Abstract We present high-cadence photometric and spectroscopic observations of supernova (SN) 2024ggi, a Type II SN with flash spectroscopy features, which exploded in the nearby galaxy NGC 3621 at ∼7 Mpc. The light-curve evolution over the first 30 hr can be fit by two power-law indices with a break after 22 hr, rising from M V ≈ −12.95 mag at +0.66 day to M V ≈ −17.91 mag after 7 days. In addition, the densely sampled color curve shows a strong blueward evolution over the first few days and then behaves as a normal SN II with a redward evolution as the ejecta cool. Such deviations could be due to interaction with circumstellar material (CSM). Early high- and low-resolution spectra clearly show high-ionization flash features from the first spectrum to +3.42 days after the explosion. From the high-resolution spectra, we calculate the CSM velocity to be 37 ± 4 km s −1 . We also see the line strength evolve rapidly from 1.22 to 1.49 days in the earliest high-resolution spectra. Comparison of the low-resolution spectra with CMFGEN models suggests that the pre-explosion mass-loss rate of SN 2024ggi falls in the range of 10 −3 –10 −2 M ☉ yr −1 , which is similar to that derived for SN 2023ixf. However, the rapid temporal evolution of the narrow lines in the spectra of SN 2024ggi ( R CSM ∼ 2.7 × 10 14 cm) could indicate a smaller spatial extent of the CSM than in SN 2023ixf ( R CSM ∼ 5.4 × 10 14 cm), which in turn implies a lower total CSM mass for SN 2024ggi. 

Extended Shock Breakout and Early Circumstellar Interaction in SN 2024ggi

Abstract The Bright Transient Survey (BTS) aims to obtain a classification spectrum for all bright ( m peak ≤ 18.5 mag) extragalactic transients found in the Zwicky Transient Facility (ZTF) public survey. BTS critically relies on visual inspection (“scanning”) to select targets for spectroscopic follow-up, which, while effective, has required a significant time investment over the past ∼5 yr of ZTF operations. We present BTSbot , a multimodal convolutional neural network, which provides a bright transient score to individual ZTF detections using their image data and 25 extracted features. BTSbot is able to eliminate the need for daily human scanning by automatically identifying and requesting spectroscopic follow-up observations of new bright transient candidates. BTSbot recovers all bright transients in our test split and performs on par with scanners in terms of identification speed (on average, ∼1 hr quicker than scanners). We also find that BTSbot is not significantly impacted by any data shift by comparing performance across a concealed test split and a sample of very recent BTS candidates. BTSbot has been integrated into Fritz and Kowalski , ZTF’s first-party marshal and alert broker, and now sends automatic spectroscopic follow-up requests for the new transients it identifies. Between 2023 December and 2024 May, BTSbot selected 609 sources in real time, 96% of which were real extragalactic transients. With BTSbot and other automation tools, the BTS workflow has produced the first fully automatic end-to-end discovery and classification of a transient, representing a significant reduction in the human time needed to scan. 

The Zwicky Transient Facility Bright Transient Survey. III. BTSbot: Automated Identification and Follow-up of Bright Transients with Deep Learning

Abstract SN 2023ixf, recently reported in the nearby galaxy M101 at a distance of 6.85 Mpc, was one of the closest and brightest core-collapse supernovae in the last decade. In this work, we present multiwavelength photometric observation of SN 2023ixf with the Multi-channel Photometric Survey Telescope (Mephisto) in the uvgr bands and with the twin 50 cm telescopes in the griz bands. We find that the bolometric luminosity reached the maximum value of 3 × 10 43 erg s −1 at 3.9 days after the explosion and fully settled onto the radioactive tail at ∼90 days. The effective temperature decreased from 3.2 × 10 4 K at the first observation and approached a constant of ∼(3000–4000) K after the first 2 months. The evolution of the photospheric radius is consistent with a homologous expansion with a velocity of 8700 km s −1 in the first 2 months, and it shrunk subsequently. Based on the radioactive tail, the initial nickel mass is about M Ni ∼ 0.098 M ⊙ . The explosion energy and the ejecta mass are estimated to be E ≃ (1.0–5.7) × 10 51 erg and M ej ≃ (3.8–16) M ⊙ , respectively. The peak bolometric luminosity is proposed to be contributed by the interaction between the ejecta and the circumstellar medium (CSM). We find a shocked CSM mass of M CSM ∼ 0.013 M ⊙ , a CSM density of ρ CSM ∼ 2.5 × 10 −13 g cm −3 , and a mass-loss rate of the progenitor of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mi>M</mml:mi> <mml:mo>̇</mml:mo> </mml:mover> <mml:mo>∼</mml:mo> <mml:mn>0.022</mml:mn> <mml:mspace width="0.25em"/> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>⊙</mml:mo> </mml:msub> <mml:mspace width="0.25em"/> <mml:msup> <mml:mi>yr</mml:mi> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:math> . 

Multiband Simultaneous Photometry of Type II SN 2023ixf with Mephisto and the Twin 50 cm Telescopes

Abstract We present UV–optical–near-infrared 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 , He i , C iii , and N iii with a narrow core and broad, symmetric wings (i.e., “IIn-like”) 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 iv/v , and O v became visible. This phenomenon is temporally consistent with a blueward shift in the UV–optical colors, both likely the result of SBO 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, respectively, 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 radiative-transfer simulations suggests a progenitor mass-loss rate of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> <mml:mo>=</mml:mo> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> <mml:mspace width="0.25em"/> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> </mml:math> yr −1 ( v w = 50 km s −1 ), confined to a distance of r &lt; 5 × 10 14 cm. Assuming a wind velocity of v w = 50 km s −1 , the progenitor star underwent an enhanced mass-loss episode in the last ∼3 yr before explosion. 

SN 2024ggi in NGC 3621: Rising Ionization in a Nearby, Circumstellar-material-interacting Type II Supernova

Early-phase Simultaneous Multiband Observations of the Type II Supernova SN 2024ggi with Mephisto

In the Halo21 absorption modeling challenge we generated synthetic absorption spectra of the circumgalactic medium (CGM), and attempted to estimate the metallicity, temperature, and density (Z, T, and nH) of the underlying gas using observational methods. We iteratively generated and analyzed three increasingly-complex data samples: ion column densities of isolated uniform clouds, mock spectra of 1--3 uniform clouds, and mock spectra of high-resolution turbulent mixing zones. We found that the observational estimates were accurate for both uniform cloud samples, with Z, T, and nH retrieved within 0.1 dex of the source value for>90% of absorption systems. In the turbulent-mixing scenario, the mass, temperature, and metallicity of the strongest absorption components were also retrieved with high accuracy. However, the underlying properties of the subdominant components were poorly constrained because the corresponding simulated gas contributed only weakly to the H I absorption profiles. On the other hand, including additional components beyond the dominant ones did improve the fit, consistent with the true existence of complex cloud structures in the source data.

pdf/the-halo21-absorption-modeling-challenge-lessons-from-1k7nbr0r.pdf

The Halo21 Absorption Modeling Challenge: Lessons From"Observing"Synthetic Circumgalactic Absorption Spectra

The early evolution of a supernova (SN) can reveal information about the environment and the progenitor star. When a star explodes in vacuum, the first photons to escape from its surface appear as a brief, hours-long shock-breakout flare1,2, followed by a cooling phase of emission. However, for stars exploding within a distribution of dense, optically thick circumstellar material (CSM), the first photons escape from the material beyond the stellar edge and the duration of the initial flare can extend to several days, during which the escaping emission indicates photospheric heating3. Early serendipitous observations2,4 that lacked ultraviolet (UV) data were unable to determine whether the early emission is heating or cooling and hence the nature of the early explosion event. Here we report UV spectra of the nearby SN 2023ixf in the galaxy Messier 101 (M101). Using the UV data as well as a comprehensive set of further multiwavelength observations, we temporally resolve the emergence of the explosion shock from a thick medium heated by the SN emission. We derive a reliable bolometric light curve that indicates that the shock breaks out from a dense layer with a radius substantially larger than typical supergiants. Using ultraviolet data as well as a comprehensive set of further multiwavelength observations of the supernova 2023ixf, a reliable bolometric light curve is derived that indicates the heating nature of the early emission.

The complex circumstellar environment of supernova 2023ixf.

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The Halo21 Absorption Modeling Challenge: Lessons From"Observing"Synthetic Circumgalactic Absorption Spectra

The complex circumstellar environment of supernova 2023ixf.