Astronomers have detected the first X-rays from a Type Ia supernova, located inside the spiral-shaped galaxy ESO 336-G009.
An image showing X-rays detected from the Type Ia supernova SN 2012ca (inside the circle). According to astronomers, SN 2012ca occurred in the spiral-shaped galaxy ESO 336-G009, located about 260 million light-years away. Image credit: Vikram Dwarkadas / Chandra X-ray Observatory.
Type Ia supernovae occur in binary star systems. One of the stars in the system must be a white dwarf star, the dense, carbon remains of a star that was about the size of our Sun. The other can be a giant star or even a smaller white dwarf.
These supernovae always have the same intrinsic brightness, so by measuring how bright they appear astronomers can determine how far away they are. Known as standard candles, they have been used for decades to measure distances across the Universe, and were also used to discover its accelerated expansion and infer the existence of dark energy.
A few years ago, astronomers began to find Type Ia supernovae with a strange optical signature that suggested they carried a very dense cloak of circumstellar material surrounding them.
Such dense material is normally only seen from Type II supernovae and is created when massive stars start to lose mass. The ejected mass collects around the star; then, when the star collapses, the explosion sends a shockwave hurtling at supersonic speeds into this dense material, producing a shower of X-rays.
Thus scientists regularly see X-rays from Type II supernovae, but they have never been seen from Type Ia supernovae.
The spiral-shaped galaxy ESO 336-G009 (center). Image credit: Digital Sky Survey.
When Dr. Vikram Dwarkadas of the University of Chicago and co-authors studied the Type Ia supernova SN 2012ca, recorded by Chandra X-ray Observatory, however, they detected X-ray photons.
“Although other Type Ia’s with circumstellar material were thought to have similarly high densities based on their optical spectra, we have never before detected them with X-rays,” said Dr. Dwarkadas, co-lead author of the study published in the Monthly Notices of the Royal Astronomical Society.
The amounts of X-rays the astronomers found were small — they counted 33 photons in the first observation a year and a half after the supernova exploded, and 10 in another about 200 days later — but present.
“SN 2012ca certainly appears to be a Ia supernova with substantial circumstellar material, and it looks as though it’s very dense. What we saw suggests a density about a million times higher what we thought was the maximum around Ia’s,” Dr. Dwarkadas said.
“It’s thought that white dwarfs don’t lose mass before they explode. The usual explanation for the circumstellar material is that it would have come from a companion star in the system, but the amount of mass suggested by this measurement was very large — far larger than one could expect from most companion stars.”
“Even the most massive stars do not have such high mass-loss rates on a regular basis. This once again raises the question of how exactly these strange supernovae form,” he said.
“It is surprising what you can learn from so few photons. With only tens of them, we were able to infer that the dense gas around the supernova is likely clumpy or in a disk,” said Caltech graduate student Chris Bochenek, co-lead author of the study.
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Christopher D. Bochenek et al. X-ray Emission from SN 2012ca: A Type Ia-CSM Supernova Explosion in a Dense Surrounding Medium. MNRAS, published online August 23, 2017; doi: 10.1093/mnras/stx2029
