Co-added Swift X-ray image of Type Ia SN
Credit: NASA/Swift/Stefan Immler

Double Degenerate?

One of the most pressing problems for today's astrophysicists: how are standard candles made? The candles, in this particular case, refer to those consistent explosions, Type Ia supernovae, that are so useful for probing the structure of the Universe. Type Ia supernova are characterized by a similar chemical composition, and the similar behavior of their "light curves", i.e. how the supernova's light varies near the time of the explosion and for weeks afterwards. These Type Ia supernova are believed to involve the destruction of a white dwarf star. White dwarfs are the remnants of the cores of intermediate-to-low mass stars (like the Sun), in which about one Sun's worth of mass is packed into an object the size of the earth. As a result, white dwarfs are incredibly dense - one teaspoon of a white dwarf's material would contain as much mass as an automobile. White dwarfs exist because of a struggle between gravity and quantum mechanics. The crush of the white dwarf's enormous gravity is opposed by a quantum mechanical force called electron degeneracy pressure. But there's an ultimate limit to how much mass electron degeneracy pressure can support. This maximum mass is called the Chandrasekhar limit, and for typical white dwarfs is about 1.4 times the mass of the Sun. Type Ia supernovae occur when a white dwarf exceeds the Chandrasekhar limit, at which time the star can explode. But exactly how this explosion occurs is, currently, a puzzle. One suggestion is that a single white dwarf can accrete matter from a nearby normal star, eventually pushing it above the Chandrasekhar limit. An alternate scenario is that the supernova could be produced by the merger of two degenerate white dwarfs in orbit around each other. To help investigate the nature of the Type Ia progenitors, astronomers have merged 53 X-ray observations of Type Ia supernova seen by NASA's Swift observatory, to look for tell-tail signs of the explosion. If the accretion scenario is correct, and the companion is a supergiant or red giant star, there should be some excess X-ray and ultraviolet emission left behind after the supernova explosion; if the merger scenario is right, there should be little excess X-ray or UV emission at the position of the supernova. The image above shows the combined Swift X-ray images of Type Ia supernovae, all centered on each individual supernova. This provides the equivalent sensitivity of a 35-day observation. The inset shows the region near the image center around the supernovae. Although there are random bright sources near the center of the image, there's no detectable X-ray excess at the position of these supernovae (marked by the white circle). These combined X-ray data, along with associated ultraviolet observations by Swift, suggest that either the "double-degenerate" merger model predominates, or that the companions to the white dwarfs are as small as or smaller than the Sun.
Published: March 26, 2012

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Page Author: Dr. Michael F. Corcoran
Last modified Sunday, 01-Apr-2012 15:14:40 EDT