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COMPTEL: Help

COMPTEL - CGRO/COMPTEL Low-Level Data and Maps


Overview

COMPTEL is both the name of the Imaging Compton Telescope on the Compton Gamma Ray Observatory (CGRO) and the name of this HEASARC database table which allows the user to select low-level data and maps produced by this instrument. Much more information on the COMPTEL instrument, data products, and data analysis techniques can be obtained from the Compton Observatory Science Support Center (COSSC) web site at
     http://cossc.gsfc.nasa.gov/
and from the article in Legacy No. 7 on the HEASARC CGRO Data Archive
     http://heasarc.gsfc.nasa.gov/docs/journal/cgro7.html
Interactions in COMPTEL occur in a two stage process: First, a Compton collision occurs in one of seven (low-Z) liquid scintillators and is usually then completely absorbed in one of fourteen (high-Z) NaI(Tl) scintillators. The unusual aspect of this detector is that the location of the gamma-ray on the sky is given by an annulus and not a point. The correlation of many events enables the localization of point sources or the creation of sky maps. Time-of-flight measurements, pulse shape discrimination and anti-coincidence shields are used to reject background events. COMPTEL has a wide field of view (about 1 steradian, equivalent to a FWHM of about 40 degrees), an angular resolution under optimal conditions of about 1 degree, and an energy resolution that ranges from 5-10% at 1 MeV. During Phase 1 of the CGRO mission, COMPTEL completed the first all-sky survey in the energy range of 0.75-30 MeV (in four energy bands). In addition, COMPTEL can measure energy spectra of solar flares or bright cosmic gamma-ray bursts between 0.1-10 MeV, and neutrons from solar flares.

Data Products

The Browse Web interface can be used both to identify data files of interest and to extract all such data files that are in FITS format. Finally, anonymous FTP can be used to retrieve both FITS and native binary data files (the latter are used by COMPASS), once the user has identified which files are appropriate.

The COMPTEL archive contains both native format and FITS data. The FITS data files contain the native binary data product (which, in its pre-FITS form is intended solely for use by COMPASS - COMPTEL's Oracle-based data analysis system) together with ancillary information extracted from the COMPASS database which is used to create the FITS header and to properly describe the data. These FITS files have only recently been made available to the public via WWW and FTP. The COMPTEL files in the FTP archive are organized first by observing cycles (phase 1, phase 2, etc.), and, for each observing cycle, by viewing period (e.g. vp337_0 for viewing period 337.0).

                    COMPTEL Data by Type and Name

      Type of Data     Name   Contents
      ---------------- ------ ----------------------------------------------
      Low Level Data   EVP    processed event
      and Housekeeping

                       TIM    Exposures

                       HKD    housekeeping (daily)

                       OAD    orbit and attitude (daily)

                       BVC    barycentric velocity correction

      Binned Events    DRE    binned event (one per energy range)
      Data and MLMs

                       DRX    binned exposure (one per energy range)

                       DRG    binned geometry (one per energy range)

                       MLM    maximum likelihood map (one per energy range)

     Burst Spectral    PBS    processed burst spectra (daily)
     Data
Except as otherwise noted and with the qualifications noted in the section entitled 'Warnings and Caveats,' each file is available on a per viewing period basis. Typically, the standard high-level maximum likelihood map (MLM) data in four energy bands are useful for quick look purposes, as they contain simple FITS skymap images which overlay counts, intensity, and statistical significance for integrated COMPTEL data. Details of the structure of an MLM FITS file can be found in its FITS header.

Warnings and Caveats

In some cases, data obtained in two or more viewing periods have been combined into one dataset (or entry in this database) by the instrument team and marked as belonging to the latest viewing period. An indication of whether a dataset contains several viewing periods can be found in the FITS file header of, for example, the maximum likelihood map (MLM) file. The parameters start_time and stop_time for an entry in this database should be used with caution, as they only refer (for entries which contain multiple observations) to the start_time and stop_time for one of these individual observations (to be precise, the observation corresponding to the viewing period implied by the obsid parameter value). Thus, it is recommended that the user not use time as a selection criterion, but rather use position (RA and Declination, or galactic latitude and longitude) or target name.

Parameters

Proposal_Cycle
The CGRO proposal and observing schedule cycle or phase.

ObsID
Tag for distinguishing COMPTEL data from other instruments data, of the form `C.VP0337_0', where C identifies the COMPTEL instrument and the `VP0337_0' indicates that (at least some of) the data were taken in the CGRO Viewing Period 337.0. See the section `Warning_and_Caveats' for a discussion of the way data for multiple observations may be combined into one entry in this database.

Target
The name of the primary target.

RA
The Right Ascension in the specified equinox of the CGRO z-axis pointing direction; in general this is different from the RA of the source itself, notice.

Dec
The Declination in the specified equinox of the CGRO z-axis pointing direction; in general this is different from the Declination of the source itself, notice.

LII
The Galactic Longitude of the CGRO z-axis pointing direction; in general this is different from the Galactic Longitude of the source itself, notice.

BII
The Galactic Latitude of the CGRO z-axis pointing direction; in general this is different from the Galactic Latitude of the source itself, notice.

Start_Time
The start time of (at least one segment of) the observation in UT (or GMT). The time displayed here is accurate to the nearest milliday (about a minute and a half), and is shown in ISO format in W3Browse (e.g., 1991-05-30 20:01 UTC) and in Year.month hr:min:sec format in BROWSE (e.g., 91.150 20:01). See the section `Warning_and_Caveats' for a discussion of the way data for multiple observations may be combined into one entry in this database.

Stop_Time
The end time of (at least one segment of) the observation in UT (or GMT). The time displayed here is accurate to the nearest milliday (about a minute and a half), and is shown in ISO format in W3Browse (e.g., 1991-05-30 20:01 UTC) and in Year.month hr:min:sec format in BROWSE (e.g., 91.150 20:01). See the section `Warning_and_Caveats' for a discussion of the way data for multiple observations may be combined into one entry in this database.

Exposure
The exposure time in seconds.


Contact Person

Questions regarding the COMPTEL database table can be addressed to the HEASARC User Hotline.
If you have any problems, please consult the help page or mail ledas-help@star.le.ac.uk
 
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