SPHEREx as a frontier for infrared transients: Classification of new Galactic FU Ori outbursts and classical novae

TL;DR

The paper tackles the challenge of characterizing infrared transients that are obscured in optical surveys by leveraging SPHEREx's all-sky, low-resolution infrared spectroscopy in conjunction with NEOWISE mid-IR transient discoveries. It demonstrates a proof-of-concept using eight Galactic transients, showing that SPHEREx spectra reveal distinct signatures—cool molecular absorption for FU Orionis outbursts and emission-line spectra for classical novae—and enabling the identification of two new FU Ori outbursts and one missed nova. This approach validates a scalable pathway for spectral classification of dusty IR transients across large samples, potentially transforming our understanding of the dynamic infrared sky. The work outlines practical challenges and future opportunities, highlighting how SPHEREx can partner with ongoing and upcoming IR time-domain surveys and missions to map the dusty transient population at unprecedented breadth.

Abstract

We demonstrate proof-of-concept of a new strategy for studying infrared (IR) transients enabled by the newly launched SPHEREx space mission, by leveraging its synergy with the NEOWISE space mission. With its fifteen year baseline and all-sky mid-IR coverage, NEOWISE provides an excellent avenue to discover thousands of slowly evolving infrared outbursts. With its all-sky spectro-photometric coverage and mid-IR sensitivity matching NEOWISE, SPHEREx is uniquely positioned to provide low-resolution IR spectra for the vast majority of these outbursts, several of which are too obscured for ground-based spectroscopic classification. As a demonstration of this approach, we present SPHEREx spectra for eight Galactic transients identified in NEOWISE. This sample includes two previously known FU Orionis-type (FUOr) outbursts whose SPHEREx spectra exhibit clear signatures of cool molecular absorption and three known classical novae showing strong emission lines in SPHEREx. Using these sources as templates, we identify two new FUOrs and one previously missed Galactic nova. Our results highlight the potential of SPHEREx for systematic explorations of the relatively underexplored dynamic infrared sky.

Figures (5)

• Figure 1: NEOWISE lightcurves of the eight transients presented in this paper. Circles mark the NEOWISE photometry, while stars indicate synthetic photometry derived from SPHEREx. The previously known transients are indicated with their published names, while the NEOWISE names of the three newly identified transients are marked in purple with their coordinates. The gray vertical shaded region marks the epochs of SPHEREx observations.
• Figure 2: SPHEREx infrared spectra for the four transients showing molecular absorption features --- two previously known and two new FU Ori-type young stellar outbursts. Plotted in red, for comparison, are medium resolution, ground-based IR spectra of known FU Ori outbursts V900 Mon, V1057 Cyg Connelley2018. For WTP16aakapb, the sharp drop in flux at $\approx3.4$$\upmu$m likely results from a bad photometric measurement (see Section \ref{['sec:spherex_spectra']}).
• Figure 3: Follow-up medium resolution K-band spectra for the two new FUOrs identified in this paper, showing CO absorption bands that are not resolved in the low-resolution SPHEREx spectra.
• Figure 4: SPHEREx infrared spectra for the four transients showing strong emission lines in their spectra --- three previously known classical novae and one likely new one. Plotted in red, for comparison, are medium resolution IR spectrum of the classical nova V5558 Sgr Rudy2025a (scaled and shifted arbitrarily).
• Figure 5: SPHEREx in the context of ongoing and planned IR time-domain surveys. SPHEREx is poised to provide low-resolution spectra for the vast majority of transients identified by these surveys.