Bright, young supernova outburst in Messier 82
BY MARK ARMSTRONG
Posted: January 23, 2014
In what could be one of the observing highlights of 2014, a very young, bright supernova has been discovered in the popular, nearby and well-placed galaxy Messier 82 in Ursa Major. This supernova is the closest such event visible to us for a number of years, since SN1987A in the Large Magellanic Cloud and SN1993J in M81 itself. As of today, 23 January, the supernova is at magnitude +10.9, so it will require a telescope to see it.
Excitingly, the outburst was caught very early and around the end of this month it could have brightened sufficiently to be visible in large binoculars. Images easily show the supernova as a bright 'star' superimposed on the cigar-shaped galaxy, to the south and west of the nucleus.
Messier 82, along with its bigger and brighter companion Messier 81, forms the best galaxy pairing in the Northern Hemisphere and they are right on our doorstep, cosmologically speaking, both lying a mere 11.4 million light-years away.
The supernova (current designation PSN_J09554214+6940260) was discovered at magnitude +11.7 on 21 January at 19.20 UT, with credit being assigned to Dr S J Fossey. Dr Fossey was assisting students during routine student training at the University of London's (UCL) observatory (ULO) at Mill Hill when the supernova was spotted. As is often the case with popular galaxies there are a number of very valuable pre-discovery images of the supernova; unfortunately discovery credit is lost if a potential supernova is not reported in time to the Central Bureau for Astronomical Telegrams (CBAT), the official IAU clearing house for all astronomical discoveries. The Liverpool Astronomical Society Supernova Search Team (LASSST) imaged M82 on 19 January, with the supernova shining around magnitude +12. The prolific nova and supernova discoverer Koichi Itagaki actually imaged the supernova on the nights of 15, 16, 17, 19, 20 and 22 January, showing a fast rise in brightness from magnitude +14.4 to +11.3. Very importantly, his image from 14 January shows no trace of the supernova down to a limiting magnitude of +17. M82 is a very dusty galaxy presented edge-on to us, making it hard to make a very early detection of a supernova in its earliest stages. The spectrum shows the supernova is 'reddened', it's brightness depleted somewhat by M82's dust. But Itagaki's image puts some time constraints on the supernova's earliest appearance.
Following a world-wide alert, professional astronomers Y Cao (Caltech), M M Kasliwal (Carnegie/Princeton), A McKay (UT Austin) and A Bradley (APO) on behalf of the intermediate Palomar Transient Factory Collaboration obtained a spectrum on 22 January 2014 at 08:47 UT with the Dual Imaging Spectrograph on the ARC 3.5m telescope. They report, "we classify this as a Type Ia supernova with a Si II velocity of 20,000 kilometres per second. The best superfit match is SN2002bo at -14d. The supernova has a red continuum and deep Na D absorption. We further note that at UT 2014 Jan 21.158, the supernova is visible in Palomar 48-inch images but it is too saturated to measure a magnitude."
What this means is that the debris from the supernova is expanding at 20,000 kilometres per second, and its light curve - its rate of brightening - best matches a previous supernova in 2002 that was observed when it was two weeks (14 days) old.
Type Ia supernovae are 'white dwarf supernovae', caused by accretion of material on the surface of the super-dense white dwarf from a companion star; once the remnant reaches a critical mass it will explode. Alternatively the merging of two white dwarfs can be the trigger for the catastrophic outburst. It is possible that the progenitor star will be found in archive professional images, most likely in those taken by the Hubble Space Telescope. M82 has produced two previous supernovae, SN2004am and 2008iz, both being type-II events caused by the explosion of a massive star.
If the supernova follows a typical Type Ia light curve then it should continue to brighten for the rest of the month at least. It could become as bright as magnitude +8.5, well within the visual range of small telescopes and large binoculars. M82 (NGC 3034) is circumpolar (never sets) from the UK and can be observed all night. It is over 40 degrees up in the north-north-eastern sky by the time the sky is astronomically dark and transits over 70 degrees above the horizon around 2am. Images and brightness estimates, visual or CCD, will be very valuable for professional astronomers from here on in, especially while the supernova is on the rise. Please send observations to the British Astronomical Association (BAA), the Society for Popular Astronomy (SPA), the Astronomer Group (TA) and Astronomy Now. For up to date supernova magnitudes and images keep an eye on David Bishop's phenomenal Latest Supernovae page at http://www.rochesterastronomy.org/snimages/