COSBRAW - COS-B Photon Events Catalog

Overview

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 (>20 deg) galactic latitudes.

The COSBRAW database table is a log of the 65 COS-B observation intervals and contains target names, sky coordinates start times and other information taken from the final COS-B database produced by ESA in 1985. 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 images at NASA/GSFC. The 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 raw data from these three COS-B files are now stored in 65 FITS files accessible with BROWSE software in the database COSBRAW. The images can be accessed and plotted using XIMAGE and other columns of the FITS file extensions can be plotted with the FTOOL FPLOT.

References

Mayer-Hasselwander H.A. 1985, "Explanatory Supplement to the COS-B Database".

Data Products

This database contains one entry for each FITS file of the original 65 COS-B events file observations. Each file consists of 3 extensions: a GTI (Good Time Intervals), an EVENTS and an EXPOSURE extension. the GTI extension contains consecutive 'up-times' within the observation. The Events extension contains pointing, timing and energy information about each detected photon. The Exposure extension contains pointing and on-time information about each exposure from the old 'OBSLI' file (see Database Overview for more information).

This data can be accessed and analyzed in two ways. The events can be plotted for quicklook observing using the BROWSE on-line command XIMAGE, or a file can be extracted from the database, then the analysis FTOOL FADMAP can be used on the file to produce source and background maps from the extracted file.

To run FADMAP, type xp # (where # is a number between 1 and 65, for the desired observation). When the file has been downloaded into your captive account area, run the FTOOL FADMAP, entering the desired FITS file name to be analysed at the prompt. If you have a FADMAP.PAR file already in your account area, you can change the hidden parameters to values other than the default by either editing that file, or by typing any parameters you want to change on the initial command line (type 'fhelp fadmap' from the command line for more details). Fadmap will create 4 files: 'source count', 'source exposure', 'modified background count' and 'modified background exposure'. These files can be further analyzed by running the FTOOL FARITH from your captive account. One option is to first divide the 'source count' map by the 'source exposure', divide the 'background count' map by the 'background exposure' map, then subtract the first resultant file by the second. This will give a fairly accurate 'intensity' map. For more information COS-B data analysis, consult the on-line "Explanatory Supplement to the COS-B database". See the references section of this dbhelp entry for more information

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 table. 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.

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.

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.

Quality of Observations

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 information 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.

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.

Parameters

RA
The Right Ascension 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 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 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 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.

Observation_ID
Original obsli observation identification number. Numbered from 00 to 64.

Time
The start time of the observation. The time is displayed in the format 'yy.ddd' where yy is the last two digits of the year and ddd is the day number within the year (cf: 79.318, is 1979, day 318)

End_Time
The stop time of the observation. The time is displayed in the format 'yy.ddd' where yy is the last two digits of the year and ddd is the day number within the year (cf: 79.318, is 1979, day 318)

Name
The source name. If one does not exists, this parameter defaults to 'N/A'.

Telapse
Elapsed time between start of an observation file and the end of an observation file. Inclusive time, in seconds of entire 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.

S3rate
The average 'scalar-3' countrate for an observation. Indicates the energetic charged particle rate seen by the scintillation-counter telescope within the experiment. It is used for a correction of the apparent variation of the instrumental background in phase with the solar cycle modulated intensity of the cosmic-ray environment of the satellite. This correction involves systematic uncertainties and a value is only derived on an observation period basis.

Rsense
The relative sensitivity is a correction factor, which is derived by 'self-calibration' using all data observed over the mission and is defined as an average value for each observation period and for the total energy range. The sensitivity is known to vary on a smaller timescale, but a sufficiently reliable correction on these smaller timescales was not feasible for the total data. If a specific investigation is sensitive to such variations, possible short term variations must be assessed as part of the investigations.

Class
The BROWSE object classification code.

File_Image
The name of the events file in the archive associated with each observation. File is of the form cb_hhmmVddP**.evt, where cb is the instrument (Cos-B), hhmm is the right ascension in degrees and minutes, V is N for positive declinations and S is for negative declination, dd is the declination in degrees, P is for 'Pointed' observations '**' is the original observation number, from 00 to 64, and evt is the FITS file 'type' ('evt' for 'events file').

Contact Person

Questions regarding the COSBRAW database table can be addressed to the HEASARC Help Desk.

Page Author: Browse Software Development Team
Last Modified: Monday, 16-Sep-2024 17:26:17 EDT