CRIRES+ observations of GY Aql in Stokes IQU

The observations are taken on the night of 2024-10-22 with CRIRES+ in spectropolarimetry mode. 3 polarimetric sequences are taken, 2 in Stokes Q and 1 in Stokes U. The 0.2 arcsecond slit was used yielding a nominal resolving power or around R = 10^5.

Each polarimetric sequence was reduced independently using a non-standard reduction. Why? Read below.

Spectropolarimetric reduction.

The goal is to find differences in the 'stellar spectrum' and the 'scattered spectrum'. The stellar spectrum is extract in a 4 pixel window around the center of the PSF (+2 pixel up and + 2 pixels down). Two 'circumstellar' spectra are extracted above and below the stellar spectrum, both with 4 pixel windows. The conversion between pixels and arcseconds on sky is as follow: 180 pixels is 10 arcseconds, 1 pixel is roughly 55 mas.

1. The first step of the reduction is to run the standard spectropolarimetry reduction with the cr2res_obs_pol recipe. This step will produce an extracted spectrum , but also the trace wave (the location of the spectrum on the detector) file, which we will use to adjust extraction positions on the detector later on. For each polarimetric sequence, the extraction is run with

esorex cr2res_obs_pol --extract_oversample=12 --save_group=1 --extract_swath_width=800 pol.sof calibs.sof

The arguments:

1. --extract_oversample=12 specify the oversampling of the slit function, 12 is the standard value for science extraction
2. --save_group=1 means that many intermediate files are saved allowing us to demodulate the Stokes signals ourselves in a better way than the DRS
3. --extract_swath_width=800 specify the swath width during extraction. 800 is a good value for high-SNR data like this
4. pol.sof and calibs.sof are the list of input files for the reductions. pol.sof contains the 8 raw science files, calibs.sof contains processed calibrations: a master flat, a bad pixel mask, and a trace wave with wavelength solution.

2. The second step is to setup the individual 3 extractions: up, star, down.. This is done by running the create_tracewaves.py which sets up the .sof files for the extractions as well as the tracewaves with vertical shifts of -4, 0, and +4 pixels, in the down/, center/, and up/ subdirectories.

3. In each subdirectories, the esorex command are created by create_reduce.sh.. This creates a script in reduce.sh with extraction command for each individual trace. The output files are named like

• A_1d_0000.fits, A_1d_0001.fits, A_1d_0002.fits

where A can be A or B (nodding position), 1 runs from 1 to 4 (polarimetric subexposure), and d can be u (up) or d (down) (polarimetric beams).

The _0002.fits files are the extracted spectra. The demodulation is done by running

• python3 gather_spec.py up/
• python3 gather_spec.py center/
• python3 gather_spec.py down/

And the final extracted and demodulated files are in center/center_cr2res_obs_pol_demodulated.fits, down/down_cr2res_obs_pol_demodulated.fits, up/up_cr2res_obs_pol_demodulated.fits