CMA - EXOSAT CMA Images/Lightcurves
The results from two EXOSAT LE telescopes. Each telescope had a channel
multiplier array, CMA, detector in the focal plane. The CMA/telescope
combination covered the 0.05-2.0 keV energy range with a field of view of 2
degrees and an on-axis angular resolution of 20 arc sec (HEW). The detectors
had no intrinsic spectral capabilities; however, different filters were
used to make broad band measurements. The most commonly used were
thin lexan (no 7), Aluminum-Parylene (no 6) and Boron (no 8).
An image was generated for each observation using a particular filter. A
detection program was used to generate one database entry per source detected
above a particular threshold. The CMA database contains a list of detected
sources, details about those detections, plus the names of the files
containing the associated image and lightcurves.
Since many objects were observed many times, there are multiple entries
De Korte P.A.J. et al., 1981, Sp. Sci. Rev., Vol. 30, p. 495.
White N.E. and Peacock A.P. 1988, in X-ray Astronomy with
EXOSAT, [eds. N.E. White and R. Pallivicini], Memoria S. A. It, p. 59.
Images and lightcurves are available for this database.
The data products were created during the post-operational phase
at the EXOSAT Observatory using a standard, automatic analysis of the data.
The LE auto-produced high-quality results in the vast majority of cases. In
some instances, however, problems were encountered. Most of the resulting
inaccuracies or errors have been removed during a systematic program of
quality checking. Although great care has been taken with this quality
checking, the monotonous nature of the work means that a few bad entries may
be lurking in the database.
The most frequent problems experienced are:
* Extended sources: Many extended sources will not have been detected and
the count rates of those that are detected can be severely underestimated.
This is due to the fact that the program used to detect sources was
optimized for point-like sources. Users interested in studying extended
sources should extract the relevant images (which are available online)
and perform a more detailed analysis.
* Confused sources: In a few cases sources were detected near a bright
object. In these cases it is possible that the lightcurve of the fainter
source is contaminated by photons from the brightest object. One such
example is the quasar EXO1102.7+2523 which lies only 2.5 arc min from the
bright AM Her-type object CW 1103+254.
* Boron filter: A special Boron PSF was used to estimate the count rate of
sources detected in this filter configuration. However, since the Boron PSF
is a function of the source's spectral shape, it is impossible to apply the
most appropriate PSF in an automatic process. This limitation leads to a
possible underestimation of the count rate which can be as large as a
factor of two for unabsorbed sources with very steep soft X-ray spectra.
* Spurious sources: There are two kinds of spurious sources - those due
to statistical fluctuation of the background and those due to the inability
of the software to cope with unusual situations. In the latter case these
are removed from the database as soon as they are recognized as such. These
sources are found near bright objects in very short exposures ( <1000
seconds), in images where a diamond filter was used, or when a bright,
extended source is present in the field of view (e.g., Cas A).
The detection threshold of a source is a complicated function of
exposure time, image background and position in the field of view.
This threshold is set so that at most one spurious detection is
expected in every 5 images. More than 3000 images were analyzed and
about 600 spurious sources are expected to be present in the LE
database across the full field of view. The vast majority of these will
be outside the central 15 arc min radius of the detector.
The reality of a source can be determined on the basis of
additional information such as significance of detection, exposure
time, background level, distance from the center, etc.
To convert LE count rates to fluxes, do the following:
* log in to the `legacy.gsfc.nasa.gov` anonymous ftp area
* go to `caldb/data/exosat/cma/cpf`
* copy the `.rmf` file for the filter instrument combination you need
* start XSPEC, then:
model wabs po (or whatever your model is)
10000 (or whatever your exposure is)
flux 0.1 2.5
The `fakeit` and `flux` commands will give the flux and count rate for the
model you have specified.
The CMA detectors were sensitive not only to X-rays, but also to ultraviolet
photons, which could cause unwanted contamination when observing O or B stars.
The different filters could be used to reduce, estimate, and eliminate UV
contamination. The program 'leuv' is used to predict the UV contamination
from a star for a given filter. Only the Boron filter is completely free
of UV contamination, although the aluminium parylene was relatively immune
(except for the very brightest and earliest stars).
The average source detection threshold for a typical 10,000-second exposure
within the central 12 arc minute radius region of the detector, using the 3000
lexan filter, is 0.002 ct/s. For a power law spectrum with an energy index of
1.0 and an interstellar column density typical when viewing a high galactic
latitude, an extragalactic source of 4x10**20 H cm**-2 gives a flux of about
7x10**-13 erg c**-2 s**-1. If the exposure time is increased by an order of
magnitude (a typical maximum observation time) the detection threshold
decreases by a factor of about 3.
Counting rate from the Al/P filter, if available from an observation of
the same source made within one day. `AL count rate error` gives
the error on the Al/P counting rate.
Al/Par Count Rate Error
The galactic latitude of the source.
Counting rate from the boron filter, if available (see the `AL count rate`
description). `BO count rate error` gives the error on boron counting rate.
Boron Count Rate Error
Source Classification. Type 'class' at the browse prompt for more info.
Source not identified as the class filed set to 'uniden'.
The `count rate` and `count rate error` are the average counting rate (counts
per sec) over the observation and its associated one sigma uncertainty.
Error on Count Rate
Detector Deadtime Correction Factor
The Dec is the Declination of the detected source.
The probability of the detection being due to random chance. Note, this is
not scaled for the independent number of search attempts in an image.
Some observation were carried out with an OBC program that sets
a 'diamond' filter. This filter electronically screened out the outer part
of the field of view. The resulting image, which is shaped as a diamond,
covers only a fraction of the 2 X 2 deg. If the diamond filter was in used than the DIAMOND_FILTER paramater is set to 1 otherwise its value is 0.
The 90-percent confidence uncertainty in arc seconds for the LE1 telescope.
Expected Variance in TA1
Expected Variance in TA2
The `exposure` is the total on-source observation time in seconds. This
includes all dead time effects, interruptions in coverage, etc.
File Name of Background Light Curve
The file_image parameter contains the file name of the associated image.
File name of the source lightcurve.
The filter numbers are
* 7 = 3000 (thin) lexan
* 6 = al/p
* 8 = boron
* 2 = PPL
* 3 = 4000 (thick) lexan
Source Order Number
The instrument paramater records which of the two telescope+detector combination,
LE1+CMA1 or LE2+CMA2, were used in the observation to detect the source.
The values are L1, for the combination LE1+CMA1, and L2, for the combination
IPC Count Rate
IPC Count Rate Error
12-micron IRAS Flux
25-micron IRAS Flux
60-micron IRAS Flux
100-micron IRAS Flux
Date of Last Record Update
Rates Buffer Identifier (First Letter)
The galactic longitude of the source.
ME Count Rate in 1-3keV Band
ME Count Rate Error in 1-3keV Band
ME Count Rate in 3-6 keV Band
ME Count Rate Error in 3-6 keV Band
Counting rate from the ME detector during this observation (warning: this may
not cover exactly the same time interval), it also covers the central field of
view and so will include all sources seen by the LE. `ME count rate error`
gives the error on the count rate.
ME Count Rate Error
The name of the EXOSAT target/field. There are many serendipitous sources
in the EXOSAT LE images, and in some cases the name given here will
reflect the original target name and not the actual source name.
21 cm NH thru the galaxy H cm**-2 from Starke et al.
Observed Variance in TA1
Observed Variance in TA2
Distance from xpix=15, ypix=15
Dead Time Correction
NH (10**19 cm-2) in the Galaxy
Pricipal Investigator Code
Point Source Flag
Date of Buffer Creation
Proposal Type (HLX etc.)
Quality flag for the LE. This is a number between 1 and 5, where 5 is
excellent, and 1 means there may be a serious problem with the entry. A value
of 3 means that the entry should be checked. For example, all boron filter
entries have a qflag of 3, to warn of the problem with the psf for soft sources
observed with this filter.
ME Quality Flag
Visual Quality of Detection
The RA is the Right Ascension of the detected source.
Radio Flux (mJy) at 5 GHz
Flag for Radio Emission
Redshift or HD Number of Counterpart
Gives the time resolution of the associated data product lightcurve in
seconds. The time resolution was 2^n seconds, where n was optimized such
that the average number of counts per bin is 0.3.
ROSAT ID (spare)
Flag for Serendipitous Sources
Archive Tape # Where Files Are Stored
The `time` of the observation refers to the start time. This is listed as
yr.day, where yr is the last two digits of the year and day is the day of
the year e.g., day 300 of 1984 is 84.300. All times are stored internally as
a short history file key (SHF), which is the number of seconds before or
after 1 January 1980 at 00:00 hrs UT. Times are accurate to the nearest
For imaging instruments, the X pixels coordinate of the source in the detector
is given by the parameter `X pixels`. The X value ranges between -1024 and 1023.
For imaging instruments, the Y pixels coordinate of the source in the detector
is given by the parameter `Y pixels`. The Y value ranges between -1024 and 1023.
Questions regarding the CMA database table can be addressed to the
HEASARC User Hotline.
If you have any problems, please consult the help
page or mail firstname.lastname@example.org