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

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.

💡 Research Summary

This paper presents a proof‑of‑concept study that leverages the newly launched SPHEREx mission together with the long‑baseline NEOWISE infrared survey to identify and spectroscopically classify infrared (IR) transients across the entire sky. NEOWISE has provided a 15‑year, all‑sky, mid‑IR (3.4 µm = W1 and 4.6 µm = W2) light‑curve database, revealing thousands of slowly evolving IR outbursts that are often heavily obscured at optical wavelengths. However, most of these sources lack spectroscopic follow‑up because of their faintness or high extinction.

SPHEREx, a space‑based spectrophotometer, surveys the sky in 0.75–5 µm with a low spectral resolution (R ≈ 40–130) and a pixel scale of 6.2″, delivering a full‑sky spectrum roughly every two weeks. Its sensitivity in the 3–5 µm range matches that of NEOWISE, making it uniquely capable of providing low‑resolution spectra for the majority of NEOWISE‑detected transients.

The authors applied image‑differencing (ZOGY) to the unWISE co‑adds to identify NEOWISE transients with S/N > 5, restricting the sample to Galactic sources (|b| < 10°) that showed detections after 2023 and were bright (W1/W2 > 10⁴ µJy). Eight of the brightest candidates were selected: two previously known FU Ori‑type (FUOr) outbursts, three known classical novae, and three previously unclassified objects.

Using the public SPHEREx Level‑2 calibrated images and the SPHEREx Spectrophotometry Tool, forced PSF photometry was performed, retaining measurements with S/N > 3 and below saturation. For most targets, >90 % of the spectrum was accumulated within a 17‑day window, ensuring that the spectra represent a quasi‑instantaneous snapshot of the outburst phase.

Spectral analysis revealed distinct signatures for the two classes. The FUOrs displayed red continua with strong H₂O vapor absorption in the 1–2 µm range and prominent ice absorption features at 3.05 µm (H₂O) and 4.25 µm (CO₂) in the mid‑IR, matching higher‑resolution ground‑based spectra of classic FUors such as V900 Mon and V1057 Cyg. The classical novae showed bright emission lines (hydrogen Br α, He I,