Version 1.2.8  

Reading and Folding ASM Data II


Reading and Folding the ASM Database Over Coffee Break

The example script found on the SITAR downloads page can be used to epoch fold the entire ASM database over coffee break. Coffee break here being defined as 22 minutes of CPU time, using a 2.8 GHz Intel Core Duo Apple laptop. In this example, for all 381 ASM data files, the data is read in, and then epoch folded over 500 periods (folded into 20 phase bins) logarithmically spaced between 5 minutes and 500 days. Plots are created for lightcurves binned on 7 days, as well as for the epoch folding statistic with linear and logarithmic x-axes.

The basic commands that are run are the following.

  • Read the ASM data, requiring an ASM solution with chi^2 of less than 1.3 (this is probably too high for very faint sources, and too low for a bright source such as Sco X-1), using Modified Julian Date for the time value:
         ev = sitar_readasm(file;minchi2=1.3,mjd);
  • Rebin the lightcurve into 7 day bins, requiring at least 7 data points per bin, and ignoring any bins with fewer data points:
         rb = sitar_rebin_rate(ev.time,7,ev.rate;minbin=7,delgap);
  • Epoch fold with 500 logarithmic periods (folded into 20 phase bins) from 5 minutes to 500 days:
         ef = sitar_epfold_rate(ev.time,ev.rate,0.0037,500.;nsrch=500,loggrid);

To run this code, download the list of ASM data files. Go to the All Sky Monitor web page and download all the data files. Note: You might have to modify the contents of asm_files, since the files available from the ASM page can change as sources are added or deleted. Finally, download the example script from the SITAR downloads page and save it as In ISIS type: > () = evalfile("fold_the_asm"); Twenty minutes later, give or take, you will have three postscript plots (rebinned lightcurve, linear and log x-axis epoch folds) for the entire ASM catalogue. (The actual epoch fold data values can be saved by modifying the script to write out the arrays.)

One of course can custom modify the script. I have run the script using 10,000 logarithmically spaced periods, instead of just 500. This took about 8 CPU hours. Selected highlights presented below.

The 30 day super-orbital period of the low mass X-ray binary and X-ray pulsar, LMC X-4:


The 180 day super-orbital period of the low mass X-ray binary, X1820-303:


The 8.965 day orbital period of the high mass X-ray binary and X-ray pulsar, Vela X-1:

Vela X-1 Epoch Fold

The following was generated by modifying the driver script used to fold the ASM lightcurve of Her X-1. Noting the extremely sharp peaks in the epoch fold results for Vela X-1 above, we have folded the Vela X-1 lightcurve on the fundamental 8.965 day period. The results are presented below, using 100 phase bins:

Vela X-1 Period Fold

This page was last updated Oct 21, 2020 by Michael Nowak. To comment on it or the material presented here, send email to
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