j_ima_iros status. May 14 2008. Niels Lund


After the correction of a bug in the calculation of the electronic
efficiency of the indivdual detector pixels the expected correlation
between pixel efficiency and pixel count was verified. As a consequence
the fits to the shadowgram count improve after application of the
efficiency corrections.

New sets of instrument parameters have been derived, based on all the
Crab calibration periods up to rev 605 which give the following results
for the Crab position and flux constancy:

JEM-X1:
position mean error: 10.9" for 516 scws with S/N > 25
flux determination rms:    8.9 %  for 450 scw with S/N > 20
the flux rms drops to 5.4 % if each calibration orbit is
normalized independently.
estimated rms statistical error on the fluxes: 3.8 %
Mean Crab position:  RA: 83.6311, dec: 22.0167

JEM-X2:
position mean error: 12.4" for 436 scws with S/N > 25
flux determination rms:   8.5 % for 424 scw with S/N > 20
the flux rms drops to 6.1 % if each calibration orbit is
normalized independently.
estimated rms statistical error on the fluxes: 3.6 %
Mean Crab position:  RA: 83.6305, dec: 22.0172

Deviation between JEM-X1 and -X2 positions: 2.8"
Common best JEM-X position for the Crab: RA: 83.6308, dec: 22.0170

Catalog position for Crab pulsar:  RA: 83.6332, dec: 22.0145
Deviation between catalog and mean JEM-X position: 12.1"

I expect that we can reduce the flux determination rms values given
above to values close to those obtained with normalization of the
individual orbit sets by the introduction of a suitable time dependent
function (we need a function which we can with some confidence apply
to all the normal observations between the Crab calibration periods).

***

The new j_ima_iros has been applied also to selected sets of
science windows with point source detections with SN>50. Using the
alignment parameters derived for the (diffuse) Crab source the
following results are obtained for the point source position accuracy:

JEM-X1:
position mean error: 7.6" for 247 scws with S/N > 50

JEM-X2:
position mean error: 8.0" for 168 scws with S/N > 50

This result is very encouraging since there are clear differences
between the alignment parameters for the Crab and the point source
sets. Apparently these differences are not so important regarding
the positioning accuracy.

****

We can note that the current j_ima_iros is not as good as earlier
versions to find many sources in complex scws: only 3 sources are
found in scw 029500560010 (5 sources found with OSA 6.1) and only
4 sources found in scw 016400760010 (also 5 srcs found with OSA 6.1).
This indicates that there is still room for improvements.