COSB - COS-B Observation Catalog
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
spark chamber telescope with approximately a 20 degree field of view.
COS-B operated in a highly eccentric polar orbit with apogee around
90000 km between 17 August 1975 and 25 April 1982. During this operational
lifetime, COS-B made 65 observations, 15 of which were devoted to high
(greater than 20 degrees) galactic latitudes.
The COSB database is a log of the 65 COS-B observation intervals and
contains target names, sky coordinates and start times taken from the
final COS-B database. This final database consisted of three basic datasets:
`OBSLI`, a dataset describing each observation period, typically a month;
`OURLI`, a dataset describing each uninterrupted observation interval,
lasting between 10 minutes and 10 hours; and `GAMLI`, a dataset containing
records for each accepted gamma-ray photon. These three data sets were
combined into FITS format pseudo-images at NASA/GSFC. The pseudo-images were
formed by making the center pixel of a 1024 x 1024 pixel image correspond
to the RA and DEC given in the `OBSLI` file. Each photon's RA and DEC was
converted to a relative pixel in the image. This was done by using Aitoff
projections. All the data from all three COS-B files are now stored in 65
FITS files accessible with BROWSE software. The pseudo-images can be accessed
and plotted using XIMAGE and other parts of the data can be plotted with the
routine FPLOT.
This database contains Aitoff projection images of each COS-B photon
in Flexible Image Transport System (FITS) format. There are 65 FITS
files, one for each COS-B observation period.
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 sparkchamber, 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 thoroughly investigated and integrated into the database. The
possible impact of their statistical and systematic uncertainties must be
considered in any type of analysis.
The COS-B mission lasted about 6.8 years and during this time the sensitivity
of the experiment and the instrumental background varied due to several
effects. Many of these effects have been taken into account when deriving the
final database; others remain embedded within it. Since the timescales of
these effects, ranging from hours to years, only allowed a partial correction,
this chapter is included to avoid the user being misled by temporal or other
artifacts in the data when searching for time variability of gamma-ray
sources, or intensities in regions of weak emission or at large incidence
angles relative to the telescope axis.
Malfunctions_and_Configuration_Changes
Throughout the mission, and especially in the early months when tuning of the
instrument took place, the triggering criteria varied. For several
observations the trigger pulse from the energy calorimeter was required to be
observed in coincidence with the triggering telescope. This obviously affected
the trigger rate and possibly the detection efficiency. As this trigger pulse
failed in the late part of the mission, it was decided to normalize all
trigger conditions "by software" to the equivalent triggering thresholds, so
that the instrument may be considered to have a single mode.
This change to software thresholds might have made the sensitivity more
susceptible to the influence of occasional electromagnetic interferences,
created by the sparkchamber discharge currents, by modifying the content of
the counters temporarily stored in the experiment electronics for readout. As
a consequence an increasing fraction of events might have been lost in the
late phases of the mission.
Alternately the observed longterm reduction of sensitivity, especially in the
second part of the mission, could be due at least partially to the slowly
decreasing efficiency of the sparkchamber, possibly connected to "cracking
products" of the quenching agent contained in the sparkchamber gas which are
produced by the spark discharges and are deposited on the sparkchamber wires.
The gain changes in the energy calorimeter were corrected using in-flight
proton data and may therefore be disregarded.
During Jan 1978 a veto PMT failed, giving an increased trigger rate in the
detector temporarily. Although the dead time factor is correctly calculated, a
reduction in efficiency was observed during this interval. A comparable effect
occurred in June 1979 when a coincidence flag from the energy calorimeter
failed. In this case the rate of triggers was reduced for a short period until
a solution could be implemented. The efficiency was thereby reduced during
this approximately 3-day interval.
Sparkchamber_Efficiency_Changes
Three effects have been observed to occur in the sparkchamber throughout the
mission: long term degeneration, odd-even gap effects and corner sparks.
Long term degeneration is a result of the sparkchamber gas composition being
altered by the spark-discharges and possibly by sedimentation of cracking
products onto the wires. A supply of gas was carried on board which allowed
the periodic renewal (flushing) of the used gas. This operation was performed
22 times during the COS-B mission, initially at 6 week intervals and
stretching to 20 weeks at the end of the mission. The decision to flush the
chamber was subjectively made, and was based upon the apparent quality of the
sparkchamber pictures. Although initially flushing restored the chamber to its
previously high efficiency, the improvement is never seen as a "step function"
in the experiment's "sensitive area". Altogether a continuous overall
reduction of the experiment is observed over the entire mission.
This is taken into account in the relative sensitivities given in the
database. However the shorter term effects of the flushings remain in the data
and must also be taken into account.
The odd-even gap-loss effect was not anticipated and occurred during several
periods, particularly in the second year of the mission. Repeatedly for a
period of time, ranging from minutes to days, the sparkchamber pictures were
missing either the odd-gap or even-gap positional information. This was a
consequence of an undue delay in one of the two spark gaps which were
triggering the two subsets of sparkchamber modules into which the sparkchamber
was divided for redundancy reasons.
After several "curing" attempts were made (either by performing a "burn-in"
procedure or by switching to redundant trigger units), the problem was
practically reduced to a level which no longer affected the efficiency over
long periods. The sporadic reappearance over relatively short time intervals
has been observed and must be kept in mind when time variability is
investigated. The loss in efficiency is corrected for on an observational
period basis within the database.
Corner sparks are parasitic sparks which appear in one or two corners of the
sparkchamber when the gas has deteriorated or, more generally, towards the end
of the mission. These have been removed from the data by rejecting events
having origins in the affected areas. The remaining contamination is
negligible. The related reduction in efficiency is at the 1% level and is
compensated by the corrections already described.
Human_Scanning_Effects
The sparkchamber analysis was performed in two parts: first, an automatic
pattern recognition program assigned the most likely gamma-ray events
(usually less than 4% of all recorded events); second, these events were
subjected to a visual scan to check for correct track assignment and
classification. The editor could make reassignment of the tracks and reject
the obvious background triggers mimicking the gamma-ray pattern.
Over 2 million events have been manually edited by about a dozen operators
from 3 institutes during 8 years; hence the editing standard is somewhat
variable. To measure the size of this variability the anticenter observation,
39, was edited in all three institutes. The differences in source intensity
derived for the Crab and Geminga in the three data sets was (approx. 10%)
although on an event-by-event basis the data showed technical differences. The
major effect of the different editing standard is in the background rejection,
some institutes being more discriminating than others. Differences in angular
resolution between the different establishments cannot be excluded, although
are probably of second order.
With this understanding of what can cause temporary variations in the COS-B
performance, it is recommended that a background sample from the same
observation period be examined before claims of source variability are made.
Instrumental_Background
Due to its eccentric polar orbit with an apogee around 90000 km, the COS-B
satellite was exposed to the full cosmic-ray flux, unshielded by the earth's
magnetic field, which is modulated by the effects of solar activity on
timescales of minutes to years, primarily in correspondence to the sun's
11-year cycle.
This orbit was chosen for technical advantages in data transmission, to obtain
long uninterrupted observation intervals (32 hours out of the 36 hour orbital
period) and to gain observation time which in a low orbit for a spinning
satellite is lost by earth occultation of the field of view.
Onboard scintillation counters combined into the "scaler-3 rate" of the
trigger telescope could be demonstrated to closely trace the cosmic-ray flux
modulated by solar activity. When all gamma-ray data were available from the
mission, the variable fraction of the COS-B gamma-ray background could be
related to the "scaler-3 rate". Unfortunately there remains the larger
fraction of the likely "instrumental" background not modulated in time. A
large modulation is expected only for low energy protons and especially
electrons, which might be of special importance here, while for highly
relativistic protons only a modulation of a few percent is occurring.
Therefore a significant time invariant instrumental background is seen which
remains indistinguishable from any possible celestial (galactic or
extragalactic) isotropic emission.
The instrumental background, when described in the form of a sky photon
intensity, need not necessarily appear "isotropic", and actually is found to
show a variation with inclination to the telescope's axis.
Name
The source name.
RA
The Right Ascension (1950) in degrees of the optimum observation direction.
With a FOV of approximately 20 degrees, photons associated with a particular
observation should be within 20 degrees of this RA.
Dec
The Declination (1950) in degrees of the optimum observation direction. With a
FOV of approximately 20 degrees, photons associated with a particular
observation should be within 20 degrees of this DEC.
LII
The Galactic Longitude (1950) in degrees of the optimum observation direction.
With a FOV of approximately 20 degrees, photons associated with a particular
observation should be within 20 degrees of this LII.
BII
The Galactic Latitude (1950) in degrees of the optimum observation direction.
With a FOV of approximately 20 degrees, photons associated with a particular
observation should be within 20 degrees of this BII.
PI
The visual scanning facility associated with a particular observation. PI
can be one of three choices: Garching , Saclay , and Noordwijk.
X_Pixels
Right Ascension reference point location.
Y_Pixels
Declination reference point location.
Time
The start time of the observation.
End_Time
The stop time of the observation.
Exposure
The sum of all the continuous, uninterrupted exposures within an
observation in seconds. Exposure is approximately the "up-time" of the
instrument during each of the 65 observations.
Photons
The number of photons collected during an observation. Dividing PHOTONS
by EXPOSURE gives an approximate value for the flux of an observation.
Count_Rate
The average scalar-3 countrate for an observation. Indicates the energetic
charged particle rate seen by the scintillation-counter telescope within
the experiment.
Class
The BROWSE object classification flag.
Mayer-Hasselwander H.A. 1985, "Explanatory Supplement to the COS-B database".
Questions regarding the COSB database table can be addressed to the
HEASARC User Hotline.
Page Author: Browse Software Development Team
Last Modified: Thursday, 18-Jul-2002 12:55:50 EDT