arXiv subjects

D. Andrew Howell

Publications and source records attributed to D. Andrew Howell.

At least 73 records · Page 4Linked to original sources

arXiv2025

Rapid follow-up observations of infant supernovae with the Gran Telescopio Canarias

The first few hours of a supernova (SN) contain significant information about the progenitor system. The most modern wide-field surveys that scan the sky repeatedly every few days can discover all kinds of transients in those early epochs. At such times, some progenitor footprints may be visible, elucidating critical explosion parameters and helping to distinguish between leading explosion models. A dedicated spectroscopic classification programme using the optical spectrograph OSIRIS mounted on the Gran Telescopio Canarias was set up to try to obtain observations of supernovae (SNe) at those early epochs. With the time awarded, we obtained spectra of 10 SN candidates, which we present here. Half of them were thermonuclear SNe, while the other half were core-collapse SNe. Most (70\%) were observed within the first six days of the estimated explosion, with two being captured within the first 48\,hr. We present a characterization of the spectra, together with other public ancillary photometry from the Zwicky Transient Facility (ZTF) and the Asteroid Terrestrial-impact Last Alert System (ATLAS). This project shows the need for an accompanying rapid-response spectroscopic programme for existing and future deep photometric wide-field surveys located at the right longitude to be able to trigger observations in a few hours after the discovery of the SN candidate.

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arXiv2025

Progenitor Insights of Type IIP SN 2018pq: A Comprehensive Photometric and Spectroscopic Study

We present high-cadence photometric and low-resolution (R $\sim$ 400--700) optical spectroscopic observations of Type IIP supernova, SN~2018pq, which exploded on the outskirts of the galaxy IC~3896A. The optically thick phase (``plateau'') lasts approximately 97 d, the plateau duration of normal Type IIP supernovae. SN~2018pq has a {\em V}-band absolute magnitude of $-16.42 \pm 0.01$ mag at 50 d, resembles normal-luminous supernova, and the V-band decline rate of 0.42$\pm$0.06 mag 50 d$^{-1}$ during the plateau phase. A steeper decline rate of 11.87$\pm$1.68 mag 100 d$^{-1}$ was observed compared to that of typical Type IIP supernovae during the transition between plateau to nebular phase. We employ detailed radiative transfer spectra modelling, TARDIS, to reveal the photospheric temperature and velocity at two spectral epochs. The well-fitted model spectra indicate SN~2018pq is a spectroscopically normal Type IIP supernova. Semi-analytical light curve modelling suggests the progenitor as a red supergiant star with an ejecta mass of $\sim$11 $M_\odot$ and an initial radius of 424 $R_\odot$. On the contrary, hydrodynamical modelling suggests a higher mass progenitor between 14--16 $M_\odot$.

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arXiv2025

Massive stars exploding in a He-rich circumstellar medium. XI. Diverse evolution of five Ibn SNe 2020nxt, 2020taz, 2021bbv, 2023utc and 2024aej

We present the photometric and spectroscopic analysis of five Type Ibn supernovae (SNe): SN 2020nxt, SN 2020taz, SN 2021bbv, SN 2023utc, and SN 2024aej. These events share key observational features and belong to a family of objects similar to the prototypical Type Ibn SN 2006jc. The SNe exhibit rise times of approximately 10 days and peak absolute magnitudes ranging from $-$16.5 to $-$19 mag. Notably, SN 2023utc is the faintest Type Ibn supernova discovered to date, with an exceptionally low r-band absolute magnitude of $-16.4$ mag. The pseudo-bolometric light curves peak at $(1-10) \times 10^{42}$ erg s$^{-1}$, with total radiated energies on the order of $(1-10) \times 10^{48}$ erg. Spectroscopically, these SNe display relatively slow spectral evolution; the early spectra are characterised by a hot blue continuum and prominent He I emission lines. Early spectra show blackbody temperatures exceeding $10000~\mathrm{K}$, with a subsequent decline in temperature during later phases. Narrow He I lines, indicative of unshocked circumstellar material (CSM), show velocities of approximately $1000~\mathrm{km~s^{-1}}$. The spectra suggest that the progenitors of these SNe underwent significant mass loss prior to the explosion, resulting in a He-rich CSM. Light curve modelling yields estimates for the ejecta mass ($M_{\rm ej}$) in the range $1-3~M_{\odot}$, with kinetic energies ($E_{\rm Kin}$) of $(0.1-1) \times 10^{50}$ erg. The inferred CSM mass ranges from $0.2$ to $1~M_{\odot}$. These findings are consistent with expectations for core-collapse events arising from relatively massive, envelope-stripped progenitors.

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arXiv2025

A Detection of Helium in the Bright Superluminous Supernova SN 2024rmj

We present extensive ultraviolet (UV), optical, and near-infrared (NIR) photometric and spectroscopic observations of the nearby hydrogen-poor superluminous supernova (SLSN-I) SN2024rmj at z = 0.1189. SN 2024rmj reached a peak absolute magnitude of Mg $\approx$ -21.9, placing it at the luminous end of the SLSN-I distribution. The light curve exhibits a pronounced pre-peak bump ($\approx$ 60 d before the main peak) and a post-peak bump ($\approx$ 55 d after the main peak). The bulk of the light curve is otherwise well fit by a magnetar spin-down model, with typical values (spin: $\approx$ 2.1 ms; magnetic field: $\approx$ 6 $\times$ 10$^{13}$ G; ejecta mass: $\approx$ 12 M$_\odot$). The optical spectra exhibit characteristic SLSN-I features and evolution, but with a relatively high velocity of $\approx$ 8,000 km s$^{-1}$ post-peak. Most significantly, we find a clear detection of helium in the NIR spectra at He I $λ$1.083 $μ$m and $λ$2.058 $μ$m, blueshifted by $\approx$ 15,000 km s$^{-1}$ (13 d before peak) and $\approx$ 13,000 km s$^{-1}$ (40 d after peak), indicating that helium is confined to the outermost ejecta; based on these NIR detections, we also identify likely contribution from He I $λ$5876 Å in the optical spectra on a similar range of timescales. This represents the most definitive detection of helium in a bright SLSN-I to date, and indicates that progenitors with a thin helium layer can still explode as SLSNe.

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arXiv2025

The Double Tidal Disruption Event AT 2022dbl Implies That at Least Some "Standard" Optical TDEs are Partial Disruptions

Flares produced following the tidal disruption of stars by supermassive black holes can reveal the properties of the otherwise dormant majority of black holes and the physics of accretion. In the past decade, a class of optical-ultraviolet tidal disruption flares has been discovered whose emission properties do not match theoretical predictions. This has led to extensive efforts to model the dynamics and emission mechanisms of optical-ultraviolet tidal disruptions in order to establish them as probes of supermassive black holes. Here we present the optical-ultraviolet tidal disruption event AT 2022dbl, which showed a nearly identical repetition 700 days after the first flare. Ruling out gravitational lensing and two chance unrelated disruptions, we conclude that at least the first flare represents the partial disruption of a star, possibly captured through the Hills mechanism. Since both flares are typical of the optical-ultraviolet class of tidal disruptions in terms of their radiated energy, temperature, luminosity, and spectral features, it follows that either the entire class are partial rather than full stellar disruptions, contrary to the prevalent assumption, or that some members of the class are partial disruptions, having nearly the same observational characteristics as full disruptions. Whichever option is true, these findings could require revised models for the emission mechanisms of optical-ultraviolet tidal disruption flares and a reassessment of their expected rates.

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arXiv2025

SN 2021hpr: A Normal Type Ia Supernova Showing Excess Emission in the Early Rising Phase

We present extensive optical observations of a nearby Type Ia supernova (SN Ia), SN 2021hpr, located in the spiral galaxy NGC 3147 at a distance of $\sim$ 45 Mpc. Our observations cover a phase within $\sim 1-2$ days to $\sim 290$ days after the explosion. SN 2021hpr is found to be a spectroscopically normal SN Ia, with an absolute B-band peak magnitude of $M_{max}(B) \approx -19.16 \pm 0.14$ mag and a post-peak decline rate of $Δm_{15}(B)= 1.00 \pm 0.01 $ mag. Early-time light curves showed a $\sim 7.0 \%$ excess emission compared to a homogeneously expanding fireball model, likely due to SN ejecta interacting with a companion or immediate circumstellar matter. The optical spectra of SN 2021hpr are overall similar to those of normal SNe Ia, but characterized by prominent detached high-velocity features (HVFs) of Si {\sc ii} and Ca {\sc ii} in the early phase. After examining a small sample of well-observed normal SNe Ia, we find that the HVFs are likely common for the subgroup with early-excess emission. The association of early bump feature with the HVFs could be attributed to density or abundance enhancement at the outer layer of the exploding star, likely as a result of interactions with companion$/$CSM or experiencing more complete burning. Nevertheless, the redshifted Fe {\sc ii} and Ni {\sc ii} lines in the nebular-phase spectra of SN 2021hpr, contrary to the blueshift trend seen in other SNe Ia showing early bump features, indicate its peculiarity in the explosion that remains to be understood.

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arXiv2025

Early Shock-Cooling Observations and Progenitor Constraints of Type IIb SN 2024uwq

We present early multi-wavelength photometric and spectroscopic observations of the Type IIb supernova SN 2024uwq, capturing its shock-cooling emission phase and double-peaked light curve evolution. Early spectra reveal broad H-alpha (v ~ 15,500 km s$^{-1}$) and He I P-Cygni profiles of similar strengths. Over time the He I lines increase in strength while the H-alpha decreases, consistent with a hydrogen envelope ($M_{env}$ = 0.7 - 1.35 $M_\odot$ ) overlying helium-rich ejecta. Analytic modeling of early shock cooling emission and bolometric light analysis constrains the progenitor to a partially stripped star with radius R = 10 - 60 $R_\odot$, consistent with a blue/yellow supergiant with an initial ZAMS mass of 12 - 20 $M_\odot$ , likely stripped via binary interaction. SN 2024uwq occupies a transitional position between compact and extended Type IIb supernovae, highlighting the role of binary mass-transfer efficiency in shaping a continuum of stripped-envelope progenitors. Our results underscore the importance of both early UV/optical observations to characterize shock breakout signatures critical to map the diversity in evolutionary pathways of massive stars. Upcoming time domain surveys including Rubin Observatory's LSST and UV missions like ULTRASAT and UVEX will revolutionise our ability to systematically capture these early signatures, probing the full diversity of stripped progenitors and their explosive endpoints.

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arXiv2025

AT 2019aalc: a Bowen Fluorescence Flare With a Precursor Flare in an Active Galactic Nucleus

AT 2019aalc is a peculiar sequence of highly variable emission events observed towards the nucleus of the broad-line AGN SDSS J152416.66+045119.0. The system exhibited two distinct UV/optical flares (the first detected in 2019, the second one in 2023). Spectra obtained following the detection of the second flare revealed prominent Bowen fluorescence and high-ionization coronal emission lines, which were much weaker, if at all detectable, in a spectrum taken following the first flare. We present and analyze a large set of multi-wavelength, multi-epoch data for this source, with particular emphasis on optical spectroscopic monitoring conducted with the Las Cumbres Observatory network. During the relatively slow dimming that followed the second optical flare, the UV/optical light-curve shows a sequence of minor rebrightening events, while the Bowen fluorescence and the coronal lines vary (roughly) in tandem with these "bumps" in the broad-band light-curve. Most of the observed behavior of AT 2019aalc links it to the growing class of Bowen fluorescence flares (BFFs) while setting it apart from canonical tidal disruption events. However, AT 2019aalc has some outstanding peculiarities, including two short flares seen in its soft X-ray light-curve during the dimming phase of the second optical flare, and which do not seem to be linked to the emission line variations. We discuss the optical and X-ray properties of the source and possible scenarios of the origin of the flare, in particular radiation pressure instabilities in the (pre-existing) AGN accretion disk.

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arXiv2025

Supernovae at Distances < 40 Mpc: II. Supernova Rate in the Local Universe

Context.This is the second paper of a series aiming to determine the birth rates of supernovae in the local Universe. Aims. In this paper, we aim to estimate the SN rates in the local universe and fit the delay-time distribution of SNe Ia to put constraints on their progenitor scenarios. Methods.We performed a Monte-Carlo simulation to estimate the volumetric rates with the nearby SN sample introduced in Paper I of the series. The rate evolution of core-collapse SNe well traces the evolution of cosmic star formation history; while that of SNe Ia involves the convolution of cosmic star-formation history and a two-component delay-time distribution including a power law and a Gaussian component. Results.The volumetric rates of type Ia, Ibc and II SNe are derived as $0.325\pm0.040^{+0.016}_{-0.010}$, $0.160\pm0.028^{+0.044}_{-0.014}$, and $0.528\pm0.051^{+0.162}_{-0.013}$ (in unit of $10^{-4} yr^{-1} Mpc^{-3} h^3_{70}$), respectively. The rate of CCSNe is consistent with previous estimates. The newly derived local SN Ia rate is larger than existing results given at redshifts 0.01 < z < 0.1, favoring an increased rate from the universe at z ~ 0.1 to the local universe. A two-component model can well fit the rate variation, with the power law component accounting for the rate evolution at larger redshifts and the Gaussian component with a delay time of 12.63$\pm$0.38 Gyr accounting for the local rate evolution. This delayed component with such a longer delay time suggests that the progenitors of these SNe Ia were formed at around 1 Gyr after the birth of the universe, which could only be explained by a double-degenerate progenitor scenario. This is evidenced by the comparison with the PTF sample of SNe Ia at z = 0.073, which reveals that the increase in SN Ia rate at z < 0.01 is primarily due to the SNe Ia of massive E and S0 galaxies with old stellar populations.

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arXiv2025

Supernovae at Distances < 40 Mpc: I.Catalogues and fractions of Supernovae in a Complete Sample

Context.This is the first paper of a series aiming to determine the fractions and birth rates of various types of supernovae (SNe) in the local Universe. Aims. In this paper, we aim to construct a complete sample of SNe in the nearby universe and provide more precise measurement of subtype fractions. Methods.We carefully selected our SN sample at a distance of < 40 Mpc mainly from wide-field surveys conducted over the years from 2016 to 2023. Results.The sample contains a total of 211 SNe, including 109 SNe II, 69 SNe Ia, and 33 SNe Ibc. With the aid of sufficient spectra, we can obtain relatively accurate subtype classifications for all SNe in this sample. After corrections for the Malmquist bias, this volume-limited sample gives fractions of SNe Ia, SNe Ibc, and SNe II as $30.4^{+3.7}_{-11.5}\%$, $16.3^{+3.7}_{-7.4}\%$, and $53.3^{+9.5}_{-18.7}\%$, respectively.In the SN Ia sample, the fraction of the 91T-like subtype becomes relatively low (~5.4\%), while that of the 02cx-like subtype shows a moderate increase (~6.8\%). In the SN Ibc sample, we find significant fractions of broadlined SNe Ic (~18.0\%) and SNe Ibn (~8.8\%). The fraction of 87A-like subtype is determined as ~2.3\% for the first time, indicating rare explosions from blue supergiant stars. We find that SNe Ia show a double peak number distribution in S0- and Sc-type host galaxies, which may serve as a straightforward evidence for the presence of "prompt" and "delayed" progenitor components giving rise to SN Ia explosions. Several subtypes of SNe such as 02cx-like SNe Ia, broadlined SNe Ic, SNe IIn (and perhaps SNe Ibn) are found to occur preferentially in less massive spiral galaxies, favoring their associations with young stellar progenitors. Moreover, the 02cx-like subtype shows a trend of exploding in the outer skirt of their hosts, suggestive of metal-poor progenitors.

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arXiv2025

A Multiwavelength Autopsy of the Interacting IIn Supernova 2020ywx: Tracing its Progenitor Mass-Loss History for 100 Years before Death

While the subclass of interacting supernovae with narrow hydrogen emission lines (SNe IIn) consists of some of the longest-lasting and brightest SNe ever discovered, their progenitors are still not well understood. Investigating SNe IIn as they emit across the electromagnetic spectrum is the most robust way to understand the progenitor evolution before the explosion. This work presents X-Ray, optical, infrared, and radio observations of the strongly interacting Type IIn SN 2020ywx covering a period $>1200$ days after discovery. Through multiwavelength modeling, we find that the progenitor of 2020ywx was losing mass at $\sim10^{-2}$--$10^{-3} \mathrm{\,M_{\odot}\,yr^{-1}}$ for at least 100 yrs pre-explosion using the circumstellar medium (CSM) speed of 120 km/s measured from our optical and NIR spectra. Despite the similar magnitude of mass loss measured in different wavelength ranges, we find discrepancies between the X-ray and optical/radio-derived mass-loss evolution, which suggest asymmetries in the CSM. Furthermore, we find evidence for dust formation due to the combination of a growing blueshift in optical emission lines and near-infrared continuum emission which we fit with blackbodies at $\sim$ 1000 K. Based on the observed elevated mass loss over more than 100 years and the configuration of the CSM inferred from the multiwavelength observations, we invoke binary interaction as the most plausible mechanism to explain the overall mass-loss evolution. SN 2020ywx is thus a case that may support the growing observational consensus that SNe IIn mass loss is explained by binary interaction.

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arXiv2025

Markarian 590: The AGN Awakens

Changing-Look AGN (CLAGN) Mkn 590 recently underwent a sudden re-ignition, marked by substantial increases in optical/UV and X-ray continuum flux since last couple of years. Swift-XRT observations revealed the re-emergence of a soft X-ray excess (SXE) as the source transitioned from a low-flux state in July 2023 to a significantly higher flux state in October 2024. This evolution was in response to an order-of-magnitude increase in extreme-UV (EUV) continuum emission, detected by Swift-UVOT. Follow-up optical spectra from FLOYDS Faulkes confirmed the enhancement of dynamically broadened Balmer lines, He II emission, and Fe II complex. As the Eddington fraction increased by a factor of $\sim$ 20 over the last 20 months, we found clear evidence of formation of a warm corona, strongly linked to the cold accretion disc underneath. Based on our multi-wavelength study on recent data, we propose that Mkn 590 is currently becoming a Seyfert-1.2, similar to its state in 1990s.

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arXiv2025

EP240801a/XRF 240801B: An X-ray Flash Detected by the Einstein Probe and Implications of its Multiband Afterglow

We present multiband observations and analysis of EP240801a, a low-energy, extremely soft gamma-ray burst (GRB) discovered on August 1, 2024 by the Einstein Probe (EP) satellite, with a weak contemporaneous signal also detected by Fermi/GBM. Optical spectroscopy of the afterglow, obtained by GTC and Keck, identified the redshift of $z = 1.6734$. EP240801a exhibits a burst duration of 148 s in X-rays and 22.3 s in gamma-rays, with X-rays leading by 80.61 s. Spectral lag analysis indicates the gamma-ray signal arrived 8.3 s earlier than the X-rays. Joint spectral fitting of EP/WXT and Fermi/GBM data yields an isotropic energy $E_{γ,\rm{iso}} = (5.57^{+0.54}_{-0.50})\times 10^{51}\,\rm{erg}$, a peak energy $E_{\rm{peak}} = 14.90^{+7.08}_{-4.71}\,\rm{keV}$, a fluence ratio $\rm S(25-50\,\rm{keV})/S(50-100\,\rm{keV}) = 1.67^{+0.74}_{-0.46}$, classifying EP240801a as an X-ray flash (XRF). The host-galaxy continuum spectrum, inferred using Prospector, was used to correct its contribution for the observed outburst optical data. Unusual early $R$-band behavior and EP/FXT observations suggest multiple components in the afterglow. Three models are considered: two-component jet model, forward-reverse shock model and forward-shock model with energy injection. Both three provide reasonable explanations. The two-component jet model and the energy injection model imply a relatively small initial energy and velocity of the jet in the line of sight, while the forward-reverse shock model remains typical. Under the two-component jet model, EP240801a may resemble GRB 221009A (BOAT) if the bright narrow beam is viewed on-axis. Therefore, EP240801a can be interpreted as an off-beam (narrow) jet or an intrinsically weak GRB jet. Our findings provide crucial clues for uncovering the origin of XRFs.

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arXiv2025

Spectropolarimetry of SN 2023ixf reveals both circumstellar material and helium core to be aspherical

We present multi-epoch optical spectropolarimetric and imaging polarimetric observations of the nearby Type II supernova (SN) 2023ixf discovered in M101 at a distance of 6.85 Mpc. The first imaging polarimetric observations were taken +2.33 days (60085.08 MJD) after the explosion, while the last imaging polarimetric data points (+73.19 and +76.19 days) were acquired after the fall from the light curve plateau. At +2.33 days there is strong evidence of circumstellar material (CSM) interaction in the spectra and the light curve. A significant level of intrinsic polarization $p_r = 1.02\pm 0.07 \% $ is seen during this phase which indicates that this CSM is aspherical. We find that the polarization evolves with time toward the interstellar polarization level during the photospheric phase, which suggests that the recombination photosphere is spherically symmetric. There is a jump in polarization ($p_r =0.45 \pm 0.08 \% $ and $p_r =0.62 \pm 0.08 \% $) at +73.19 and +76.19 days when the light curve falls from the plateau. This is a phase where polarimetric data is sensitive to non-spherical inner ejecta or a decrease in optical depth into the single scattering regime. We also present spectropolarimetric data that reveal line (de)polarization during most of the observed epochs. In addition, at +14.50 days we see an ``inverse P Cygni" profile in the H and He line polarization, which clearly indicates the presence of asymmetrically distributed material overlying the photosphere. The overall temporal evolution of polarization is typical for Type II SNe, but the high level of polarization during the rising phase has only been observed in SN 2023ixf.

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arXiv2025

The origin and evolution of the normal Type Ia SN 2018aoz with infant-phase reddening and excess emission

SN~2018aoz is a Type Ia SN with a $B$-band plateau and excess emission in the infant-phase light curves $\lesssim$ 1 day after first light, evidencing an over-density of surface iron-peak elements as shown in our previous study. Here, we advance the constraints on the nature and origin of SN~2018aoz based on its evolution until the nebular phase. Near-peak spectroscopic features show the SN is intermediate between two subtypes of normal Type Ia: Core-Normal and Broad-Line. The excess emission could have contributions from the radioactive decay of surface iron-peak elements as well as ejecta interaction with either the binary companion or a small torus of circumstellar material. Nebular-phase limits on H$α$ and He~I favour a white dwarf companion, consistent with the small companion size constrained by the low early SN luminosity, while the absence of [O~I] and He~I disfavours a violent merger of the progenitor. Of the two main explosion mechanisms proposed to explain the distribution of surface iron-peak elements in SN~2018aoz, the asymmetric Chandrasekhar-mass explosion is less consistent with the progenitor constraints and the observed blueshifts of nebular-phase [Fe~II] and [Ni~II]. The helium-shell double-detonation explosion is compatible with the observed lack of C spectral features, but current 1-D models are incompatible with the infant-phase excess emission, $B_{\rm max}-V_{\rm max}$ color, and absence of nebular-phase [Ca~II]. Although the explosion processes of SN~2018aoz still need to be more precisely understood, the same processes could produce a significant fraction of Type Ia SNe that appear normal after $\sim$ 1 day.

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arXiv2025

SN 2021foa: the bridge between SN IIn and Ibn

We present the long-term photometric and spectroscopic analysis of a transitioning SN~IIn/Ibn from $-$10.8 d to 150.7 d post $V$-band maximum. SN~2021foa shows prominent He {\sc i} lines comparable in strength to the H$α$ line around peak, placing SN~2021foa between the SN~IIn and SN~Ibn populations. The spectral comparison shows that it resembles the SN~IIn population at pre-maximum, becomes intermediate between SNe~IIn/Ibn and at post-maximum matches with SN~IIn 1996al. The photometric evolution shows a precursor at $-$50 d and a light curve shoulder around 17d. The peak luminosity and color evolution of SN 2021foa are consistent with most SNe~IIn and Ibn in our comparison sample. SN~2021foa shows the unique case of a SN~IIn where the narrow P-Cygni in H$α$ becomes prominent at 7.2 days. The H$α$ profile consists of a narrow (500 -- 1200 km s$^{-1}$) component, intermediate width (3000 -- 8000 km s$^{-1}$) and broad component in absorption. Temporal evolution of the H$α$ profile favours a disk-like CSM geometry. Hydrodynamical modelling of the lightcurve well reproduces a two-component CSM structure with different densities ($ρ$ $\propto$ r$^{-2}$ -- $ρ$ $\propto$ r$^{-5}$), mass-loss rates (10$^{-3}$ -- 10$^{-1}$ M$_{\odot}$ yr$^{-1}$) assuming a wind velocity of 1000 km s$^{-1}$ and having a CSM mass of 0.18 M$_{\odot}$. The overall evolution indicates that SN~2021foa most likely originated from a LBV star transitioning to a WR star with the mass-loss rate increasing in the period from 5 to 0.5 years before the explosion or it could be due to a binary interaction.

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arXiv2025

Asymmetries and Circumstellar Interaction in the Type II SN 2024bch

We present a comprehensive multi-epoch photometric and spectroscopic study of SN 2024bch, a nearby (19.9 Mpc) Type II supernova (SN) with prominent early high ionization emission lines. Optical spectra from 2.9 days after the estimated explosion reveal narrow lines of H I, He II, C IV, and N IV that disappear by day 6. High cadence photometry from the ground and TESS show that the SN brightened quickly and reached a peak M$_V \sim$ $-$17.8 mag within a week of explosion, and late-time photometry suggests a $^{56}$Ni mass of 0.050 M$_{\odot}$. High-resolution spectra from day 8 and 43 trace the unshocked circumstellar medium (CSM) and indicate a wind velocity of 30--40 km s$^{-1}$, a value consistent with a red supergiant (RSG) progenitor. Comparisons between models and the early spectra suggest a pre-SN mass-loss rate of $\dot{M} \sim 10^{-3}-10^{-2}\ M_\odot\ \mathrm{yr}^{-1}$, which is too high to be explained by quiescent mass loss from RSGs, but is consistent with some recent measurements of similar SNe. Persistent blueshifted H I and [O I] emission lines seen in the optical and NIR spectra could be produced by asymmetries in the SN ejecta, while the multi-component H$α$ may indicate continued interaction with an asymmetric CSM well into the nebular phase. SN 2024bch provides another clue to the complex environments and mass-loss histories around massive stars.

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arXiv2025

Shock-cooling Constraints via Early-time Observations of the Type IIb SN 2022hnt

We report the results of a rapid follow-up campaign on the Type IIb Supernova (SN) 2022hnt. We present a daily, multi-band, photometric follow-up using the Las Cumbres Observatory, the Zwicky Transient Facility, the orbiting \textit{Swift} observatory, and the Asteroid Terrestrial-impact Last Alert System (ATLAS). A distinctive feature in the light curve of SN 2022hnt and other IIb SNe is an early narrow peak prior to the ${}^{56}$Ni peak caused by rapid shock cooling of the hydrogen envelope, which can serve as an important probe of the properties of the massive progenitor star in the moments before explosion. Using SN 2022hnt as a case study, we demonstrate a framework of considerations for the application of shock cooling models to type IIb SNe, outlining a consistent procedure for future surveys of Type IIb SNe progenitor and explosion properties. \hll{We fit several recent models of shock-cooling emission and obtain progenitor radii between $\sim50$ and $\sim100$ $R_\odot$, as well as hydrogen-enriched envelope masses between $\sim0.01$ and $\sim0.1$ $M_\odot$, both consistent with values for other IIb SNe. One of these models is the model of \cite{Morag2023}, marking the first time this model has been applied to a Type IIb SN.} We evaluate contrasting predictions between shock-cooling models to construct a fiducial parameter set which can be used for comparison to other SNe. Finally, we investigate the possibility of extended wind breakout or precursor emission captured in the earliest detections.

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