Example: XSPEC Fit

bash-3.2$ xspec

XSPEC version: 12.6.0z
Build Date/Time: Thu Feb 3 13:48:36 2011

XSPEC12>data GRB080916C.pha
***Warning: No TLMIN keyword value for response matrix FCHAN column.
Will assume TLMIN = 1.

1 spectrum in use

Spectral Data File: GRB080916C.pha Spectrum 1
Net count rate (cts/s) for Spectrum:1 2.342e-01 +/- 2.197e-02
Assigned to Data Group 1 and Plot Group 1
Noticed Channels: 1-30
Telescope: GLAST Instrument: LAT Channel Type: PI
Exposure Time: 555 sec
Using Response (RMF) File GRB080916C.rsp for Source 1

XSPEC12>response GRB080916C.rsp
Response successfully loaded.
XSPEC12>model powerlaw

Input parameter value, delta, min, bot, top, and max values for ...
1 0.01 -3 -2 9 10
1:powerlaw:PhoIndex>
1 0.01 0 0 1e+24 1e+24
2:powerlaw:norm>

========================================================================
Model powerlaw<1> Source No.: 1 Active/On
Model Model Component Parameter Unit Value
par comp
1 1 powerlaw PhoIndex 1.00000 +/- 0.0
2 1 powerlaw norm 1.00000 +/- 0.0
________________________________________________________________________

Chi-Squared = 7.473424e+12 using 30 PHA bins.
Reduced chi-squared = 2.669080e+11 for 28 degrees of freedom
Null hypothesis probability = 0.000000e+00
Current data and model not fit yet.
XSPEC12>fit

Chi-Squared Lvl Par # 1 2
120.045 -1 1.05564 1.77028e-06
118.301 -1 1.11316 5.31205e-06
115.983 -1 1.1698 1.60958e-05
113.124 -1 1.22375 4.66069e-05
109.86 -1 1.27429 0.000124845
106.324 -1 1.32154 0.000306229
102.572 -1 1.36598 0.000690929
98.6134 -1 1.40811 0.00144643
94.4765 -1 1.44831 0.00282752
90.2912 -1 1.48683 0.00517865
Number of trials exceeded: continue fitting?
86.3611 -1 1.52381 0.00891145
83.1066 -1 1.55924 0.0144702
79.5541 -1 1.59294 0.0240463
75.5373 -1 1.62449 0.0401131
71.5122 -1 1.65392 0.0651534
67.6934 -1 1.68146 0.102268
64.1442 -1 1.70729 0.155275
60.8704 -1 1.73161 0.228751
57.8604 -1 1.75453 0.32802
55.0982 -1 1.77619 0.459118
Number of trials exceeded: continue fitting?
52.5671 -1 1.79666 0.628736
50.2504 -1 1.81605 0.844153
48.1315 -1 1.83444 1.11314
46.1949 -1 1.85188 1.44387
44.4253 -1 1.86844 1.84479
42.8086 -1 1.88418 2.32453
41.3315 -1 1.89916 2.89177
39.9816 -1 1.91342 3.55514
38.7476 -1 1.92701 4.3231
37.6191 -1 1.93997 5.20383
Number of trials exceeded: continue fitting?
36.5863 -1 1.95233 6.20516
35.6408 -1 1.96413 7.33446
34.7744 -1 1.97541 8.59859
33.9799 -1 1.98619 10.0038
33.251 -1 1.99651 11.5559
32.5815 -1 2.00639 13.2596
31.9663 -1 2.01584 15.1195
31.4004 -1 2.02491 17.1389
30.8794 -1 2.0336 19.3209
30.3994 -1 2.04194 21.6675
Number of trials exceeded: continue fitting?
29.9568 -1 2.04995 24.1802
29.5484 -1 2.05764 26.8596
29.1711 -1 2.06503 29.706
28.8224 -1 2.07213 32.7186
28.4999 -1 2.07896 35.8963
28.2013 -1 2.08553 39.2372
27.9247 -1 2.09186 42.739
27.6682 -1 2.09795 46.3987
27.4303 -1 2.10382 50.2131
27.2093 -1 2.10947 54.1784
Number of trials exceeded: continue fitting?
27.0041 -1 2.11492 58.2902
26.8133 -1 2.12018 62.544
26.6358 -1 2.12525 66.935
26.5962 -2 2.16551 102.631
25.6113 -2 2.19477 153.727
24.7647 -2 2.21582 210.292
24.38 -2 2.23179 265.035
24.205 -2 2.2442 315.251
24.1154 -2 2.25393 359.968
24.0656 -2 2.26157 398.872
Number of trials exceeded: continue fitting?
24.0366 -2 2.26759 432.075
24.0193 -2 2.27234 459.973
24.0186 -3 2.28715 548.814
==============================
Variances and Principal Axes
1 2
6.0047E-05| -1.0000 0.0002
5.0230E+05| -0.0002 -1.0000
------------------------------

========================
Covariance Matrix
1 2
1.424e-02 8.440e+01
8.440e+01 5.023e+05
------------------------

========================================================================
Model powerlaw<1> Source No.: 1 Active/On
Model Model Component Parameter Unit Value
par comp
1 1 powerlaw PhoIndex 2.28715 +/- 0.119332
2 1 powerlaw norm 548.814 +/- 708.735
________________________________________________________________________

Chi-Squared = 24.02 using 30 PHA bins.
Reduced chi-squared = 0.8578 for 28 degrees of freedom
Null hypothesis probability = 6.805513e-01