'nusplitsc' decomposes the NuSTAR SCIENCE_SC (06) event files into a maximum of seven distinct event files corresponding to the CHU combinations descrived below.
NuSTAR SCIENCE_SC (06) observing mode events refer to scientific data collected during time intervals in which an aspect solution from the on-board star tracker located on the X-ray optics bench (Camera Head Unit 4, CHU4), the primary method for determining the absolute pointing, is not available. During these time intervals, the aspect reconstruction used to calculate sky coordinates is derived using data from the three star trackers located on the spacecraft bus (Camera Head Units 1-2-3, CHU1/CHU2/CHU3). In these cases the accuracy of the sky coordinates degrades to about 2 arcminutes, mainly due to thermal flexion of the spacecraft bus star cameras. Moreover, the three star tracker camera units data can be integrated in seven different combinations (CHU1 only, CHU2 only, CHU3 only, CHU1&2,CHU1&3, CHU2&3, and CHU1&2&3) that are characterized by a specific aspect reconstruction. Due to the switching between different CHU combinations, the sky images can then show multiple centroids of the same source precluding an accurate scientific analysis of these data.
'nusplitsc' first generates a set of seven GTI files based on CHU1/CHU2/CHU3 housekeeping parameters stored in the input file 'chu123hkfile'. The GTI files are created by running the multi-mission task 'maketime'. The GTI selection expressions, corresponding to the seven CHU combinations, are read from the input file 'chu123rangefile' (CALDB by default). In the second step, the task splits the input event file using the calculated GTIs using the multi-mission package 'xselect'. Finally, 'nusplitsc' applies the dead time correction to the temporal keywords of the output event files by running the 'nulivetime' task (see the 'nulivetime' help file for additional details).
The event files generated by 'nusplitsc' need to be carefully checked before the scientific analysis. In particular, it is necessary to verify that the target source in the sky images appears as a well defined centroid. If multiple centroids are present the corresponding event file should not be used for the generation of high-level data products (i.e. energy spectra and light-curves). In these cases, the user can set the input parameter 'splitmode' (='NORMAL' by default) to 'STRICT' to remove some CHU1/CHU2/CHU3 flickering which can be at the origin of the multiple centroids. As an additional option, multiple centroids in some cases can be removed by setting the 'timecut' input parameter (='no' by default) to 'yes'.
By default the task applies the stem of the input files to the output files (input parameter 'stemout=DEFAULT'). The naming convention of the generated event files in the case 'splitmode=NORMAL' is the following:
nu10012001002A06_chu1_N_cl.evt L2 FPMA 06 event file for CHU1 only nu10012001002A06_chu2_N_cl.evt L2 FPMA 06 event file for CHU2 only nu10012001002A06_chu3_N_cl.evt L2 FPMA 06 event file for CHU3 only nu10012001002A06_chu12_N_cl.evt L2 FPMA 06 event file for CHU1 and CHU2 nu10012001002A06_chu13_N_cl.evt L2 FPMA 06 event file for CHU1 and CHU3 nu10012001002A06_chu23_N_cl.evt L2 FPMA 06 event file for CHU2 and CHU3 nu10012001002A06_chu123_N_cl.evt L2 FPMA 06 event file for CHU1 and CHU2 and CHU3In the case 'splitmode=STRICT' in the output file names the _N_ string is replaced by _S_.
To extract the high-level data products from the output event files the 'nuproducts' software module with standard command lines can be used. Note that each CHU1/CHU2/CHU3 combination has a specific aspect reconstruction with a typical scatter of about 2 arcminutes. For this reason, a set of specific spatial source and backgroud extraction regions should be used to extract the scientific products.
> nusplitsc infile=nu60101035002A06_cl.evt hkfile=nu60101035002A_fpm.hk chu123hkfile=nu60101035002_chu123.fits