Supernova SN 2012ap is a ‘missing link’ between stellar explosions that generate gamma-ray bursts and those that don’t, says a group of astronomers led by Dr Den Milisavljevic of the Harvard-Smithsonian Center for Astrophysics.
Images of SN 2012ap and its host galaxy, NGC 1729. Image credit: D. Milisavljevic et al.
SN 2012ap is a so-called core-collapse supernova. It was first detected by the Lick Observatory Supernova Search with the 0.76-m Katzman Automatic Imaging Telescope in the galaxy NGC 1729 on February 10, 2012.
The object is located some 23,150 light-years in projection from the NGC 1729’s center along the outer periphery of a spiral arm.
Core-collapse supernovae occur when the nuclear fusion reactions at the core of a very massive star no longer can provide the energy needed to hold up the core against the weight of the outer parts of the star. The core then collapses catastrophically into a superdense neutron star or a black hole. The rest of the star’s material is blasted into space in a supernova explosion.
The most common type of such a supernova blasts the star’s material outward in a nearly-spherical bubble that expands rapidly, but at speeds far less than that of light. These explosions produce no burst of gamma rays.
In a small percentage of cases, the infalling material is drawn into a short-lived swirling disk surrounding the new neutron star or black hole. This accretion disk generates jets of material that move outward from the disk’s poles at speeds approaching that of light.
This combination of a swirling disk and its jets is called an ‘engine,’ and this type of explosion produces gamma-ray bursts.
In an ordinary core-collapse supernova with no ‘central engine,’ ejected material expands outward nearly spherically, left. At right, a strong central engine propels jets of material at nearly the speed of light and generates a gamma-ray burst. The lower panel shows an intermediate supernova like SN 2012ap, with a weak central engine, weak jets, and no gamma-ray burst. Image credit: Bill Saxton / NRAO / AUI / NSF.
A new study by Dr Milisavljevic and his colleagues shows, however, that not all ‘engine-driven’ supernova explosions produce gamma-ray bursts.
The scientists found that SN 2012ap has many characteristics expected of one that generates a powerful burst of gamma rays, yet no such burst occurred.
“This is a striking result that provides a key insight about the mechanism underlying these explosions. This object fills in a gap between gamma-ray bursts and other supernovae of this type, showing us that a wide range of activity is possible in such blasts,” said team member Dr Sayan Chakraborti, also of the Harvard-Smithsonian Center for Astrophysics.
The results have been submitted to the Astrophysical Journal (arXiv.org preprint).
_____
D. Milisavljevic et al. 2015. The Broad-Lined Type Ic SN 2012ap and the Nature of Relativistic Supernovae Lacking a Gamma-ray Burst Detection. ApJ, submitted for publication; arXiv: 1408.1606
