Simulating an ACIS Spectrum for an On-axis Point Source, Including Pileup
Proposer Threads (Cycle 18)
- 1. Thread Overview
- 2. Preliminary Considerations
- 3. Simulate Spectrum with Source in Low State with WebSpec
- 4. Simulate Spectrum with Source in High State
- 5. Complete Target Form
This thread shows how to simulate a simple ACIS spectrum, including pileup. We assume in this example that the target is a highly variable AGN previously observed with other observatories (e.g Rosat and ASCA) and instruments. The goal of the proposal is to monitor this source (with two observations) to characterize its variability. Please note that monitor observations are time constrained. We need to determine the length of an exposure required to constrain the power law spectral index to within 15%.
Because the source is so variable, we need to simulate the spectrum in both its low and high flux states. We know from previous observations that in the low state the spectral slope has a power law photon index of 1.6, and the 2-10 keV flux is 2.7E-14 ergs/s/cm^2. The source has been observed to vary by at least a factor of 30 on timescales of weeks. For the high state, we will assume a 2-10 keV flux of 2.7E-12 ergs/s/cm^2. This is 100 times the low state flux. The count rate in the high state is sufficiently high that pileup is a concern. Pileup occurs when two or more photons hit the same region of the detector and are detected as a single event. Pileup has several undesirable consequences, including a distortion of the energy spectrum and an underestimate of the count rate. More information about pileup can be found in the CIAO "Why" Docuement An Overview of Pileup.
Here are some of the things you should consider when you begin the proposal process.
- Check for previous observations of the same target. You will need to justify additional observations if any are found. For detailed instructions, please refer to the "Proposing to Observe an On-Axis Point Source with ACIS" thread.
- Check for other bright sources in the field-of-view. The Observation Visualizer is available for this task. For further information, please refer to the the "Proposing to Observe an On-Axis Point Source with ACIS" thread.
- This proposal requires two observations of the same source (i.e. it is a monitoring observation). Monitoring observations are time constrained (see the FAQ for Constrained and Coordinated Proposals).
- Find the column density of galactic neutral hydrogen, NH. The tool colden is available at: http://cxc.harvard.edu/toolkit/colden.jsp. For this source, we obtained a galactic Nh of 2.57e20 cm-2.
- This thread uses WebSpec, the web version of the X-ray spectral fitting package XSPEC. WebSpec has the advantage that the proposer does not need to download and install software. The CIAO modeling and fitting package Sherpa can also be used to simulate a spectrum and the Sherpa thread Simulating Chandra ACIS-S Spectra with Sherpa explains the required steps. The command line version of XSPEC can of course be used, but must be previously installed.
WebSpec can be found at the following URL: http://heasarc.gsfc.nasa.gov/webspec/webspec.html. The first page sets the telescope/instrument combination and the source model:
- Mission/Instrument is Chandra ACIS-S on axis
- Check the box to apply photoelectic absorption
- Highlight "Power Law" in the Available Models box
- Click on "I'm ready to set model parameters"
The next steps simulate a spectrum, using what is known about the source from ROSAT.
- Set exposure time to be 80000s (80ks). This is a guess which may need to be refined later.
- Set the upper and lower bounds of "Calculating flux over energies" to be 2.0 and 10.keV
- Set the hydrogen column to be 0.0257
- Set the Photon Index to be 1.6
- Check the box to freeze the model normalization, photon index and hydrogen column (leave first redshift parameter free).
- All other parameters (including normalization) can be left at their default values.
- Hit "Do the simulation!"
The simulation button creates a simulated 1-D dataset from the source model and and instrument model, and then adds Poisson noise to the modeled data. The simulated data file currently does not have the correct normalization--the flux of the simulated data is incorrect because the power law normalization was arbitrarily set to 1.0. To correct the flux we need to adjust the normalization.
The 2-10 keV flux in the simulated spectrum (given in the "Resulting Fluxes" box) is 4.7652E-9 erg cm-2 s-1. The 2-10 keV flux of this source has been measured at 2.70E-14 erg cm-2 s-1 in the low state. Therefore the correct normalization is 2.7E-14/4.7652E-9=5.666E-6. Hit the "back" button to go back to the previous page and change the model normalization from 1.0 to 5.666E-6 and repeat the simulation. With the new normalization, the simulated flux is correctly set at the measured flux of 2.7E-14 erg cm-2 s-1.
Now that the normalization is correct, we can estimate how well the spectral index can be measured. Hit the back button and be sure that the "Freeze parameter" box for the photon index is NOT set for the photon index and that the "Compute the error" box IS checked. Also change the "Calculating flux over energies" box to be set to 0.2-10.5 keV. The resulting simulation has a count rate of about 0.005 counts/s, for a total of 0.005*800000=400 counts. The uncertainties on the photon index are 1.51 (+0.118,-0.114). The actual simulated values will vary, but with this many counts, the photon index can be measured to better than 15%, as required. This means that even if the source is in the lowest observed state the slope can be meaningfully constrained. If the source is brighter, then the uncertainties will be reduced.
In the high state the flux is 100 times that of the low state and pileup must be considered. The degree to which the source is piled up can be estimated from PIMMS. Set the following parameters:
- Input -- "Flux"
- Flux -- "Absorbed"
- Input energy 2.0-10 keV
- Model -- power law, galactic nH=2.57E20, photon index=1.6, absorbed flux=2.7E-12
- Output Mission: current Chandra cycle
- Output Detector/Grating/Filter: ACIS-S/None/None
The pileup estimate from PIMMS is 34% and the predicted count rate is 0.15 cts/sec. To illustrate the effect of pileup we have plotted the spectrum with and without pileup in Figure 1. The unpiled spectrum has a peak count rate of slightly more than 0.3 counts/sec/keV. In contrast, the piled spectrum has a peak count rate of 0.1 counts/sec/keV because many photons are rejected and/or many low energy photons are mistakenly counted as high energies.
The effect of pileup on a spectrum can also be evaluated using Webspec:
- Mission/Instrument is Chandra ACIS-S on axis
- Check the "Pile-up" and "Photoelectric Absorption" boxes
- Choose the model Power Law
Fill in the model parameters as above and hit "I'm ready to set model parameters". Then fill in the following values: Photon Index=1.6, Hydrogen Column=0.0257, Normalization=5.666E-6). The exposure is 80ks and the "Calculating flux over energies" boxes should be 2.0 and 10.0 keV. All parameters can be frozen, except for the first redshift. Please note that the pileup component must be removed to calculate fluxes. Addition of the pile-up component will show the effect of pile-up on the spectrum. It is possible to download the simulated spectra and responses from the WebSpec results page and read them into command line versions of Sherpa or Xspec. The effect of pile-up on a spectrum with pile-up (pile-up box checked) can then be compared to the spectrum without the effects of pile-up (pile-up box not checked).
There are a number of ways to configure ACIS to minimize pileup. These are discussed in the POG and include offset pointing, using a transmission grating, using CC mode, alternating exposures, and reducing the frame time with a subarray. For our target we do not know in advance whether the spectrum will be piled. The source might actually be quite faint. In this case, we wouldn't use the first four options because they are not optimized for faint sources. We are therefore left with only two choices -- either use a subarray or do not take any steps to mitigate pileup. Most of the ACIS field-of-view is lost if a subarray is chosen. This would be a big disadvantage if there were other sources in the field which formed part of the proposal. Since the target is a point source, a subarray is a good choice.
Sometimes, regardless of whether or not a subarray has been used, the pileup fraction is larger than anticipated. In this case, it is possible to estimate the pileup and recover the source spectrum using the pileup model described earlier in this section.
More information about pileup can also be found in the CIAO "Why" Document An Overview of Pileup.
A subarray reduces pileup by shortening the frame time. With a shorter frame time, the probability that two photons are detected as a single event decreases. Here we simulate a 1/8 subarray, which corresponds to a frame time of 0.4 sec. The frame time for standard subarrays is also provided in Chapter 6 of the POG. For example, for the default 1/8 subarray on ACIS-S with one chip turned on, the CCD is exposed for 0.4 sec per frame instead of 3.2 sec per frame. Use PIMMS to estimate the reduced pileup by setting the "Frame Time" to be 0.4 sec. All other parameters are as described above. The pileup is reduced to 5%, which is acceptable. The effect of 5% pile-up on the spectrum can be evaluated by WebSpec. The parameters are set as above, except that the frame time (fr_time) is set to 0.4s.
For general instructions on how to submit a proposal, please see the "Using RPS to Prepare and Submit a Chandra Proposal" thread.
For this observation, we will use ACIS chip S3 and the Very Faint telemetry format. Instrument configuration and telemetry formats are described in detail in the ACIS chapter of the POG.
RPS requires both the source count rate and the total field count rate (sources plus background). The purpose of these fields is to check that the total count rate does not exceed the telemetry limit. We therefore give the source count rate in the high state as 0.5 cts/sec.
In calculating the total field count rate, we need to consider all contributions to the background. Estimates of the background as a function of energy band are provided in the ACIS total background section of the POG. Table 6.11 gives the total background rate for the S3 BI chip to be 6.8 cts/sec/chip, for the energy band 0.3-13 keV. Since we are using a 1/8 subarray, our total background rate is 6.8/8=0.85 counts/sec. Therefore, the total field count rate is 0.85+0.5=1.35 cts/s.
The RPS target form should have the following parameter values for this observation. If a parameter isn't listed here, use the default RPS value or leave the field blank. Please note that this program consists of 2 time constrained observations (see the FAQ for Constrained and Coordinated Proposals). The Constraints/Slewtax button in RPS can be used to estimate how these constrained observations are classified (i.e. easy, average or difficult).
- Target Name -- Fake Target
- Total Observing Time (ksec) -- 160 ks
- Count Rate -- 0.5
- Total Field Count rate -- 1.35
- Exposure Mode -- TE
- Event Telemetry Format -- Very Faint
- CCD's On -- ONLY S3 should be checked Y
- Subarray Type -- Standard 1/8 subarray
- The monitoring table should be filled out as follows:
- First observation Exposure Time=80ks
- Second observaion Exposure Time=80ks, Minimum Time Interval=30 days, Maximum Time Interval=60 days