1: Introduction

The HXD covers the highest energy range of the Astro-E2 band-pass, from 10 to 600 keV. The detector is a collimated well-type phoswich array designed to achieve the lowest background, and therefore the highest sensitivity ever achieved, in this band.

In spite of the low detector background, the sensitivity of the HXD is dominated by the systematic error of the instrumental background estimates. We suggest you fully take into account this error in all simulations.

To restate: only objects brighter than the systematic error of the background estimate at the energy band in question are detectable.

The background systematic error will have an energy dependence as well as a time dependence. Although the calibration requirement for this error is 10% at 1 sigma, we will improve this value after launch by analyzing actual data. We quote an initial error of 10% as a realistic target to be achieved within the first year or two after launch.

Further improvements in the analysis method will enable us to re-analyze data obtained in the early phase with better accuracy.

The HXD team is continuously studying the background behavior to improve the systematic error. Details on our understanding and the current status will be posted in the future on the Astro-E2 web site.

2: Responses and Background Files

• hxd_pin_ao1.rsp: PIN response file
• hxd_pin_ao1_bgd.pha: PIN background file
• hxd_gso_ao1.rsp: GSO response file
• hxd_gso_bgd_ao1.pha: GSO background file

Please note that we do not have separate rmf and arf files for the HXD.

3: Simulation procedure of HXD data for Cycle 1

In the simulation using XSPEC, HXD team strongly suggest you check the effect of background systematic error (BGD-sys-err) as follows:

• Estimate your target flux and compare it with the HXD background level.
• Check at what energy band your target source flux is larger than the BGD-sys-err (which is currenly 10%-flat). This routine is performed by using the correction file utility in XSPEC.
• Ignore the energy band where the above conditions are not satisfied.
• Check the effect of BGD-sys-err by modifying the BGD value using "cornorm" command in XSPEC.

Following is the typical flux detectable given the backgroud systematic error as currently estimated. We will update this table according to our experience in orbit. Again we note that this value is not a limit to which the HXD can ultimately detect sources, but the limit to which analysis can be supported in the early days of the mission.

4: Simulating the background sys-error on XSPEC

In the following, we describe a method to handle the systematic error in the simulation using XSPEC, with several additional commands.

The method we suggest is to

• A: Simulate your source spectra with "fakeit" command
• B: Compare the HXD background with your source spectra
• C: Change the background itself according to the energy dependent BGD-sys-err upper limit value provided by the HXD team (currently +10% flat) and see at what energy bands the source is detectable.
• D: Ignore the energy bands in which the source is undetectable.

  If you wish do determine the parameter range (e.g. power law index) of the simulated data, you can then

• E: Change the background itself according to the upper and lower limits of the BGD-sys-err. The envelope of these errors is the total error range including both the statistical and systematic errors. You can also adjust the energy band to within which the systematic error caused by BGD-sys-err is minimized.

Note: The fitting procedure in step E above is not completely true in logical sense, but it does show what the effect of BGD-sys-err will be.

Note: We only provide the on-axis responses of PIN and GSO detectors, and typical background pha files. Off-axis response is not available yet. These response files are only to be used for Cycle 1 proposals. The format is somewhat different from the acutual response file.

We provide detailed step-by-step examples for a 100 mCrab source and a 10 mCrab source in hxd_ao1_ckbk_100mc.com and hxd_ao1_ckbk_10mc.com.