 ```TIMESKEW (Jan96) xanadu.xronos TIMESKEW (Jan96) NAME timeskew -- Calculates time skewness function of a time series using a slow algorithm USAGE timeskew file(s)+options window dtnb nbint nintfm rebin outfile plot plotdev DESCRIPTION This task calculates a time skewness function for one time series, plots and outputs the results (in a FITS file). The time skewness is computed using a slow algorithm. The input file format is FITS using the BINTABLE extension. Both binned data format and event format are input (FILELIST and INPUT FILE OPTIONS). Time, Phase, Intensity and Exposure windows (See WINDOW) allow for data screening. Input data can be rebinned and divided into Intervals and Frames (See GENERAL XRONOS TERMINOLOGY). The results of the time skewness from several intervals can be averaged in a frame, and the averaged results can be rebinned. The standard plot output is the time skewness function versus time delay (See PLOT). The time skewness error bars are obtained either by propagating the theoretical error bars of the time skewness from individual intervals (in turn obtained by propagating the newbin errors bars through the time skewness formula) or by evaluating directly the standard deviation of the average of the time skewness in each time delay bin from different intervals. This depends on whether the specified number of intervals per frame is, respectively, smaller or larger than the value of "errorbars" parameter (default =5). Different normalizations can be used for the time skewness function by changing the parameter "normalization" (See normalization). normalization The "normalization" paramater has the following meanings (and values) : * =0 time skewness is normalised by dividing by the number of good newbins in each interval. * =1 (d/f) time skewness is normalised by dividing by the number of good newbins and the third moment of newbins in each interval. No correction for the third moment due to Poisson statistics in newbins is calculated. If other values than those listed above are used, they will be treated as =0. GENERAL XRONOS TERMINOLOGY Within XRONOS tasks, BINS and NEWBINS control the binning used in the analysis, INTERVALS the subdivision of the time series and FRAME the grouping of the output results: BINS : these are the time bins of the time series being analysed. More than one input file can have different bin durations, e.g. two consecutive time series, one with 0.5 s bins and the other with 2 s bins. The original bin time is the value stored in the input file in the keyword TIMEDEL. If the data are stored in each row as an array with 1CTYPn = 'TIME', the original bin is set to the value stored in the keyword 1CDLTn (where n is the column number). NEWBINS : these correspond to the time resolution at which the analysis is carried out. Note that: (i) newbins cannot be shorter than the longest bin duration of the time series being analysed; (ii) in many XRONOS applications (e.g. powspec, autocor, crosscorr) the newbin duration is forced to be an integer multiple of the longest bin duration. INTERVAL : an interval is defined by the number of newbins over which the analysis is carried out. Note that in applications using FFT algorithms (e.g. powspec, autocor and crosscor set in fast mode) the number of newbins in an interval is a power of 2. FRAME : a frame consists of the average of the results of the analysis of one or more contiguous intervals. Note that in 'lcurve', 'efsearch' and 'lcstats' a frame consists always of one interval. WINDOWS If any window is required during the analysis, a window file containing the relevant windows must be created with the application XRONWIN, before running a XRONOS task. There are 4 different types of windows : * Time Windows : consist of up to 1000 time intervals * Phase Windows : consist of an Epoch, a Period and up to 10 phase intervals * Intensity Windows : consist of up to 10 intensity in bin, newbin and interval * Exposure Windows : consist of up to 1 exposure in bin, newbin and interval Intensity and Exposure Windows can be specified independently for: (i) Bins , (ii) New Bins , (iii) Intervals. When dealing with more than one time series, Intensity and Exposure Windows must be specified separately for each series. Time and Phase windows are applied to Bins. Intensity and Exposure windows are applied first to Bins, then Newbins and finally to Intervals as specified. For time and phase windows, only those bins whose center time is within the start and stop of a time window or phase window (for a specified epoch and period) are accepted. Intensity windows must be ordered with increasing intensity and if set for newbins can be used in conjunction with "Special Newbin Windows" (see below). Exposure Windows consist of a minimum and a maximum exposure level. Units are such that 1 means 100% exposure. The Newbin Exposure is obtained by propagating the bin exposures to each newbin. For example, if in a 30 s newbin the total exposure (due to the sum of the individual exposure of the bins contributing to the given newbin) is 18 s then its exposure is 60%. The Interval Exposure is the ratio of accepted to expected newbins: for example, if a 128 newbin long interval contains only 32 accepted newbins, then its exposure is 25%. Many XRONOS application use some default exposure windows, which are designed to avoid analysing data sets which are too inhomogeneous with respect to their statistical properties. The minimum default Exposure windows in an Interval is set to 0.0 in the lcurve, efold and efsearch and to 0.5 (i.e. 50% exposure ) in all the other tasks. Note that exposures can be higher than 100% (e.g. if the newbin time is not a multiple of the bin time, then "beats" are generated which might bring the exposure of a newbin to values >100%; or if two or more input files for the same time series overlap in part, some of the newbins will be more than 100% exposed). IMPORTANT NOTE WHEN TIME WINDOWS ARE SET IN THE WINDOW FILE: The time used within XRONOS tasks is Truncated Julian Days (TJD= JD-2440000.5) if either (1) the keyword MJDREF is present in the header or (2) if the TIMESYS value is one of the following strings MJD or JD or TJD. If (2), the time values are expected to be stored as JD, MJD or TJD in the header keywords and in the TIME column in which case the MJDREF keyword is not used (it should not be present). When Time windows are set using XRONWIN, they must be compatible with the values in header of the timing keywords and/or the values in the TIME column. An additional window type called "Special Newbin Window" can be set directly from the parameter file. Special Newbin Windows are used to exclude the parts of a light curve which immediately follow or precede a burst or a background event which has been rejected by intensity windows in newbins. The Special Window operates on newbins in conjunction with intensity windows (in newbins) and are specified by changing to positive values the parameters 'spwinbefore' and 'spwindowafter'. Their use is the following: if e.g. spwinbefore is set =10, all newbins, whose center time is within 10 second before the center time of a newbin rejected by intensity windows, will also be rejected; if e.g. spwindowafter is set =20, all newbins, whose center time is within 20 second after the center time of a newbin rejected by intensity windows, will also be rejected. FILELIST and INPUT FILE OPTIONS To input multiple files for each time series, a file containing the list of files is needed (Filelist). The Filelist is input in the program as '@Filelist'. The format of this file list is ascii and contains one filename+options per line. Files from different times series are separated by '///' mark. Below is an example of the Filelist containing 2 files for 3 different times series. file1_ser1 file2_ser1 /// file1_ser2 file2_ser2 /// file1_ser3 file2_ser3 The Input File Options (up to 10) can be specified for each file in the same input string. They consist of 2 characters followed by a numerical constant (up to 8 character long). There are two groups of options. The first allows data selection within a FITS extension. The available options within this group are : frN= start reading input file from row number N (first row) lrN= stop reading input file from row number N (last row) vxN= use column number N as x-axis (i.e. time axis, default name is TIME) vyN= use column number N as y-axis (default names are COUNT or RATE) vsN= use column number N as error for y-axis (default name is ERROR) veN= use column number N as exposure (default name FRACEXP). If the input file is an event list, exposure is by default calculated using the GTI extension. In this case, N=0 turns off the usage of the GTI extension for the exposure calculation, and N > 0 specifies the GTI extension to use. feN= select data (either binned or events) from channel number N (First Energy). For an event list channel selection is made using the column named 'PHA' leN= select data (either binned or events) to channel number N (Last Energy). For event list the default column channel name searched is 'PHA'. The option 'vcN' allows the choice of a channel column name different from 'PHA' (es. 'PI'). vcN= use column number N for channel selection (valid only for event lists). rtN= use extension N of the FITS file to read the data. The first extension is N=1 (the primary array is irrelevant). To specify the extension the following also can be used: filename[N] or filename+N. of = The MJDREF keyword is not used. The time is calculated using the TIME column and the TIMEZERO keyword. The second group of options performs algebraic operations on individual input files. They are applied in the same order in which are specified. For event files they are applied after the data are binned. The available options within this group are: stX = Shift all Time in input file by X days ssX = Shift all times in input file by X Seconds muX= multiply data and errors by X (MUltiply) mdX= multiply data by X (Multiply Data) meX= multiply errors by X (Multiply Errors maX= as muX but exposure is divided by X diX= divide data and errors by X (DIvide) ddX= divide data by X (Divide Data) deX= divide errors by X (Divide Errors) daX= as diX but exposure is multiplied by X aaX= add data and errors with X (Add All) adX= add data with X (Add Data) aeX= add errors with X (Add Errors) saX= subtract data and errors with X (Subtract All) sdX= subtract data with X (Subtract Data) seX= subtract errors with X (Subtract Errors) qaX= add to data the square of data muliplied by X and add to errors the product of data and error multiplied by X qdX= as above but for data only qeX= as above but for error only Below is an example of the Filelist containing 2 files for 3 different times series where the different options are applied to the input files for different time series. file1_ser1 aa4 add to data and error 4 file2_ser1 aa4 " " " " /// file1_ser2 rt2 aa2 read 2nd extension; add to data and error 2 file2_ser2 rt2 aa2 " " " " " " " /// file1_ser3 rt2 vy4 vs5 read 2nd extension; use column 4 and 5 for Y-axis and Error file2_ser3 rt2 vy4 vs5 " " " " " " " " " " PLOT The array of results from each XRONOS task can be plotted. The PLT routine (See also the QDP/PLT manual) provides the interactive plotting and fitting functions. The plotting function is available for the following tasks : autocor, crosscor, efold, efsearch, lcurve, powspec and timeskew. The 'timeskew' plot is the time skewness function function versus time delay (positive) given in seconds. A number of commands can be entered from the 'PLT>' prompt to allow plot customisation (e.g. Add/remove labels; Plot data with various combinations of lines, markers, and error bars; Change text fonts; Change X-axis and/or Y-axis; Change number of plotted panels; Define and Fit models to data). Useful QDP Commands are: PLT> r x xlow xhigh * Rescale X axis PLT> r y ylow yhigh * Rescale Y axis PLT> r xlow xhigh ylow yhigh * Rescale both PLT> log x * X axis is plot in logarithmic scale PLT> log y * Y axis is plot in logarithmic scale PLT> log off * Turn off the logarithmic axes (both X and Y) PLT> dev /xxx * Change the current plot device PLT> mo ? * List available model PLT> mo cons linr * Define model= constant plus linear plt> fit * Fit the defined model PLT> hardcopy filename * Hardcopy of the current plot (postscript) PLT> exit * EXit: to exit from the PLT subroutine type: The QDP/PLT software is provided and maintained by Allyn Tennant (Marshall Space Flight Center). The PLT software uses the PGPLOT Graphics Subroutine Library for plotting, written by T.J. Pearson (California Institute of Technology). OUTPUT The analysis results are output in a FITS file. Two different FITS layouts are available (see parameter outfiletype). The first stores one interval (or frame) of interval results per FITS table row, and the output file will have a single extension. The second stores one interval (or frame) per FITS extension, and the output file will have as many extensions as the number of intervals (or frames). The output file contains, besides the array of results, a number of statistical variables (and errors if appropriate) associated with each interval (or frame) for each time series. These are: 1- average count/s in frame (in a frame this is the average of the averages in intervals); 2- fractional exposure in frame (from average of fractional exposures in intervals, the latter is the ratio of good newbins to the total number of expected newbins/interval); 3-variance in frame (average of variances in intervals); 4- expected variance in frame (average of the expected variances in intervals, the latter is calculated from the error bars of the newbins); 5- 3rd moment in frame (average of 3rd moments in intervals); 6- minimum count/s in frame; 7- maximun count/s in frame; 8- excess variance in frame (average of excess variances in intervals, the latter is calculated as variance-expected variance); 9- chi-square in frame (average of chi-squares in intervals); 10-rms fractional variation in frame (average of rms fractional variation in intervals, the latter is calculated as the square root of the excess variance divided by the average. When "outfiletype"=2, the output file contains in each extension the following columns: time delay, half width of time delay bin, time skewness and error. Not many applications can currently deal with the output files produced when "outfiletype"=1 The format will be described in future releases. PARAMETERS cfile1 (filename(s) first series+options) [string] Input filename(s) for the first time series + options. The valid input files are in FITS format using the BINTABLE extension. Xronos tasks read for each time series many consecutive input files (up to 50). Additional flexibility is provided by Input File Options which are used to perform algebraic operations on individual input files (either on the 'times' or on the 'count' or 'count/s' values). The Input file Options are also used to select columns and rows within a FITS file. If the first character of the input string is '@', the rest of the string is taken to be a filename containing the list of input files (Filelist). The Filelist can contain filenames for more than one series. See description of "FILELIST and INPUT FILE OPTIONS". window (name of window file) [string] Filename of the xronos window file. The window file is an ASCII file and by default a standard window file is used, where only exposure windows are set. To modify the standard file or create a new file used the script XRONWIN. dtnb (integration time) [double] The duration in seconds of the NEWBIN time. For binned input files the NEWBIN duration can not be shorter than the longest bin duration in the input file. In a number of XRONOS tasks the NEWBIN time must be an integer multiple of the minimum newbin time. The task internally calculates (and prints on the screen) a default value such that a single interval is produced with a fixed number of newbins (typically between 128 and 4096 depending on the task and on the time interval length see also "nbdf" parameter). Typing 'INDEF' forces the task to use as NEWBIN time the value calculated by the program. NOTE: By pressing return the task will use as NEWBIN time the value found in the parameter file used in a previous run. nbint (number of points per interval) [integer] The number of newbins per interval used in the analysis. The "nbint" together with the NEWBIN duration determines the length in time of an interval and therefore the total number of intervals within the start and stop time over which the analysis will be carried out. Typing 'INDEF' forces the task to use the default value (see parameter "nbdf"). NOTE: By pressing return "nbint" is set to the value found in the parameter file used in a previous run. nintfm (number of intervals per frame) [integer] The results of the analysis from individual intervals can be averaged to produce a frame. The "nintfm" is used to specify the number of intervals averaged in a frame. Typing 'INDEF' forces the task to use the default value (all intervals will be use in one frame). NOTE: By pressing return "nintfm" is set to the value found in the parameter file used in a previous run. rebin (rebin analysis results) [double] The results of the analysis of individual frames can be rebinned either with a constant rebinning factor or as a geometrical series (to produce nearly equispaced bins in a logX representation). The default value (0) is not to rebin the results. Values greater than 1 indicate a constant rebinning factor. Geometrical series rebinning is obtained by entering a value less than -1, whose modulus represents the step of the geometrical series (Values between -1.4 and -1.2 are adequate in most cases). itre (Flag for trend removal) [integer] A polynomial trend, up to 4th-order, can be removed from input time series. Setting the parameter "itre" equal to 1 or 2 or 3 or 4 remove a 1st, 2nd, 3rd, 4th-order polynomial trend, respectively. The trend is determined separately for each interval of each series by using a least-square technique. The value 0 does not cause the removal of any trend from the input series and is the default value. NOTE The trend removal is not available for the efold and efsearch tasks. itremo (Mode for trend removal) [integer] Specify how the trend removal is applied to the data (available only id "itre" is higher than 0). The trend can be subtracted from the time series (itremo =1) , or the time series can be divided by the trend (itremo = 2), or the time series can be replaced with the trend (itremo = 3). By default the trend is subtracted (value set to 1). outfile (output filename) [string] Name of the output file. If only the root is given, the program adds an extension of 3 characters, '.fXX', where f stands for FITS and XX is a suffix which identifies the program that creates the output. The XX values are : 'ac' (autocor), 'cc' (crosscor), 'ef (efold), 'es' (efsearch), 'lc' (lcurve), 'ps' (powspec), 'ts' (timeskew). Typing '-' forces the program to generate a default value for the output filename (see outfileroot parameter). By typing blank characters at the prompt no output file will be created. outfileroot (default value) [string] This parameter used in conjunction with "outfile" generates a default output filename. If the value of "outfileroot" is 'default' and "outfile" is '-', the output filename will have the same root name of the first input file of the fits series with a '.fXX' extension (see outfile for XX values). outfiletype (Type of FITS layout) [integer] Define the FITS structure for the output file. Two types are available. The first type (outfiletype=1) has 1 interval of results per FITS table row. The output file will have only one extension. Each row of the results column contains an array of values with a size equal to the number of points per interval (or frame) and a number of rows equal to the valid number of intervals analysed. The second type (outfiletype=2) has 1 interval of results per FITS extension. The output file will have as many extension as the number of intervals (or frames) analysed. The column containing the results has a single element value in each row and the number of rows is equal to the number of points in the interval. The default value of "outfiletype" is 2 (one interval per FITS extension). plot (if plot) [boolean] Flag specifying whether or not the results of the analysis are plotted via PLT (default = yes). NOTE: After plotting the results the XRONOS task is left in the interactive plotting/fitting mode (PLT> prompt) use command `exit` to return and finish the XRONOS task. If the "plot" parameter is set = no, the other queries regarding the plotting are disabled ("plotdev", "plotfile", "plotdum"). plotdev (device) [string] Change the plot device. The most common plot devices are /xw (X window), /tek (tektronix), /vt (vt125), /ps (PostScript). Setting "plotdev" as '?', forces the program before plotting to list the available plot devices to the terminal and to prompt for this parameter again. plotfile (.pco file) [string] Name of the Plt COmmands file. For each task an appropriate default Plt COmmand file is automatically loaded by the program setting the parameter "plotfile" as '-'. The user can insert additional commands using his own '.pco' file at this level ( or within the PLT> prompt).The user commands are appended to the default command file and therefore executed last. The default '.pco' files are stored in the directory defined by the parameter 'dpath'. tchat (terminal chattiness) [integer] Set terminal chattiness: (0-4) only little information is output in running XRONOS task ; chattiness 5 is the default value; (6-7) more details on input files, windows, intervals statistics, etc.; (<8) mostly for debugging purposes. lchat (log file chattiness) [integer] Set log file and chattiness in the log file: = 0 the log file is not written; for all other values, information is written in the log file. The chattiness levels are the same as for the terminal. logname (log filename) [string] Name for the log file. The default name is xronos.log. clobber [boolean] Flag specifying whether or not a pre-existing file with the same name as that requested for an output file in the current task will be overwritten. Default value = yes. (dpath = XRDEFAULTS) [string] This string parameter gives the path to the Xronos 'defaults' directory, which contains the default '.pco' file (used for plotting) and the defaults window file 'default_win.wi'. Ordinarily, the user may leave this parameter set to the string 'XRDEFAULTS', which causes Xronos to use the environment variable XRDEFAULTS to locate these files. XRDEFAULTS is set by the mkftools script to point to the appropriate directory for the current distribution of Xronos (for FTOOLS v3.6 this is /ftools/xronos/defaults/). If the user wishes to modify these files, he or she may make and edit copies, and change the XRDEFAULTS variable appropriately using setenv, but the original files should not be changed. gapfill (running mean gap filling) [integer] Replace gaps in input series with running mean. If =0 (the default) data gaps are not filled. If =n newbin data gaps in input series are filled in with running mean values calculated over n newbins. Note that a gap newbin is filled in only if the corresponding running mean is calculated over n/4 points at least (this means that in order to bridge a gap of m newbins n must be >1.35m). This global parameter is ignored in epoch folding applications (efold and efsearch). forcestart (flag for start time) [boolean] If = yes the first interval will be forced to start at the time of the first time window otherwise (=no default) the center time of the first qualified newbin is used as the start time. errorbars (Error bar Evaluation) [integer] This parameter defines the way in which the error bars of the analysis results are calculated. If the number of the intervals per frame ("nintfm") is higher than "errorbars" value (default=5), the error bars are evaluated by using the standard deviation of the average (based on the measured scatter). Otherwise the error bars are evaluated by propagating the theoretical error bars through an averaging process. Note that for several XRONOS applications (e.g. `autocor`, `crosscor`) only the former way of evaluating error bars is available. For example, if "errorbars" is 5 in the application `powspec`: (a) if a frame contains the average of 5 or fewer power spectra, then the error bars in the average power spectrum will be calculated by propagating through the average the theoretical error bars associated with each power spectrum (in turn obtained from the relevant chi-square distribution); (b) if a frame contains the average of 6 or more power spectra, then the error bars in the average power spectrum will be calculated by evaluating the standard deviation of the average power for each frequency. By adjusting the value of "errorbars" it is possible, e.g. to evaluate error bars as in (a), also in the case in which a large number of intervals per frame has been specified. Values < 5 are not recommended (at least 5-6 measures are necessary to reliably evaluate the standard deviation of the average from the scatter around it). NOTE: not applicable for 'lcurve', 'lcstats' and 'efsearch'. exposure (flag for analysis of exposure profile) [boolean] If =yes the exposure profile(s) (i.e. newbin values are set =1, gaps and rejected newbins are set =0) is/are analysed (instead of the input series). Default value is = no. normalization (type of normalization) [integer] Flag to specify the type of normalization to apply to the results. This parameter is only relevant for the following tasks: powspec, autocor, efold, crosscor, timeskew). The standard normalization corresponds to a value of 1 (the default) in all XRONOS applications. Other normalization value flags are described for each application (See normalization). NOTE: not applicable for 'lcurve' and 'efsearch'. simultaneous (flag for simultaneity) [boolean] If =yes a strict simultaneity is forced between the input series in applications which use more then one series (i.e. if the n-th newbin of a series is a gap or is rejected, then the n-th newbin of all other series will be also rejected). This flag is ignored in the efold applications. Default value is = no. spwinbefore (special window start) [double] Special newbin window : number of seconds before. If a value > 0 is used , e.g. 10.0, then all the newbins within 10.0 seconds before a newbin rejected by an intensity window will also be rejected. The default (=0) is not to apply this type of special newbin window. spwinafter (special window stop) [double] Special newbin window : number of seconds after. If a value >0 is used , e.g. 10.0, then all the newbins within 10.0 seconds after a newbin rejected by an intensity window will also be rejected. The default (=0) is not to apply this type of special newbin window. rescale (rescaling for results) [double] Rescaling factor applied to result variables and errors. The rescaling is applied just before writing the output file (this to avoid affecting the statistical variables for the frame). Default value for "rescale" is set to 1. offset (additive constant for results) [double] Additive constant summed to result variables. Result error bars are left unchanged. The additive constant is added just before writing the output file (this is to avoid affecting the statistical variables for the frame). Note that if a rescaling factor is also specified (different from 1), then the results are first multiplied by the rescaling factor. Default value for "rescale" is set to 0. fast (Flag for fast algorithm) [boolean] This parameter sets the type of algorithm used for the Fourier transform. IMPORTANT NOTE: This parameter can be set by the user only in the `powspec`, `autocor` and `crosscor` tasks (it is a query parameter in these tasks). In all the other tasks "fast" is a non-query parameter, and should not be changed from the default setting. nser (Number of time series) [integer] Number of input time series simultaneously processed. IMPORTANT NOTE: This parameter, the values of which range between 1 and 4, can be set by the user only in the "lcurve" and "efold" tasks (it is a query parameter in these two cases). In the other tasks "nser" is a non query parameter, and should not be changed from the default setting. ipow2 (Flag if power of 2) [integer] Internal Flag used to decide if the current task must used with a power of 2 of number of points (ipow2=1) per interval or not (ipow2=0). IMPORTANT NOTE: This parameter should not be changed by the user. iavgreb (Flag if average interval) [integer] Internal Flag used to decide if the current task allows the averaging of intervals in frame and/or the rebinning of the analysis results. If "iavgreb" is set to -1 the results in an interval can not be either averaged or rebinned, if set to -2, the results in an interval can be averaged but not rebinned. IMPORTANT NOTE: This parameter should not be changed by the user. nbdf (Default No. Bins) [integer] Set an internal default value for the number of newbins per Interval. This value is used to calculate the default newbin integration time to have one interval with nbdf points. Different "ndbf" values have been set for different XRONOS task. IMPORTANT NOTE: With caution this parameter can be changed by the user. EXAMPLES 1. From a time series of 10000 sec with a binning of 6.25e-2 seconds, calculate the time skewness of 8192 points (equivalent to an interval in time of 512 seconds) and average every 5 intervals in one frame (4 frames in total). Apply no rebinning to the results, make a plot and output the results in a file. > timeskew cfile1="mydata.lc" window="-" dtnb=6.25e-2 nbint=8192 nintfm=5 rebin=0 plot=yes plotdev="/xw" outfile="-" 2. Same as above but all the intervals are averaged in one frame > timeskew cfile1="mydata.lc" window="-" dtnb=6.25e-2 nbint=8192 nintfm=INDEF rebin=0 plot=yes plotdev="/xw" outfile="-" SEE ALSO autocor, crosscor, efold, efsearch, lcurve, lcstats, listdata, powspec, xronwin, fits2qdp, ascii2lc. BUGS Report problems to angelini@lheavx.gsfc.nasa.gov and xanprob@athena.gsfc.nasa.gov. Provide a detailed description of the problem (with a log file if possible). ```   Please send reports of errors to : xanprob@athena.gsfc.nasa.gov HEASARC Home | Observatories | Archive | Calibration | Software | Tools | Students/Teachers/Public Last modified: Thursday, 06-May-2004 13:47:25 EDT HEASARC support for unencypted FTP access will be ending possibly as early as July 31, 2019. Please see this notice for details.