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

Abstract: Calcium-strong transients (CaSTs) are a subclass of faint and rapidly evolving supernovae (SNe) that exhibit strong calcium features and notably weak oxygen features. The small but growing population of CaSTs exhibits some aspects similar to thermonuclear supernovae and others that are similar to massive star core-collapse events, leading to intriguing questions on the physical origins of CaSTs. SN 2025coe is one of the most nearby CaSTs discovered to date, and our coordinated multi-wavelength observations obtained days to weeks post-explosion reveal new insights on these enigmatic transients. With the most robust NIR spectroscopic time-series of a CaST collected to date, SN 2025coe shows spectral signatures characteristic of Type Ib SNe (SNe Ib, i.e. He-rich stripped-envelope SNe). SN~2025coe is the third X-ray detected CaST and our analysis of the \textit{Swift} X-ray data suggest interaction with 0.12 $\pm\,0.11\ M_{\odot}$ of circumstellar material (CSM) extending to at least $2 \times 10^{15}$cm ($\sim 30,000\ R_{\odot}$), while our analysis of the 1-240 GHz radio non-detections gives an outer radius of that CSM of at most $\sim 4\times 10<sup>{15}$ cm. This inferred nearby high-density CSM extending out to $3\pm 1 \times10<sup>{15}$ cm is similar to that seen in the other two X-ray detected CaSTs, and its presence suggests that either intensive mass-loss or some polluting mechanism may be a common feature of this subclass. Our work also expands upon recent studies on the optical properties of SN 2025coe and explores our current understanding of different progenitor systems that could possibly produce CaSTs.

• The paper presents the first near-infrared (NIR) spectral time series of a Calcium-Strong Transient (CaST), SN 2025coe, combining it with X-ray and radio observations to reveal several key insights.
• X-ray detections and subsequent non-detections hint at a transient CSM interaction, terminating around 10^15 cm, and favor lower-energy, velocity dispersion-poor massive stars as core-collapse progenitors.
• He-rich ejecta are evinced via consistent NIR observations at speeds as high as 2Å, while suggesting interesting merger candidates.

Context and motivation

Calcium-strong transients (CaSTs) are a rare subclass of faint, rapidly evolving supernovae (SNe) characterized by prominent [Ca II] emission at both photospheric and nebular phases, accompanied by notably weak oxygen features. Roughly 40 members are known, with an estimated rate of about two per decade within 25 Mpc, and their progenitors remain contested: observed properties straddle the boundary between core-collapse (CC) explosions of highly stripped massive stars and thermonuclear explosions of white dwarfs (WDs). The two previously nearest CaSTs, SNe 2019ehk and 2021gno, both showed X-ray detections and double-peaked light curves consistent with circumstellar material (CSM) interaction.

This paper presents coordinated X-ray, near-infrared (NIR), and radio observations of SN 2025coe, a CaST associated with NGC 3277 at $25 \pm 9.3$ Mpc and offset $\sim$40 kpc from its host center—the third-nearest CaST discovered to date. The dataset comprises Swift-XRT monitoring over 2–100 days post-explosion, three NIR spectra from Keck/NIRES and MMT/MMIRS spanning +10 to +45 days post-explosion (the most robust NIR spectral time series of any CaST), and radio non-detections with SMA, uGMRT, and VLA across 1–240 GHz out to 153 days. Optical properties are drawn from the companion analysis of Ravi et al., which establishes a double-peaked light curve, ejecta mass of 0.4–0.5 $M_\odot$, and $^{56}$Ni mass of $\sim$0.014 $M_\odot$.

X-ray detections and CSM constraints

SN 2025coe is only the third CaST detected at X-ray wavelengths, with significant detections ($S/N > 2.5$) at 2.98 and 8.17 days post-explosion, both at count rates of $\sim 2.4 \times 10^{-3}$ counts s$^{-1}$ and unabsorbed fluxes of $\sim 3 \times 10^{-13}$ erg cm$\sim$0 s$\sim$1, corresponding to luminosities of order $\sim$2 erg s$\sim$3. Spectral fitting yields a power-law photon index $\sim$4 and a large intrinsic absorbing column of $\sim$5 cm$\sim$6—far exceeding the Galactic column of $\sim$7 cm$\sim$8—implying substantial local absorption. Notably, the implied thermal temperature of $\sim$95 keV is softer than the $M_\odot$010 keV emission seen in SNe 2019ehk and 2021gno, which the authors attribute plausibly to reverse-shock emission, though the error bars preclude confirmation.

Converting the emission measure ($M_\odot$1 cm$M_\odot$2) into a CSM mass under assumptions of spherical geometry, solar abundances, full ionization, and a shock speed of $M_\odot$330,000 km s$M_\odot$4 yields $M_\odot$5 at a radius of $M_\odot$6$M_\odot$7 cm, extending to at least $M_\odot$8 cm based on the day-8 detection. These radii depend directly on the assumed shock speed; a slower 15,000 km s$M_\odot$9 shock would bring them into agreement with the blackbody radii from UV/optical SED fitting in the companion paper. If the CSM is helium-rich rather than solar-composition, the inferred mass roughly doubles. The implied mass-loss rate, if this material was lost within the final 100–200 days before explosion, exceeds $^{56}$0—three or more orders of magnitude above typical stripped-envelope supernova (SESN) values of $^{56}$1–$^{56}$2. The authors also note that the absence of narrow photoionized lines in the optical spectra suggests the X-ray-emitting CSM may be asymmetric or clumped; introducing a filling factor of 0.1–0.5 would raise clump densities by factors of 2–10.

The key structural result is radial confinement: after day 8, no further X-ray detections occur, and the non-detections from 20–92 days coincide temporally with the first radio non-detection, indicating the dense CSM terminates near $^{56}$3$^{56}$4 cm. The day-8 detection is also the latest X-ray detection ever for a CaST, implying CSM extending to larger radii than in either SN 2019ehk or 2021gno, while the total CSM mass appears roughly 20 times larger than those objects (albeit with large uncertainties and contingent on the constant-density assumption).

NIR spectroscopy: helium-rich ejecta

The three NIR spectra reveal strong He I P-Cygni profiles at 1.083 and 2.058 $^{56}$5m, identified definitively by the alignment of emission peaks with zero velocity. MCMC Gaussian fits show declining absorption velocities consistent with photospheric recession into homologously expanding ejecta. Two findings stand out:

Velocity comparison: He absorption velocities in SN 2025coe exceed those of SNe 2019ehk and 2021gno by a factor of $^{56}$61.5–2 and instead resemble those of the normal SN Ib 2008D. If explosion energies were similar, this could indicate lower ejecta mass, but the optical estimate of 0.4–0.5 $^{56}$7 is only marginally below the 0.6–0.7 $^{56}$8 inferred for the other two CaSTs—insufficient to explain the velocity difference. The discrepancy between the 1.083 $^{56}$9m and 2.058 $\sim$0m velocities is most likely attributable to blending of the 1 $\sim$1m feature with Mg II $\sim$21.0927 (and possibly C I $\sim$31.069), though formation at different optical depths cannot be excluded.

Template comparisons: Across 1–2.3 $\sim$4m, the spectra match He-rich SESN templates well at all epochs but bear little resemblance to normal SN Ia templates—lacking Fe/Co lines and the H-band break—and diverge substantially from the hybrid He/C/O + C/O WD merger model of Zenati et al., which predicts much weaker He features and over-predicts intermediate-mass-element and calcium line strengths. On this basis the authors conclude the NIR data favor a core-collapse interpretation over thermonuclear models. One ambiguous feature—a boxy $\sim$510,000 km s$\sim$6 profile in He I 2.058 $\sim$7m at +34 days—could indicate interaction with a dense CSM shell, coinciding with a claimed third light-curve peak reported by Chen et al. but not confirmed by Ravi et al.; the authors appropriately decline to draw firm conclusions here and note that NIR radiative transfer modeling is required.

Radio non-detections

All six radio observations from 20 to 153 days post-explosion yielded non-detections, with limits as deep as 27 $\sim$8Jy at 10 GHz. No CaST has ever been detected at radio wavelengths. Under standard Chevalier synchrotron self-absorption plus free-free absorption assumptions, the 240 GHz SMA limit at 20 days constrains $\sim$9 for a 1000 km s$M_\odot$0 wind, bounding the outer CSM radius at $M_\odot$1 cm. Combined with the X-ray results, the CSM extends to $M_\odot$2 cm—dense nearby material with an abrupt outer edge, a configuration not seen in any normal SESN. The authors note that low-density CSM below the radio sensitivity threshold ($M_\odot$3) cannot be fully ruled out as a source of the boxy NIR profile.

Progenitor implications

The synthesis favors a stripped-envelope CC explosion that overran a compact, very dense CSM shell, but the paper is explicit that neither scenario is settled. In the massive-star picture, the extreme mass-loss rate combined with the small CSM radius is difficult to reconcile with known SESN progenitors; such a progenitor might be expected to resemble a SN Ibn, yet SN 2025coe lacks the hallmark narrow He emission lines. The authors speculate that CaSTs may occupy a transitional regime of less intense, shorter-duration mass loss than Ibn events, with more intact helium layers.

In the thermonuclear picture, the CSM could arise naturally from pollution by a recurrent helium nova in an AM CVn-like system preceding a He/C/O + C/O WD detonation. However, the strong, high-velocity He P-Cygni absorption in the NIR—matching normal SESNe where helium belongs to the ejecta rather than the CSM—places a demanding constraint on such models. The paper advances a pointed argument: no thermonuclear SN has ever been conclusively detected at X-rays, so if these three X-ray-bright CaSTs are thermonuclear, they would be the first of any subclass; alternatively, X-ray brightness may itself discriminate, with genuinely thermonuclear CaSTs being intrinsically X-ray faint—as suggested by the deep non-detection of the CaST-Ia SN 2016hnk.

Limitations and open questions

Several caveats qualify the quantitative results. The CSM mass and radius rest on assumed spherical geometry, solar composition, full ionization, and a shock speed adopted from other interacting SESNe; each assumption carries factor-of-several uncertainty, and the density profile is unconstrained because only two early X-ray detections exist. The late-time X-ray non-detections have higher flux limits than the initial detections due to short exposures, so they do not strongly constrain late mass-loss. The boxy He I 2.058 $M_\odot$4m profile remains unexplained pending non-LTE NIR radiative transfer simulations including non-thermal excitation of helium. The host-galaxy association itself is uncertain, since fainter satellite candidates near the transient's location could be the true host. Open questions left by the work include whether the soft X-ray spectrum indeed traces reverse-shock emission, what mechanism produces the high ejecta velocities relative to the other two CaST-Ib/IIb events, and whether the claimed third photometric peak is real.

Conclusion

This study delivers the first NIR spectral time series of a CaST, the third X-ray detection of the class, and the deepest radio constraints to date on any member. The combined picture—an X-ray-luminous first week powered by interaction with $M_\odot$5 of CSM confined within $M_\odot$6 cm, followed by an abrupt cessation of interaction, alongside robust He-rich ejecta signatures matching SESN templates—is consistent with a core-collapse origin, though the extreme mass-loss rates, compact CSM extent, weak oxygen lines, and large host-galaxy offset remain difficult to accommodate in any single progenitor model. The convergence of X-ray detections in all three nearest CaSTs suggests early CSM interaction is common to the subclass and provides a practical trigger for rapid multiwavelength follow-up, while the NIR helium constraints now set a concrete benchmark that future thermonuclear merger and double-detonation models must reproduce.