Astro Jargon for Statisticians

8/31/2004

Note: An equivalent list, with Statistics terms described for Astronomers, is available at www.ics.uci.edu/~dvd/Astro/stat-jargon-for-astro.pdf

[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, and [Å]=12.3985/[keV].

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 "metals".)

ARF

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.

Continuum

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.

Dither

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).

Emissivity

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].

Event

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.

Image

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

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.

Lines

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

1. 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.
2. 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.
3. "PHA File": Standard file type for a histogram of counts vs. spectral channel (PHA, ADU, diffraction angle, wavelength, or other).

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 and RMF.

Pileup

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.

Plasma

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 energy photons.

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.

RMF

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.

Sources

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 stellar winds.
• 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 gas.
• 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".

Spectrum

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.

Units

• 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], [photons/sec/cm^2/keV], etc.
• 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).

WCS

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.

• Chandra
  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.
• Compton
  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.
• Einstein
  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
• GLAST
  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.
• ROSAT
  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.
• XMM-Newton
  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