Cookbook

HostSub_GP adopts preprocessed 2D spectra from upstream pipelines, such as PypeIt. It then rectifies the 2D spectra onto a regular grid in wavelength and spatial direction and coadds multiple exposures. With GPs it models the 2D galaxy spectrum and provides estimates of the local background at the transient position in the original 2D spectra.

Run with PypeIt

Note

HostSub_GP has been tested on PypeIt v1.17, but should work with v1.14 and later.

  1. Install HostSub_GP by following the instructions in Installation.

  2. Make sure that your spectrograph is supported by HostSub_GP (see Spectrographs) and PypeIt (see Spectrographs).

  3. Reduce your longslit data with PypeIt, following the instructions in the PypeIt documentation. A couple of special setups are recommended:

    • Lower the significance threshold for arcs to be used in tracing wavelength tilts (to bump up the number of lines used to model the tilts and improve the 2D wavelength solution).

    • Turn off the internal local sky modeling of PypeIt (bright pixels from the host galaxy can be identified as cosmic rays and mistakenly masked).

    You can include these settings in your PypeIt configuration file,

    [calibrations]
    [[tilts]]
        tracethresh = 5.0
[reduce]
    [[skysub]]
        no_local_sky = True

    Note

    With no_local_sky turned on, PypeIt may have trouble finding and tracing the transient, especially if the trace is significantly curved. Please carefully inspect the spec2d objects.

  4. In the PypeIt output directory, run

    hostsub_setup pypeit -c YOUR_PYPEIT_FILE.pypeit --target TARGET

    This will read your .pypeit file and generate a folder named HostSub_OBJ containing a template input file hostsub.txt. See Input File for details of the input file format and parameters.

  5. Edit the input file hostsub.txt for a couple of key parameters. As a minimum, you need to specify:

    • The coordinates of the target (ra and dec) – in some spectrographs, the coordinates in the header may be incorrect (e.g., LRIS).

    • The spatial resolution, i.e., seeing (spat_resln), otherwise the FWHM of the standard star will be used.

    • The path to the raw data (raw_dir).

    • In [host_prior], the survey and filters of the archival images to be used (by default, survey = PS1, filters = grizy).

    • In [host_emission], a rough redshift of the host (z).

  6. Run the main processing script

    hostsub hostsub.txt
    This will

    • Download archival images of the host galaxy from the specified survey (e.g., Pan-STARRS1, Legacy Surveys, and SDSS) and save them in SURVEY_cutout/ (SURVEY = ps1, ls, or sdss).

    • Conduct the modeling and throw a bunch of QA plots in QA/.

    • Save the background-subtracted 2D spectra in the Science/ folder generated by PypeIt, with the suffix _hostsub.fits.

    This concludes the main HostSub_GP processing.

  7. Coming back to PypeIt, coadd the *_hostsub.fits files and a 1D spectrum and will be automatically extracted from the coadded 2D spectrum. With that you can proceed with your usual analysis.

    Note

    pypeit_coadd_2dspec sometimes complains about the *_hostsub.fits files not having the right objects to be used in the frame alignment. In this case, add

    [coadd2d]
    offsets = 0,0,0

    to your coadd2d configuration file, assuming you have three exposures to be coadded.

Other Pipelines

Support under development. Please raise an issue if you are interested in using HostSub_GP with other upstream pipelines.