X-Atlas Data Processing

The HETG consists of two grating arms: the High Energy Grating (HEG), optimized for a spectral range of approximately 1.2 to 15 Å, and the Medium Energy Grating (MEG), sensitive up to around 31 Å \citep{cxc05}. The dispersed spectra produced by these HETG arms cross at their respective zeroth orders, creating the elongated 'x' for which X-Atlas is named.

All archival data used to create X-Atlas have undergone Standard Data Processing (SDP) during the most recent Chandra reprocessing run, which began in February 2006. In addition,the reduction of HETG observations and extraction of the HEG and MEG spectra was performed using the latest version (3.3) of the Chandra Interactive Analysis of Observations (CIAO) software. To achieve uniform processing, we have created a pipeline of the appropriate 'threads' to reduce each observation. In addition to removing various instrumental distortions, the X-Atlas pipeline locates the zeroth-order image and the dispersed spectra of each gratings arm, classifies the dispersed photons by spectral order, and coadds the positive and negative gratings spectral orders.

Data preparation for a given observation begins with the level 1 event file, a list of the arrival times and energies of incoming photons that is retrieved from the Chandra data archive. Some portion of the events are caused by cosmic ray interactions in the CCD, and if not removed, these 'afterglow' events can be interpreted as faint sources. SDP flags possible cosmic ray events to filter them out in the level 2 event file, but it has been shown that 3-5% of valid source photons may also be rejected by this processing \citep{ciao06}. Reductions for X-Atlas therefore use CIAO threads to reset the afterglow flags on the level 1 event file and re-identify afterglows. Despite the significant increase in processing time per observation, far fewer actual source events are marked as afterglows, and pixels that are hot or have bad bias values are flagged as well.

In addition to afterglow detection, SDP also randomizes the coordinates of events within the pixel where they are detected to avoid aliasing effects and remove the 'gridded' appearance of the data. For X-Atlas reductions, we removed this randomization. Other corrections applied include a gain adjustment to compensate for time-dependent drifts in the detectors and an adjustment for the charge transfer inefficiency (CTI) of the detectors as charge is moved from pixel to pixel during readout.

With the data prepped, the extraction of the dispersed photons begins. Various CIAO threads detect the zeroth-order image, determine the locations of the grating arms, and assign events from each arm to the appropriate spectral orders. At this point, a level 2 event file is created by filtering out the bad grades previously marked as afterglows and hot pixels, removing the effects of readout streaking, and selecting only data falling within the good time intervals determined by SDP.

The actual grating spectra are extracted from the destreaked level 2 event file. In addition, we create gratings Redistribution Matrix Files (RMFs) to map between detector pulse height and energy space and Ancillary Response Files (ARFs) to measure the instrumental area and quantum efficiency of the HETG/ACIS-S configuration. The positive and negative spectral orders of each arm are then coadded and stored in six pulse height amplitude (PHA2) spectral files, each of which corresponds to either the HEG or MEG arm of HETG and to one of the coadded 1st, 2nd, or 3rd orders.