There is something strange is lurking in the galactic neighborhood. An unknown object in galaxy M82 12 million light-years away has started sending out radio waves, and the emission does not look like anything seen anywhere in the universe before except perhaps by Ford Prefect. M82 is starburst galaxy five times as bright as the Milky Way and one hundred times as bright as our galaxy's center. "We don't know what it is," says co-discoverer Tom Muxlow of Jodrell Bank Centre for Astrophysics near Macclesfield, UK. But its apparent sideways velocity is four times the speed of light. This "superluminal" motion occurs usually in high-speed jets of material bursting out by black holes.
The new object, which appeared in May 2009, has left us scratching our heads - we’ve never seen anything quite like this before,” said Dr Muxlow. “The object turned on very rapidly within a few days and shows no sign of decaying in brightness over the first few months of its existence. The new young supernova explosions that we were expecting to see in M82 brighten at radio wavelengths over several weeks and then decay over several months, so that explanation seems unlikely." The object was discovered while Muxlow and his colleagues were monitoring an unrelated stellar explosion in M82 using the MERLIN network of radio telescopes in the UK. Unlike supernova emissions, which usually get brighter over a few weeks and then fade away over months, the enigmatic source has hardly changed in brightness over the course of a year, and its spectrum is steady.
Yet it does seem to be moving – and fast: its apparent sideways velocity is four times the speed of light. Such apparent "superluminal" motion has been seen before in high-speed jets of material squirted out by some black holes. The stuff in these jets is moving towards us at a slight angle and travelling at a fair fraction of the speed of light, and the effects of relativity produce a kind of optical illusion that makes the motion appear superluminal.
Could the object be a black hole? It is not quite in the middle of M82, where astronomers would expect to find the kind of supermassive central black hole that most other galaxies have. Which leaves the possibility that it could be a smaller-scale "microquasar".
A microquasar is formed after a very massive star explodes, leaving behind a black hole around 10 to 20 times the mass of the sun, which then starts feeding on gas from a surviving companion star. Microquasars do emit radio waves – but none seen in our galaxy is as bright as the new source in M82. Microquasars also produce plenty of X-rays, whereas no X-rays have been seen from the mystery object. "So that's not right either", Muxlow told New Scientist.
M82 is an irregular galaxy in a nearby galaxy group located 12 million lightyears from Earth. Despite being smaller than the Milky Way, it harbors a vigorous central starburst in the inner few hundred lightyears. In this stellar factory more stars are presently born than in the entire Milky Way. M82 is often called an 'exploding galaxy', because it looks as if being torn apart in optical and infrared images as the result of numerous supernova explosions from massive stars (see Fig. 1, left). Many remnants from previous supernovae are seen on radio images of M82 and a new supernova explosion was long overdue. For a quarter of century astronomers have tried to catch this cosmic catastrophe in the act and have started to wonder why the galaxy has been so silent in recent years.
The new discovery was first made in April 2009 when the MPIfR's Dr. Andreas Brunthaler examined data just taken (on April 8) with the Very Large Array (VLA) of the National Radio Astronomy Observatory, an interferometer of 27 identical 25 meter telescopes in New Mexico, USA. "I then looked back into older data we had from March and May last year, and there it was as well, outshining the entire galaxy!", he says (see Fig 1, top). Observations taken before 2008 showed neither pronounced radio nor X-ray emission at the position of this supernova.
On the other hand, observations of M82 taken last year with optical telescopes to search for new supernovae showed no signs of this explosion. Furthermore, the supernova is hidden on ultraviolet and X-ray images. The supernova exploded close to the center of the galaxy in a very dense interstellar environment. This could also reveal the mystery about the long silence of M82: many of these events may actually be something like "underground explosions", where the bright flash of light is covered under huge clouds of gas and dust and only radio waves can penetrate this dense material. "This cosmic catastrophe shows that using our radio telescopes we have a front-row seat to observe the otherwise hidden universe", Prof. Heino Falcke from Radboud University/Nijmegen & ASTRON explains. If not obscured, the explosion could have been visible even in a medium-sized amateur telescope.
Radio emission can be detected only from core collapse supernovae, where the core of a massive star collapses and produces a black hole or a neutron star. It is produced when the shock wave of the explosion propagates into dense material surrounding the star, usually material that was shed from the massive progenitor star before it exploded.
By combining data from the ten telescopes of the Very Long Baseline Array (VLBA), the VLA, the Green Bank Telescope in the USA, and the Effelsberg 100m telescope in Germany, using the technique of Very Long Baseline Interferometry (VLBI), the team was able to produce images that show a ring-like structure expanding at more than 40 million km/h or 4% of the speed of light, typical for supernovae. "By extrapolating this expansion back in time, we can estimate the explosion date. Our current data indicate that the star exploded in late January or early February 2008.", explains Dr. Andreas Brunthaler.
Only three months after the explosion, the ring was already 650 times larger than Earth's orbit around the Sun. It takes the extremely sharp view of VLBI observations to resolve this structure which is as large as a 1 Euro coin seen from a distance of 13.000 km.
The asymmetric appearance of the supernova on the VLBI images indicates also that either the explosion was highly asymmetric or the surrounding material unevenly distributed. "Using the super sharp vision of VLBI we can follow the supernova expanding into the dense interstellar medium of M82 over the coming years and gain more insight on it and the explosion itself.", says Prof. Karl Menten, director at the MPIfR.
Discoveries like this supernova will be routine with the next generation of radio telescopes, such as the Low Frequency Array (LOFAR) which is currently under construction in Europe, the Allen Telescope Array (ATA) in the USA, or the planned Square Kilometer Array (SKA). These will have the capability to observe large parts of the sky continously.