Stellar outburst: X-raying this year's supernova

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  • Published: Feb 1, 2014
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: Stellar outburst: X-raying this year's supernova

Serendipitous supernova

The new supernova. This view showing SN 2014J (arrow) merges three exposures taken on Jan. 22, 2014. Mid-ultraviolet light is shown in blue, near-UV light in green, and visible light in red. The image is 17 arcminutes across, or slightly more than half the apparent diameter of a full moon. Credit: NASA/Swift/P. Brown, TAMU

Astronomers have planned observations using the Hubble Space Telescope operated by NASA and the European Space Agency as well as NASA's Chandra X-ray Observatory, Nuclear Spectroscopic Telescope Array (NuSTAR), Fermi Gamma-ray Space Telescope, and Swift missions in order to glean as much information about the recent supernova flare-up SN 2014J, in galaxy M82, the "cigar galaxy", which lies in the constellation Ursa Major, as possible.

An exceptionally close stellar explosion was spotted on 21st January 2014; "close" of course being entirely relative. The focus of astronomical attention by amateur and academic observatories around and above the globe is on a blast site designated "SN 2014J", a supernova in the galaxy M82, which lies 12 million light-years away (about 1.1x1020 km from Earth). Nevertheless, this is the closest optical supernova seen in two decades and as far as active space missions are concerned might well be the closest type Ia supernova.

A supernova is a stellar explosion of far greater energy than a nova; a nova being the cataclysmic nuclear explosion on a white dwarf star in a binary system, which leads to sudden transient brightening of the star. They are thought to be a result of accretion of hydrogen by one of the pair leading to a runaway fusion reaction on the surface. A supernova by contrast, is caused by re-ignition of a dying star's nuclear fuel or gravitational collapse of a massive star resulting in an enormously violent outburst. A type Ia supernova represents the total destruction of a white dwarf star by one of two possible scenarios. In the first, the white dwarf orbiting a normal star, pulls a stream of matter from it, and gains mass until it reaches a critical threshold and explodes, in the second, the results from a collision between two orbiting white dwarfs in a binary system. Either way, the explosion produces a superheated shell of plasma that expands outward into space at tens of millions of kilometres per hour. There have been no supernovae observed in the Milky Way Galaxy since "Kepler's Star" of 1604 (SN 1604).

To make the most of the appearance of SN 2014J, all cosmic hands are to the pumps, as it were, with astronomers recruiting numerous instruments to make visual observations, to record X-ray emissions and spectroscopic data from the supernova. Data from Swift have already been recorded, just a day after the first sighting of the phenomenon; Swift's Ultraviolet/Optical Telescope (UVOT) capturing the supernova and its host galaxy.

A pizza, the action

As ever, hindsight is a remarkable thing and SN 2014J can be seen in images of M82 recorded up to seven days before anyone noticed the presence of this new bright object. Apparently, because it was overcast Steve Fossey was giving his undergraduate students - Ben Cooke, Tom Wright, Matthew Wilde and Guy Pollack - a ten-minute lesson on how to use their telescope's CCD (charge-coupled device), its "digital camera". The telescope in question is the 0.35-metre instrument at the University of London Observatory (a University College London teaching facility). The group focused on M82 in a patch of clear sky that night and noticed a bright "star" they hadn't seen before in their observations. Observations with a second telescope proved it wasn't an artefact just as the night became completely overcast. Fossey quickly prepared a report for the International Astronomical Union’s Central Bureau for Astronomical Telegrams, the organisation that catalogues supernovae. He also alerted a US-based supernova search, team who have access to spectroscopic facilities. The students were very excited to be a part of this new discovery. "It was a surreal and exciting experience taking images of the unidentified object as Steve ran around the observatory verifying the result," Pollack. "I'm very chuffed to have helped in the discovery of the M82 Supernova." His team mate Wright was equally enthusiastic: “One minute we’re eating pizza then five minutes later we’ve helped to discover a supernova. I couldn’t believe it. It reminds me why I got interested in astronomy in the first place."

Stellar work

It is crucial to spread the word urgently so that astronomers can start recording data and spectra as soon as possible and before a supernova begins to wane. "Finding and publicizing new supernova discoveries is often the weak link in obtaining rapid observations, but once we know about it, Swift frequently can observe a new object within hours," explains Neil Gehrels of NASA's Goddard Space Flight Center in Greenbelt, Maryland. Although the explosion is unusually close, the supernova's light is attenuated by thick dust clouds in its galaxy, which may slightly reduce its apparent peak brightness. Interstellar dust preferentially scatters blue light, which is why Swift's UVOT sees SN 2014J brightly in visible and near-ultraviolet light but barely at all at mid-ultraviolet wavelengths. However, this super-close supernova provides astronomers with an important opportunity to study how interstellar dust affects the light crossing interstellar space. As a class, type Ia supernovae explode with remarkably similar intrinsic brightness, a property that makes them useful "standard candles" - some astronomers refer to them instead as "standard bombs" - for exploring the distant universe.

Of course, it is worth noting that X-rays have never been definitively observed from type Ia supernovae, detection by Swift's X-ray Telescope, Chandra or NuSTAR would therefore represent a significant discovery, as would Fermi detection of high-energy gamma rays. Astronomers predict that SN 2014J will get brighter well into the first week of February and it may become visible to amateur astronomers with small telescopes or even binoculars.

"Results from the AAVSO website indicate it has peaked and is starting to fade. Typical fade rates for these objects are about a factor 2.5 every fortnight. While this requires urgent observations now, this actually means that we will be studying this object for a long time to come, and it will remain visible in amateur and small-telescope imaging for many weeks for sure. Professional facilities will follow it for months (and it is well placed in the sky to do so)," Fossey told SpectroscopyNOW.

At the time of writing, early visual evidence was mounting that a second supernova event had occurred in another galaxy M99.

Related Links

NASA Goddard, online: "Exceptionally close stellar explosion discovered"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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