The Chandra X-ray Observatory, previously known as the Advanced X-ray
Astrophysics Facility, is a space observatory 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 2014.
Chandra is one of the Great Observatories, along with the Hubble
Space Telescope, Compton Gamma Ray Observatory, and the Spitzer Space
Telescope. The telescope is named after astrophysicist Subrahmanyan
Chandrasekhar. History
In 1976 the Chandra X-ray Observatory was proposed to NASA by Riccardo
Giacconi and Harvey Tananbaum. Preliminary work began the following
year at Marshall Space Flight Center and the Smithsonian Astrophysical
Observatory. In the meantime, in 1978, NASA launched the first imaging X-ray
telescope, Einstein, into orbit. Work continued on the AXAF project throughout
the 1980s and 1990s. In 1992, to reduce costs, the spacecraft was redesigned.
Four of the twelve planned mirrors were eliminated, as were two of the six
scientific instruments. AXAF's planned orbit was changed to an elliptical one,
reaching one third of the way to the Moon's at its farthest point. This
eliminated the possibility of improvement or repair by the space
shuttle but put the observatory above the Earth's radiation belts for most of
its orbit. AXAF was assembled and tested by TRW in Redondo Beach, California.
AXAF was renamed Chandra as part of a contest held by NASA in 1998, which drew
more than 6,000 submissions worldwide. The contest winners, Jatila van der Veen
and Tyrel Johnson, suggested the name in honor of Nobel Prize–winning
Indian-American astrophysicist Subrahmanyan Chandrasekhar. He is known
for his work in determining the maximum mass of white dwarf stars, leading to
greater understanding of high energy astronomical phenomena such as neutron
stars and black holes. Originally scheduled to be launched in
December 1998, the spacecraft was delayed several months, eventually being
launched in July 1999 by Space Shuttle Columbia during STS-93. At 22,753
kilograms, it was the heaviest payload ever launched by the shuttle, a
consequence of the two-stage Inertial Upper Stage booster rocket system needed
to transport the spacecraft to its high orbit.
Chandra has been returning data since the month after it launched. It is
operated by the SAO at the Chandra X-ray Center in Cambridge, Massachusetts, with
assistance from MIT and Northrop Grumman Space Technology. The ACIS CCDs suffered
particle damage during early radiation belt passages. To prevent further
damage, the instrument is now removed from the telescope's focal plane during
passages. Although Chandra was initially given an
expected lifetime of 5 years, on September 4, 2001 NASA extended its
lifetime to 10 years "based on the observatory's outstanding results."
Physically Chandra could last much longer. A study performed at the Chandra
X-ray Center indicated that the observatory could last at least 15
years. In July 2008, the International X-ray Observatory, a joint project
between ESA, NASA and JAXA, was proposed as the next major X-ray observatory but
was later cancelled. ESA later resurrected the project as the Advanced
Telescope for High Energy Astrophysics with a proposed launch in 2028.
Discoveries The data gathered by Chandra has 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.
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.
The first X-ray emission was seen from the supermassive black hole, Sagittarius
A*, at the center of the Milky Way. Chandra found much more cool gas than
expected spiraling 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 super
massive black holes. X-ray emission lines were associated for
the first time with a gamma-ray burst, Beethoven Burst GRB 991216.
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 super massive black hole were observed in the Perseus Cluster.
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.
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.
Hubble constant measured to be 76.9 kmMpc using Sunyaev-Zel'dovich effect.
2006 Chandra found strong evidence that dark matter exists by observing super
cluster collision 2006 X-ray emitting loops, rings and
filaments discovered around a super massive 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. Jupiter's x-rays coming from poles, not
auroral ring. A large halo of hot gas was found
surrounding the Milky Way. Extremely dense and luminous dwarf
galaxy M60-UCD1 observed. On January 5, 2015, NASA reported that
CXO observed an X-ray flare 400 times brighter than usual, a record-breaker,
from Sagittarius A*, a supermassive black hole in the center of the Milky
Way galaxy. The unusual event may have been caused by the breaking apart of an
asteroid falling into the black hole or by the entanglement of magnetic field
lines within gas flowing into Sagittarius A*, according to
astronomers. Technical description
Unlike optical telescopes which possess simple aluminized parabolic surfaces,
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; 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 most distant
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, Chandra possesses a
resolution over 1000 times better than that of the first orbiting X-ray
telescope. = Instruments=
The Science Instrument Module holds the two focal plane instruments, the AXAF
CCD Imaging Spectrometer and the High Resolution Camera, 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 works
over 0.4–10 keV and has a spectral resolution of 60–1000. The Low Energy
Transmission Grating Spectrometer has a range of 0.09–3 keV and a resolution of
40–2000. Gallery
See also Great Observatories program
List of space observatories NuSTAR
X-ray astronomy References
Further reading External links
Chandra X-ray Observatory at NASA.gov Chandra X-ray Observatory at Harvard.edu
Chandra podcast by Astronomy Cast