernova remnant created by a star that went supernova during the US’ Civil War. This is significant because of two facts:

1. Prior to this, the most recent supernova we’ve known of in the Milky Way is over 400 years old.
2. The statistics of stellar populations tells us that there should be seeing about three supernova in the Milky Way each century - meaning we have roughly a 12 supernova deficit (well, 11 after yesterday’s announcement) since that 400 year old remnant.

The thing is, we understood already why we don’t see these things go off - even though they should all be as bright as the brightest planets at the distances within our own galaxy. Basically, the hypothesis is that lots of these supernova aren’t visible due to extinction - obscuration by the huge loads of galactic dust that get in the way of our seeing them.

So we’ve understood that supernova must be going off fairly frequently for a long time now. But we’ve also understood why we can’t easily see them. So now they find a remnant that helps confirm that (a) they are going off like we thought, and (b) we can’t easily see them for precisely the reasons that we thought.

Yeah, it’s a cool discovery. I’m fine with that. But it is cool because it shows the predictive power of the scientific method. We saw a discrepancy in the data. We made some discoveries that might explain the discrepancy. Then, at length, we confirmed that yep, that data does indeed explain the discrepancies.

But as far as the object itself, I’m completely unable to get worked up about it. Maybe it’s because it isn’t in the solar system. Maybe I’m just still a little blown up from my trip.

Whatever. If you want to learn about it, you apparently will get no opportunity from my apathy-riddled brain; instead, the Bad Astronomer has excellent coverage.

For years, astronomers have tried to catch a supernova in the earliest stages of its explosion. They’ve gone to the extremes of building highly automated telescopes to search hundreds of galaxies each night for new supernova, often revisiting the same galaxy several times per night just so that if one did happen to be going off, they could catch it several times early in the process. Researchers in the highly competitive field have even collaborated with each other to insure they are getting most efficient coverage possible.

Catching a supernova early on in its detonation is of interest to astronomers for two reasons. The first reason is that certain supernova are useful standard candles for calibrating intergalactic distances. The better the measurement of supernova’s brightness over time, the better the distance measurement will be. So there’s an incentive to observe a supernova regularly over as much of its lifetime as possible. The other reason is that the very earliest stages of the explosion should tell us interesting things about how supernovae explode. We’ve got the broad strokes of supernova formation pretty well figured out, but watching a supernova in the early stages will allow us to figure out the details.

Astronomers studying supernovae have had some success with their strategies. They’ve caught quite a few supernovae on their rise to brightness, but they’ve never really achieved their holy grail - observing a supernova from the earliest stages of its detonation.

Back on January 9, that all changed. On that day, I got an e-mail from the ATEL service reporting that the Swift satellite had discovered a bright X-ray transient while it was observing a two-week old supernova in the galaxy NGC 2770:

Follow-up observations of SN2007uy with the Swift/XRT reveal a new transient source about 95 arcsec away from the SN position at RA=09:09:30.7, Dec=+33:08:19 (J2000). This position coincides with the outskirts of the host galaxy of SN2007uy (NGC 2770).

This sort of thing is fairly routine. I get ATEL messages, and alerts from other services, primarily because some of the astronomers who run my software use those services to report and find targets of opportunity. It is nice to know what my clients and colleagues are up to - makes me feel like I’m still in the loop, even though I’m not doing the day-to-day thing at an observatory anymore.

After the initial report, several other astronomers chimed in, some with negative observations, and the early thinking was that the object was a flare from a soft gamma-ray repeater, or a weak X-ray flash - two classes of phenomena that are pretty interesting in their own right, but nothing that would keep me up nights.

But within a few hours, an optical counterpart had been discovered by J. Deng and Y. Zhu at the National Astronomical Observatory of China:

We took a 60s unfiltered CCD image and a 600s R-band image of NGC 2770 on 2008 Jan 10 18:19 UT and 18:22 UT with the BFOSC [Beijing Faint Object Spectrograph and Camera] instrument of the NAOC 2.16m telescope at the Xinglong Observatory, China. An optical source was clearly seen in both images which was conincident in position with the X-ray transient reported in GCN 7159. It was roughly 2 mag fainter than SN 2007uy in the same galaxy….. We suspect that this source is the SN counterpart of the X-ray transient (a XRF). Spectroscopic observations are in progress despite the cloudy weather condition.

Just fifteen hours later, Stefano Valenti reported:

Following the report of J. Deng and Y. Zhu (GCN7160) that the transient was roughly 2 magnitudes fainter than SN2007uy on Jan. 10.76, in R band, it seems that the luminosity of the transient is rising rapidly, supporting the very fortunate possibility that Swift has observed, in real time, the explosion of a massive star.

This was exciting - electrifying - because in the past, nothing like this had ever been done. For forty years, astronomers have theorized that when a supernova goes off, you should first get a burst of neutrinos, then some gravitational waves, and then some X-rays, all within seconds or minutes of each other. A couple days after that, you’ll start to get the visible light, so the best that all the astronomers using optical telescopes to hunt for supernova could possibly accomplish was to discover something that had exploded a day or two before.

It looked pretty good for this being the first discovery of a supernova at the very moment of the explosion. One of the things that astronomers were looking for was something called “shock breakout radiation” - a big burst of x-rays and ultraviolet light that results from the explosion’s shock wave emerging through the stellar surface. Shock breakout radiation had not, to the best of my knowledge, ever been observed before. Then several astronomers reported on ATEL that:

the optical transient was clearly detected in the U, V, and B bands of the UVOT. The source was marginally seen in the UVW1 image. The very rough B and V magnitude is 18.8 and 18.4, respectively. The Swift observations indicate that the optical transient becomes brighter in the past 24 hours. We note that the optical transient was not seen in the UVOT observations taken on 2008 Jan 9.

The “U” band referred to is ultraviolet light, so this was a good sign - they could not see the supernova at nearly the moment of detonation, but they could see it, and measured the brightness of the UV radiation from it, two days later. That’s almost exactly what theory predicted they should be able to do!

Over the next few days, a flood of messages about this newly-discovered object came down the pike, and after a while, the excitement died down. Almost a month ago, the observers submitted a paper to Nature, and the preprint appeared on the arXiv server. Everyone has been very responsible about not discussing it due to the embargo while the paper undergoes peer review, but now Science News has published a story, and given my small readership, I don’t see the point of being quiet about it any more.

The data suggests that this was a Type Ibc supernova discovered at the moment of detonation, that the progenitor was a Wolf-Rayet star, that shock breakout radiation was observed, that radio, optical, and long-lasting x-ray counterparts were observed, that fast ejecta was observed, and that all the observed phenomena fit very nicely into a model of how supernova explode.

Furthermore, the observations are inconsistent with one hypothesis about how supernova generate their radiation. The “relativistic outflow” model proposes that a lot of supernova emission is the result of particles that are accelerated to a substantial fraction of the speed of light, something that did not happen in this case.

And finally, it looks as though a wide-angle X-ray instrument similar to the one the Swift astronomers were using would allow scientists to detect several hundred core-collapse (Type Ib, Ic, and Type II) supernovae each year at the instant of explosion. This would not only allow the model to be tested with a lot of different supernovae, but scientists using neutrino and gravitational wave detectors might benefit from having some constraints on the position and time of the events these sensors are designed to detect.

The bottom line is, these results are very much what science is all about. Some of the hypotheses about supernovae made predictions that didn’t stand up to the observations of SN2008D very well at all, but others fit the observations just about perfectly. It opened up new lines of inquiry and research, and suggested some things that astronomers could do to do even better science in the future. And last but not least, it showed where we still have some things to learn about supernova that aren’t fully explained yet.

It is a good time to be an astronomer!