We present UVopticalnearinfrared observations and modeling of supernova SN 2024ggi a type II supernova SN II located in NGC 3621 at 7.2 Mpc. Earlytime flash spectroscopy of SN 2024ggi within 0.8 days of discovery shows emission lines of H I He I C III and N III with a narrow core and broad symmetric wings i.e. IInlike arising from the photoionized optically thick unshocked circumstellar material CSM that surrounded the progenitor star at shock breakout SBO. By the next spectral epoch at 1.5 days SN 2024ggi showed a rise in ionization as emission lines of He II C IV N IVV and O V became visible. This phenomenon is temporally consistent with a blueward shift in the UVoptical colors both likely the result of SBO in an extended dense CSM. The IInlike features in SN 2024ggi persist on a timescale of t SUBIInSUB  3.8  1.6 days at which time a reduction in CSM density allows the detection of Dopplerbroadened features from the fastest SN material. SN 2024ggi has peak UVoptical absolute magnitudes of M SUBw2SUB  18.7 mag and M SUBgSUB  18.1 mag respectively that are consistent with the known population of CSMinteracting SNe II. Comparison of SN 2024ggi with a grid of radiation hydrodynamics and nonlocal thermodynamic equilibrium radiativetransfer simulations suggests a progenitor massloss rate of inlineformula mmlmath overflowscrollmmlmover accenttruemmlmrowmmlmiMmmlmimmlmrowmmlmrowmmlmommlmommlmrowmmlmovermmlmommlmommlmsupmmlmrowmmlmn10mmlmnmmlmrowmmlmrowmmlmommlmommlmn2mmlmnmmlmrowmmlmsupmmlmspace width0.25emmmlmspacemmlmsubmmlmrowmmlmiMmmlmimmlmrowmmlmrowmmlmommlmommlmrowmmlmsubmmlmath inlineformula yrSUP1SUP v SUB w SUB  50 km sSUP1SUP confined to a distance of r lt 5  10SUP14SUP cm. Assuming a wind velocity of v SUB w SUB  50 km sSUP1SUP the progenitor star underwent an enhanced massloss episode in the last 3 yr before explosion.
['Core-collapse supernovae', 'Type II supernovae', 'Circumstellar matter', 'Radiative transfer', 'Ultraviolet astronomy', '304', '1731', '241', '1335', '1736', 'Astrophysics - High Energy Astrophysical Phenomena', 'Astrophysics - Solar and Stellar Astrophysics']
SN 2024ggi in NGC 3621 Rising Ionization in a Nearby Circumstellarmaterialinteracting Type II Supernova
{'SN 2024ggi in NGC 3621: Rising Ionization in a Nearby, CSM-Interacting Type II Supernova': "W. V. J/a.pc/c.pc/o.pc/b.pc/s.pc/o.pc/n.pc-G/a.pc/l.pc '/a.pc/n.pc, 1, 2 K. W. D/a.pc/v.pc/i.pc/s.pc, 3 C. D. K/i.pc/l.pc/p.pc/a.pc/t.pc/r.pc/i.pc/c.pc/k.pc, 4, 5 L. D/e.pc/s.pc/s.pc/a.pc/r.pc/t.pc, 6 R. M/a.pc/r.pc/g.pc/u.pc/t.pc/t.pc/i.pc, 1, 7 R. C/h.pc/o.pc/r.pc/n.pc/o.pc/c.pc/k.pc, 1 R. J. F/o.pc/l.pc/e.pc/y.pc, 3 P. A/r.pc/u.pc/n.pc/a.pc/c.pc/h.pc/a.pc/l.pc/a.pc/m.pc, 3 K. A/u.pc/c.pc/h.pc/e.pc/t.pc/t.pc/l.pc, 3, 8 C. R. B/o.pc/m.pc, 9 R. C/a.pc/r.pc/t.pc/i.pc/e.pc/r.pc, 10 D. A. C/o.pc/u.pc/l.pc/t.pc/e.pc/r.pc, 11 G. D/i.pc/m.pc/i.pc/t.pc/r.pc/i.pc/a.pc/d.pc/i.pc/s.pc, 12 D. D/i.pc/c.pc/k.pc/i.pc/n.pc/s.pc/o.pc/n.pc, 13 M. R. D/r.pc/o.pc/u.pc/t.pc, 14 A. T. G/a.pc/g.pc/l.pc/i.pc/a.pc/n.pc/o.pc, 15 C. G/a.pc/l.pc/l.pc, 16 B. G/a.pc/r.pc/r.pc/e.pc/t.pc/s.pc/o.pc/n.pc, 13 L. I/z.pc/z.pc/o.pc, 16, 17 D. O. J/o.pc/n.pc/e.pc/s.pc, 18 N. L/e.pcB/a.pc/r.pc/o.pc/n.pc, 1 H.-Y. M/i.pc/a.pc/o.pc, 19 D. M/i.pc/l.pc/i.pc/s.pc/a.pc/v.pc/l.pc/j.pc/e.pc/v.pc/i.pc/c.pc, 13 Y.-C. P/a.pc/n.pc, 19 A. R/e.pc/s.pc/t.pc, 20, 21 C. R/o.pc/j.pc/a.pc/s.pc-B/r.pc/a.pc/v.pc/o.pc, 3 A. S/a.pc/n.pc/t.pc/o.pc/s.pc, 9 H. S/e.pc/a.pc/r.pc/s.pc, 4, 5 B. M. S/u.pc/b.pc/r.pc/a.pc/y.pc/a.pc/n.pc, 13 K. T/a.pc/g.pc/g.pc/a.pc/r.pc/t.pc, 3 /a.pc/n.pc/d.pc S. T/i.pc/n.pc/y.pc/a.pc/n.pc/o.pc/n.pc/t.pc 22  \n1 Department of Astronomy, University of California, Berkeley, CA 94720-3411, USA  \n2 NSF Graduate Research Fellow  \n3 Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA  \n4 Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), Northwestern University, Evanston, IL 60202, USA  \n5 Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208, USA  \n6 Institut d'Astrophysique de Paris, CNRS-Sorbonne Universit´e, 98 bis boulevard Arago, F-75014 Paris, France  \n7 Department of Physics, University of California, Berkeley, CA 94720, USA  \n8 School of Physics, The University of Melbourne, VIC 3010, Australia  \n9 Centro Brasileiro de Pesquisas F´ısicas, Rua Dr. Xavier Sigaud 150, 22290-180 Rio de Janeiro, RJ, Brazil  \n10 Instituto de Estudios Astrof´ısicos, Facultad de Ingenier´ıa y Ciencias, Universidad Diego Portales, Av. Ej´ercito Libertador 441, Santiago, Chile 11  \nSpace Telescope Science Institute, Baltimore, MD 21218, USA  \n12 School of Physics, Trinity College Dublin, The University of Dublin, Dublin 2, D02 PN40, Ireland  \n13 Department of Physics and Astronomy, Purdue University, 525 Northwestern Avenue, West Lafayette, IN 47907, USA  \n14 David A. Dunlap Department of Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, Ontario, M5S 3H4, Canada  \n15 The NSF AI Institute for Artificial Intelligence and Fundamental Interactions  \n16 DARK, Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen, Denmark  \n17 Osservatorio Astronomico di Capodimonte, INAF, Salita Moiariello 16, Napoli, 80131, Italy  \n18 Institute for Astronomy, University of Hawai'i, 640 N. A'ohoku Pl., Hilo, HI 96720, USA  \n19 Graduate Institute of Astronomy, National Central University, 300 Zhongda Road, Zhongli, Taoyuan 32001, Taiwan  \n20 Space Telescope Science Institute, Baltimore, MD 21218  \n21 Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD 21218, USA  \n22 National Astronomical Research Institute of Thailand, 260 Moo 4, Donkaew, Maerim, Chiang Mai, 50180, Thailand", 'ABSTRACT': "Wepresent UV/optical/NIR observations and modeling of supernova (SN) 2024ggi, a type II supernova (SN II) located in NGC 3621 at 7.2 Mpc. Early-time ('flash') spectroscopy of SN 2024ggi within +0.8 days of discovery shows emission lines of H /i.pc, He /i.pc, C /i.pc/i.pc/i.pc, and N /i.pc/i.pc/i.pc with a narrow core and broad, symmetric wings (i.e., IInlike) arising from the photoionized, optically-thick, unshocked circumstellar material (CSM) that surrounded the progenitor star at shock breakout. By the next spectral epoch at +1.5 days, SN 2024ggi showed a rise in ionization as emission lines of He /i.pc/i.pc, C /i.pc/v.pc, N /i.pc/v.pc//v.pc and O /v.pc became visible. This phenomenon is temporally consistent with a blueward shift in the UV/optical colors, both likely the result of shock breakout in an extended, dense CSM. The IIn-like features in SN 2024ggi persist on a timescale of 𝑡 IIn = 3 . 8 ± 1 . 6 days at which time a reduction in CSM density allows the detection of Doppler broadened features from the fastest SN material. SN 2024ggi has peak UV/optical absolute magnitudes of 𝑀 w2 = -18 . 7 mag and 𝑀 g = -18 . 1 mag that are consistent with the known population of CSM-interacting SNe II. Comparison of SN 2024ggi with a grid of radiation hydrodynamics and non-local thermodynamic equilibrium (nLTE) radiative-transfer simulations suggests a progenitor mass-loss rate of / 𝑀 = 10 -2 M ⊙ yr -1 ( 𝑣 𝑤 = 50 kms -1 ), confined to a distance of 𝑟 < 5 × 10 14 cm. Assuming a wind velocity of 𝑣 𝑤 = 50 kms -1 , the progenitor star underwent an enhanced mass-loss episode in the last ∼ 3 years before explosion.  \nCorresponding author: Wynn Jacobson-Gal'an (he, him, his)  \nKeywords: supernovae:general - supernovae: individual (SN 2024ggi) - surveys - red supergiants - CSM", '1. INTRODUCTION': "Shock breakout (SBO) from a red supergiant (RSG) star is characterized by an optical depth of 𝜏 ≈ 𝑐 / 𝑣 sh, where 𝑐 is the speed of light and 𝑣 sh is the shock velocity. Consequently, the location and timescale of SBO photon escape is highly dependent on the density and extent of circumstellar material (CSM) that borders the RSG prior to explosion. In addition to light travel effects during SBO (Waxman & Katz 2017; Goldberg et al. 2022), the SBO signal can be significantly enhanced and elongated by the presence of high density CSM directly above the stellar surface (Chevalier & Irwin 2011; Dessart et al. 2017; Haynie & Piro 2021). Once the shock has 'broken out,' the associated burst of high-energy radiation will 'flash ionize' the surrounding medium - observationally this manifests as a hot supernova (SN) continuum riddled with recombination lines from ionization CSM. However, to overcome the recombination timescale of the 'flash ionized CSM' ( 𝑡 rec ∝ 1 / 𝑛 𝑒 ≈ hours-days for 𝑛 ≈ 10 7 -10 cm -3 , 𝜌 ≈ 10 -14 -10 -17 g cm -3 at 𝑟 < 2 𝑅 ★ , where 𝑅 ★ is progenitor radius), SN ejecta interaction with dense CSM provides continuous photoionization of the medium for sufficiently large CSM densities (e.g., 𝜌 ≳ 10 -14 g cm -3 ).  \nFor type II supernovae (SNe II) interacting with such dense CSM, high-energy photons will be emitted from the shock front as the post-shock gas cools primarily through free-free emission ( 𝑇 sh ≈ 10 5 -8 K; Chevalier & Irwin 2012; Chevalier &Fransson 2017). This process then prolongs the formation of high-ionization recombination lines (e.g., He /i.pc/i.pc, N /i.pc/i.pc/i.pc//i.pc/v.pc//v.pc, C/i.pc/i.pc/i.pc//i.pc/v.pc) present during the 'flash ionization' phase. Intriguingly, as recombination line photons try to exit the CSM, they electron-scatter off of free electrons in the ionized gas, which broadens the observed emission lines that will then appear as the combination of a narrow core and Lorentzian wings (i.e., 'IIn-like features'; Chugai 2001; Dessart et al. 2009; Huang &Chevalier 2018a). However, as the CSM density and optical depth to electron-scattering drops, these electron-scattering profiles will fade on a characteristic timescale ( 𝑡 IIn), with the SN photosphere then revealing the fastest moving SN material (Dessart et al. 2017; Dessart & Jacobson-Gal'an 2023; Jacobson-Gal'an et al. 2023, 2024).  \nGiven the rapid evolution of CSM-interacting SNe II in their first hours-to-days, ultra-rapid ('flash') spectroscopy is essential to both capture the SBO signal and persistent photoionization of dense CSM, but also to constrain the composition and mass-loss history of the progenitor star in its final year(s) before collapse (Gal-Yam et al. 2014; Khazov et al. 2016; Yaron et al. 2017; Terreran et al. 2022; JacobsonGal'an et al. 2023). To date, all-sky transient surveys have allowed for systematic discovery of SNe II within days of first  \nlight. Through modeling of early-time SN II spectra showing IIn-like features with non-LTE radiative transfer codes (e.g., CMFGEN ; Hillier & Dessart 2012; Dessart et al. 2015), numerous single-object studies indicate enhanced RSG massloss ( / 𝑀 ≈ 10 -3 - 10 -2 M ⊙ yr -1 , 𝑣 𝑤 ≈ 50-100 km s -1 ) in the final years before explosion (e.g., PTF11iqb, Smith et al. 2015; SN 2013fs, Yaron et al. 2017; Dessart et al. 2017; SN 2014G, Terreran et al. 2016; SN 2016bkv, Hosseinzadeh et al. 2018; Nakaoka et al. 2018; SN 2017ahn, Tartaglia et al. 2021; SN 2018zd, Zhang et al. 2020; Hiramatsu et al. 2021; SN2019nyk, Dastidar et al. 2024; SN 2020pni, Terreran et al. 2022; SN 2020tlf, Jacobson-Gal'an et al. 2022; SN 2022jox, Andrews et al. 2023; SN 2023ixf, Jacobson-Gal'an et al. 2023; Bostroem et al. 2023; Teja et al. 2023; Smith et al. 2023; Zimmerman et al. 2024). Furthermore, sample studies suggest that > 40% of SNe II discovered within 2 days of first light show IIn-like features from interaction with dense CSM (Bruch et al. 2021, 2023). Additionally, relative to standard SNe II, events with early-time IIn-like features are incredibly bright in the ultraviolet (Irani et al. 2023; Jacobson-Gal'an et al. 2024).  \nIn this paper we present, analyze, and model photometric and spectroscopic observations of SN 2024ggi, first discovered by the Asteroid Terrestrial-impact Last Alert System (ATLAS) (Srivastav et al. 2024; Tonry et al. 2024; Chen et al. 2024) on 2024-04-11 (MJD 60411.14). SN 2024ggi was classified as a Type II SN (Hoogendam et al. 2024; Zhai et al. 2024) and is located at 𝛼 = 11 h 18 m 22 . 09 s , 𝛿 = -32 · 50 ' 15 . 26 '' in host galaxy NGC 3621. We adopt a time of first light of MJD 60410 . 80 ± 0 . 34 days, which is based on the average between the times of last non-detection ( 𝑚 𝐿 = 19 . 5 mag) and first detection 𝑚 𝑜 = 18 . 9 mag. To validate this estimate of first light, we first fit the ATLAS 𝑜 -band light curve with a two-component power-law in the REDBACK software package (Sarin et al. 2023) and derive a time of first light of MJD 60411 . 07 ± 0 . 01. Additionally, using a uniform prior distribution derived from the last ATLAS nondetection on MJD 60410.45, we fit the bolometric light curve of SN 2024ggi to a suite of hydrodynamical models (e.g., see Moriya et al. 2023; Subrayan et al. 2023) which constrains the time of first light to be MJD 60410 . 56 + 0 . 07 -0 . 12 . Both methods yield consistent explosion epochs to that derived above from the phases of last non-detection and first detection. All phases reported in this paper are with respect to this adopted time of first light ( 𝛿𝑡 ). In this paper, we use a redshift-independent host-galaxy distance of 7 . 24 ± 0 . 20 Mpc (Saha et al. 2006) and adopt a redshift of 𝑧 = 0 . 002215 based on Na /i.pc D absorption in high-resolution spectra of SN 2024ggi obtained  \nwith the NEID Spectrograph on the WIYN Telescope (private communication).  \nGiven detection and classification during its infancy, SN 2024ggi represents an incredible opportunity to study the SBO phase of a CSM-interacting SN II in unprecedented detail. In § 2 we describe UV, optical, and NIR observations of SN 2024ggi. In § 3 we present analysis, comparisons and modeling of SN 2024ggi's optical photometric and spectroscopic properties. Finally, in § 4 we discuss the progenitor environment and mass-loss history prior to SN 2024ggi. Conclusions are drawn in § 5. All uncertainties are quoted at the 68% confidence level (c.l.) unless otherwise stated.", '2.1. Photometric Observations': "The Ultraviolet Optical Telescope (UVOT; Roming et al. 2005) onboard the Neil Gehrels Swift Observatory (Gehrels et al. 2004) started observing SN 2024ggi on 11 April 2024 ( 𝛿𝑡 = 0 . 79 days). We performed aperture photometry with a 5 '' region radius with uvotsource within HEAsoft v6.33 (Nasa High Energy Astrophysics Science Archive Research Center (Heasarc) 2014) /one.sup , following the standard guidelines from Brown et al. (2014) /two.sup . In order to remove contamination from the host galaxy, we employed pre-explosion images to subtract the measured count rate at the location of the SN from the count rates in the SN images and corrected for pointspread-function (PSF) losses following the prescriptions of Brown et al. (2014). We note that not all early-time UVOT observations are included in this analysis given the degree of saturation.  \nWe obtained 𝑢𝑔𝑟𝑖 imaging of SN 2024ggi with the Las Cumbres Observatory (LCO) 1 m telescopes from 12 April to 26 April 2024 (Program ANU2024A-004). After downloading the BANZAI -reduced images from the LCO data archive (McCully et al. 2018), we used photpipe (Rest et al. 2005) to perform DoPhot PSF photometry (Schechter et al. 1993). All photometry was calibrated using PS1 stellar catalogs described above with additional transformations to SDSS 𝑢 -band derived from Finkbeiner et al. (2016). For additional details on our reductions, see Kilpatrick et al. (2018). We also obtained photometry using a 0.7 meter Thai Robotic Telescope at Cerro Tololo Inter-American Observatory in the 𝑢𝑔𝑟𝑖𝑧 bands. Images are bias subtracted and field flattened. Absolute photometry is obtained using stars in the 10 ' × 10 ' field of view. We also observed SN 2024ggi with the Lulin 1 m telescope in 𝑔𝑟𝑖𝑧 bands. Standard calibrations for bias and flat-fielding were performed on the images using IRAF ,  \nand we reduced the calibrated frames in photpipe using the same methods described above for the LCO images.  \nWe also observed SN 2024ggi in 𝑔𝑟𝑖𝑧𝐻 -bands with the Rapid Eye Mount (REM; Antonelli et al. 2003) telescope located in La Silla, Chile. REM is equipped with two cameras, which can observe simultaneously the same field of view (10' × 10') in the optical and near-IR. Single images have been initially corrected for dark and flat frames observed each night. Then, images obtained with the same setup have been stacked and finally corrected for cosmic rays, all using a dedicated pipeline written in Python. Magnitudes were measured with aperture photometry, with a variable aperture size according to the seeing of the night, and calibrated against selected field stars from the Skymapper DR4 (Onken et al. 2024) that also have 2MASS 𝐽𝐻𝐾 -band photometry. We also obtained 𝑢𝑔𝑟𝑖𝑧 imaging of SN 2024ggi with the 0.8 m T80S telescope at Cerro Tololo Inter-american Observatory, Chile. These were processed using the S-PLUS Transient Extension Program pipeline (Santos et al. 2024), including non-linearity to recover detections of SN 2024ggi close to the saturation level. Additionally, we include 𝑜 -band photometry by ATLAS that was downloaded from the forced photometry server (Tonry et al. 2018; Smith et al. 2020; Shingles et al. 2021). The complete multi-color light curve of SN 2024ggi is presented in Figure 1.  \nThe Milky Way (MW) 𝑉 -band extinction and color excess along the SN line of sight is 𝐴 𝑉 = 0 . 22 mag and E(B-V) = 0.07 mag (Schlegel et al. 1998; Schlafly & Finkbeiner 2011), respectively, which we correct for using a standard Fitzpatrick (1999) reddening law ( 𝑅 𝑉 = 3.1). In addition to the MW color excess, we estimate the contribution of galaxy extinction in the local SN environment. Using a high-resolution Kast spectrum of SN 2024ggi at 𝛿𝑡 = 5 . 5 days, we calculate Na /i.pc D2 and D1 equivalent widths (EWs) of 0.18 and 0.13 ˚ A, respectively; these values are confirmed in a follow-up high resolution Gemini spectrum at 𝛿𝑡 = 9 . 2 days. We use 𝐴 host 𝑉 = ( 0 . 78 ± 0 . 15 ) mag ×( EWNaID/ ˚ A) from Stritzinger et al. (2018) to convert these EWs to an intrinsic host-galaxy 𝐸 ( 𝐵 -𝑉 ) and find a host galaxy extinction of 𝐸 ( 𝐵 -𝑉 ) host = 0 . 084 ± 0 . 018 mag, also corrected for using the Fitzpatrick (1999) reddening law.", '2.2. Spectroscopic Observations': "SN2024ggi was observed with Shane/Kast (Miller & Stone 1993) and the Goodman spectrograph (Clemens et al. 2004) at the Southern Astrophysical Research (SOAR) telescope between 𝛿𝑡 = 1 . 5 -9 . 3days. For all these spectroscopic observations, standard CCD processing and spectrum extraction were accomplished with IRAF /three.sup . The data were extracted using the optimal algorithm of Horne (1986). Low-order polynomial  \nFigure 1. Left: Multi-color light curve of SN 2024ggi (circles) with respect to time since first light (MJD 60410 . 80 ± 0 . 34) from Swift , ATLAS, LCO, TRT, REM, T80s, and Lulin telescopes. Observed photometry is presented in the AB magnitude system and has been corrected for host galaxy and MW extinction. CMFGEN m1em2 and r1w6a model light curves are shown as solid and dashed lines, respectively. Right: Early-time, reddening-corrected 𝑊 2 -𝑉 color plot for SN 2024ggi (magenta stars) and SN 2023ixf (cyan stars) with respect to gold/silver-sample objects (red, yellow, blue lines) and comparison sample objects (black dashed lines) (Jacobson-Gal'an et al. 2024). SN 2024ggi shows a blueward shift in color within the first ∼ day since first light that is consistent with a rise in temperature and ionization (e.g., Fig. 2).  \n<!-- image -->  \nfits to calibration-lamp spectra were used to establish the wavelength scale and small adjustments derived from nightsky lines in the object frames were applied. SN 2024ggi spectra were also obtained Gemini Multi-Object Spectrograph (GMOS) at Gemini South Observatory at 𝛿𝑡 = 9 . 2 days and reduced with the Data Reduction for Astronomy from Gemini Observatory North and South (DRAGONS) pipeline (Labrie et al. 2023). Spectra were also obtained with the TripleSpec4.1 NIR Imaging Spectrograph (TSpec) at the SOAR telescope, and reduced using a modified version of Spextool (Cushing et al. 2004). Telluric corrections were applied using xtellcor presented in Vacca et al. (2003). Additional modifications to calibrations are described in Kirkpatrick et al. (2011).  \nIn Figure 2 we present the complete series of optical spectroscopic observations of SN 2024ggi from 𝛿𝑡 = 0 . 8 -9 . 3 days. In this plot, we also show the classification spectrum of SN 2024ggi at + 0 . 8 days from the Lijiang 2.4m telescope (Zhai et al. 2024), which we only use for narrow line identification. The complete optical/NIR spectral sequence is shown in Figures 2 & 3 and the log of spectroscopic observations is presented in Appendix Table A1.", '3.1. Photometric Properties': "Wepresent the complete early-time, multi-band light curve of SN 2024ggi in Figure 1. Given the estimated time of first light, SN 2024ggi was first detected by ATLAS at 𝛿𝑡 = 0 . 3 days with absolute magnitude of 𝑀 𝑜 = -10 . 7 mag and then quickly increased in luminosity to 𝑀 𝑜 = -15 . 3 mag by 𝛿𝑡 = 1 . 2 days. We fit high-order polynomials to 𝑤 2 , 𝑚 2 , 𝑤 1 , 𝑢, 𝑔, 𝑟, 𝑖 -band light curves in order to estimate the peak luminosity and rise-time of SN 2024ggi. All measurements are reported in Table 1, with the uncertainty in peak magnitude being the 1 𝜎 error from the fit and the uncertainty in the peak phase being found from adding the uncertainties in both the time of peak magnitude and the time of first light in quadrature. We find that SN 2024ggi has UV and optical peak absolute magnitudes of 𝑀 𝑤 2 = -18 . 7 ± 0 . 07 mag and 𝑀 𝑔 = -18 . 1 ± 0 . 06 mag, respectively. Using the adopted time of first light, we estimate UV and optical rise-times of 𝑡 𝑤 2 = 3 . 0 ± 0 . 3 days and 𝑡 𝑔 = 6 . 5 ± 0 . 9 days.  \nIn Figure 4, we compare the observed peak absolute magnitudes of SN 2024ggi to a sample of 74 SNe II from Jacobson-  \nFigure 2. Early-time optical spectral series of SN 2024ggi (black) together with r1w6 (red) and r1w5r (blue) models from Dessart et al. (2017). The r1w6 and r1w5r models are characterized by wind mass-loss rates of / 𝑀 = ( 0 . 5 -1 ) × 10 -2 M ⊙ yr -1 that extends to a CSM radius of 𝑅 CSM = ( 2 -5 ) × 10 14 cm. Model spectra have been smoothed with a Gaussian kernel to match the spectral resolution of the data. Line identifications shown in green. The appearance of He /i.pc/i.pc, C /i.pc/v.pc, N /i.pc/v.pc//v.pc and O /v.pc after the 𝛿𝑡 = + 0 . 8 day spectrum indicates a rise in ionization and temperature in SN 2024ggi as the breakout pulse and the subsequent continuous release of radiation from the shock diffuses through the CSM.  \n<!-- image -->  \nFigure 3. Left/Right: NIR spectra of SN 2024ggi (black) and SN 2017ahn (orange) compared to r1w5r (blue) and r1w6 (red) models from Dessart et al. (2017). Line identifications shown in green.  \n<!-- image -->  \nGal'an et al. (2024). This sample includes 39 SNe II with detected IIn-like features in their early-time spectra: 'goldsample' objects have spectra at 𝛿𝑡 < 2 days and 'silversample' objects only have spectra obtained at 𝛿𝑡 > 2 days. As discussed in Jacobson-Gal'an et al. (2024) and delineated by color in Figure 4, the gold/silver-sample objects are classifed in three main groups: 'Class 1' (blue) show emission lines of N /i.pc/i.pc/i.pc, He/i.pc/i.pc, and C/i.pc/v.pc (e.g., SNe 1998S, 2017ahn, 2018zd, 2020pni, 2020tlf, 2023ixf), 'Class 2' (yellow) have no N /i.pc/i.pc/i.pc emission but do show He /i.pc/i.pc and C /i.pc/v.pc (e.g., SNe 2014G, 2022jox), and 'Class 3' (red) only show weaker, narrow He/i.pc/i.pc emission superimposed with a blueshifted, Dopplerbroadened He /i.pc/i.pc (e.g., SN 2013fs, 2020xua). However, this classification scheme is epoch dependent because emission lines of O /v.pc//v.pc/i.pc and N /i.pc/v.pc//v.pc are also present in some objects such as SN 2013fs at 𝑡 < 1 day owing to a more compact CSM than other CSM-interacting SNe II (Yaron et al. 2017; Dessart et al. 2017). Additionally, we present the comparison sample of 35 SNe II with spectra obtained at 𝛿𝑡 < 2 days but no detected IIn-like features.  \nAs shown in Figure 4, SN 2024ggi is more luminous than most in the comparison sample SNe II without IIn-like features in all UV/optical filters. Furthermore, SN 2024ggi has a longer rise-time in UV filters than comparison sample objects but comparable rise-times in optical filters. However, SN 2024ggi shows consistent UV/optical luminosities and rise-times to gold/silver-sample objects such as iPTF11iqb (Smith et al. 2015) and SN 2014G (Terreran et al. 2016). Additionally, SN 2024ggi has a comparable optical luminosity to SN 2023ixf but is less luminous in the UV by ∼ 0.7 mags. In the right panel of Figure 1, we also compare the early-time ( 𝛿𝑡 < 10 days) 𝑤 2 -𝑣 colors of SN 2024ggi to gold, silver and comparison objects. Interestingly, SN 2024ggi shows a dramatic red-to-blue 𝑤 2 -𝑣 color evolution of -0 . 46 mag to -1 . 56 mag between 𝛿𝑡 = 0 . 8 -1 . 2 days, followed by  \nconsistently blue colors as it evolves redward in its first week. This unusual color evolution is also observed in other SNe II with IIn-like features such as SN 2023ixf (Hosseinzadeh et al. 2023; Hiramatsu et al. 2023; Li et al. 2024; Zimmerman et al. 2024), which was proposed as evidence for SBO in an extended dense CSM. This phenomenon is a product of the breakout pulse and the subsequent continuous release of radiation from the shock diffusing through the CSM (Dessart et al. 2017) and also corresponds to the phase during which the photosphere moves outward, initially at 𝑅 ★ and then out to the location of the electron scattering photosphere ( 𝑅 𝜏 = 1; Dessart & Jacobson-Gal ' an 2023). We also fit a blackbody model to the SN 2024ggi UV/optical SED and find blackbody temperature[radii] during this red-to-blue color evolution of ∼ 20 kK[5 . 3 × 10 13 cm] at 𝛿𝑡 = 0 . 8 days and ∼ 23 kK[1 . 1 × 10 14 cm] at 𝛿𝑡 = 1 . 2 days. Furthermore, we find that the blackbody temperature of SN 2024ggi peaks at ∼ 34 kK on 𝛿𝑡 = 1 . 4 days. This is a similar peak blackbody temperature to the 34.3 kK found for SN 2023ixf at 𝛿𝑡 = 3 . 51 days (Zimmerman et al. 2024). However, we note that these blackbody temperatures correspond to the temperature at the thermalization depth and that the blackbody radius is not equivalent to the location of the photosphere (i.e., 𝑅 BB << 𝑅 phot).", '3.2. Spectroscopic Properties': "We present our sample of optical observations for SN2024ggi spanning from 𝛿𝑡 = + 0 . 8 to + 9 . 3 days in Figure 2. In the earliest spectrum at 𝛿𝑡 = + 0 . 8 days, SN 2024ggi shows IIn-like features of H /i.pc ( 𝜒 = 13 . 6 eV), He /i.pc ( 𝜒 = 24 . 6 eV), N/i.pc/i.pc/i.pc ( 𝜒 = 47 . 4 eV) and C /i.pc/i.pc/i.pc ( 𝜒 = 47 . 9 eV). However, by the next spectral observation at 𝛿𝑡 = + 1 . 5 days, SN 2024ggi shows prominent emission lines of He /i.pc/i.pc ( 𝜒 = 54 . 5 eV), N/i.pc/i.pc/i.pc//i.pc/v.pc//v.pc ( 𝜒 = 47 . 4 / 77 . 5 / 97 . 9 eV), C /i.pc/v.pc ( 𝜒 = 64 . 5 eV) and O/v.pc ( 𝜒 = 113 . 9 eV), indicating a dramatic rise in ionization  \nFigure 4. Left: Peak g -band absolute magnitude versus rise time (Left) and duration of IIn-like features (Right). Gold/silver samples shown as blue/yellow/red circles and the comparison sample is shown as black squares. Solid colored points represent the subsample of objects at 𝐷 > 40 Mpc. Parameters from the CMFGEN model grid ( § 3.3) are plotted as colored stars, polygons, diamonds and plus signs with the CSM densities at 10 14 cm (in g cm -3 ) for each model displayed in parentheses. SNe 2024ggi and 2023ixf are shown as a magenta star and a cyan cross, respectively.  \n<!-- image -->  \nand temperature within ∼ 14 hours that is temporally consistent with the blueward evolution in 𝑤 2 -𝑣 colors (e.g., Fig. 1). There may also be a detection of O /i.pc/v.pc 𝜆 3410 but the lower S/N in that spectral region makes the line identification uncertain. Interestingly, the detection of O /i.pc/v.pc//v.pc has only been confirmed in one other SN II, 2013fs, and in that object the timescale of this emission was < 0 . 5 days (Yaron et al. 2017). Notably, the shift in ionization seen in SN 2024ggi was also observed in SN 2023ixf but at later phases of + 1 . 1 -2 . 4 days (JacobsonGal'an et al. 2023). A two-component Lorentzian model fit to the H 𝛼 profile in the +1.5 d Kast spectrum shows a narrow component full width at half maximum (FWHM) velocity of < 270 kms -1 that traces the unshocked CSM combined with a broad symmetric component with velocity of ∼ 1320 km s -1 that is caused by scattering of recombination line photons by free, thermal electrons in the ionized CSM (Chugai 2001; Dessart et al. 2009; Huang & Chevalier 2018b). However, the narrow component is likely affected by radiative acceleration of the CSM that causes the width of the narrow component to  \nbe larger than the true velocity of the progenitor wind (Dessart et al. 2015, 2017; Tsuna et al. 2023).  \nIn addition to optical spectroscopy, we present NIR spectra of SN 2024ggi in Figure 3 that extend from 0 . 9 -2 . 4 𝜇 m. The first spectrum at 𝛿𝑡 = 2 . 2 days shows IIn-like features of He/i.pc/i.pc as well as Hydrogen Paschen and Brackett transitions of P 𝛿 , P 𝛾 , P 𝛽 and B 𝛾 . We then compare this spectrum to SN II 2017ahn (Tartaglia et al. 2021), one of the only SN with IIn-like features to have a NIR spectrum during the dense CSM-interaction phase. Overall, the NIR spectrum of SN 2024ggi at 𝛿𝑡 = 2 . 2 days shows similar narrow emission lines to SN 2017ahn with the exception being that SN 2017ahn shows prominent He /i.pc emission while SN 2024ggi only shows He/i.pc/i.pc.  \nAs shown in Figure 5, the IIn-like features in SN 2024ggi are already fading by the +2.2 day epoch, with possible blueshifted, Doppler-broadened He /i.pc/i.pc emission as the fastest moving ejecta and/or the dense shell starts to become visible given a decrease in optical depth. Then, by 𝛿𝑡 = 5 . 5 days, the IIn-like features have vanished and SN 2024ggi shows  \nFigure 5. Left: H 𝛼 velocity evolution in SN 2024ggi (black) from 𝛿𝑡 = 0 . 8 -9 . 3 days with respect to r1w6 model spectra (red), which has been scaled to the emission line peaks of SN 2024ggi and smoothed with a Gaussian filter to better compare with the data. Early-time spectral profiles are shaped by electron-scattering in the dense CSM. The transition shown from black to blue lines ( 𝑡 IIn = 3 . 7 ± 1 . 8 days) marks the emergence of broad absorption features derived from the fastest moving SN ejecta. Middle: H 𝛽 velocity evolution, also showing that the electron-scattering line profiles subside by ∼ 5 . 5 days. Right: He/i.pc/i.pc 𝜆 4686 velocity evolution reveals that the electron-scattering profile fades by ∼ 5 . 5 days, suggesting a significant decrease in CSM density.  \n<!-- image -->  \na broad, blueshifted absorption profile in all Balmer transitions as well as a blue-shifted He /i.pc/i.pc profile extending out to ∼ ( 10 -12 ) × 10 4 kms -1 (i.e., 'ledge feature'; Dessart et al. 2016; Hosseinzadeh et al. 2022; Pearson et al. 2023; Chugai & Utrobin 2023; Shrestha et al. 2024). Based on this evolution, we estimate the duration of the IIn-like line profiles to be 𝑡 IIn = 3 . 8 ± 1 . 6 days, which marks the transition point at which the optical depth to electron-scattering has dropped enough to see the emerging fast-moving SN ejecta. At 𝛿𝑡 > 5 days, we observe broad absorption profiles in the H 𝛼 and H 𝛽 transitions that extend to ∼ 12000 km s -1 , which provides a decent estimate of the velocities of the fastest moving H-rich material at the shock front. Using the estimated 𝑡 IIn and a shock velocity of 𝑣 sh = 12000 kms -1 , the transition to lower density CSM likely occurs at a radius of 𝑟 = 𝑅 ★ + 𝑣 sh × 𝑡 IIn = ( 4 . 3 ± 1 . 7 ) × 10 14 cm(for 𝑅 ★ = 500 R ⊙ ) and at a CSM density of 𝜌 = ( 𝜅 T × 𝑡 IIn × 𝑣 sh ) -1 = ( 7 . 4 ± 3 . 1 )× 10 -15 gcm -3 , for 𝜏 = 1 and Thompson opacity of 𝜅 T = 0 . 34 cm 2 g -1 . This likely indicates a more confined CSM and/or lower mass-loss rate for SN 2024ggi than Class 1 gold/silver sample SNe II that have 𝑡 IIn > 5 days e.g., SNe 2017ahn, 2018zd, 2020pni, 2020tlf, 2023ixf. However, as shown in Figure 4, the duration of IIn-like features in SN 2024ggi is similar to Class 2/3 objects, which reveals that if earlier time spectra (e.g., 𝛿𝑡 < 1 day) had  \nbeen obtained for these objects, they may have shown N /i.pc/i.pc/i.pc emission like Class 1 objects and SN 2024ggi at 𝛿𝑡 < 1 day.", '3.3. Model Matching': "In order to quantify the CSM properties of SN 2024ggi, we compared the spectral and photometric properties of SN 2024ggi to a model grid of radiation hydrodynamics and non-LTE radiative transfer simulations covering a wide range of progenitor mass-loss rates ( / 𝑀 = 10 -6 -10 0 M ⊙ yr -1 ; 𝑣 𝑤 = 50 kms -1 ) and CSM radii ( 𝑅 = 10 14 -10 16 cm), all in spherical symmetry. Density profiles for a representative set of models are present in Figure 6. Simulations of the SN ejecta-CSM interaction were performed with the multigroup radiation-hydrodynamics code HERACLES (Gonz'alez et al. 2007; Vaytet et al. 2011; Dessart et al. 2015), which consistently computes the radiation field and hydrodynamics. Then, at selected snapshots in time post-explosion, the hydrodynamical variables are imported into the non-LTE radiativetransfer code CMFGEN (Hillier & Dessart 2012; Dessart et al. 2015) for an accurate calculation of the radiative transfer, which includes a complete model atom, ∼ 10 6 frequency points, and treatment of continuum and line processes as well as non-coherent electron scattering. For each model, we adopt an explosion energy of 1 . 2 × 10 51 erg, a 15M ⊙ progenitor with  \na radius ranging from 𝑅 ★ ≈ 500 -700 R ⊙ , and a CSM composition set to the surface mixture of a RSG progenitor (Dessart et al. 2017). For the simulations presented in this work, the CSM extent is much greater than 𝑅 ★ ( ∼ 500-1200 R ⊙ for a RSG mass range of ∼ 10-20 M ⊙ ) and 𝑅 ★ has little impact during phases of ejecta-CSM interaction. The progenitor radius plays a more significant role on the light curve evolution during the plateau phase (e.g., see Dessart et al. 2013; Jacobson-Gal'an et al. 2022), i.e., once the interaction phase is over and the emission from the deeper ejecta layers dominate the SN luminosity. Specific methods for each simulation can be found in Dessart et al. (2016, 2017); Jacobson-Gal'an et al. (2022); Dessart & Jacobson-Gal'an (2023).  \nWe determine the best-matched model to SN 2024ggi through direct spectral matching and peak/rise-time in all available UV/optical filters (see Jacobson-Gal'an et al. 2024 for specifics of the model matching procedure). As shown in Figure 2, the early-time evolution of SN 2024ggi is most consistent with the r1w6 model, suggesting a mass-loss rate of / 𝑀 = 10 -2 M ⊙ yr -1 and a dense CSM (i.e., optically thick to electron scattering) that extends to ∼ 5 × 10 14 cm. In Figure 5 we show that the line profiles in SN 2024ggi during the first few days of SN 2024ggi are decently matched by the r1w6 model and that a lower mass-loss rate model (e.g., / 𝑀 = 5 × 10 -3 M ⊙ yr -1 ; r1w5r) cannot match the duration of the IIn-like features. However, notable differences include the lack of N /i.pc/i.pc/i.pc and the strong He /i.pc/i.pc at +0.8 days that is not present in SN 2024ggi and inability of the r1w6 model to adequately reproduce the complete N /i.pc/i.pc/i.pc/He /i.pc/i.pc complex i.e., the model profile is too narrow as was observed in SN 2023ixf (Jacobson-Gal'an et al. 2023), which could be due to the fact that these simulations assume spherical symmetry and/or require higher kinetic energies. After 𝑡 IIn, the r1w6 model spectra show a narrow P-Cygni profile combined with broad absorption that extends to ∼ 7000 km s -1 in Balmer transitions. However, while this is qualitatively the behavior observed in SN 2024ggi, there is no narrow P Cygni observed until the + 9 . 3 day spectra and the absorption profiles in SN 2024ggi extend to much larger velocities of ∼ 12000 kms -1 . This mismatch is likely due to increased ejecta deceleration in the models that is not present in SN 2024ggi and/or the CSM of SN2024ggi is asymmetric, allowing for some parts of the SN ejecta that only encounter low density CSM to retain higher velocities. Asymmetric CSM has been proposed for similar events such as SNe 1998S and 2023ixf (Leonard et al. 2000; Vasylyev et al. 2023) based on spectropolarimetry. Similar to SN 2023ixf, CSM asymmetry may need to be invoked if the densities inferred from X-ray detections (Margutti & Grefenstette 2024; Zhang et al. 2024) deviate from those inferred from the early-time UV/optical light curve and spectra. Furthermore, it is expected that the dense shell formed from post-shock gas should break up, even for a spherical explosion  \nFigure 6. CSMdensities and radii for a subset of the CMFGEN model grid used in Jacobson-Gal'an et al. (2024). Best-matched models to SN 2024ggi ( § 3.3) are r1w6 (solid red line), m1em2 (dashed limegreen line), r1w4 (solid dark green line) and r1w5r (solid orange line). Blackbody radii between 𝛿𝑡 = 0 . 8 -4 . 7 days shown in light blue shaded region. Transition point where the CSM goes from optically thick to thin to electron scattering shown as magenta star ( § 3.2).  \n<!-- image -->  \nand spherical CSM, which would lead to a much more complicated structure than is obtained when spherical symmetry is imposed.  \nIn addition to direct spectral comparison, we also use the peak absolute magnitudes and rise-times in all UV/optical filters to determine the best-matched models from the CMFGEN grid. Using the residuals between the model predictions and observables, we find that SN 2024ggi is best matched by the r1w4, m1em2, r1w6 and r1w6b models, which have a massloss range of / 𝑀 = ( 0 . 1 -1 ) × 10 -2 M ⊙ yr -1 and radii of dense CSM that ranges from 𝑟 = 4 -8 × 10 14 cm. We note that the m1em2 model (Dessart & Jacobson-Gal'an 2023) does not have a confined shell of dense CSM but rather has a continuous 𝑟 -3 density profile that extends to 10 16 cm (e.g., see Fig. 6). Furthermore, the r1w4 model ( / 𝑀 = 10 -3 M ⊙ yr -1 ) cannot reproduce the spectral evolution of SN 2024ggi because the IIn-like features are not sustained for a long enough timescale. Additionally, matching the observed 𝑡 IIn in SN 2024ggi to the model grid produces only three consistent models: m1em2, r1w6, r1w6a, all with / 𝑀 = 10 -2 M ⊙ yr -1 . As shown in Figure 1, the complete m1em2 and r1w6a model light curves provide a decent match to the multi-band SN 2024ggi light curve at maximum light but cannot capture the steep rise to peak. This inconsistency is likely the result of the CSM density profile immediately beyond 𝑅 ★ and/or the limitations of the simulations at very early phases (e.g., 𝛿𝑡 < 1 day) e.g., light travel time effects which are considered in the radiation hydrodynamics simulations but are not included in the computation of the resulting spectrum.  \nFigure 7. Duration of IIn-like features versus mass-loss rates (left) and CSM densities at 𝑟 = 10 14 cm (right) derived from direct spectral matching for gold/silver- (blue, yellow and red circles) and comparison- (black squares) sample objects presented in Jacobson-Gal'an et al. (2024). Solid colored points represent the subsample of objects at 𝐷 > 40 Mpc. Best-matched mass-loss rate and CSM densities at 𝑟 = 10 14 cm for SN 2024ggi from spectral matching ( § 3.3) shown as magenta star. SNe 1998S and 2023ixf are shown for reference as a green triangle and cyan star, respectively.  \n<!-- image -->  \n/circledot  \nTable 1. Main parameters of SN 2024ggi  \n- a Saha et al. (2006)\n- b Schlegel et al. (1998); Schlafly & Finkbeiner (2011)\n- c Stritzinger et al. (2018)\n- d Mass-loss within 𝑟 < 10 15 cm\n- e Not varied in model grid", '4. DISCUSSION': "Observations of SN 2024ggi for 2 weeks after first light have helped to constrain both the explosion dynamics/physics as well as progenitor activity prior to core-collapse. Specifically, the observed change in ionization present in the ∼ 0 . 8 -1 . 5day spectra (Fig. 5) and the rise in temperature shown by the redto-blue color evolution (Fig. 1) suggests that we caught the SN as the shock broke out of the optically-thick envelope and into the extended, dense, and initially cold CSM. Rather than a prompt rise in temperature to ∼ 10 5 K, the CSM heats up to a lower temperature and becomes progressively more ionized (e.g., He /i.pc//i.pc/i.pc, C /i.pc/i.pc/i.pc//i.pc/v.pc, or N /i.pc/i.pc/i.pc//i.pc/v.pc//v.pc) on a ∼ 1 -2 day timescale as the radiative precursor, and then the radiation from the shock/ejecta, diffuses through the CSM. Furthermore, comparison of the spectral series, 𝑡 IIn, peak luminosities and rise-times to a grid of CMFGEN models indicates that the progenitor of SN 2024ggi was likely a RSG with a massloss rate of / 𝑀 ≈ 10 -2 M ⊙ yr -1 , which created dense CSM extending to 𝑟 ≈ 5 × 10 14 cm that contained a total mass of 𝑀 CSM ≈ 0 . 02 -0 . 04 M ⊙ . This range of CSM masses is consistent with simulations of RSG mass-loss through energy injection within convective RSG envelopes (Tsang et al. 2022), pre-SN outbursts (Takei et al. 2022), and RSG 'boiloff' (Fuller & Tsuna 2024). We also find that the observed light curve can be matched with standard explosion energy (1 . 2 × 10 51 erg) and that the IIn-like signatures in SN 2024ggi can be modeled with a CSM composition that matches typical RSG surface abundances, which suggests that significant N or He enrichment to the CSM may not be required.  \nFor a wind velocity of ∼ 50 kms -1 , the proposed CSM extent translates to a period of enhanced mass-loss in the last ∼ 3 years years prior to core-collapse. Similarly, for the observed 𝑡 IIn and a shock velocity of 𝑣 sh = 12000 kms -1 , the dense CSM would extend to 𝑟 ≈ ( 4 . 3 ± 1 . 7 ) × 10 14 cm and would indicate enhanced mass-loss in the final ∼ 1 . 4 -3 . 6 years before first light, for a progenitor wind velocity of 50 kms -1 . However, it should be noted that this timescale maynotbeaccurate for enhanced wind scenarios (e.g., Davies et al. 2022) because the timescales for wind acceleration to a terminal velocity can take longer than the above pre-SN timescale. From the spectral evolution of SN 2024ggi in its first two weeks, it is clear that a reduction in CSM density did occur at ∼ 3 . 8 days because as the CSM optical depth to electron-scattering drops below unity, the Lorentzian wings of the IIn-like features disappear and absorption profiles from the fastest moving material become visible. However, the open question is whether the CSM detected in SN 2024ggi represents the only high-density shell of CSM (i.e. only one phase of enhanced mass-loss) that extends to 𝑟 < 5 × 10 14 cm (e.g., r1w6 model density profile) or if SN 2024ggi sustained enhanced mass-loss for several decades before explosion (e.g., m1em2 model density profile). As was done for SN 2023ixf, future multi-wavelength monitoring will be able to determine the CSM density at larger scales as the SN shock samples material that was liberated from the progenitor star yearsto-decades before explosion (Grefenstette et al. 2023; Berger et al. 2023; Chandra et al. 2024; Panjkov et al. 2023, Nayana et al., in prep). Overall, both the confined high-density CSM shell and the extended low-density wind may have made the RSG progenitor star quite dust-obscured prior to explosion (Davies et al. 2022), similar to what was revealed by preexplosion imaging of the SN 2023ixf RSG progenitor (Kilpatrick et al. 2023; Jencson et al. 2023; Qin et al. 2023; Soraisam et al. 2023; Van Dyk et al. 2023; Niu et al. 2023).  \nAs shown in Figures 4 & 7, the observables and bestmatched CSM properties of SN 2024ggi reside naturally within the continuum of SNe II, both with and without spectroscopic signatures of interaction with dense CSM. SN 2024ggi's best-matched / 𝑀 and CSM density at 10 14 cm is lower than that inferred for more extreme Class 1 events such as SNe 1998S, 2017ahn, 2018zd, 2020tlf, 2020pni and 2023ixf, all of which likely have higher mass-loss rates, and/or dense CSM that extends to larger radii. Nonetheless, SN 2024ggi's evolution requires a progenitor mass-loss rate that is significantly higher than what is inferred for standard SNe II as well as what is measured in local RSGs (Beasor et al. 2020). Overall, this continues to point towards a phase of enhanced mass-loss in the final years before explosion for some significant fraction of RSGs.", '5. CONCLUSIONS': "In this paper we have presented UV/optical/NIR observations of the nearby SN II, 2024ggi located in nearby spiral host galaxy NGC 3621 at 𝐷 = 7 . 2 Mpc. Below we summarize the primary observational findings of SN 2024ggi:  \n- · The first optical spectrum of SN 2024ggi shows prominent narrow emission lines of H /i.pc, He /i.pc, N /i.pc/i.pc/i.pc and C /i.pc/i.pc/i.pc that result from the photo-ionization of dense, optically thick CSM. Between 𝛿𝑡 = 0 . 8 -1 . 5 days, SN 2024ggi exhibits a detectable rise in temperature and ionization that manifests as the appearance of higher ionization species such as He /i.pc/i.pc, N /i.pc/v.pc//v.pc, C /i.pc/v.pc, and O /v.pc. This spectral evolution is temporally consistent with a dramatic red-to-blue evolution in 𝑤 2 -𝑣 colors and an increasing blackbody temperature. These phenomena suggest that SN 2024ggi was observed during the initial precursor associated with shock breakout inside dense CSM.\n- · SN 2024ggi displayed electron-scattering broadened profiles (i.e., IIn-like) that persist on a characteristic timescale of 𝑡 IIn = 3 . 8 ± 1 . 6 days, after which time a decrease in optical depth to electron scattering due to a reduction in CSM density allows for the detection of broad absorption profiles from the fastest H-rich SN ejecta.\n- · Interaction of the SN 2024ggi ejecta with dense, confined CSM yielded peak UV/optical absolute magnitudes (e.g., 𝑀 w2 = -18 . 7 mag, 𝑀 g = -18 . 1 mag) that are consistent with other SNe II with IIn-like features presented in sample studies (e.g., Jacobson-Gal'an et al. 2024).\n- · Comparison of SN 2024ggi's spectral evolution, peak absolute magnitudes, rise-times and duration of IInlike profiles to a grid of CMFGEN simulations suggests a CSM that has a composition typical of a solarmetallicity RSG, confined to 𝑟 < 5 × 10 14 cm, and that is formed from a progenitor mass-loss rate of / 𝑀 = 10 -2 M ⊙ yr -1 (i.e., 𝜌 ≈ 10 -12 g cm -3 at 𝑟 = 10 14 cm). Adopting a wind velocity of 𝑣 𝑤 = 50 kms -1 , this scenario corresponds to a period of enhanced mass-loss during the last ∼ 3 years years before core-collapse.\n- · SN2024ggiissimilar to SN 2023ixf in its rise in ionization within ∼ days of first light and the high-ionization species present in its early-time spectra, both objects likely observed during shock breakout within dense CSM. However, SN 2024ggi has a shorter timescale of IIn-like features ( ∼ 4 days vs. ∼ 7 days) and a more compact CSM ( < 5 × 10 14 cm vs ( 0 . 5 -1 ) × 10 15 cm) despite a similar mass-loss rate and CSM density at 10 14 cm.  \nLike SN 2023ixf, SN 2024ggi represents a unique opportunity to study the long-term, multi-wavelength evolution of a SN II that interact with dense, confined CSM in exquisite detail. Future studies and multi-wavelength observations will probe the nature and late-stage evolution of the progenitor star as well as uncover the properties of the large-scale circumstellar environment, both of which tracing an unconstrained period of RSG evolution before core-collapse.", '6. ACKNOWLEDGEMENTS': "Research at UC Berkeley is conducted on the territory of Huichin, the ancestral and unceded land of the Chochenyo speaking Ohlone people, the successors of the sovereign Verona Band of Alameda County. Shane 3-m observations were conducted on the land of the Ohlone (Costanoans), Tamyen and Muwekma Ohlone tribes.  \nW.J.-G. is supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. DGE-1842165. W.J.-G. acknowledges support through NASA grants in support of Hubble Space Telescope program GO-16075 and 16500. This research was supported in part by the National Science Foundation under Grant No. NSF PHY-1748958. R.M. acknowledges support by the National Science Foundation under Award No. AST-2221789 and AST-2224255. The Margutti team at UC Berkeley is partially funded by the Heising-Simons Foundation under grant # 2018-0911 and #2021-3248 (PI: Margutti).  \nMRD acknowledges support from the NSERC through grant RGPIN-2019-06186, the Canada Research Chairs Program, and the Dunlap Institute at the University of Toronto. Parts of this research were supported by the Australian Research Council Discovery Early Career Researcher Award (DECRA) through project number DE230101069. Y.-C.P. is supported by the National Science and Technology Council (NSTC grant 109-2112-M-008-031-MY3. CG is supported by a VILLUM FONDEN Young Investigator Grant (project number 25501). D.M. acknowledges NSF support from grants PHY-2209451 and AST-2206532. A.S. acknowledges the financial support from CNPq (402577/2022-1). G.D. is supported by the H2020 European Research Council grant no.758638. CRB acknowledges the financial support from CNPq (316072/2021-4) and from FAPERJ (grants 201.456/2022 and 210.330/2022) and the FINEP contract 01.22.0505.00 (ref. 1891/22). This research is based on observations made with the Thai Robotic Telescope under program ID TRTC11B006, which is operated by the National Astronomical Research Institute of Thailand (Public Organization). Part of the data were obtained with the REM telescope, located in Chile and operated by the d'REM team for INAF. This work is supported by the National Science Foundation under Cooperative Agreement PHY-2019786 (The NSF AI  \nInstitute for Artificial Intelligence and Fundamental Interactions, http://iaifi.org/).  \nThe UCSC team is supported in part by NASA grants NNG17PX03C and 80NSSC22K1518, NSF grant AST1815935, and by a fellowship from the David and Lucile Packard Foundation to R.J.F.  \nYSE-PZ (Coulter et al. 2023) was developed by the UC Santa Cruz Transients Team with support from NASA grants NNG17PX03C, 80NSSC19K1386, and 80NSSC20K0953; NSFgrantsAST-1518052, AST-1815935, and AST-1911206; the Gordon & Betty Moore Foundation; the Heising-Simons Foundation; a fellowship from the David and Lucile Packard Foundation to R.J.F.; Gordon and Betty Moore Foundation postdoctoral fellowships and a NASA Einstein fellowship, as administered through the NASA Hubble Fellowship program and grant HST-HF2-51462.001, to D.O.J.; and a National Science Foundation Graduate Research Fellowship, administered through grant No. DGE-1339067, to D.A.C.  \nAmajor upgrade of the Kast spectrograph on the Shane 3 m telescope at Lick Observatory, led by Brad Holden, was made possible through generous gifts from the Heising-Simons Foundation, William and Marina Kast, and the University of California Observatories. Research at Lick Observatory is partially supported by a generous gift from Google.  \nThis work has made use of data from the Asteroid Terrestrial-impact Last Alert System (ATLAS) project. The Asteroid Terrestrial-impact Last Alert System (ATLAS) project is primarily funded to search for near earth asteroids through NASA grants NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575; byproducts of the NEO search include images and catalogs from the survey area. This work was partially funded by Kepler/K2 grant J1944/80NSSC19K0112 and HST GO-15889, and STFC grants ST/T000198/1 and ST/S006109/1. TheATLASscienceproductshavebeenmade possible through the contributions of the University of Hawaii Institute for Astronomy, the Queen's University Belfast, the Space Telescope Science Institute, the South African Astronomical Observatory, and The Millennium Institute of Astrophysics (MAS), Chile.  \nThis research was supported by the Munich Institute for Astro-, Particle and BioPhysics (MIAPbP) which is funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany's Excellence Strategy EXC-2094 - 390783311.  \nIRAFis distributed by NOAO, which is operated by AURA, Inc., under cooperative agreement with the National Science Foundation (NSF).  \nFacilities: ATLAS, Swift UVOT, Gemini-South Observatory (GMOS), SOAR (Goodman/TSpec), Shane Telescope (Kast), Thailand Robotic Telescope, Las Cumbres Observatory, Lulin Telescope, Cerro Tololo Inter-american Observatory, Rapid Eye Mount Telescope  \nSoftware: IRAF (Tody 1986, Tody 1993), photpipe (Rest et al. 2005), DoPhot (Schechter et al. 1993), HOTPANTS (Becker 2015), YSE-PZ (Coulter et al. 2022, 2023), CMFGEN  \n(Hillier & Dessart 2012; Dessart et al. 2015), HERACLES (Gonz'alez et al. 2007; Vaytet et al. 2011; Dessart et al. 2015), HEAsoft (v6.33; HEASARC 2014), DRAGONS (Labrie et al. 2023)", 'APPENDIX': 'Here we present a log of optical spectroscopic observations of SN 2024ggi in Table A1.  \nTable A1. Optical Spectroscopy of SN 2024ggi'}