Time History of the HRC Background

The HRC background is dominated by the effects of charged particles. In the period from launch to the end of 2000, the overall background rate in the HRC dropped. Since, then the rate has risen slightly and possibly reached a plateau. This behavior is correlated with the high-energy particle flux in which the Chandra X-ray Observatory (CXO) is immersed, as measured by the EPHIN. Figure 1 shows the time history of the EPHIN integral channel flux along with the HRC anticoincidence detector rate and MCP total event rate. Each point is a daily average. In order to minimize the contribution from the earth's radiation belts to the EPHIN data only times taken when the CXO was at a geocentric distance greater than 80,000 km have been included. The HRC data are taken from the "secondary science" rate data. The total event rate measures all triggers of the MCP, including those from events that are vetoed on-board. No separation has been made in the MCP total rate data between times when the HRC-I or the HRC-S was selected as the active detector. Except for the times when the HRC-S is operated in imaging mode, the HRC-I and HRC-S active areas are roughly equal and as a result we expect their total event rates will be roughly equal.

EPHIN Integral Channel and
HRC Rates vs Time
Figure 1: EPHIN integral channel flux and HRC detector rates as a function of time. All points are daily averages. The top two panes show EPHIN data: in the first using a log range to show the days with elevated rates due to solar activity, and in the second using a linear range but scaled to show the bulk of the data and the decreasing flux trend. The third pane is the HRC anticoincidence detector rate. The bottom pane is the MCP total event rate, both the HRC-I and HRC-S combined. Both the HRC anticoincidence rate and MCP total event rate exhibit the same decreasing trend as observed in the EPHIN data.

The HRC and EPHIN data exhibit the same trend. The quality of the correlations are better seen by directly plotting the HRC rate versus the EPHIN flux. Figure 2 is a scatter plot of the HRC anticoincidence detector rate versus the EPHIN integral channel flux; figure 3 is similar but for the HRC MCP total event rate. The HRC anticoincidence detector rate is rather well correlated with the high-energy particle flux. The dashed line over-plotted in figure 2 is from a fit to the scatter, clipped to eliminate the effects of outliers.

HRC Antico Shield Rate vs
      EPHIN Inetgral Channel Flux
Figure 2: Comparison of daily averages of HRC anticoincidence detector rate to EPHIN integral channel flux.

The HRC MCP total event rate is less well correlated with the high-energy particle flux. Some of this can be due to the contribution from x-rays from observations of bright targets and some can be due to the flaring background that appears to be attributable to lower energy particles. The cluster of low total event rate (30-60 count/s) points are from running the HRC-S in imagaing mode, where only 1/3 of the dectetor area is active. The dashed line in the scatter plot is a "fit-by-eye" correlation to the higher density of points.

HRC MCP Total event Rate vs
EPHIN Integral Channel Flux
Figure 2: Comparison of daily averages of HRC MCP total event rate to EPHIN integral channel flux.


Mike Juda
Last modified: Mon Oct 29 11:21:14 EST 2001