arXiv subjects

Christoffer Fremling

Publications and source records attributed to Christoffer Fremling.

At least 55 records · Page 3Linked to original sources

arXiv2025

IIb or not IIb: A Catalog of ZTF Kilonova Imposters

Among the various classes of fast optical transients (FOTs), kilonovae (KNe), which can emerge as a result of neutron star mergers, are extremely challenging to observe because of not only the rapid timescale on which they fade (on the order of days), but also due to the relative scarcity of their occurrence. This scarcity is compounded by the large number of other FOTs that may initially resemble the characteristic rise of a KNe. While these objects can be ruled out as candidate KNe by taking spectroscopy, a method of confidently ruling out transients based on photometric analysis alone would be incredibly valuable. We describe the compilation of various "imposter" transients, including a plurality of IIb SNe, and investigate a number of comparative metrics by which one might be able to remove transients from consideration without the use of spectroscopy. We provide a list of these objects and their classifications as well as a glossary of the transient types included in the sample.

astro-ph.HE↗

arXiv2025

Interstellar comet 3I/ATLAS: discovery and physical description

We describe the physical characteristics of interstellar comet 3I/ATLAS, discovered on 2025 July 1 by the Asteroid Terrestrial-impact Last Alert System. The comet has eccentricity, $e$ $\simeq$ 6.08 and velocity at infinity, v$_{\infty}$ $\simeq$ 57 km/s, indicating an interstellar origin. \textbf{We obtained B,V, R, I, g, r, i, and z photometry with the Kottamia Astronomical Observatory 1.88-m telescope, the Palomar 200-inch telescope, and the Astrophysical Research Consortium 3.5-m telescope on 2025 July 2, 3, and 6. We measured colour indices B-V=0.98$\pm$0.23, V-R=0.71$\pm$0.09, R-I=0.14$\pm$0.10, g-r=0.84$\pm$0.05 mag, r-i=0.16$\pm$0.03 mag, i-z=-0.02$\pm$0.07 mag, and g-i=1.00$\pm$0.05 mag and a spectral slope of 16.0$\pm$1.9 $\%$/100 nm.} We calculate the dust cross-section within 10,000 km of the comet to be 184.6$\pm$4.6 km$^2$, assuming an albedo of 0.10. 3I/ATLAS's coma has FWHM$\simeq$2.2 arcsec and A(0$^\circ$)f$ρ$=280.8$\pm$3.2 cm. \textbf{We estimate that 3I/ATLAS's \textmu m-scale to mm-scale dust is ejected at $\sim$0.01-1 m/s, implying a dust production of $\sim$0.1 - 1.0 kg/s.

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arXiv2025

Palomar and Apache Point Spectrophotometry of Interstellar Comet 3I/ATLAS

On July 1st 2025 the third interstellar object, 3I/ATLAS or C/2025 N1 (ATLAS), was discovered, with an eccentricity of $e=6.15 \pm 0.01$ and perihelion of $q=1.357\pm0.001$ au. We report our initial visible to near-infrared (420-1000 nm) spectrophotometry of 3I/ATLAS using both the Palomar 200 inch telescope and Apache Point Observatory. We measure 3I/ATLAS to have a red spectral slope of 19 %/100 nm in the 420-700 nm range, and a more neutral 6 %/100 nm slope over 700-1000 nm. We detect no notable emission features such as from C$_2$.

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arXiv2025

Twin peaks: SN 2021uvy and SN 2022hgk in the landscape of double-peaked stripped envelope supernovae

In recent years, a class of stripped-envelope supernovae (SESNe) showing two distinct light-curve peaks has emerged, where the first peak cannot be attributed to shock cooling emission. Such peculiar SNe are often studied individually, explained by a combination of powering mechanisms, but are rarely discussed broadly as a group. In this paper, we attempt to form a picture of the landscape of double-peaked SESNe and their powering mechanisms by adding two more objects -- SN 2021uvy and SN 2022hgk. SN 2021uvy is a broad, luminous SN Ib with an unusually long first peak rise and constant color evolution with rising photospheric temperature during the second peak. Though its first peak resembles SN 2019stc, their second peaks differ, making SN 2021uvy unique. SN 2022hgk shows photometric similarity to SN 2019cad and spectroscopic similarity to SN 2005bf, both proposed to be powered by a double-nickel distribution in their ejecta. We analyze their light curves and colors, compare them with a sample of double-peaked SESNe from the ZTF archive, and analyze the light curve parameters of the sample. We observe a correlation (p-value~0.025) between the peak absolute magnitudes of the first and second peaks. No single definitive powering mechanism applies to the whole sample, as it shows variety in the photometric and spectroscopic properties. However, sub-groups of similarity exist that can be explained by mechanisms like the double-nickel distribution, magnetar central engine, interaction, and fallback accretion. We also map out the duration between the peaks ($Δt^{21}$) vs the difference between peak absolute magnitudes ($ΔM^{21}$) as a phase-space that could potentially delineate the most promising powering mechanisms for the double-peaked SESNe.

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arXiv2025

A Massive Black Hole 0.8 kpc from the Host Nucleus Revealed by the Offset Tidal Disruption Event AT2024tvd

Tidal disruption events (TDEs) that are spatially offset from the nuclei of their host galaxies offer a new probe of massive black hole (MBH) wanderers, binaries, triples, and recoiling MBHs. Here we present AT2024tvd, the first off-nuclear TDE identified through optical sky surveys. High-resolution imaging with the \textit{Hubble Space Telescope} shows that AT2024tvd is $0.914\pm 0.010^{\prime\prime}$ offset from the apparent center of its host galaxy, corresponding to a projected distance of $0.808\pm 0.009$ kpc at $z=0.045$. Chandra and VLA observations support the same conclusion for the TDE's X-ray and radio emission. AT2024tvd exhibits typical properties of nuclear TDEs, including a persistent hot UV/optical component that peaks at $L_{\rm bb}\sim 6\times 10^{43}\,{\rm erg\,s^{-1}}$, broad hydrogen lines in its optical spectra, and delayed brightening of luminous ($L_{\rm X,peak}\sim 3\times 10^{43}\,{\rm erg\,s^{-1}}$), highly variable soft X-ray emission. The MBH mass of AT2024tvd is $10^{6\pm1}\,M_\odot$, at least 10 times lower than its host galaxy's central black hole mass ($\gtrsim 10^8\,M_\odot$). The MBH in AT2024tvd has two possible origins: a wandering MBH from the lower-mass galaxy in a minor merger during the dynamical friction phase or a recoiling MBH ejected by triple interactions. Combining AT2024tvd with two previously known off-nuclear TDEs discovered in X-rays (3XMM J2150 and EP240222a), which likely involve intermediate-mass black holes in satellite galaxies, we find that the parent galaxies of all three events are very massive ($\sim 10^{10.9}\,M_\odot$). This result aligns with expectations from cosmological simulations that the number of offset MBHs scales linearly with the host halo mass.

astro-ph.GA↗

arXiv2025

The BTSbot-nearby discovery of SN 2024jlf: rapid, autonomous follow-up probes interaction in an 18.5 Mpc Type IIP supernova

We present observations of the Type IIP supernova (SN) 2024jlf, including spectroscopy beginning just 0.7 days ($\sim$17 hours) after first light. Rapid follow-up was enabled by the new $\texttt{BTSbot-nearby}$ program, which involves autonomously triggering target-of-opportunity requests for new transients in Zwicky Transient Facility data that are coincident with nearby ($D<60$ Mpc) galaxies and identified by the $\texttt{BTSbot}$ machine learning model. Early photometry and non-detections shortly prior to first light show that SN 2024jlf initially brightened by $>$4 mag/day, quicker than $\sim$90% of Type II SNe. Early spectra reveal weak flash ionization features: narrow, short-lived ($1.3 < τ~\mathrm{[d]} < 1.8$) emission lines of H$α$, He II, and C IV. Assuming a wind velocity of $v_w=50$ km s$^{-1}$, these properties indicate that the red supergiant progenitor exhibited enhanced mass-loss in the last year before explosion. We constrain the mass-loss rate to $10^{-4} < \dot{M}~\mathrm{[M_\odot~yr^{-1}]} < 10^{-3}$ by matching observations to model grids from two independent radiative hydrodynamics codes. $\texttt{BTSbot-nearby}$ automation minimizes spectroscopic follow-up latency, enabling the observation of ephemeral early-time phenomena exhibited by transients.

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arXiv2025

Keck and Gemini characterization of $Hayabusa2\#$ rendezvous target 1998 KY$_{26}$

Near-earth object (NEO) 1998 KY$_{26}$ is a target of the $Hayabusa2\#$ spacecraft, which it will rendezvous with in July 2031. The asteroid is a rapid rotator and has a large out-of-plane nongravitational acceleration. We present deep $g$ and $R$ band imaging obtained with the Keck I/Low Resolution Imaging Spectrometer and visible spectroscopy from Gemini North/Gemini Multi-Object Spectrograph taken of 1998 KY$_{26}$ on 2024 June 8-9 when the asteroid was $\sim$0.037 au from the Earth. The asteroid lacks evidence of a dust coma in the deep images and its spectrum most closely resembles Xe-type asteroids, possessing a spectral slope of 6.71$\pm$0.43 $\%$ 100 nm$^{-1}$, and colors $g$-$r$ = 0.63$\pm$0.03, $r$-$i$ = 0.15$\pm$0.03, $i$-$z$ = 0.05$\pm$0.04, and implies a diameter of $\sim$10 m. From our images, we compute a 3$σ$ upper limit on the dust production of 1998 KY$_{26}$ of $<$10$^{-5}$ kg s$^{-1}$, $<$10$^{-2}$ kg s$^{-1}$, and $<$10$^{-1}$ kg s$^{-1}$ assuming $\mathrmμ$m, mm, and cm size dust particles. Additionally, we compare the orbit of 1998 KY$_{26}$ and large nongravitational parameters asteroids to NEO population models and find that the majority, including 1998 KY$_{26}$, likely originated from the inner Main Belt, while the second most numerous group originates from the outer Main Belt, followed by a third group originating from the Jupiter Family Comet population. Given its inner Main Belt origin, its Xe-type spectrum, and rapid rotation, we hypothesize that the nongravitational acceleration of 1998 KY$_{26}$ may be caused by the shedding of large dust grains from its surface due to its rotation rather than H$_2$O vapor outgassing.

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arXiv2025

CCSNscore: A multi-input deep learning tool for classification of core-collapse supernovae using SED-Machine spectra

Supernovae (SNe) come in various flavors and are classified into different types based on emission and absorption lines in their spectra. SN candidates are now abundant with the advent of large systematic sky surveys like the Zwicky Transient Facility (ZTF), however, the identification bottleneck lies in their spectroscopic confirmation and classification. Fully robotic telescopes with dedicated spectrographs optimized for SN follow-up have eased the burden of data acquisition. However, the task of classifying the spectra still largely rests with the astronomers. Automating this classification step reduces human effort and can make the SN type available sooner to the public. For this purpose, we have developed a deep-learning based program for classifying core-collapse supernovae (CCSNe) with ultra-low resolution spectra from the SED-Machine spectrograph on the Palomar 60-inch telescope. The program consists of hierarchical classification task layers, with each layer composed of multiple binary classifiers running in parallel to produce a reliable classification. The binary classifiers utilize RNN and CNN architecture and are designed to take multiple inputs to supplement spectra with $g$- and $r$-band photometry from ZTF. On non-host-contaminated and good quality SEDM spectra ("gold" test set) of CCSNe, CCSNscore is ~94% accurate in distinguishing between hydrogen-rich (Type II) and hydrogen-poor (Type Ibc) CCSNe. With light curve input, CCSNscore classifies ~83% of the gold set with high confidence (score $\geq 0.8$ and score-error $<0.05$), with ~98% accuracy. Based on SNIascore's and CCSNscore's real-time performance on bright transients ($m_{pk}\leq18.5$) and our reporting criteria, we expect ~0.5% (~4) true SNe Ia to be misclassified as SNe Ibc and ~6% (~17) of true CCSNe to be misclassified between Type II and Type Ibc annually on the Transient Name Server.

astro-ph.IM↗

arXiv2025

A Luminous Red Optical Flare and Hard X-ray Emission in the Tidal Disruption Event AT2024kmq

We present the optical discovery and multiwavelength follow-up observations of AT2024kmq, a likely tidal disruption event (TDE) associated with a supermassive ($M_{\rm BH}\sim 10^{8} M_\odot$) black hole in a massive galaxy at $z=0.192$. The optical light curve of AT2024kmq exhibits two distinct peaks: an early fast (timescale 1 d) and luminous ($M\approx-20$ mag) red peak, then a slower (timescale 1 month) blue peak with a higher optical luminosity ($M\approx-22$ mag) and featureless optical spectra. The second component is similar to the spectroscopic class of "featureless TDEs" in the literature, and during this second component we detect highly variable, luminous ($L_X\approx 10^{44}$ erg s$^{-1}$), and hard ($f_ν\propto ν^{-1.5}$) X-ray emission. Luminous ($10^{29} $erg s$^{-1}$ Hz$^{-1}$ at 10 GHz) but unchanging radio emission likely arises from an underlying active galactic nucleus. The luminosity, timescale, and color of the early red optical peak can be explained by synchrotron emission, or alternatively by thermal emission from material at a large radius ($R\approx\mathrm{few}\times10^{15}$ cm). Possible physical origins for this early red component include an off-axis relativistic jet, and shocks from self-intersecting debris leading to the formation of the accretion disk. Late-time radio observations will help distinguish between the two possibilities.

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arXiv2025

Variability of Central Stars of Planetary Nebulae with the Zwicky Transient Facility. I. Methods, Short-Timescale Variables, Binary Candidates, and the Unusual Nucleus of WeSb 1

Over the past several decades, time-series photometry of CSPNe has yielded significant results including, but not limited to, discoveries of nearly 100 binary systems, insights into pulsations and winds in young white dwarfs, and studies of stars undergoing very late thermal pulses. We have undertaken a systematic study of optical photometric variability of cataloged CSPNe, using the light curves from the Zwicky Transient Facility (ZTF). By applying appropriate variability metrics, we arrive at a list of 94 highly variable CSPN candidates. Based on the timescales of the light-curve activity, we classify the variables broadly into short- and long-timescale variables. In this first paper in this series, we focus on the former, which is the majority class comprising 83 objects. We report periods for six sources for the first time, and recover several known periodic variables. Among the aperiodic sources, most exhibit a jitter around a median flux with a stable amplitude, and a few show outbursts. We draw attention to WeSb 1, which shows a different kind of variability: prominent deep and aperiodic dips, resembling transits from a dust/debris disk. We find strong evidence for a binary nature of WeSb 1 (possibly an F-type subgiant companion). The compactness of the emission lines and inferred high electron densities make WeSb 1 a candidate for either an EGB 6-type planetary nucleus, or a symbiotic system inside an evolved planetary nebula, both of which are rare objects. To demonstrate further promise with ZTF, we report three additional newly identified periodic sources that do not appear in the list of highly variable sources. Finally, we also introduce a two-dimensional metric space defined by the von Neumann statistics and Pearson Skew and demonstrate its effectiveness in identifying unique variables of astrophysical interest, like WeSb 1.

astro-ph.SR↗

arXiv2024

Prospects for Systematic Planetary Nebulae Detection with the Census of the Local Universe Narrowband Survey

We investigate the efficacy of a systematic planetary nebula (PN) search in the Census of the Local Universe (CLU) narrowband (H$α$) survey that covers a considerably larger sky region of above declination $-20^\circ$ than most previous surveys. Using PNe observed by the Isaac Newton Telescope Photometric H$α$ Survey (IPHAS) as validation, we are able to visually recover 432 out of 441 cataloged PNe (98\%) within the CLU dataset, with 5 sources having unusable CLU images and 4 missed due to limitations of imaging quality. Moreover, the reference PNe are conventionally divided into three PN classes in decreasing order of identification confidence given their spectra and morphologies. We record consistently high recovery rate across all classes: 95\% of True, 71\% of Likely, and 81\% of Possible sources are readily recovered. To further demonstrate the ability of CLU to find new PNe, we undertake a preliminary search of compact PNe within a sub-region of the validation catalog, mainly utilizing the significance of narrow-band colors ($Σ$) as a metric for identification. In a $200\,\rm deg^2$ region, we search the CLU source catalog and find 31 PN candidates after automated and visual scrutiny, of which 12 are new sources not appearing in previous studies. As a demonstration of our ongoing follow-up campaign, we present medium-resolution optical spectra of six candidates and notice that four of them show emission signatures characteristic of confirmed PNe. As we refine our selection methods, CLU promises to provide a systematic catalog of PNe spanning $2/3$ of the sky.

astro-ph.SR↗

arXiv2024

ZTF SN Ia DR2: Overview

We present the first homogeneous release of several thousand Type Ia supernovae (SNe Ia), all having spectroscopic classification, and spectroscopic redshifts for half the sample. This release, named the "DR2", contains 3628 nearby (z < 0.3) SNe Ia discovered, followed and classified by the Zwicky Transient Facility survey between March 2018 and December 2020. Of these, 3000 have good-to-excellent sampling and 2667 pass standard cosmology light-curve quality cuts. This release is thus the largest SN Ia release to date, increasing by an order of magnitude the number of well characterized low-redshift objects. With the "DR2", we also provide a volume-limited (z < 0.06) sample of nearly a thousand SNe Ia. With such a large, homogeneous and well controlled dataset, we are studying key current questions on SN cosmology, such as the linearity SNe Ia standardization, the SN and host dependencies, the diversity of the SN Ia population, and the accuracy of the current light-curve modeling. These, and more, are studied in detail in a series of articles associated with this release. Alongside the SN Ia parameters, we publish our force-photometry gri-band light curves, 5138 spectra, local and global host properties, observing logs, and a python tool to ease use and access of these data. The photometric accuracy of the "DR2" is not yet suited for cosmological parameter inference, which will follow as "DR2.5" release. We nonetheless demonstrate that the multi-thousand SN Ia Hubble Diagram has a typical 0.15 mag scatter.

astro-ph.CO↗

arXiv2024

Optical and Radio Analysis of Systematically Classified Broad-lined Type Ic Supernovae from the Zwicky Transient Facility

We study a magnitude-limited sample of 36 Broad-lined Type Ic Supernovae (SNe Ic-BL) from the Zwicky Transient Facility Bright Transient Survey (detected between March 2018 and August 2021), which is the largest systematic study of SNe Ic-BL done in literature thus far. We present the light curves (LCs) for each of the SNe, and analyze the shape of the LCs to derive empirical parameters, along with the explosion epochs for every event. The sample has an average absolute peak magnitude in the r band of $M_r^{max}$ = -18.51 $\pm$ 0.15 mag. Using spectra obtained around peak light, we compute expansion velocities from the Fe II 5169 Angstrom line for each event with high enough signal-to-noise ratio spectra, and find an average value of $v_{ph}$ = 16,100 $\pm$ 1,100 km $s^{-1}$. We also compute bolometric LCs, study the blackbody temperature and radii evolution over time, and derive the explosion properties of the SNe. The explosion properties of the sample have average values of $M_{Ni}$ = $0.37_{-0.06}^{+0.08}$ solar masses, $M_{ej}$ = $2.45_{-0.41}^{+0.47}$ solar masses, and $E_K$= $4.02_{-1.00}^{+1.37} \times 10^{51}$ erg. Thirteen events have radio observations from the Very Large Array, with 8 detections and 5 non-detections. We find that the populations that have radio detections and radio non-detections are indistinct from one another with respect to their optically-inferred explosion properties, and there are no statistically significant correlations present between the events' radio luminosities and optically-inferred explosion properties. This provides evidence that the explosion properties derived from optical data alone cannot give inferences about the radio properties of SNe Ic-BL, and likely their relativistic jet formation mechanisms.

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arXiv2024

How accurate are transient spectral classification tools? -- A study using 4,646 SEDMachine spectra

Accurate classification of transients obtained from spectroscopic data are important to understand their nature and discover new classes of astronomical objects. For supernovae (SNe), SNID, NGSF (a Python version of SuperFit), and DASH are widely used in the community. Each tool provides its own metric to help determine classification, such as rlap of SNID, chi2/dof of NGSF, and Probability of DASH. However, we do not know how accurate these tools are, and they have not been tested with a large homogeneous dataset. Thus, in this work, we study the accuracy of these spectral classification tools using 4,646 SEDMachine spectra, which have accurate classifications obtained from the Zwicky Transient Facility Bright Transient Survey (BTS). Comparing our classifications with those from BTS, we have tested the classification accuracy in various ways. We find that NGSF has the best performance (overall Accuracy 87.6% when samples are split into SNe Ia and Non-Ia types), while SNID and DASH have similar performance with overall Accuracy of 79.3% and 76.2%, respectively. Specifically for SNe Ia, SNID can accurately classify them when rlap > 15 without contamination from other types, such as Ibc, II, SLSN, and other objects that are not SNe (Purity > 98%). For other types, determining their classification is often uncertain. We conclude that it is difficult to obtain an accurate classification from these tools alone. This results in additional human visual inspection effort being required in order to confirm the classification. To reduce this human visual inspection and to support the classification process for future large-scale surveys, this work provides supporting information, such as the accuracy of each tool as a function of its metric.

astro-ph.IM↗

arXiv2024

A cosmic formation site of silicon and sulphur revealed by a new type of supernova explosion

The cores of stars are the cosmic furnaces where light elements are fused into heavier nuclei. The fusion of hydrogen to helium initially powers all stars. The ashes of the fusion reactions are then predicted to serve as fuel in a series of stages, eventually transforming massive stars into a structure of concentric shells. These are composed of natal hydrogen on the outside, and consecutively heavier compositions inside, predicted to be dominated by helium, carbon/oxygen, oxygen/neon/magnesium, and oxygen/silicon/sulphur. Silicon and sulphur are fused into inert iron, leading to the collapse of the core and either a supernova explosion or the direct formation of a black hole. Stripped stars, where the outer hydrogen layer has been removed and the internal He-rich layer (in Wolf-Rayet WN stars) or even the C/O layer below it (in Wolf-Rayet WC/WO stars) are exposed, provide evidence for this shell structure, and the cosmic element production mechanism it reflects. The types of supernova explosions that arise from stripped stars embedded in shells of circumstellar material (most notably Type Ibn supernovae from stars with outer He layers, and Type Icn supernovae from stars with outer C/O layers) confirm this scenario. However, direct evidence for the most interior shells, which are responsible for the production of elements heavier than oxygen, is lacking. Here, we report the discovery of the first-of-its-kind supernova arising from a star peculiarly stripped all the way to the silicon and sulphur-rich internal layer. Whereas the concentric shell structure of massive stars is not under debate, it is the first time that such a thick, massive silicon and sulphur-rich shell, expelled by the progenitor shortly before the SN explosion, has been directly revealed.

astro-ph.HE↗

arXiv2024

Probing pre-supernova mass loss in double-peaked Type Ibc supernovae from the Zwicky Transient Facility

Eruptive mass loss of massive stars prior to supernova (SN) explosion is key to understanding their evolution and end fate. An observational signature of pre-SN mass loss is the detection of an early, short-lived peak prior to the radioactive-powered peak in the lightcurve of the SN. This is usually attributed to the SN shock passing through an extended envelope or circumstellar medium (CSM). Such an early peak is common for double-peaked Type IIb SNe with an extended Hydrogen envelope but is uncommon for normal Type Ibc SNe with very compact progenitors. In this paper, we systematically study a sample of 14 double-peaked Type Ibc SNe out of 475 Type Ibc SNe detected by the Zwicky Transient Facility. The rate of these events is ~ 3-9 % of Type Ibc SNe. A strong correlation is seen between the peak brightness of the first and the second peak. We perform a holistic analysis of this sample's photometric and spectroscopic properties. We find that six SNe have ejecta mass less than 1.5 Msun. Based on the nebular spectra and lightcurve properties, we estimate that the progenitor masses for these are less than ~ 12 Msun. The rest have an ejecta mass > 2.4 Msun and a higher progenitor mass. This sample suggests that the SNe with low progenitor masses undergo late-time binary mass transfer. Meanwhile, the SNe with higher progenitor masses are consistent with wave-driven mass loss or pulsation-pair instability-driven mass loss simulations.

astro-ph.HE↗

arXiv2024

SN 2023zaw: an ultra-stripped, nickel-poor supernova from a low-mass progenitor

We present SN 2023zaw $-$ a sub-luminous ($\mathrm{M_r} = -16.7$ mag) and rapidly-evolving supernova ($\mathrm{t_{1/2,r}} = 4.9$ days), with the lowest nickel mass ($\approx0.002$ $\mathrm{M_\odot}$) measured among all stripped-envelope supernovae discovered to date. The photospheric spectra are dominated by broad He I and Ca NIR emission lines with velocities of $\sim10\ 000 - 12\ 000$ $\mathrm{km\ s^{-1}}$. The late-time spectra show prominent narrow He I emission lines at $\sim$1000$\ \mathrm{km\ s^{-1}}$, indicative of interaction with He-rich circumstellar material. SN 2023zaw is located in the spiral arm of a star-forming galaxy. We perform radiation-hydrodynamical and analytical modeling of the lightcurve by fitting with a combination of shock-cooling emission and nickel decay. The progenitor has a best-fit envelope mass of $\approx0.2$ $\mathrm{M_\odot}$ and an envelope radius of $\approx50$ $\mathrm{R_\odot}$. The extremely low nickel mass and low ejecta mass ($\approx0.5$ $\mathrm{M_\odot}$) suggest an ultra-stripped SN, which originates from a mass-losing low mass He-star (ZAMS mass $<$ 10 $\mathrm{M_\odot}$) in a close binary system. This is a channel to form double neutron star systems, whose merger is detectable with LIGO. SN 2023zaw underscores the existence of a previously undiscovered population of extremely low nickel mass ($< 0.005$ $\mathrm{M_\odot}$) stripped-envelope supernovae, which can be explored with deep and high-cadence transient surveys.

astro-ph.HE↗

arXiv2024

WTP19aalnxx: Discovery of a bright mid-infrared transient in the emerging class of low luminosity supernovae revealed by delayed circumstellar interaction

While core-collapse supernovae (SNe) often show early and consistent signs of circumstellar (CSM) interaction, some exhibit delayed signatures due to interaction with distant material around the progenitor star. Here we present the discovery in NEOWISE data of WTP19aalnxx, a luminous mid-infrared (IR) transient in the outskirts of the galaxy KUG 0022-007 at $\approx 190$ Mpc. First detected in 2018, WTP19aalnxx reaches a peak absolute (Vega) magnitude of $\approx-22$ at $4.6 \, μ$m in $\approx3$ yr, comparable to the most luminous interacting SNe. Archival data reveal a $\gtrsim 5\times$ fainter optical counterpart detected since 2015, while follow-up near-IR observations in 2022 reveal an extremely red ($Ks-W2 \approx 3.7$ mag) active transient. Deep optical spectroscopy confirm strong CSM interaction signatures via intermediate-width Balmer emission lines and coronal metal lines. Modeling the broadband spectral energy distribution, we estimate the presence of $\gtrsim 10^{-2}$ M$_\odot$ of warm dust, likely formed in the shock interaction region. Together with the lack of nebular Fe emission, we suggest that WTP19aalnxx is a missed, low (optical) luminosity SN in an emerging family of core-collapse SNe distinguished by their CSM-interaction-powered mid-IR emission that outshines the optical bands. Investigating the Zwicky Transient Facility sample of SNe in NEOWISE data, we find $17$ core-collapse SNe ($\gtrsim 3$% in a volume-limited sample) without early signs of CSM interaction that exhibit delayed IR brightening, suggestive of dense CSM shells at $\lesssim 10^{17}$cm. We suggest that synoptic IR surveys offer a new route to revealing late-time CSM interaction and the prevalence of intense terminal mass loss in massive stars.

astro-ph.HE↗