Why is an extreme gamma-ray source behaving so strangely?

Analysis: Researchers are assessing what's behind unusual behaviour in the Cassiopeia constellation involving the changing brightness of stars

During the clear night we can see lots of beautiful stars in the sky. The more massive the star, the brighter it is shining. A star ten times brighter than the Sun shines as three thousand solar mass’s stars. Such an extreme brightness leads to the strong winds from the massive stars. On average, a ten solar mass star loses its mass ten thousand times faster than our Sun.

Despite the relatively low number of such massive stars (less than 0.1 percent of total number of stars), their strong winds play an important role on the star formation in the Universe. These winds might trigger the collapse of surrounding clouds of gas and dust to form new stars or, conversely, blast the clouds away leaving them no chance to produce new stars.

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Star formation by collapse of molecular clouds

Despite their important role, the detailed structure of these winds from massive stars remains poorly understood. One of the ways to learn more about them is to study systems in which another star with a strong wind is orbiting around a massive star. It can be either another massive star, or a rapidly rotating neutron star. Neutron stars are one of the possible end products of stellar evolution, and are known to emit beams of radio emission and a wind of high-energy electrons out of its magnetic poles.

This radiation can be observed only when a beam of emission is pointing toward Earth - which is why such an object is called a pulsar. Interaction of the pulsar wind with the massive star outflow will lead to the production of photons over a million times more energetic than the visible light. Their energy can exceed the energies reached by the most powerful human-built particle accelerator Large Hadron Collider in CERN.

From Science Channel, how does the Large Hardon Collider work?

Such a situation has been observed in the case of an unusual binary system in the Cassiopeia constellation called LSI +61 303. In this system, a neutron star is orbiting around a young star at least 10 times more massive than our Sun. As the neutron star rotates around the massive star over an oval shaped path, it is going through the wind regions with different densities which lead to the different brightness of the system. Thus at most energies (radio, optical, X-rays), we see periodic change of the system brightness, with the maximum when the neutron star is passing near the massive star.

We observe this behaviour with various modern telescopes allowing us to study the Universe at frequencies varying from MHz (radio emission) up to frequencies higher by a factor of 1020 (very high energy gamma-rays). Such a situation is illustrated on the following video, where the black oval represents the path of the neutron star, and the size of the blue spotis proportional to the brightness of the system. In this case the brightness varies with the orbital period and the emission is always the highest when the pulsar is passing near the star.

The Fermi Gamma-ray Space Telescope is a space observatory being used to perform gamma-ray astronomy observations. Its main instrument is the Large Area Telescope, an imaging high-energy gamma-ray telescope covering the energy range from about 20 MeV to more than 300 GeV. Such gamma rays are emitted only in the most extreme conditions by particles moving very nearly at the speed of light.

The LAT's field of view covers about 20% of the sky at any time, and it scans continuously, covering the whole sky every three hours. The LAT is the ideal instrument for an all-sky survey aimed to study various astrophysical and cosmological high-energy phenomena.

Continuous monitoring of LSI +61 303 by the gamma-ray FERMI/LAT telescope for almost 15 years has allowed a team of international scientists from Ireland, Germany and France to make a surprising discovery. It turned out that the variability period at some energies is slightly longer than the orbital period.

Because of this, the maximum brightness of the system gradually varies with time, shifting from the position closest to the star to the furthest possible position. This is clearly seen in the video above, where the brightness of the system varies with a slightly longer period than the orbital one, so that the position of the maximum brightness changes with time.

Such a change in behaviour came as a big surprise as it had never been observed in any other binary system. We don’t know how to explain it yet, but it may be related to the periodic change of clumpy structure and instabilities in the stellar wind. Further observations of the system should help us to better understand the structure of winds from massive stars and the details of their interaction with the neutron star.

The views expressed here are those of the author and do not represent or reflect the views of RTÉ

Why is an extreme gamma-ray source behaving so strangely?

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Masha Chernyakova