An international team of astronomers, led Dr Matteo Bachetti of the University of Toulouse in France, has detected what they say is the most powerful pulsar ever spotted, with the energy of about 10 million suns.
A rare and mighty pulsar (magenta) can be seen at the center of Messier 82 in this image. Image credit: NASA / JPL-Caltech / SAO / NOAO.
Pulsars are dense stellar remnants left over from supernova explosions. They are typically between one and two times the mass of our Sun.
The newly-discovered object falls in that same range but shines about 100 times brighter than theory suggests something of its mass should be able to.
It is located in a nearby galaxy called Messier 82 (also known as NGC 3034, Cigar Galaxy or M82), about 12 million light-years away.
“This compact little stellar remnant is a real powerhouse. We’ve never seen anything quite like it. We all thought an object with that much energy had to be a black hole,” Prof Fiona Harrison from California Institute of Technology, Pasadena, a co-author of the paper published in the journal Nature.
Dr Bachetti, Prof Harrison and their colleagues identified the pulsar in the Messier 82’s nuclear region through NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR), a pair of orbiting telescopes that detect high-energy X-rays from far-off galaxies. They detected pulsations with an average period of 1.37 seconds and a 2.5-day sinusoidal modulation.
The exceptional brightness of this object classifies it as an ultraluminous X-ray (ULX) source – an object so bright that it defies any known process of stellar radiation.
Indeed, ULXs are scientific curiosities, and astronomers have proposed that such objects may be intermediate-mass black holes: not as small as stellar black holes, which have a mass five to 50 times that of our Sun, but not as big as supermassive black holes, which are 100,000 to 1 billion times as massive as the Sun.
“For decades everybody has thought these ultraluminous X-ray sources had to be black holes. But black holes don’t have a way to create this pulsing,” Prof Harrison said.
“But pulsars do. They are like giant magnets that emit radiation from their magnetic poles. As they rotate, an outside observer with an X-ray telescope, situated at the right angle, would see flashes of powerful light as the beam swept periodically across the observer’s field of view, like a lighthouse beacon.”
Prof Deepto Chakrabarty, head of the Astrophysics Division at Massachusetts Institute of Technology and a co-author of the discovery, added: “there are a number of ULX sources known, and until now, most people have assumed that they are black holes, and pretty massive. Now there may be other, similar ULX pulsars. And that would mean the whole picture that was being built up to try and explain this whole class of weird objects is wrong.”
With the pulsar and its location within Messier 82 identified, there are still many questions left to answer.
The object is many times higher than the Eddington limit, a basic physics guideline that sets an upper limit on the brightness that an object of a given mass should be able to achieve.
“This is the most extreme violation of that limit that we’ve ever seen. We have known that things can go above that by a small amount, but this blows that limit away,” said co-author Dr Dom Walton of California Institute of Technology, Pasadena.
_____
M. Bachetti et al. 2014. An ultraluminous X-ray source powered by an accreting neutron star. Nature 514, 202–204; doi: 10.1038/nature13791
