For the first time, astronomers have witnessed a double stellar explosion at exactly the same position on the sky suggesting the death of a massive star.

SN 2006jc was discovered in UGC 4904 on 2006 October 9.75 UT. The early spectrum was that of a hydrogen-poor event with strong, narrow HeI emission lines superimposed on a broad-line spectrum of a type Ic supernova. In 2004, an optical transient was reported which, when retrospectively compared with the SN 2006jc discovery images, appeared to be spatially coincident with the new supernova. The 2004 transient was much fainter than SN 2006jc and it remained visible for only a few days after discovery. Given the new, bright supernova discovery, the nature of this transient, named UGC 4904-V1, has become intriguing.

A global collaboration of astronomers analised the images containing the two transients using differential astrometry of 21 nearby stars, and found that UGC 4904-V1 and SN 2006jc are indeed coincident to within the uncertainties. So they decided to follow-up and study SN2006jc with a wide range of large telescopes, among them, the William Herschel Telescope.

Sequence of images of UGC 4904 rebinned to a pixel scale of 0.53 arcsec. Left: detection of UGC 4904-V1 on 2004 October 16. Middle: another image obtained on 2006 September 21 showing no transient detection. Right: R-band frame taken on 2006 October 29. The two transients are coincident to within 0.1 arcsec, and the total error budget is 0.3 arcsec. Extracted from Figure 1 of Pastorello, A., et al., 2007, Nature, 447, 829 [ JPEG ].

The most likely explanation for the 2004 explosion was probably a giant outburst of a very massive star like Eta-Carinae, and only the most massive stars can produce this type of outburst. So the 2006 supernova must have been the death of the same star, possibly a star 50 to 100 times more massive than the Sun. And it turns out that SN2006jc is a very weird supernova – unusually rich in the chemical element helium which supports the idea of a massive star outburst then death.

References:

• Pastorello, A., et al., 2007, "A giant outburst two years before the core-collapse of a massive star", Nature, 447, 829.
• "Double explosion heralds the death of a very massive star", STFC press release, 14th June, 2007.
• "Un lampo, poi la fine. Un bagliore nel cielo annuncia con due anni di anticipo l’esplosione di una Supernova", INAF press release, 14th June 2007.
• "Chronique d'une mort annoncée: une explosion d'étoile précédée d'un flash lumineux", IAP press release, 14th June 2007.
• "Une rare explosion d'étoile précédée d’un flash lumineux", CEA press release, 2007.
• "Coup de semonce pour un cataclysme", CEA/DSM/DAPNIA press release, 14th June 2007.
• K. Cooper, 2007, "The supernova that exploded twice", Astronomy Now, 21, 15.