2.3
Data reduction
Analysis of the raw data received from the telescopes mentioned above requires many pieces of software. An overview is given of the software used for many data reduction and analysis tasks and the methods they employ. The focus will be on the data reduction steps for Suzaku’s XIS, but the general steps are the same for each telescope. The reduction and analysis of data from any given telescope is usually done using some form of software, which generally perform one or more steps in the following tasks:
• screening: filtering of photon detections (events) such as removing dead/hot pixels and “grading” events (explained below) and removing periods of high background; • region selection: filtering of events based on the area of the detector done to produce
source and background data sets;
• event selection: filtering for investigation purposes such as splitting up a longer observation into parts;
• producing data products: such as images, spectra and/or light curves for analysis; • generation of response files which are used to account for the detector.
2.3.1
Suzaku
data reduction
The data used in the subsequent chapters comes from a variety of telescopes. The steps taken in the reduction of XIS data are reasonably representative of all X-ray observatories used in this work – depending on the source/telescope there may be some subtle differences.
On completion of an observation, the PHA events and any relevant meta-data are output into a multi-layered FITS (Flexible Image Transport System) file, these event files are then processed via the telescope’s pipeline and made available to the researcher and
after a period of time (depending on the telescope) is then made available to the public via on-line archives (such as heasarc). Before these “raw” event files can be useful for analysis they must first undergo several steps of screening which be presented next.
2.3.1.1 Screening
Raw data need to be screened to remove contaminants from the data. Contaminants come in many forms, from dead/hot pixels to charged particles and cosmic rays. Screening is done to remove contamination form the events. The Suzaku team present a list of criteria for science data which are followed to remove most “bad” data; these vary from pointing/orbiting information to instrumental information. A detector-level constraint comes from events being spatially resolved over the CCD; this allows events to be “graded”5 (which is the spatial
distribution of charge) as photons will distribute charge differently to charged particles for instance.
One of the orbital constraints is due to the South Atlantic Anomaly (SAA) which is a region of the atmosphere where the Van Allen radiation belts extend closer to the Earth’s surface. As this contains high energy particles no useful data can be obtained by the low Earth telescopes (such as Suzaku and NuSTAR); this can be seen in table 2.1 with "SAA_HXD==0" and "T_SAA_HXD>436", the second of which gives time for radioactive products of the particle bombardment to decay. Other obvious constrains on the telescope pointing are making sure the telescope pointing is with a small angular distance of the source. Also the pointing must not be too close to bright pollutants such as the Sun, along with the Earth and Moon which can scatter some solar X-rays.
2.3 Data reduction 39
Criterion Comments
GRADE=0, 2, 3, 4, 6 ASCA grades indicating X-ray events STATUS=0:524287 Bad columns, charge injection rows removed
cleansis Flickering pixels are removed
ANG_DIST<1.5 Instantaneous pointing within 1.5 arcmin of mean Sn_DRATE<3 Telemetry rate SuperHigh, High, or Medium
SAA_HXD==0 Satellite is outside SAA
T_SAA_HXD>436 Time since SAA passage >436 sec ELV>5 Pointing direction >5 deg above Earth
DYE_ELV>20 Pointing direction >20 deg above sunlit limb of Earth
Table 2.1: Screening criteria of events for the XIS. SAA: South Atlantic anomaly
2.3.1.2 High-level data products
This is done as a part of the initial pipeline so that clean event files can be downloaded from archives such as heasarc. These event files can be used for producing scientific products. To produce high-level scientific products a series of event selection steps take place, these steps are carried out using xselect. The first of which is spatial event selection, this is done by producing an image of the events and selecting regions for the source (typically a circle with a radius ∼ 1.50) and background (typically one or more circles with an equal or greater area). For each of these regions light curves can be extracted, these can be investigated on their own or used to select periods of interesting behaviour or high background count rate. After any time filtering spectra are also extracted for each region.
Before spectra can be analysed, two response files are generated, these are: the Redistribution Matrix File (RMF) and the Ancilliary Response File (ARF) and are generated by using xisrmfgen6 and xissimarfgen7 respectively (which are packaged within HEAsoft ).
6
NASA’s help page for xisrmfgen: http://heasarc.gsfc.nasa.gov/ftools/fhelp/xisrmfgen.txt 7
The RMF is used to define the probability of a photon of given energy being detected by a particular channel, hence it encodes the energy resolution and mapping of the channels into energy. The ARF is defined as the energy-dependent effective area of the telescope and detector system (in cm2). Both RMF and ARF are indispensable components in the overall instrument response and therefore required to produce the flux spectrum.
Channels can then be grouped to increase signal to noise within a bin, this is performed using the grppha tool. Another use of grppha is to set headers in the FITS file of the source pointing to the background and response files, this is done so that xspec can import them together.