XSPEC User's Guide

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4 allow significant new features to be added, and support the analysis of spectra from coded-mask instruments. 1.4 Acknowledgements This project would not have been possible without ignoring the advice of far more people than can be mentioned here. We would like to thank all our colleagues for their suggestions, bug reports, and (especially) source code. The GSFC X-ray astronomy group are particularly thanked for their patience exhibited while functioning as the beta test site for new releases. The initial development of XSPEC was funded by a Royal Society grant to Prof. Andy Fabian and subsequent development was partially funded by the European Space Agency's EXOSAT project and is now funded by the HEASARC at NASA/GSFC. 1.5 References Arnaud, K.A., 1996, Astronomical Data Analysis Software and Systems V, eds. G. Jacoby and J. Barnes,p17, ASP Conf. Series volume 101. Dorman, B., and Arnaud, K. A. 2001, Astronomical Data Analysis Software and Systems X eds. F.R. Harnden, Jr., F.A. Primini, and H. E. Payne, vol. 238, p. 415 Dorman, B., Arnaud, K. A., and Gordon, C. A. XSPEC12: Object-Oriented X-Ray Data Analysis, AAS HEAD meeting No. 35, #22.10
5 2. Spectral Fitting and XSPEC 2.1 Introduction This chapter comprises a brief description of the basics of spectral fitting, their application in XSPEC, and some helpful hints on how to approach particular problems. We then provide more details on the way XSPEC provides flexibility in its approach to the minimization problem. We also describe the data formats accepted. 2.2 The Basics of Spectral Fitting Although we use a spectrometer to measure the spectrum of a source, what the spectrometer obtains is not the actual spectrum, but rather photon counts (C) within specific instrument channels, (I). This observed spectrum is related to the actual spectrum of the source (f(E)) by: CI ( ) f( ERIEdE ) ( , ) 0 Where R(I,E) is the instrumental response and is proportional to the probability that an incoming photon of energy E will be detected in channel I. Ideally, then, we would like to determine the actual spectrum of a source, f(E), by inverting this equation, thus deriving f(E) for a given set of C(I). Regrettably, this is not possible in general, as such inversions tend to be non-unique and unstable to small changes in C(I). (For examples of attempts to circumvent these problems see Blissett & Cruise 1979; Kahn & Blissett 1980; Loredo & Epstein 1989). The usual alternative is to choose a model spectrum, f(E), that can be described in terms of a few parameters (i.e., f(E,p1,p2,...)), and match, or “fit” it to the data obtained by the spectrometer. For each f(E), a predicted count spectrum (C p (I)) is calculated and compared to the observed data (C(I)). Then a “fit statistic'” is computed from the comparison and used to judge whether the model spectrum “fits” the data obtained by the spectrometer. The model parameters then are varied to find the parameter values that give the most desirable fit statistic. These values are referred to as the best-fit parameters. The model spectrum, f b (E), made up of the best-fit parameters is considered to be the best-fit model. 2 The most common fit statistic in use for determining the “best-fit” model is , defined as follows: 2 2 2 ( CI ( )- Cp( I)) /( ( I)) where σ(I) is the (generally unknown) error for channel I (e.g., if C(I) are counts then σ(I) is usually estimated by CI ( ); see e.g. Wheaton et.al. 1995 for other possibilities). Once a “best-fit” model is obtained, one must ask two questions: 1. How confident can one be that the observed C(I) can have been produced by the best-fit model f b (E) The answer to this question is known as the “goodness-of-fit”
Page 1 and 2: An X-Ray Spectral Fitting Package U
Page 3 and 4: iii 3.9.1 What to do when you have
Page 5 and 6: v 5.7.3 iplot: make a plot, and lea
Page 7 and 8: vii 6.2.67 step: step function conv
Page 9 and 10: Updates to the manual July 2009 (v1
Page 11 and 12: 1 1. XSPEC XSPEC is a command-drive
Page 13: 3 1.2 How to find out more informat
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Page 19 and 20: 9 may be ignored. This can be chang
Page 21 and 22: 11 • Auxiliary Responses: callib/
Page 23 and 24: 13 Bevington, P.R., 2002, 3 rd Edit
Page 25 and 26: 15 Model commands define and manipu
Page 27 and 28: 17 3.7 Data Commands XSPEC is desig
Page 29 and 30: 19 3.8.1 Models with multiple respo
Page 31 and 32: 21 additive model components on dat
Page 33 and 34: 23 #LOCAL_MODEL_DIRECTORY: /path/to
Page 35 and 36: 25 # USE_NUMERICAL_DIFFERENTIATION:
Page 37 and 38: 27 %xspec XSPEC12>data s54405.pha 1
Page 39 and 40: 29 cutoffpl disk diskbb diskline di
Page 41 and 42: 31 Figure B: The result of the comm
Page 43 and 44: 33 Figure C: The result of the comm
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Page 47 and 48: 37 XSPEC12>mo pha(br) Model: phabs[
Page 49 and 50: 39 XSPEC12>fit ... ----------------
Page 51 and 52: 41 XSPEC12>thaw 4 Number of variabl
Page 53 and 54: 43 Null hypothesis probability = 0.
Page 55 and 56: 45 mission; however, it has now bee
Page 57 and 58: 47 1 1 1 phabs nH 10^22 0.2574 +/-
Page 59 and 60: 49 Here, the first group makes up t
Page 61 and 62: 51 --------------------------------
Page 63 and 64: 53 using response (RMF) file... s0c
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55 The first thing we'll do is chan
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57 selecting the "PHA" output optio
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59 ________________________________
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61 effects of the coded-mask. The l
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63 particular background model empl
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65 One might then make a second pas
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67 5. XSPEC commands 5.1 Summary of
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69 Command Category Description goo
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71 Command Category Description rth
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73 //This setting essentially disab
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75 If no is given, then the file s
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77 XSPEC12> show rparameters // All
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79 8. search failed in +ve directio
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81 solab stat statmethod steppar st
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83 5.4 Data Commands 5.4.1 arf: cha
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85 is equivalent to no correction f
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87 For files with multiple spectra
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89 which assumes that at least two
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91 number of fakeit spectra produce
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93 confusion can arise when the row
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95 If integers are given for the ch
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97 =:: [[:]] ..., and is the name
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99 Either format is readable when u
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101 For high-chatter settings, addi
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103 XSPEC12>chain temperature .8 //
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105 5.5.5 freeze: set parameters as
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107 margin (for 1-D and 2-D distrib
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109 where =:: [sourceNum:] | -- T
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111 where n is the component number
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113 is range of positions in the m
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115 Detector channel energies 5.0 -
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117 Similarly, an array extension c
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119 the energy values are two separ
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121 XSPEC commands for editing/view
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123 // Spectrum 3 response will ret
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125 Syntax: lmod [directory] As fo
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127 5.6.14 mdefine: Define a simple
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129 ***Warning: bb is a pre-defined
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131 makes the model named active (
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133 Applying multiple models: Assum
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135 (XSPEC will reject attempts to
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137 5.7 Plot Commands 5.7.1 cpd: se
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139 This command works like the plo
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141 eemodel Plot the current incide
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143 area, noarea After setplot area
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145 /KRM3 /TK4100 /VT125DEC /XDISP
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147 poiss-3 is the arithmetic mean
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149 Mg 3.80e-05 3.80e-05 3.95e-05 3
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151 XSPEC12.0 includes version94.1
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XSPEC Models 153 5.8.6 xset: set va
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XSPEC Models 155 WMAP. SUZPSF-MIXFA
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XSPEC Models 157 5.9 Tcl Scripts Th
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XSPEC Models 159 5.9.5 writefits: w
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XSPEC Models 161 6. XSPEC V12 Model
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XSPEC Models 163 Model Description
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XSPEC Models 165 Model Description
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XSPEC Models 167 Thermal broadening
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XSPEC Models 169 The bapec model us
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XSPEC Models 171 The zbbody variant
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XSPEC Models 173 par4 E c , the e-f
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XSPEC Models 175 par2 par3 Energy s
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XSPEC Models 177 hydrogen density (
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XSPEC Models 179 par2 par3 par4 par
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XSPEC Models 181 The region of para
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XSPEC Models 183 6.2.19 compTT: Com
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XSPEC Models 185 6.2.23 Diskir: Irr
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XSPEC Models 187 6.2.27 diskpbb: ac
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XSPEC Models 189 Borkowski, Lyerly
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XSPEC Models 191 The vgnei variant
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XSPEC Models 193 par2 second powe
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XSPEC Models 195 6.2.38 kerrdisk: a
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XSPEC Models 197 elements. Abundanc
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XSPEC Models 199 par1 plasma temper
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XSPEC Models 201 6.2.45 nei, vnei:
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XSPEC Models 203 par4 Lower limit o
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XSPEC Models 205 6.2.48 nsagrav: NS
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XSPEC Models 207 par1 par2 par3 A =
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XSPEC Models 209 6.2.53 pegpwrlw: p
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XSPEC Models 211 6.2.56 plcabs: pow
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XSPEC Models 213 6.2.59 pshock, vps
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XSPEC Models 215 par1 par2-par13 pa
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XSPEC Models 217 1.1 but uses APED
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XSPEC Models 221 6.2.66 sresc: sync
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XSPEC Models 223 par6 par7 Number o
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XSPEC Models 225 XSPEC12>model etab
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XSPEC Models 227 source frame. Futu
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XSPEC Models 229 par1= n H equivale
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XSPEC Models 231 par1= E c par2= f
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XSPEC Models 233 6.3.26 tbabs, ztba
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XSPEC Models 235 The zvarabs varian
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XSPEC Models 237 varies between 1 a
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XSPEC Models 239 energy ranges of t
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XSPEC Models 241 par2= max Maximum
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XSPEC Models 243 where: par1= par2
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XSPEC Models 247 If multiple observ
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XSPEC Models 249 6.6.5 xmmpsf: xmm
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A-251 Appendix A Appendices The Use
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A-253 120 XSPEC12> This would produ
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A-255 After being executed, many tc
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A-257 in order to start a C-shell.
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A-259 Note that the model string, w
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A-261 Appendix B Fitting with few c
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A-263 1 C y0( x ) i y0( xi) y0(
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A-265 Appendix C Adding models to X
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A-267 first should be set to 1 if m
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A-269 const string& init Initializa
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Appendix D Overview of PLT A-271 As
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A-273 Freeze GAp Grid Hardcopy HElp
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A-275
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A-277 The directory ftools/callib/s
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A-279 csmgh0 Get the cosmology H0 s
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Appendix G Adding a Custom Chain Pr
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A-283 model parameter values. The o
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A-285 (and can be ignored if not re
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A-287 Additionally, we have withdra
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A-289 There are minor plotting
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