Supernova observed just after explosion

The observation was early enough to determine for the first time what happens in the early stages of a supernova.

Supernovae are explosions that happen at the end of a massive star’s life. Even though these stars are already 10,000 times brighter than the Sun during their lifetime and even brighter in death, witnessing the early stages of these explosions is very rare. If by chance one is found a few days later, the supernova ejecta has already destroyed any information about it's immediate environment. But Ofer Yaron and colleagues had the chance to witness a supernova much earlier, providing new insights into just what happens when a supernova is in its infancy. The observations are presented in a new Nature Physics study.

The dying star was a red supergiant that exploded into a Type II supernova, the most common supernova type. In the days leading up to the explosion, the star lost mass, rapidly ejecting material and creating a dense shell of material around it.

We spoke with Yaron about the unique observation.

ResearchGate: How did you observe this?

Ofer Yaron: To set the stage, it is important to explain the advances in technology that enabled us to perform “day one” supernova observations. Until several years ago, catching a supernova even a week after explosion was regarded as early. This is not the case anymore, and with new wide-field automated and fully robotic sky surveys, we have begun catching events a day or less after explosion. Palomar Transient Factory (PTF) at the Palomar observatory can scan about a tenth of the visible sky on a typical night.

With the telescope automatically scanning the sky during night time in Palomar, we exploit the time zone differences, and collaborators of the PTF survey in Europe perform regular human monitoring of the transient candidates that are streaming in, live from the telescope. If an interesting transient is found, especially if it was not detected in the same field the day before, the human scanner can immediately send alerts to the other collaborators to trigger follow-up observations.

That is what happened with the supernova reported here – iPTF13dqy.

iPTF13dqy (SN2013fs) exploded in a relatively nearby (~160 million light years) spiral galaxy on Oct 6 2013, and was detected by the iPTF (Palomar Transient Factory) sky survey a mere 3 hours after explosion. Credit: Ofer Yaron.

RG: Can you give us a brief insight into what you observed?

Yaron: With the last non-detection occurring less than 24 hours before the detection, we immediately called Dan Perley, who was coincidently performing spectroscopic observations with the Keck telescope in Hawaii, and asked him to take a spectrum of this likely young transient. He took a sequence of four spectra, not just one. Analysis we performed soon afterwards by following the rise of the supernova light curve revealed that these spectra were between six and ten hours from the actual explosion!

We also obtained very early multiband photometry, including UV and X-ray with the Swift satellite, all from day one after the explosion. It was all of this that enabled us to perform the thorough analysis and mapping of the distribution of the circum-stellar matter that was ejected by the star before going supernova.

RG: What new insights did you garner from this?

Yaron: The early spectra revealed that the progenitor star is engulfed with a spherical shell of circum-stellar material (CSM), a disk of matter surrounding the star. By analyzing the very early spectra, we were able to obtain estimates on the radial distance of this emitting shell, the density of the matter there, and the mass-loss rate with which it was expelled from the star before the final supernova explosion.

Direct observations of the very early and limited time window before the region is swept up by supernova ejecta can significantly improve our understanding of the late stages of stellar evolution of massive stars—composition, density profiles, and mass-loss history—particularly just before the supernova explosion.

Last but not least, the fact that 13dqy was found to be a fairly normal type II with circum-stellar material means that this phenomenon of elevated—maybe even eruptive—mass-loss just before the final supernova explosion is common among core-collapse supernovae.

RG: Are these events rare in of themselves or just rare to witness? Why is that?

Yaron: These Type II core-collapse supernovae are not rare. The tricky part is to detect them early and perform significant prompt follow-up observations. So the event here is not unique; it’s the observational dataset we managed to obtain and the conclusions we could derive from these that are.

Featured image of supernova remnant courtesy of NASA.