Scientists suggest explanation for mysterious radio flashes
Mysterious bright radio flashes that appear for only a brief moment on the sky and do not repeat could be the final farewell greetings of a massive star collapsing into a black hole, astronomers from Nijmegen and Potsdam argue.
Radio telescopes have picked up some bright radio flashes that appear
for only a brief moment on the sky and do not repeat. Scientists have
since wondered what causes these unusual radio signals. An article in
this week's issue of Science suggests that the source of the flashes
lies deep in the early cosmos, and that the short radio burst are
extremely bright. However, the question of which cosmic event could
produce such a bright radio emission in such a short time remained
unanswered. The astrophysicists Heino Falcke (Radboud University
Nijmegen/ ASTRON) and Luciano Rezzolla (Max Planck Institute for
Gravitational Physics in Potsdam) provide a solution for the riddle.
They propose that the radio bursts could be the final farewell greetings
of a supramassive rotating neutron star collapsing into a black hole.
Spinning star withstands collapse
Neutron stars are the ultra-dense remains of a star that has undergone a supernova explosion. They are the size of a small city but have up to two times the mass of our Sun. However, there is an upper limit on how massive neutron stars can become. If they are formed above a critical mass of more than two solar masses, they are expected to collapse immediately into a black hole. Falcke & Rezzolla now suggest that some stars could postpone that final death through fast rotation for millions of years. Like a ballerina spinning around her own axis, centrifugal forces could stabilize these overweight neutron stars against collapse and leave them in a ‘half-dead' state for up to a few million years. Nonetheless, the star is just buying time and even with this trick it cannot avoid the inevitable. Neutron stars have extremely strong magnetic fields threading their environment like huge propeller blades.
Any left-over matter in the surrounding will be blown away by this
magnetic fan and rotational energy is radiated away. Thus, while the
half-dead star ages, it also slows down and becomes more and more
compact, with gravity playing an ever stronger role. At some point the
tired star can no longer withstand gravity's pull. It will cross the
ultimate death-line and suddenly collapse to a black hole while
transmitting a strong radio flash.
Emission disappears in black hole
Astrophysicists normally expect a gravitational collapse to be accompanied by bright fireworks of optical and gamma-ray radiation from the imploding matter. This characteristic emission, however, is not seen in the newly found fast radio bursts. Falcke & Rezzolla suggest that this is because the neutron star has already cleaned out its surroundings and the remaining stellar surface is quickly covered by the emerging event horizon.
‘All the neutron star has left is its magnetic field, but black holes
cannot sustain magnetic fields, so the collapsing star has to get rid of
them,' explains Prof. Falcke. When the black hole forms, the magnetic
fields will be cut off from the star and snap like rubber bands. As we
show, this can indeed produce the observed giant radio flashes. All
other signals you normally would expect - gamma rays, x-rays - simply
disappear behind the event horizon of the black hole.'
Because of its single, ultra-rapid and unrepeatable signal, Falcke and
Rezzolla named these objects ‘blitzars', from the German blitz (flash).
This is opposed to pulsars, which are rotating neutron stars that are
flashing repeatedly like cosmic lighthouses and simply fade away. Prof.
Rezzolla adds: ‘These fast radio bursts could be the first evidence of
the birth of a black hole, whose formation is therefore accompanied by
an intense, almost pure, radio-wave emission. Interestingly, a blitzar
is at the same time the farewell signal of a dying neutron star and the
first message of from a newly born black hole.'
The new theory proposed by Falcke & Rezzolla provides a first solid
interpretation of the previously mysterious radio bursts. Their work has
been submitted to the journal ‘Astronomy & Astrophysics' and was
posted on the arxiv.org preprint archive. To further test their
proposal, more observations of the so far elusive radio bursts are
required. Falcke and his colleagues plan to use telescopes like the new
LOFAR radio telescope to detect more of these dying stars in the future.
This would allow them to locate the events quicker and more precisely,
and to observe this new formation channel of black holes in the depths
of the cosmos with keen ‘radio eyes'.
'A population of fast radio bursts at cosmological distances'; Science, July 5
D. Thornton, B. Stappers, M. Bailes, B. Barsdell, S. Bates, N. D. R. Bhat, M. Burgay, S. Burke-Spolaor, D. Champion, P. Coster, N. D'Amico, A. Jameson, S. Johnston, M. Keith, M. Kramer, L. Levin, S. Milia, C. Ng, A. Possenti, & W. van Straten.
More information:
Fast radio bursts: the last sign of supramassive neutron stars - Submitted to Astronomy & Astrophysics by Heino Falcke and Luciano Rezzolla
Contact:
Prof. Heino Falcke - h.falcke@astro.ru.nl Tel: +31 24-36-52020 / Mobile: +49 151 23040365 / Secretary (Esther Gebhardt) +31 24-36-52080
Prof. Luciano Rezzola - Luciano.Rezzolla@aei.mpg.de
Photo above: Still from pulsar animation. Image credit: NASA.
