Example of Proposal Planning - Observe an On-axis Point Source with ACIS

Check for Previous or Planned Observations by Chandra

If the target has already been observed, additional observations must be justified (variability, longer exposure, etc.). To determine if any previous observations exist, you can use WebChaSeR, available at http://cda.harvard.edu/chaser.

Enter the RA and Dec of the object. The source 1RXS J152417.7-200856 is at RA: 15 24 17.70 and Dec: -20 08 56.5. Verify that these boxes are checked: Archived, Observed, Scheduled, and Unobserved.

The screen is shown in Figure 1.

Click the Search button. There are no observations returned, so there are no other Chandra observations to consider in this proposal.

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Target Visibility and Bright Sources

ObsVis is a tool to aid in observation planning which allows the inspection of sky images with overlaid instrument fields-of-view (FoV). ObsVis interfaces with the SAOimage DS9 image display tool.

ObsVis is available as part of the CIAO software or as a standalone software package. If you included ObsVis when you downloaded and installed CIAO, start CIAO and type "obsvis". If you do not have CIAO, download the standalone ObsVis.

Detailed information, examples and screen shots can be found on the ObsVis webpage

To view the sky in the region of 1RXS J152417.7-200856:

• start ciao (if you have installed it)
• run obsvis on the command line
• enter the coordinates into the "Target Coordinate" box Figure 2
• turn on desired chips using the toggle buttons on the right hand side of the screen.
• click the "New FOV" button

The Digital Sky Survey image for the region around 1RXS J152417.7-200856 will appear in a DS9 window. The resulting sky image with chips I0-I3 is in Figure 3. Turn on desired chips using the toggle buttons on the right hand side of the screen (see Optional CCDs for suggestions on optional CCDs).The instrument FOVs can be adjusted from the obsvis window or by directly manipulating DS9.

To check for X-ray sources, navigate "Analysis --> Catalogs --> High Energy --> Second ROSAT PSPC" within DS9. No sources should be within the ACIS FOV. There are no other strong sources in the field of view, as shown in Figure 3, so a roll constraint is not required.

Planning an observation with Chandra is complicated by the fact that the maximum exposure time for a given target depends on the solar pitch angle (i.e. the angle between the viewing direction and the direction to the sun).

Pitch angle restrictions are described in detail in the POG. The pitch angle of a celestial source changes throughout the year. Therefore, the maximum exposure time might also change througout the Cycle. There may be times when it is impossible to schedule a particular target. Proposers who require time constraints should use the PRoVis tool to check that the constraints are feasible. The exposure time for this target is very short and the observation is not time constrained. Therefore pitch angle restrictions are not important for this proposal.

Using PIMMS

PIMMS is appropriate for sources with simple spectra.

The web version of PIMMS is at: http://cxc.harvard.edu/toolkit/pimms.jsp . Select the following values for the PIMMS Input fields.

Select the following values for the PIMMS Output fields.

We have chosen to use ACIS-S. This is because we are working with a ROSAT source, and ROSAT has a lower energy bandpass than Chandra. ACIS-S includes two Backside-Illuminated (BI) chips, which provide better low-energy response compared to the Frontside-Illuminated (FI) chips. This is illustrated in chapter 6 of the POG.

To estimate a Chandra count rate from a ROSAT count rate, it is necessary to assume a model for the source's spectral energy distibution. Since this is unknown in the Chandra bandpass, we will calculate the count rate for two common models, a Power Law model with photon index = 1.0, and a 5 keV Bremsstrahlung model.

For the Power Law model, select the following fields.

Select these fields at the bottom of the screen.

Click CALCULATE. Shown in Figure 6 are the PIMMS results for the Power Law model. PIMMS predicts an ACIS-S count rate of 0.3265 cts/sec with 35% pileup. PIMMS also calculates a background count rate of 2.4e-5 cts/sec.

For the Bremsstrahlung model, use the same values for the Input/Output fields and adjust the model parameters.

For this model, PIMMS predicts an ACIS-S count rate of 0.2160 cts/sec with 24% pileup, and a background count rate of 2.4e-5 cts/sec.

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ACIS Pileup

The PIMMS results show that 1RXS J152417.7-200856 is likely to be affected by ACIS pileup. Pileup results when the count rate is so high that two or more photons are detected as a single event. Pileup and pileup mitigation are discussed in the ACIS chapter of the POG. Since 1RXS J152417.7-200856 is expected to be a point source, selecting a subarray is a good approach for reducing pileup. When a subarray is selected only part of the chip is read out, allowing for a shorter integration time.

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. The shorter frame time reduces the probablity that two or more photons will hit the same detect region in a given frame interval.

What is the pileup fraction for a 1/8 subarray using a single chip? Using the Bremsstrahlung model, enter 0.4 in the PIMMS Frame Time field and click CALCULATE. New PIMMS results in Figure 7 shows that the pileup fraction is now reduced to 3%.

A pileup value of 3% is acceptable for many scientific purposes. If one of the science objectives is to carry out a detailed spectral fit, you might need to reduce pileup still further, for example, by using Continuous Clocking mode. It is desirable for spectral analysis to minimize pileup because some low-energy piled photons can be erroneously cast to higher energies, which distorts the spectrum. In this example we are assuming that 3% pileup is acceptable.