The Crab cal simulation on SwiMM (1)

M.Suzuki 03/Nov/2004

(1) Motivation

Swift mass model (SwiMM) is the full simulator of Swift.

This simulator have an advantage to be able to modify BAT detector response matrix (batdrmgen) before Swift/BAT calibration phase. Also, after the launch, it is important for us to make use of the simulation data for a understanding of the calibration data. Because the simulation become successful to make the almost same Swift geometry and the detector response. In each case, this full simulator is the powerful tool to understand the detector response.

So, we simulated the on-axis Crab calibration observation on the SwiMM. Then we have made the way to get the simulation data into the right format for the BAT ground ftools, and fitted it on Xspec.

(2) Data Process Overview

1.Output Simulated Fits Data
2.FilterSwiMM (Fits Convert Program) 
3.batmaskwtevt 
4.batbinevt 
5.batdrmgen

1. Simulation

The simulation input some parameters (i.g. an input energy, a number of photons, a shape of photon beam, etc.) into SwiMM.

In this Crab cal simulation, the parallel rays of photon are irradiated above Swift, the spectrum is the same power-law spectrum of the Crab (Photon Index = 2.10). And the generated photons are proper for the exposure of 1k sec. (See About Swift Mass Model)

2. FilterSwiMM

This program can convert the simulation data into the BAT event fits file.

3. batmaskwtevt

Including the MASK_WEIGHT value to the new event file.

[excerpt from batmaskwtevt help page]
Applying mask weighting is equivalent to background subtraction. Detectors
which are fully shadowed are assigned a -1 weight, and fully illuminated
detectors a +1 weight, and partially illuminated detectors are assigned an
prorated value. Thus, a weighted sum of the counts will automatically subtract
the background. (In detail, the weight values are moreover divide by the number 
of CZTs and cosine theta, now. The theta is an incident angle to the detector.)

4.batbinevt

Generate the PHA file which is rebinned to the 80 energy channels.

5.batdrmgen

Make the response file from the PHA file.

(3) Results

Fitting Result

Figure.1:fitting result

The fitting result looks to match the powerlaw model in almost whole energy range.

This is because we apply for the follow effects to the simulation data.

• CZT charge trapping effect
• Electrical noise
• Mask modulation

In other words, the model generated by batdrmgen is much better. It is successful to reproduce the detector response with high speed processing and the simple calculating formulas.

---------------------------------------------------------------------------
Model:  powerlaw<1>
Model Fit Model Component  Parameter  Unit     Value
par   par comp
  1    1    1   powerlaw   PhoIndex            2.17470     +/-  0.311339E-02
  2    2    1   powerlaw   norm                11.3431     +/-  0.116983
---------------------------------------------------------------------------

Chi-Squared =      319.7562     using    76 PHA bins.
Reduced chi-squared =      4.321030     for     74 degrees of freedom

I calculate the output flux with the above phoIndex, normalization and the same energy range. In the result, as follows,

Input  flux:  1.49 [photons/cm2/sec] 
Output flux:  1.46 [photons/cm2/sec] 

The fluxes are much the same.

DPI image

Figure.2:DPI image

This image pdf file is here. [pdf(123KB)]

(4) Summary

• We simulated the Crab on-axis spectrum (1k sec) on SwiMM.
• We make the analysis method which processes the simulation data by the BAT ground software.

(5) Next step

We will simulate the 7x5 grid observations in the on-orbit Crab calibration by SwiMM.