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ABSTRACT Spectroscopy is an important tool for providing insights into the structure of core-collapse supernova explosions. We use the Monte Carlo radiative transfer code artis to compute synthetic spectra and light curves based on a two-dimensional explosion model of an ultra-stripped supernova. These calculations are designed both to identify observable fingerprints of ultra-stripped supernovae and as a proof of principle for using synthetic spectroscopy to constrain the nature of stripped-envelope supernovae more broadly. We predict characteristic spectral and photometric features for our ultra-stripped explosion model, and find that these do not match observed ultra-stripped supernova candidates like SN 2005ek. With a peak bolometric luminosity of $$6.8\times 10^{41}\, \mathrm{erg}\, \mathrm{s}^{-1}$$, a peak magnitude of $$-15.9\, \mathrm{mag}$$ in R band, and Δm15,R = 3.50, the model is even fainter and evolves even faster than SN 2005ek as the closest possible analogue in photometric properties. The predicted spectra are extremely unusual. The most prominent features are Mg ii lines at $$2 {,}800\, {\mathring{\rm A}}$$ and $$4 {,}500\, {\mathring{\rm A}}$$ and the infrared Ca triplet at late times. The Mg lines are sensitive to the multidimensional structure of the model and are viewing-angle dependent. They disappear due to line blanketing by iron group elements in a spherically averaged model with additional microscopic mixing. In future studies, multi-D radiative transfer calculations need to be applied to a broader range of models to elucidate the nature of observed Type Ib/c supernovae. 

We present the first results of a comprehensive supernova (SN) radiative-transfer (RT) code-comparison initiative (StaNdaRT), where the emission from the same set of standardised test models is simulated by currently used RT codes. We ran a total of ten codes on a set of four benchmark ejecta models of Type Ia SNe. We consider two sub-Chandrasekhar-mass (M_tot= 1.0M_⊙) toy models with analytic density and composition profiles and two Chandrasekhar-mass delayed-detonation models that are outcomes of hydrodynamical simulations. We adopt spherical symmetry for all four models. The results of the different codes, including the light curves, spectra, and the evolution of several physical properties as a function of radius and time are provided in electronic form in a standard format via a public repository. We also include the detailed test model profiles and several Python scripts for accessing and presenting the input and output files. We also provide the code used to generate the toy models studied here. In this paper, we describe the test models, radiative-transfer codes, and output formats in detail, and provide access to the repository. We present example results of several key diagnostic features.