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Compton Gamma-Ray Observatory Science Support Center

OSSE High Level Product Descriptions

The Nature of SDB Files

Graphical Products

The graphical products described below can be displayed and downloaded as PostScript files, through the High Level Selector pages.
Averaged Count & Photon Spectrum
The Averaged Count Spectrum displays the the background-subtracted photon counts by energy received by the OSSE detectors over the duration of the observation. The averaged count spectrum is deconvolved through the Detector Response Matrix (DRM) to generate the averaged photon spectrum. The mathematical nature of this deconvolution requires the photon spectrum be generated from the count spectrum and DRM using a Forward Folding process. For this spectrum, the model assumed is an E^(-2.0) power law.

Detector Pointing Strategy
This plot illustrates the pointing strategy used by the four OSSE detectors for collection of source and background spectra. This is useful for recognizing possible confusing sources in the field-of-view.

Daily Count Spectra
This plots the background-subtracted count spectrum by detector averaged over each day of the observation,

Light Curves by Energy Band
These are plots of the daily-averaged, background-subtracted counts over the duration of the observation. It is separated into four energy bands: 0.05-0.15 MeV, 0.15-0.30 MeV, 0.30-0.80 MeV, and 0.80-1.50 MeV. This is useful for studying time-variability (on day time-scales) in the source.

Count Spectra Averaged by Detector (two versions)
These are background-subtracted count spectra averaged over the observation for each detector. Each plot is performed in both linear and logarithmic energy scaling and with two different energy binnings.

Data Files

The data files described below are available through the High Level Selector pages. These files are useful for more extensive analysis than was performed for the graphical products.
Products for use with XSPEC
The Averaged Count Spectrum and the Detector Response Matrix (two separate files) are available in FITS format compatible with the XSPEC program available from HEASARC.

Data files in text format
These files can be viewed directly on the Web or easily downloaded as text files.

  • Averaged Photon Spectrum: The observation-averaged photon spectrum (already deconvolved using the E^-2 model) is available in a text format suitable for loading into your own plotting routine.
  • Observation Logs: These contain information on the observation such as source confusion issues along with spacecraft and detector status. Researchers should always read these as part of their interpretation of the data presented in this archive.


High Level SDB Files
OSSE Spectral Data Base (SDB) files of background-subtracted spectra packages with various time resolutions (from two-minute dwells to the entire viewing period) are available for more detailed processing using IGORE.

  • "Bottom-Line" SDB File: This file contains the observation-averaged, background-subtracted photon spectra already deconvolved through the E-2 model.
  • Totalled SDB File: This file contains the observation-averaged, background-subtracted count spectra by detector, along with the observation-averaged detector response matrix.
  • Good-Summed SDB File: This file contains daily-averaged, background-subtracted count spectra by detector.
  • Daily Difference SDB Files: There are generally multiple files of this type, containing background-subtracted spectra by detector for each dwell (normally about two minutes) that a detector is pointing on-source.
A cautionary note: SDB files are designed for use on the VMS operating system with IGORE. Special steps must be taken to download usable SDB files from this archive. Click here to connect to a page of special instructions for downloading these files.

Using the High Level Data Products

Some data products, particularly the graphical products, can be utilized directly after downloading from the archive. However, more detailed analyses, such as forward-fold fitting of photon spectra (Say that three times fast!), more work must be done.

Users familiar with XSPEC can use the files generated for just this purpose. To perform model fitting with IGORE, you need to run FITIT using the Totalled SDB (*TOT.SDB) file which contains the observation-averaged count spectra (by detector) and the response matrix.

If you wish to break down the observation into daily, or multi-day spectra, you must use the Good-Summed SDB (*GSM.SDB) file with the FFIT_PREP routine in IGORE. You can then perform model fits on the daily spectra using FITIT.

Contact the COSSC or click on this link to FTP a copy of the GRO/OSSE Guest Investigator Guide which has detailed instructions on using IGORE, FFIT_PREP, and FITIT.
Beyond the High Level Products

Why might you need to analyze data beyond the High Level Products?
  • Your observation might fall under one of the categories described under What is not in this Archive.


  • You need to correct for instrument systematics that were either unknown or uncorrectable at the time the High Level Products were generated.


  • You have a highly specialized application in mind which I have no clue of how to describe.


In these situations, you may have no choice but to conduct your data analysis starting with the low-level data products and IGORE.

Contact the COSSC for assistance and a copy of the GRO/OSSE Guest Investigator Guide which has more detailed information on the use of IGORE and his friends.
What is not in the High Level Archive

At present, there are two basic types of observations that are not included in this archive at the present time:
  • Staring/Timing Observations: These observations generally have no background pointing and are generally used for pulsar or time-series type analyses.


  • Serious unresolved systematics: If the observation contains major systematic errors that currently cannot be resolved, it may be excluded from this part of the archive.


In all the above cases, you must examine the low-level data products.


If you have a question about CGRO, please contact us via the Feedback form.

This page was last modified on Monday, 01-Aug-2005 13:42:43 EDT.

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