14 DAY TRIAL // Astronomers have identified in amazing detail a type of cosmic phenomenon never seen before: a pulsar plunges into the solar wind of its companion star twice every 3.4 years. As the pulsar rotates around its companion star on an elongated orbit, the second plunge is only a few months apart from the first.
The companion star is a weird type of star called 'Be' due to certain spectral characteristics. This star is only few times more massive than our Sun but rotates around its axis at a staggering speed. This causes it to have an elongated shape and to have an equatorial ring made of gas material flung off from the star. These rings look like Saturn's rings but, of course, they are totally different in nature.
The pulsar is a neutron star that emits pulses of radio waves. It is an incredibly dense star (around the same mass as the Sun but only 20 km in diameter) made from neutrons and spinning very fast.
A long standing dilemma was the nature of the solar wind ejected by such stars. Scientists argued about how energetic the winds are and whether they consist of protons or electrons. But the opportunity to decide the matter empirically didn't exist - until now.
"Despite countless observations, the physics of pulsar winds have remained an enigma," said lead author Masha Chernyakova, of the Integral Science Data Centre, Versoix, Switzerland. "Here we had the rare opportunity to see pulsar wind clashing with stellar wind. It is analogous to smashing something open to see what's inside."
When the two winds collide, twice each rotation of the neutron star around the 'Be' star, gamma rays and X-rays are emitted. The X-rays have now been detected using the European Space Agency's XMM-Newton spacecraft and the gamma rays using the HESS, the High Energy Stereoscopic System, a new ground-based gamma-ray telescope in Namibia. The new XMM-Newton data was collected nearly simultaneously with a HESS observation and they revealed the origin and content of the pulsar winds.
"For most of the 3.4-year orbit, both sources are relatively dim in X-rays and it is not possible to identify characteristics in the pulsar wind," said co-author Andrii Neronov. "As the two objects draw closer together, sparks begin to fly."
Both telescopes found the same thing: when the two stars approached 'periastron', the points in orbit where they are closest to each other, the bursts of gamma rays and X-rays happened. By combining these two observations with the radio observations of the pulsar the scientists put together a complete picture of this binary system.
The scientists managed to observe the energy of X-rays and gamma rays as the pulsar dug through the Be star's disk. They concluded the pulsar wind is made of electrons although they are not yet certain whether these electrons come directly from the pulsar or are being produced by protons emited from the pulsar. Moreover, they have found that the X-ray radiation, although energetic, is about 1000 times less energetic than expected. The wind of electrons producing the gamma rays seems to be 1000 times more energetic than the wind of electrons producing the X-rays. What happens here? The observation solves a set of problems and rises another one.
Chernyakova said that the model explaining the behavior of this system all the way from radio waves to X-rays to gamma rays is still "under construction".
"The only fact that is crystal clear at the moment is that this is the pulsar system to watch if we want to understand pulsar winds," said Chernyakova. "Never have we seen pulsar wind in such detail. We are continuing with theoretical models now."
Image: Artist's impression of the XMM-Newton telescope. Credit: ESA. Picture: High Energy Stereoscopic System (HESS) gamma ray observatory. Credit: South African Agency for Science and Technology Advancement.
