jeudi 23 août 2018



Astronomical observations in X-rays

An interesting aspect of the life of the neutron stars was discovered when the first observations of the sky in the X-ray field took place.

The terrestrial atmosphere being opaque to these rays, it was necessary to overtake it. It was first in the 1960s telescopes placed in balloons or rockets and could study the sky for short periods. Then, in 1970, it was Uhuru, the first satellite in the X domain, that showed more than one hundred very powerful sources.

Since then, many other X domain study satellites have given us a deeper view. In particular, the Einstein observatories in 1978, ROSAT in 1990, and Chandra and XMM-Newton, both launched in 1999.

The accretion disk of neutron stars

Most X-ray sources are binary stars in which a process of mass transfer and creation of an accretion disk occurs. However, in this case, instead of a white dwarf, it is around a neutron star that everything happens.

Subject to tremendous gravity, the material that accumulates is very dense and its temperature extremely high. It therefore emits thermal radiation at very short wavelengths, in X-rays.

A view of the globular cluster NGC 6266 taken by the Chandra satellite in X-rays. Most visible points are binary systems containing either a white dwarf or a neutron star that devours the matter of his companion. Credit: NASA / CXC / MIT / D. Pooley

X-ray pulsars

Most of the time, this emission is continuous without sudden variation. But some X sources are variable, with a period of a few seconds. In this case, the gas which falls on the neutron star is subjected to the influence of the magnetic field and goes towards the two poles. The impact of the gas at these points is extremely violent and gives rise to a very powerful radiation.

As for the radio emission of pulsars, this radiation is located in a rather narrow beam which periodically sweeps the sky. If the Earth is by chance on the trajectory of this beam, it sees periodically a small flare of radiation, hence the name of X-ray pulsar.

X-ray bursts

A phenomenon similar to nova can also occur. This is the case when the material of the accretion disk is not affected by the magnetic field and is distributed over the entire surface of the star. Due to the extreme conditions, nuclear fusion reactions of hydrogen to helium occur continuously. There is thus creation, without violent event, of a layer of helium on the surface of the star.

Finally, when the temperature and density are sufficient, the combustion of helium is triggered and a phenomenal explosion occurs. This gives rise to a huge rush of radiation called a burst of X-rays. The phenomenon is much faster than for the novae. It only lasts a few seconds, explosion and return to normal understood.

This article is translated: astronomes

X-RAY SOURCES Reviewed by INVENTION on août 23, 2018 Rating: 5 X-RAY SOURCES  Astronomical observations in X-rays An interesting aspect of the life of the neutron stars was discovered when the...