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HRC-I Background PI Spectra

V. Kashyap & J. Posson-Brown

May 2010

This page describes a new calibration product: HRC-I background PI spectra, located in the CALDB at

/data/CALDB/data/chandra/hrc/bgspec/hrciDYYYY-MM-DDpibgspecN0001.fits .

There is one background PI spectrum per year. The spectra can be used to calculate a custom PI filter to reduce background, as explained in the HRC-I Background Spectra CIAO thread. The files were first introduced in CALDB 4.3.


The background PI spectra are created from HRC-I calibration observations of AR Lac taken yearly at 21 locations on the detector. For each year, level 2 event lists are created by reprocessing the data with the latest CIAO/CALDB version. Time-dependent gain corrections are applied to the scaled SUMAMPS values to create PI.

For each year, we extract events in a 800x800 pixel box at the 17 observation locations < 10 arcminutes from the aimpoint, excluding the location where the source is located. We sum the PI spectra from each location, weighing by exposure time, to create the final PI spectrum for the year. Finally, the PI spectra are Loess-smoothed with a third degree polynomial fit over ranges 10 channels wide.

The following figure shows the PI spectra for each year.

File Contents

Each background spectrum is a FITS table with one extension, SPECTRUM, with 4 columns and 1024 rows. The columns are:


The spectra can be used to calculate a custom PI filter to reduce background, as explained in the HRC-I Background Spectra CIAO thread. Examples of PI filters for different spectral models are shown in Section 7.10.3 of the Proposers' Observatory Guide (see Figures 7.19 through 7.21).

Comparison with HRC-I Stowed Background Event Lists

In addition to the background spectra, there are HRC-I background event lists available in the CALDB. The background event lists comprise data taken while the HRC is in a stowed position. For more information about the files, see the High Resolution Camera Stowed Background Study. The usage of these files is described in the HRC-I Background Event Files CIAO thread.

Here, we compare spectra extracted from the stowed event lists to the background spectra derived from the AR Lac observations. First, we compare total counts in the background AR Lac spectra to total counts extracted in the same regions from the stowed event lists, after normalizing by the appropriate exposure time. We find that the counts can differ by +/- 20%.

Comparison of Background Intensity: AR Lac vs Stowed
Stowed/AR Lac0.790.881.191.030.990.811.

Next, we compare the shape of the AR Lac and stowed background spectra after normalizing each to total counts. We find that the shapes are similar, with reduced chi-square values for PI=40:300 ranging from 0.99-1.72. Over a PI=1:400, the reduced chi-square values are a bit larger.

Comparison of Spectral Shape: AR Lac vs Stowed
red. chi-sq 40:3001.721.201.201.411.371.410.991.161.081.07
red. chi-sq 1:4002.111.671.451.841.851.481.631.501.371.31

We suspect that the differences in intensity and spectral shape between the AR Lac and stowed backgrounds are because the stowed event lists are summed over an entire year, while the AR Lac backgrounds sample only a few consecutive days of each year.

Last modified: 09/27/12

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