Date: Thursday 7 Mar 1996 14:58 PST
From: Richard Rothschild (

HEXTE Detector PWB2 loses spectral capability and automatic gain control

Richard E. Rothschild, HEXTE Principal Investigator

The HEXTE instrument team at UCSD has discovered that the pulse height analyzer for the detector #3 (out of 4) in cluster B of the HEXTE on the Rossi X-ray Timing Explorer (RXTE) failed on 1996 March 6 at 11:27:12 UT. The failure mode is that all pulse heights of good events from that detector are now digitized to be in either PHA channel 1 or 2. This means that the value of the pulse height in telemetry is no longer scientifically useful for this detector. The phoswich detector and photomultiplier tube, as well as the event selection logic, including pulse shape analysis, are still functioning properly.

As a direct consequence of loss of pulse height data, the automatic gain control (AGC) no longer functions, so the gain for this detector will vary over time. The magnetic shielding, however, should minimize variations that are due to the changing magnetic field vector experienced as the RXTE orbits the Earth. Earlier data showed gain variation corrections by the AGC system to be about 2% peak-to-peak.

The detector data can still be used for timing studies, as long as pulse height (spectral) information is not required. For instance, one can still use this detector to measure 15-250 keV fluxes as a function of time.

Diagnostic work with the HEXTE Engineering Cluster on the ground has shown that the problem can be reproduced by shorting out the input to the Harris op-amp, or by opening the custom delay line that precedes the charging of the pulse height analyzer capacitor. The diagnostic work is continuing.

The configuration of the HEXTE has not been altered since the discovery of the problem, other than for some tests. No data from the other 7 detectors are affected.

High Energy X-ray Timing Experiment

Send questions or comments to Philip Blanco