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Last modified: 18 January 2017


Characterizing the Spatial Distribution of Elements in Groups and Clusters

Proposer Threads (Cycle 19)


1. Thread Overview

Groups of galaxies are generally X-ray sources and the origin of the X-ray emission is thought to be thermal emission from hot gas. From ROSAT observations, the gas is known to be an optically thin plasma with a temperature of about 1keV, with the flux dominated by emission lines from Fe, Ni, O, Si, Mg and S. In this thread, we consider a proposal to determine the spatial distribution of these elements in the example group SS2b153. We chose this target because of its high X-ray luminosity and because a substantial fraction of the cluster will fit into the ACIS-S array.

Our observational goal is to determine the Fe abundance to about 10% in 6 spatial regions.

2. Preliminary Considerations

  • Check for previous observations of SS2b153 using WebChaser. This target was approved for 31 ks in Cycle 3. Additional observations need to be justified.
  • The CXC tool ProVis can be used to determine when this source has maximum visibility and a "good" pitch angle. This is not required unless a target has constraints (and SS2b153 has none) but may be useful background information.
  • Check for other bright sources in the field of view using the Observation Visualizer. The ObsVis tool is available as part of CIAO. The J2000 coordinates for SS2b153 are: RA = 10 50 26.9, Dec = -12 50 26.0.
  • The "Simulating an ACIS Spectrum for an On-axis Point Source, Including Pileup" thread gives detailed instructions on how to simulate an ACIS spectrum for proposal purposes. See also the Sherpa FAKE_PHA thread.

3. Estimate Count Rate and Perform Spectral Simulations

In this section, we estimate the total count rate expected from SS2b153 in both ACIS-S and ACIS-I. We will then perform simple spectral simulations to decide the following:
  • Which detector (ACIS-I or ACIS-S) is best for determining abundances. (Note that the result of this calculation is only one factor we will consider when selecting which CCDs to use; this is discussed further below.)
  • How many counts are necessary to constrain the Fe abundance to better than 10% in a single extraction region. The number of counts required from each region plus the number of regions we divide our group into will set the overall exposure time.

Estimate Total Count Rate

Use COLDEN to estimate the galactic line-of-sight NH to SS2b153. Here we use the coordinates RA = 10 50 26.9, Dec = -12 50 26.0 (J2000). The resulting NH returned by COLDEN is 4.50E20.

SS2b153 has a ROSAT PSPC count rate of about 0.28 counts/sec. We assume an optically thin plasma model, temperature 1 keV = 1.16E7 K. To get the predicted ACIS-S count-rate, run PIMMS with the following inputs.

Input: Count Rate
Input Mission -- ROSAT
Input Detector/Grating/Filter -- PSPC/None/OPEN
Energy Input -- default to default keV
Output: Count Rate
Output Mission -- Chandra [current cycle]
Output Detector/Grating/Filter -- ACIS-S/None/None
Output Energy -- default to default keV
Model -- Plasma/Raymond Smith
NH -- 4.5E20
Abundance -- 0.2
Log T -- 7.05 (0.9669 keV)
Count Rate -- 0.28

PIMMS gives an estimated ACIS-S count rate of 0.3569 counts/sec.

Repeat the command with ''Output Detector/Grating/Filter'' set to ''ACIS-I/None/None'' to get an estimated ACIS-I count rate of 0.2049 counts/sec.

ACIS Simulations

In this section we simulate an ACIS spectrum with WebSpec. Sherpa can also be used (see the Sherpa FAKE_PHA thread). The following steps will create a correctly normalized spectrum and determine the uncertainty in the Fe abundance:

  • Simulate a spectrum with the normalization set to 1.0.
  • Adjust the normalzation so that the count rate of the simulated spectrum is equal to the predicted (PIMMS) count rate
  • Determine the uncertainty in the Fe abundance in the simulated spectrum. It may be necessary to repeat this step with different exposure times until the uncertainty in the Fe abundance is within desired limits.

Go to the WebSpec homepage and fill out the form as follows:

  • Choose Mission/Instrument Chandra ACIS-S 2' off axis (away from ACIS-I). Our initial choice of ACIS-S (rather than ACIS-I) is motivated by the fact that PIMMS predicts a higher countrate with ACIS-S. We choose a response that is 2 arcmins off axis because our source is extended. There are two possible responses that depend on whether a source is displaced toward or away from ACIS-I. Our cluster is extended over an area that is covered by both responses. We therefore determine an exposure time that gives enough counts in the least sensitive part of the chip.
  • Do NOT check the pileup box. Pile-up can be ignored because the source is extended.
  • Check the box to apply photoelectric absorption.
  • We choose a thermal model appropriate for hot gas. In this case, we need "Mekal (abund)". The plasma code for this model is based on the calculations of Mewe, Kaastra and Liedahl. This version allows individual abundances to be set.
  • Click "I'm ready to set parameters".

Set the parameters as follows:

  • Set the exposure to be 20000 seconds. This is an initial guess. We may need to refine the exposure time to obtain enough counts to measure the Fe abundance to 10%
  • Set the galactic hydrogen column to be 0.045. This parameter should be frozen and "Compute the error" not checked.
  • Here, the "redshift" parameter of the photoelectric absoption model refers to the redshift of the absorbing column. In this case, the absorption is within the galaxy so set this to zero.
  • Set the hydrogen density to be 0.001. This is a value appropriate for the outer regions of a group. (The value of the hydrogen density does not strongly affect this calculation.)
  • Set all abundances to 30% solar (i.e 0.3).
  • Set the redshift parameter for the plasma model to be 0.0153, the redshift of the cluster.
  • All other parameters not specified can retain their default values, including normalization = 1.0. Click "Do the Simulation".

The results page gives the fake spectrum. Scroll down page to find the count rate in the fake spectrum (76.7887 cts/sec). The predicted ACIS-S count rate from PIMMS was 0.3569 counts/sec. We therefore need to set the normalization to be 0.3569/76.7887 = 4.64E-3. Navigate back to return to the inputs page. Set the normalization = 4.64E-3.

In addition to resetting the normalization, we need to "thaw" the Fe abundance, i.e. uncheck "freeze parameter" and check "Compute the error". When this new simulation is run, XSPEC will create another fake spectrum and calculate the uncertainties on the Fe abundance. To be more precise, XSPEC will create a fake spectrum by convolving the current model with the response matrix, adding noise appropriate to the integration time. It will then attempt to fit the model to the simulated data, calculating confidence limits for "free" parameters. In this case, the free parameters are temperature, normalization and Fe abundance.

We get the following results:

This is a (Photoelectric Absorption)((Mekal with all abundances specified)) simulation of a CHANDRA.ACISS.off2away observation of 20000 seconds. The actual model expression used by xspec was zphabs*vmekal .

This model resulted in a C-statistic of  and a count rate of 0. cts/s over the fitted energy range.

The best fit parameters and errors for this simulated data set are:

For the Photoelectric Absorption component:
	Hydrogen Column 	0.045 	10^22 atoms/cm^2
	Redshift 	0.0 	
For the Mekal with all abundances specified component:
	Plasma Temperature 	1.00005 +0.024 -0.026	keV
	Hydrogen Density 	0.001 cm^
	He Abundance 	0.3 	
	C Abundance 	0.3 	
	N Abundance 	0.3 	
	O Abundance 	0.3 	
	Ne Abundance 	0.3 	
	Na Abundance 	0.3 	
	Mg Abundance 	0.3 	
	Al Abundance 	0.3 	
	Si Abundance 	0.3 	
	S Abundance 	0.3 	
	Ar Abundance 	0.3 	
	Ca Abundance 	0.3 	
	Fe Abundance 	0.291199 	+0.0282 -0.0265
	Ni Abundance 	0.3 	
	Redshift 	0.0153 	
	Switch 	        1 		0
	Normalization 	0.00425465 	

The measured Fe abundances will be 0.291 +/-0.027 solar (averaging upper and lower bounds). This simulation shows that with about 0.3569*20000 = 7,138 counts, it is possible to constrain the Fe abundance within about 10% with ACIS-S.

The ACIS-I predicted count rate is somewhat lower than ACIS-S (0.2049 vs. 0.3569 counts/sec). Therefore a longer exposure time is required to obtain the same number of counts. Setting an initial guess at the exposure time to be (0.3569/0.2049)*20000 = 34,836 seconds, we can repeat the above simulation to find that the Fe abunance can also be constrained to within about 10% with ACIS-I.

Determine Total Exposure Time

From the above simulations we know that we need about 7138 counts to get the required constraint on the Fe abundance in a single region. Remember that 7138 is the number of counts from the entire cluster in a 20ks observation. Our goal is to measure abundances in 6 regions, which can be of varying size. Since the spatial distribution isn't well known, we assume that each region will produce the same count rate and that we will need a total of 21414 counts from the entire group to determine the Fe abundances in the 6 regions. This implies a total ACIS-S exposure time of approximately 120 ks and an ACIS-I exposure time of approximately 204 ks.

What About Background?

The above simulations do not include the effect of background which, in this case, is dominated by the diffuse X-ray background. The POG Table 6.9 gives background rates for ACIS-S positioned at the aimpoint. Assuming we are using chip S3 with an exposure time of 120 ks and the default energy range of 0.3-10 keV, the number of background counts is 1.0 counts/sec/chip * 120,000 sec = 120000 counts. The total background depends on the size of the region that the counts are extracted from. The chip is 1024*1024 pixels. If our smallest region is 100x100 pixels, then the total number of background counts in this region 100^2/1024^2 * 120000, or about 1144 counts.

The background counts must be subtracted from the source counts; the uncertainty in the background is only 17 counts. If we extract counts from a region 10 times larger, then the uncertainly in the background rises to about 53 counts. This is still small compared to our 7138 source counts. These are ''worst case'' estimates, since (1) the S3 chip has a high background relative to other chips and (2) if we use VF telemetry format we can further reduce the background by using the "clean55" algorithm developed by Alexey Vikhlinin to eliminate charged particles. This is now incorporated into the CIAO tool acis_process_events (see the "ACIS VFAINT Background Cleaning" why topic for more information). The POG Section 6.15.2 provides a detailed description of ACIS telemetry formats. For this observation, we do not expect background to be a problem in our analysis.

Summary of Simulations

We have performed a simple simulation to determine the number of counts required to constrain the Fe abundance to about 10%. We find that about 7138 counts are required to do this in both the ACIS-I and ACIS-S detectors. We therefore conclude that for the purpose of measuring Fe abundance either array will do. If we choose ACIS-I we will need a higher total exposure time. The cosmic X-ray background will not impact our analysis if we have a minimum of 7138 extracted counts.

4. Select Instrument Configuration


Both arrays have advantages and disadvantages for this observation. As described above, ACIS-I or ACIS-S could be used to constrain the Fe abundance within our goal, although the exposure time for ACIS-I will be longer. ACIS-I has a larger field of view and lower background than ACIS-S, but suffers from the disadvantage of degraded CTI (see the POG Section 6.7). The main result of degraded CTI is that the spectral resolution is position dependent, which can complicate analysis. Since the higher background in S3 should not impact our analysis, we choose ACIS-S to avoid CTI complications.

Exposure Mode, Telemetry Format, Choice of Chips and Optional Chips

There are only two ACIS modes available: Timed Exposure (TE) and Continuous Clocking (CC). TE mode is appropriate for imaging observations. There are three telemetry formats available for TE mode: Faint, Graded, and Very Faint. The Very Faint format provides pixel values in a 5x5 island around the event position; less information is telemetered in the other two formats. The extra information provided by the Very Faint format can be used to reduce the number of background photons by using a screening algorithm. Since we are extracting spectra from large areas of the chip, the Very Faint format is the best choice.

The chips most commonly used for ACIS-S imaging are S1-S4, I2, and I3. In order to reduce the probability of telemetry saturation, we choose to turn off chips furthest from the source on S3 (i.e. turn off S1, S4 and I2). The telemetry saturation limit is less for Very Faint than for the Faint and Graded formats. As long as we do not exceed the telemetry limit, we would prefer to use the Very Faint format because of the detailed information provided for each event.

The telemetry saturation limit for Very Faint mode is 68.8 cts/sec. From the Table 6.10, the background rates for the on chips are (at 10 keV): FI chips: 6.0 cts/s and BI chips: 5.5 cts/s. If S2, S3 and I3 are turned on we have two FI chips (S2 and I3) and one BI chip (S3). So the total background rate is 17.5 cts/s. If we add the source count rate of 0.3569 cts/s to the background rate, a good estimate for the maximum total count rate is 17.8569 cts/sec. This is significantly less than the saturation limit, so we can safely choose VF format.

Starting in Cycle 10, observers are being asked to identify "optional" chips. These chips can be turned off (without warning!) should the ACIS Power Supply and Mechanism Controller be in danger of overheating (see the ACIS chapter of the POG). We choose to designate I3 as optional because this chip is farthest from the aimpoint and will have the worst PSF.

Offset Pointing

The default ACIS-S aimpoint is close to the boundary between chips S3 and S2. This will put the peak of the group X-ray emission at the point of highest spatial resolution. This is shown in Figure 1. However, it will also put the outer edges of the group emission onto S2. We choose to move the peak X-ray emission into the CENTER of S3, thus minimizing overlap onto other chips. For SS2b153 this corresponds to a Y offset of -2 arcmins, as shown in Figure 2. This has the slight disadvantage that the X-ray core will be moved away from the area of highest spatial resolution. But since we will be extracting spectra from a number of resolution elements, this is not a major concern.

[The field of view is displayed in ds9. Regions are overlaid to indicate the chip edges and the default aimpoint.]
[Print media version: The field of view is displayed in ds9. Regions are overlaid to indicate the chip edges and the default aimpoint.]

Figure 1: ObsVis: the default ACIS-S aimpoint

The default aimpoint is close to the boundary between chips S3 and S2.

5. Complete Target Form

For general instructions on how to submit a proposal, please see the "Using RPS to Prepare and Submit a Chandra Proposal" thread. 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 as required..

  • Target Name -- SS2b153
  • RA -- 10 50 26.9
  • Dec. -- -12 50 26.0
  • Y Detector Offset (arcmin) -- -2
  • Total Observing Time (ksec) -- 120
  • Count Rate -- 0.3569
  • Total Field Count rate -- 17.8569
  • Is target an extended source? -- Y
  • Exposure Mode -- TE
  • Event Telemetry Format -- Very Faint
  • CCDs On -- chips S2, S3 should be checked Y
  • Optional Chips -- I3 designated Off1

6. Thread Summary

In this thread, we design an observation to measure the spatial distribution of the Fe abundance in the galaxy group SS2b153. The goal is to measure the Fe abundance to about 10% in 6 spatial regions. We perform simple simulations to show that a total of 7138 counts in a single spatial region will constrain the Fe abundance to the required accuracy on either ACIS-I or ACIS-S. Emission from the X-ray background will not impact our analysis.

We choose to use ACIS-S in imaging mode for this observation to avoid problems due to CTI on the ACIS-I array. The total exposure time required to measure the Fe abundance in 6 regions is 120 ks. We choose TE mode and Very Faint telemetry format. Very Faint format has the advantage that it can be used to eliminate background photons; however, using this format can also result in telemetry saturation. We choose to turn off S1, S4, and I2 to lower the probability of telemetry saturation. Finally, we offset the pointing direction by Y = -2 arcmin to center the group emission in the S3 chip.

Last modified: 18 January 2017
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