Instruments: HRC

Another undocumented feature of the HRC has been uncovered recently. First, some background information. In addition to the primary science data for individual events, the rate of microchannel plate triggers (total rate) and triggers that pass on-board validity tests (valid rate) are telemetered to the ground. The valid rate is used to correct the primary rate for deadtime and telemetry saturation effects. As long as the primary rate is below saturation, the primary rate itself can be used to make the small ($\le$1%) correction, since the event processing dead-time is known. However, when the event rate exceeds saturation (a not uncommon occurrence because of the flaring background from low energy protons) the valid rate is necessary to correct the event rate. Unfortunately, the total and valid event rates are overestimated by about 15% for normal operation of the HRC-S . In the HRC-S ``imaging" (aka ``timing") mode these rates are overestimated by a few percent, at most. The problem is caused by an overshoot in occasional large trigger pulses. This results in double counting in the total and valid event on-board scalers. The primary science event is not affected, since once event processing starts with the initial trigger pulse, a gate rejects further pulses until processing is complete. The HRC-I does not have the overshoot problem. We suspect that rise-time differences in the trigger pulses in I and S are the reason for the difference in the responses. We are studying ways of mitigating the problem. Raising the trigger threshold level reduces the double counting but we need to evaluate the consequence on detector QE (quantum efficiency) and uniformity. The simplest and least ``invasive" solution would be to apply a correction factor based on the fraction of large pulses determined from the pulse height distribution obtained during an observation. We are studying this solution.

A Calibration Review was held on October 30-31, 2001. The HRC reports presented can be found at http://cxc.harvard.edu/cal/calreview/.

The HRC remains robust and healthy. Detector temperatures and voltages remain nominal. The HRC-I temporal stability has been examined and a longer baseline will be required to perceive any temporal drift in the QE. There is, however, a clear, slight downward drift in the HRC-I gain of about 2% per year. In the future, if this trend continues and affects performance, the gain can be raised by raising the detector voltage.

  

Figure 5: An HRC image of the central region of Cas A. The central point source is in the center of the image. The double arrow is 20 arcsec wide.

LINK TO POSTSCRIPT FILE for Figure 5

Is the compact source at the center of Cas A pulsed?

X-ray observations of the supernova remnant Cas A using the Einstein, ROSAT and ASCA observatories failed to detect a central point source. However, with the 6000 second ``first light" observation of Cas A by ACIS, a central point source ``stuck out like a sore thumb". The object is likely to be a neutron star or black hole that remained after the explosion of the progenitor of the supernova; efforts to find a radio or optical counterpart have failed so far. Surmising that a rotating neutron star would produce periodic pulses, a team led by Steve Murray made a 50 ks observation of Cas A using the HRC-S in the ``imaging" (aka ``timing") mode. An additional 50 ks observation was made with the ACIS in order to obtain a spectrum of the source. Fig. 5 shows an image of the central source. The team, after extensive analysis of the data, found several plausible, though not highly statistically significant, periodic signals. Using spectral and luminosity data and the age of the remnant to select the ``most probable of the plausible", the team suggests a period of 12.155526 ms but emphasizes that evidence for the existence of the pulsar is statistically weak. The team has recently made a follow-up observation to try to determine the true period, if one exists. Stay tuned. An ApJ article describing these results will be published in February 2002 (preprint: astro-ph/0106516, S. Murray, S. Ransom, M. Juda, U. Huang, and S. Holt)

M15

Figure 6 on the left shows a 30 ks HRC-I observation of M15 by Phil Charles and his team. M15 is perhaps the densest of all globular star clusters in our Milky Way galaxy and has undergone a process known as core collapse. The previously known source AC211 (4U2127), a low-mass X-ray binary (LXMB), is on the left, the ``other" source is labeled XRB2. The width of the arrow is 2 arcsec. The image on the right is a simulated image that was part of their original proposal. The intensity ratio chosen for the simulation was 1:2, whereas the actual measured ratio is about 1:3. The team concludes that this resolves the ``problem" of the X-ray bursts from M15, observed by the Japanese satellite Ginga, which are almost certainly from XRB2. These two sources have also been observed by Nicholas White and Lorella Angelini using ACIS/HETGS.

  

Figure 6: The image on the left is an HRC-I image of M15 by Phil Charles and his team. On the right is a simulation made prior to the observation. See text for discussion.

LINK TO POSTSCRIPT FILE for Figure 6

- Martin Zombeck for the HRC Team