Astro Jargon for Statisticians
Note: An equivalent list, with Statistics terms
described for Astronomers, is available at
- [Å] and [keV]
- Ångstrom, a unit of length equal to 10-8 cm. X-ray
wavelengths geneally range from O(1) to O(100) Å. The equivalent
unit of photon energy is [keV], where 1 eV = 1.609 10-12 ergs,
- Abundance and Metallicity
- The relative number of an element wrt that of Hydrogen is
the abundance. It is usually written A(Z)=N(Z)/N(H) for
a simple ratio, or [Z]=log10(A(Z)/A(Z)Sun)
when denoting abundance relative to the Solar composition. The
metallicity is the abundance of Fe, which is the most dominant
source of emission. Abundance variations are usually reported
with reference to Fe, e.g., [Ne/Fe], [O/Fe], etc. (Note
that astronomers refer to all elements other than H and He as
- Auxiliary Response File (aka Ancillary Response File). Encodes
the telescope effective area. Includes the combined
telescope/filter/detector areas and efficiencies as a function of
energy averaged over time. When the input flux spectrum is multiplied
by the ARF the result is the distribution of counts that would be seen
by a detector with perfect (i.e. infinite) energy resolution. The
RMF is then needed to produce the final observed spectrum.
- Spectra usually consist of emission lines, absorption features,
and continua. The continuum can arise due to blackbody emission,
atomic bound-free transitions, bremsstrahlung radiation, cyclotron
or synchrotron emission.
- While observing a celestial source, X-ray telescopes do not
maintain a fixed pointing direction, but instead `dithers' around
the Sky. Chandra's dither pattern is a Lissajous
figure, with irrational periods in the cardinal directions to
make a non-closed Lissajous pattern.
- Effective Area
- The product of the telescope mirror geometric area,
reflectivity (which is a strong function of energy), off-axis
vignetting (also a function of energy as well as off-axis
angle), detector quantum efficiency (including any filters),
which depends on energy and position on the detector, and
[if applicable] diffraction grating efficiency (which is a
function of order and energy).
- The intrinsic strength of an atomic transition that produces
a spectral line. The term encapsulates all the atomic data
information needed to calculate the flux. It is often generalized
to also refer to continuum processes. Units are
usually [10-23 ergs cm3 s-1].
Sometimes also presented as [ph cm3 s-1],
and especially for continuum emissivities,
[(erg|ph) cm3 s-1 Å-1]
- Emission Measure
- A measure of the "amount of material" of a plasma available
to produce the observed flux, the product of the square of the
electron number density and the volume of emission, with units
[cm-3]. Often, because observations are carried out
along a line of sight, the cross-section area is taken out of
the expression and the units become [cm-5].
- X-ray astronomy instruments record a distinct signal from
each individual photon they detect. As a result X-ray data are
stored event by event, which retains all the information and
allows great flexibility of analysis. Every X-ray "event"
(a general term for a detection; may refer to a celestial photon
or a background cosmic ray) is characterized by a "pulse height"
(PHA) that encodes the energy of the incoming photon; a time of
arrival; a grade describing the quality of the event; and typically
two position coordinates.
- Exposure maps and Instrument maps
- An array specifying the `amount of exposure' at each
image pixel for a given observation. For Chandra,
the exposure map includes the effective area and
has units of [cm2] or [cm2 s]. This
quantity is essential to determine the flux or brightness
of a celestial source from the observed counts. A related
item is the Instrument map, which describes the efficiency of the
detector at each detector pixel. Generally, an exposure map is
derived after applying aspect dither to the instrument map.
- A two-way frequency table of observed counts as a function
of location. The location coordinates are either rooted in
physical detector space, or in inferred direction of arrival
of the photons in angular coordinates on the sky.
- lambda refers to the wavelength of a photon, usually in
[Å], and E refers to its energy, usually in [keV].
For grating data, because the RMF is almost diagonal, these also
refer to the detector bin. Low-resolution spectra are placed in
channel bins, whose boundaries are mapped to approximate ranges of
photon energies via a gain map.
- Atoms, ions, and molecules emit photons at characteristic
energies, with each producing a unique set of lines that serve
as "bar codes" that identify the element. Lines can be seen
either in emission (as enhancements over a smooth continuum)
or in absorption (as dips from a smooth continuum).
- PHA and PI
- Pulse Height Amplitude/Pulse Invariant Channel
- Engineering unit describing the integrated charge per
pixel from an event recorded in a detector. In early
electronic devices, this was the size of the pulse.
- The PHA value in event files is the total pulse
height of an event. For a given location, a gain table is
used to map the PHA of an event to a nominal energy value,
converting the PHA into a Pulse Invariant (PI) channel.
- "PHA File": Standard file type for a histogram of counts
vs. spectral channel (PHA, ADU, diffraction angle, wavelength,
- Photons v/s Counts
- By convention, the term "photon" usually refers to the
photons before they pass through the telescope, while
"counts" refer to the observed signal in the detector.
That is, "counts" are the result of "photons" passing through
the telescope/detector system. Thus, a count spectrum is the
incident photon spectrum modfified by the instrument ARF
- Pileup occurs when two or more photons are detected in a CCD
pixel within one read-put period, so the detector electronics are
fooled into mixing them into a single event.
- A highly ionized state of matter achieved either by heating
some material to very high temperatures (hundreds and thousands
of degrees), or bombarding a material by a strong flux of high
- PSF (or PRF)
- The Point Spread Function describes the shape of the image
produced on the detector by a delta function (point) source. Also
known as `Point Response Function' or PRF. A related term is the
Line Spread Function (LSF), which applies to the response of a grating
to a spectral line of delta-function shape.
- Quantum Efficiency (QE)
- The QE is the fraction of incident photons registered by a
detector. A strong, highly structured function of energy, originally
used to describe CCD detectors such as Chandra's ACIS,
and generalized to include other types of detectors such as multichannel
plates (e.g., HRC).
- Quantum Efficiency Uniformity (QEU)
- The QE is usually defined as a function of energy for a single
point on the detector, and deviations from it at different detector locations
are mapped in a QEU file.
- Redistribution Matrix File, maps from photon space into
detector counts (pulse height or position) space. The
redistribution matrix contains the information about how the incoming
photons are spread out over detector channels by the
detector resolution. In high resolution instruments (e.g., diffraction
gratings such as HETG and LETG) the matrix is almost diagonal. In
proportional counters the matrix elements are non-zero over a large
area. CCD detectors, such as ACIS, are an intermediate case, with most
of the response being almost diagonal, but escape peaks and low energy
tails add significant contributions.
- Some types of common astronomical X-ray sources are described below.
- Hot Stars: Massive stars of spectral class earlier
than type A (bluer stars like Vega) produce X-rays via shocks in
- Cool Stars: Stars like the Sun that show evidence of
magnetic activity have hot atmospheres confined by magnetic
fields; these hot atmospheres (aka coronae) are thought to be
heated via magnetic reconnection.
- Supernova Remnants: The diffuse emission from the detritus
of a supernova explosion, heated by shocks as the ejecta plows
into the interstellar medium.
- X-ray binaries: Powered by accretion of matter from a
companion star, which forms a disk around a central compact
object and is heated (mostly) by viscous dissipation.
- QSO/Quasar: Highly luminous (luminosities a trillion times
greater than that of the Sun) unresolved emission from the core of
galaxies at high redshifts.
- Active Galactic Nuclei: Galaxies with high luminosity
of the central, compact region, emitting into the entire
spectral energy range from radio to X-rays and in some cases
gamma-rays. Outflows and jets observed often.
- Cluster of galaxies: Group of galaxies at nearly the same
distance from the Earth, exhibits X-ray emission from hot intergalactic
- Cooling Flows: Hot intergalactic gas falling in towards
the central galaxy of a cluster, increasing in density and cooling
in the process.
- Spectral Class
- Most stars are classified into a sequence of types organized by
their photospheric (surface) temperature, denoted by the letters
O, B, A, F, G, K, and M. The Sun is a G type star. Hotter stars
(like O, B, etc.) are also called "earlier type stars" and cooler
stars (like M, K) are also called "later type stars".
- A frequency distribution of observed counts as a function of
detector channels. For low-resolution spectra, the detector channels
are PHA or PI (mapped onto photon energy space via
the RMF). For high-resolution grating spectra, the detector
channels are wavelengths derived from pixel location.
- Astronomers tend to use mostly CGS units, such as [ergs] for energy,
[ergs/s/cm2] for flux, etc.
- Spectra are displayed as functions of either energy (in
[keV], for low-resolution spectra) or wavelength (in
[Å] for high-resolution grating spectra). Spectral
intensities are variously shown in units of [counts/channel], [counts/sec/keV],
- Angular separations are measured in [degrees], [arcminutes], and
[arcseconds], while angular locations (positions on the sky) are denoted
either in decimal degrees or sexagesimal notation as Right Ascension
([hours:minutes:seconds] of time; 15 degrees of arc == 1 hour of time)
and Declination ([degrees:minutes:seconds] of arc).
- World Coordinate System, a standardized format for storing
coordinate information in image files, that allows the
translation of pixel positions to true sky coordinates.
- X-Ray Telescope
- Unlike in the optical, it is difficult to make mirrors or
lenses to focus X-ray light. X-ray mirrors are usually
built as nested paraboloid and hyperboloids that bring photons
to a focus by deflecting them at small angles. Often, mirrors
are entirely dispensed with and collimators are used. Also,
because the Earth's atmosphere absorbs X-rays, X-ray telescopes
must be placed in outer space. Some selected X-ray and gamma-ray
missions are listed below.
The Chandra X-Ray Observatory, one of the series of
Great Observatories launched by NASA (others are: Hubble,
Compton, SIRTF). Named after Prof. Subrahmanya Chandrasekhar,
the Indian-American Nobel laureate in Physics.
See http://chandra.harvard.edu/about/ for more details.
Chandra has two detectors on board, the Advanced Camera for
Imaging and Spectroscopy (ACIS), a CCD detector, and the High Resolution
Camera (HRC), a multichannel plate detector. There are two gratings that
may be optionally used to obtain high resolution spectra: the HETGS
(High Energy Transmission Grating Spectrometer) and the LETGS (Low Energy
Transmission Grating Spectrometer). The HRMA (High Resolution Mirror
Assembly) consists of 4 shells of nested mirrors with 2 segments -
paraboloid and hyperboloid - each.
The Compton Gamma-Ray Observatory (CGRO) was the first
gamma ray observatory, launched in 1991 and deliberately "deorbited" in
June 2000. Carried telescopes COMPTEL (0.5-30 MeV) and EGRET (30 MeV-100 GeV)
that surveyed the whole sky.
The first true imaging X-ray telescope, launched in 1978, first
non-solar X-ray telescope to map diffuse and point sources.
Energy Range : 2 keV - 100 MeV
Very large area, large field of view gamma-ray experiment,
expected to be launched in late 2005, covering an energy range
of 20 MeV to 300 GeV.
First X-ray telescope to do all-sky survey; "pathfinder" for Chandra.
Lifetime : 1 June 1990 - 12 February 1999
Energy Range : X-ray 0.1 - 2.5 keV , EUV 62-206 eV
- TRACE and SOHO
Transition Region and Coronal Explorer and the Solar and Heliospheric
Observatory, EUV telescopes devoted to solar observations, and are unique
in that they use normal incidence mirrors over narrow wavelength bands
to make images of the Sun.
The X-Ray Multi-Mirror Mission, a large area X-ray telescope
operated by ESA. Has higher count rates and energy range than
Chandra, but less spectral resolution.
- See also:
- The Chandra/CIAO Dictionary: http://asc.harvard.edu/ciao/dictionary/
- The CIAO Why Topics: http://asc.harvard.edu/ciao/why/
- The Astronomy Cafe: http://www.astronomycafe.net/
- Imagine the Universe: http://imagine.gsfc.nasa.gov/
- Astro Picture of the Day: http://antwrp.gsfc.nasa.gov/apod/astropix.html
CHASC: The California-Harvard AstroStatistics Collaboration