Observing Universe with X-Rays

    

    Observing the universe with a normal telescope in the visible spectrum has many limitations. The motion in Earth's atmosphere distorts most of the radiations. Furthermore, the atmosphere blocks ultraviolet, infrared and higher frequency radiation. Ground-based telescopes cannot catch the radiations coming from distant stars or galaxies. Even if we perform observations through telescopes orbiting the Earth in the visible spectrum, many radiations of different frequencies will still not be detected. The addition of X-rays in astronomy bought a new revolution. The use of X-rays in astronomy began in the 1920s and by 1940 special instruments capable of detecting X-rays were completely in use. From 1920 to 1980, the capability of these detectors was greatly increased. 

    X-rays are expected to be emitted from astronomical objects with very high temperatures, in the range of millions of kelvins. The sun is a large source of X-rays but earlier it was not detected because these radiations were blocked by the atmosphere. Until 1960, X-rays sources were discovered within the solar system. In 1962, the first cosmic X-ray source, beyond the solar system was detected. The source of these X-rays was Scorpius X-1, located 9000 light-years away in the constellation Scorpius. The X-ray energy output of this source is 100,000 greater than the energy output of the Sun in all wavelengths.

Special High Altitude Balloons for X-ray Astronomy:

    Special balloons are designed which will carry X-ray instruments. these balloons are usually filled with helium or hydrogen and can fly up to an altitude of 30 to 40 km above sea level, in the stratosphere which is above 99.99% of the Earth's atmosphere. Balloons can stay floating for a longer period. However, at this altitude X-rays with energy, less than 35 keV cannot be detected.

    These balloons are launched in 'near space', an area defined above the Armstrong limit (20 km from the sea level) where pressure is not survivable for a human being. However, they always stay below the Karman Line (85 km above sea level), where simple balloons cannot maintain normal flight. Rockets and spacecraft, constructed using the laws of astrodynamics can only maintain flight above the Karman Line. 

    The first successful balloon-based X-ray detection from a distant cosmic body was made in 1964 when a balloon launched from Texas detected hard x-rays coming from the Crab Nebula. Thus the first X-ray observation of the Crab nebula was recorded.

High Energy Focusing Telescope:

    HEFT (High Energy Focusing Telescope) is a balloon-borne experiment for hard X-ray imaging. HEFT uses tungsten silicone multilayer coatings to extend the reflectivity of mirrors beyond 10 keV. HEFT was launched in May 2005 from New Mexico, USA. It performed a 25-hour long flight and observed a supernova SN 1054 or the Crab Nebula. Apart from this many large samples of extragalactic objects were also observed using HEFT. The main purpose of this experiment was to observe the Ti-44 emission from the young supernova remnants using hard X-rays.

Rockoon:

    Balloons had their own limitations. They were not able to continue their flight above a certain height. So astronomers performed a new experiment known as Rockoon (Rocket + Balloon). Rockoon uses a solid-fuel rocket attached to a balloon. Rather than being ignited while on land, here the rocket is automatically ignited when it is separated from the balloon. Once the balloon has reached its maximum height, the rocket will automatically ignite. This will give extra height to the X-ray instruments. 

    The concept was originally developed by Lee Lewis, G. Halvorson, S. Singer and James van Allen in 1949. The rockets used in Rockoon are known as 'van Allen's Rockoon'. During the first flight, the balloon reached a height of 70,000 ft but the rocket did not fire. The possible theory behind this was that the extreme cold at high altitudes have made it impossible for a rocket to ignite automatically. In the next flight, van Allen heated orange juice cans fixed them in Rockoon's gondola and wrapped the whole body for insulation. This time the rocket successfully ignited. 

    The first successful flight of rockoon was made on 9th Aug 1955. It was followed by another successful flight on 30th Aug, the same year. In September 1957, 36 rockoons were launched from USS Glacier in the Pacific, Antarctic and Atlantic regions. This experiment was again headed by van Allen. Today many private space agencies from different countries like UK, Spain, the USA etc are developing different shaped rockoons to carry microsatellites in low orbit.   

X-ray Instruments:

       X-ray telescopes have played a major role in the advancement of modern astronomy. The basic elements of x-ray telescopes are detectors (which detect and measure the incident radiations) and optics (which collets the incoming radiations). 

(1) Optics: 

    Special x-ray mirrors allow the incident radiation to focus on the detector plane. Today, the Wolter telescope is most widely used for X-ray astronomy. Typical telescope designs do not work well for X-rays. Hans Walter in 1952 proposed three ways of building mirrors that work perfectly for x-rays. These are known as alter Telescopes of type I, II and III. Each design has its own unique advantage and disadvantage. Walter's design successfully created an x-ray telescope with a wide field of view.

    Such optical design provides both, high-resolution images and high sensitivity telescopes. The mirrors used in these telescopes are mainly made up of ceramic or metal foil with a thin layer of reflective metal like gold or iridium. Mirrors used in x-ray telescope works on the basis of total internal reflection. Earlier telescopes, constructed in the 2000s (Chandra and XMN Newton) had a limit of 15 keV light. However, using multi-coated mirrors, the limit of the NuSTAR telescope (launched in 2012) was extended up to 79 keV. To achieve such a high limit, the mirrors were multi-coated with tungsten/ silicon or platinum/silicon carbide. 

(2) Detectors:

    Detectors are another important element of the X-ray telescope. X-rays range from 8 nm to 8 pm in wavelength, 50 PHz to 50 EHz in frequency and 0.12 keV to 120 keV in energy. X-rays between 0.12 keV and 12 keV are classified as soft x-rays and that between 12 to 120 keV are hard x-rays. Earth's atmosphere blocks almost all the x-rays coming from stars and supernovae. Hence to catch any x-ray, detectors must fly above the Earth's atmosphere. These can be done through balloons, rockets and satellites as discussed above. 

    X-ray telescopes use special detectors called the Scintillation detector. A scintillator is a material that exhibits luminescence when excited by ionizing radiations. Earlier due to the limitation of technology it was hard to differentiate between gamma and x-rays. However, today with modern advanced detectors, they are easier to differentiate.