Instruments: HRC

Surprise -- the HRC works as built, not as designed. Martin Weisskopf and Allyn Tennant were analyzing his Crab data last Spring and found something peculiar. The Crab pulsar pulse profile was broader than expected from ROSAT data and even more interesting, the surrounding nebula was also pulsing! Suppressing the natural urge to send off an IAU telegram, they quickly realized that something was probably amiss and asked the HRC team to investigate.

The laboratory investigation used the laboratory proof-of-concept HRC (The ``POC''), a mechanically chopped X-ray beam to simulate the Crab pulsar and an unobstructed X-ray beam to simulate the nebula. Only by combining a pulsed and a non pulsed X-ray component were their results reproduced. After much head scratching, the HRC team found the cause of this problem; it turned out to be a subtle wiring problem in the HRC electronics.

The wiring problem causes an X-ray event's time to be associated with the next event that triggers the HRC detector. If all detector triggers were in the telemetry stream one could recover all the timing information by a mere event-time sequence shift. This shift could be performed once the data were ``on the ground.'' In flight many real X-ray times are associated with cosmic rays events and these are vetoed by the on-board anti-coincidence shield. It is possible to operate the anticoincidence shield so that it only ``flags'' events and does not veto them. Unfortunately, since the present cosmic-ray rate plus any X-ray event rate exceeds the telemetry limit of 184 c s^-1 many of the events needed to perform the sequence shift are lost.

Fortunately, the HRC Team has worked out a special operating mode that allows high precision timing to be achieved. This new timing mode uses only the central segment of the HRC-S. At present, temporarily disabling the outer two segments lowers the total background event rate to about 50 s^-1. Since the anticoincidence is disabled for HRC-S, and if the telemetry is not saturated, there is a high probability that all events will be telemetered. In this case, once the time tag of each event has been appropriately shifted, the original timing accuracy (16 $\mu$s) can be recovered. In using this approach, it is prudent to be sure that the total counting rate (background plus source) is somewhat below the telemetry saturation limit to avoid the effects of statistical fluctuations. The HRC-S when used in this ``Imaging Mode'' provides a field of view about 7 arc minutes in one direction and 30 arc minutes along the long axis of the segment. The center segment of the HRC-S has about the same sensitivity as the HRC-I. Observers wishing to use this mode should make sure that their observations are properly scheduled. In some cases a change from HRC-I to HRC-S (Imaging Mode) will be necessary. Mission Planning will work with the observers to make the necessary changes.

Nature may also come to the rescue. Lately, the total event rate for both the HRC-I and HRC-S has dropped to about 150 c s^-1.

Special software for shifting the event times and also screening the data to reduce background has been developed by the HRC Team and can be found at the HRC Web Site (http://hea-www.harvard.edu/HRC/screening/screening.html.)

- The HRC Team