Subject: Extremely bright transients

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Dette er et udkast.

niels

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Hello Peter:

***************  Re: Very bright transients  ******************

For JEM-X the main issue for the very bright transient is the risk
that the instrument will automatically switch off through the
hardware switch-off logic. This vill happen after about 8 seconds
if a fifty Crab (JEMX) source is observed on-axis. This has happened
in the past when observing Sco X-1. Apart from the immediate loss
of data, the instrument calibration will be complicated for the
first few hours after HV recovery.

Even for a very bright transient we will prefer to stay in normal
full imaging mode. The detector response exhibits very large
variations across the detector area, so going to any of the non-
imaging modes will entail a serious loss of data quality.

The grey filter mechanism can reduce the input rate by up to a factor
30, and this ensures that we do not run into start-stop interruptions
of the data. The data rate is limited by the processing of the events
by the on-board CPU. This limiting rate is just under 1000 cnts/s
(8 Crabs (JEMX), and since we can transmit about 100 cnts/s we need
at most a factor 10 reduction so we are safe here. But the grey filter
has no effect on the hardware protection circuit.

We can reduce the JEM-X rate by pointing off the source and/or by
disabling three of the four anode segments.

A) Disabling anode segments ?, ? and ? (JEM-X1) and ?. ? and ? (JEM-X2)

This will reduce the input event rate by about a factor 4. This meaans
that we can look straight into a 150 Crab source without problems.
We prefer disabling the anode segments compared to offpointing because
we believe this has a minimal effect on the instrument calibration.

B) Offpointing INTEGRAL from the transient.

Offpointing by 2.17 degrees reduces the count rate in JEM-X to 60% of
the on-axis rate, offpointing by 2.17*sqrt(2) reduces the rate to 40%.
OFfpointing to 4.34 degrees limits JEM-X to 12%, but we are concerned
about the calibration of observations taken so far off axis.

We would like to know what dithering strategy will be suggested from
IBIS and SPI for such observations. It seems to us that source varia-
bility could be a serious complication of this type of one shot event
if a long duration dither pattern is chosen - but in this case JEM-X
may not at all be interested in having the source close to the axis !

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We believe that if such a very unique event occurs the instrument
teams should send their experts to MOC/ISDC to be available for
consultations in real time.

We are also looking into the possibility of disabling our hardware
switch-off functionality. This is possible, but requires a patch
to the onboard software - then the activation/deactivation could
be done using a modification of the command now used to select
the DFEE processor speed. We will return to this later.

With best regards

niels