SN 2024ggi: Nearby Type II-P Supernova

• SN 2024ggi is a nearby hydrogen-rich core-collapse Type II-P supernova distinguished by its extremely early detection and direct progenitor identification.
• Early flash spectroscopy and multiwavelength follow-up reveal a dense, compact circumstellar material consistent with enhanced mass loss in the final years before explosion.
• Nebular-phase spectra, along with non-detections in millimeter, gamma-ray, and neutrino bands, provide key constraints on explosion energy, ejecta mixing, and nucleosynthesis.

SN 2024ggi is a nearby hydrogen-rich core-collapse supernova in NGC 3621, generally classified as a Type II-P event and distinguished by extremely early discovery, direct progenitor identification, dense and compact circumstellar material revealed by flash spectroscopy, and unusually broad multiwavelength follow-up. Published studies place it at about $6.64$–$7.24$ Mpc, and several works explicitly describe it as one of the closest core-collapse supernovae of the decade or the last decade. Its observational record spans pre-explosion HST and Spitzer imaging, hour-cadence early photometry and spectroscopy, plateau-phase optical-to-mid-infrared spectroscopy, late nebular optical, near-infrared, and JWST data, and non-detections or limits at millimeter, gamma-ray, and neutrino energies (Chen et al., 2024, Xiang et al., 2024).

1. Discovery, classification, and observational setting

SN 2024ggi was discovered by ATLAS on 2024 April 11, with papers quoting discovery times such as 2024-04-11.14 UT, MJD 60411.14, or JD 2460411.64, and explosion or first-light estimates clustered around MJD $60410.8$–$60410.90$. Several analyses therefore place the discovery only a few hours after explosion: $5.8^{+1.9}_{-2.9}$ hours in one early photometric study, about $6$ hours in the high-resolution spectroscopic analysis, and within hours in later summary papers (Chen et al., 2024, Pessi et al., 2024).

The event was rapidly identified as a young Type II supernova with flash features. Early spectra showed narrow, high-ionization emission lines from H, He, C, N, and O, while later evolution developed the broad Balmer P-Cygni profiles and plateau morphology characteristic of a Type II-P supernova. Early bolometric analyses described an initial luminous peak of order $10^{43}\,\mathrm{erg\,s^{-1}}$, followed by a plateau of order $10^{42}\,\mathrm{erg\,s^{-1}}$ lasting roughly $120$ days, and then a radioactive tail interpreted through $^{56}\mathrm{Co}\rightarrow{}^{56}\mathrm{Fe}$ decay (Zhang et al., 2024, Buccheri et al., 20 Mar 2026).

The early light curve rose unusually quickly. One study reported a $7.24$0 magnitude rise in $7.24$1 hours in the combined $7.24$2- and $7.24$3-band light curves, while another found rapid $7.24$4, $7.24$5, and $7.24$6-band rises of about $7.24$7 mag day$7.24$8 and a $7.24$9-band peak near $60410.8$0 mag in about $60410.8$1 days. The rapid blueward color evolution and the absence of a short optical spike expected from shock breakout at a bare stellar surface were interpreted as evidence for shock breakout inside dense circumstellar material rather than at the naked photosphere of the progenitor (Chen et al., 2024, Chen et al., 2024).

2. Progenitor identification and mass estimates

Pre-explosion HST and Spitzer imaging identified the progenitor as a red, bright variable star consistent with a red supergiant. Direct spectral-energy-distribution fitting using stellar atmosphere and dust radiative-transfer models yielded $60410.8$2, $60410.8$3, and $60410.8$4, implying an initial mass of $60410.8$5. The same study found that the progenitor brightened in F814W from absolute magnitude $60410.8$6 mag in 1995 to $60410.8$7 mag in 2003, and that the historical mid-infrared light curves imply a pulsational period of about $60410.8$8 days (Xiang et al., 2024).

That direct-imaging analysis also inferred a relatively low long-term mass-loss rate, $60410.8$9, compared with the much higher rates implied by flash spectroscopy and X-ray constraints. A plausible implication is that the progenitor underwent a substantial enhancement in mass loss only in the last few years before explosion, rather than sustaining an extreme wind over the longer interval probed by the pre-explosion infrared data (Xiang et al., 2024).

A separate environmental analysis used HST imaging of stars within $60410.90$0 pc of the SN site. That region contains $60410.90$1 stars after quality cuts and excluding the progenitor candidate, and is described as uniform and non-clumpy. Hierarchical Bayesian star-formation-history fitting recovered three age components, of which the youngest, $60410.90$2 at $60410.90$3, or $60410.90$4 Myr, is the only one young enough to host a core-collapse progenitor; its corresponding initial mass is $60410.90$5 (Hong et al., 2024).

Other methods give higher values. One hydrodynamical study designed to remain consistent with pre-explosion detections favored $60410.90$6, while another hydrodynamical analysis with full plateau coverage preferred $60410.90$7. Nebular [O I]/continuum modeling yielded $60410.90$8, with a likely range of $60410.90$9–$5.8^{+1.9}_{-2.9}$0. This spread suggests that SN 2024ggi has become a methodological test case in which direct detection, environmental dating, hydrodynamical fits, and nebular diagnostics do not yet define a single progenitor mass, and the discrepancy has been explicitly linked to circumstellar extinction, pulsational variability, and model dependence in the age–mass conversion or line-diagnostic framework (Aryan et al., 14 Aug 2025, Ertini et al., 3 Mar 2025, Hueichapán et al., 4 Aug 2025).

3. Shock breakout and circumstellar medium

The earliest spectra captured a rapid transformation of the flash-ionized circumstellar environment. High-resolution MIKE spectra at $5.8^{+1.9}_{-2.9}$1 and $5.8^{+1.9}_{-2.9}$2 hours after first light resolved ions of H I, He I, He II, N III, C III, Si IV, N IV, and C IV. He I lines were detected only in the first spectrum, while He II and higher-ionization species strengthened later, indicating rapid ionization changes. The narrow Gaussian components had average blueshifts of $5.8^{+1.9}_{-2.9}$3 and $5.8^{+1.9}_{-2.9}$4$5.8^{+1.9}_{-2.9}$5 at the two epochs, whereas the broad Lorentzian components averaged $5.8^{+1.9}_{-2.9}$6 and $5.8^{+1.9}_{-2.9}$7$5.8^{+1.9}_{-2.9}$8, implying an increase of about $5.8^{+1.9}_{-2.9}$9$6$0 for the narrow components and about %%%%5$7.24$5%%%%2 for the broad components over only $6$3 hours. That behavior was interpreted as radiative acceleration of dense CSM (Pessi et al., 2024).

A denser hour-to-day spectroscopic series followed the same flash phase across the first $6$4 hours. It showed an initial low-ionization spectrum with He I, C III, and N III, followed by a rapid rise in ionization to He II, N IV, C IV, and O IV–O V by about $6$5 hours. The duration of the IIn-like spectra was used to infer a dense and relatively confined CSM extending to $6$6 cm, and comparison with the Dessart et al. model grid required a mass-loss rate exceeding $6$7 to reproduce the low-ionization emission (Zhang et al., 2024).

The velocity of the unshocked outer CSM is itself model dependent but consistently slow. One study measured $6$8 from high-resolution spectra, while the H$6$9 line-shift analysis placed the velocity of the unshocked outer CSM between $10^{43}\,\mathrm{erg\,s^{-1}}$0 and $10^{43}\,\mathrm{erg\,s^{-1}}$1, explicitly matching a typical red-supergiant wind (Soker, 2024, Zhang et al., 2024).

Published CSM parameterizations differ substantially. Early light-curve modeling favored a confined CSM radius $10^{43}\,\mathrm{erg\,s^{-1}}$2 cm, a mass-loss rate $10^{43}\,\mathrm{erg\,s^{-1}}$3 for an assumed $10^{43}\,\mathrm{erg\,s^{-1}}$4 wind, and a corresponding CSM mass of $10^{43}\,\mathrm{erg\,s^{-1}}$5. A hydrodynamical study that fits the full plateau instead preferred a two-component CSM with $10^{43}\,\mathrm{erg\,s^{-1}}$6, $10^{43}\,\mathrm{erg\,s^{-1}}$7, and $10^{43}\,\mathrm{erg\,s^{-1}}$8, while a MESA+STELLA analysis found either a steady-wind CSM with $10^{43}\,\mathrm{erg\,s^{-1}}$9 out to $10^{42}\,\mathrm{erg\,s^{-1}}$0 cm or an accelerated-wind CSM with $10^{42}\,\mathrm{erg\,s^{-1}}$1 out to $10^{42}\,\mathrm{erg\,s^{-1}}$2 cm. By contrast, a semi-analytic electromagnetic-plus-neutrino model inferred $10^{42}\,\mathrm{erg\,s^{-1}}$3, $10^{42}\,\mathrm{erg\,s^{-1}}$4 cm, and a density slope $10^{42}\,\mathrm{erg\,s^{-1}}$5. This range suggests that the inferred CSM structure depends strongly on whether the modeling emphasizes flash spectroscopy, the very early optical rise, the plateau, or the later bolometric evolution (Chen et al., 2024, Ertini et al., 3 Mar 2025, Aryan et al., 14 Aug 2025, Buccheri et al., 20 Mar 2026).

One theoretical interpretation argued that the dense compact CSM is better explained by an “effervescent zone” produced by convection and pulsation plus a regular RSG wind, rather than by a purely radiative extended wind-acceleration zone, and further suggested that the explosion energy may favor the jittering jets explosion mechanism over the delayed neutrino mechanism. In the literature on SN 2024ggi, this remains an interpretive proposal rather than a settled description (Soker, 2024).

For a steady wind, the usual parameterization is

$10^{42}\,\mathrm{erg\,s^{-1}}$6

and several SN 2024ggi studies use either this form directly or an accelerated-wind generalization to describe the dense inner CSM (Aryan et al., 14 Aug 2025).

4. Plateau-phase evolution and explosion parameters

The optical and bolometric light curves place SN 2024ggi within the Type II-P class but with unusually well-sampled early behavior. One full-plateau hydrodynamical study derived $10^{42}\,\mathrm{erg\,s^{-1}}$7 mag, $10^{42}\,\mathrm{erg\,s^{-1}}$8 mag per $10^{42}\,\mathrm{erg\,s^{-1}}$9 d, $120$0 mag per $120$1 d, cooling duration $120$2 d, plateau duration $120$3 d, and optically thick duration $120$4 d. In that analysis, the preferred explosion model had $120$5, pre-SN mass $120$6, pre-SN radius $120$7, explosion energy $120$8 erg, and $120$9Ni mass below $^{56}\mathrm{Co}\rightarrow{}^{56}\mathrm{Fe}$0 (Ertini et al., 3 Mar 2025).

A different hydrodynamical program tied to the directly detected progenitor instead favored a lower-mass solution. Its preferred $^{56}\mathrm{Co}\rightarrow{}^{56}\mathrm{Fe}$1 progenitor model lies within the HST progenitor error box and implies $^{56}\mathrm{Co}\rightarrow{}^{56}\mathrm{Fe}$2, $^{56}\mathrm{Co}\rightarrow{}^{56}\mathrm{Fe}$3, explosion energy $^{56}\mathrm{Co}\rightarrow{}^{56}\mathrm{Fe}$4 erg, and $^{56}\mathrm{Co}\rightarrow{}^{56}\mathrm{Fe}$5, with the broader allowed ranges $^{56}\mathrm{Co}\rightarrow{}^{56}\mathrm{Fe}$6 and $^{56}\mathrm{Co}\rightarrow{}^{56}\mathrm{Fe}$7 once $^{56}\mathrm{Co}\rightarrow{}^{56}\mathrm{Fe}$8 is admitted (Aryan et al., 14 Aug 2025).

A semi-analytic electromagnetic model linked to neutrino predictions returned yet another parameter set: $^{56}\mathrm{Co}\rightarrow{}^{56}\mathrm{Fe}$9 foe, $7.24$00, $7.24$01 cm, $7.24$02, $7.24$03 cm, and $7.24$04. This model interprets the early optical peak as shock breakout plus CSM interaction, the plateau as recombination of the hydrogen-rich envelope, and the late decline as a radioactive tail (Buccheri et al., 20 Mar 2026).

These differences are large enough that the explosion properties of SN 2024ggi remain model dependent. Published energies range from about $7.24$05 erg to $7.24$06 erg, and published $7.24$07Ni masses range from below $7.24$08 to about $7.24$09. One theoretical paper explicitly argued that energies of $7.24$10 erg may be difficult for the delayed neutrino mechanism and might instead favor jittering jets; other hydrodynamical studies described the event as compatible with more standard Type II-P explosion parameters (Soker, 2024).

At $7.24$11 days, a panchromatic $7.24$12 data set from JWST and ground-based spectroscopy found a plateau-phase spectrum dominated by H I emission. A toy PHOENIX/1D model reproduced the continuum especially well redward of $7.24$13, and no evidence for a mid-infrared excess or dust was found at these epochs; the observed flux was matched out to $7.24$14 without invoking an additional dust component (Baron et al., 24 Jul 2025).

5. Nebular ejecta, nucleosynthesis, molecules, and geometry

Nebular spectroscopy extended the progenitor-mass debate into the transparent phase. Optical and near-infrared data between $7.24$15 and $7.24$16 days were used to compare the [O I] $7.24$17 doublet with model grids, yielding $7.24$18 and a likely range of $7.24$19. An independent nebular optical study at $7.24$20 days likewise found $7.24$21 from [O I] flux, [O I]/[Ca II], and model comparisons, and derived a synthesized $7.24$22Ni mass of $7.24$23 from the late bolometric light curve (Hueichapán et al., 4 Aug 2025, Ferrari et al., 30 Jul 2025).

The line profiles indicate a stratified but nontrivial geometry. The broad H$7.24$24 line has FWHM $7.24$25 and is close to a symmetric Gaussian centered at zero velocity, while the [O I] doublet is blue-shifted by about $7.24$26. In the near-infrared, Mg I and [Fe II] lines show persistent double-peaked emission with characteristic blue and red peaks of order $7.24$27, and this pattern is not seen in the hydrogen lines. The nebular optical study emphasizing late-time symmetry therefore described the ejecta as roughly symmetrical overall, with a possible clump of oxygen-rich material moving toward the observer; the more line-specific NIR analysis instead identified a bipolar or strongly clumpy inner core rich in O, Mg, Fe, and Ni, embedded inside a comparatively symmetric H-rich envelope. Taken together, these results point to a largely spherical outer envelope and a more asymmetric metal-rich core (Hueichapán et al., 4 Aug 2025, Ferrari et al., 30 Jul 2025).

Panchromatic nebular spectroscopy from $7.24$28 to $7.24$29 at about $7.24$30 and $7.24$31 days added an infrared view of the inner ejecta. In that study, all major atomic and ionic lines had essentially the same width, implying efficient macroscopic chemical mixing of the inner ejecta at $7.24$32 and little mixing of $7.24$33Ni to larger velocities. A molecule-free radiative-transfer model based on a standard red-supergiant explosion with $7.24$34 erg, $7.24$35 of $7.24$36Ni, and a $7.24$37 progenitor provided a satisfactory match across the optical and infrared. The same work identified stable Ni through lines such as [Ni II] $7.24$38, and argued that stable Ni is probably a common product of massive-star explosions (Dessart et al., 8 Jul 2025).

Molecular emission was detected repeatedly. Near-infrared spectra revealed first-overtone CO emission at $7.24$39 from $7.24$40 to $7.24$41 days, and the absence of strong band heads was used to infer $7.24$42 K. The panchromatic optical-to-infrared nebular study found that the CO fundamental radiates about $7.24$43 of the total SN luminosity and that this fraction is comparable to the decay power absorbed in the model C/O-rich shell. That study therefore suggested negligible microscopic mixing, with about $7.24$44 of the SN luminosity still radiated by atoms and ions (Hueichapán et al., 4 Aug 2025, Dessart et al., 8 Jul 2025).

Late-time interaction appears weak or absent. One nebular study explicitly reported no signatures of circumstellar-material interaction up to $7.24$45 days after explosion, despite the strong flash-ionization and X-ray evidence for dense material at very early times. This suggests that the immediate CSM was radially confined and had already been overrun by the shock well before the nebular phase (Ferrari et al., 30 Jul 2025).

6. Millimeter, gamma-ray, and neutrino constraints

Early ALMA Band 6 observations at $7.24$46, $7.24$47, and $7.24$48 days yielded non-detections with $7.24$49 limits of $7.24$50, $7.24$51, and $7.24$52 mJy, corresponding to a representative luminosity limit of about $7.24$53. Synchrotron-interaction modeling showed that a simple steady-wind CSM does not align with the flash-spectroscopic evidence, whereas an eruptive model with a peak mass-loss rate $7.24$54 can satisfy both the optical constraints and the ALMA non-detections. This study therefore favored a compact, highly absorbing eruptive shell over a single $7.24$55 wind (Hu et al., 2024).

Very-high-energy gamma-ray observations with H.E.S.S. accumulated $7.24$56 hours of data over about a month after explosion. No significant excess was detected. The resulting upper limits exclude bright gamma-ray emission in the first day after explosion, while later upper limits are consistent with the wind densities derived from optical observations. In the context of model comparisons used in that paper, the H.E.S.S. limits disfavor the brightest early-time TeV scenarios but do not rule out later, fainter hadronic emission from ejecta–CSM interaction (Borowska-Naguszewska et al., 3 Oct 2025).

Semi-analytic neutrino modeling similarly predicts that the high-energy neutrino signal should remain below current detector sensitivity. Using the same interaction framework that fits the bolometric light curve, one study found a TeV-dominated fluence at Earth of about $7.24$57, with the instantaneous muon-neutrino flux peaking near $7.24$58 TeV during the first $7.24$59 days. The expected number of IceCube muon-neutrino events above $7.24$60 TeV is only $7.24$61, far below detectability, although the paper identifies SN 2024ggi-like events as relevant targets for next-generation facilities such as IceCube-Gen2 and KM3NeT/ARCA (Buccheri et al., 20 Mar 2026).

Across these non-detections and limits, SN 2024ggi remains consistent with a picture in which a nearby Type II-P supernova interacts strongly with dense but compact CSM at very early times, yet still fails to produce an observable millimeter, gamma-ray, or neutrino signal once realistic absorption, geometry, and detector sensitivity are taken into account. This suggests that the event’s greatest long-term importance lies less in a direct high-energy detection than in its role as a tightly constrained bridge between red-supergiant pre-supernova mass loss, flash-ionized circumstellar structure, and the later optical-infrared signatures of the ejecta.