ACIS background

• 1. Background flares
  • 1.1. Frequency of flares
  • 1.2. What to do with the flares
• 2. Quiescent background
  • 2.1. Spectrum
  • 2.2. Time dependence
  • 2.3. Spatial nonuniformity
  • 2.4. How to subtract the background

• Pre-flight data
• In-flight data with HRMA door closed (F. Baganoff, postscript)

The ACIS background consists of a relatively soft Cosmic X-ray Background (CXB) contribution and cosmic ray-induced events with a hard spectrum. Most cosmic ray events can be filtered out by applying a grade filter (e.g., rejecting ASCA grades 1,5,7). After such filtering, the CXB component dominates below ~2 keV (during the quiescent background intervals, see below) and the cosmic ray component dominates above ~5 keV, consistently with the pre-launch estimates.

1. Background flares

A phenomenon not anticipated prior to launch that can seriously affect the scientific value of an observation is background flares, when the count rate can increase by a factor of up to 100. Such flares have been observed anywhere in the orbit, including near the apogee. They are most prominent in the BI chips but also affect the FI chips. The nature of these flares is under investigation (ask S. Virani and P. Plucinsky for an update), but they generally correlate with the increased EPHIN rate for soft electrons. The flares are easily seen in the ACIS light curves; several examples are shown below.

The figure below shows an observation moderately affected by the flares (OBSID 1190). The horizontal axis gives Chandra time; the rates shown are for the BI chip S3 (upper curve) and the FI chip I2 (lower curve), for ASCA grades 02346 and energies below 10 keV. Although celestial sources were not excluded from these light curves, the flux is dominated by the background:

(PS)

The rate is constant most of the time but there are several strong flares. Note how chip S3 is affected more strongly by these flares.

Below is an example of an extremely bad observation (OBSID 1232 taken on 1999 Aug 27). The first figure shows the rate for S3 (only grades 02346 and energies 0.3-10 keV are included; celestial sources are excluded). Compare to the average rate of 1.2 cts/s/chip during the quiescent intervals:

(PS)

And these are light curves for the FI chips S2 (upper curve) and I2 (lower curve) from the same observation; compare to the quiescent rates of 0.5-0.7 cts/s/chip:

(PS)

The figure below shows the background spectra during a moderate flare and during a quiescent interval (OBSID 1226) in the area of chip S3 free of sources. Their exposures are similar (9-10 ks):

(PS)

The next figure shows a difference of the above spectra normalized by their exposures, to give an idea of the spectrum of the additional background component arising during the high rate intervals:

(PS)

The telescope effective area drops off much more steeply than this spectrum does, thus the component is not due to X-rays reflected by the mirror.

1.1. Frequency of flares

The figure below shows cumulative probability for the ACIS background rate to be a certain factor above the respective quiescent rate (see below), for an FI chip S2 and a BI chip S3. Only the good grades 1,5,7 and energies 0.3-10 keV are used. Celestial sources were masked out whenever possible, otherwise the target flux was roughly estimated and subtracted, but most included observations were dominated by the background anyway. All observations are made without gratings.

The data were divided into 3 time intervals, August 1999 (about 200 ks in total), September - November 1999 (350 ks for chip S3 and 480 ks for chip S2) and December 1999 - January 2000 (about 400 ks). No effort was made to create a "representative" set of observations, but the observations sample all points of the observatory orbit and are selected more or less randomly. The histograms were made by binning the light curves into 260 s bins (80 times the ACIS readout time) so the Gaussian scatter is rather small.

(PS)

Other FI and BI chips have similar flare probabilities. From the above histograms, 20-30% of the time the background rate in the BI chips is more than twice the quiescent rate, and 5-15% of the time, it is 10 times the quiescent rate. For the FI chips, 5-15% of the time the background rate is twice the quiescent rate. It appears that the flares have become somewhat less frequent with time since August 1999, although the current tendency is not clear.

1.2. What to do with the flares

Unless the target is a bright point source, it will usually be best to discard the time intervals with flares. Even though this can significantly shorten the exposure, the flares are so bright that it may still increase the signal to noise ratio.

The Calibration team has produced experimental quiescent background event files (see below), created by excluding time intervals with rates outside a factor of 1.2 of the quiescent rate, to keep the background uncertainty below 20%. To use those files, the observation will have to be cleaned similarly. Vertical dotted lines in panels above show this factor of 1.2. For S3, the fraction above this cutoff is between 30-40%, and for S2, it is 10-20%. This is the average fraction of the data that has to be discarded in order to use those background datasets.

2. Quiescent background

Table 1: ACIS-I in aimpoint:

-------------------------------------------------------------
E,keV    I0    I1    I2    I3    S2    S3      I0123 average 

0.3-10   0.31  0.31  0.31  0.29  0.32  0.89    0.31
0.5-2    0.07  0.07  0.07  0.07  0.07  0.17    0.07
0.5-7    0.19  0.19  0.19  0.18  0.20  0.40    0.19
  5-10   0.15  0.15  0.15  0.14  0.15  0.51    0.15
-------------------------------------------------------------

---------------------------------------------
E,keV    S0    S1    S2    S3    S4    S5

0.3-10   ...   1.41  0.33  0.86  1.04  0.35
0.5-2    ...   0.17  0.07  0.16  0.52  0.10
0.5-7    ...   0.45  0.19  0.38  0.68  0.22
  5-10   ...   0.97  0.15  0.50  0.16  0.15
---------------------------------------------

---------------------------------------------
E,keV    S0    S1    S2    S3    S4    S5

0.3-10   0.27  1.55  0.38  1.03  1.13  0.33
0.5-2    0.07  0.18  0.10  0.18  0.57  0.08
0.5-7    0.19  0.51  0.26  0.53  0.77  0.21
  5-10   0.10  1.08  0.16  0.56  0.16  0.15
---------------------------------------------

As the tables show, the rates for the same chip with either ACIS-I or ACIS-S in aimpoint are close (at least for chips S2 and S3). The rates for different FI chips are similar, except for S4 that has a defect (a very high background below 2 kev in the form of streaks along the chip x axis). The BI chip S3 has a significantly lower high-energy background than the other BI chip, S1. Finally, the ACIS-S rates with and without HETG are close (but not identical).

The picture for the FI chips is complicated by the large CTI increase during August 1999. It causes some of the good grades to migrate to grades 5,7 and be rejected by the standard event screening. The BI chips are unaffected by this; however, there is also indication of a genuine decrease of the background rate since August 1999, see below.

For completeness, the table below gives approximate total (prior to any screening) quiescent rates, in cts/s/chip, for the most common ACIS mode with the onboard PHA cut at 3750 ADU (about 15 kev) and the standard onboard grade filtering. These rates exclude flare periods but include everything else (the targets had negligible flux in the observations used for this estimate).

Table 4: Total rates:

------------------------------------------------------------------
chip       I0   I1   I2   I3     S0    S1    S2    S3    S4    S5

rate       8    8    9    8      11    11    9     10    10    8 
------------------------------------------------------------------

2.1. Spectrum

The plots below illustrate contributions of the CXB and cosmic ray components to the quiescent background for an FI chip S2 (top) and a BI chip S3 (bottom), with ACIS-S in aimpoint, after the standard grade and hot pixel cleaning and exclusion of obvious celestial sources. Black shows the data prior to the mirror aft door opening (OBSID 62706) and thus only includes the cosmic ray component, and red shows the total background (a sum of several August 1999 observations after the mirror doors opening).

(PS)

(PS)

The cosmic ray component dominates above ~5 keV, and it is lower in the FI chips than in the BI chips. A memo by F. Baganoff provides more details on the cosmic ray component.

2.2. Time dependence

The plots below show the quiescent rates in different observations vs. time, for the ACIS-I and ACIS-S3 chips in soft and hard energy bands. Obvious celestial sources are excluded. Filled symbols show "good" observations included in the composite background datasets (see below), while open symbols show several observations not included in those datasets either because of their anomalously high soft background, or too high Galactic absorption, or because the target occupied a large area of the field (in which case the rates were corrected for the excluded area). Vertical dotted lines separate periods with different ACIS focal plane temperature. Horizontal lines show average rates for black dots during the t=-110C period. The energy bands are defined using the nonstandard 1/20/2000 gain tables for all observations. These tables are not applicable for the early, low-CTI ACIS-I data at -100C, thus the comparison between different periods in the upper two (ACIS-I) panels is not very meaningful.

(PS)

(PS)

The plots show that the high-energy rate, where the background is dominated by its cosmic ray component, has been remarkably constant (to better than +-10%) since September 1999, after having dropped by up to 30% between August-September (when the appropriate gain tables are applied to the FI chip data, a similar drop is seen). The low-energy rate is more variable, apparently due to the celestial diffuse component.

2.3. Spatial nonuniformity

The cosmic ray component of the background is spatially nonuniform, as illustrated by the plots below. They show chip x and chip y projections of the 5-10 keV background images for chips S3 and I0 (for the September 1999 -- January 2000 period):

(PS)

(PS)

There are variations by more than 30%. The spatial structure appears to stay relatively constant during the t=-110C period, but it is different in the earlier, higher-background (and lower-CTI) period. At lower energies, the background is more uniform.

2.3. How to subtract the background

Spatial nonuniformity of the background precludes its accurate subtraction using source-free sky regions of the same observation. On the other hand, the observed weak dependence of the quiescent background on time, especially at high energies where it is most important for the analysis of celestial sources, lets us use other observations (cleaned of the flares) to model the background, normalizing them by the ratio of exposures.

A number of source-free observations have been combined to create experimental quiescent background event files for the two distinct periods in the time-dependence plots above -- the August 1999 (t=-100C) and September 1999 to January 2000 (t=-110C) periods. These datasets, their detailed description and tools for their use can be found at this link.