COSBMAPS - COS-B Map Product 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.

This database is a collection of maps created from the 65 COS-B observation files. The original observation files can be accessed within BROWSE by changing to the COSBRAW database. For each of the COS-B observation files, the analysis package FADMAP was run and the resulting maps, plus GIF images created from these maps, were collected into this database. Each map is a 120 x 120 pixel FITS format image with 0.5 degree pixels. The user may reconstruct any of these maps within the captive account by running FADMAP from the command line after extracting a file from within the COSBRAW database.

The parameters used for selecting data for these product map files are embedded keywords in the FITS maps themselves. These parameters are set in FADMAP, and for the maps in this database are set as 'wide open' as possible. That is, except for selecting on each of 4 energy ranges, all other FADMAP parameters were set using broad criteria. To find more information about how to run FADMAP on the raw event's file, the user can access help files within the COSBRAW database or can use the 'fhelp' facility from the command line to gain information about FADMAP.

References

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

Data Products

This database contains 12 FITS images for each of the original 65 COS-B photon events files. This database was created by running the FTOOL FADMAP on the events files 4 times, in 4 different energy ranges (low, medium, high and 'full'). Running FADMAP on an events file produces four product maps: a "source count" map, a "source exposure" map, a "corrected background count" map and a "corrected background exposure" map. Retained here for the database are only the source count and source exposure maps.

The source count map was divided by the source exposure map using the FTOOL FARITH to produce a "modified flux" map. For quicklook observations this product is the most useful. A GIF image was also created for each file in this database. Thus, for each pointing there exists a "source counts". a "source exposure" and a "modified flux" map in each of the 4 energy ranges, plus of GIF of each of these maps.

The energy ranges selected for COS-B are as follows: "low" (70-150 MeV), "medium" (150-300 MeV), "high" (300-5000 MeV) and "full" (50-7800 MeV).

For each database entry, you will be able to extract any of the 12 maps for viewing or further analysis. Reproduction of any of these files, using the original events files can be done by running FADMAP on them with user selected parameters or the defaults. This method can also be used to reproduce the "background count" and "background exposure" maps for analysis. To do this, change databases to COSBRAW, extract the desired events file, and run FADMAP on that file. See the database COSBRAW for more details.

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.

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 "scalar-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 "scalar-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

BII
The Galactic Latitude in degrees of the optimum observation direction.

Class
The BROWSE object classification code.

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.

E_Max
The maximum energy in this file. The program FADMAP was run on the raw data and photons were selected in one of 4 energy ranges. E MIN and E MAX show the energy boundary for a given file.

E_Min
The minimum energy in this file. The program FADMAP was run on the raw data and photons were selected in one of 4 energy ranges. E MIN and E MAX show the energy boundary for a given file.

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)

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.

File_Image
The name of the events file in the archive associated with each observation. File is of the form cb_hhmmVddP**t_b.xxx, 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, t is the datafile 'type' (b for 'background', s for 'source'), b is the 'band' (l for 'low', m for 'medium', h for 'high', and t for 'total') and xxx is the FITS file 'type' (img for 'image', exp for 'exposure', flux for 'modified flux map' or gif for 'graphical image format'). There will be one GIF image for the source image, exposure map and flux map. These GIFs will have the form cb_hhmmVddP**t_b_X.gif, where all the values are the same as for the ordinary FITS maps, but the value 'X' will distinguish which file is represented in the GIF. 'X' will be 'i' (for image), 'e' (for exposure) and 'f' (for flux). An example filename would be cb_1721s44p24s_l_i.gif. This file is a COS-B point centered on RA=17h21m, DEC=-44, the original COS-B point number 24, source image file in the 'low' energy band, in GIF format.

LII
The Galactic Longitude in degrees of the optimum observation direction.

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

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

Photons
The number of photons collected during an observation. Dividing PHOTONS by EXPOSURE gives an approximate value for the flux of an observation.

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.

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

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)

Contact Person

Questions regarding the COSBMAPS 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