---
title: 'SN 2025kg: Engine-Driven Ic-BL Supernova'
url: https://www.emergentmind.com/topics/sn-2025kg
type: topic
---

# SN 2025kg: Engine-Driven Ic-BL Supernova

Searching arXiv for recent papers on SN 2025kg / EP250108a to ground the article in current literature.
SN 2025kg is a luminous stripped–envelope core-collapse supernova classified as a broad-lined Type Ic supernova (SN Ic-BL) and associated with the fast X-ray transient EP250108a, discovered on 2025 January 8 by the Einstein Probe. It is one of the rare supernovae linked to an X-ray flash or fast X-ray transient, and it has become a reference event for the study of engine-driven stripped-envelope explosions with double-peaked optical light curves, early blue thermal emission, and possible dense circumstellar interaction [2507.20457]. Its observational interpretation is not unique: published analyses variously emphasize hydrodynamical magnetar powering, cocoon cooling from a weak or choked jet, and interaction with extended circumstellar material (CSM), but they converge on SN 2025kg as an unusually well-observed Ic-BL event at the interface between low-luminosity GRB-like explosions, X-ray flashes, and fast X-ray transients [2504.17516].

## 1. Discovery, identification, and classification

EP250108a was detected by the Einstein Probe as a fast X-ray transient or X-ray flash on **2025 Jan 8, 12:30:28.34 UT**, and the detection time is commonly taken as the explosion epoch in subsequent modeling [2507.20457]. Follow-up optical observations revealed the transient later designated SN 2025kg. Spectroscopy established a broad-lined Type Ic classification, with hydrogen- and helium-free optical spectra in the canonical classification sense and broad absorption lines indicating high expansion velocities [2507.20457].

A secure redshift of \(z = 0.17641 \pm 0.0003\) was measured from host-galaxy emission lines, with a luminosity distance \(D_L = 880.6\ {\rm Mpc}\) or \(D_L \simeq 881\ \mathrm{Mpc}\) under the cosmological assumptions adopted in the discovery analyses [2504.08886]. At this redshift SN 2025kg became the closest known supernova discovered following an Einstein Probe fast X-ray transient [2504.08889].

The event occupies a specific phenomenological niche. It is not a classical long GRB, because no prompt gamma-ray counterpart was detected, yet its X-ray and optical properties place it close to low-luminosity GRB and X-ray-flash supernovae such as SN 2006aj/XRF 060218 [2504.17516]. This has made SN 2025kg central to the emerging view that some Ic-BL explosions host weak, low-efficiency, or choked jets whose prompt high-energy signatures appear primarily in soft X-rays rather than in gamma rays [2504.08886].

## 2. Photometric evolution and multi-wavelength behavior

The defining photometric property of SN 2025kg is its **double-peaked light curve**. The first peak occurs during the first \(\lesssim 6\) days and is described as an early cooling or interaction bump; the second is the main supernova peak around \(\sim 10\)–15 rest-frame days [2507.20457]. Around the main peak, different bolometric reconstructions agree well, whereas at very early times they diverge, likely because of differing treatments of UV flux and bolometric corrections [2507.20457].

The first peak was very blue and very luminous. One analysis quotes \(M_g \approx -19.5\) mag and \(M_r \approx -19.1\) mag for the initial bump, with \(g-r \approx -0.4\) mag, followed by reddening as the first peak faded [2504.17516]. The main radioactive peak reached \(M_r = -19.39 \pm 0.02\) and \(M_g = -18.95 \pm 0.06\), brighter than the mean Ic-BL population but still within the observed range [2504.17516]. Another study reports a bolometric peak luminosity \(L_{\rm peak}\approx 2\times10^{43}\,\mathrm{erg\,s^{-1}}\) at \(\sim 14.5\) days after explosion in the rest frame [2507.18544].

Early UV–optical spectral energy distributions are well described by a rapidly expanding cooling blackbody. Representative fits give \(T_{\rm bb} \approx 2.05^{+1.05}_{-0.55}\times10^4\) K and \(R_{\rm bb} \approx 0.79^{+0.35}_{-0.25}\times10^{15}\) cm at 0.97 days, with \(\log L_{\rm bb,bol} \approx 43.90^{+0.59}_{-0.22}\), while by 3.31 days the temperature had cooled to \(1.22\times10^4\) K and the radius had expanded to \(1.23\times10^{15}\) cm [2504.08886]. A time-dependent fit implies an average expansion speed over the first 0.5 days of \(\bar{v}\sim 0.45c\), suggesting mildly relativistic ejecta during the earliest phase [2504.08886].

The associated X-ray transient was soft and long-lived compared with classical GRBs. Reported values include a duration of \(\sim 2200\) s or \(>2500\) s, peak 0.5–4 keV luminosity of order \(10^{46}\ {\rm erg\ s^{-1}}\), and total radiated X-ray energy of order \(10^{49}\ {\rm erg}\) [2504.08886]. Radio follow-up yielded only upper limits, including MeerKAT non-detections at 3.06 GHz and VLA limits at 10 GHz, excluding standard bright on-axis GRB-like afterglows but remaining consistent with low-energy jets, choked jets, or low-density environments [2504.17516].

This combination—soft X-ray prompt emission, weak or absent gamma rays, blue early optical cooling, and a GRB-SN-like main peak—has motivated comparisons to both XRF-associated supernovae and engine-driven Ic-BL events more generally [2504.08889].

## 3. Spectroscopy, velocities, helium, and hydrogen signatures

Optical spectroscopy shows the transition from a blue continuum-dominated transient to a typical Ic-BL spectral sequence [2504.17516]. Broad absorption features attributed to Fe II and Si II appear prominently, and template matching links SN 2025kg closely to SN 2002ap, SN 2006aj, and SN 1998bw at different epochs [2504.08889]. The Fe II \(\lambda5169\) velocity evolution, commonly used as a photospheric proxy in stripped-envelope supernovae, declines from \(32440 \pm 3388\ \mathrm{km\ s^{-1}}\) at \(\sim 4.6\) days to \(10816 \pm 2373\ \mathrm{km\ s^{-1}}\) at \(\sim 25.6\) days [2504.17516].

Near maximum light, JWST/NIRSpec prism spectroscopy from 0.5 to 5 \(\mu\mathrm{m}\) revealed two unusual features for an Ic-BL event: weak He I absorption near \(1.0830\,\mu{\rm m}\) and \(2.0581\,\mu{\rm m}\), and a broad unidentified emission structure around 4–4.5 \(\mu\mathrm{m}\) [2504.08889]. Multi-component fitting disfavors interpreting the \(2\,\mu\mathrm{m}\) absorption as only C I and Mg II, because that would imply velocities \(24{,}000–41{,}000\) km s\(^{-1}\), substantially larger than the optical Fe/Si velocities; a model including He I yields more consistent velocities of \(20{,}000–22{,}000\) km s\(^{-1}\) [2504.08889]. The reported conclusion is that He I is almost certainly present, though likely at low to moderate strength, with a conservative estimate \(M_{\rm He} \lesssim 0.5\,M_\odot\) [2504.08889].

The 4–4.5 \(\mu\mathrm{m}\) emission remains unidentified. Thermal dust or r-process-powered emission is rejected because a blackbody explanation would require \(R \sim 2\times10^{16}\,{\rm cm}\) and \(v \sim 0.9c\) by \(\sim 15\) rest-frame days, which is inconsistent with the measured ejecta kinematics [2504.08889]. A plausible suggestion in the published analysis is that it may arise from an Fe-group nebular blend, but no definitive identification is given [2504.08889].

A further distinctive feature is transient broadened H\(\alpha\) at \(\sim 42.5\) days. A broad H\(\alpha\)-like component was detected in one epoch but not in neighboring spectra [2507.18544]. Its line width implies \(v_{\rm H\alpha}\sim 3540\,\mathrm{km\,s^{-1}}\), comparable to the Si II velocity at the same phase, which argues that the hydrogen resides in inner, low-velocity ejecta rather than in a distant high-velocity interaction zone [2507.18544]. Other analyses interpret the same phenomenon as evidence for interaction with hydrogen-rich material at \(\sim 10^{16}\) cm, perhaps in a thin shell or clumps [2504.08889]. These interpretations are not mutually exclusive, but they locate the hydrogen in a geometrically or temporally localized component rather than a canonical extended hydrogen envelope.

## 4. Models for the early peak: cooling emission, cocoon emission, and circumstellar interaction

The early optical bump has been modeled in several ways. One line of work describes the first \(\sim 6\) days as a rapidly expanding cooling blackbody and argues that the observed X-ray and radio properties are consistent with a collapsar-powered jet that is low energy \((\lesssim10^{51}\ {\rm erg})\) and/or fails to break out of dense surrounding material [2504.08886]. In that interpretation, the early optical emission is favored to arise from a **shocked cocoon** rather than from ordinary supernova ejecta alone [2504.08886].

Analytic cocoon fits to the early light curve yield a cocoon mass \(M_{\rm cocoon} \approx 0.04–0.15\, M_\odot\), cocoon kinetic energy \(E_{\rm cocoon} \sim 4–45\times10^{50}\ {\rm erg}\), opening angle \(\theta_{\rm cocoon} \sim 20–25^\circ\), and shock breakout radius \(R_{\rm sh} \sim 1.9–3.5\, R_\odot\) [2504.08886]. These values are consistent with shock breakout from a compact Wolf–Rayet-like progenitor and with the requirement that only \(10^{-4}–10^{-3}\,M_\odot\) of material at \(\beta>0.1\) is needed to reproduce the prompt X-ray light curve in jet-driven models [2504.08886].

A second line of work jointly fits the optical light curve with radioactive powering plus an early shock-cooling or cocoon component and compares three scenarios: SN ejecta interacting with extended CSM, a cocoon from a jet choked in the stellar envelope, and a cocoon from a jet choked in extended CSM [2504.17516]. In the **extended CSM interaction** model, the inferred envelope parameters are \(M_e = 0.11^{+0.02}_{-0.03}\,M_\odot\), \(R_e = 5.37^{+2.75}_{-2.41}\times 10^{13}\ \mathrm{cm}\), and \(E_e = 3.63^{+3.78}_{-2.22}\times 10^{51}\ \mathrm{erg}\) [2504.17516]. In the **jet-choked-in-CSM** model, the corresponding values are \(M_{\rm CSM} = 0.07^{+0.06}_{-0.04}\,M_\odot\), \(R_{\rm CSM} = 3.71^{+8.58}_{-2.13}\times 10^{13}\ \mathrm{cm}\), and \(E_{\rm eng} = 5.75^{+20.00}_{-4.96}\times 10^{51}\ \mathrm{erg}\) [2504.17516].

Both of those CSM-based scenarios naturally reproduce a soft X-ray shock breakout with \(L_{\rm SBO}\sim 10^{46}\ \mathrm{erg\ s^{-1}}\), duration \(\sim 10^3\) s, and characteristic observed temperature near 1.2 keV, close to the observed EP250108a properties [2504.17516]. By contrast, a cocoon confined to the stellar envelope in the Nakar–Piran formalism does not by itself naturally yield such a soft, long-lived prompt X-ray signal [2504.17516].

Hydrodynamical modeling offers a related but numerically distinct picture. In a preferred “Mag+CSM” solution, the early cooling phase is reproduced by a dense wind-like CSM with \(M_{\rm CSM} = 0.27~M_\odot\), \(R_{\rm CSM} = 500~R_\odot\), and \(\rho(r)\propto r^{-2}\), attached to a compact progenitor of radius \(R_\star \lesssim 5~R_\odot\) [2507.20457]. This larger inferred CSM radius reflects differences in modeling assumptions and parameterization rather than a settled consensus.

These variations underscore a central point: the early bump is widely attributed to **shock-heated extended material**, but whether that material is best described as a cocoon, a dense CSM shell, a wind-like CSM, or some hybrid remains unresolved.

## 5. Power source of the main peak: radioactive nickel versus magnetar injection

The main peak of SN 2025kg poses a luminosity problem for purely radioactive models. In 1D radiation–hydrodynamical calculations, a Ni-only model reproduces the main peak and intermediate-time photospheric velocities with \(E = 2.2~\mathrm{foe}\), \(M_{\rm ej} = 1.9~M_\odot\), and \(M_{\rm Ni} = 0.85~M_\odot\) [2507.20457]. The implied ratio \(M_{\rm Ni}/M_{\rm ej} \approx 0.45\) is explicitly identified as problematic, because it requires nearly half of the ejecta to be radioactive nickel [2507.20457].

A related semi-analytic treatment reaches a similar conclusion: matching \(L_{\rm peak}\approx 2\times10^{43}\,\mathrm{erg\,s^{-1}}\) and \(M_{\rm ej}\sim 2\,M_\odot\) with radioactivity alone implies \(M_{\rm Ni}\sim 0.8\,M_\odot\) and \(f_{\rm Ni}\sim 0.4\), above the typical range for SNe Ic-BL and even above the 0.025–0.3 range quoted from recent collapsar simulations [2507.18544]. This is the main reason published studies have explored central-engine power.

The preferred hydrodynamical alternative is a **magnetar + CSM** model with \(E \approx 2.4~\mathrm{foe}\), \(M_{\rm ej} = 3.4~M_\odot\), \(M_{\rm Ni} = 0.2~M_\odot\), magnetar spin period \(P = 2.9~\mathrm{ms}\), and dipole magnetic field \(B = 2.8\times 10^{15}~\mathrm{G}\) [2507.20457]. This model reproduces the main bolometric peak, early decline, and Fe II velocity evolution while requiring a much more ordinary nickel mass [2507.20457]. The close similarity of these parameters to those inferred for the GRB-SN 2023pel is explicitly cited as support for a magnetar scenario [2507.20457].

A separate magnetar-based study, combining magnetar injection, radioactive decay, and outer-cocoon cooling, derives tighter semi-analytic parameters from MCMC fitting: \(P_{\rm i}=1.66^{+0.03}_{-0.02}\,\mathrm{ms}\), \(B_{\rm p}=2.01^{+0.01}_{-0.01}\times10^{15}\,\mathrm{G}\), \(M_{\rm ej}=2.53^{+0.04}_{-0.04}\,M_\odot\), \(v_{\rm oc,min}=0.26^{+0.01}_{-0.01}c\), and \(r_\star=4.95^{+0.38}_{-0.29}\,R_\odot\) [2507.18544]. In that framework the magnetar initial rotational energy is \(E_{\rm rot,i}\sim7\times10^{51}\left(P_{\rm i}/1.7\mathrm{\,ms}\right)^{-2}\mathrm{erg}\), and \(\gtrsim98\%\) of that energy is converted into ejecta kinetic energy, yielding a total kinetic energy \(\sim 8\times10^{51}\,\mathrm{erg}\) [2507.18544].

By contrast, purely radioactive fits in the observational papers give more modest and model-dependent results. An Arnett-like fit to the second peak yields \(M_{\rm Ni} = 0.57^{+0.60}_{-0.30}\,M_\odot\), \(M_{\rm ej} = 1.66^{+0.79}_{-0.49}\,M_\odot\), and \(E_{\rm K} = 2.91^{+1.36}_{-0.86}\times 10^{51}\ \mathrm{erg}\) [2504.17516], while another study reports a conservative nickel mass range \(M_{\rm Ni} \approx 0.2 - 0.6\,M_\odot\) depending on whether one uses a one-zone Arnett model or a multi-zone Ni-mixing model [2504.08889].

The technical disagreement is therefore not about whether radioactivity contributes—it certainly does—but about whether it can plausibly dominate the peak. Hydrodynamical and engine-based studies argue that the required nickel fraction is too high, favoring magnetar input; more phenomenological light-curve fits can reproduce the data with radioactive models, but only within broad parameter uncertainties.

## 6. Progenitor system, central engine, and relation to other events

SN 2025kg has been linked to several progenitor scenarios, all involving a stripped massive star. Hydrodynamical models use compact H-free CO-core progenitors with \(R_\star \lesssim 5~R_\odot\), favoring a progenitor with \(M_{\rm ZAMS}=18~M_\odot\) in the preferred magnetar+CSM solution [2507.20457]. Observational and semi-analytic studies instead emphasize a low-mass helium star with an extended helium envelope and pre-supernova mass \(\sim 4\,M_\odot\), consistent with the He I detections and the inferred radius \(r_\star\sim 5\,R_\odot\) [2507.18544].

The binary-evolution interpretation is especially explicit in one Letter, which argues that near-solar host metallicity disfavors quasi-chemically homogeneous evolution and instead points to a close helium-star + main-sequence binary formed through isolated binary evolution [2507.18544]. In that picture, tidal torques in an orbit with \(P_{\rm orb}\lesssim 2\) days can spin up the helium star sufficiently to form a millisecond magnetar, while a main-sequence companion can also supply hydrogen-rich material that later produces broad H\(\alpha\) [2507.18544]. A plausible implication is that SN 2025kg probes not only central-engine physics but also the role of binary angular-momentum transfer in metal-rich collapsars.

The event is repeatedly compared with SN 2006aj, SN 2020bvc, SN 2023pel, and EP240414a/SN 2024gsa. Hydrodynamical comparison finds that SN 2025kg and SN 2023pel have strikingly similar bolometric light curves and Fe II velocities, with almost identical best-fit parameters in a magnetar scenario [2507.20457]. By contrast, SN 2006aj and SN 2020bvc can be modeled with Ni+CSM solutions alone, whereas SN 2025kg is too luminous for its velocities and ejecta mass if powered only by Ni [2507.20457].

SN 2025kg also serves as a comparison standard for later XRF-SN discoveries. In the study of XRF 241001A/SN 2024aiiq, SN 2025kg is treated as a benchmark NIR spectral analogue: the JWST spectrum of SN 2024aiiq shows excellent agreement with SN 2025kg in continuum shape and broad features, especially from 1.5 to 5 \(\mu\mathrm{m}\), and both objects display a broad \(1\,\mu\mathrm{m}\) feature and a shallower \(2\,\mu\mathrm{m}\) absorption that may indicate a small amount of helium in the ejecta [2604.20346]. This suggests that “SN 2025kg-like” events may define a subclass of soft X-ray transient–associated Ic-BL supernovae with GRB-SN-like optical properties but unusually informative NIR signatures.

Population arguments remain provisional, but early Einstein Probe discoveries imply that FXT-SNe may be more common than successful GRB jets. One observational paper states that the sample of EP FXT SNe supports rate estimates that low-luminosity jets seen through FXTs are more common than successful GRB jets and that similar FXT-like signatures are likely present in at least a few percent of the brightest Ic-BL SNe [2504.08889]. This suggests that SN 2025kg may be representative not of an isolated peculiarity but of a broader, previously under-sampled channel of engine-driven stellar collapse.

## 7. Open questions and interpretive tensions

Several aspects of SN 2025kg remain unsettled. The first concerns the **main power source** of the supernova peak. Hydrodynamical and engine-based models regard a Ni-only solution as formally possible but physically suspect because of the extreme required nickel fraction [2507.20457], whereas more empirical light-curve models recover radioactive parameters within the broad Ic-BL/GRB-SN range [2504.17516]. The absence of very late-time photometry and nebular spectroscopy is repeatedly identified as limiting tighter constraints on \(M_{\rm Ni}\) [2507.18544].

The second concerns the **geometry and composition of the extended material**. Some models favor a wind-like CSM with \(0.27\,M_\odot\) extending to \(500\,R_\odot\) [2507.20457]; others infer \(\sim 0.1\,M_\odot\) at radii of a few \(\times 10^{13}\) cm [2504.17516]; cocoon fits prefer much smaller shocked masses but still invoke angularly confined mildly relativistic ejecta [2504.08886]. These discrepancies reflect different assumptions about whether the early optical peak is dominated by shocked CSM, a cocoon, or both.

A third tension lies in the **classification versus composition**. SN 2025kg is spectroscopically Ic-BL, yet JWST spectroscopy provides evidence for weak He I and late-time H\(\alpha\) [2504.08889]. This does not invalidate the Ic-BL classification, which depends primarily on the absence of strong optical H and He in the classical sequence, but it does suggest a more compositionally complex progenitor than the label alone implies.

Finally, the **nature of EP250108a** remains debated. One model interprets it as off-axis cooling emission from an inner cocoon viewed at \(\sim45^\circ\) [2507.18544], whereas other studies prefer soft X-ray shock breakout in extended CSM [2504.17516] or a trapped or low-energy jet whose shocked cocoon powers both the X-rays and the early blue optical light [2504.08886]. All of these frameworks are engine-driven; the controversy is about breakout conditions, viewing geometry, and the radial structure of the surrounding material rather than about whether a central engine was present.

Taken together, the literature establishes SN 2025kg as a pivotal event for understanding broad-lined Type Ic supernovae associated with soft high-energy transients. It combines GRB-SN-like optical luminosity and kinematics with an X-ray-flash-like prompt counterpart, unusually rich infrared spectroscopy, and clear evidence that the outer stellar environment played an essential role in shaping the observed transient [2504.08889].

Source: https://www.emergentmind.com/topics/sn-2025kg