Spectroscopy

THATCH extracts spectra from two HST spectroscopic modes:

STIS Slit Spectroscopy

For STIS observations, THATCH uses the pipeline-extracted 1D spectra (x1d/sx1 FITS files) directly from MAST. These are flux-calibrated by the calstis pipeline.

Supported gratings: G140L, G230L, G230LB, G430L, G750L (R ~ 500–10,000)

SN 2011fe: 71 STIS Spectra

THATCH extracted 71 STIS spectra spanning days -14 to +3996 relative to B-maximum, covering UV through optical wavelengths:

HST/STIS spectral sequence of SN 2011fe showing evolution from pre-maximum through the nebular phase. UV (G230LB), optical (G430L), and red (G750L) gratings are combined at each epoch.

WFC3/IR Grism Spectroscopy

For slitless grism data (G102, G141), THATCH uses grizli for forward-model extraction with contamination subtraction. The workflow:

1. Pair each grism FLT with its contemporaneous direct image FLT
2. Build a segmentation map at the transient position
3. Initialize grizli's GrismFLT model with grism trace calibrations
4. Compute the contamination model and perform optimal 1D extraction
5. Flux-calibrate using the grism sensitivity curve

Supported grisms: G102 (0.8–1.15 μm, R~210), G141 (1.1–1.7 μm, R~130)

AT2017gfo: Grism Extraction

THATCH extracted flux-calibrated G102+G141 spectra at 4 epochs (+5, +7, +9, +10 days post-merger):

WFC3/IR grism spectra of AT2017gfo extracted with grizli. Each epoch combines 4 dithered exposures.

Combined UV-to-NIR Spectra

By combining STIS UV spectroscopy with WFC3/IR grism data, THATCH produces continuous 0.15–1.7 μm spectral coverage from HST alone:

AT2017gfo at ~5 days post-merger: STIS G230L (UV) + WFC3 G102 + G141 (NIR), with X-shooter ground-based spectrum for comparison.

Output Format

Spectra are stored in HDF5, with each spectrum as a group containing:

• wavelength — array in Angstroms
• flux — array in erg/s/cm²/Å
• flux_err — array in erg/s/cm²/Å
• Metadata attributes: mjd, instrument, grating, exptime_s, proposal_id