An international team of astronomers using ESA’s XMM-Newton Space Observatory has identified a pulsar that is able to dramatically change the way in which it shines – the object can quiet its radio waves while at the same time it makes its X-ray emissions much brighter.
Artist’s impression of a pulsar in radio-bright, left, and X-ray-bright/radio-quiet mode (ESA / ATG medialab)
Pulsars are small spinning stars about 20 km in diameter. They emit oppositely directed beams of radiation from their magnetic poles. Some pulsars produce radiation across the entire electromagnetic spectrum, including at X-ray and radio wavelengths. Despite being discovered more than 45 years ago the exact mechanism by which pulsars shine is still unknown.
It has been known for some time that some radio-emitting pulsars flip their behavior between two states, changing the pattern and intensity of their radio pulses. The moment of flip is both unpredictable and sudden. It is also known from satellite-borne telescopes that a handful of radio pulsars can also be detected at X-ray frequencies. However, the X-ray signal is so weak that nothing is known of its variability.
To find out if the X-rays could also flip the scientists studied a particular pulsar called PSR B0943+10, one of the first to be discovered. It has radio pulses which change in form and brightness every few hours with some of the changes happening within about a second.
“The behavior of this pulsar is quite startling, it’s as if it has two distinct personalities. As PSR B0943+10 is one of the few pulsars also known to emit X-rays, finding out how this higher energy radiation behaves as the radio changes could provide new insight into the nature of the emission process,” explained Dr Ben Stappers of the University of Manchester’s School of Physics and Astronomy, who co-author a paper published in the journal Science.
Since the source is a weak X-ray emitter, the team used the most sensitive X-ray telescope in existence, the XMM-Newton on board a spacecraft orbiting the Earth. The observations took place over six separate sessions of about six hours in duration. To identify the exact moment of flip in the pulsar’s radio behavior the X-ray observations were tracked simultaneously with two of the largest radio telescopes in the world, LOFAR and the GMRT.
What the scientists found was that whilst the X-rays did indeed change their behavior at the same time as the radio emission, as might have been expected, in the state where the radio signal is strong and organized the X-rays were weak, and when the radio emission switched to weak the X-rays got brighter.
“To our surprise we found that when the brightness of the radio emission halved, the X-ray emission brightened by a factor of two! Furthermore the intense X-rays have a very different character from those in the radio-bright state, since they seem to be thermal in origin and to pulse with the neutron star’s rotation period,” said lead author Prof Wim Hermsen of the Netherlands Institute for Space Research.
_______
Bibliographic information: W. Hermsen et al. 2013. Synchronous X-ray and Radio Mode Switches: A Rapid Global Transformation of the Pulsar Magnetosphere. Science, vol. 339, no. 6118, pp. 436-439; doi: 10.1126/science.1230960
