SPIRITS 15c and SPIRITS 14buu: Two Obscured Supernovae in the Nearby Star-forming Galaxy IC 2163

Abstract

SPitzer InfraRed Intensive Transients Survey-SPIRITS-is an ongoing survey of nearby galaxies searching for infrared (IR) transients with Spitzer/IRAC. We present the discovery and follow-up observations of one of our most luminous (M . Pre-discovery ground-based imaging revealed an associated, shorter-duration transient in the optical and near-IR (NIR). NIR spectroscopy showed a broad (≈8400 km s −1 ), double-peaked emission line of HeI at 1.083 μm, indicating an explosive origin. The NIR spectrum of SPIRITS 15c is similar to that of the Type IIb SN2011dh at a phase of ≈200days. Assuming an A V =2.2 mag of extinction in SPIRITS 15c provides a good match between their optical light curves. The NIR light curves, however, show some minor discrepancies when compared with SN2011dh, and the extreme [3.6]-[4.5] color has not been previously observed for any SNIIb. Another luminous (M 4.5 = −16.1 ± 0.4 mag) event, SPIRITS 14buu, was serendipitously discovered in the same galaxy. The source displays an optical plateau lasting 80days, and we suggest a scenario similar to the low-luminosity TypeIIP SN2005cs obscured by A V ≈1.5 mag. Other classes of IR-luminous transients can likely be ruled out in both cases. If both events are indeed SNe, this may suggest that 18% of nearby core-collapse SNe are missed by currently operating optical surveys.

The SPIRITS Sample of Luminous Infrared Transients: Uncovering Hidden Supernovae and Dusty Stellar Outbursts in Nearby Galaxies*

“…SPIRITS 15c, spectroscopically confirmed as an SN Ib/IIb in Jencson et al (2017), is fully consistent with the sample of stripped-envelope SNe in its [4.5] light curve. At [3.6] − [4.5] = 3.0 at peak light, it is the reddest strippedenvelope SN yet observed by Spitzer.…”

“…As discussed in detail in Jencson et al (2017), SPIR-ITS 15c is a clear example of a confirmed, moderately obscured, CCSN. Our assessment is based primarily on near-IR spectroscopy showing distinct similarity to the Type IIb SN 2011dh, and in particular the presence of a broad (≈ 8400 km s 1 ) double-peaked emission line of He I at 1.0830 µm.…”

“…We list basic properties for these objects in Table 1. One source, SPIRITS 14buu (first presented in Jencson et al 2017), was identified in the first epoch of SPIRITS imaging of the galaxy IC 2163. As such, we have no constraint on the age of this object from SPIRITS data, and thus exclude it from the primary sample.…”

Revealing Dusty Supernovae in High-redshift (Ultra)Luminous Infrared Galaxies through Near-infrared Integrated Light Variability

“…Another example is the ongoing SPitzer InfraRed Intensive Transients Survey (SPIRITS; Kasliwal et al (2017)), which searches for Ch1/2 transients in 190 nearby galaxies. SPIRITS has also led to the discovery of new SNe: Jencson et al (2017) report two SNe in IC 2163 (not a LIRG) separated by less than two years. This survey, which does not include (U)LIRGS, is confined to the local universe (< 15 Mpc) and only aims to discover transients that can be individually resolved.…”

An ALMA/HST Study of Millimeter Dust Emission and Star Clusters

“…It is possible for this source to be a dust-obscured supernova as it would only take an A V on the order of a few magnitudes to dim the event enough to not be detectable in optical surveys (see e.g. Jencson et al 2017). Ultimately, deeper imaging at many wavelengths coupled with sensitive spectroscopic information (e.g., JWST) is needed to more fully understand this enigmatic source.…”