Transient Treasures: Discovering Explosive Events in JWST’s Infrared Dark Field

The James Webb Space Telescope (JWST), best known for its dazzling images of distant galaxies, is also a powerful time-domain observatory—meaning it can catch objects in the act of changing over time. In this study, Haojing Yan and collaborators used JWST’s NIRCam instrument to look for “transients,” or brief astronomical events, in a field known as the JWIDF (James Webb Infrared Dark Field). This region of sky, continuously visible to JWST and with very little background light, is perfect for spotting dim and fast-changing objects. Using three sets of NIRCam images taken roughly six months apart, along with supporting data from the Hubble Space Telescope (HST), the team discovered 21 new transients—some possibly ordinary supernovae, others more mysterious.

Catching Cosmic Flickers

The team used JWST's NIRCam in four infrared filters, covering a rectangular patch of the sky about 14 arcminutes squared. This region was observed three times across JWST’s first year of science operations. By subtracting images taken at different times, they could detect small changes in brightness—signatures of transients. The transients were identified in three ways: those that were fading over time (found by subtracting later images from earlier ones), those that were brightening (by subtracting earlier images from later ones), and those peaking in the middle. The most sensitive filter, F356W, served as the basis for detection. To ensure these were real and not image artifacts, the team examined each candidate by eye and matched them with nearby host galaxies when possible.

Classifying the Cosmic Flashers

Of the 21 detected transients, five had no visible host galaxies, which could mean their hosts were too faint to detect or that the transients occurred in truly isolated regions. The remaining 16 were either deeply embedded within galaxies or located near their outskirts. Using the colors and brightness of each transient over time—measured across the different filters—the team built a picture of how each event evolved. Some transients remained bright over several months, while others faded quickly. For the 16 with visible hosts, the researchers also used the galaxies’ colors to estimate how far away they were, with redshifts ranging from very nearby (z ≈ 0.04) to very distant (z > 2.6).

Supernovae and “Gap” Transients

A few transients, particularly those in more distant galaxies (z > 1.6), had properties consistent with known types of supernovae—especially Type Ia, which are important for measuring cosmic distances. One of these (T-D31-E) was confirmed as a Type Ia supernova at redshift z = 1.64 using JWST’s NIRSpec spectrograph. However, the fainter and closer events (z < 0.4) did not match typical supernova brightness and instead fell into what astronomers call the “gap” category—events dimmer than supernovae but brighter than classical novae. These might be examples of rare phenomena like supernova impostors or luminous red novae, or they might represent entirely new classes of transients.

Why It Matters

This study highlights the power of JWST in exploring time-domain astronomy in the infrared—an area where earlier telescopes struggled. Because the JWIDF can be observed all year, it offers a unique opportunity for long-term monitoring. The transients found here are just the beginning: with more frequent observations and follow-up spectra, astronomers could unravel the true nature of the “gap” events and perhaps discover entirely new kinds of stellar explosions.

Looking Ahead

The authors argue that the best way to uncover the secrets of these transients is to observe the JWIDF more often and in more wavelengths. They also recommend more follow-up using NIRSpec to get detailed spectra of the transients while they are still bright. By doing this, JWST can continue to shed light not only on the distant universe’s past but also on its rapidly changing present.

Source: Yan