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The SAS-2 and COS-B
Paul Barrett & Brendan Perry
In Volume 5 of the `Annual Review of Astronomy and Astrophysics' (1967),
Giovanni Fazio wrote that "... up until now, no photons of energy greater than
100 keV originating from beyond the solar system have definitely been
detected." During the next few years, many groups using balloon-borne detectors
reported detections of gamma-ray sources. However, such experiments were
hampered by the high level of atmospheric gamma-rays due to cosmic ray
interactions in the atmosphere, making the statistical significance of such
sources low and their existence doubtful. Though several gamma-ray detectors
were placed in orbit, the first certain detection of celestial gamma-rays came
from OSO-3, which detected emission of gamma-rays with energies greater than 50
MeV from the galactic disk with a peak intensity toward the galactic center.
The first unambiguous detection of high-energy (>50 MeV) gamma-ray sources
was achieved with the SAS-2 satellite. Unfortunately, the lifetime of the SAS-2
mission was cut short after about six months by the failure of a low-voltage
power supply. The European Space Agency's COS-B satellite was the next major
high-energy gamma-ray detector. It's lifetime was considerably longer than
that of SAS-2 and was turned off after about 8 years when on-board resources
were exhausted. The second COS-B catalog containing the positions of 25
sources was the most complete listing of high-energy gamma-ray sources until
the recent release of the Compton/EGRET catalog (Fichtel et al. 1993).
Since the launch of the Compton Observatory in April 1991, there has been
renewed interest in the SAS-2 and COS-B data. For example, the data has been
used to check for long-term (of order years) variability of sources discovered
in the EGRET all-sky survey and to improve the ephemeris of the Geminga pulsar
(see e.g. Legacy, No.2) by extending its timeline. Therefore, HEASARC
has decided to make this data available publicly and this article may be
considered an announcement of its availability. The intention of this article
is to give an overview of the two datasets. Details of the restoration and FITS
formats for the data, and associated calibration data, can be found in other
OGIP documents located on the FTP server legacy (see the article in
Legacy, No.3 by Drake and O'Neel).
2. The SAS-2 Mission
The second NASA Small Astronomy Satellite (SAS-2) was dedicated to gamma-ray
astronomy in the energy range above 35 MeV. The satellite carried a single
telescope using a 32-level wire spark-chamber. The satellite was spin
stabilized with the telescope axis along the spin axis. SAS-2 was launched on
1972 November 15 and became operational on 1972 November 19. On 1973 June 8, a
failure of the low-voltage power supply ended the collection of data. During
the approximately six months of the mission, 27 pointed observations, typically
of a week duration, were made resulting in about 55 percent of the sky being
observed, including most of the galactic plane. The field-of-view of the
telescope is about 35 degrees (full width at half maximum) with an angular
resolution of a few degrees. In addition to the general galactic emission,
high-energy gamma-rays were also seen from the Crab and Vela pulsars (see e.g.
Fichtel et al 1975).
The low fluxes involved in the study of gamma-ray sources make it desirable to
minimize the background flux from cosmic-rays. Therefore a low Earth equatorial
orbit was chosen having a 2 degree inclination, an apogee and perigee of 610 km
and 440 km, respectively, and an orbital period of about 95 minutes. The
sensitivity of the sparkchamber was noticed to decrease during the lifetime of
the mission as the sparks from the gamma-ray event caused the gas to crack
producing unwanted by-products. Do to the degradation of the gas and the
inability to flush and replace it with new gas, the lifetime of the mission
would not have been more than about 9 months, in any case. The calibration of
the SAS-2 experiment was done using both the flight unit and an identical
flight spare unit. The range of energy studied at the National Bureau of
Standards (NBS) Synchrotron, Gaithersburg, Maryland, was approximately 20 to
114 MeV. The energy range between 200 to 1000 MeV was studied at the Deutsches
Elektronen-Synchrotron (DESY), Hamburg, West Germany.
The format of the SAS-2 data is based on a ROSAT Events FITS file. Briefly,
such a file contains a primary header and four extensions. No data is
contained in the primary header. All data resides in the four extensions which
are named ALLGTI, STDGTI, EVENTS, and TSI. The ALLGTI and STDGTI extensions
contain the exposure information. The ALLGTI gives the time interval when the
instrument was on and ready to receive data, whereas the STDGTI gives the time
interval when data, deemed to be good by the standard processing software, were
received by the instrument. Therefore, the total exposure from the STDGTI is
usually less than that from the ALLGTI. The EVENTS extension contains a list of
all the events for an observation, and the TSI extension is for the
housekeeping data or the instrumental status.
In order to restore fully the SAS-2 data (i.e.. provide event, exposure, and
housekeeping data), access to the original data was necessary. The SAS-2 event
list was commonly used by researchers, but the exposure information was never
made available. The original data provided us with the orbital parameters and
status of the spacecraft, from which we were able to calculate the necessary
exposure data and extract the auxiliary housekeeping data. Thus, for the first
time the SAS-2 data has been put into a form that is compatible with that of
COS-B and EGRET data. The SAS-2 data has been divided into 27 files, one for
each pointing. The total number of events that satisfied the trigger logic of
the spark-chamber and post-flight human and software selection was 13056.
In addition to the event data, calibration data also has been made available.
There are four types of calibration files: 1) the redistribution matrix or
energy dispersion data, 2) the effective area or sensitivity data, 3) the
point-spread-function data, and 4) the energy boundary data. The calibration
data was not available in the original data, but was gleaned from the
literature (see Fichtel et al. 1975). More detailed information about these
files can be obtained from the `SAS-2 Calibration Guide' in the legacy
3. The COS-B Mission
The European Space Agency's satellite COS-B was dedicated to gamma-ray
astronomy in the energy range 50 MeV to 5 GeV and carried a single experiment:
a spark-chamber telescope, developed by six European institutes in
collaboration. The experiment became operational on 1975 August 17 and was
switched off on 1982 April 25 when on-board resources were exhausted. During
this period, 65 observations, typically of a month's duration, were performed.
The satellite was spin-stabilized with the telescope axis along the spin axis.
Circular sky regions of about 40 degrees in diameter were covered in each
observation. The majority of the pointings were distributed along the galactic
equator, 15 observations were devoted to regions at high (>20 degree)
galactic latitudes. Several regions of specific interest were observed
The highly eccentric polar orbit of COS-B, with an apogee around 90,000 km
(chosen to maximize useful observation time while allowing real-time data
transmission) exposed the experiment to the solar modulated interplanetary
cosmic-ray flux. The unexpectedly long operational life of the experiment,
specifically of the spark-chamber, was accompanied by a long-term degradation
and by short-term disturbances of its performances and, consequently, of the
experimental sensitivity. The variation and sensitivity of the instrumental
background were investigated thoroughly and integrated into the database. The
possible impact of their statistical and systematic uncertainties must be
considered in any type of analysis.
The format of the COS-B data, like the previously discussed SAS-2 data, is
based on a ROSAT Events FITS file. The COS-B FITS files are based on the final
COS-B database (see H.A. Meyer-Hasselwander et al., 1986 for a detailed
description). The original database contained three files: the observation
file, the exposure file, and the event-list file, with indices in one file
pointing to records in another file. The design of this database is similar to
the extensions in a ROSAT Events FITS file, making the creation of the COS-B
FITS files easier than those of SAS-2. The completed database contains 65 files
corresponding to the 65 observation periods containing all of the exposure and
event information for that period.
During the processing of the data, we decided to scan each of the three
original files for data integrity. For the event-list file, we checked the
following parameters for their proper values (as described in the "Explanatory
Supplement to the COS-B Final Database" by H.A. Meyer-Hasselwander): gamma
class, edit class, photon energy, right ascension, and declination. We found
that some events have a `photon energy' of 0 MeV, a `declination' of +95
degrees and an `edit class' larger than three. These 750 events were obviously
not meaningful and were deleted from the database. Additionally, the `gamma
class' was found to have more than the three cases (2, 22, and 3) as specified
in the "Explanatory Supplement." Since the energy and coordinate of the event
was acceptable, these events were not deleted from the database. Details of
other minor discrepancies can be found in the "HEASARC COS-B Database
Document", which is currently in preparation.
4. Using the Data
The main reason for restoring the SAS-2 and COS-B datasets is to make them
available in a standard FITS format that is useful to the High Energy
Astrophysics community. The rationalized FITS format developed by the HEASARC
and the ROSAT Guest Observer Facility was adopted as the standard. The ROSAT
and ASCA missions are using this format as their standard and EGRET data also
will be translated into this format, as will most future HEA photon event data.
The availability of SAS-2, COS-B, and EGRET data in this format allows the data
to be imported easily into the IRAF/PROS software package and XANADU software
package, including XSELECT, a set of tasks for analyzing HEA data in FITS
In addition a program called FADMAP, which is based on a program of the same
name for analyzing COS-B data, will be made available soon. FADMAP produces
three maps or images: a background counts map, a source counts map, and an
exposure map. A fourth map, a flux map, is produced from these three maps. By
using FTOOLS tasks, it will be possible to combine data from SAS-2, COS-B, and
EGRET into a FITS file that then can be read into FADMAP or any of the other
previously mentioned data analysis software.
The data are currently available via HEASARC's anonymous ftp account and are
located in the public directories, cosb/ and sas2/, on the
host, legacy.gsfc.nasa.gov (220.127.116.11). Each directory contains
several subdirectories, data/, doc/, and
calib_data/, along with .message and README files
describing the contents and status of the directory. The calib_data/
subdirectory also has several subdirectories including a doc/
subdirectory that currently contains LaTeX and PostScript files of the SAS-2
and COS-B Calibration Guide.
Access to these data is possible via anonymous ftp or Gopher. For more
information about using ftp or Gopher to access the HEASARC anonymous ftp
account, see the article by Drake et al. in this issue of Legacy.
5. Future Directions
The SAS-2 and COS-B FITS files may undergo some revision in the future to make
the data more accessible to the high-energy astrophysics community, though we
expect these changes to be minor. Aspects that may affect these changes are:
1) the compatibility of the two datasets with each other. Because SAS-2 and
COS-B were different instruments from different experimental groups, the event
data and housekeeping data were likewise different. Some incompatibility still
exists between the two datasets mostly due to differences of the housekeeping
data. Such changes may be resolved in the future allowing data analysis
software to access the data in a more consistent manner.
2) the compatibility of the two datasets with the Compton Gamma-Ray Observatory
Science Support Center's data analysis software. Some of the data analysis
software is designed to analyze EGRET data. Changes to the SAS-2 and COS-B FITS
files also would allow this data to be analyzed using the same data analysis
In a future issue of Legacy, we plan to describe how to use the tasks
available in the XANADU software package, including FADMAP, to analyze the
COS-B and SAS-2 data.
"The HEASARC's Newly Consolidated Anonymous FTP Account", 1993, S. Drake &
B. O'Neel, Legacy, 3, 53.
"Gamma Radiation from Celestial Objects", 1967, G.G. Fazio, , Ann. Rev.
Astr. Ap., 5, 481.
"High-Energy Gamma-Ray Results from the Second Small Astronomy Satellite",
1975, C.E. Fichtel, R.C. Hartman, D.A. Kniffen, D.J. Thompson, G.F. Bignami, H.
Ogelman, M.E. Ozel, & T. Tumer, Ap. J., 198, 163.
"First Energetic Gamma-Ray Experiment Telescope Catalog",1993, C.E. Fichtel et
al. , Ap. J. Supp., submitted.
"Explanatory Supplement to the COS-B Final Database", 1986, H.A.
MeyerHasselwander et al., in Proc. Cosmic Ray Conf., La Jolla, ESA
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