2. XSPEC: Part 1

Note: No background file is necessary to perform a spectrum analysis of a Gamma-ray burst; only a PHA file with the spectrum and an RSP file with the DRM are required. It is expected that all non-burst sources will produce less than 1 photon in the extraction region during the burst. The following details only the simplest example of fitting a spectrum with XSPEC; for further details, consult the XSPEC manual.

Prerequisites

You must have the two files necessary to analyze the burst spectrum with XSPEC:

• A PHA file with the spectrum (e.g., GRB080916C.pha)
• An RSP file with the DRM (e.g., GRB080916C.rsp)

Procedure

1. To start XSPEC, enter:

xspec

Note: If you are running on a Windows machine, use
X-Win32 as your X server.

Tip: To see a summary of xspec commands at any time,
enter:

help
?

2. Load in the data:

XSPEC12>data GRB080916C.pha

3. Load in the DRM:

XSPEC12>response GRB080916C.rsp

4. Load in a power law model

XSPEC12>model powerlaw

5. Choose the parameters' initial values.

Note: Power law indices tend to be near 2 for gamma-ray sources; in this tutorial, the default values were used.

6. Fit the data:

XSPEC12>fit

Tip: When queried "Number of trials exceeded: continue fitting?", hit the enter key until the fit has completed. (See Example: XSPEC Fit.)

7. Change plotting device (e.g., pgplot X-Window server /xw):

XSPEC12>cpd /xw

8. To look at the fit using energy as the x-axis, enter:

XSPEC12>setplot energy

XSPEC12>plot ldata

The following plot will be displayed. (Note: If you're running on a Windows machine, be sure you're using an X-Win32 terminal window.)

Note: You may be prompted for the lower and upper energy boundaries; these are provided in units of kev. Issues to consider are:

• The number of photons in the spectrum.
• The number of counts per bin (and therefore the number of bins).
• The energy range that should be fit.
• The fitting statistic

9. To perform rebinning, additional fits, and view additional plots, etc.; continue this session and refer to Run XSPEC: Part 2.