arXiv subjects

C. Fremling

Publications and source records attributed to C. Fremling.

At least 19 recordsLinked to original sources

arXiv2026

SN 2024afyu interpreted as a Pair Instability Supernova

Pair-instability supernovae (PISNe) are the predicted explosions of very massive stars triggered by electron-positron pair production. Numerous transients have been proposed as PISN candidates, yet none has provided unambiguous confirmation of this explosion mechanism. The predicted strengths of nebular emission lines offer a powerful means of testing the PISN scenario. We investigate the nature of SN 2024afyu, a nearby (z = 0.0085), long-lived (trise = 85 +- 11.7 days) SN with peculiar spectral evolution, with the aim of identifying its powering mechanism. We analyse multi-band photometry and optical and near-infrared spectroscopy from shortly after explosion to the nebular phase (around 500 days past peak). Besides early appearance of [Ca II] features, we identify a number of sulfur and silicon emission lines, for which we estimate electron temperatures and elemental masses. SN 2024afyu has an inferred 56Ni mass of around 0.4 to 1.0 solar masses and an inferred sulfur mass of the order of 3 solar masses, substantially larger than expected for conventional core-collapse explosions. SN 2024afyu is photometrically similar (although fainter, M_Peak(r) = -18.9 +- 0.04 mag) but spectroscopically distinct to other proposed PISNe. Yet, existing PISN models broadly reproduce several key characteristics, including the overall spectral appearance and broad photometric evolution. SN 2024afyu is a strong PISN candidate, since alternative scenarios would struggle to explain the combination of broad light curve, large intermediate-mass-element abundance, and general spectroscopic evolution. The discrepancies between the observations and currently available theoretical models highlight the need for new PISN calculations spanning a wider range of progenitor masses, metallicities, mixing prescriptions, and circumstellar environments.

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arXiv2026

Peak 'Nebular' Emission and Early Flux Excesses in Ca-strong Supernovae

Calcium-strong supernovae (Ca-strong SNe) are a class of thermonuclear transient characterised by rapid photometric evolution (rise times of <= 16 d) and nebular-phase spectra dominated by strong [Ca II]7291,7324 emission, in comparison to the strength of [O I]6300,6364 emission. Despite their distinctive spectra and association with remote environments, their origins remain uncertain. Using new discoveries from surveys such as the Zwicky Transient Facility, supplemented by archival data, we compile a sample of 46 Ca-strong SNe - the largest sample to date. We present a comprehensive optical photometric and spectroscopic study of Ca-strong SNe, aimed at better constraining their diversity, progenitor systems, and explosion mechanisms. Of the 35 Ca-strong SNe with spectra near peak light, we find that 32 objects exhibit significant [Ca II] emission within 10 days of peak. Forbidden [Ca II] emission is typically expected to emerge after the ejecta have expanded, cooled, and reached a sufficiently low density for forbidden transitions to occur. The prevalence of these early features in Ca-strong SNe, however, remains unexplained by current explosion models of these objects. In several objects, we identify complex, multi-component line profiles that suggest a transition between two distinct velocity regimes. These transitional features, combined with the common presence of early forbidden emission, lead us to favour a scenario in which extended material surrounds the progenitor and pre-dates the SN itself. Our results suggest that the diverse spectroscopic behaviours of Ca-strong SNe are consistent with white dwarfs exploding in extended environments polluted by prior activity of the progenitor system.

astro-ph.HE↗

arXiv2026

Bridging the gap between SLSNe and SE-SNe. Multi-wavelength analysis of the SLSN-Ib SN 2024jlc

The Type I super-luminous supernova SN~2024jlc (ZTF24aapadbb) exploded on the 25th of May 2024 at $z = 0.039$. Being the closest supernova of this class discovered in recent years and one of the closest ever, represented a rare opportunity to study in detail this type of objects. We performed a multi-wavelength analysis, spanning ten orders of magnitude in frequency, including optical/UV photometry and spectroscopy, soft and hard X-rays, and high-energy $\gamma$-rays. We characterized the event as a slow-evolving and He-rich supernova, with one of the lowest peak luminosities reported for a super-luminous event $M_g\sim-19.37$ mag, and a light curve evolution compatible with both circumstellar interaction and magnetar spin-down models, with noticeable contribution from $^{56}$Ni decay. No significant excess was found in the soft and hard X-ray bands, for which we provide upper-limits on the flux. Additionally, we analyzed two years of \textit{Fermi}-LAT data, from which we report an intriguing hint of a $\gamma$-ray signal at the $\sim 3.6 \sigma$ level, although no firm detection can be claimed. The gamma to optical efficiency ratio, $\eta = 0.38$, is suggestive of the presence of a central-engine scenario, similar to SN~2017egm. Our analysis suggests that SN~2024jlc could bridge the gap between SLSNe and classical stripped-envelope supernovae. While still poorly populated, this bridge could consist of all SLSN-Ib supernovae, with the key difference residing in the powering mechanism.

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arXiv2026

Early Multiwavelength Observations of AT 2026fgk: The Luminous Afterglow to Sub-luminous GRB 260310A, Identified Independently of a Gamma-ray Trigger

The origins of sub-luminous ($L_\mathrm{\gamma,\mathrm{iso}} < 10^{49.5}$\,erg\,s$^{-1}$) gamma-ray bursts (GRBs) associated with broad-lined Type~Ic supernovae (Ic-BL SNe) are poorly understood, in part due to the low discovery rate and faint afterglows. Here we present the identification of the optical afterglow of Fermi-GBM-detected GRB\,260310A (AT\,2026fgk) as a rapidly rising ($>1\,$mag\,d$^{-1}$), red ($g-r=0.4$\,mag) transient using the Gravitational-wave Optical Transient Observatory, Large Array Survey Telescope, and Zwicky Transient Facility (ZTF) data streams. We present multiwavelength follow-up observations from the first 50\,days, which reveal that GRB 260310A/AT\,2026fgk was sub-luminous ($L_\mathrm{\gamma,iso}=10^{48.8}\,$erg\,s$^{-1}$); it was the most nearby ($z=0.153$) afterglow identified blindly by an optical survey; and that it is one of the brightest afterglows ever observed at X-ray, optical, and radio (cm to mm) wavelengths. We spectroscopically confirm an underlying Ic-BL SN with properties typical of GRB-SNe ($M_\mathrm{ej}\approx3\,M_\odot$, $E_{\rm K}\approx 10^{52}\,$erg). With basic modeling of the afterglow, including the long optical rise ($\approx10^{3}\,$s), we infer either a low initial Lorentz factor ($\Gamma_0\approx40$) or a slightly off-axis viewing angle ($\lesssim3^\circ$). The host galaxy's mass and star formation rate are similar to the hosts of other sub-luminous GRBs. ZTF's flux-limited survey gives a volumetric rate of AT\,2026fgk-like events of $0.30^{+1.37}_{-0.29}\,$Gpc\,$^{-3}$\,yr$^{-1}$, which is consistent with the on-axis, high luminosity ($L_{\rm \gamma,iso}>10^{49.5}$\,erg\,s$^{-1}$) long-GRB rate. The similarity in the rates strongly constrains the prevalence of low-$\Gamma_0$ bursts and the beaming of the initial relativistic material in GRBs.

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arXiv2026

Late-Time HST UV Detections Reveal Eruptive Mass Loss and Circumstellar Interaction in a Quarter of Stripped-Envelope Supernovae

We present HST WFC3/UVIS F275W near-UV imaging of 91stripped-envelope supernovae (SE SNe; Types IIb, Ib, Ic) from Snapshot program SNAP-16657, observed at phases of 270-1845 days (median 952 days) after first optical detection. We detect UV counterparts in 13 SE~SNe, of which 6 are classified as secure and 7 as ambiguous after comparison to nearby H\textsc{ii} regions, interpreting the secure sources as signatures of interaction with circumstellar material (CSM). Independent WISE W1/W2 light curves show $>300$ day mid-IR excesses in two of the secure UV sources, corroborating the interaction interpretation, and reveal two additional IR-only candidates without UV counterparts, indicating dust-obscured interaction episodes missed by the UV survey. A forward-modeling MCMC analysis using a physics-based CSM interaction model with three free parameters, the interaction fraction $f_\mathrm{CSM}$, shell mass $M_\mathrm{CSM}$, and thickness fraction $f_\mathrm{thick}$, yields $f_\mathrm{CSM} = 0.23^{+0.17}_{-0.09}$, $M_\mathrm{CSM} \approx 0.013~M_\odot$, and $f_\mathrm{thick} \approx 0.07$. The inferred thin-shell geometry implies an ejection duration of $\sim$6 yr for an outflow velocity of $300$ km s$^{-1}$, two to three orders of magnitude shorter than the thermal timescale of stable Roche-lobe overflow. This result disfavors steady binary mass transfer as the origin of the detected CSM and instead points to eruptive pre-supernova mass ejection in the final years before core collapse, either from wave-driven outbursts or from mass transfer triggered by late-stage progenitor re-expansion.

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arXiv2026

Nebular Phase Evolution of SN 2023ixf (I): From Circumstellar Infrared Echo to the onset of in-situ Dust Formation in a Type II Supernova

We present optical and near-infrared (NIR) photometric and spectroscopic observations of the Type II supernova SN 2023ixf spanning 150 to 750 days, combined with published early-time optical and infrared photometry, and JWST NIRSpec and MIRI spectroscopy, to disentangle circumstellar echo emission from newly formed internal dust. The combined dataset reveals an early infrared excess by 1.8 days, a broad secondary NIR rebrightening over about 89 to 175 days, progressive attenuation of the red wing of H-alpha from about 132 days, and CO emission detected by about 217 days. We identify the onset of H-alpha asymmetry as the first direct signature for internal dust formation, and modeling of the H-alpha profile over 140 to 418 days yields an internal silicate-equivalent dust mass of about 1.5e-6 to 6e-5 solar masses. By contrast, the early infrared evolution is best interpreted as echo-dominated: the 1.8 to 33.6 day excess is consistent with a radiative-flash infrared echo from pre-existing circumstellar dust, while the 89 to 175 day rebrightening is more naturally explained by a more extended echo arising from structured wind material. JWST spectral energy distribution modeling further reveals a multi-component infrared continuum in which a cold graphite component traces lingering echo emission, while a colder silicate-bearing component grows to about 2e-3 solar masses, providing the strongest late-time spectral energy distribution evidence that internal CDS/ejecta dust becomes substantial. SN 2023ixf therefore provides one of the clearest time-resolved case studies of dust signatures in a Type II supernova, linking early circumstellar reprocessing with increasingly important in situ dust formation.

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arXiv2025

The hydrogen-free circumstellar interaction in the Type Ib supernova 2021efd: A clue to the mechanism of the helium-layer stripping

Stripped-envelope supernovae (SESNe), including Type IIb, Ib, and Ic supernovae (SNe), originate from the explosions of massive stars whose outer envelopes have been largely removed during their lifetimes. The main stripping mechanism for the hydrogen (H) envelope in the progenitors of SESNe is often considered to be interaction with a binary companion, while that for the helium (He) layer is unclear. We conducted photometric and spectroscopic observations of the Type Ib SN 2021efd, which shows signs of interaction with H-free circumstellar material (CSM). Around 30 days after the r-band light curve (LC) peak, until at least ~ 770 days, its LCs display excessive luminosity compared to regular SESNe and at least three distinct peaks. The light curve evolution is similar to that of SN 2019tsf, whose previously unpublished spectrum at 400 days is also presented here. The nebular spectrum of SN 2021efd shows narrow emission lines (~ 1000 km/s) in various species. Our observations suggest that SN 2021efd is a Type Ib SN interacting with CSM with the following parameters: The estimated CSM mass, composition, and distribution are at least a few times 0.1 M_sun, H-free, and clumpy, respectively. Based on the estimated ejecta properties, we conclude that this event is a transitional SN whose progenitor was experiencing He-layer stripping at the epoch of the explosion, and was on the way to becoming a carbon-oxygen star from a He star. The estimated CSM properties suggest that the progenitor had some episodic mass ejections with a rate of ~ 0.001-0.01 M_sun/yr for the last decade and slightly smaller before this final phase at least from ~ 200 years before the explosion, for the assumed CSM velocity of 100 km/s. For the case of ~ 1000 km/s, the necessary mass-loss rate would be increased by a factor of ten, and the timescales decreased by a factor of ten.

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arXiv2025

A thermonuclear supernova interacting with hydrogen- and helium-deficient circumstellar material. SN 2020aeuh as a SN Ia-CSM-C/O?

Identifying the progenitors of thermonuclear supernovae (Type Ia supernovae; SNe Ia) remains a key objective in contemporary astronomy. The rare subclass of SNe Ia that interacts with circumstellar material (Type Ia-CSM) allows for studies of the progenitor's environment before explosion, and generally favours single-degenerate progenitor channels. The case of SN Ia-CSM PTF11kx clearly connected thermonuclear explosions with hydrogen-rich CSM-interacting events, and the more recent SN 2020eyj connected SNe Ia with helum-rich companion progenitors. Here we present a study of SN 2020aeuh, a Type Ia-CSM with delayed interaction. We analyse photometric and spectroscopic data that monitor the evolution of SN 2020aeuh and compare its properties with those of peculiar SNe Ia and core-collapse SNe. At early times, the evolution of SN 2020aeuh resembles a slightly overluminous SN Ia. Later, the interaction-dominated spectra develop the same pseudocontinuum seen in Type Ia-CSM PTF11kx and SN 2020eyj. However, the later-time spectra of SN 2020aeuh lack hydrogen and helium narrow lines. Instead, a few narrow lines could be attributed to carbon and oxygen. We fit the pseudobolometric light curve with a CSM-interaction mode, yielding a CSM mass of 1-2 M$_{\odot}$. We propose that SN 2020aeuh was a Type Ia supernova that eventually interacted with a dense medium which was deficient in both hydrogen and helium. Whereas previous SNe Ia-CSM constitute our best evidence for nondegenerate companion progenitors, the CSM around SN 2020aeuh is more difficult to understand. We include a hydrodynamical simulation for a double-degenerate system to showcase how the dynamical evolution of such a progenitor scenario could produce the CSM observed around SN 2020aeuh. It is clear that SN 2020aeuh challenges current models for stellar evolution leading up to a SN Ia explosion.

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arXiv2025

Luminous, rapidly declining supernovae as stripped transitional objects in low metallicity environments: the case of SN 2022lxg

We present an analysis of the optical and near-infrared properties of SN 2022lxg, a bright ($\rm M_{g\, \mathrm{peak}}=-19.41$ mag) and rapidly evolving SN. It was discovered within a day of explosion, and rose to peak brightness in 10 d. Two distinct phases of circumstellar interaction are evident in the data. The first is marked by a steep blue continuum (T $>15,000$ K) with flash-ionisation features due to hydrogen and He II. The second, weaker phase is marked by a change in the colour evolution accompanied by changes in the shapes and velocities of the spectral line profiles. Narrow P-Cygni profiles (~ $150$ km s$^{-1}$) of He I further indicate the presence of slow-moving unshocked material and suggesting partial stripping of the progenitor. The fast decline of the light curve from peak (3.48$\pm$ 0.26 mag $\rm (50\,d)^{-1}$ in $g$-band) implies that the ejecta mass must be low. Spectroscopically, until $+35$ d there are similarities to some Type IIb SNe but then there is a transition to spectra that are more reminiscent of an interacting SN II. However, metal lines are largely absent in the spectra, even at epochs of 80 d. Its remote location from the presumed host galaxy, a dwarf with $\rm M_B$ ~ $-14.4$ mag, is consistent with our metallicity estimate - close to the SMC value - obtained from scaling relations. Furthermore, several lines of evidence (including intrinsic polarisation of $p$ ~ (0.5-1.0) %) point to deviations from spherical symmetry. We suggest that a plausible way of uniting the observational clues is to consider a binary system that underwent case C mass transfer. This failed to remove the entire H-envelope of the progenitor before it underwent core-collapse. In this scenario, the progenitor itself would be more compact and perhaps straddle the boundary between blue and yellow supergiants, tying in with the early spectroscopic similarity to Type IIb SNe.

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arXiv2025

The ambiguous AT2022rze: Changing-look AGN mimicking a supernova in a merging galaxy system

AT2022rze is a luminous, ambiguous transient located South-East of the geometric center of its host galaxy at redshift z = 0.08. The host appears to be formed by a merging galaxy system. The observed characteristics of AT2022rze are reminiscent of active galactic nuclei (AGN), tidal disruption events (TDEs), and superluminous supernovae (SLSNe). The transient reached a peak absolute magnitude of -20.2 +- 0.2 mag, showing a sharp rise (trise,1/e = 27.5 +- 0.6 days) followed by a slow decline (tdec,1/e = 382.9 +- 0.6). Its bumpy light curve and narrow Balmer lines indicate the presence of gas (and dust). Its light curve shows rather red colors, indicating that the transient could be affected by significant host extinction. The spectra reveal coronal lines, indicative of high-energy (X-ray/UV) emission. Archival data reveal no prior activity at this location, disfavoring a steady-state AGN, although an optical spectrum obtained prior to the transient is consistent with an AGN classification of the host. Based on this, we conclude that the transient most likely represents a Changing-look AGN at the center of the smallest component of the merging system.

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arXiv2024

Eruptive mass loss less than a year before the explosion of superluminous supernovae: I. The cases of SN 2020xga and SN 2022xgc

We present photometric and spectroscopic observations of SN 2020xga and SN 2022xgc, two hydrogen-poor superluminous supernovae (SLSNe-I) at $z = 0.4296$ and $z = 0.3103$, respectively, which show an additional set of broad Mg II absorption lines, blueshifted by a few thousands kilometer second$^{-1}$ with respect to the host galaxy absorption system. Previous work interpreted this as due to resonance line scattering of the SLSN continuum by rapidly expanding circumstellar material (CSM) expelled shortly before the explosion. The peak rest-frame $g$-band magnitude of SN 2020xga is $-22.30 \pm 0.04$ mag and of SN 2022xgc is $-21.97 \pm 0.05$ mag, placing them among the brightest SLSNe-I. We used high-quality spectra from ultraviolet to near-infrared wavelengths to model the Mg II line profiles and infer the properties of the CSM shells. We find that the CSM shell of SN 2020xga resides at $\sim 1.3 \times 10^{16}~\rm cm$, moving with a maximum velocity of $4275~\rm km~s^{-1}$, and the shell of SN 2022xgc is located at $\sim 0.8 \times 10^{16}~\rm cm$, reaching up to $4400~\rm km~s^{-1}$. These shells were expelled $\sim 11$ and $\sim 5$ months before the explosions of SN 2020xga and SN 2022xgc, respectively, possibly as a result of luminous-blue-variable-like eruptions or pulsational pair instability (PPI) mass loss. We also analyzed optical photometric data and modeled the light curves, considering powering from the magnetar spin-down mechanism. The results support very energetic magnetars, approaching the mass-shedding limit, powering these SNe with ejecta masses of $\sim 7-9~\rm M_\odot$. The ejecta masses inferred from the magnetar modeling are not consistent with the PPI scenario pointing toward stars $> 50~\rm M_\odot$ He-core; hence, alternative scenarios such as fallback accretion and CSM interaction are discussed.

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arXiv2024

The Palomar twilight survey of 'Ayl\'o'chaxnim, Atiras, and comets

Near-sun sky twilight observations allow for the detection of asteroid interior to the orbit of Venus (Aylos), the Earth (Atiras), and comets. We present the results of observations with the Palomar 48-inch telescope (P48)/Zwicky Transient Facility (ZTF) camera in 30 s r-band exposures taken during evening astronomical twilight from 2019 Sep 20 to 2022 March 7 and during morning astronomical twilight sky from 2019 Sep 21 to 2022 Sep 29. More than 46,000 exposures were taken in evening and morning astronomical twilight within 31 to 66 degrees from the Sun with an r-band limiting magnitude between 18.1 and 20.9. The twilight pointings show a slight seasonal dependence in limiting magnitude and ability to point closer towards the Sun, with limiting magnitude slightly improving during summer. In total, the one Aylo, (594913) 'Ayl\'o'chaxnim, and 4 Atiras, 2020 OV1, 2021 BS1, 2021 PB2, and 2021 VR3, were discovered in evening and morning twilight observations. Additional twilight survey discoveries also include 6 long-period comets: C/2020 T2, C/2020 V2, C/2021 D2, C/2021 E3, C/2022 E3, and C/2022 P3, and two short-period comets: P/2021 N1 and P/2022 P2 using deep learning comet detection pipelines. The P48/ZTF twilight survey also recovered 11 known Atiras, one Aylo, three short-period comes, two long-period comets, and one interstellar object. Lastly, the Vera Rubin Observatory will conduct a twilight survey starting in its first year of operations and will cover the sky within 45 degrees of the Sun. Twilight surveys such as those by ZTF and future surveys will provide opportunities for discovering asteroids inside the orbits of Earth and Venus.

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arXiv2024

Sample of hydrogen-rich superluminous supernovae from the Zwicky Transient Facility

Hydrogen-rich superluminous supernovae (SLSNe II) are rare. The exact mechanism producing their extreme light curve peaks is not understood. Analysis of single events and small samples suggest that CSM interaction is the main responsible for their features. However, other mechanisms can not be discarded. Large sample analysis can provide clarification. We aim to characterize the light curves of a sample of 107 SLSNe II to provide valuable information that can be used to validate theoretical models. We analyze the gri light curves of SLSNe II obtained through ZTF. We study peak absolute magnitudes and characteristic timescales. When possible we compute g-r colors, pseudo-bolometric light curves, and estimate lower limits for their total radiated energy. We also study the luminosity distribution of our sample and estimate the percentage of them that would be observable by the LSST. Finally, we compare our sample to other H-rich SNe and to H-poor SLSNe I. SLSNe II are heterogeneous. Their median peak absolute magnitude is -20.3 mag in optical bands. Their rise can take from two weeks to over three months, and their decline from twenty days to over a year. We found no significant correlations between peak magnitude and timescales. SLSNe II tend to show fainter peaks, longer declines and redder colors than SLSNe I. We present the largest sample of SLSNe II light curves to date, comprising of 107 events. Their diversity could be explained by considering different CSM morphologies. Although, theoretical analysis is needed to explore alternative scenarios. Other luminous transients, such as Active Galactic Nuclei, Tidal Disruption Events or SNe Ia-CSM, can easily become contaminants. Thus, good multi-wavelength light curve coverage becomes paramount. LSST could miss 30 percent of the ZTF events in the its footprint in gri bands. Redder bands become important to construct complete samples.

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arXiv2024

Spectroscopic observations of progenitor activity 100 days before a Type Ibn supernova

Obtaining spectroscopic observations of the progenitors of core-collapse supernovae is often unfeasible due to an inherent lack of knowledge as to which stars will go supernova and when they will explode. In this letter, we present photometric and spectroscopic observations of the progenitor activity of SN 2023fyq in the preceding 150 days before the He-rich progenitor exploded as a Type Ibn supernova. The progenitor of SN 2023fyq shows an exponential rise in flux prior to core-collapse. Complex He I emission line features are observed, with a P-Cygni like profile, as well as an evolving broad base with velocities on the order of 10,000 km/s, possibly due to electron scattering. The luminosity and evolution of SN 2023fyq are consistent with a faint Type Ibn, reaching a peak r-band magnitude of 18.1 mag, although there is some uncertainty in the distance to the host, NGC 4388, located in the Virgo cluster. We present additional evidence of asymmetric He-rich material being present prior to the explosion of SN 2023fyq, as well as after, suggesting this material has survived the ejecta-CSM interaction. Broad [O I] and the Ca II triplet lines are observed at late phases, confirming that SN 2023fyq was a genuine supernova rather than a non-terminal interacting transient. SN 2023fyq provides insight into the final moments of a massive star's life, highlighting that the progenitor is likely highly unstable before core-collapse.

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arXiv2023

SN 2021adxl: A luminous nearby interacting supernova in an extremely low metallicity environment

SN 2021adxl is a slowly evolving, luminous, Type IIn supernova with asymmetric emission line profiles, similar to the well-studied SN 2010jl. We present extensive optical, near-ultraviolet, and near-infrared photometry and spectroscopy covering ~1.5 years post discovery. SN 2021adxl occurred in an unusual environment, atop a vigorously star-forming region that is offset from its host galaxy core. The appearance of Ly-alpha, O II, as well as the compact core, would classify the host of SN 2021adxl as a Blueberry galaxy, analogous to the higher redshift Green Pea galaxies. Using several abundance indicators, we find a metallicity of the explosion environment of only 10% solar, the lowest reported metallicity for a Type IIn SN environment. SN 2021adxl reaches a peak magnitude of r ~ -20.2 mag and since discovery, SN 2021adxl has faded by only ~4 magnitudes in the r band with a cumulative radiated energy of ~1.5e50 erg over 18 months. SN 2021adxl shows strong signs of interaction with a complex circumstellar medium, seen by the detection of X-rays, revealed by the detection of coronal emission lines, and through multi-component hydrogen and helium profiles. In order to further understand this interaction, we model the H{\alpha} profile using a Monte-Carlo electron scattering code. The blueshifted high-velocity component is consistent with emission from a radially thin, spherical shell resulting in the broad emission components due to electron scattering. Using the velocity evolution of this emitting shell, we find that the SN ejecta collide with circumstellar material of at least 5 Msun, assuming a steady-state mass-loss rate of 4-6e-3 Msun per year for the first ~200 days of evolution. Continuing the observations of SN 2021adxl may reveal signatures of dust formation or an infrared excess, similar to that seen for SN 2010jl.

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arXiv2023

Resolving the explosion of supernova 2023ixf in Messier 101 within its complex circumstellar environment

Observing a supernova explosion shortly after it occurs can reveal important information about the physics of stellar explosions and the nature of the progenitor stars of supernovae (SNe). When a star with a well-defined edge explodes in vacuum, the first photons to escape from its surface appear as a brief shock-breakout flare. The duration of this flare can extend to at most a few hours even for nonspherical breakouts from supergiant stars, after which the explosion ejecta should expand and cool. Alternatively, for stars exploding within a distribution of sufficiently dense optically thick circumstellar material, 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 heating. The difficulty in detecting SN explosions promptly after the event has so far limited data regarding supergiant stellar explosions mostly to serendipitous observations that, owing to the lack of 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 observations of SN 2023ixf in the nearby galaxy M101, covering the early days of the event. Using UV spectroscopy from the Hubble Space Telescope (HST) as well as a comprehensive set of additional multiwavelength observations, we trace the photometric and spectroscopic evolution of the event and are able to temporally resolve the emergence and evolution of the SN emission.

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arXiv2023

Palomar discovery and initial characterization of naked-eye long period comet C/2022 E3 (ZTF)

Long-period comets are planetesimal remnants constraining the environment and volatiles of the protoplanetary disc. We report the discovery of hyperbolic long-period comet C/2022 E3 (ZTF), which has a perihelion $\sim$1.11 au, an eccentricity $\gtrsim$1 and an inclination $\sim$109$^{\circ}$, from images taken with the Palomar 48-inch telescope during morning twilight on 2022 Mar 2. Additionally, we report the characterization of C/2022 E3 (ZTF) from observations taken with the Palomar 200-inch, the Palomar 60-inch, and the NASA Infrared Telescope Facility in early 2023 February to 2023 March when the comet passed within $\sim$0.28 au of the Earth and reached a visible magnitude of $\sim$5. We measure g-r = 0.70$\pm$0.01, r-i = 0.20$\pm$0.01, i-z = 0.06$\pm$0.01, z-J = 0.90$\pm$0.01, J-H = 0.38$\pm$0.01 and H-K = 0.15$\pm$0.01 colours for the comet from observations. We measure the A(0$^\circ$)f$\rho$ (0.8~$\mu$m) in a 6500~km radius from the nucleus of 1483$\pm$40~cm, and CN, C$_3$, and C$_2$ production of 5.43$\pm0.11\times$10$^{25}$~mol/s, 2.01$\pm0.04\times$10$^{24}$, and 3.08$\pm0.5\times$10$^{25}$~mol/s, similar to other long period comets. We additionally observe the appearance of jet-like structures at a scale of $\sim$4,000 km in wide-field g-band images, which may be caused by the presence of CN gas in the near-nucleus coma.

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arXiv2023

The broad-lined Type-Ic supernova SN 2022xxf with extraordinary two-humped light curves

We report on our study of supernova (SN) 2022xxf based on observations obtained during the first four months of its evolution. The light curves (LCs) display two humps of similar maximum brightness separated by 75 days, unprecedented for a broad-lined (BL) Type Ic supernova (SN IcBL). SN 2022xxf is the most nearby SN IcBL to date (in NGC 3705, $z = 0.0037$, at a distance of about 20 Mpc). Optical and near-infrared photometry and spectroscopy are used to identify the energy source powering the LC. Nearly 50 epochs of high signal-to-noise-ratio spectroscopy were obtained within 130 days, comprising an unparalleled dataset for a SN IcBL, and one of the best-sampled SN datasets to date. The global spectral appearance and evolution of SN 2022xxf points to typical SN Ic/IcBL, with broad features (up to $\sim14000$ km s$^{-1}$) and a gradual transition from the photospheric to the nebular phase. However, narrow emission lines (corresponding to $\sim1000-2500$ km s$^{-1}$) are present in the spectra from the time of the second rise, suggesting slower-moving circumstellar material (CSM). These lines are subtle, in comparison to the typical strong narrow lines of CSM-interacting SNe, for example, Type IIn, Ibn, and Icn, but some are readily noticeable at late times such as in Mg I $\lambda$5170 and [O I] $\lambda$5577. Unusually, the near-infrared spectra show narrow line peaks in a number of features formed by ions of O and Mg. We infer the presence of CSM that is free of H and He. We propose that the radiative energy from the ejecta-CSM interaction is a plausible explanation for the second LC hump. This interaction scenario is supported by the color evolution, which progresses to the blue as the light curve evolves along the second hump, and the slow second rise and subsequent rapid LC drop. (Abstract abridged)

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