by S. Prasad Ganti

All the X-ray telescopes are in space. None on the ground. There are optical, infra-red, radio telescopes on the ground, but not X-rays. For the simple reason that X-rays coming from outer space cannot penetrate the earth’s atmosphere. Fortunately for us, else we would have been toast. No life would have been possible on earth. 

Chandra is an X-ray telescope, up in space since 1999. It recently completed 25 years in space. It was expected to last only five years, but had performed excellently for the last quarter of a century. X-rays, in minor doses, help with medical imaging of body inners. But in outer space, the dosages are very high and result from a supernova remnant or a black hole. An X-ray telescope helps in studying such cosmic objects. 

Chandra is an engineering marvel which has the cleanest pairs of mirrors. There are 4 pairs which are polished to the smoothness of a few atoms and are precisely aligned. X-ray telescopes require much higher levels of precision than optical telescopes because the wavelength of X-rays is much smaller than visible light. The unevenness of the surface of the mirror should be much less than the wavelength. Chandra’s great accomplishments have been the discovery of Cassiopeia A, which is the remnant of a supernova. The discovery of the Bullet cluster provided evidence of dark matter. Hundreds of black holes were also discovered.   

There are 2 X-ray observatories which have been launched in the last three years. To study the polarization of X-rays from extremely bright objects (not visual but in X-ray terms). Polarimeter is an instrument which is used to study the polarization of the X-rays, basically the alignment of electric and magnetic fields. They are always perpendicular to each other, but which direction is the electric component is the question. Polarization of X-rays can reveal the source from where they are coming and also the magnetic fields and the geometry of the source region. Initial observations of the Crab nebula, powered by a neutron star at its core; and the Cygnus X1 black hole X-ray binary are very encouraging. 

Another X-ray observatory called the Einstein probe, is a joint mission by the Chinese Academy of Sciences, European Space Agency (ESA) and the Max Planck Institute for Extraterrestrial Physics. Its goals are to identify and study new and transient sources of high-energy X-rays. The events are very short lived and shift from location to location, with very little chance of repeating at the same location. Such events are triggered by inactive black holes and merger of neutron stars. It was launched in Jan, 2024. 

The Einstein probe  has a Wide-field X-ray Telescope (WXT), with a lobster like eye with a 180 degree field of view. The telescope inspired by the crustacean eyes works by reflection rather than refraction. The wide field helps the telescope to observe a large swath of the sky at a time.   

The X-ray observatories form a part of the multi messenger astronomy where observations are made in several parts of the electromagnetic spectrum like radio waves, infrared waves, light, x-rays, and gamma rays. The instruments and the observatories are different engineering for each part of the electromagnetic spectrum. Except for x-rays and gamma rays, the rest of them can be observed using ground based observatories as well. 

Given below is the picture of Chandra X-ray telescope, courtesy NASA.