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

TL;DR

SN 2025coe, a nearby Ca-Strong Transient, was observed across X-ray, NIR, and radio wavelengths to probe its progenitor and circumstellar environment. X-ray emission from ejecta-CSM interaction indicates a dense, nearby CSM extending to about $2\times 10^{15}$ cm and implies a substantial recent mass loss, while radio non-detections constrain CSM beyond $\sim 4\times 10^{15}$ cm. The robust NIR spectral time series reveals strong He I lines and velocities akin to He-rich SESNe, supporting a core-collapse origin, though the extreme CSM properties pose questions about the progenitor system. Taken together, SN 2025coe points to a CCSN in a system with unusually dense, radially confined CSM, highlighting the importance of multiwavelength follow-up for CaSTs and guiding future modeling of their progenitors.

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 $\sim 4\times 10^{15}$ cm. This inferred nearby high-density CSM extending out to $3\pm 1 \times10^{15}$ 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.

Figures (10)

• Figure 1: The X-ray detections of SN 2025coe in context with the other two X-ray detected calcium-rich transients SN 2019ehk and 2021gno JacobsonGalan2020_19ehkJacobsonGalan2022. We also show the non-detection of the CaST-Ia SN 2016hnk Bell_2018WJG_2020 and the X-ray evolution of the SN Ib 2008D. Vertical dashed lines denote epochs of radio non-detections for SN 2025coe.
• Figure 2: The absorbed power-law fit to the combined X-ray spectrum of SN 2025coe. Data are combined from epochs at 2.98 and 8.17 days post-explosion.
• Figure 3: All NIR spectra of 2025coe compared to other Ca-strong transients. To date, 2025coe has the most robust NIR spectral times series of any CaST. All phases are listed with respect to optical maximum of the secondary, nickel-powered light curve peak. At earlier times, SN 2025coe does not show many similarities to Ca-rich transients Ca-Ic 2022oqm Yadavalli2024 or Ca-Ia 2016hnk Galbany2019, but later evolves to resemble Ca-strong transients Ca-IIb 2021gno JacobsonGalan2022Crawford2025Ertini2023 and Ca-Ib 2019ehk JacobsonGalan2020_19ehk. SN 2022joj is an SN Ia that has been identified as a potential helium shell detonation candidate PadillaGonzalez2024, and is included here for comparison.
• Figure 4: A view of the upper limits on radio luminosity for 3 CaSTs: SN 2019ehk, 2021gno and 2025coe. SNe 2019ehk and 2021gno data are from JacobsonGalan2020_19ehkJacobsonGalan2022. To date, no CaST has ever been detected at radio wavelengths. VLA data are S/C/X band (3-10 GHz), GMRT are 1.25 GHz and SMA data are 240 GHz. The dashed line denotes the last X-ray detection of SN 2025coe. We show the typical radio luminosity space of type Ib and IIb supernovae as well. The radio luminosity constraints are quite deep, and an order of magnitude lower than the typical luminosity of a detected SESNe. These non-detections clearly suggest that SN 2025coe was not interacting with dense CSM at late times, similar to SN 2019ehk/2021gno JacobsonGalan2020_19ehkJacobsonGalan2022.
• Figure 5: A comparison between SN 2025coe optical and NIR He line profiles at the 3 epochs at which NIR spectra were taken, with all phases indicated with respect to r-band peak. At earlier phases, the optical and NIR He features exhibit similar P-Cygni profiles. As SN 2025coe evolves over time, the NIR He line profiles begin to take on a more boxy shape that is most evident in the 2 $\mu$m He feature at +34 days.