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

This paper demonstrates a new strategy for discovering and classifying infrared transients by leveraging the synergy between NEOWISE's long-term monitoring and SPHEREx's all-sky spectro-photometric capabilities, successfully identifying new FU Orionis-type outbursts and a classical nova.

Original authors: Viraj Karambelkar, Kishalay De, Lynne A. Hillenbrand, J. L. Sokoloski, Danielle Frostig, Julianne Dalcanton

Published 2026-02-10

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CC BY 4.0

Original authors: Viraj Karambelkar, Kishalay De, Lynne A. Hillenbrand, J. L. Sokoloski, Danielle Frostig, Julianne Dalcanton

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Cosmic "Hidden World" Detectives: A New Way to See the Unseen

Imagine you are trying to watch a massive, colorful fireworks display, but someone has draped a thick, heavy velvet curtain over the entire sky. You can’t see the bright flashes of light or the vibrant colors with your eyes. You’re essentially "blind" to the show.

For a long time, that is how astronomers have viewed the "infrared" universe. Most of our telescopes look for visible light (the kind our eyes see), but many of the most exciting events in space—like stars being born or stars exploding—are wrapped in thick clouds of cosmic dust. This dust acts like that velvet curtain, soaking up visible light and hiding the action.

However, infrared light (which is essentially heat radiation) can "ghost" through that dust, much like how a thermal camera can see a person standing in a dark, smoky room.

This paper describes a new "detective duo" of space telescopes that are working together to finally pull back that curtain.

The Detective Duo

To solve the mystery of these hidden cosmic events, we need two different types of tools:

The Lookout (NEOWISE): Think of NEOWISE as a security camera that has been recording the sky for 15 years. It doesn't see fine details, but it is excellent at noticing when something suddenly gets "brighter" or "hotter" in the infrared. It’s great at spotting a "blip" on the radar, but it can't tell you exactly what caused the blip. Is it a star exploding? A baby star growing? A cosmic collision? NEOWISE knows something happened, but it doesn't know the identity of the culprit.
The Forensic Expert (SPHEREx): This is the newly launched specialist. If NEOWISE is the security camera that spots the movement, SPHEREx is the forensic scientist who arrives on the scene with a high-tech scanner. SPHEREx doesn't just see a bright spot; it takes a "spectral fingerprint" (a spectrum). By looking at the specific patterns of light, it can tell us the chemical makeup and the temperature of the object.

The "Proof of Concept"

The researchers used this duo to test a new strategy. They took the "blips" found by NEOWISE and ran them through the SPHEREx "fingerprint scanner." Here is what they found:

The "Hungry Babies" (FU Orionis stars): They found young stars that are having massive "growth spurts." These stars are surrounded by disks of ice and dust. SPHEREx saw the "fingerprints" of water and CO2 ice, confirming these were indeed young stars eating up material from their surroundings.
The "Cosmic Grenades" (Classical Novae): They found stars that underwent violent explosions. These explosions leave behind specific chemical signatures (like hydrogen and carbon) that glow brightly. SPHEREx was able to identify these even when they were hidden behind so much dust that regular telescopes couldn't see them at all.

Why This Matters

Before this, if a star was hidden behind a thick cloud of dust, it was often a "lost cause" for astronomers—we knew something was there, but we couldn't study it.

By combining the long-term memory of NEOWISE with the chemical detective skills of SPHEREx, we are opening a new window into the universe. We are no longer just seeing "blips" in the dark; we are starting to understand the life stories of the most obscured, dusty, and dramatic objects in our galaxy.

In short: We’ve stopped just watching the shadows and started reading the fingerprints left behind.

Technical Summary: SPHEREx as a Frontier for Infrared Transients

Title: SPHEREx as a frontier for infrared transients: Classification of new Galactic FU Ori outbursts and classical novae
Authors: Viraj Karambelkar et al. (February 2026)

1. The Problem: The "Obscured" Gap in Time-Domain Astronomy

While the last decade has seen a revolution in time-domain astronomy, most wide-field surveys (e.g., ZTF, Rubin) operate in the optical spectrum. This creates a significant selection bias: the most dust-enshrouded stellar eruptions—such as young stellar object (YSO) outbursts and certain classical novae—emit primarily in the infrared (IR).

Historically, IR transient studies have been limited by the high cost of IR detectors and a lack of systematic, all-sky spectroscopic follow-up. While the NEOWISE mission has identified thousands of mid-IR transients, many are too obscured by dust for ground-based optical or near-IR spectroscopy to provide definitive classifications. There is a critical need for a mission capable of providing low-resolution spectroscopy for all-sky IR transients to bridge this gap.

2. Methodology: Synergistic Multi-Mission Approach

The authors propose and demonstrate a "proof-of-concept" strategy that leverages the synergy between two space missions:

NEOWISE (Discovery): Utilizes the 15-year baseline of the Wide-field Infrared Survey Explorer to identify slowly evolving or bright mid-IR outbursts using image-differencing techniques (ZOGY algorithm) on the unWISE dataset.
SPHEREx (Classification): Uses the newly launched SPHEREx mission to provide low-resolution ( $R \sim 40–130$ ) spectrophotometry across the $0.7–5\mu\text{m}$ range.

Workflow:

Selection: The authors identified eight of the brightest Galactic IR outbursts detected by NEOWISE in the final two years of its survey (post-2023) within $|b| < 10^\circ$ .
Extraction: Using the SPHEREx Spectrophotometry Tool, they performed forced PSF photometry on Level 2 calibrated images to extract spectra.
Validation: They used known transients (FU Orionis-type outbursts and classical novae) as templates to validate that SPHEREx could recover hallmark spectral features.
Identification: They applied these spectral templates to previously unclassified NEOWISE transients.

3. Key Contributions and Results

The study successfully demonstrates that SPHEREx can distinguish between different classes of dusty transients based on specific spectral signatures:

FU Orionis-type (FUOr) Outbursts:
- Signatures: Red continua in the near-IR ( $1–2\mu\text{m}$ ) with water vapor absorption, and mid-IR ( $2–5\mu\text{m}$ ) absorption features from $\text{H}_2\text{O}$ ice ( $\sim3.05\mu\text{m}$ ) and $\text{CO}_2$ ice ( $\sim4.25\mu\text{m}$ ).
- New Discoveries: The authors identified two new FUOrs (WTP16aakapb and WTP15aaavof). Follow-up medium-resolution K-band spectroscopy from the IRTF confirmed the presence of CO absorption bands, a definitive FUOr diagnostic.
Classical Novae:
- Signatures: Strong emission lines (Hydrogen, Carbon, Aluminium, Calcium). A notable finding was a strong emission feature around $\sim4.5\mu\text{m}$ (potentially $\text{Mg II}$ , $\text{C I}$ , or $\text{Al IX}$ ), which is inaccessible from the ground due to atmospheric $\text{CO}_2$ absorption.
- New Discovery: The authors identified one previously missed Galactic nova (WTP24abacpo) by matching its fast-evolving lightcurve and $4.5\mu\text{m}$ emission feature to known novae.

4. Significance and Future Outlook

This paper establishes SPHEREx as a transformative tool for infrared time-domain astronomy.

Systematic Exploration: It proves that even with modest spectral resolution, SPHEREx can effectively classify the "hidden" population of dusty transients that optical surveys miss.
Complementarity: SPHEREx acts as a spectroscopic bridge, providing the initial classification for discoveries made by other surveys (PGIR, WINTER, PRIME) and setting the stage for future high-sensitivity missions like NEO Surveyor and the Nancy Grace Roman Space Telescope.
Scientific Impact: By enabling the systematic study of FUOrs and novae in highly obscured environments, SPHEREx will provide unprecedented insights into star formation processes and the physics of eruptive stellar phenomena in the Milky Way.