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When main-sequence stars expand into red giants, they are expected to engulf close-in planets 1 – 5 . Until now, the absence of planets with short orbital periods around post-expansion, core-helium-burning red giants 6 – 8 has been interpreted as evidence that short-period planets around Sun-like stars do not survive the giant expansion phase of their host stars 9 . Here we present the discovery that the giant planet 8 Ursae Minoris b 10 orbits a core-helium-burning red giant. At a distance of only 0.5  au from its host star, the planet would have been engulfed by its host star, which is predicted by standard single-star evolution to have previously expanded to a radius of 0.7  au . Given the brief lifetime of helium-burning giants, the nearly circular orbit of the planet is challenging to reconcile with scenarios in which the planet survives by having a distant orbit initially. Instead, the planet may have avoided engulfment through a stellar merger that either altered the evolution of the host star or produced 8 Ursae Minoris b as a second-generation planet 11 . This system shows that core-helium-burning red giants can harbour close planets and provides evidence for the role of non-canonical stellar evolution in the extended survival of late-stage exoplanetary systems. The giant planet 8 Ursae Minoris b seems to have avoided engulfment by its giant host star through a stellar merger that either affected the evolution of the host star or produced 8 Ursae Minoris b as a second-generation planet.

Classical novae are thermonuclear explosions that occur on the surfaces of white dwarf stars in interacting binary systems 1 . It has long been thought that the luminosity of classical novae is powered by continued nuclear burning on the surface of the white dwarf after the initial runaway 2 . However, recent observations of gigaelectronvolt γ-rays from classical novae have hinted that shocks internal to the nova ejecta may dominate the nova emission. Shocks have also been suggested to power the luminosity of events as diverse as stellar mergers 3 , supernovae 4 and tidal disruption events 5 , but observational confirmation has been lacking. Here we report simultaneous space-based optical and γ-ray observations of the 2018 nova V906 Carinae (ASASSN-18fv), revealing a remarkable series of distinct correlated flares in both bands. The optical and γ-ray flares occur simultaneously, implying a common origin in shocks. During the flares, the nova luminosity doubles, implying that the bulk of the luminosity is shock powered. Furthermore, we detect concurrent but weak X-ray emission from deeply embedded shocks, confirming that the shock power does not appear in the X-ray band and supporting its emergence at longer wavelengths. Our data, spanning the spectrum from radio to γ-ray, provide direct evidence that shocks can power substantial luminosity in classical novae and other optical transients. Simultaneous optical and gamma-ray observations of nova V906 Carinae reveal correlated flares in both wavelength ranges that can be linked to shocks in the nova ejecta. Weak X-ray emission suggests that the shocks are deeply embedded, but they contribute substantially to the luminosity of the nova.

Classical novae are runaway thermonuclear burning events on the surfaces of accreting white dwarfs in close binary star systems, sometimes appearing as new naked-eye sources in the night sky 1 . The standard model of novae predicts that their optical luminosity derives from energy released near the hot white dwarf, which is reprocessed through the ejected material 2 – 5 . Recent studies using the Fermi Large Area Telescope have shown that many classical novae are accompanied by gigaelectronvolt γ-ray emission 6 , 7 . This emission likely originates from strong shocks, providing new insights into the properties of nova outflows and allowing them to be used as laboratories for the study of the unknown efficiency of particle acceleration in shocks. Here, we report γ-ray and optical observations of the Milky Way nova ASASSN-16ma, which is among the brightest novae ever detected in γ-rays. The γ-ray and optical light curves show a remarkable correlation, implying that the majority of the optical light comes from reprocessed emission from shocks rather than the white dwarf 8 . The ratio of γ-ray to optical flux in ASASSN-16ma directly constrains the acceleration efficiency of non-thermal particles to be around 0.005, favouring hadronic models for the γ-ray emission 9 . The need to accelerate particles up to energies exceeding 100 gigaelectronvolts provides compelling evidence for magnetic field amplification in the shocks. A tight correlation between gamma rays and optical emission in nova ASASSN-16ma indicates that the optical light comes from reprocessed emission from shocks in the ejecta, rather than an energy release near the hot white dwarf, as in the standard model.

We present the discovery that ATLAS18mlw was a tidal disruption event (TDE) in the galaxy WISEA J073544.83+663717.3, at a luminosity distance of 334 Mpc. Initially discovered by the Asteroid Terrestrial Impact Last Alert System (ATLAS) on 2018 March 17.3, the TDE nature of the transient was uncovered only recently with the re-reduction of a SuperNova Integral Field Spectrograph (SNIFS) spectrum. This spectrum, taken by the Spectral Classification of Astronomical Transients (SCAT) survey, shows a strong blue continuum and a broad H\\(\\alpha\\) emission line. Here we present roughly six years of optical survey photometry beginning before the TDE to constrain AGN activity, optical spectroscopy of the transient, and a detailed study of the host galaxy properties through analysis of archival photometry and a host spectrum. ATLAS18mlw was detected in ground-based light curves for roughly two months. From a blackbody fit to the transient spectrum and bolometric correction of the optical light curve, we conclude that ATLAS18mlw is best explained by a low-luminosity TDE with a peak luminosity of log(L [erg s\\(^{-1}\\)]) = \\(43.5 \\pm 0.2\\). The TDE classification is further supported by the quiescent Balmer strong nature of the host galaxy. We also calculated the TDE decline rate from the bolometric light curve and find \\(\\Delta L_{40} = -0.7 \\pm 0.2\\) dex, making ATLAS18mlw a member of the growing class of ``faint and fast'' TDEs with low peak luminosities and fast decline rates.

We present observations of ASASSN-20hx, a nearby ambiguous nuclear transient (ANT) discovered in NGC 6297 by the All-Sky Automated Survey for Supernovae (ASAS-SN). We observed ASASSN-20hx from \\(-\\)30 to 275 days relative to peak UV/optical emission using high-cadence, multi-wavelength spectroscopy and photometry. From Transiting Exoplanet Survey Satellite (TESS) data, we determine that the ANT began to brighten on 2020 June 22.8 with a linear rise in flux for at least the first week. ASASSN-20hx peaked in the UV/optical 30 days later on 2020 July 22.8 (MJD = 59052.8) at a bolometric luminosity of \\(L = (3.15 \\pm 0.04) \\times 10^{43}\\) erg s\\(^{-1}\\). The subsequent decline is slower than any TDE observed to date and consistent with many other ANTs. Compared to an archival X-ray detection, the X-ray luminosity of ASASSN-20hx increased by an order of magnitude to \\(L_{x} \\sim 1.5 \\times 10^{42}\\) erg s\\(^{-1}\\) and then slowly declined over time. The X-ray emission is well-fit by a power law with a photon index of \\(\\Gamma \\sim 2.3 - 2.6\\). Both the optical and near infrared spectra of ASASSN-20hx lack emission lines, unusual for any known class of nuclear transient. While ASASSN-20hx has some characteristics seen in both tidal disruption events (TDEs) and active galactic nuclei (AGNs), it cannot be definitively classified with current data.

In November 2020, the Swift team announced an update to the UltraViolet and Optical Telescope calibration to correct for the loss of sensitivity over time. This correction affects observations in the three near ultraviolet (UV) filters, by up to 0.3 mag in some cases. As UV photometry is critical to characterizing tidal disruption events (TDEs) and other peculiar nuclear outbursts, we re-computed published Swift data for TDEs and other singular nuclear outbursts with Swift photometry in 2015 or later, as a service to the community. Using archival UV, optical, and infrared photometry we ran host SED fits for each host galaxy. From these, we computed synthetic host magnitudes and host-galaxy properties. We calculated host-subtracted magnitudes for each transient and computed blackbody fits. In addition to the nuclear outbursts, we include the ambiguous transient ATLAS18qqn (AT2018cow), which has been classifed as a potential TDE on an intermediate mass black hole. Finally, with updated bolometric light curves, we recover the relationship of \\citet{hinkle20a}, where more luminous TDEs decay more slowly than less luminous TDEs, with decreased scatter as compared to the original relationship.

We compare the luminosity, radius, and temperature evolution of the UV/optical blackbodies for 21 well-observed tidal disruption events (TDEs), 8 of which were discovered by the All-Sky Automated Survey for Supernovae. We find that the blackbody radii generally increase prior to peak and slowly decline at late times. The blackbody temperature evolution is generally flat, with a few objects showing small-scale variations. The bolometric UV/optical luminosities generally evolve smoothly and flatten out at late times. Finally, we find an apparent correlation between the peak luminosity and the decline rate of TDEs. This relationship is strongest when comparing the peak luminosity to its decline over 40 days. A linear fit yields \\(\\log_{10}( L_{peak}) = (44.1^{+0.1}_{-0.1}) + (1.6^{+0.4}_{-0.2})(\\Delta L_{40} + 0.5)\\) in cgs, where \\(\\Delta L_{40} = \\log_{10}(L_{40} / L_{peak})\\).

We present a detailed analysis of nearly two decades of optical/UV and X-ray data to study the multi-wavelength pre-explosion properties and post-explosion X-ray properties of nearby SN2023ixf located in M101. We find no evidence of precursor activity in the optical to UV down to a luminosity of \\(\\lesssim 7\\times10^{4}\\, \\rm L_{\\odot}\\), while X-ray observations covering nearly 18 years prior to explosion show no evidence of luminous precursor X-ray emission down to an absorbed 0.3 - 10.0 keV X-ray luminosity of \\(\\sim6\\times10^{36}\\) erg s\\(^{-1}\\). Extensive Swift observations taken post-explosion did not detect soft X-ray emission from SN2023ixf within the first \\(\\sim\\)3.3 days after first light, which suggests a mass-loss rate for the progenitor of \\(\\lesssim5\\times10^{-4}\\,\\rm M_{\\odot}\\) yr\\(^{-1}\\) or a radius of \\(\\lesssim4\\times10^{15}\\) cm for the circumstellar material. Our analysis also suggests that if the progenitor underwent a mass-loss episode, this had to occur \\(>\\) 0.5 - 1.5 years prior to explosion, consistent with previous estimates. Swift detected soft X-rays from SN2023ixf \\(\\sim4.25\\) days after first light, and it rose to a peak luminosity of \\(\\sim10^{39}\\) erg s\\(^{-1}\\) after 10 days and has maintained this luminosity for nearly 50 days post first light. This peak luminosity is lower than expected, given the evidence that SN2023ixf is interacting with dense material. However, this might be a natural consequence of an asymmetric circumstellar medium. X-ray spectra derived from merging all Swift observations over the first 50 days are best described by a two-component bremsstrahlung model consisting of a heavily absorbed and hotter component similar to that found using NuSTAR, and a less-absorbed, cooler component. We suggest that this soft component arises from cooling of the forward shock similar to that found in Type IIn SN2010jl.

Light echoes occur when light from a luminous transient is scattered by dust back into our line of sight with a time delay due to the extra propagation distance. We introduce a novel approach to estimating the distance to a source by combining light echoes with recent three-dimensional dust maps. We identify light echoes from the historical supernovae Cassiopeia A and SN 1572 (Tycho) in nearly a decade of imaging from the All-Sky Automated Survey for Supernovae (ASAS-SN). Using these light echoes, we find distances of \\(3.6\\pm0.1\\) kpc and \\(3.2^{+0.1}_{-0.2}\\) kpc to Cas A and Tycho, respectively, which are generally consistent with previous estimates but are more precise. These distance uncertainties are primarily dominated by the low distance resolution of the 3D dust maps, which will likely improve in the future. The candidate single degenerate explosion donor stars B and G in Tycho are clearly foreground stars. Finally, the inferred reddening towards each SN agrees well with the intervening HI column density estimates from X-ray analyses of the remnants.

In November 2020, the Swift team announced a major update to the calibration of the UltraViolet and Optical Telescope (UVOT) data to correct for the gradual loss of sensitivity over time. Beginning in roughly 2015, the correction affected observations in the three near ultraviolet (UV) filters, reaching levels of up to 0.3 mag immediately prior to the correction. Over the same time period, an increasing number of Type I superluminous supernovae (SLSNe-I) were discovered and studied. Many SLSNe-I are hot (T\\(_\\textrm{eff}\\) \\(\\approx 10,000\\) K) near peak, and therefore accurate UV data are imperative towards properly understanding their physical properties and energetics. We re-compute Swift UVOT photometry for SLSNe-I discovered between 2014 and 2021 with at least 5 Swift observations in 2015 or later. We calculate host-subtracted magnitudes for each SLSN and fit their spectral energy distributions with modified blackbodies to obtain the radius and temperature evolution. We also fit multi-band photometry using the Modular Open Source Fitter for Transients (MOSFiT) to obtain key parameters such as the spin period (P), magnetic field strength (B), ejecta mass (M\\(_\\textrm{ej}\\)), and kinetic energy (E\\(_\\textrm{kin}\\)). From our MOSFiT modeling, we also estimate the peak UV/optical luminosity (L\\(_\\textrm{peak}\\)) and total radiative energy (E\\(_\\textrm{rad}\\)). Under the assumption of magnetar-powered SLSNe we find several strong trends, including anti-correlations between P and both L\\(_\\textrm{peak}\\) and E\\(_\\textrm{rad}\\), a correlation between E\\(_\\textrm{kin}\\) and E\\(_\\textrm{rad}\\), and an anti-correlation between B and E\\(_\\textrm{rad}\\).