Search

We present a new photometric pipeline for the detection of pre-supernova (pre-SN) emission in the Young Supernova Experiment (YSE) sky survey. The method described is applied to SN 2020tlf, a type II SN (SN II) with precursor emission in the last ~100 days before first light. We re-analyze the YSE griz-band light curves of SN 2020tlf and provide revised pre-explosion photometry that includes a robust list of confident detection and limiting magnitudes. Compared to the results of Jacobson-Galan et al. 2022a, this new analysis yields fewer total r/i/z-band pre-SN detections at phases > -100 days. Furthermore, we discourage the use of the blackbody modeling of the pre-explosion spectral energy distribution, the pre-SN bolometric light curve and the blackbody model parameters presented in Jacobson-Galan et al. 2022a. Nevertheless, binned photometry of SN 2020tlf confirms a consistent progenitor luminosity of ~10$^{40}$ erg s$^{-1}$ before explosion., Comment: 5 pages, 1 figure. Published in RNAAS

The Mid-infrared ELT Imager and Spectrograph (METIS) is a first-generation instrument for the Extremely Large Telescope (ELT), Europe's next-generation 39 m ground-based telescope for optical and infrared wavelengths. METIS will offer diffraction-limited imaging, low- and medium-resolution slit spectroscopy, and coronagraphy for high-contrast imaging between 3 and 13 microns, as well as high-resolution integral field spectroscopy between 3 and 5 microns. The main METIS science goals are the detection and characterisation of exoplanets, the investigation of proto-planetary disks, and the formation of planets. The Single-Conjugate Adaptive Optics (SCAO) system corrects atmospheric distortions and is thus essential for diffraction-limited observations with METIS. Numerous challenging aspects of an ELT Adaptive Optics (AO) system are addressed in the mature designs for the SCAO control system and the SCAO hardware module: the complex interaction with the telescope entities that participate in the AO control, wavefront reconstruction with a fragmented and moving pupil, secondary control tasks to deal with differential image motion, non-common path aberrations and mis-registration. A K-band pyramid wavefront sensor and a GPU-based Real-Time Computer (RTC), tailored to the needs of METIS at the ELT, are core components. This current paper serves as a natural sequel to our previous work presented in Hippler et al. (2018). It includes updated performance estimations in terms of several key performance indicators, including achieved contrast curves. We outline all important design decisions that were taken, and present the major challenges we faced and the main analyses carried out to arrive at these decisions and eventually the final design. We also elaborate on our testing and verification strategy, and, last not least, comprehensively present the full design, hardware and software., Comment: 51 pages, 29 figures, accepted to Experimental Astronomy

We present the analysis of optical data of a bright and extremely-rapidly evolving transient, AT2024wpp, whose properties are similar to the enigmatic AT2018cow (aka the Cow). AT2024wpp rose to a peak brightness of c=-21.9mag in 4.3d and remained above the half-maximum brightness for only 6.7d. The blackbody fits to the multi-band photometry show that the event remained persistently hot (T>20000K) with a rapidly receding photosphere (v~11500km/s) until the end of the photometric dataset at +16.1d post-discovery. This behaviour mimics that of AT2018cow, albeit with a several times larger photosphere. The spectra are consistent with blackbody emission throughout our spectral sequence ending at +21.9d, showing a tentative, very broad emission feature at 5500{\AA} -- implying that the optical photosphere is likely within a near-relativistic outflow. Furthermore, reports of strong X-ray and radio emission cement the nature of AT2024wpp as a likely Cow-like transient. AT2024wpp is only the second event of the class with optical polarimetry. Our BVRI observations obtained from +6.1 to +14.4d show a low polarisation of P<0.5% across all bands, similar to AT2018cow that was consistent with P~0% during the same outflow-driven phase. In the absence of evidence for a preferential viewing angle, it is unlikely that both events would have shown low polarisation in the case that their photospheres were aspherical. As such, we conclude that the near-relativistic outflows launched in these events are likely highly spherical, but polarimetric observations of further events are crucial to constrain their ejecta geometry and stratification in detail., Comment: 10 pages, 6 figures. Submitted to MNRAS

A device filled with pure xenon first demonstrated the ability to operate simultaneously as a bubble chamber and scintillation detector in 2017. Initial results from data taken at thermodynamic thresholds down to ~4 keV showed sensitivity to ~20 keV nuclear recoils with no observable bubble nucleation by $\gamma$-ray interactions. This paper presents results from further operation of the same device at thermodynamic thresholds as low as 0.50 keV, hardware limited. The bubble chamber has now been shown to have sensitivity to ~1 keV nuclear recoils while remaining insensitive to bubble nucleation by $\gamma$-rays. A robust calibration of the chamber's nuclear recoil nucleation response, as a function of nuclear recoil energy and thermodynamic state, is presented. Stringent upper limits are established for the probability of bubble nucleation by $\gamma$-ray-induced Auger cascades, with a limit of $<1.1\times10^{-6}$ set at 0.50 keV, the lowest thermodynamic threshold explored., Comment: 10 pages, 7 figures

SN 2023tsz is a Type Ibn supernova (SNe Ibn) discovered in an extremely low-mass host. SNe Ibn are an uncommon subtype of stripped-envelope core-collapse SNe. They are characterised by narrow helium emission lines in their spectra and are believed to originate from the collapse of massive Wolf-Rayet (WR) stars, though their progenitor systems still remain poorly understood. In terms of energetics and spectrophotometric evolution, SN 2023tsz is largely a typical example of the class, although line profile asymmetries in the nebular phase are seen, which may indicate the presence of dust formation or unshocked circumstellar material. Intriguingly, SN 2023tsz is located in an extraordinarily low-mass host galaxy that is in the 2nd percentile for SESN host masses and star formation rates (SFR). The host has a radius of 1.0 kpc, a $g$-band absolute magnitude of $-12.73$, and an estimated metallicity of $\log(Z_{*}/Z_{\odot}$) = $-1.56$. The SFR and metallicity of the host galaxy raise questions about the progenitor of SN 2023tsz. The low SFR suggests that a star with sufficient mass to evolve into a WR would be uncommon in this galaxy. Further, the very low-metallicity is a challenge for single stellar evolution to enable H and He stripping of the progenitor and produce a SN Ibn explosion. The host galaxy of SN 2023tsz adds another piece to the ongoing puzzle of SNe Ibn progenitors, and demonstrates that they can occur in hosts too faint to be observed in contemporary sky surveys at a more typical SN Ibn redshift., Comment: 14 pages, 10 figures, submitted to MNRAS

We present ultraviolet, optical and near-infrared photometric and optical spectroscopic observations of the luminous, fast blue optical transient (LFBOT), CSS161010:045834-081803 (CSS161010). The transient was found in a low-redshift (z=0.033) dwarf galaxy. The light curves of CSS161010 are characterized by an extremely fast evolution and blue colours. The V-band light curve shows that CSS161010 reaches an absolute peak of M$_{V}^{max}=-20.66\pm0.06$ mag in 3.8 days from the start of the outburst. After maximum, CSS161010 follows a power-law decline $\propto t^{-2.8\pm0.1}$ in all optical bands. These photometric properties are comparable to those of well-observed LFBOTs such as AT 2018cow, AT 2020mrf and AT 2020xnd. However, unlike these objects, the spectra of CSS161010 show a remarkable transformation from a blue and featureless continuum to spectra dominated by very broad, entirely blueshifted hydrogen emission lines of velocities of up to 10% of the speed of light. The persistent blueshifted emission and the lack of any emission at the rest wavelength of CSS161010 are unique features not seen in any transient before CSS161010. The combined observational properties of CSS161010 and its M$_{*}\sim10^{8}$ M$_\odot$ dwarf galaxy host favour the tidal disruption of a star by an intermediate-mass black hole as its origin., Comment: 29 pages (including the appendix); 8 figures in the main text, 4 figures and 8 tables in the appendix. Accepted for publication in ApJ

We present the results from an extensive follow-up campaign of the Tidal Disruption Event (TDE) ASASSN-15oi spanning $\delta t \sim 10 - 3000$ d, offering an unprecedented window into the multiwavelength properties of a TDE during its first $\approx 8$ years of evolution. ASASSN-15oi is one of the few TDEs with strong detections at X-ray, optical/UV, and radio wavelengths and featured two delayed radio flares at $\delta t \sim 180$ d and $\delta t \sim 1400$ d. Our observations at $> 1400$ d reveal an absence of thermal X-rays, a late-time variability in the non-thermal X-ray emission, and sharp declines in the non-thermal X-ray and radio emission at $\delta t \sim 2800$ d and $\sim 3000$ d, respectively. The UV emission shows no significant evolution at $>400$ d and remains above the pre-TDE level. We show that a cooling envelope model can explain the thermal emission consistently across all epochs. We also find that a scenario involving episodic ejection of material due to stream-stream collisions is conducive to explaining the first radio flare. Given the peculiar spectral and temporal evolution of the late-time emission, however, constraining the origins of the second radio flare and the non-thermal X-rays remains challenging. Our study underscores the critical role of long-term, multiwavelength follow-up., Comment: 36 pages, 13 Figures, 8 Tables. Submitted to ApJ

Fast X-ray Transients (FXTs) are a new observational class of phenomena with no clear physical origin. This is at least partially a consequence of limited multi-wavelength follow up of this class of transients in real time. Here we present deep optical ($g-$ and $i-$ band) photometry with Keck, and prompt radio observations with the VLA of FXT 210423 obtained at ${\delta t \approx 14-36}$ days since the X-ray trigger. We use these multi-band observations, combined with publicly available data sets, to constrain the presence and physical properties of on-axis and off-axis relativistic jets such as those that can be launched by neutron-star mergers and tidal disruption events, which are among the proposed theoretical scenarios of FXTs. Considering a wide range of possible redshifts $z\le3.5$, circumstellar medium (CSM) density $n={10^{-6}-10^{-1}\,\rm{cm^{-3}}}$, isotropic-equivalent jet kinetic energy $E_{k,iso}={10^{48}-10^{55}\,\rm{erg}}$, we find that we can rule out wide jets with opening angle ${\theta_{j}=15^{\circ}}$ viewed within ${10^{\circ}}$ off-axis. For more collimated jets (${\theta_{j}=3^{\circ}}$) we can only rule out on-axis (${\theta_{obs}=0^{\circ}}$) orientations. This study highlights the constraining power of prompt multi-wavelength observations of FXTs discovered in real time by current (e.g., Einstein Probe) and future facilities., Comment: 14 pages, 6 figures

Luminous Fast Blue Optical Transients (LFBOTs) are a class of extragalactic transients notable for their rapid rise and fade times, blue colour and accompanying luminous X-ray and radio emission. Only a handful have been studied in detail since the prototypical example AT2018cow. Their origins are currently unknown, but ongoing observations of previous and new events are placing ever stronger constraints on their progenitors. We aim to put further constraints on the LFBOT AT2023fhn, and LFBOTs as a class, using information from the multi-wavelength transient light-curve, its host galaxy and local environment. Our primary results are obtained by fitting galaxy models to the spectral energy distribution of AT2023fhn's host and local environment, and by modelling the radio light-curve of AT2023fhn as due to synchrotron self-absorbed emission from an expanding blast-wave in the circumstellar medium. We find that neither the host galaxy nor circumstellar environment of AT2023fhn are unusual compared with previous LFBOTs, but that AT2023fhn has a much lower X-ray to ultraviolet luminosity ratio than previous events. We argue that the variety in ultraviolet-optical to X-ray luminosity ratios among LFBOTs is likely due to viewing angle differences, and that the diffuse, yet young local environment of AT2023fhn - combined with a similar circumstellar medium to previous events - favours a progenitor system containing a massive star with strong winds. Plausible progenitor models in this interpretation therefore include black hole/Wolf-Rayet mergers or failed supernovae., Comment: Accepted for publication in A&A. 15 pages, 12 figures, 8 tables

Time-domain astrophysics continues to grow rapidly, with the inception of new surveys drastically increasing data volumes. Democratised, distributed approaches to training sets for machine learning classifiers are crucial to make the most of this torrent of discovery -- with citizen science approaches proving effective at meeting these requirements. In this paper, we describe the creation of and the initial results from the $\textit{Kilonova Seekers}$ citizen science project, built to find transient phenomena from the GOTO telescopes in near real-time. $\textit{Kilonova Seekers}$ launched in July 2023 and received over 600,000 classifications from approximately 2,000 volunteers over the course of the LIGO-Virgo-KAGRA O4a observing run. During this time, the project has yielded 20 discoveries, generated a `gold-standard' training set of 17,682 detections for augmenting deep-learned classifiers, and measured the performance and biases of Zooniverse volunteers on real-bogus classification. This project will continue throughout the lifetime of GOTO, pushing candidates at ever-greater cadence, and directly facilitate the next-generation classification algorithms currently in development., Comment: 20 pages, 15 figures. Accepted in MNRAS

We report on spin variations in the intermediate polar and cataclysmic variable CC Scl, as seen by the Transiting Exoplanet Survey Satellite (TESS). By studying both the spin period and its harmonic, we find that the spin has varied since it was first observed in 2011. We find the latest spin value for the source to be 389.473(6)s, equivalent to 0.00450779(7) days, 0.02s shorter than the first value measured. A linear fit to these and intermediate data give a rate of change of spin ~-4.26(2.66)e10^-11 and a characteristic timescale tau~2.90e10^5 years, in line with other known intermediate polars with varying spin. The spin profile of this source also matches theoretical spin profiles of high-inclination intermediate polars, and furthermore, appears to have changed in shape over a period of three years. Such `spin-up' in an intermediate polar is considered to be from mass accretion onto the white dwarf (the primary), and we note the presence of dwarf nova eruptions in this source as being a possible catalyst of the variations., Comment: 5 pages, 6 figures. Accepted into MNRAS Letters

We present detailed radio observations of the tidal disruption event (TDE) ASASSN-19bt/AT2019ahk, obtained with the Australia Telescope Compact Array (ATCA), the Atacama Large Millimeter/submillimeter Array (ALMA), and the MeerKAT radio telescopes, spanning 40 to 1464 days after the onset of the optical flare. We find that ASASSN-19bt displays unusual radio evolution compared to other TDEs, as the peak brightness of its radio emission increases rapidly until 457 days post-optical discovery and then plateaus. Using a generalized approach to standard equipartition techniques, we estimate the energy and corresponding physical parameters for two possible emission geometries: a non-relativistic spherical outflow and a relativistic outflow observed from an arbitrary viewing angle. We find that the non-relativistic solution implies a continuous energy rise in the outflow from $E\sim10^{46}$ erg to $E\sim10^{49}$ erg with $\beta \approx 0.05$, while the off-axis relativistic jet solution instead suggests $E\approx10^{52}$ erg with $\Gamma\sim10$ erg at late times in the maximally off-axis case. We find that neither model provides a holistic explanation for the origin and evolution of the radio emission, emphasizing the need for more complex models. ASASSN-19bt joins the population of TDEs that display unusual radio emission at late times. Conducting long-term radio observations of these TDEs, especially during the later phases, will be crucial for understanding how these types of radio emission in TDEs are produced., Comment: 25 pages. Submitted to ApJ

This book was created as part of the SIRIUS B VERGE program to orient students to astrophysics as a broad field. The 2023-2024 VERGE program and the printing of this book is funded by the Women and Girls in Astronomy Program via the International Astronomical Union's North American Regional Office of Astronomy for Development and the Heising-Simons Foundation; as a result, this document is written by women in astronomy for girls who are looking to pursue the field. However, given its universal nature, the material covered in this guide is useful for anyone interested in pursuing astrophysics professionally., Comment: Introductory guide for students interested in pursuing astrophysics; to be submitted to BAAS

We present the first deep X-ray observations of a luminous FBOT AT2018cow, at $\sim3.7\,\rm{yr}$ since discovery, together with the re-analysis of the observation at $\delta t\sim 220$ d. X-ray emission is significantly detected at a location consistent with AT2018cow. The very soft X-ray spectrum and sustained luminosity are distinct from the spectral and temporal behavior of the LFBOT in the first $\sim100$ d, and would possibly signal the emergence of a new emission component, although a robust association with AT2018cow can only be claimed at $\delta t \sim220$ d, while at $\delta t \sim1350$ d contamination of the host galaxy cannot be excluded. We interpret these findings in the context of the late-time panchromatic emission from AT2018cow, which includes the detection of persistent, slowly-fading UV emission with $\nu L_{\nu}\approx 10^{39}\,\rm{erg\,s^{-1}}$. Similar to previous works, (and in analogy with arguments for Ultra-Luminous X-ray sources --ULXs), these late-time observations are consistent with thin-disks around Intermediate Mass Black Holes (IMBHs, with $M_{\bullet}\approx 10^3-10^4\, \rm{M_{\odot}}$) accreting at sub-Eddington rates. However, differently from previous studies, we find that smaller-mass BHs with $M_{\bullet}\approx 10-100\,\rm{M_{\odot}}$ accreting at $\gtrsim$ the Eddington rate cannot be ruled out, and provide a natural explanation for the inferred compact size ($R_{\rm out} \approx 40\,R_{\odot}$) of the accretion disk years after the optical flare. Most importantly, irrespective of the accretor mass, our study lends support to the hypothesis that LFBOTs are accretion-powered phenomena and that, specifically, LFBOTs constitute electromagnetic manifestations of super-Eddington accreting systems that evolve to $\lesssim$ Eddington over a $\approx 100$ days time scale., Comment: 17 pages, 3 figures, accepted for publication in ApJL

Luminous Fast Blue Optical Transients (LFBOTs) - the prototypical example being AT2018cow - are a rare class of events whose origins are poorly understood. They are characterised by rapid evolution, featureless blue spectra at early times, and luminous X-ray and radio emission. LFBOTs thus far have been found exclusively at small projected offsets from star-forming host galaxies. We present Hubble Space Telescope, Gemini, Chandra and Very Large Array observations of a new LFBOT, AT2023fhn. The Hubble Space Telescope data reveal a large offset (greater than 3.5 half-light radii) from the two closest galaxies, both at a redshift of 0.24. The location of AT2023fhn is in stark contrast with previous events, and demonstrates that LFBOTs can occur in a range of galactic environments., Comment: Accepted for publication in MNRASL. 7 pages, 4 figures, 2 tables

We present the results from a multi-year radio campaign of the superluminous supernova (SLSN) 2017ens, which yielded the earliest radio detection of a SLSN to date at the age of $\sim$3.3 years after explosion. SN2017ens was not detected at radio frequencies in the first $\sim$300\,d of evolution but reached $L_{\nu}\approx 10^{28}\,\rm{erg\,s^{-1}\,cm^{-2}}$ at $\nu\sim 6$ GHz, $\sim1250$ days post-explosion. Interpreting the radio observations in the context of synchrotron radiation from the supernova shock interaction with the circumstellar medium (CSM), we infer an effective mass-loss rate of $\approx 10^{-4}\,\rm{M_{\odot}yr^{-1}}$ at $r\sim 10^{17}$ cm from the explosion's site, for a wind speed of $v_w=50-60\,\rm{km\,s^{-1}}$ measured from optical spectra. These findings are consistent with the spectroscopic metamorphosis of SN2017ens from hydrogen-poor to hydrogen-rich $\sim190$ d after explosion reported by Chen et al., 2018. SN2017ens is thus an addition to the sample of hydrogen-poor massive progenitors that explode shortly after having lost their hydrogen envelope. The inferred circumstellar densities, implying a CSM mass up to $\sim0.5\,\rm{M_{\odot}}$, and low velocity of the ejection point at binary interactions (in the form of common envelope evolution and subsequent envelope ejection) playing a role in shaping the evolution of the stellar progenitors of SLSNe in the $\lesssim 500$ yr preceding core collapse., Comment: 9 pages, 4 figures, 1 table

A tidal disruption event (TDE) occurs when a star is destroyed by a supermassive black hole. Broadband radio spectral observations of TDEs trace the emission from any outflows or jets that are ejected from the vicinity of the supermassive black hole. However, radio detections of TDEs are rare, with less than 20 published to date, and only 11 with multi-epoch broadband coverage. Here we present the radio detection of the TDE AT2020vwl and our subsequent radio monitoring campaign of the outflow that was produced, spanning 1.5 years post-optical flare. We tracked the outflow evolution as it expanded between $10^{16}$ cm to $10^{17}$ cm from the supermassive black hole, deducing it was non-relativistic and launched quasi-simultaneously with the initial optical detection through modelling the evolving synchrotron spectra of the event. We deduce that the outflow is likely to have been launched by material ejected from stream-stream collisions (more likely), the unbound debris stream, or an accretion-induced wind or jet from the supermassive black hole (less likely). AT2020vwl joins a growing number of TDEs with well-characterised prompt radio emission, with future timely radio observations of TDEs required to fully understand the mechanism that produces this type of radio emission in TDEs., Comment: 15 pages, 9 figures, accepted for publication in MNRAS

GRB 221009A ($z=0.151$) is one of the closest known long $\gamma$-ray bursts (GRBs). Its extreme brightness across all electromagnetic wavelengths provides an unprecedented opportunity to study a member of this still-mysterious class of transients in exquisite detail. We present multi-wavelength observations of this extraordinary event, spanning 15 orders of magnitude in photon energy from radio to $\gamma$-rays. We find that the data can be partially explained by a forward shock (FS) from a highly-collimated relativistic jet interacting with a low-density wind-like medium. Under this model, the jet's beaming-corrected kinetic energy ($E_K \sim 4\times10^{50}$ erg) is typical for the GRB population. The radio and mm data provide strong limiting constraints on the FS model, but require the presence of an additional emission component. From equipartition arguments, we find that the radio emission is likely produced by a small amount of mass ($\lesssim6\times10^{-7} M_\odot$) moving relativistically ($\Gamma\gtrsim9$) with a large kinetic energy ($\gtrsim10^{49}$ erg). However, the temporal evolution of this component does not follow prescriptions for synchrotron radiation from a single power-law distribution of electrons (e.g. in a reverse shock or two-component jet), or a thermal electron population, perhaps suggesting that one of the standard assumptions of afterglow theory is violated. GRB 221009A will likely remain detectable with radio telescopes for years to come, providing a valuable opportunity to track the full lifecycle of a powerful relativistic jet., Comment: Accepted for publication in the Astrophysical Journal Letters

We present a detailed compilation and analysis of the X-ray phase space of low- to intermediate-redshift ($ 0\le z \le 1$) transients that consolidates observed light curves (and theory where necessary) for a large variety of classes of transient/variable phenomena in the 0.3--10 keV energy band. We include gamma-ray burst afterglows, supernovae, supernova shock breakouts and shocks interacting with the environment, tidal disruption events and active galactic nuclei, fast blue optical transients, cataclysmic variables, magnetar flares/outbursts and fast radio bursts, cool stellar flares, X-ray binary outbursts, and ultraluminous X-ray sources. Our overarching goal is to offer a comprehensive resource for the examination of these ephemeral events, extending the X-ray duration-luminosity phase space (DLPS) to show luminosity evolution. We use existing observations (both targeted and serendipitous) to characterize the behavior of various transient/variable populations. Contextualizing transient signals in the larger DLPS serves two primary purposes: to identify areas of interest (i.e., regions in the parameter space where one would expect detections, but in which observations have historically been lacking) and to provide initial qualitative guidance in classifying newly discovered transient signals. We find that while the most luminous (largely extragalactic) and least luminous (largely Galactic) part of the phase space is well-populated at $t > 0.1$ days, intermediate luminosity phenomena (L$_x = 10^{34} - 10^{42}$ erg s$^{-1}$) represent a gap in the phase space. We thus identify L$_x = 10^{34} - 10^{42}$ erg s$^{-1}$ and $t = 10^{-4} - 0.1$ days as a key discovery phase space in transient X-ray astronomy., Comment: 12 figures, 13 tables; version accepted to ApJ

The Scintillating Bubble Chamber (SBC) Collaboration is developing liquid-noble bubble chambers for the quasi-background-free detection of low-mass (GeV-scale) dark matter and coherent scattering of low-energy (MeV-scale) neutrinos (CE$\nu$NS). The first physics-scale demonstrator of this technique, a 10-kg liquid argon bubble chamber dubbed SBC-LAr10, is now being commissioned at Fermilab. This device will calibrate the background discrimination power and sensitivity of superheated argon to nuclear recoils at energies down to 100 eV. A second functionally-identical detector with a focus on radiopure construction is being built for SBC's first dark matter search at SNOLAB. The projected spin-independent sensitivity of this search is approximately $10^{-43}$ cm$^2$ at 1 GeV$/c^2$ dark matter particle mass. The scalability and background discrimination power of the liquid-noble bubble chamber make this technique a compelling candidate for future dark matter searches to the solar neutrino fog at 1 GeV$/c^2$ particle mass (requiring a $\sim$ton-year exposure with non-neutrino backgrounds sub-dominant to the solar CE$\nu$NS signal) and for high-statistics CE$\nu$NS studies at nuclear reactors., Comment: 35 pages, 12 figures, contributed white paper to Snowmass 2021 (final version for Snowmass proceedings)

We present multi-band ATLAS photometry for SN 2019tsf, a stripped-envelope Type Ib supernova (SESN). The SN shows a triple-peaked light curve and a late (re-)brightening, making it unique among stripped-envelope systems. The re-brightening observations represent the latest photometric measurements of a multi-peaked Type Ib SN to date. As late-time photometry and spectroscopy suggest no hydrogen, the potential circumstellar material (CSM) must be H-poor. Moreover, late (>150 days) spectra show no signs of narrow emission lines, further disfavouring CSM interaction. On the contrary, an extended CSM structure is seen through a follow-up radio campaign with Karl G. Jansky Very Large Array (VLA), indicating a source of bright optically thick radio emission at late times, which is highly unusual among H-poor SESNe. We attribute this phenomenology to an interaction of the supernova ejecta with spherically-asymmetric CSM, potentially disk-like, and we present several models that can potentially explain the origin of this rare Type Ib supernova. The warped disc model paints a novel picture, where the tertiary companion perturbs the progenitors CSM, that can explain the multi-peaked light curves of SNe, and here we apply it to SN 2019tsf. This SN 2019tsf is likely a member of a new sub-class of Type Ib SNe and among the recently discovered class of SNe that undergo mass transfer at the moment of explosion, Comment: 23 pages, Comments are welcome, Submitted to ApJ

We present the results from our seven-year long broad-band X-ray observing campaign of SN\,2014C with \emph{Chandra} and \emph{NuSTAR}. These coordinated observations represent the first look at the evolution of a young extragalactic SN in the 0.3-80 keV energy range in the years after core collapse. We find that the spectroscopic metamorphosis of SN\,2014C from an ordinary type Ib SN into an interacting SN with copious hydrogen emission is accompanied by luminous X-rays reaching $L_x\approx 5.6\times10^{40}\, \rm{erg\,s^{-1}}$ (0.3--100 keV) at $\sim 1000$ days post explosion and declining as $L_x\propto t^{-1}$ afterwards. The broad-band X-ray spectrum is of thermal origin and shows clear evidence for cooling after peak, with $T(t)\approx 20 \,{\rm keV}(t/t_{\rm pk})^{-0.5}$. Soft X-rays of sub-keV energy suffer from large photoelectric absorption originating from the local SN environment with $NH_{\rm int}(t)\approx3\times 10^{22}(t/400 \,\rm{days})^{-1.4}\,\rm{cm^{-2}}$. We interpret these findings as the result of the interaction of the SN shock with a dense ($n\approx 10^{5}-10^{6}\,\rm{cm^{-3}}$), H-rich disk-like circumstellar medium (CSM) with inner radius $\sim2\times 10^{16}$ cm and extending to $\sim 10^{17}$ cm. Based on the declining $NH_{\rm int}(t)$ and X-ray luminosity evolution, we infer a CSM mass of $\sim(1.2\,f$--2.0$\sqrt{f}) \rm{M_{\odot}}$, where $f$ is the volume filling factor. Finally, we place SN\,2014C in the context of 119 core-collapse SNe with evidence for strong shock interaction with a thick circumstellar medium and we highlight the challenges that the current mass-loss theories (including wave-driven mass loss, binary interaction and line-driven winds) face when interpreting the wide dynamic ranges of CSM parameters inferred from observations., Comment: We recognize that the supernova within our sample is non-exhaustive, please email daniel_brethauer@berkeley.edu for suggestions of additional core-collapse interacting supernovae

We present extensive multi-frequency VLA and VLBA observations of the radio-bright supernova (SN) IIb SN 2004C that span $\sim(40-2793)$ days post-explosion. We interpret the temporal evolution of the radio spectral energy distribution (SED) in the context of synchrotron self-absorbed (SSA) emission from the explosion's forward shock as it expands in the circumstellar medium (CSM) previously sculpted by the mass-loss history of the stellar progenitor. VLBA observations and modeling of the VLA data point to a blastwave with average velocity $\sim0.06c$ that carries an energy of $\sim 10^{49}$ erg. Our modeling further reveals a flat CSM density profile $\rho_{\rm{CSM}} \propto R^{-0.03 \pm0.22}$ up to a break radius $R_{br} \approx (1.96 \pm 0.10) \times 10^{16}$ cm, with a steep density gradient following $\rho_{\rm{CSM}} \propto R^{-2.3 \pm 0.5}$ at larger radii. We infer that the flat part of the density profile corresponds to a CSM shell with mass $\sim0.021 M_{\odot}$, and that the progenitor's effective mass-loss rate varied with time over the range $(50-500) \times 10^{-5} M_{\odot} \rm{yr}^{-1}$ for an adopted wind velocity $v_w =1000$ km $s^{-1}$ and shock microphysical parameters ${\epsilon}_e = 0.1, {\epsilon}_B = 0.01$. These results add to the mounting observational evidence for departures from the traditional single-wind mass-loss scenarios in evolved, massive stars in the centuries leading up to core collapse. Potentially viable scenarios include mass loss powered by gravity waves and/or interaction with a binary companion., Comment: 32 pages, 10 figures, submitted to ApJ

We present deep X-ray and radio observations of the Fast Blue Optical Transient (FBOT) AT2020xnd/ZTF20acigmel at $z=0.2433$ from $13$d to $269$d after explosion. AT2020xnd belongs to the category of optically luminous FBOTs with similarities to the archetypal event AT2018cow. AT2020xnd shows luminous radio emission reaching $L_{\nu}\approx8\times10^{29}$ergs$^{-1}$Hz$^{-1}$ at 20GHz and $75$d post explosion, accompanied by luminous and rapidly fading soft X-ray emission peaking at $L_{X}\approx6\times10^{42}$ergs$^{-1}$. Interpreting the radio emission in the context of synchrotron radiation from the explosion's shock interaction with the environment we find that AT2020xnd launched a high-velocity outflow ($v\sim$0.1-0.2$c$) propagating into a dense circumstellar medium (effective $\dot M\approx10^{-3}M_{\rm{sol}}$yr$^{-1}$ for an assumed wind velocity of $v_w=1000$kms$^{-1}$). Similar to AT2018cow, the detected X-ray emission is in excess compared to the extrapolated synchrotron spectrum and constitutes a different emission component, possibly powered by accretion onto a newly formed black hole or neutron star. These properties make AT2020xnd a high-redshift analog to AT2018cow, and establish AT2020xnd as the fourth member of the class of optically-luminous FBOTs with luminous multi-wavelength counterparts., Comment: 21 pages, 6 figures, 6 tables. Submitted to ApJ

We present panchromatic observations and modeling of supernova (SN) 2020tlf, the first normal type II-P/L SN with confirmed precursor emission, as detected by the Young Supernova Experiment transient survey with the Pan-STARRS1 telescope. Pre-explosion emission was detected in $riz-$bands at 130 days prior to SN 2020tlf and persisted at relatively constant flux until first light. Soon after discovery, "flash" spectroscopy of SN 2020tlf revealed prominent narrow symmetric emission lines ($v_w < 300$ km s$^{-1}$) that resulted from the photo-ionization of unshocked circumstellar material (CSM) shedded in progenitor mass loss episodes in the final weeks to months before explosion. Surprisingly, this novel display of pre-SN emission and associated mass loss occurred in a RSG progenitor with ZAMS mass of only 10-12 M$_{\odot}$, as inferred from nebular spectra. Modeling of the light curve and multi-epoch spectra with the non-LTE radiative transfer code CMFGEN and radiation-hydrodynamical (RHD) code HERACLES suggests a dense CSM limited to $r \approx 10^{15}$ cm, and mass loss rate of $10^{-2}$ M$_{\odot}$ yr$^{-1}$. The subsequent luminous light-curve plateau and persistent blue excess indicates an extended progenitor, compatible with a RSG model with $R_{\star} = 1100$ R$_{\odot}$. Inferences from the limits on the shock-powered X-ray and radio luminosity are consistent with these conclusions and suggest a CSM density of $\rho < 2 \times 10^{-16}$ g cm$^{-3}$ for distances of $r \approx 5 \times 10^{15}$ cm, as well as a mass loss rate of $\dot M<1.3 \times 10^{-5}\,\rm{M_{\odot}\,yr^{-1}}$ at larger distances. A promising power source for the observed precursor emission is the ejection of stellar material following energy disposition into the stellar envelope as a result of gravity waves emitted during either neon/oxygen burning or a nuclear flash from silicon combustion., Comment: 36 pages, 20 figures. Accepted for publication in ApJ

We present a population of 19 radio-luminous supernovae (SNe) with emission reaching $L_{\nu}{\sim}10^{26}-10^{29}\,\rm{erg\,s^{-1}Hz^{-1}}$ in the first epoch of the Very Large Array Sky Survey (VLASS) at $2-4$GHz. Our sample includes one long Gamma-Ray Burst, SN 2017iuk/GRB171205A, and 18 core-collapse SNe detected at $\approx (1-60)$years after explosion. No thermonuclear explosion shows evidence for bright radio emission, and hydrogen-poor progenitors dominate the sub-sample of core-collapse events with spectroscopic classification at the time of explosion (79\%). We interpret these findings into the context of the expected radio emission from the forward shock interaction with the circumstellar medium (CSM). We conclude that these observations require a departure from the single wind-like density profile (i.e., $\rho_{\rm{CSM}}\propto r^{-2}$) that is expected around massive stars and/or a departure from a spherical Newtonian shock. Viable alternatives include the shock interaction with a detached, dense shell of CSM formed by a large effective progenitor mass-loss rate $\dot M \sim (10^{-4}-10^{-1})$ M$_{\odot}$ yr$^{-1}$ (for an assumed wind velocity of $1000\,\rm{km\,s^{-1}}$); emission from an off-axis relativistic jet entering our line of sight; or the emergence of emission from a newly-born pulsar-wind nebula. The relativistic SN 2012ap that is detected 5.7 and 8.5 years after explosion with $L_{\nu}{\sim}10^{28}$ erg s$^{-1}$ Hz$^{-1}$ might constitute the first detections of an off-axis jet+cocoon system in a massive star. However, none of the VLASS-SNe with archival data points are consistent with our model off-axis jet light curves. Future multi-wavelength observations will distinguish among these scenarios.Our VLASS source catalogs, which were used to perform the VLASS cross matching, are publicly available at https://doi.org/10.5281/zenodo.4895112., Comment: Published on December 21, 2021 Comments are welcome 17 pages, 4 figures, 1 table

We present multiwavelength observations of the Type II SN 2020pni. Classified at $\sim 1.3$ days after explosion, the object showed narrow (FWHM $<250\,\textrm{km}\,\textrm{s}^{-1}$) recombination lines of ionized helium, nitrogen, and carbon, as typically seen in flash-spectroscopy events. Using the non-LTE radiative transfer code CMFGEN to model our first high resolution spectrum, we infer a progenitor mass-loss rate of $\dot{M}=(3.5-5.3)\times10^{-3}$ M$_{\odot}$ yr$^{-1}$ (assuming a wind velocity of $v_w=200\,\textrm{km}\,\textrm{s}^{-1}$), estimated at a radius of $R_{\rm in}=2.5\times10^{14}\,\rm{cm}$. In addition, we find that the progenitor of SN 2020pni was enriched in helium and nitrogen (relative abundances in mass fractions of 0.30$-$0.40, and $8.2\times10^{-3}$, respectively). Radio upper limits are also consistent with a dense CSM, and a mass-loss rate of $\dot M>5 \times 10^{-4}\,\rm{M_{\odot}\,yr^{-1}}$. During the first 4 days after first light, we also observe an increase in velocity of the hydrogen lines (from $\sim 250\,\textrm{km}\,\textrm{s}^{-1}$ to $\sim 1000\,\textrm{km}\,\textrm{s}^{-1}$), which suggests a complex CSM. The presence of dense and confined CSM, as well as its inhomogeneous structure, suggest a phase of enhanced mass loss of the progenitor of SN 2020pni during the last year before explosion. Finally, we compare SN 2020pni to a sample of other shock-photoionization events. We find no evidence of correlations among the physical parameters of the explosions and the characteristics of the CSM surrounding the progenitors of these events. This favors the idea that the mass-loss experienced by massive stars during their final years could be governed by stochastic phenomena, and that, at the same time, the physical mechanisms responsible for this mass-loss must be common to a variety of different progenitors., Comment: 19 pages, 14 figures

Fast-rotating pulsars and magnetars have been suggested as the central engines of super-luminous supernovae (SLSNe) and fast radio bursts, and this scenario naturally predicts non-thermal synchrotron emission from their nascent pulsar wind nebulae (PWNe). We report results of high-frequency radio observations with ALMA and NOEMA for three SLSNe (SN 2015bn, SN 2016ard, and SN 2017egm), and present a detailed theoretical model to calculate non-thermal emission from PWNe with an age of about 1-3 yr. We find that the ALMA data disfavors a PWN model motivated by the Crab nebula for SN 2015bn and SN 2017egm, and argue that this tension can be resolved if the nebular magnetization is very high or very low. Such models can be tested by future MeV-GeV gamma-ray telescopes such as AMEGO., Comment: 7 pages, 4 figures, 2 tables

For the first $\sim3$ years after the binary neutron star merger event GW 170817 the radio and X-ray radiation has been dominated by emission from a structured relativistic off-axis jet propagating into a low-density medium with n $< 0.01\,\rm{cm^{-3}}$. We report on observational evidence for an excess of X-ray emission at $\delta t>900$ days after the merger. With $L_x\approx5\times 10^{38}\,\rm{erg\,s^{-1}}$ at 1234 days, the recently detected X-ray emission represents a $\ge 3.2\,\sigma$ (Gaussian equivalent) deviation from the universal post jet-break model that best fits the multi-wavelength afterglow at earlier times. In the context of JetFit afterglow models, current data represent a departure with statistical significance $\ge 3.1\,\sigma$, depending on the fireball collimation, with the most realistic models showing excesses at the level of $\ge 3.7\,\sigma$. A lack of detectable 3 GHz radio emission suggests a harder broad-band spectrum than the jet afterglow. These properties are consistent with the emergence of a new emission component such as synchrotron radiation from a mildly relativistic shock generated by the expanding merger ejecta, i.e. a kilonova afterglow. In this context, we present a set of ab-initio numerical-relativity BNS merger simulations that show that an X-ray excess supports the presence of a high-velocity tail in the merger ejecta, and argues against the prompt collapse of the merger remnant into a black hole. Radiation from accretion processes on the compact-object remnant represents a viable alternative. Neither a kilonova afterglow nor accretion-powered emission have been observed before, as detections of BNS mergers at this phase of evolution are unprecedented., Comment: 30 pages, 13 figures, Accepted for publication in ApJL

We present X-ray and radio observations of what may be the closest type Iax supernova (SN) to date, SN 2014dt (d=12.3-19.3 Mpc) and provide tight constraints on the radio and X-ray emission. We infer a specific radio luminosity of < (1.0-2.4)E25 erg/s/Hz at a frequency of 7.5 GHz and a X-ray luminosity < 1.4E38 erg/s (0.3-10 keV) at ~38-48 days post-explosion. We interpret these limits in the context of Inverse Compton (IC) emission and synchrotron emission from a population of electrons accelerated at the forward shock of the explosion in a power-law distribution $N_e(\gamma_e)\propto \gamma_e^{-p}$ with p=3. Our analysis constrains the progenitor system mass-loss rate to be smaller than 5E-6 solar masses per year at distances where r <= 1E16 cm for an assumed wind velocity v=100 km/s, and a fraction of post-shock energy into magnetic fields and relativistic electrons of epsilon_B=0.01 and epsilon_e=0.1, respectively. This result rules out some of the parameter space of symbiotic giant star companions, and it is consistent with the low mass-loss rates expected from He-star companions. Our calculations also show that the improved sensitivity of the next generation Very Large Array (ngVLA) is needed to probe the very low-density media characteristic of He stars that are the leading model for binary stellar companions of white dwarfs giving origin to type Iax SNe., Comment: Submitted to MNRAS, 9 pages and 7 figures

The physics reach of a low threshold (100 eV) scintillating argon bubble chamber sensitive to Coherent Elastic neutrino-Nucleus Scattering (CE$\nu$NS) from reactor neutrinos is studied. The sensitivity to the weak mixing angle, neutrino magnetic moment, and a light $Z'$ gauge boson mediator are analyzed. A Monte Carlo simulation of the backgrounds is performed to assess their contribution to the signal. The analysis shows that world-leading sensitivities are achieved with a one-year exposure for a 10 kg chamber at 3 m from a 1 MW$_{th}$ research reactor or a 100 kg chamber at 30 m from a 2000 MW$_{th}$ power reactor. Such a detector has the potential to become the leading technology to study CE$\nu$NS using nuclear reactors., Comment: 9 pages, 5 figures. v2: figures added, minor changes. Matches published version

Time domain science has undergone a revolution over the past decade, with tens of thousands of new supernovae (SNe) discovered each year. However, several observational domains, including SNe within days or hours of explosion and faint, red transients, are just beginning to be explored. Here, we present the Young Supernova Experiment (YSE), a novel optical time-domain survey on the Pan-STARRS telescopes. Our survey is designed to obtain well-sampled $griz$ light curves for thousands of transient events up to $z \approx 0.2$. This large sample of transients with 4-band light curves will lay the foundation for the Vera C. Rubin Observatory and the Nancy Grace Roman Space Telescope, providing a critical training set in similar filters and a well-calibrated low-redshift anchor of cosmologically useful SNe Ia to benefit dark energy science. As the name suggests, YSE complements and extends other ongoing time-domain surveys by discovering fast-rising SNe within a few hours to days of explosion. YSE is the only current four-band time-domain survey and is able to discover transients as faint $\sim$21.5 mag in $gri$ and $\sim$20.5 mag in $z$, depths that allow us to probe the earliest epochs of stellar explosions. YSE is currently observing approximately 750 square degrees of sky every three days and we plan to increase the area to 1500 square degrees in the near future. When operating at full capacity, survey simulations show that YSE will find $\sim$5000 new SNe per year and at least two SNe within three days of explosion per month. To date, YSE has discovered or observed 8.3% of the transient candidates reported to the International Astronomical Union in 2020. We present an overview of YSE, including science goals, survey characteristics and a summary of our transient discoveries to date., Comment: ApJ, in press; more information at https://yse.ucsc.edu/

We present the largest and deepest late-time radio and millimeter survey to date of superluminous supernovae (SLSNe) and long duration gamma-ray bursts (LGRBs) to search for associated non-thermal synchrotron emission. Using the Karl G. Jansky Very Large Array (VLA) and the Atacama Large Millimeter/submillimeter Array (ALMA), we observed 43 sources at 6 and 100 GHz on a timescale of $\sim 1 - 19$ yr post-explosion. We do not detect radio/mm emission from any of the sources, with the exception of a 6 GHz detection of PTF10hgi (Eftekhari et al. 2019), as well as the detection of 6 GHz emission near the location of the SLSN PTF12dam, which we associate with its host galaxy. We use our data to place constraints on central engine emission due to magnetar wind nebulae and off-axis relativistic jets. We also explore non-relativistic emission from the SN ejecta, and place constraints on obscured star formation in the host galaxies. In addition, we conduct a search for fast radio bursts (FRBs) from some of the sources using VLA Phased-Array observations; no FRBs are detected to a limit of $16$ mJy ($7\sigma$; 10 ms duration) in about 40 min on source per event. A comparison to theoretical models suggests that continued radio monitoring may lead to detections of persistent radio emission on timescales of $\gtrsim {\rm decade}$., Comment: 30 pages; 12 figures; accepted to ApJ

We present panchromatic observations and modeling of the Calcium-rich supernova 2019ehk in the star-forming galaxy M100 (d$\approx$16.2 Mpc) starting 10 hours after explosion and continuing for ~300 days. SN 2019ehk shows a double-peaked optical light curve peaking at $t = 3$ and $15$ days. The first peak is coincident with luminous, rapidly decaying $\textit{Swift}$-XRT discovered X-ray emission ($L_x\approx10^{41}~\rm{erg~s^{-1}}$ at 3 days; $L_x \propto t^{-3}$), and a Shane/Kast spectral detection of narrow H$\alpha$ and He II emission lines ($v \approx 500$ km/s) originating from pre-existent circumstellar material. We attribute this phenomenology to radiation from shock interaction with extended, dense material surrounding the progenitor star at $r<10^{15}$ cm and the resulting cooling emission. We calculate a total CSM mass of $\sim$ $7\times10^{-3}$ $\rm{M_{\odot}}$ with particle density $n\approx10^{9}\,\rm{cm^{-3}}$. Radio observations indicate a significantly lower density $n < 10^{4}\,\rm{cm^{-3}}$ at larger radii. The photometric and spectroscopic properties during the second light curve peak are consistent with those of Ca-rich transients (rise-time of $t_r =13.4\pm0.210$ days and a peak B-band magnitude of $M_B =-15.1\pm0.200$ mag). We find that SN 2019ehk synthesized $(3.1\pm0.11)\times10^{-2} ~ \rm{M_{\odot}}$ of ${}^{56}\textrm{Ni}$ and ejected $M_{\rm ej} = (0.72\pm 0.040)~\rm{M_{\odot}}$ total with a kinetic energy $E_{\rm k}=(1.8\pm0.10)\times10^{50}~\rm{erg}$. Finally, deep $\textit{HST}$ pre-explosion imaging at the SN site constrains the parameter space of viable stellar progenitors to massive stars in the lowest mass bin (~10 $\rm{M_{\odot}}$) in binaries that lost most of their He envelope or white dwarfs. The explosion and environment properties of SN 2019ehk further restrict the potential WD progenitor systems to low-mass hybrid HeCO WD + CO WD binaries., Comment: 51 pages, 27 figures. Accepted for publication in ApJ

We present a classification of galaxies in the Pan-STARRS1 (PS1) 3$\pi$ survey based on their recent star formation history and morphology. Specifically, we train and test two Random Forest (RF) classifiers using photometric features (colors and moments) from the PS1 data release 2. Labels for the morphological classification are taken from Huertas-Company+2011, while labels for the star formation fraction (SFF) are from the Blanton+2005 catalog. We find that colors provide more predictive accuracy than photometric moments. We morphologically classify galaxies as either early- or late-type, and our RF model achieves a 78\% classification accuracy. Our second model classifies galaxies as having either a low-to-moderate or high SFF. This model achieves an 89\% classification accuracy. We apply both RF classifiers to the entire PS1 $3\pi$ dataset, allowing us to assign two scores to each PS1 source: $P_\mathrm{HSFF}$, which quantifies the probability of having a high SFF, and $P_\mathrm{spiral}$, which quantifies the probability of having a late-type morphology. Finally, as a proof of concept, we apply our classification framework to supernova (SN) host-galaxies from the Zwicky Transient Factory and the Lick Observatory Supernova Search samples. We show that by selecting on $P_\mathrm{HSFF}$ or $P_\mathrm{spiral}$ it is possible to significantly enhance or suppress the fraction of core-collapse SNe (or thermonuclear SNe) in the sample with respect to random guessing. This result demonstrates how contextual information can aid transient classifications at the time of first detection. In the current era of spectroscopically-starved time-domain astronomy, prompt automated classification is paramount.

We present X-ray and radio observations of the Fast Blue Optical Transient (FBOT) CRTS-CSS161010 J045834-081803 (CSS161010 hereafter) at t=69-531 days. CSS161010 shows luminous X-ray ($L_x\sim5\times 10^{39}\,\rm{erg\,s^{-1}}$) and radio ($L_{\nu}\sim10^{29}\,\rm{erg\,s^{-1}Hz^{-1}}$) emission. The radio emission peaked at ~100 days post transient explosion and rapidly decayed. We interpret these observations in the context of synchrotron emission from an expanding blastwave. CSS161010 launched a mildly relativistic outflow with velocity $\Gamma\beta c\ge0.55c$ at ~100 days. This is faster than the non-relativistic AT2018cow ($\Gamma\beta c\sim0.1c$) and closer to ZTF18abvkwla ($\Gamma\beta c\ge0.3c$ at 63 days). The inferred initial kinetic energy of CSS161010 ($E_k\gtrsim10^{51}$ erg) is comparable to that of long Gamma Ray Bursts (GRBs), but the ejecta mass that is coupled to the mildly relativistic outflow is significantly larger ($\sim0.01-0.1\,\rm{M_{\odot}}$). This is consistent with the lack of observed gamma-rays. The luminous X-rays were produced by a different emission component to the synchrotron radio emission. CSS161010 is located at ~150 Mpc in a dwarf galaxy with stellar mass $M_{*}\sim10^{7}\,\rm{M_{\odot}}$ and specific star formation rate $sSFR\sim 0.3\,\rm{Gyr^{-1}}$. This mass is among the lowest inferred for host-galaxies of explosive transients from massive stars. Our observations of CSS161010 are consistent with an engine-driven aspherical explosion from a rare evolutionary path of a H-rich stellar progenitor, but we cannot rule out a stellar tidal disruption event on a centrally-located intermediate mass black hole. Regardless of the physical mechanism, CSS161010 establishes the existence of a new class of rare (rate $<0.4\%$ of the local core-collapse supernova rate) H-rich transients that can launch mildly relativistic outflows., Comment: Accepted to ApJL

For decades cataclysmic variables (CVs) were thought to be one of the few classes of accreting compact objects to not launch jets, and have consequently been used to constrain jet launching models. However, recent theoretical and observational advances indicate that CVs do in fact launch jets. Specifically, it was demonstrated that their accretion-outflow cycle is analogous to that of their higher mass cousins -- the X-ray Binaries (XRBs). Subsequent observations of the CV SS Cygni confirmed this and have consistently shown radio flaring equivalent to that in the XRBs that marks a transient jet. Based on this finding and the emission properties, several studies have concluded that the radio emission is most likely from a transient jet. Observations of other CVs, while not conclusive, are consistent with this interpretation. However, the issue is not yet settled. Later observations have raised a number of questions about this model, as well as about potential alternative radio emission mechanisms. CVs are non-relativistic and many have well-determined distances; these properties would make them ideal candidates with which to address many of our outstanding questions about fundamental jet physics. Here we review the case for jets in CVs, discuss the outstanding questions and issues, and outline the future work necessary to conclusively answer the question of whether CVs launch jets., Comment: To appear in New Astronomy Reviews special volume '100 Years of Astrophysical Jets' (eds. Rob Fender and Ralph Wijers)

We report on VLBI observations of the fast and blue optical transient (FBOT), AT 2018cow. At ~62 Mpc, AT 2018cow is the first relatively nearby FBOT. The nature of AT 2018cow is not clear, although various hypotheses from a tidal disruption event to different kinds of supernovae have been suggested. It had a very fast rise time (3.5 d) and an almost featureless blue spectrum although high photospheric velocities (40,000 km s$^{-1}$) were suggested early on. The X-ray luminosity was very high, ~$1.4 \times 10^{43}$ erg s$^{-1}$, larger than those of ordinary SNe, and more consistent with those of SNe associated with gamma-ray bursts. Variable hard X-ray emission hints at a long-lived "central engine." It was also fairly radio luminous, with a peak 8.4-GHz spectral luminosity of ~$4 \times 10^{28}$ erg s$^{-1}$ Hz$^{-1}$, allowing us to make VLBI observations at ages between 22 and 287 d. We do not resolve AT 2018cow. Assuming a circularly symmetric source, our observations constrain the average apparent expansion velocity to be <0.49$c$ by t = 98 d (3$\sigma$ limit). We also constrain the proper motion of AT 2018cow to be <0.51$c$. Since the radio emission generally traces the fastest ejecta, our observations make the presence of a long-lived relativistic jet with a lifetime of more than one month very unlikely., Comment: Accepted for publication in MNRAS 3 figurs]es

We present Chandra and VLA observations of GW170817 at ~521-743 days post merger, and a homogeneous analysis of the entire Chandra data set. We find that the late-time non-thermal emission follows the expected evolution from an off-axis relativistic jet, with a steep temporal decay $F_{\nu}\propto t^{-1.95\pm0.15}$ and a simple power-law spectrum $F_{\nu}\propto \nu^{-0.575\pm0.007}$. We present a new method to constrain the merger environment density based on diffuse X-ray emission from hot plasma in the host galaxy and we find $n\le 9.6 \times 10^{-3}\,\rm{cm^{-3}}$. This measurement is independent from inferences based on the jet afterglow modeling and allows us to partially solve for model degeneracies. The updated best-fitting model parameters with this density constraint are a fireball kinetic energy $E_0 = 1.5_{-1.1}^{+3.6}\times 10^{49}\,\rm{erg}$ ($E_{iso}= 2.1_{-1.5}^{+6.4}\times10^{52}\, \rm{erg}$), jet opening angle $\theta_{0}= 5.9^{+1.0}_{-0.7}\,\rm{deg}$ with characteristic Lorentz factor $\Gamma_j = 163_{-43}^{+23}$, expanding in a low-density medium with $n_0 = 2.5_{-1.9}^{+4.1} \times 10^{-3}\, \rm{cm^{-3}}$ and viewed $\theta_{obs} = 30.4^{+4.0}_{-3.4}\, \rm{deg}$ off-axis. The synchrotron emission originates from a power-law distribution of electrons with $p=2.15^{+0.01}_{-0.02}$. The shock microphysics parameters are constrained to $\epsilon_{\rm{e}} = 0.18_{-0.13}^{+0.30}$ and $\epsilon_{\rm{B}}=2.3_{-2.2}^{+16.0} \times 10^{-3}$. We investigate the presence of X-ray flares and find no statistically significant evidence of $\ge2.5\sigma$ of temporal variability at any time. Finally, we use our observations to constrain the properties of synchrotron emission from the deceleration of the fastest kilonova ejecta with energy $E_k^{KN}\propto (\Gamma\beta)^{-\alpha}$ into the environment, finding that shallow stratification indexes $\alpha\le6$ are disfavored., Comment: version accepted for publication in ApJL, 13 pages, 6 figures

We present a revised and complete optical afterglow light curve of the binary neutron star merger GW170817, enabled by deep Hubble Space Telescope (HST) F606W observations at $\approx\!584$ days post-merger, which provide a robust optical template. The light curve spans $\approx 110-362$ days, and is fully consistent with emission from a relativistic structured jet viewed off-axis, as previously indicated by radio and X-ray data. Combined with contemporaneous radio and X-ray observations, we find no spectral evolution, with a weighted average spectral index of $\langle \beta \rangle = -0.583 \pm 0.013$, demonstrating that no synchrotron break frequencies evolve between the radio and X-ray bands over these timescales. We find that an extrapolation of the post-peak temporal slope of GW170817 to the luminosities of cosmological short GRBs matches their observed jet break times, suggesting that their explosion properties are similar, and that the primary difference in GW170817 is viewing angle. Additionally, we place a deep limit on the luminosity and mass of an underlying globular cluster of $L \lesssim 6.7 \times 10^{3}\,L_{\odot}$, or $M \lesssim 1.3 \times 10^{4}\,M_{\odot}$, at least 4 standard deviations below the peak of the globular cluster mass function of the host galaxy, NGC4993. This limit provides a direct and strong constraint that GW170817 did not form and merge in a globular cluster. As highlighted here, HST (and soon JWST) enables critical observations of the optical emission from neutron star merger jets and outflows., Comment: 11 pages, 6 figures, 2 tables. Accepted to ApJ Letters

We present comprehensive multiwavelength radio to X-ray observations of GRB 181201A spanning from $\approx150$ s to $\approx163$ days after the burst, comprising the first joint ALMA-VLA-GMRT observations of a gamma-ray burst (GRB) afterglow. The radio and mm-band data reveal a distinct signature at $\approx3.9$ days, which we interpret as reverse shock (RS) emission. Our observations present the first time that a single radio-frequency spectral energy distribution can be decomposed directly into RS and forward shock (FS) components. We perform detailed modeling of the full multiwavelength data set, using Markov Chain Monte Carlo sampling to construct the joint posterior density function of the underlying physical parameters describing the RS and FS synchrotron emission. We uncover and account for all degeneracies in the model parameters. The joint RS-FS modeling reveals a weakly magnetized ($\sigma\approx3\times10^{-3}$), mildly relativistic RS, from which we derive an initial bulk Lorentz factor of $\Gamma_0\approx103$ for the GRB jet. Our results support the hypothesis that low-density environments are conducive to the observability of RS emission. We compare our observations to other events with strong RS detections, and find a likely observational bias selecting for longer lasting, non-relativistic reverse shocks. We present and begin to address new challenges in modeling posed by the present generation of comprehensive, multi-frequency data sets., Comment: Accepted version

Broad-line type Ic Supernovae (BL-Ic SNe) are characterized by high ejecta velocity ($\gtrsim 10^4$ km s$^{-1}$) and are sometimes associated with the relativistic jets typical of long duration ($\gtrsim 2$ s) Gamma-Ray Bursts (L-GRBs). The reason why a small fraction of BL-Ic SNe harbor relativistic jets is not known. Here we present deep X-ray and radio observations of the BL-Ic SN2014ad extending from $13$ to $930$ days post explosion. SN2014ad was not detected at either frequency and has no observational evidence of a GRB counterpart. The proximity of SN2014ad ($d\sim 26$ Mpc) enables very deep constraints on the progenitor mass-loss rate $\dot{M}$ and on the total energy of the fast ejecta $E$. We consider two synchrotron emission scenarios for a wind-like circumstellar medium (CSM): (i) uncollimated non-relativistic ejecta, and (ii) off-axis relativistic jet. Within the first scenario our observations are consistent with GRB-less BL-Ic SNe characterized by a modest energy budget of their fast ejecta ($E \lesssim 10^{45}$ erg), like SNe 2002ap and 2010ay. For jetted explosions, we cannot rule out a GRB with $E \lesssim 10^{51}$ erg (beam-corrected) with a narrow opening angle ($\theta_j \sim 5^{\circ}$) observed moderately off-axis ($\theta_{\rm obs} \gtrsim 30^{\circ}$) and expanding in a very low CSM density ($\dot{M}$ $\lesssim 10^{-6}$ M$_{\odot}$ yr$^{-1}$). Our study shows that off-axis low-energy jets expanding in a low-density medium cannot be ruled out even in the most nearby BL-Ic SNe with extensive deep observations, and might be a common feature of BL-Ic SNe., Comment: 9 pages, 5 figures, accepted in ApJ

We present comprehensive observations and analysis of the energetic H-stripped SN 2016coi (a.k.a. ASASSN-16fp), spanning the $\gamma$-ray through optical and radio wavelengths, acquired within the first hours to $\sim$420 days post explosion. Our campaign confirms the identification of He in the SN ejecta, which we interpret to be caused by a larger mixing of Ni into the outer ejecta layers. From the modeling of the broad bolometric light curve we derive a large ejecta mass to kinetic energy ratio ($M_{\rm{ej}}\sim 4-7\,\rm{M_{\odot}}$, $E_{\rm{k}}\sim 7-8\times 10^{51}\,\rm{erg}$). The small [\ion{Ca}{ii}] \lam\lam7291,7324 to [\ion{O}{i}] \lam\lam6300,6364 ratio ($\sim$0.2) observed in our late-time optical spectra is suggestive of a large progenitor core mass at the time of collapse. We find that SN 2016coi is a luminous source of X-rays ($L_{X}>10^{39}\,\rm{erg\,s^{-1}}$ in the first $\sim100$ days post explosion) and radio emission ($L_{8.5\,GHz}\sim7\times 10^{27}\,\rm{erg\,s^{-1}Hz^{-1}}$ at peak). These values are in line with those of relativistic SNe (2009bb, 2012ap). However, for SN 2016coi we infer substantial pre-explosion progenitor mass-loss with rate $\dot M \sim (1-2)\times 10^{-4}\,\rm{M_{\odot}yr^{-1}}$ and a sub-relativistic shock velocity $v_{sh}\sim0.15c$, in stark contrast with relativistic SNe and similar to normal SNe. Finally, we find no evidence for a SN-associated shock breakout $\gamma$-ray pulse with energy $E_{\gamma}>2\times 10^{46}\,\rm{erg}$. While we cannot exclude the presence of a companion in a binary system, taken together, our findings are consistent with a massive single star progenitor that experienced large mass loss in the years leading up to core-collapse, but was unable to achieve complete stripping of its outer layers before explosion., Comment: Submitted to ApJ. Main text: 21 pages; Appendix: 15 pages; 12 figures

On 2019 April 25.346 and 26.640 UT the LIGO and Virgo gravitational wave (GW) observatories announced the detection of the first candidate events in Observing Run 3 that contain at least one neutron star. S190425z is a likely binary neutron star (BNS) merger at $d_L = 156 \pm 41$ Mpc, while S190426c is possibly the first NS-BH merger ever detected, at $d_L = 377 \pm 100$ Mpc, although with marginal statistical significance. Here we report our optical follow-up observations for both events using the MMT 6.5-m telescope, as well as our spectroscopic follow-up of candidate counterparts (which turned out to be unrelated) with the 4.1-m SOAR telescope. We compare to publicly reported searches, explore the overall areal coverage and depth, and evaluate those in relation to the optical/NIR kilonova emission from the BNS merger GW170817, to theoretical kilonova models, and to short GRB afterglows. We find that for a GW170817-like kilonova, the partial volume covered spans up to about 40% for S190425z and 60% for S190426c. For an on-axis jet typical of short GRBs, the search effective volume is larger, but such a configuration is expected in at most a few percent of mergers. We further find that wide-field $\gamma$-ray and X-ray limits rule out luminous on-axis SGRBs, for a large fraction of the localization regions, although these searches are not sufficiently deep in the context of the $\gamma$-ray emission from GW170817 or off-axis SGRB afterglows. The results indicate that some optical follow-up searches are sufficiently deep for counterpart identification to about 300 Mpc, but that localizations better than 1000 deg$^2$ are likely essential., Comment: The first six authors contributed equally to this work

Whether supernovae are a significant source of dust has been a long-standing debate. The large quantities of dust observed in high-redshift galaxies raise a fundamental question as to the origin of dust in the Universe since stars cannot have evolved to the AGB dust-producing phase in high-redshift galaxies. In contrast, supernovae occur within several millions of years after the onset of star formation. This white paper focuses on dust formation in supernova ejecta with US-Extremely Large Telescope (ELT) perspective during the era of JWST and LSST., Comment: Astro2020 Science White Paper: Dust Formation in Supernovae with US-ELT Perspective

The Large Synoptic Survey Telescope (LSST) will open a discovery frontier for faint and fast transients with its ability to detect variable flux components down to $\sim$24.5 mag in a $\sim$30 second exposure. Spectroscopic follow-up of such phenomena - which are necessary for understanding the physics of stellar explosions - can require a rapid response and several hours with a 8-10m telescope, making it both expensive and difficult to acquire. The future Extremely Large Telescopes (ELTs) would be able to provide not only spectroscopy but capabilities such as spectropolarimetry and high-resolution diffraction-limited imaging that would contribute to future advances in our physical understanding of stellar explosions. In this white paper we focus on several specific scientific impacts in the field of explosive transient astrophysics that will be generated by the combination of LSST's discovery abilities and ELTs' follow-up capacities. First, we map the uncharted frontier of discovery phase-space in terms of intrinsic luminosity and timescales for explosive transients, where we expect the unexpected. We then focus on six areas with open science questions for known transients: the progenitors of thermonuclear supernovae (SNe), mass loss prior to core collapse, asymmetry in stellar explosions, light echoes, high-$z$ transients, and strongly lensed SNe. We conclude with a brief discussion of the practical aspects of ELT & LSST synergy., Comment: 8 pages, 4 figures, submitted to Astro2020 call for Science White Papers

Supernovae are among the most powerful and influential explosions in the universe. They are also ideal multi-messenger laboratories to study extreme astrophysics. However, many fundamental properties of supernovae related to their diverse progenitor systems and explosion mechanisms remain poorly constrained. Here we outline how late-time spectroscopic observations obtained during the nebular phase (several months to years after explosion), made possible with the next generation of Extremely Large Telescopes, will facilitate transformational science opportunities and rapidly accelerate the community towards our goal of achieving a complete understanding of supernova explosions. We highlight specific examples of how complementary GMT and TMT instrumentation will enable high fidelity spectroscopy from which the line profiles and luminosities of elements tracing mass loss and ejecta can be used to extract kinematic and chemical information with unprecedented detail, for hundreds of objects. This will provide uniquely powerful constraints on the evolutionary phases stars may experience approaching a supernova explosion; the subsequent explosion dynamics; their nucleosynthesis yields; and the formation of compact objects that may act as central engines., Comment: 9 pages, 3 figures, Astro2020 Science White Paper

The field of time-domain astrophysics has entered the era of Multi-messenger Astronomy (MMA). One key science goal for the next decade (and beyond) will be to characterize gravitational wave (GW) and neutrino sources using the next generation of Extremely Large Telescopes (ELTs). These studies will have a broad impact across astrophysics, informing our knowledge of the production and enrichment history of the heaviest chemical elements, constrain the dense matter equation of state, provide independent constraints on cosmology, increase our understanding of particle acceleration in shocks and jets, and study the lives of black holes in the universe. Future GW detectors will greatly improve their sensitivity during the coming decade, as will near-infrared telescopes capable of independently finding kilonovae from neutron star mergers. However, the electromagnetic counterparts to high-frequency (LIGO/Virgo band) GW sources will be distant and faint and thus demand ELT capabilities for characterization. ELTs will be important and necessary contributors to an advanced and complete multi-messenger network., Comment: White paper submitted to the Astro2020 Decadal Survey

As of today, we have directly detected exactly one source in both gravitational waves (GWs) and electromagnetic (EM) radiation, the binary neutron star merger GW170817, its associated gamma-ray burst GRB170817A, and the subsequent kilonova SSS17a/AT 2017gfo. Within ten years, we will detect hundreds of events, including new classes of events such as neutron-star-black-hole mergers, core-collapse supernovae, and almost certainly something completely unexpected. As we build this sample, we will explore exotic astrophysical topics ranging from nucleosynthesis, stellar evolution, general relativity, high-energy astrophysics, nuclear matter, to cosmology. The discovery potential is extraordinary, and investments in this area will yield major scientific breakthroughs. Here we outline some of the most exciting scientific questions that can be answered by combining GW and EM observations., Comment: submitted to Astro2020