The Chandra X-ray Observatory is a space telescope launched on STS-93 by NASA on July 23, 1999. Chandra is sensitive to X-ray sources 100 times fainter than any previous X-ray telescope, enabled by the high angular resolution of its mirrors. Since the Earth's atmosphere absorbs the vast majority of X-rays, they are not detectable from Earth-based telescopes; therefore space-based telescopes are required to make these observations. Chandra is an Earth satellite in a 64 hour orbit, and its mission is ongoing as of 2012.
Chandra Observatory is the third of NASA's four Great Observatories. The first was Hubble Space Telescope; the second was the Compton Gamma Ray Observatory, launched in 1991; and the last is the Spitzer Space Telescope. Of the four, Compton ended in 2000 and the other three continue. Chandra has been described as being as revolutionary to astronomy as Galileo's first telescope.
It was named in honor of the Nobel-prize winning physicist Subrahmanyan Chandrasekhar who worked for University of Chicago from 1937 until he died in 1995. He was known for determining the maximum mass for white dwarfs. "Chandra" means "moon" in Sanskrit. Before 1998, it was known as AXAF, the Advanced X-ray Astrophysics Facility. AXAF was assembled and tested by TRW (now Northrop Grumman Aerospace Systems) in Redondo Beach, California.
Unlike optical telescopes which possess simple aluminized parabolic surfaces (mirrors), X-ray telescopes generally use a Wolter telescope consisting of nested cylindrical paraboloid and hyperboloid surfaces coated with iridium or gold. X-ray photons would be absorbed by normal mirror surfaces, so mirrors with a low grazing angle are necessary to reflect them.
Chandra uses four pairs of nested mirrors, together with their support structure, called the High Resolution Mirror Assembly (HRMA); the mirror substrate is 2 cm-thick glass, with the reflecting surface a 33 nm iridium coating, and the diameters are 65 cm, 87 cm, 99 cm and 123 cm. The thick substrate and particularly careful polishing allowed a very precise optical surface, which is responsible for Chandra's unmatched resolution: between 80% and 95% of the incoming X-ray energy is focused into a one-arcsecond circle. However, the thickness of the substrates limit the proportion of the aperture which is filled, leading to the low collecting area compared to XMM-Newton.
Chandra's highly elliptical orbit allows it to observe continuously for up to 55 hours of its 65 hour orbital period. At its furthest orbital point from earth, Chandra is one of the furthest from earth earth-orbiting satellites. This orbit takes it beyond the geostationary satellites and beyond the outer Van Allen belt.With an angular resolution of 0.5 arcsecond (2.4 µrad), Chandra possesses a resolution over 1000 times better than that of the first orbiting X-ray telescope.
The Science Instrument Module (SIM) holds the two focal plane instruments, the Advanced CCD Imaging Spectrometer (ACIS) and the High Resolution Camera (HRC), moving whichever is called for into position during an observation.
ACIS consists of 10 CCD chips and provides images as well as spectral information of the object observed. It operates in the range of 0.2–10 keV. HRC has two micro-channel plate components and images over the range of 0.1–10 keV. It also has a time resolution of 16 microseconds. Both of these instruments can be used on their own or in conjunction with one of the observatory's two transmission gratings.
The transmission gratings, which swing into the optical path behind the mirrors, provide Chandra with high resolution spectroscopy. The High Energy Transmission Grating Spectrometer (HETGS) works over 0.4–10 keV and has a spectral resolution of 60–1000. The Low Energy Transmission Grating Spectrometer (LETGS) has a range of 0.09–3 keV and a resolution of 40–2000.
The data gathered by Chandra have greatly advanced the field of X-ray astronomy.
- The first light image, of supernova remnant Cassiopeia A, gave astronomers their first glimpse of the compact object at the center of the remnant, probably a neutron star or black hole. (Pavlov, et al., 2000)
- In the Crab Nebula, another supernova remnant, Chandra showed a never-before-seen ring around the central pulsar and jets that had only been partially seen by earlier telescopes. (Weisskopf, et al., 2000)
- The first X-ray emission was seen from the supermassive black hole, Sagittarius A*, at the center of the Milky Way. (Baganoff, et al., 2001)
- Chandra found much more cool gas than expected spiralling into the center of the Andromeda Galaxy.
- Pressure fronts were observed in detail for the first time in Abell 2142, where clusters of galaxies are merging.
- The earliest images in X-rays of the shock wave of a supernova were taken of SN 1987A.
- Chandra showed for the first time the shadow of a small galaxy as it is being cannibalized by a larger one, in an image of Perseus A.
- A new type of black hole was discovered in galaxy M82, mid-mass objects purported to be the missing link between stellar-sized black holes and supermassive black holes. (Griffiths, et al., 2000)
- X-ray emission lines were associated for the first time with a gamma-ray burst, Beethoven Burst GRB 991216. (Piro, et al., 2000)
- High school students, using Chandra data, discovered a neutron star in supernova remnant IC 443.
- Observations by Chandra and BeppoSAX suggest that gamma-ray bursts occur in star-forming regions.
- Chandra data suggested that RX J1856.5-3754 and 3C58, previously thought to be pulsars, might be even denser objects: quark stars. These results are still debated.
- Sound waves from violent activity around a supermassive black hole were observed in the Perseus Cluster (2003).
- TWA 5B, a brown dwarf, was seen orbiting a binary system of Sun-like stars.
- Nearly all stars on the main sequence are X-ray emitters. (Schmitt & Liefke, 2004)
- The X-ray shadow of Titan was seen when it transitted the Crab Nebula.
- X-ray emissions from materials falling from a protoplanetary disc into a star. (Kastner, et al., 2004)
- Hubble constant measured to be 76.9 km/s/Mpc using Sunyaev-Zel'dovich effect.
- 2006 Chandra found strong evidence that dark matter exists by observing supercluster collision
- 2006 X-ray emitting loops, rings and filaments discovered around a supermassive black hole within Messier 87 imply the presence of pressure waves, shock waves and sound waves. The evolution of Messier 87 may have been dramatically affected.
- Observations of the Bullet cluster put limits on the cross-section of the self-interaction of dark matter.
- "The Hand of God" photograph of PSR B1509-58.
Organization: NASA, SAO, CXC
Major contractors: TRW, Northrop Grumman
Launch date: 23 July 1999
Launched from: Kennedy Space Center, USA
Launch vehicle: Space Shuttle Columbia
Mission length planned: 5 years
elapsed: 13 years, 2 months and 4 days
Mass: 4,790 kg (10,600 lb)
Orbit height: apogee 133,000 km (83,000 mi)
perigee 16,000 km (9,900 mi)
Orbit period: 64.2 hours
Wavelength: X-ray (0.1 - 10 keV)
Diameter: 1.2 m (3.9 ft)
Collecting area: 0.04 m2 (0.43 sq ft) at 1 keV
Focal length: 10 m (33 ft)