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This Legacy journal article was published in Volume 4, February 1994, and has not been updated since publication. Please use the search facility above to find regularly-updated information about this topic elsewhere on the HEASARC site.

All-Sky Monitor Databases Update

Laura A. Whitlock and Pat Tyler

HEASARC


As previously reported (Whitlock, Lochner, and Rhode 1992; hereafter WLR), light curves from the Vela 5B and Ariel 5 all-sky monitor (ASM) experiments were being generated for a number of sources to be put on-line at the HEASARC. This effort is now complete and the files are beginning to be accessed by users via the captive XRAY account. A complete listing of the sources for which light curves were generated is given in Table 1. Note that this is an updated version of the listing given in WLR. Below is updated information about the files put on-line for the 2 missions currently included in the ASM data bases. Also included are answers to some commonly asked questions about each data base.

Vela 5B Update

The BROWSE database VELA5B contains the listing of the available Vela 5B FITS files and includes general information for each file. Database parameters are as follows:

  • Name - The name of the source
  • Alt Name - An alternate name for the source, if available
  • RA - The Right Ascension of the source
  • Dec - The Declination of the source
  • Lii - The Galactic longitude of the source
  • Bii - The Galactic latitude of the source
  • Count Rate - An average of counts per second over all observations in file
  • Energy - Energy is 3-12 keV for channel 1 and 6-12 keV for channel 2
  • Bins - Bins are one second, 56 hour, or 112 hour
  • Time - The start time of the observation
  • End Time - The end time of the observation
  • File Lcurve - The unique portion of the FITS filename for the source
  • Date - The FITS file creation date
  • Class - A BROWSE classification flag, if applicable

General information regarding each file is accessed within the VELA5B database with the dsam (display sample) and dall (display all) commands. The dsam, or ds, command produces a table listing as follows:

VELA5B_TOTAL_DEC > ds

table listing

<CR> continues, to exit type any character <CR> or <EOF> d

--------------------------------------------------------------------------------

The dall command may then be used to examine other parameters not listed in the line summary for any particular entry or for a range of entries. For example,

________________________________________________________________________________ VELA5B_TOTAL_DEC > dall 9
______ 9 ___________________________________

DATE 04/01/93 NAME 4U 1254-690 ALT NAME RA (1950) 12 54 19.7 DEC (1950)-69 00 54.0 LII 303.4799931678365 BII -6.419953608587102 COUNT RATE 0.7221077848208485 ENERGY 6-12 keV BINS 112 hr TIME 1969/147 10:59 END TIME 1979/148 8: 0 FILE LCURVE 4U1254_CH2_MAP CLASS 0 SPARE ________________________________________________________________________________

There is one data file for each entry listed with dsam. These Vela 5B FITS files are named by source, channel (energy), and bins. Files with bins of one second are dat files; those with bins of 56 hours are bin files; and those with bins of 112 hours are map files. The dat and bin files are associated with sources considered alone in the FOV; the map files result from the mathematical deconvolution of sources from crowded FOVs. The parameter FILE LCURVE as shown in the example above demonstrates the use of those labels in the naming convention. The actual FITS filename consists of the prefix vela5b_ followed by FILE LCURVE and the extension .fits. In this particular instance, the file for the source 4U 1254-690 in energy range 6-12 keV and with binning of 112 hours is named vela5b_4u1254_ch2_map.fits.

These files may be extracted using the BROWSE xp command and transferred onto the user's home machine with the send command. They are also available by anonymous ftp and are located in the directory /vela5b/data/rates.

Other Vela 5B Notes

At present, no on-line analysis tools are available that are proper for use on the Vela 5B files. This is due to the data being unevenly spaced in time. However, a tool is in development for XTE that also will be available for timing analyses on the Vela 5B files.

An FTOOL is being developed to allow a selection of the data within a file to remove the temperature effect present in the count rate. This known problem is discussed in detail in both WLR and in the Vela 5B Calibration/User's Guide.

A second Vela 5B FTOOL is being developed to allow users to "clean" the data in a file by setting acceptance levels on count rate and/or count rate error. It is of note that essentially all data associated with a given source are included in the on-line files, even if the values are obviously corrupted. It is left for each user to decide which observations should be kept or rejected before submitting the data to any sort of analysis.

The Calibration/User's Guide for the Vela 5B data will be available from the HEASARC in January 1994. It will contain a detailed description of everything that has ever happened to the Vela 5B data (at least everything that has happened since 1985).

Finally, below is a series of frequently asked questions and their answers. We hope these responses will allow future users benefit from the 'learning curve' of past researchers.

Q: What is the source detection flux limit for Vela 5B?

A: In the 3-12 keV detector, 1Vela ct/sec is ~4.5x10-10 ergs/sec/cm2. Perhaps a better rule to remember is the following: for a 1 day average, the 3[[sigma]] detection limit is 250 UFU (Priedhorsky and Holt 1987).

Q: Why do some light curves in the 'map' category have obviously spurious large positive and negative counts in the light curves?

A: If there are no counts above the noise for two close (< 2deg. apart) sources being fitted in a map, the result is often that the light curves for these two sources will be strongly anti-correlated, with enormously high/low count rates . In fact, several important points should be made concerning the analysis of Vela 5B data when considering the observation of low-signal sources from a region of the sky containing more than one X-ray source in the field-of-view. First, when performing the deconvolution of source contributions to a crowded region, it is critically important to know which sources to include and which to exclude. Including sources whose intensity is below the Vela 5B detection limit results in intensity being taken away from legitimate sources. Often, the decision can be made only by performing fits with and without a given source and evaluating the 'goodness of fit' which results in each case. This 'by-hand' comparison was not made for every source in every map put on-line. Secondly, 'poor' fits can be generated by other causes such as the following: 1) putting the sources at incorrect positions due to uncertainties in their position; 2) data coverage in the map is intrinsically poor. Uncertainties in the source position can be solved only by examining the goodness of fit for various trial positions. We have tried to handle poor data coverage by requiring a certain number of coordinate boxes to contain data before performing a fit.

A description of the 'fitting' or 'mapping' of light curves for sources in crowded regions of the sky can be found in WLR or the Vela 5B Calibration/User's Guide.

Q: Why is the average count rate for some light curves a negative number?

A: This phenomena is the result of the background fitting and removal. The algorithm worked well, but it was not perfect.

Q: Can the light curves for sources not currently on-line be extracted from the data base?

A: Yes, e-mail one of us at the addresses given below and we will work with you to extract the data of interest and put it on-line for you.

Q: Can I get a light curve for an on-line source with a different time binning than what is already there?

A: Yes, e-mail one of us at the addresses given below and we will do our best to accommodate your needs.

Ariel 5 Update

Parameters for the BROWSE database ARIEL5 are as follows:

  • Name - The Name of the source
  • Alt Name - An alternate name for the source, if available
  • RA - The Right Ascension of the source
  • Dec - The Declination of the source
  • Lii - The Galactic longitude of the source
  • Bii - The Galactic latitude of the source
  • Flux - An average of the flux
  • Flux Corr - An average of occultation-corrected flux
  • Time - The start time of the observation
  • End Time - The end time of the observation
  • File Lcurve - The unique portion of the FITS data filename
  • FITS File Name - The name of the FITS data file
  • Class - A BROWSE classification flag, if applicable

Again, the BROWSE commands dsam and dall may be used to list the available files and access general information for each.

As for VELA5B, there is one data file for each entry in the database listing. Names for the files are taken from a sequentially and arbitrarily numbered listing of the sources and are given by the parameter FITS FILE NAME. These files can also be extracted with the xp command or obtained by anonymous ftp. They will be located in the directory ariel5/data/rates.

Other Ariel 5 Notes

As was discussed for Vela 5B, no on-line analysis tools are currently available for the Ariel 5 files. A user will have to copy the data back to their home area and use their own algorithms for doing time series analysis on these unevenly spaced data. However, the tool under development to work on XTE and Vela 5B will work for the Ariel 5 data files as well.

The Ariel 5 Calibration/User's Guide will be available in late January or early February 1994.

Lastly, a few common Ariel 5 questions and their answers:

Q: Can I get the light curves for other sources put on-line?

A: If the source was in the 3U catalog, we can extract the light curve for it from the final production tape made by Steve Holt many years ago. There is one caveat, however, namely that the sources already put on-line are known to be the brightest sources, and yet we know that some of these data files contain noise without signal. Thus, it is unclear that any science will be found in the additional files. Nevertheless, if you request it, we will extract the file for you....as long as the programs on the IBM continue to work.

Q: Are the coordinates given in the RA_OBJ and DEC_OBJ keywords the 3U or 4U values?

A: They are the 4U values. This was done to insure that a user doing a coordinate search for an object would find the proper data file. However, it must not be forgotten that the intensity deconvolutions that generated the light curves used the 3U source locations. To enable to user to determine if this situation could affect the data for a given source, the difference in Ra and Dec between the 3U and 4U source locations is given in the comment section of each FITS file.

Q: The data file I looked at has a time resolution of a few days. Do you have higher time resolution data?

A: No. The only data we have for the Ariel 5 ASM light curve of a given source is the multiple-orbit binned data you currently find on-line.

Summary

The first two data bases, Ariel 5 and Vela 5B, have been put on-line in the ASM partition at the HEASARC. The files are heavily commented in the FITS header record. Additional documentation will be available in early 1994. The e-mail addresses for help with the Vela 5B or Ariel 5 data bases are:

Laura Whitlock: whitlock@lheavx.gsfc.nasa.gov

Pat Tyler: tyler@heagip.gsfc.nasa.gov

References

Whitlock, L.A., Lochner, J.C., and Rhode, K., Legacy, 2, 25, 1992.

Priedhorsky, W.C. and Holt, S.S., Sp. Sci. Rev., 45, 291, 1987.

List of sources for which light curves were generated


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