JEM-X MONDAY MEETING #105: Monday 24th July 2006
		

Present: 
	Niels Lund, Soeren Brandt, Carl Budtz-Joergensen, Carol Anne Oxborrow

Instrument status: SB satisfied with performance and requested HV drop (one 
step down) has been performed at beginning of revolution 459.
There is some discussion with MOC about where JEM-X should be switched on and
off compare to the radiation belts. No recent cases of the hardware trigger
limit shutting down the instrument, though the trigger rate has been high.
Now takes instrument (currently JEM-X1) about three hours to stabilize after a
switch on after normal radiation belt passage (i.e. not in eclipse season). How
can this time be minimized? Could we do some experimentation to see to what
extent HV could be switched on during the passage to stop ion relaxation
occurring in the glass? Radiation belt passage only accounts for about 10% of
an orbit, so presumably instrument damage won't have that long to occur in.
But particle flux is about 1000 times higher in the belts than in the rest of
the orbit, so potential for 1000 times the damage. CBJ worried about increased
rate of gas contamination. How much HV is needed to hold the ions in place in
the glass plate? With a little more telemetry (say, during OMC flat-fielding)
we could experiment a bit with JEM-X2. BUT that is total shut down and not even
DPE is running. SB points out that we really MUST remember that short switch-
offs result in a large overshoot in gain, which is potentially far more 
damaging than current undershoot. So experimentation, if any is allowed, must
be done with caution.

Vignettting in j_ima_iros: SP and ISDC are doing an exercise whereby they
extract source fluxes from NL's images. A consistent fall-off of flux is found
in all bands with increasing off-axis angle -- about a factor of 2 in going 
from 2 deg. to 4-5 deg. NL already takes into account smaller effective area
with increase angle etc., but maybe there's room for an empirical vignetting
factor here - but it must be done carefully because tilting source position
diagonally across collimator grid gives different vignetting than tilting along
one or other of the collimator axes. NL's flux finding method doesn't use all
the available events, only those that are most certain to have come from a
particular source. This cuts out a lot of background counts compared to CBJ's
method, but the degree of certainty about a given event clearly decreases with
increasing off-axis angle. Unfortunately, instrument was designed to optimise
performance for on-axis sources, which we rarely see because pointing and
dithering is driven by the needs of the two main instruments. SB suggests doing
another mini-dither exercise at next calibration, but this time with the source
several degrees off axis. This could help pin down the exact nature of any
empirical vignetting factor.

OSA 6.0: New j_cor_gain not available yet, so NJW's current programs find gain
for each event by dividing PI by PHA - this will be needed for cases where the
gain output table is missing (backward compatibility). Work continues on all
components.

Moscow Meeting: Very satisfactory, with fewer but longer contributions.
Interesting French/Russian analysis of galactic-rich emission in the galactic
play that correlates very well with star density, indicating that the diffuse
emission is due to unresolved point sources. Unfortunately, many galactic
centre sources that are point sources for ISGRI are diffuse for JEM-X and 
reduce the instrument's sensitivity to them. Unfortunately a very strong burst
that was seen by JEM-X was analysed without our good data because the observers
hadn't ASKED for JEM-X data, so they used poorer data from another all-sky
monitor instead. This means that much unclaimed AO3 JEM-X data on interesting
sources might be available for the taking if PIs don't ask for it.

CAO 26/7/2006