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A multiwavelength view of the nearby Calcium-Strong Transient SN 2025coe in the X-Ray, Near-Infrared, and Radio Wavebands

This paper presents a multiwavelength analysis of the nearby Calcium-Strong Transient SN 2025coe, revealing its Type Ib spectral characteristics and the presence of dense circumstellar material that supports a massive star progenitor or exotic pre-supernova mass ejection scenario.

Original authors: Sahana Kumar, Raphael Baer-Way, Aravind P. Ravi, Maryam Modjaz, Poonam Chandra, Stefano Valenti, Lindsey A. Kwok, Samaporn Tinyanont, Ryan J. Foley, D. Andrew Howell, Daichi Hiramatsu, Jennifer E. And

Published 2026-07-21

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CC BY 4.0

Original authors: Sahana Kumar, Raphael Baer-Way, Aravind P. Ravi, Maryam Modjaz, Poonam Chandra, Stefano Valenti, Lindsey A. Kwok, Samaporn Tinyanont, Ryan J. Foley, D. Andrew Howell, Daichi Hiramatsu, Jennifer E. Andrews, K. Azalee Bostroem, Collin Christy, Noah Franz, Brian Hsu, Jeniveve Pearson, David J. Sand, Manisha Shrestha, Nathan Smith, Bhagya Subrayan

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

Imagine the universe as a giant, cosmic stage where stars are the actors. Most of the time, when a massive star dies, it puts on a predictable show called a supernova, leaving behind a specific set of clues that astronomers can read like a script. But sometimes, the universe throws a curveball: a "Calcium-strong Transient" (or CaST). These are the weirdos of the supernova world. They are faint, they fade away incredibly fast, and instead of the usual heavy elements, their final performance is dominated by a loud, bright shout of calcium. For years, scientists have been scratching their heads over these cosmic oddballs. Are they the explosive deaths of massive stars that got stripped of their outer layers? Or are they the result of two white dwarf stars (the dense, dead cores of smaller stars) crashing into each other? It's a mystery that has kept astronomers up at night, because solving it means understanding how different types of stars live and die.

Now, enter SN 2025coe, the newest and most famous suspect in this cosmic whodunit. This paper is like a detective story where the investigators finally get to look at the crime scene with three different pairs of high-tech glasses: X-ray, near-infrared, and radio. The team didn't just look at the light; they listened to the radio waves and felt the heat of the X-rays to figure out what happened. They found that SN 2025coe is a very close neighbor in cosmic terms, making it the perfect subject for a deep dive. By combining all these different views, the researchers are trying to answer the big question: Is this a massive star that lost its hair before it died, or a double-star system that exploded?

The investigation began with a look at the X-rays, which are like the "smoke" left behind after a fire. The team spotted SN 2025coe glowing in X-rays just 3 and 8 days after it exploded. This glow suggested that the exploding star slammed into a thick cloud of gas (called circumstellar material, or CSM) that was hanging out very close by. By crunching the numbers, they estimated this cloud weighed about 0.12 ± 0.11 times the mass of our Sun and stretched out to a distance of at least 2 × 10¹⁵ cm (which is roughly 30,000 times the distance from the Sun to the Earth). However, the story has a twist: when they looked again later, the X-ray glow disappeared. This suggests the cloud wasn't a giant, endless fog, but rather a dense, compact shell that the explosion ran through and then left behind. The radio observations confirmed this, showing no signs of a massive, extended cloud further out.

Next, the team turned their "infrared glasses" on the event to look at the chemical makeup of the explosion. In the world of supernovas, near-infrared light is special because it can see through the dust to spot helium, a key ingredient. The spectra (the chemical fingerprints) of SN 2025coe were packed with strong helium signals, looking almost exactly like a Type Ib supernova—a star that has been stripped of its outer hydrogen layers. This is a big clue. The paper argues that these helium features are much stronger and faster than what models of exploding white dwarfs (thermonuclear explosions) predict. While the data points strongly toward a massive star that had been stripped of its skin, leaving a helium-rich core that exploded, the authors note that the origin is still debated. They find the evidence consistent with a core-collapse event, but acknowledge that the strange mass-loss history and the location far from the galaxy center make it difficult to fully rule out exotic white dwarf scenarios.

So, what is the final verdict? The paper suggests that SN 2025coe is likely a massive star that lost a huge amount of its mass in the final months before it died, creating a dense, nearby shell of gas. When the star exploded, it hit this shell, creating the X-ray glow, and then raced through the helium-rich remains of the star. While the location of the explosion was far from the center of its host galaxy (which usually makes scientists think it's a white dwarf), the physical evidence from the X-rays and the infrared light tells a different story. The authors conclude that this event is most consistent with a core-collapse of a massive star, but one that behaved very strangely compared to its cousins. It's a reminder that even in a universe full of predictable patterns, there are still some cosmic characters that refuse to follow the script, and we need to keep watching them closely to understand the full story of how stars end their lives.

Technical Summary: A Multiwavelength View of the Nearby Calcium-strong Transient SN 2025coe

Problem and Context
Calcium-strong transients (CaSTs) represent a growing but enigmatic subclass of faint, rapidly evolving supernovae (SNe) characterized by prominent [Ca II] emission and weak oxygen features. Their physical origins remain debated, with properties suggesting either the core-collapse of a low-mass massive star or a thermonuclear explosion of a white dwarf (WD). While optical studies have identified CaSTs, their origins are difficult to constrain due to the lack of multiwavelength data, particularly in the near-infrared (NIR), X-ray, and radio regimes. Previous NIR studies of CaSTs (e.g., iPTF15eqv, SN 2019ehk, SN 2021gno) have been limited, and only two CaSTs had prior X-ray detections. The nature of the circumstellar material (CSM) surrounding these events—critical for distinguishing between progenitor scenarios—is poorly understood.

Methodology
The authors present a coordinated multiwavelength study of SN 2025coe, the third-nearest CaST discovered (distance $25 \pm 9.3$ Mpc), observed from days to weeks post-explosion. The study integrates:

X-ray Observations: Utilizing the Neil Gehrels Swift Observatory (XRT) at 11 epochs (2–100 days post-explosion), reduced to five distinct epochs with sufficient exposure. The authors performed spectral fitting using power-law and thermal bremsstrahlung models to derive absorption columns and luminosities.
Near-Infrared (NIR) Spectroscopy: Obtaining the most robust NIR spectral time series for a CaST to date, using Keck II/NIRES (10 and 23 days post-explosion) and MMT/Magellan Infrared Spectrograph (45 days post-explosion). The authors identified He I lines (1.083 and 2.058 $\mu$ m) and compared them against synthetic templates of He-rich stripped-envelope SNe (SESNe), Type Ia SNe, and hybrid WD models.
Radio Observations: Conducting non-detection monitoring from 20 to 153 days post-explosion across 1.25–240 GHz using the SMA, GMRT, and VLA. These data were used to place upper limits on CSM density and mass-loss rates via synchrotron self-absorption (SSA) and free-free absorption (FFA) modeling.
Optical Context: Leveraging a companion paper (A. P. Ravi et al. 2026) for optical light curve modeling, explosion timing, and optical spectral analysis to provide a holistic view.

Key Results

X-ray Detection and CSM Interaction: SN 2025coe is the third X-ray-detected CaST. X-ray emission was detected at $\sim$ 3 and 8 days post-explosion with an unabsorbed luminosity of $\sim 3 \times 10^{40}$ erg s $^{-1}$ . Modeling suggests interaction with $\sim 0.12 \pm 0.11 M_\odot$ of CSM extending to at least $2 \times 10^{15}$ cm. The inferred mass-loss rate ( $\sim 0.2\text{--}0.5 M_\odot$ yr $^{-1}$ ) implies intensive mass loss in the months to years preceding the explosion, a rate significantly higher than typical SESNe.
NIR Spectral Evolution: The NIR spectra reveal strong He I P Cygni profiles at 1.083 and 2.058 $\mu$ m, evolving from photospheric to nebular phases. The spectral morphology, particularly the strength and velocity of helium lines, closely matches He-rich SESNe (Type Ib/IIb) and is distinct from normal Type Ia SNe or hybrid WD merger models (e.g., Y. Zenati et al. 2023), which fail to reproduce the observed He line strengths.
Radio Constraints: Radio observations yielded non-detections from 20 to 153 days. These limits constrain the CSM density, suggesting that while dense CSM exists within $\sim 3.5 \times 10^{15}$ cm (consistent with X-ray data), there is no evidence for extended dense CSM beyond $\sim 5 \times 10^{15}$ cm. The lack of radio emission at late times argues against a continuous, high-density wind extending far from the progenitor.
Progenitor Velocity: NIR helium absorption velocities in SN 2025coe ( $\sim 10,000\text{--}12,000$ km s $^{-1}$ ) are higher than those in other nearby CaSTs (SN 2019ehk, SN 2021gno) but consistent with normal Type Ib SNe.

Significance and Claims
The paper claims that SN 2025coe provides the most comprehensive multiwavelength dataset for a CaST to date, offering critical constraints on their progenitor systems.

Core-Collapse vs. Thermonuclear: The presence of strong He I P Cygni lines in the NIR and the detection of X-ray emission favor a core-collapse origin (specifically a stripped-envelope massive star) over a thermonuclear origin. The authors note that no thermonuclear SN has been conclusively detected in X-rays; the detection in three nearby CaSTs (including SN 2025coe) suggests X-ray emission may be a common feature of core-collapse CaSTs, while thermonuclear CaSTs (like SN 2016hnk) may be intrinsically X-ray faint.
CSM Characteristics: The study identifies a unique CSM configuration: high-density material confined to a small radius ( $\sim 10^{15}$ cm) with an extreme mass-loss rate, distinct from the extended winds of typical interacting SNe (e.g., Type Ibn or IIn). This suggests a transitional progenitor scenario, possibly a low-mass massive star undergoing intensive, short-duration mass loss, or a binary system with a specific pre-explosion ejection history.
Future Directions: The authors emphasize that the large offset of SN 2025coe from its host galaxy complicates a standard massive star origin, necessitating future high-sensitivity X-ray and radio observations closer to explosion to better constrain CSM properties. They also call for advanced NIR radiative transfer simulations including non-thermal excitation to better quantify helium masses and distinguish between core-collapse and WD merger models.

In summary, the paper argues that SN 2025coe is likely a core-collapse event with a compact, high-density CSM environment, and that the multiwavelength properties of CaSTs (specifically X-ray and NIR signatures) are essential for resolving their long-standing progenitor debate.

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