5. The ``README FIRST" of Suzaku data analysis

One of the XIS units with an FI chip, XIS2, suffered a catastrophic damage on 2006 November 9. Since then, no astronomically useful data have been obtained with XIS2, although some diagnostic mode data are taken. Users should therefore expect no cleaned event files for XIS2 in observations taken after that 2006 November 9.

Since December 20, the default telemetry allocation to XIS units have been updated to 3:1:3:3 for XIS0:1:2:3. Observations of bright objects before and after this dates may be affected differently by telemetry saturation.

5.2.2 Contamination

In late November 2005, contamination in the optical path of each sensor became apparent. Spectra of celestial sources show that the contaminant is predominantly carbon. Monitoring of 1E 0102.2-7219 and RX J1856.5-3754 shows that the contamination is increasing at a different rate for each sensor, from less than 0.3 to 0.9Êmg cm $^{\rm -2}$ day $^{\rm -1}$ leading to a equivalent additional column density of C of 6 $\times$ 10 $^{\rm 18}$ cm $^{\rm -2}$ (as of April 2006; see Figure 5.1). Contamination build-appears to have saturated for XIS0 and XIS1.

Observations of the bright earth show that the contaminant is twice as thick at the center of the field of view than at the edge, a pattern that tracks the temperature distribution on the optical blocking filter (OBF). This suggests that the contaminant is on the spacecraft side of the OBF, rather than on the CCD detector surfaces. Recent studies suggest that the contaminant is DEHP (C $_{\rm 24}$ H $_{\rm 38}$ O $_{\rm 4}$ , or C/O = 6 by number) although the XIS team is still investigating the material's exact composition.

**Figure:** An empirical model for the on-axis contamination evolution, assuming DEHP (C24H38O4, or C/O = 6 by number) as contaminant. The data points are derived from repeated observations of E010272. The solid lines show the model of the time evolution of the contamination as provided in calibration files dated 2007-09-20, while dotted lines show the earlier model (2006-10-16).
$\includegraphics[height=12.5cm]{Figures/e0102_rates_20071212.ps}$

The ARF generator xissimarfgen contains the current information to take into account the contamination. xissimarfgen does have the broken line (as shown in the figure above) approximation of contamination built-in. However, that the current calibration (20070920) is inaccurate for XIS0 data taken since Apr 2007.

5.2.3 SCI

The cumulative effect of in-orbit radiation damage creates charge traps in CCDs, leading to an ever increasing charge transfer inefficiency (CTI). This changes the PHA-Energy conversion factor as a function of the number of transfers before charges can be read out, hence on the position on the CCD. This also degrades the spectral resolution.

The ability to inject artificial charges into the CCD chips has been designed into Suzaku XIS. This can be used to fill the charge traps and therefore ameliorate the effects of the CTI. This operation is called spaced-row charge injection, or SCI:
http://suzaku.gsfc.nasa.gov/docs/suzaku/analysis/sci.html In 2006 September, SCI has been used for selected observations to test its effectiveness. Once this has been confirmed, it was offered as an option to all users in 2006 November. Since 2007 April (Cycle 2 observations), the use of the SCI is the default.

With Version 2 processing, this is purely a data quality issue. Users need not take action, since information about the SCI operation is encoded in the CI keyword, which is read by Suzaku FTOOLS as necessary and appropriate calibration is used. However, users may wish to check the value of this keyword. In most cases, the value of the CI keyword should be 0 for observations without SCI, and 2 for observations with.

5.2.4 Energy scale for non-SCI data taken in 2006

The accumulation of radiation damage inflicted on-board changes the charge transfer inefficiency (CTI) of the CCDs. That is, a fraction of the charge from an X-ray photon is lost during the read-out process, changing the PHA to Energy conversion. Spaced-row charge injection (SCI) fills the charge traps created by radiation damage and reduces the CTI significantly. The energy scale calibrations for both SCI and non-SCI data are included in Version 2 processing, and in general achieves an accuracy of 0.2% at 6 keV.

However, for some non-SCI data taken in 2006, the energy scale calibration is noticeably worse than stated above. Discrepancies of up to 40 eV have been noticed among different XIS units.

5.2.5 Flux calibration for small extraction regions

The flux calibration for XIS data is most accurate for large, circular extraction regions centered on the source. For example, a radius of 250 pixels (260 arcsec) ensures that 99% of the point source flux is included in the extraction region. Users should use such large extraction regions if at all possible.

The current calibration is noticeably less accurate for small extraction regions. This is not a major issue for

150 arcsec radius, which results in agreement among XIS units to better than a few percent. The problem becomes serious for

100 arcsec. For 50 arcsec region, XIS1 in particular will measure fluxes that are 20-30% lower than it should (i.e., compared to the value measured with a large extraction region). XIS0 appears most reliable in this respect, followed by XIS3 and XIS2.

5.2.6 Updated calibration of energy scale for SCI data

The XIS team has updated the CTI and gain calibration of XIS data taken with SCI. This has been implemented in processing version 2.1.6.16. Data processed with V2.1.6.15 or earlier suffer from time and energy dependent effects in energy scale calibration, and should therefore be reprocessed. This can be done by running xispi on unscreened files (running it on screened file will lead to inaccurate results, since new calibration changes event grades, which is used for screening). The updated files must then be screened to produce a new screened files. See:
http://suzaku.gsfc.nasa.gov/docs/suzaku/analysis/sci_gain_update.html for more detailed instructions.

5.2.7 Timing mode

Although timing mode data are processed and distributed in V2, they are poorly calibrated and cannot be used for publication quality spectral fitting.

5.3 HXD

5.3.1 GSO background

As of January 2008, the HXD team has just released the V2 non X-ray background files for GSO. The analysis of GSO data will be documented on the GOF web pages and included in the next version of the ABC guide.

5.3.2 PIN Background

The HXD background is distributed as simulated event files tailored to each observation. At present the PIN background for 1-day long observations is reproducible at the 4% level (15-40 keV). If there is an energy band with no detectable source counts, this fact can be used to adjust the normalization of the PIN background model. This improves the reproducibility to the 3% level.

Users should be aware of the fact that cosmic X-ray background is not included in the background files distributed by the HXD team. Each user must add this component to the background spectrum or as an extra model component in spectral fitting.

There are two epochs (the initial operation period, up to 2005 September 1, as well as the period 2006 March 23 to May 13, both within epoch 1) during which the V2 PIN non X-ray background (NXB) files are known to show $\sim$ 10% systematic offset, caused by minor changes of operational parameters. For these two periods, the HXD team suggests the workaround of using the Ver 1.2 bgd_d (METHD=LCFIT) NXB files until new NXB files with appropriate correction for these two pereiods can be provided. For the initial operation phase, additional care is required as theoperation as well as the background build-up was not stable. Users can check the validity of using archival data and/or contact the HXD team.

5.3.3 PIN responses by epoch

The bias voltage on-board and low energy threshold in ground processing of various subsets of PIN units have been adjusted since launch to reduce noise events. This changes the characteristics of these PIN units in several discrete steps.

With Version 2 processing, data from all PIN units should be analyzed together. However, due to the changes in the bias voltage and the software threshold, response matrices appropriate for the epoch of the observation (see Table 7.3 in Chapter 7) should be used in spectral fitting.

5.3.4 Energy range

The current response matrix cannot reproduce the Crab spectrum below 12 keV. The instrument team has been studying the energy scale of individual PIN diodes, in parallel with fine-tuning the response matrix but this study of the response is still ongoing.

5.4 Cross calibration

The observations of the Crab has been used to study the cross-normalization of XIS and HXD/PIN. With Version 2 processed data, the normalization of PIN data relative to XIS0 data is 1.06-1.09 for observations at the XIS nominal position, and 1.11-1.13 for those at the HXD nominal position. These cross normalization factors should be taken into account in joint spectral fits of XIS and PIN data.