Particle background observations: dark Moon and ACIS stowed

Maxim Markevitch (maxim % head.cfa.harvard.edu), last updated 15 Jun 06


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1. Dark Moon

To separate the CXB and instrumental components of the ACIS background, Chandra observed the dark Moon in July and September 2001 in two series of short pointings (to track the fast-moving Moon). Chips S2, S3, I2 and I3 were on and telemetered data in VF mode. Technical difficulties were encountered in these two runs, related mostly to the fact that Chandra aspect camera cannot be used near the Moon, which prevented further dark Moon observations. In addition, during the July 2001 run, optical flux from the illuminated region of the lunar disk produced a detectable bias error. The effect was most severe in chip I2, but small in S2 and negligible in S3. Therefore, only the S2 and S3 data from that run are useful. A correction to each event's PHA was calculated individually by averaging the lowest 16 pixels of the 5x5 pixel VF mode event island. The bias error in S2 and S3 is well below the threshold of affecting the event grades, so this problem did not result in any loss of events due to the onboard grade rejection (as was the problem for chip I2 in this dataset). The average correction to the energy for events in chip S3 was within a few eV and for S2, within 10 eV.

We combined the observation periods when the particular interesting chip was fully covered by dark Moon, and excluded several periods of background flares. Table 1 lists the dark Moon observations (all archived) included in the final datasets (available upon request):

Table 1.   Dark Moon exposures 
-------------------------------------------
 date       obsid  Clean exposures, s
                    I2    I3    S2    S3
-------------------------------------------
 2001-7-26  2469               2930  2811
 2001-7-26  2487               1921  2982
 2001-7-26  2488               2324  2748
 2001-7-26  2489               2736  2736
 2001-7-26  2490               2739   978
 2001-7-26  2493               2573   950
 2001-9-22  2468   3157  3157  1219   519
 2001-9-22  3368   2223  2223
 2001-9-22  3370   4772  4772
 2001-9-22  3371   3995  3995
-------------------------------------------
 total            14147 14147 16442 13724
-------------------------------------------

The dark Moon spectrum for S3 was compared to the Event Histogram Mode data (a mode with HRC-I is in the focal plane and ACIS stowed inside the detector housing, transmitting limited information, see this memo by B. Biller). At that SIM position, ACIS is not exposed to sky X-rays but the particle background is expected to be the same. As Fig. 1 shows (see also figures in B. Biller's memo), the dark Moon and the EHM spectra are in good agreement, except for the expected residual line emission from the external calibration source.



Fig. 1.   Spectra of dark Moon (black) and Event Histogram Mode (red) for chip S3. Energy scale is approximate (spectra are extracted in PHA channels); gain correction, VF mode or bad pixel cleaning were not applied in order for the spectra to be directly comparable.

An example of using the dark Moon S3 background for science analysis is given in our CXB paper (astro-ph/0209441). However, as shown in Wargelin et al. (astro-ph/0402247), there is a small time-variable soft component in the dark Moon data for chips I2 and I3, so it shouldn't be used for detector background modeling. For all practical purposes, the dark Moon background is superseded by ACIS-stowed data described below.

2. Observations with ACIS stowed

The above EHM - dark Moon comparison strongly suggests that particle background inside the detector housing is the same as in the focal position. With that in mind, starting from 2002, we are performing calibration measurements in a stowed position (moving ACIS further away from the external calibration source than in the HRC-I position, in order to avoid the residual line illumination seen in Fig. 1). ACIS runs in the normal imaging VF mode (as opposed to EHM) so that all the usual filtering and gain correction can be applied. In addition to the normal event grades, flight grade 66 was transmitted for experimental work by the ACIS team; it can be removed by applying the usual ASCA grade filter. Chips I0, I2, I3, S1, S2, S3 were on. The four I chips are sufficiently similar so that after some manipulations, I0 data can be used as a substitute for chip I1, as discussed in in this memo. The usual canned background event file which combines these observations (and includes fake I1 events) can be found in this directory (see the accompanying README file).

As of summer 2006, 7 observations were performed:

Table 2.   ACIS-stowed exposures 
---------------------------
 date       obsid  exp, ks
---------------------------
 2002-08-06   4286   9
 2002-09-03  62850  53
 2003-05-04  62848  48
 2003-12-08  62846  46
 2004-11-04  62836  46
 2005-06-10  62831  47
 2005-11-13  62824  47
 2006-06-01  62823  45
---------------------------
 combined good     333
---------------------------
The short obsid 4286 exhibited strange spectral deviations, only marginally significant statistically but at such energies that they might bias scientific results. It was therefore decided not to include this observation in the final background file. A 50 ks exposure in Dec 2004 was taken in a wrong mode due to a planning error.

Figure 2 below shows background spectra from different chips from the first long exposure (obsid 62850). VF mode cleaning is applied. Spectra from chips I0, I2, I3 are similar (so they are shown together), chip S2 has a slightly higher background than the I chips. Chip S1 has a higher background, especially at E>5 keV, then the other BI chip S3.



Fig. 2.   Average of chips I023 (red), chip S2 (black), S3 (green) and S1 (blue), for the 2002 exposure.


2.1. Comparison to dark Moon

Figures 3, 4 and 5 below compare the ACIS-stowed spectra (Sep 2002 piece, obsid 62850) with the dark Moon observations of 2001, after a small renormalization by the ratio of the high-energy fluxes (in the 2500-3000 ADU interval or 9.5-12 keV energy interval, as recommended in our 2002 CXB paper and 2005 CXB paper):



Fig. 3.   Chip S3, ACIS stowed (black; obsid 62850) compared to dark Moon (red).



Fig. 4.   Chip S2, ACIS stowed (black; obsid 62850) compared to dark Moon (red).



Fig. 5.   Chips I23, ACIS stowed (black; obsid 62850) compared to dark Moon (red). The Moon exhibits a significant excess at 0.6-0.9 keV.

The S3 and S2 dark Moon and stowed spectra are in good agreement. However, the I23 spectra of the Moon show a significant excess between 0.6-1 keV. As seen in Table 1, the I23 Moon spectra are obtained in September 2001, while almost all the S3 and S2 Moon data are obtained in July 2001. This time-variable excess has sky origin and is discussed in Wargelin et al. (2004).

2.2. Comparison with sky background

The two figures below compare the ACIS-stowed background from chips I023 (Sep 02, obsid 62850) and the sky background from empty fields with most point sources excluded. Again, a small renormalization was applied using the ratio of fluxes in the 2500-3000 ADU (10-12 keV) interval. Fig. 6 shows a comparison with an observation from the same time period (Aug 2002), and Fig. 7, with the long composite background dataset for year 2001. Both sky background spectra correspond to regions of low Galactic soft X-ray brightness.



Fig. 6.   Chips I023, ACIS stowed (obsid 62850, black) compared to a contemporary blank sky observation (obsid 4357, red) with point sources removed. VF mode cleaning is not applied; the difference at E<2 keV is the diffuse sky background.



Fig. 7.   Chips I023, ACIS stowed (obsid 62850, black) compared to the composite blank-sky background from 2001 (red) with point sources removed. VF cleaning is applied.

At E>2 keV where particle background is dominant, the two sky background spectra are indistinguishable (figure not shown). In addition, as shown below in Sec. 2.3, there has been no change of the spectral shape of the detector background during 2002-2004. Thus, the 2002-2004 ACIS-stowed background can be used in the analysis of the 2001 science observations (and probably -120C data from 2000 as well).

2.3. Comparison of 2002-2005 ACIS-stowed observations

Figures below compare spectra from four long exposures (Table 2), after a renormalization to match their 9-12 kev rates. The three 2002-2003 exposures have very similar high-energy rates, while the Nov 2004 exposure has a 1.3 times higher rate, in line with the observed background increase during 2004. After the renormalization, all spectra, including the latter one, are remarkably similar; across the whole energy range the differences are within the statistical scatter or within +-5% from the average almost everywhere. The 4 exposures are combined into a 189 ks dataset which can be found here.





Fig. 8.   Comparison of May 02 (black), Sep 02 (red), Dec 03 (green), and Nov 04 (blue) ACIS-stowed spectra after the renormalization by 9-12 keV rates. Upper pannel shows FI chips (0236), lower panel shows S3. VF cleaning and time-dependent gain correction are applied.


Fig. 9 shows the constancy of the spectral shape in wider bands for the ACIS-I chips (also includes the dark moon; reproduced from astro-ph/0512542). When normalized to 9-12 keV, the fluxes in these bands are the same to +-2%, and are the same as those for the moon (except at E<1 keV where the moon data have astrophysical charge exchange signal, Wargelin et al. 2004).




Fig. 9.   Ratios of rates in several energy bands to 9.5-12 keV, for dark Moon and the 5 pieces of the stowed dataset, for I chips (note that moon I23 data are compared to I023 for stowed). The lower points are the 9.5-12 keV rates. Pairs of lines show +-2% around the mean for the stowed data.


The ratio of the Moon to stowed fluxes (normalized to their respective 9.5-12 keV rates) are consistent with 1 (Table 3), indicating that there are no missing components in the stowed background. The 2-7 keV average for the two independent (and separated in time) FI datasets, chips I23 and S2, is 1.00+-0.02. A marginally significant excess in the S3 moon data above 2 keV is consistent with low-level flares in the BI chip when it is not shielded by the detector housing.

Table 3.   Ratios of moon to stowed normalized rates 
-----------------------------
 chips  band      moon/stowed
-----------------------------
 I23    1-2 keV   0.97+-0.05
 S2     1-2 keV   0.97+-0.06
 S3     1-2 keV   1.01+-0.04

 I23    2-7 keV   1.01+-0.03
 S2     2-7 keV   0.99+-0.04
 S3     2-7 keV   1.04+-0.03
-----------------------------