SPIRITS: Uncovering Unusual Infrared Transients With Spitzer

TL;DR

The SPIRITS survey systematically searches for infrared transients in nearby galaxies, discovering 43 transients including 14 unusual events called SPRITEs that fill the luminosity gap between novae and supernovae, with diverse properties and origins.

Contribution

This study introduces the SPIRITS survey and characterizes a new class of infrared transients, SPRITEs, with detailed analysis of their properties and potential origins.

Findings

Discovered 43 transients in the first year of SPIRITS.

Identified 14 SPRITEs with unique infrared properties.

Detailed case study of SPIRITS 14ajc showing shock-excited molecular hydrogen.

Abstract

We present an ongoing, systematic search for extragalactic infrared transients, dubbed SPIRITS --- SPitzer InfraRed Intensive Transients Survey. In the first year, using Spitzer/IRAC, we searched 190 nearby galaxies with cadence baselines of one month and six months. We discovered over 1958 variables and 43 transients. Here, we describe the survey design and highlight 14 unusual infrared transients with no optical counterparts to deep limits, which we refer to as SPRITEs (eSPecially Red Intermediate Luminosity Transient Events). SPRITEs are in the infrared luminosity gap between novae and supernovae, with [4.5] absolute magnitudes between -11 and -14 (Vega-mag) and [3.6]-[4.5] colors between 0.3 mag and 1.6 mag. The photometric evolution of SPRITEs is diverse, ranging from < 0.1 mag/yr to > 7 mag/yr. SPRITEs occur in star-forming galaxies. We present an in-depth study of one of them, SPIRITS 14ajc in Messier 83, which shows shock-excited molecular hydrogen emission. This shock may have been triggered by the dynamic decay of a non-hierarchical system of massive stars that led to either the formation of a binary or a proto-stellar merger.