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: When you select a mission for input or output, then clicking on the corresponding "Detector/Grating/Filter" button will display the available options for that mission.

Flux or Flux Density

For normalizing the source spectrum, choose whether to specify For flux or flux density normalization, specify whether to take the value as diminished by line-of-sight absorption or not: The FLUX choices are:

Energy Range

Model

Select the spectral shape to convolve with the effective area curve of the instrument. PIMMS supports the following one-parameter models:

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-5 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-17 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-17 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:

  1. 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.

  2. 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.

  3. 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.]

10.6.1 Tuesday, November 28, 2017
The Chandra X-Ray Center (CXC) is operated for NASA by the Smithsonian Astrophysical Observatory. 60 Garden Street, Cambridge, MA 02138 USA.   Email: cxcweb@head.cfa.harvard.edu Smithsonian Institution, Copyright © 1998-2017. All rights reserved.