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Core-collapse Supernovae: shock physics, chemical enrichment, and progenitor masses

When a massive star has exhausted its supply of nuclear fuel, it can no longer support itself against gravitational collapse, and the process of collapse begins. The detailed physics of this process are still uncertain, but what is clear from observations is that a large amount of material is thrown off after the collapse, perhaps after a “bounce” of material hitting a core. This high-velocity (roughly 10,000 km/sec) ejecta runs into the dense material which surrounded the progenitor star. Massive stars are known to have dense, slow-moving winds, which are responsible for this material. As the ejecta plow into the circumstellar material, two shock fronts are thought to form: one which moves slightly faster and shocks the circumstellar material and one which moves slightly slower and shocks the ejecta. It is from this shocked ejecta that the X-ray emission originates, and from an age of a few months the X-ray emission can dominate the total radiative output of a supernova (8).

Few young supernovae (ages of days to years after explosion) have been observed in X-rays. In the 30 years of X-ray astronomy before the era of Chandra and XMM-Newton, only a dozen had ever been detected in X-rays. Since 1999, this number has more than tripled. I have been directly involved in observations of 21 of these with Chandra, XMM-Newton, and Swift (76,83,68,46,78,48,47,102,89,80,74,79,77). Although the number is still small, a few trends seem to be emerging with respect to supernova subtype (of which there are several based on optical lightcurves and spectra) and X-ray luminosity. However, given the paucity of detections, the most exciting results are from individual supernovae.

The earliest noteworthy result was my detection of numerous heavy element emission features in the X-ray spectrum of the type IIn supernova 1998S (83). This was the first detection in X-rays of Ne, Al, Si, S, Ar, and Fe features from a young supernova. Because these emission lines are almost certainly from the shocked ejecta, we were able to roughly determine the elemental abundance yields of the supernova explosion based on the strengths of these emission lines. This was interesting for two important reasons. First, it gave an estimate of the chemical enrichment due to a core collapse supernovae. Second, one of our co-investigators, Ken'ichi Nomoto, has performed detailed numerical calculations of the expected abundance yields of core collapse supernovae as a function of progenitor mass (72). Comparing the observed abundance ratios to his calculations, we were able to constrain the progenitor mass of 1998S. This was a first for X-ray astronomy.

The most exciting supernova observed so far has been 2006gy (102). This was the most luminous supernova ever observed3, with MV = -22, and it had a much slower evolution than any other supernova. Based on its optical spectrum, there was a strong possibility that it would be highly X-ray luminous as well, so I requested and was awarded a Director's Discretionary Time Chandra observation. The supernova was indeed detected in X-rays but at a much lower level than we anticipated. We therefore concluded that circumstellar interaction was not powering the optical light curve, and it had to be powered by the radioactive decay of 56Ni. The amount of Ni required was enormous ($\sim$22 Msun). All evidence pointed to the death of an extremely massive star, and we suggested that 2006gy could have been a pair-production instability supernova. If this proves correct, it will be the first one ever seen. This was big news, and it resulted in a NASA Science Update that was aired on NASA TV and CSPAN2, segments of which ran on numerous local news stations. The story made the front pages of several major news outlets (USA Today, Washington Post,, and and had a two-page feature in Time magazine. It was even mentioned on Late Show with David Letterman and National Public Radio's “Wait, Wait, Don't Tell Me.” Recently, it was named Time's third most important scientific discovery of 2007, after stem cells but ahead of a 400-year-old clam. (HA! Take that, mollusc!)

Currently, I am compiling all available X-ray data on detections and non-detections of two particularly rare and energetic classes of supernovae (the type Ib/c and the type IIn). We are at a stage when such compilations are needed, and both classes are very intriguing. The progenitors of type Ib/c supernovae are thought to be high mass stars (34,18), and this subtype is also associated with gamma-ray bursts (106). Their X-ray properties show differences of many orders of magnitude at timescales of both days and years. The type IIn supernovae are the most X-ray luminous and are poorly understood theoretically. The high luminosity is thought to result from the interaction of the ejecta with a clumpy, high-density circumstellar medium (10).

I recently finished the Ib/c investigation (74), and it was extremely fruitful. I discovered six new X-ray emitting Ib/c supernovae, which is 1/3 of the current total, by searching through 7.3 Msec of archival data (nearly five hundred separate observations). The IIn investigation is well underway. After that, an investigation of all other type II supernovae is planned, and finally a “fishing expedition” for type Ia supernovae will be attempted.

The prospects for advancement in this field are quite good. Two of the main hindrances to date have been the temporal lag between discovery of the supernova and the first X-ray observations and the scarcity of X-ray observations, which result in poorly sampled X-ray light curves. However, the Swift satellite has made supernova observations a priority during the times when they are not following up gamma-ray bursts, and I am part of an informal advisory group to discuss possible supernova targets. Given the current discovery rate, we expect there to be a good target every year or two. The next time a nearby (less than 20 Mpc) X-ray bright core-collapse supernova does occur, we will obtain excellent, well-sampled X-ray coverage. It promises to be an exciting event.