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XSPEC Commands N-S


Adjust one or more of the model parameters.

newpar   [<index range> [<param spec list> | <coupling list>]]

newpar   0


<param spec list> =:: <param value><delta><param range spec>
<param range spec> =:: <hard min><soft min><soft max><hard max>
<coupling list> =:: = <index> [expression]

The model parameters are accessed through their model parameter indices. For example, the first parameter of the first model component generally is model parameter 1, etc. The first command line argument, <index range>, gives the indices' parameters to be modified by the newpar command. The default value is the range from the previous invocation of newpar. The remaining arguments can be used to update the parameter specification. If the parameter specification is omitted from the command line, then the user is explicitly prompted for it. The first two arguments of the parameter specification are:

<param value>     !  the trial value of the parameter used initially in the fit;
<delta>     !  the ``step size'' used in the numerical determination of the derivatives used in the fitting process, when delta is set to zero, the parameter is not adjustable during the fit.

The four arguments of the range specification determine the range of acceptable values for the parameter. The soft limits should include the range of expected parameter behavior. Between the hard and soft limits, the parameter is made stiffer to adjustment by the minimization routine invoked by the fit command. The parameter is never allowed to have a value at or outside the hard limits. (One exception to this rule : the parameter can equal one of the hard limits if the soft limits have been placed outside the hard limit). A slash (/) will set all the six parameter specification values (value, delta, range specification) to the previous value (default for a new model, current value if the parameter has previously been set or fit). The sequence /* leaves all parameters unchanged. Coupling of parameters allows two or more parameters in a model to always have the same value or to be linearly relate. The coupled parameters are nevertheless free in the fit. An equal sign (``='') after the first index indicates that coupling of parameters is desired. The coupling expression is of the form

x   = {*} a {/ b }(+|-) c

The initial '*' (indicating a following multiplicative factor) is optional if the next token is a real value. That is, newpar x = a $\alpha$ produces a coupling such that parameter x is a * $\alpha$, whereas newpar x = a -$\alpha$ produces a constant offset between x and a (even if the intention was to give a sign to $\alpha.$

newpar 0 is a special form of the command, which simply prints out the current values of all the parameters.


The total number of model parameters for the example is four.

XSPEC>newpar 2 0.1     !  The value of the second parameter is set to 0.1.
XSPEC>newpar 3     !  The program will prompt for a specification for the 3rd parameter (comp gives the name of the corresponding model component
comp:param3>0.001,0     !  which has its value set to 0.001 and its delta set to zero, fixing it in later fits. The program now prompts for a specification for the 4th parameter,
comp:param4>21     !  which is set to 21. As there is no 5th parameter, the program displays a summary and returns to command level.
XSPEC>newpar ,,.001     !  The value of the delta of the 3rd parameter (which is the default index as it was the first parameter modified in the previous newpar invocation) is set to 0.001, allowing it to be adjusted during any fits.


The total number of parameters for this example is six.

XSPEC>newpar 4 = 1     !  The value of parameter 4 is set to the value of parameter 1. This has the consequence of model parameter 4 being frozen at the value of parameter 1 during subsequent fitting procedures. If model parameter 1 is a free parameter, then both parameters 1 and 4 change their values simultaneously in the fit procedure.
XSPEC>newpar 4 = 1 * 4/5 + 6.7     !  The value of parameter 4 is set to 4/5 = 0.8 times the value of parameter 1 plus 6.7
XSPEC>newpar 6 = 3 .1 - 9.5     !  The value of parameter 6 is set to 0.1 times the value of parameter 3 minus 9.5
XSPEC>newpar 5 = 2 + 5.     !  The value of parameter 5 is set to the value of parameter 2 plus 5.
XSPEC>newpar 8 = 1 / 4.6     !  parameter 8 is set to parameter 1 divided by 4.6
XSPEC>untie 6     !  Makes parameter 6 independent of parameter 3 and a free parameter.


Notice data channels. (See also ignore.)

notice   <range1> <range2> ... <rangeN>

where <rangeI> =:: {<data set range>:<channel range> | <channel range>}. If no <data set range> is given, then the previous range is used (the initial default range is file one (1) only).<data set range> =:: {<init data set> - <last data set> | <data set>}, where <init data set>, <last data set>, and <data set> are data set numbers, in the order they were input with the data command.<channel range> =:: {<initial channel> - <last channel> | <channel>}. If <channel range> are integers then channels will be used or if reals then energies (or wavelenths if setplot wave has been specified). If only the last channel is indicated, then a default value of 1 is used for the initial channel. Channels remain noticed until they are explicitly ignored with the ignore command. When a data set is replaced by another data set with a different number of PHA channels, all input channels automatically are noticed. When a channel is noticed, the response matrix must be recreated to include the new channel's response. Thus all the response files are re-read (this might take a while). The energy range covered by the current response matrix may thereby be increased.


Assume that 4 data sets have been read in, the first 2 having 100 channels and the last 2 having 50 channels. Assume also that channels 1-10 of all four data sets are ignored and that channels 80-100 of data sets 1 and 2 are ignored.

XSPEC>notice $\ast$$\ast$:1-10     !  The first 10 channels of all 4 data sets are noticed.
XSPEC>notice 80-$\ast$$\ast$     !  An attempt will be made to notice channels $\geq$80 in all 4 data sets (as that was the last data set range specified) but the result is that only channels 80-100 will be noticed for data sets 1 and 2, with no change for data sets 3 and 4 as they have no channels greater than 50.
XSPEC>notice 1:1-5     !  No channels are noticed, as these channels were noticed in the beginning.


Make one or more plots to the current plot device (see setplot device).

plot   <plot type> [<plot type>]

<plot type> is a keyword describing the various plots allowed. Any of the options counts, data, or ldata can be followed by a second option, which should be one of chisq, delchi, ratio, residuals, or none. If one of these second options is given, then a two-pane plot is drawn with the first option in the upper pane and the second option in the lower. (see also iplot)


Plot contributions to chisq. The contribution is plotted +ve or -ve depending on whether the residual is +ve or -ve.


Plot a two-dimensional fit-statistic contour plot of the last steppar run.

XSPEC>plot contour [<min fit stat> [<# levels> [<levels>]]]

where <min fit stat> is the minimum fit statistic relative to which the delta fit statistic is calculated, <# levels> is the number of contour levels to use and <levels> := <level1> ... <levelN> are the contour levels in the delta fit statistic. contour will plot the fit statistic grid calculated by the last steppar command (which should have gridded on two parameters). A small plus sign (``+'') will be drawn on the plot at the parameter values corresponding to the minimum found by the most recent fit. The fit statistic confidence contours are often drawn based on a relatively small grid (i.e., 5x5). To fully understand what these plots are telling you, it is useful to know a couple of points concerning how the software chooses the location of the contour lines. The contour plot is drawn based only on the information contained in the sample grid. For example, if the minimum fit statistic occurs when parameter 1 equal 2.25 and you use steppar 1 1.0 5.0 4, then the grid values closest to the minimum are 2.0 and 3.0. This could mean that there are no grid points where delta-fit statistic is less than your lowest level (which defaults to 1.0). As a result, the lowest contour will not be drawn. This effect can be minimized by always selecting a steppar range that causes XSPEC to step very close to the true minima. For the above example, using steppar 1 1.25 5.25 4, would have been a better selection. The location of a contour line between grid points is designated using a linear interpolation. Since the fit statistic surface is often quadratic, a linear interpolation will result in the lines being drawn inside the true location of the contour. The combination of this and the previous effect sometimes will result in the minimum found by the fit command lying outside the region enclosed by the lowest contour level.


XSPEC>step 2 0.5 1. 4 3 1. 2. 4     !  Create a 5$\ast$5 grid for parameters 2 and 3
XSPEC>plot contour     !  Plot out a grid with three contours with delta fit statistic of 2.3, 4.61 and 9.21
XSPEC>pl cont,,4,1.,2.3,4.61,9.21     !  same as above, but with a delta fit statistic = 1 contour.


Plot the data (with the folded model, if defined) with the y-axis being numbers of counts in each bin.


Plot the data (with the folded model, if defined).


Plot the residuals in terms of sigmas with error bars of size one.


Plot the relative contributions of plasma at different temperatures for multi-temperature models. Note that it plots the emission measure integrated over each temperature bin (not really the DEM which should be emission measure per unit temperature interval). For the cooling flow models the plot is of the emission measure divided by the mass flow rate.


Plot the current incident model spectrum in E2f(E) (Note: This is NOT the same as an ``unfolded'' spectrum.) or, if wavelength plotting has been set, $\lambda^2$f($\lambda$). The contributions of the various additive components also are plotted.


Plot the unfolded spectrum and the model in E2f(E) or, if wavelength plotting has been set, $\lambda^2$f($\lambda$). The contributions to the model of the various additive components are also plotted. This corresponds to the $\nu$-f$_\nu$ plot beloved of AGN researchers. WARNING ! This plot is not model-independent and your unfolded model points will move if the model is changed.


Plot the total response efficiency versus incident photon energy. Only currently noticed channels are included in the calculation of the efficiency.


Plot the current incident model spectrum in Ef(E) (Note: This is NOT the same as an unfolded spectrum.) or, if wavelength plotting has been set, $\lambda$f($\lambda$). The contributions of the various additive components also are plotted.


Plot the unfolded spectrum and the model in Ef(E) or, if wavelength plotting has been set, $\lambda$f($\lambda$). The contributions to the model of the various additive components also are plotted. WARNING ! This plot is not model-independent and your unfolded model points will move if the model is changed.


Integrated counts and folded model. The integrated counts are normalized to one.


Plot the insensitivity of the current data set to changes in the incident spectra (experimental).


Plot the data (with the folded model, if defined) with a logarithmic y-axis.


Plot the results of MCMC runs. Takes two arguments specifying the parameters to be plotted. If one of these arguments is zero then the fit statistic is plotted instead of a parameter.


Plot the current incident model spectrum (Note: This is NOT the same as an unfolded spectrum.) The contributions of the various additive components also are plotted.


Plot the data divided by the folded model.


Plot the data minus the folded model.


Plot the sensitivity of the current data set to changes in the incident spectra (experimental).


A pretty plot of the data and residuals against both channels and energy.


Plot the unfolded spectrum and the model. The contributions to the model of the various additive components also are plotted. WARNING ! This plot is not model-independent and your unfolded model points will move if the model is changed.


Switch on/off the continue fitting questions.

query   <option>

where <option> is yes, no, or on. If on then the continue fitting question in fit, steppar, and error will be asked when the number of trials is exceeded. Also when the number of trials to find the error is exceeded a question will be asked. For either of the other two options the questions will not be asked but the answer will be assumed to be yes or no depending on the <option> set. So to ensure that fitting continues without any questions being asked use the command query yes.


The same as exit



Enable/disable gnu readline facility.

readline   [on | off]

This command is used to enable or disable the gnu readline command editing/history facility. Giving the command with no arguments will print the current status (enabled or disabled). Readline is enabled by default. For more information on using readline see Appendix D.


Adjusts the indicated data sets' correction norm by a single multiplicative factor that minimizes the fit statistic. This approach to fitting the correction norm is considered an INTERIM solution.

recornrm   [<file range>...]

where <data set range> := <low data set> [-<high data set>] All the data sets specified by the one or more data set ranges on a single invocation of the command are multiplied by a single number, which is chosen to best reduce the fit statistic. If no ranges are given, then the last SINGLE range input is used. If you wish to fit a data set's correction norm individually, then refer only to that data set. Note : The use of the recornrm command requires that a response and model be defined. It is perhaps best if a preliminary fit is performed. The user then should alternate between fit's and recornrm's until a stable solution is achieved. If the model is not a good fit, this process may not converge.


XSPEC>recornrm $\ast$$\ast$     !  All the files' correction norms are adjusted by a single number.
XSPEC>recornrm 1-3 5     !  Files 1,2,3, and 5 are adjusted.
XSPEC>recornrm     !  File 5 (the last range input) is adjusted by itself.
XSPEC>recornrm 2     !  File 2 is adjusted


Renormalize model, or change renorm conditions.

renorm   [AUTO | NONE | PREFIT]

The renorm command will adjust the normalizations of the model by a single multiplication factor, which is chosen to minimize the fit statistic. Such a renorm will be performed explicitly whenever the command is used without a key-word, or during certain XSPEC commands, as determined by the following key-words:


Modify one or more of the matrices used to describe the response of the associated data set to incident X-rays.

response   [<filespec>...]

where <filespec> =:: [<data set num>] <file name>..., and where <file name> is the name of the response file to be used for the response of the associated data set. <data set num> is the data set number for the first<file name> in the specification, <data set num>+1 is the number for the next file, and so on. If no <data set num> is given in the first <filespec>, it is assumed to be one (1). If no file specifications are entered, then none of the data set responses are modified. An error message is printed if the data set number is greater than the current number of data sets (as determined from the last use of the data command. A file name none indicates that no response is to be used for that data set. This situation means that any incident spectrum will produce no counts for those particular channels. If a file is not found or can not be opened for input, then the user is prompted for a replacement response file. An <EOF> at this point is equivalent to using none as the response. See the data command for ways to totally remove the data set from consideration. The user is also prompted for a replacement if the response file has a different number of PHA channels than the associated data set. A warning will be printed out if the response detector ID is different from the associated data set's. The current ignore status for channels is not affected by the command. (See the ignore and notice commands).

Examples: It is assumed that there are currently three data sets:

XSPEC>response a,b,c     !  New files for the response are given for all three files.
XSPEC>response 2 none     !  No response will be used for the second file.
XSPEC>response ,d     !  d.pha becomes the response for the second file.


Save aspects of the current state to a command file.

save   <option> <filename>

If no <file name> is given, then the file savexspec.xcm is created. If you don't give the extension to the file name the default is .xcm. The values of <option> allowed are model, files and all. The model option writes out commands to recreate the current model and parameter values, the files option writes out commands to read-in the current data sets, and the all option does both of the above. The default option is model. To recover the saved context use the command source filename.


XSPEC>save,,fname     !  Write out model commands to the file fname.xcm.
XSPEC>save     !  Same as above into file savexspec.xcm.
XSPEC>save files fname     !  Write out data file commands.


Open a script file.

script   <script file>

where <script file> is the name of the file to be opened (default extension is .xcm). If no arguments are on the line, then the default file name is xspec.xcm. If <script file> matches the string none, then the current script file is closed. The script file saves all commands that are input. This command is useful for users who use the same set of commands repeatedly. Once a script file is written and saved, the user then can re-run the same set of commands on other data by souce <script file>.


XSPEC>script     !  Turn on the script file (default xspec.xcm).
XSPEC>script none     !  Close the script file.
XSPEC>script myscript     !  Open the script file ( myscript.xcm).


Set one of the various plot options.

XSPEC>setplot <subcommand string>

where <subcommand string> is a keyword followed in some cases by arguments.


Show individual additive model components on the data plots.

XSPEC>setplot add

The opposite is setplot noadd.


Change the x-axis on data and residual plots to channels.

XSPEC>setplot channel


Add a PLT command to the command list.

XSPEC>setplot command <PLT command>

where <PLT command> is any valid PLT command. Every time you use setplot command, that command is added to the list that is passed to PLT when you use plot or iplot. The most common use of setplot command is to add a common label to all plots produced. You should be careful when using this command, because XSPEC does not check to see if you have entered a valid PLT command. These commands are appended to the list that XSPEC creates to generate the plot and so setplot command will override these values (this can either be a bug or a feature, depending on what you have done!) See also setplot delete and setplot list.


XSPEC>setp c LA OT Crab     !  Add the label ``Crab'' to future plots.


Delete a PLT command from the command list.

XSPEC>setplot delete <command #>

where <command #> is the number of a PLT command that had been entered previously using setplot command. This command is used to delete commands from the list passed to PLT when you use the XSPEC plot or iplot commands.


XSPEC>setp c LA OT Testing     !  PLT label command
XSPEC>setp c LWidth 5     !  PLT line-width command
XSPEC>setplot lis     !  List the PLT command stack.
1: LAbel OT Testing     !   
2: LWidth 5     !   
XSPEC>setplot del 1     !  Delete the first command in the stack.
XSPEC>setplot lis     !   
1: LWidth 5     !   


Set the device used for plots.

XSPEC>setplot device {<PGPLOT device> | NONE}

where <PGPLOT device> is a string that gives an (optional) file name and device plot type (see below and the PGPLOT documentation for options available). Entering the device none will close the current plot device and disable plotting until a subsequent setplot device is issued. Note that if an environment (UNIX) or logical (VMS) variable called PGPLOT_TYPE exists, then XSPEC automatically sets it as the plot device on invocation.

PGPLOT devices

A number of plot device types are supported in XSPEC. PGPLOT devices available on Unix machines are :

 /GIF        Graphics Interchange Format file, landscape orientation
 /VGIF       Graphics Interchange Format file, portrait orientation
 /NULL       Null device, no output
 /PPM        Portable Pixel Map file, landscape orientation
 /VPPM       Portable Pixel Map file, portrait orientation
 /PS         PostScript file, landscape orientation
 /VPS        PostScript file, portrait orientation
 /CPS        Colour PostScript file, landscape orientation
 /VCPS       Colour PostScript file, portrait orientation
 /TEK4010    Tektronix 4010 terminal
 /GF         GraphOn Tek terminal emulator
 /RETRO      Retrographics VT640 Tek emulator
 /GTERM      Color gterm terminal emulator
 /XTERM      XTERM Tek terminal emulator
 /ZSTEM      ZSTEM Tek terminal emulator
 /V603       Visual 603 terminal
 /KRM3       Kermit 3 IBM-PC terminal emulator
 /TK4100     Tektronix 4100 terminals
 /VT125      DEC VT125 and other REGIS terminals
 /XDISP      pgdisp or figdisp server
 /XWINDOW    X window window@node:display.screen/xw
 /XSERVE     A /XWINDOW window that persists for re-use


XSPEC>setplot device /xt     !  sets the device to the xterm.
XSPEC>setplot device none     !  closes the plot file.


Change the x-axis on plots to energy in units of keV.

XSPEC>setplot energy


Define a range of data sets to be in the same group for plotting purposes only.

XSPEC>setplot group <data set range>...

where <data set range> is a range of contiguous data sets to be treated as a single data set for plotting purposes. The data sets still are fit individually. If multiple ranges are given, each range becomes a single group. Initially, all data sets read in are treated as single data sets.


Assume that there are five data sets currently read in, all of them ungrouped initially.

XSPEC>setplot group 1-4     !  The first four data sets are treated as one group, with the fifth data sets on its own. Thus all plots will appear to have two data sets.
XSPEC>setplot group 1 2 3 4     !  The data sets are reset to each be in their own group.
XSPEC>setplot group 2-3 4-5     !  Now there are three plot groups, being data set 1, by itself, and data sets 2-3 and 4-5 as groups.
XSPEC>setplot group 1-$\ast$$\ast$     !  All the data sets are placed in a single plot group.


Switch on plotting of line IDs.

XSPEC>setplot id <temperature> <emissivity limit> <redshift>

The IDs are taken from the APEC line list for the temperature given by the first argument. The plot only shows those lines with emissivities above the limit set and the lines are redshifted by the amount specified. Currently the APEC version used is 1.3.1. If the command xset has been used to set APECROOT then the identify command uses the APECROOT value to define the new atomic physics data files. See the documentation for the apec model for details.


List all the PLT commands in the command list.

XSPEC>setplot list

See setplot delete for an example of use.


Do not show individual additive model components on the data plots.

XSPEC>setplot noadd

The opposite is setplot add.


Switch off plotting of line IDs.


Define characteristics used in rebinning the data (for plotting purposes ONLY).

XSPEC>setplot rebin <min significance> <max # bins> <plot group> <error type>

In plotting the data from a data set (or group of data sets, see setplot group), adjacent bins are combined until they have a significant detection at least as large as <min significance> (in sigma). However, no more than <max # bins> may be so combined. Initial values are 0. and 1, respectively. This argument effects only the presentation of the data in plots. It does not change the fitting, in particular the number of degrees of freedom. The values given are applied to all the plotted data in the plot group specified as the final argument. To simultaneously change the rebinning for all the plot groups give a negative value of the plot group.

The <error type> argument specifies how to calculate the error bars on the new bins. The default is quad which sums in quadrature the errors on the original bins. sqrt uses $\sqrt{N}$, where N is the number of counts in the new bin, poiss-1 uses $1+\sqrt{N+0.75}$, poiss-2 uses $\sqrt{N-0.25}$, and poiss-3 is the arithmetic mean of poiss-1 and poiss-2. If background is present its error is calculated by the same method then added in quadrature to the source error.


XSPEC>setplot rebin 3 5 1     !  Bins in plot group 1 are plotted that have at least 3 sigma, or are grouped in sets of 5 bins.
XSPEC>setplot rebin 5 5     !  The significance is increased to 5 sigma.
XSPEC>setplot rebin,,10,-1     !  All plotted bins can be grouped into up to 10 bins in reaching the 5 sigma significance criterion.
XSPEC>setplot rebin,,,,sqrt     !  Uses $\sqrt{N}$ to calculate error bars.


Change the x-axis on plots to wavelength in units of angstroms.

XSPEC>setplot wave

This command also makes ignore and notice operate in terms of wavelength rather than energies.


List selected information to the user's terminal (and the log file, if open).

show   [<selection>]

where <selection> is a key word to select the information to be printed. If omitted, it is the information last asked for. Initially, the default selection is all. Selections are:

all     !  All the information.
allfile     !  All file information = files+noticed+rates.
control     !  XSPEC control information.
free     !  Free parameters.
files     !  File names, associated coefficients.
fit     !  Fit information.
model     !  The model specification.
noticed     !  Channel ranges noticed for each file.
parameters     !  Current model parameter values.
parameters <n>     !  Current settings for the <n>th model parameter.
pha     !  Current data, error and model values for each channel.
rates     !  Folded model, correction rates for each file.

Note that show pha requires the chatter level to be set to $\geq$ 11.


Change the fit statistic in use.

statistic   [chi | cstat]

The options are chi-squared (chi) or C statistic (cstat). Note that the C statistic has a couple of limitations : a) if the calculated model goes to zero in any channel then the format statistic goes to infinity - in practice this is trapped and a large value of the statistic should result; b) the C statistic can be used to estimate parameter values and confidence regions but does not provide a goodness-of-fit. See Arnaud (2001, ApJ submitted) for further details.


Add tcl procedures in script file to command set. This command is intended to be used for developing new procedures, after which the script should be added to the user's $XSPEC_HOME directory and be source'd automatically on startup.

source   <file name>

The full file name must be specified. There are no default extensions as in previous versions of XSPEC. The script may use the full power of the TCL scripting language. It is recommended that full command names be used when writing scripts. This will cause the script files to run more efficiently.

N.B. The alternative syntax for executing files containing xspec commands,


is recommended for xspec scripts not containing tcl procedures, for example output from the 'save' command. This is because the tcl command processor uses a 'newline' character to determine when a command that is not in curly parentheses is ended. However, this is not correct for xspec's multiple-line commands (i.e. those which when run interactively prompt the user for input such as model). Hence scripts that contain such commands may not process correctly. In contrast, the '@' command will detect such conditions and process the script properly.


Perform a fit while stepping the value of a parameter through a given range. Useful for determining confidence ranges in situations where greater control is needed than given with the error command.

steppar   <step spec> [<step spec>...]

where <step spec> ::= [{log | nolog}] [{current | best}] <param index>
<low value><high value><# steps>. The parameter is stepped from <low value> to<high value> in <# steps> plus one trials. The stepping is either linear or log. Initially, the stepping is linear but it can be changed by the optional string log before the parameter index. nolog will force the stepping to be returned to the linear form. The number of steps is set initially to 10. At each value, the parameter is frozen, a fit performed, and the resulting value of chi-squared given. If best is given as an argument then the non-stepped parameters are reset to the best-fit values at each grid point. Alternatively, if current is given as an argument then the non-stepped parameters are started at their values after the last grid point (the default).

If multiple <step spec> are given for different parameters, then a raster scan of the parameter ranges is performed. At the end of the set, the parameters and chi-squared are restored to the values they had initially.


Assume that the current model has four parameters:

XSPEC>steppar 3 1.5 2.5     !  Step parameter 3 from 1.5 to 2.5 in steps of .1.
XSPEC>steppar log     !  Repeat the above, only use multiplicative steps of 1.0524.
XSPEC>step nolog 2 -.2 .2 20     !  Step parameter 2 linearly from -.2 to .2 in steps of 0.02.


Get advice on what to do with bug reports and enhancement requests.


If the suggestion is the result of a catastrophic error, please give as much information as possible. Clarity and completeness will improve the response to your suggestion.


Execute command in a shell.

syscall   [<shell command>]

This command executes its arguments by passing them to the users current shell for execution. Thus file name globbing (ie. wildcard expansion) are performed on the command before execution. This is in contrast to the exec command, which executes commands directly, without first passing them on to a shell.

If no arguments are given, then the command will start an interactive subshell.


systematic   [<model systematic error>]

Set a systematic error term on the model to be added in quadrature to that on the data when evaluating chi-squared. The default value is zero.

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Ben Dorman