As of 1998-04-30 the ASCA archive contains 2299 sequences with over 66,000 kiloseconds of exposure time. The majority of these data (1768 sequences) has been released to the public. The data are distributed among target types as shown in table 1.
The total data volume in the archive is given in table 2. The mean size of a single sequence is 120 megabytes, though sequences sizes vary widely depending on exposure time, observing modes, and source brightness. ASCA produces 4.1 kilobytes of archived products for every second of useful exposure time. Processed products occupy about twice as much space as the raw telemetry. Table 2 -- Distribution of data volume for all sequences in the archive. Note this is the size of gzip compressed files. Compression has reduced the file sizes by roughly a factor of three.
sub-directory Volume (gigabytes) aux 62.0 (dominated by house keeping files) calib 9.5 images 1.8 lcurves 7.6 spectra 14.5 screened 8.8 unscreened 84.8 telem 86.8 --------------------------------------------- total 275.8Figure 1 shows the distribution of archived ASCA observations on the sky. You may notice several prominent objects as clusters of observations, such as The Large and Small Magellanic Clouds toward the lower right of the plot and the Cygnus Loop to the left and just below the Galactic plane. The Large Scale Survey (LSS) is visible as a cluster of dots near the North Galactic Pole, and a line of dots in the lower left corner is a series of observations of the Moon. An interactive version of Figure 1, color-coded by sequence type is available at: http://adfwww.gsfc.nasa.gov/asca/asca_coord_seq.html
Figure 1 -- The distribution of ASCA observations on the sky. The plot is an Aitoff projection in Galactic coordinates with the North Galactic Pole at the top, the Galactic Center in the middle and Galactic longitude increasing to the left.
As our understanding of ASCA and its instruments has improved, the level of sophistication in the processing of the archived products has increased. Major changes in processing are known as "revisions" with the current processing known as "rev2". The major differences between rev1 and rev2 are that we have added a number of image, spectrum and light curve products which are useful for "quick-look". We have also reduced the number of files per sequence by a factor of several by deleting the "raw" event files in favor of the "unfiltered" event files, which are concatenations of the former. Finally, we have added a number of HTML formatted documents describing each sequence. The scientific content of the data, though, should be the same, except where reprocessing with an updated calibration file has made a difference. For more information on rev2 and how it differs from rev1, see the ASCA Getting Started Guide at http://adfwww.gsfc.nasa.gov/asca/processing_doc/GS/getting_started.html Also, the ASCA Processing Documentation at http://adfwww.gsfc.nasa.gov/asca/processing_doc/proc/processing.html gives a detailed description of processing and the differences between various versions.Each time a revision changes, we reprocess the entire archive. Rev2 reprocessing began on 1997-06-18. Currently all sequences have been reprocessed with rev2, but as of this writing we are making a second pass through the archive to make a few minor adjustments. The most significant of these is a change in the SIS screening criteria (see the ABC guide /docs/asca/abc/abc.html ). This second pass through the archive will be complete by June 1998. There are several ways to determine which processing version (and revision) was used on a given sequence. For rev2 data, you can check the processing header page "ad*_hdr_page.html", which is usually in the "aux" directory. For all processing versions, you can check the PROCVER keyword in any FITS file. Processing versions 5.x.x are rev1, and 7.x.x are rev2. Versions 6.x.x refer to a hybrid between rev1 and rev2 called rev1+.
The ASCA archive can be accessed via the World-Wide Web, via the 'xray' captive account, or via anonymous ftp (see the "Access method" section of the "HEASARC Online Service" article in this issue). The anonymous ftp method is useful if you already know the sequence number, or for obtaining trend data (see below).The database to search in w3browse is the ASCA Master Catalog (ASCAMASTER); this will list all approved, observed, and/or public sequences. If the field 'public_date' is empty, the sequence is proprietary; otherwise, the sequence is public and the data can be downloaded. (If the 'observed_date' field is blank, then the observation has not taken place; it is possible that the target will never be observed, if a target was approved at priority C and was not carried over to the subsequent cycles.) In browse, the 'ascapublic' database should be used for the time being for downloading archived ASCA data. The processing team also maintains a separate, complimentary set of web pages, which give information about each sequence and can be used to access the archive. These are available at http://adfwww.gsfc.nasa.gov/asca/asca.html A unique feature of these web pages is that for public sequences you can use them to view bitmapped versions of all the plots and to browse the HTML pages produced in processing. There is also a "thumbnail" index where you can view a page of small versions of these plots: http://adfwww.gsfc.nasa.gov/cgi-bin/asca_icon_list?
This can be useful for doing things like selecting a number of observations with strong iron lines, or with obvious flares, or with multiple (possibly serendipitous) sources.The HEASARC archive contains both public and proprietary data. Both may be freely down-loaded, but the proprietary data have been encrypted using PGP. The names of encrypted files end in ".pgp" and such files may only be decrypted by someone (the PI) who possesses the encryption key. For more on down-loading proprietary data see http://adfwww.gsfc.nasa.gov/asca/pgp/pgp.html The proprietary period for an observation depends on the nationality of the Primary PI. Japanese sequences are released 18 months plus two weeks after they are distributed to the PI, and US and European sequences are released one year and two weeks after being distributed. A few calibration and special sequences are released immediately. See /docs/asca/rules.html
In addition to the sequence-oriented data, the archive also contains "trend" data, which are a set of public data organized by file type instead of sequence. These files can be used for monitoring the instruments and for calibration.The trend data are located at:
This directory contains four sub-directories of interest:
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