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The Release of the BBXRT Event Data

Alan P. Smale (LHEA)

1. Introduction

The Broad Band X-ray Telescope, BBXRT, was flown on the Space Shuttle Columbia as part of the Astro-1 payload in December 1990. During this mission it performed 157 observations of 80 celestial X-ray sources ranging from stars, CVs and X-ray binaries to clusters of galaxies and the X-ray background. BBXRT has an energy range considerably broader than that of the Einstein SSS (0.3-12 keV) and a spectral resolution a factor of four improved over the EXOSAT and Tenma Gas Scintillation Proportional Counters (9% at 1 keV; 2.5% at 6 keV). In addition to the many scientific results derived from BBXRT data, the mission also tested the on-orbit behavior of the novel thin-foil conical mirror assembly and the broad-band solid-state detectors.

A previous article (Smale, 1992) gives a full description of the BBXRT instrument, the mission, the list of sources observed, the detector response, and a schedule for the release of BBXRT data to the community. This reference also contains more detailed information about the point spread function, detector background and alignment, and other analysis issues. It is assumed here that the reader is familiar with this material.

2. The Data

The full BBXRT mission dataset is now available to the community. All products are in standard FITS formats defined by the HEASARC, and the FITS headers contain considerable explanatory information.

All items in the BBXRT archive are available via anonymous FTP from heasarc.gsfc.nasa.gov (128.183.8.233), and can be found in the `bbxrt' directory. In addition, the observation catalog, spectra and light curves can be examined using BROWSE in the HEASARC's on-line system, accessible on Internet at ndadsa.gsfc.nasa.gov (128.183.36.17) or on DECnet at NDADSA (node 15761). From BROWSE, spectral fitting can be performed directly using XSPEC, or the data products can be extracted and transferred back to the user's home machine.

One data product has been generated per observation, slew or calibration interval - 324 files per data type. The following types are available:

Event data

The `bbxrt/events' directory contains complete event lists for the whole mission. The files contain one record per detected event; each record includes the time, instrument, detector, PHA channel, and five data quality flags.

Housekeeping data

The `bbxrt/hk' directory contains housekeeping (hk) files for the relevant time interval. The format for these files is identical to the standard ASCA format; one record per parameter, with the values of the parameters listed at the beginning of each file and only recorded thereafter when a value changes. The parameters available are listed in Table 1. The maximum available time resolution is 32 seconds for the first six parameters, 8 seconds for the last two. For further information about these parameters, see Smale (1992).

Table 1


     MNEMONIC  PARAMETER		       ACCEPTABLE RANGE

     DETTA     Temperature of the A detector   100K to 128 K
     DETTB     Temperature of the B detector   100K to 128 K
     COVERA    Cover A position monitor	       -5 to 110 % (100% = `cover on')
     COVERB    Cover B position monitor	       -5 to 110 % (100% = `cover on')
     CALIBA    Calibration source A position   -5 to 110 % (100% = `source in')
     CALIBB    Calibration source B position   -5 to 110 % (100% = `source in')
     GUARDA    Guard rate A		       > 0
     GUARDB    Guard rate B		       > 0

Files containing attitude information can be found in the `bbxrt/pointing' directory. The time resolution is 1 second, and the parameters available are RA and DEC (degrees, 1950.0) and the ROLL angle. These pointing data have an accuracy of 10 arcminutes.

Mission Quality data

The `bbxrt/missqual' directory contains files with information about various parameters that are important in assessing data quality. Available parameters are listed in Table 2. The time resolution is 1 second. See Smale (1992) for further details.

Table 2

     SUN ANGLE	         Sun angle from target in degrees (Target-BBXRT-Sun).
     MOON ANGLE	         Moon angle from target in degrees (Target-BBXRT-Moon).
     EARTH ANGLE	 Earth angle from target in degrees (Target-BBXRT-Earth
		         center).
     Bright/Dark         Brightness/darkness of nearest limb.
     Shuttle Day/Night	 Shuttle in day/night.
     RAM ANGLE	         Velocity vector angle from target in degrees.

Spectra, background spectra, light curves and response matrices

The `bbxrt/spectra' directory contains source and background spectra, one per pixel per observation, for each of the 157 source observations. The spectral format conforms to the OGIP/HEASARC standard format (Arnaud et al. 1992). Light curves are similarly available in the `bbxrt/rates' directory.

BBXRT response matrix files are classified by off-axis angle rather than by observation, since there were many observations performed at each given angle. The `bbxrt/responses' directory contains one spectral response matrix per pixel per unit off-axis angle between zero and 10 arcminutes, in the standard FITS RMF (redistribution matrix file) format (George et al. 1992). The correct off-axis angle for each spectrum can be found in its FITS file header.

Compressed files

To assist archival investigators who wish to examine large subsets of the BBXRT archive, groups of the above files are available in compressed tarfiles. The tar utility (allowing the collection and storage of a large number of files in a single `tape archive' file) and the compression utility used are the standard versions available on unix/ultrix systems. The `tar' directory contains two subdirectories, `by_filetype' and `by_observation'. The former contains one compressed tarfile per filetype for the following filetypes: spectra, background spectra, light curves, HK files, mission quality files, pointing files, and response matrices for a given off-axis angle. In the `by_observation' directory users will find 157 tarfiles, each containing the complete event list, HK data, mission quality data, pointing data, source and background spectra, and rates files for one observation.

3. Analysis tools

The adoption of the FITS format makes the entire BBXRT dataset available in a system-independent, self-describing format, and many institutions will have their own suites of programs capable of reading data in this format. The FTOOLS software, designed and written by the HEASARC/ASCA project, are now available to help users manipulate FITS files and analyze the data they

contain. The software runs both under IRAF and as a series of standalone tasks, and a user-friendly command-driven top level program called XSELECT is currently being tested.

References

Arnaud, K.A., George, I.M. & Tennant, A.F., 1992, Legacy, 2, 65.
George, I.M., Arnaud, K.A., Pence, B. & Ruamsuwan, L., 1992, Legacy, 2, 51.
Smale, A.P., 1992, Legacy, 2, 17.

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