Specutils: Combining Voigt1D + Exponential1D in fitting gives strange behavior with units

[mishra-n]

Description

I combine a Voigt and Exponential Profile to model absorption spectra. And when I do the model works as expected, giving proper units. But after I run

Expected behavior

After fitting, the fitted model should just give the same model but with fitted and changed parameters but it doesn't.

Actual behavior

"Exponential1D: Units of input 'x', (dimensionless), could not be converted to required input units of Angstrom (length)"

Steps to Reproduce

import astropy.units as u
from astropy.modeling import models, fitting

from specutils.spectra import Spectrum1D
from specutils.fitting import fit_lines

import numpy as np

v = models.Voigt1D(amplitude_L=2., x_0=100 * u.Angstrom, fwhm_L=1*u.Angstrom, fwhm_G=1*u.Angstrom)
e = models.Exponential1D(amplitude=1., tau=-1., fixed={'tau': True, 'amplitude': True})

ve = v | e

x = np.linspace(95,105,1000) * u.Angstrom
y_model = ve(x)
y_data = ve(x) + np.random.normal(0, 0.2, size=x.shape)

spec_data = Spectrum1D(flux=y_data, spectral_axis=x)

ve_fitted = fit_lines(spec_data, ve)

ve_fitted(x)

System Details

Linux-4.18.0-372.26.1.el8_6.x86_64-x86_64-with-redhat-8.6-Ootpa Python 3.7.13 (default, Mar 29 2022, 02:18:16) [GCC 7.5.0] Numpy 1.21.5 pyerfa 2.0.0 astropy 4.3.1 Scipy 1.7.3 Matplotlib 3.5.2 Specutils 1.8.1

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[pllim]

I am moving this to specutils and see if this is a downstream issue first. Thanks!

[ivkram]

If think that the proper way of creating a compound model would be to use + instead of |, i.e.:

ve = v + e,

since | stands for a bitwise OR. The exponential model should be updated respectively by adding a unit to tau:

e = models.Exponential1D(amplitude=1., tau=-1. * u.Angstrom, fixed={'tau': True, 'amplitude': True})

After these updates, ve_fitted(x) works as expected.