Swift UVOT Grism Observations of Nearby Type Ia Supernovae - I. Observations and Data Reduction

TL;DR

This paper presents the largest UV spectroscopic dataset of nearby Type Ia supernovae, obtained with Swift, including a new data reduction method that enhances spectral quality and consistency with Hubble observations.

Contribution

It introduces a comprehensive UV spectral dataset of 39 SNe Ia and a novel data reduction technique for Swift UV grism data, enabling better analysis of supernova properties.

Findings

Largest UV spectral sample of SNe Ia to date.

New data reduction procedure improves spectral quality.

Swift spectra are consistent with Hubble UV spectra.

Abstract

Ultraviolet (UV) observations of Type Ia supernovae (SNe Ia) are useful tools for understanding progenitor systems and explosion physics. In particular, UV spectra of SNe Ia, which probe the outermost layers, are strongly affected by the progenitor metallicity. In this work, we present 120 Neil Gehrels Swift Observatory UV spectra of 39 nearby SNe Ia. This sample is the largest UV (lambda < 2900 A) spectroscopic sample of SNe Ia to date, doubling the number of UV spectra and tripling the number of SNe with UV spectra. The sample spans nearly the full range of SN Ia light-curve shapes (delta m(B) ~ 0.6-1.8 mag). The fast turnaround of Swift allows us to obtain UV spectra at very early times, with 13 out of 39 SNe having their first spectra observed >~ 1 week before peak brightness and the earliest epoch being 16.5 days before peak brightness. The slitless design of the Swift UV grism complicates the data reduction, which requires separating SN light from underlying host-galaxy light and occasional overlapping stellar light. We present a new data-reduction procedure to mitigate these issues, producing spectra that are significantly improved over those of standard methods. For a subset of the spectra we have nearly simultaneous Hubble Space Telescope UV spectra; the Swift spectra are consistent with these comparison data.