OCCcm08886  January 13, 2009

Anomaly
-------
On day 44 of 2009, obsid 10275 stopped sending event data from ACIS 
FEP 1. Telemetry from the remaining FEPs proceeded normally.

Analysis
--------
Software housekeeping reported a reset on FEP_1 at 2009:044:14:08.
The time delay of 449 seconds between the last two event packets
is characteristic of a FEP watchdog timeout after the FEP's DEA
board stopped sending data. The same FEP and the same DEA board 
ran without problems in subsequent observations.

Engineering data showed the expected slight drop in DPA_A current
after the FEP reset. Otherwise, engineering looked normal, but
we would not expect a clear drop in current in the DEA from a single 
sequencer shutting down. See also MIT's ACIS Software Problem Report 
M09021301.

Conclusion
----------
This anomaly is most similar to the one affecting obsid 5008 on 2004:175.
In that case, all five FEPs watchdogged, and the ACIS team attributed
the anomaly to noise on the BEP to DEA command channel. Another 
instance affecting all five chips occurred during obsid 15474 on
2013:211.

The ACIS SI team concluded that this anomaly was due to a radiation
event.