Proposal Planning Toolkit: PIMMS Help
PIMMS: Portable, Interactive Multi-Mission Simulator
This is a help page specific to using PIMMS online. For more information, please consult the CIAO PIMMS help page here.
Mission & Detector/Grating/Filter
The missions supported through the Toolkit GUI are:- ASCA
- HITOMI
- CHANDRA - The effective area curves are provided for the
current and all previous NRAs, and can be viewed through the
Effective Area File Viewer.
More information is provided on the Chandra Proposal Planning Calibration Files page.
Note that the CHANDRA effective area files include the sum of source count rates in positive and negative orders
(i.e. the 1st order files contain orders +/-1 and the letghi file contains orders +/-2 through +/-11). - EINSTEIN
- EXOSAT
- GINGA
- NUSTAR
- ROSAT
- SAX
- SWIFT
- SUZAKU
- XMM
- XTE
Flux or Flux Density
For normalizing the source spectrum, choose whether to specify- its flux over a specified energy range;
- its flux density at a specified energy;
- the count rate it would produce in a selected instrument.
- Absorbed Flux (or Flux Density) - Use flux from the source spectrum at the telescope aperture (before application of instrumental effects, and including absorption in the ISM).
- Unabsorbed Flux (or Flux Density) - Use flux from the source spectrum at the telescope aperture (before application of instrumental effects, and as if there were no absorption in the ISM).
Energy Range
- Input Energy - Specify the energy bounds of the source spectrum, in keV. If you are normalizing the source spectrum by specifying the count rate it would produce in a selected instrument, you may elect to use the default energy range of the instrument. For flux density, specify the energy, in keV, at which the flux density is evaluated.
- Output Energy - Specify the output energy range of interest, in keV. You may elect to use the default energy range of the detecting instrument. For flux density, specify the energy, in keV, at which the flux density is evaluated.
Model
Select the spectral shape to convolve with the effective area curve of the instrument. PIMMS supports the following one-parameter models:- Power Law - The parameter is Photon Index, defined such that AE**(-(photon index)) is the flux in photons / cm**2 / s .
- Black Body - The parameter is energy (kT) in keV.
- Thermal Bremsstrahlung - The parameter is energy (kT) in keV. The model includes the Gaunt factor.
- Plasma - APEC/MEKAL/Raymond-Smith
The parameters are temperature, abundance and NH.PIMMS v4.0 is distributed with APEC and Raymond-Smith models at 59 temperatures (logT of 5.60 to 8.50 in an increment of 0.05) times 5 abundance (0.2 to 1.0 in an increment of 0.2), while MEKAL has a narrower temperature range starting with logT of 6.0 (51 temperatures to logT=8.50). For MEKAL and APEC, ``solar'' abundances are those due to Anders & Grevesse, while the solar standard for the Raymond-Smith grid has become unclear due to passage of time. PIMMS will select the nearest temperature and abundance that is supported by the grid in use.
The Raymond-Smith models provided through the GUI support up to a maximum energy of 8 keV. Any inputs exceeding 8 keV are reset within the code to 8 keV. [8 keV is used as the maximum energy for normalizing the source spectrum (i.e., as the maximum energy contributing to the "input" flux or count rate), and also as the maximum energy contributing to the "output" count rate. The errors caused by these truncations are opposite in sense (the input truncation overestimates the source, the output truncation underestimates the count rate that that source would produce). The errors are generally small, as the effective areas are diminishing rapidly at these high energies.]
Galactic NH
Neutral hydrogen column density. If the value is 30.0 or less, it is interpreted as log10(NH). Otherwise the value is NH, in cm**(-2), specified with Fortran-style exponent (e.g., 2.5e+21). The range accepted is 0.0 through 6.3E25. NOTE: only photoelectric absorption is taken into account. At levels above ~1e24, the material becomes thick to Compton scattering and the results are not expected to be accurate.Redshift
Optionally, all components may be redshifted using a common z (in which case, Redshifted NH values are interpreted as an intrinsic absorber, with the same z) with an optional (always z=0) Galactic NH.Redshifted NH
The neutral hydrogen column density of an intrinsic absorber at the redshift of the source.Photon Index
Parameter for Power Law models, defined such that AE**(-(photon index)) is the flux in photons / cm**2 / s . The photon index may have any value. A negative value specifies a power law that increases with increasing energy in photon space.kT
Parameter for the Black Body and Thermal Bremsstrahlung models, the energy (kT) in keV. The range is 0.01 keV through 107 keV .Abundance
Parameter for Raymond-Smith models, the abundance, either 0.2, 0.4, 0.6, 0.8, or 1.0 solar (as defined by Allen).log T | keV
Parameter for Raymond-Smith models, the base 10 logarithm of the temperature in degrees Kelvin, or the equivalent energy in keV. Values of logT supported range from 5.60 through 8.50, in increments of 0.05.Count Rate
Count rate in the instrument selected for normalizing the source spectrum, in counts / s . The range is 1.0e-7 through 1.0e+6.Absorbed Flux
Flux, after line-of-sight absorption, in ergs / cm**2 / s, for normalizing the source spectrum. The range is 1.0e-18 through 1.0e-6.Unabsorbed Flux
Flux, disregarding line-of-sight absorption, in ergs / cm**2 / s, for normalizing the source spectrum. The range is 1.0e-18 through 1.0e-6. For unabsorbed flux, both intrinsic and Galactic absorption will temporarily be set to 0.Absorbed Flux Density
Flux Density, after line-of-sight absorption, in ergs / cm**2 / s / keV, for normalizing the source spectrum. The range is 1.0e-18 through 1.0e-9.Unabsorbed Flux Density
Flux Density, disregarding line-of-sight absorption, in ergs / cm**2 / s / keV, for normalizing the source spectrum. The range is 1.0e-18 through 1.0e-9.Source
When the mission selected for output is CHANDRA and background calibration data are available for the selected detector/grating combination, the Toolkit will provide information about instrument-specific background count rates (not included in the PIMMS prediction). Specify whether the source is a point source or an extended source. Extended source size specification affects only background determination.Size
For an extended source to be observed with CHANDRA, specify the area of the source in arcsec**2, to enable estimation of the background count rate. (The background estimate for a source smaller than 7.0 arcsec**2 is the same as for a point source.)Frame Time
The frame time is the fundamental unit of exposure for ACIS. The option exists to either "Specify" the frame time, or to have the routine "Calculate" the frame time.When specifying frame time, the valid range is from 0.2 to 10.0 seconds, in 0.1 second increments for simplicity. The default value is 3.2 seconds. Frame times greater than the default will INCREASE the probability of the occurrence of pileup.
When calculating frame time, it is necessary to enter the Number of Chips, and Subarray, described below. The choice of instrument (ACIS-I or ACIS-S) is also used in the frame time calculation.
Please refer to the Specific Help for Chandra RPS (Target Form -> ACIS Parameters -> Parameters that affect PILEUP -> Subarray: Frame Time) and the Proposers' Observatory Guide (ACIS chapter) for more information regarding the selection of frame time and how its value affects pileup.
Number of Chips
This item appears when Frame Time is set to "Calculate". It is the number of ACIS chips set. The choices are 1-6. The default is 6.Subarray
This item appears when Frame Time is set to "Calculate". A subarray is a reduced region of the CCDs (all of the CCDs that are turned on) that will be read. A reduced region may also help to reduce the effects of pulse pile-up. The choices are None, 1/2, 1/4, 1/8. The default is None.PIMMS Prediction
The predicted count rate (in the absence of pileup for ACIS), in counts / s. This is 100% of the count rate for the full field of view.When viewing the full results through the "View Output" button, the Model Normalization is provided. If using the Power Law model, the units of Model Normalization are photons/cm^2/s/keV at 1 keV.
% Pileup
The estimated pileup percentage, defined as the ratio of the number of frames with two or more events to the number of frames with one or more events times 100, for the predicted count rate (assuming a point source) with ACIS. The ACIS spectrum will be "piled up" by this amount. Pileup predictions are currently not available for extended sources.Pileup is calculated using nine 3x3-pixel detect cells. The inner cell is assumed to have an encircled energy fraction of 0.886; the remaining energy is distributed equally to the surrounding eight cells. The number of counts per frame is determined by multiplying the frame time by the count rate, and expected count rates are calculated for each cell given its encircled energy fraction.
The cts/frame output is the summation of the individual detect cell values.
Pileup warnings:
- The pileup fraction assumes the user is calculating the flux
over the *entire* bandpass of the ACIS-I or ACIS-S instrument.
If the energy
range is restricted, the pileup fraction calculated will be
*under*-estimated because fewer photons will be included in the
pileup calculation than the full bandpass. The severity of
the effects of pileup
depends upon the spectral energy distribution (SED) of a source;
however, the PIMMS calculation does not take the SED into account.
- The current pileup model begins to break down and becomes
increasingly invalid as a predictor for pileup fractions greater
than about 75%. 75% pileup is reached approximately when the PIMMS
output is equivalent to about 3 counts/frame or higher. Because the
model does break down above 75%, the "cts/frame" and "cts/s" results
may appear anomalous when the pileup percentage is this high.
Please refer to the Proposers' Observatory Guide for information on
mitigating the pileup effect.
- For grating observations, the pileup estimate is an upper limit based on the undispersed 1st order count rate. Improved pileup estimates may be obtained by reducing the energy range being considered; however, users who need accurate simulations in order to ensure pileup will not be a problem are advised to use the MARX simulation tool.
Piled counts per frame
The estimated number of observed counts per frame assuming that ACIS pileup occurs at the estimated percentage (see % Pileup for a detailed explanation of the PIMMS pileup model) .Piled count rate
The estimated observed count rate (number of counts per second) assuming that ACIS pileup occurs at the estimated percentage. This quantity is the "Piled counts per frame" divided by the input "frame time" The estimated observed count rate (number of counts per second) assuming that ACIS pileup occurs at the estimated percentage. This quantity is the "Piled counts per frame" divided by the input "frame time" (see % Pileup for a detailed explanation of the PIMMS pileup model).Background Count Rate
If background calibration data are available for the selected CHANDRA output instrument, the Toolkit will present either an estimate of the total background in the source area, in counts / s, over the default energy range for the instrument (regardless of the energy range selected for the source count rate); or information to enable the user to make a qualitative estimate of the background. The computations assume a 1.5" radius point source circle, which encloses 90% of the total energy (as quoted in the PIMMS Prediction (see above)) at 6.4 keV. For extended sources, the count rate is normalized to the input area. [A background count rate is provided for all non-grating cases i.e. */None/None, and background information is provided for the HRC-S/LETG/* cases. For background information on grating observations, the user is advised to consult the appropriate chapter in the Proposers' Observatory Guide.]
