Search

Quasi-periodic Eruptions (QPEs) are luminous bursts of soft X-rays from the nuclei of galaxies, repeating on timescales of hours to weeks. The mechanism behind these rare systems is uncertain, but most theories involve accretion disks around supermassive black holes (SMBHs), undergoing instabilities or interacting with a stellar object in a close orbit. It has been suggested that this disk could be created when the SMBH disrupts a passing star, implying that many QPEs should be preceded by observable tidal disruption events (TDEs). Two known QPE sources show long-term decays in quiescent luminosity consistent with TDEs, and two observed TDEs have exhibited X-ray flares consistent with individual eruptions. TDEs and QPEs also occur preferentially in similar galaxies. However, no confirmed repeating QPEs have been associated with a spectroscopically confirmed TDE or an optical TDE observed at peak brightness. Here we report the detection of nine X-ray QPEs with a mean recurrence time of approximately 48 hours from AT2019qiz, a nearby and extensively studied optically-selected TDE. We detect and model the X-ray, ultraviolet and optical emission from the accretion disk, and show that an orbiting body colliding with this disk provides a plausible explanation for the QPEs.

We present multi-wavelength data of SN2020acct, a double-peaked stripped-envelope supernova (SN) in NGC2981 at ~150 Mpc. The two peaks are temporally distinct, with maxima separated by 58 rest-frame days, and a factor of 20 reduction in flux between. The first is luminous (M$_{r}$ = -18.00 $\pm$ 0.02 mag), blue (g - r = 0.27 $\pm$ 0.03 mag), and displays spectroscopic signatures of interaction with hydrogen-free circumstellar material. The second peak is fainter (M$_{r}$ = -17.29 $\pm$ 0.03 mag), and spectroscopically similar to an evolved stripped-envelope SNe, with strong blended forbidden [Ca II] and [O II] features. No other known double-peak SN exhibits a light curve similar to that of SN 2020acct. We find the likelihood of two individual SNe occurring in the same star-forming region within that time to be highly improbable, while an implausibly fine-tuned configuration would be required to produce two SNe from a single binary system. We find that the peculiar properties of SN2020acct match models of pulsational pair instability (PPI), in which the initial peak is produced by collisions of shells of ejected material, shortly followed by a terminal explosion. Pulsations from a star with a 72 M$_{\odot}$ helium core provide an excellent match to the double-peaked light curve. The local galactic environment has a metallicity of 0.4 Z$_{\odot}$, a level where massive single stars are not expected retain enough mass to encounter the PPI. However, late binary mergers or a low-metallicity pocket may allow the required core mass. We measure the rate of SN 2020acct-like events to be $<3.3\times10^{-8}$ Mpc$^{-3}$ yr$^{-1}$ at z = 0.07, or <0.1% of the total core-collapse SN rate., Comment: 31 pages, 14 figures. Submitted to ApJL, comments welcome

We present a comprehensive analysis of the photometric and spectroscopic evolution of SN~2021foa, unique among the class of transitional supernovae for repeatedly changing its spectroscopic appearance from hydrogen-to-helium-to-hydrogen-dominated (IIn-to-Ibn-to-IIn) within 50 days past peak brightness. The spectra exhibit multiple narrow ($\approx$ 300--600~km~s$^{-1}$) absorption lines of hydrogen, helium, calcium and iron together with broad helium emission lines with a full-width-at-half-maximum (FWHM) of $\sim 6000$~km~s$^{-1}$. For a steady, wind-mass loss regime, light curve modeling results in an ejecta mass of $\sim 8$ M$_{\odot}$ and CSM mass below 1 M$_{\odot}$, and an ejecta velocity consistent with the FWHM of the broad helium lines. We obtain a mass-loss rate of $\approx 2$ M$_{\odot} {\rm yr}^{-1}$. This mass-loss rate is three orders of magnitude larger than derived for normal Type II SNe. We estimate that the bulk of the CSM of SN~2021foa must have been expelled within half a year, about 15 years ago. Our analysis suggests that SN~2021foa had a helium rich ejecta which swept up a dense shell of hydrogen rich CSM shortly after explosion. At about 60 days past peak brightness, the photosphere recedes through the dense ejecta-CSM region, occulting much of the red-shifted emission of the hydrogen and helium lines, which results in observed blue-shift ($\sim -3000$~km~s$^{-1}$). Strong mass loss activity prior to explosion, such as those seen in SN~2009ip-like objects and SN~2021foa as precursor emission, are the likely origin of a complex, multiple-shell CSM close to the progenitor star., Comment: Revised. Accepted in ApJ

We present UV/optical/NIR observations and modeling of supernova (SN) 2024ggi, a type II supernova (SN II) located in NGC 3621 at 7.2 Mpc. Early-time ("flash") spectroscopy of SN 2024ggi within +0.8 days of discovery shows emission lines of H I, He I, C III, and N III with a narrow core and broad, symmetric wings (i.e., IIn-like) arising from the photoionized, optically-thick, unshocked circumstellar material (CSM) that surrounded the progenitor star at shock breakout. By the next spectral epoch at +1.5 days, SN 2024ggi showed a rise in ionization as emission lines of He II, C IV, N IV/V and O V became visible. This phenomenon is temporally consistent with a blueward shift in the UV/optical colors, both likely the result of shock breakout in an extended, dense CSM. The IIn-like features in SN 2024ggi persist on a timescale of $t_{\rm IIn} = 3.8 \pm 1.6$ days at which time a reduction in CSM density allows the detection of Doppler broadened features from the fastest SN material. SN 2024ggi has peak UV/optical absolute magnitudes of $M_{\rm w2} = -18.7$ mag and $M_{\rm g} = -18.1$ mag that are consistent with the known population of CSM-interacting SNe II. Comparison of SN 2024ggi with a grid of radiation hydrodynamics and non-local thermodynamic equilibrium (nLTE) radiative-transfer simulations suggests a progenitor mass-loss rate of $\dot{M} = 10^{-2}$M$_{\odot}$ yr$^{-1}$ ($v_w$ = 50 km/s), confined to a distance of $r < 5\times 10^{14}$ cm. Assuming a wind velocity of $v_w$ = 50 km/s, the progenitor star underwent an enhanced mass-loss episode in the last ~3 years before explosion., Comment: Accepted for publication in ApJ. arXiv admin note: text overlap with arXiv:2306.04721, arXiv:2403.02382

We present LAISS (Lightcurve Anomaly Identification and Similarity Search), an automated pipeline to detect anomalous astrophysical transients in real-time data streams. We deploy our anomaly detection model on the nightly ZTF Alert Stream via the ANTARES broker, identifying a manageable $\sim$1-5 candidates per night for expert vetting and coordinating follow-up observations. Our method leverages statistical light-curve and contextual host-galaxy features within a random forest classifier, tagging transients of rare classes (spectroscopic anomalies), of uncommon host-galaxy environments (contextual anomalies), and of peculiar or interaction-powered phenomena (behavioral anomalies). Moreover, we demonstrate the power of a low-latency ($\sim$ms) approximate similarity search method to find transient analogs with similar light-curve evolution and host-galaxy environments. We use analogs for data-driven discovery, characterization, (re-)classification, and imputation in retrospective and real-time searches. To date we have identified $\sim$50 previously known and previously missed rare transients from real-time and retrospective searches, including but not limited to: SLSNe, TDEs, SNe IIn, SNe IIb, SNe Ia-CSM, SNe Ia-91bg-like, SNe Ib, SNe Ic, SNe Ic-BL, and M31 novae. Lastly, we report the discovery of 325 total transients, all observed between 2018-2021 and absent from public catalogs ($\sim$1% of all ZTF Astronomical Transient reports to the Transient Name Server through 2021). These methods enable a systematic approach to finding the "needle in the haystack" in large-volume data streams. Because of its integration with the ANTARES broker, LAISS is built to detect exciting transients in Rubin data., Comment: 44 pages (68 pages with Appendix), 15 figures, accepted to ApJ

We present ultraviolet/optical/near-infrared observations and modeling of Type II supernovae (SNe II) whose early-time ($\delta t < 2$ days) spectra show transient, narrow emission lines from shock ionization of confined ($r < 10^{15}$ cm) circumstellar material (CSM). The observed electron-scattering broadened line profiles (i.e., IIn-like) of HI, He I/II, C III/IV, and N III/IV/V from the CSM persist on a characteristic timescale ($t_{\rm IIn}$) that marks a transition to a lower-density CSM and the emergence of Doppler-broadened features from the fast-moving SN ejecta. Our sample, the largest to date, consists of 39 SNe with early-time IIn-like features in addition to 35 "comparison" SNe with no evidence of early-time IIn-like features, all with ultraviolet observations. The total sample consists of 50 unpublished objects with 474 previously unpublished spectra and 50 multiband light curves, collected primarily through the Young Supernova Experiment and Global Supernova Project collaborations. For all sample objects, we find a significant correlation between peak ultraviolet brightness and both $t_{\rm IIn}$ and the rise time, as well as evidence for enhanced peak luminosities in SNe II with IIn-like features. We quantify mass-loss rates and CSM density for the sample through matching of peak multiband absolute magnitudes, rise times, $t_{\rm IIn}$ and optical SN spectra with a grid of radiation hydrodynamics and non-local thermodynamic equilibrium (nLTE) radiative-transfer simulations. For our grid of models, all with the same underlying explosion, there is a trend between the duration of the electron-scattering broadened line profiles and inferred mass-loss rate: $t_{\rm IIn} \approx 3.8[\dot{M}/(0.01 \textrm{M}_{\odot} \textrm{yr}^{-1})]$ days., Comment: 58 pages, 24 figures, submitted to ApJ. Supplementary figures available on Github (https://github.com/wynnjacobson-galan/Flash_Spectra_Sample). Data release following publication

We report the All-Sky Automated Survey for SuperNovae discovery of the tidal disruption event (TDE) ASASSN-23bd (AT 2023clx) in NGC 3799, a LINER galaxy with no evidence of strong AGN activity over the past decade. With a redshift of $z = 0.01107$ and a peak UV/optical luminosity of $(5.4\pm0.4)\times10^{42}$ erg s$^{-1}$, ASASSN-23bd is the lowest-redshift and least-luminous TDE discovered to date. Spectroscopically, ASASSN-23bd shows H$\alpha$ and He I emission throughout its spectral time series, and the UV spectrum shows nitrogen lines without the strong carbon and magnesium lines typically seen for AGN. Fits to the rising ASAS-SN light curve show that ASASSN-23bd started to brighten on MJD 59988$^{+1}_{-1}$, $\sim$9 days before discovery, with a nearly linear rise in flux, peaking in the $g$ band on MJD $60000^{+3}_{-3}$. Scaling relations and TDE light curve modelling find a black hole mass of $\sim$10$^6$ $M_\odot$, which is on the lower end of supermassive black hole masses. ASASSN-23bd is a dim X-ray source, with an upper limit of $L_{0.3-10\,\mathrm{keV}} < 1.0\times10^{40}$ erg s$^{-1}$ from stacking all \emph{Swift} observations prior to MJD 60061, but with soft ($\sim 0.1$ keV) thermal emission with a luminosity of $L_{0.3-2 \,\mathrm{keV}}\sim4\times10^{39}$ erg s$^{-1}$ in \emph{XMM-Newton} observations on MJD 60095. The rapid $(t < 15$ days) light curve rise, low UV/optical luminosity, and a luminosity decline over 40 days of $\Delta L_{40}\approx-0.7$ make ASASSN-23bd one of the dimmest TDEs to date and a member of the growing ``Low Luminosity and Fast'' class of TDEs., Comment: 17 pages, 13 figures, submitted to MNRAS

We present the Keck Infrared Transient Survey (KITS), a NASA Key Strategic Mission Support program to obtain near-infrared (NIR) spectra of astrophysical transients of all types, and its first data release, consisting of 105 NIR spectra of 50 transients. Such a data set is essential as we enter a new era of IR astronomy with the James Webb Space Telescope (JWST) and the upcoming Nancy Grace Roman Space Telescope (Roman). NIR spectral templates will be essential to search JWST images for stellar explosions of the first stars and to plan an effective Roma} SN Ia cosmology survey, both key science objectives for mission success. Between 2022 February and 2023 July, we systematically obtained 274 NIR spectra of 146 astronomical transients, representing a significant increase in the number of available NIR spectra in the literature. The first data release includes data from the 2022A semester. We systematically observed three samples: a flux-limited sample that includes all transients $<$17 mag in a red optical band (usually ZTF r or ATLAS o bands); a volume-limited sample including all transients within redshift $z < 0.01$ ($D \approx 50$ Mpc); and an SN Ia sample targeting objects at phases and light-curve parameters that had scant existing NIR data in the literature. The flux-limited sample is 39% complete (60% excluding SNe Ia), while the volume-limited sample is 54% complete and is 79% complete to $z = 0.005$. All completeness numbers will rise with the inclusion of data from other telescopes in future data releases. Transient classes observed include common Type Ia and core-collapse supernovae, tidal disruption events (TDEs), luminous red novae, and the newly categorized hydrogen-free/helium-poor interacting Type Icn supernovae. We describe our observing procedures and data reduction using Pypeit, which requires minimal human interaction to ensure reproducibility.

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

We present the Young Supernova Experiment Data Release 1 (YSE DR1), comprised of processed multi-color Pan-STARRS1 (PS1) griz and Zwicky Transient Facility (ZTF) gr photometry of 1975 transients with host-galaxy associations, redshifts, spectroscopic/photometric classifications, and additional data products from 2019 November 24 to 2021 December 20. YSE DR1 spans discoveries and observations from young and fast-rising supernovae (SNe) to transients that persist for over a year, with a redshift distribution reaching z~0.5. We present relative SN rates from YSE's magnitude- and volume-limited surveys, which are consistent with previously published values within estimated uncertainties for untargeted surveys. We combine YSE and ZTF data, and create multi-survey SN simulations to train the ParSNIP and SuperRAENN photometric classification algorithms; when validating our ParSNIP classifier on 472 spectroscopically classified YSE DR1 SNe, we achieve 82% accuracy across three SN classes (SNe Ia, II, Ib/Ic) and 90% accuracy across two SN classes (SNe Ia, core-collapse SNe). Our classifier performs particularly well on SNe Ia, with high (>90%) individual completeness and purity, which will help build an anchor photometric SNe Ia sample for cosmology. We then use our photometric classifier to characterize our photometric sample of 1483 SNe, labeling 1048 (~71%) SNe Ia, 339 (~23%) SNe II, and 96 (~6%) SNe Ib/Ic. YSE DR1 provides a training ground for building discovery, anomaly detection, and classification algorithms, performing cosmological analyses, understanding the nature of red and rare transients, exploring tidal disruption events and nuclear variability, and preparing for the forthcoming Vera C. Rubin Observatory Legacy Survey of Space and Time., Comment: Accepted to ApJS; 64 pages; 35 figures; 10 tables

We present optical and near-infrared (NIR) observations of the Type Icn supernova (SN Icn) 2022ann, the fifth member of its newly identified class of SNe. Its early optical spectra are dominated by narrow carbon and oxygen P-Cygni features with absorption velocities of 800 km/s; slower than other SNe Icn and indicative of interaction with a dense, H/He-poor circumstellar medium (CSM) that is outflowing slower than a typical Wolf-Rayet wind velocity of $>$1000 km/s. We identify helium in NIR spectra obtained two weeks after maximum and in optical spectra at three weeks, demonstrating that the CSM is not fully devoid of helium. We never detect broad spectral features from SN ejecta, including in spectra extending to the nebular phase, a unique characteristic among SNe~Icn. Compared to other SNe Icn, SN 2022ann has a low luminosity, with a peak o-band absolute magnitude of -17.7, and evolves slowly. We model the bolometric light curve and find it is well-described by 1.7 M_Sun of SN ejecta interacting with 0.2 M_sun of CSM. We place an upper limit of 0.04 M_Sun of Ni56 synthesized in the explosion. The host galaxy is a dwarf galaxy with a stellar mass of 10^7.34 M_Sun (implied metallicity of log(Z/Z_Sun) $\approx$ 0.10) and integrated star-formation rate of log(SFR) = -2.20 M_sun/yr; both lower than 97\% of the galaxies observed to produce core-collapse supernovae, although consistent with star-forming galaxies on the galaxy Main Sequence. The low CSM velocity, nickel and ejecta masses, and likely low-metallicity environment disfavour a single Wolf-Rayet progenitor star. Instead, a binary companion star is likely required to adequately strip the progenitor before explosion and produce a low-velocity outflow. The low CSM velocity may be indicative of the outer Lagrangian points in the stellar binary progenitor, rather than from the escape velocity of a single Wolf-Rayet-like massive star., Comment: 21 pages, 13 figures, to be submitted to MNRAS

NGC 5273 is a known optical and X-ray variable AGN. We analyze new and archival IR, optical, UV, and X-ray data in order to characterize its long-term variability from 2000 to 2022. At least one optical changing-look event occurred between 2011 and 2014, when the AGN changed from a Type 1.8/1.9 Seyfert to a Type 1. It then faded considerably at all wavelengths, followed by a dramatic but slow increase in UV/optical brightness between 2021 and 2022. Near-IR (NIR) spectra in 2022 show prominent broad Paschen lines that are absent in an archival spectrum from 2010, making NGC 5273 one of the few AGNs to be observed changing-look in the NIR. We propose that NGC 5273 underwent multiple changing-look events between 2000 and 2022 -- starting as a Type 1.8/1.9, NGC 5273 changes-look to a Type 1 temporarily in 2002 and again in 2014, reverting back to a Type 1.8/1.9 by 2005 and 2017, respectively. In 2022, it is again a Type 1 Seyfert. We characterize the changing-look events and their connection to the dynamic accretion and radiative processes in NGC 5273, and propose that the variable luminosity (and thus, Eddington ratio) of the source is changing how the broad line region (BLR) reprocesses the continuum emission., Comment: 19 pages, 13 figures, 5 tables, accepted to MNRAS

We present the Spectroscopic Classification of Astronomical Transients (SCAT) survey, which is dedicated to spectrophotometric observations of transient objects such as supernovae and tidal disruption events. SCAT uses the SuperNova Integral-Field Spectrograph (SNIFS) on the University of Hawai'i 2.2-meter (UH2.2m) telescope. SNIFS was designed specifically for accurate transient spectrophotometry, including absolute flux calibration and host-galaxy removal. We describe the data reduction and calibration pipeline including spectral extraction, telluric correction, atmospheric characterization, nightly photometricity, and spectrophotometric precision. We achieve $\lesssim 5\%$ spectrophotometry across the full optical wavelength range ($3500-9000~\r{A}$) under photometric conditions. The inclusion of photometry from the SNIFS multi-filter mosaic imager allows for decent spectrophotometric calibration ($10-20\%$) even under unfavorable weather/atmospheric conditions. SCAT obtained $\approx 640$ spectra of transients over the first 3 years of operations, including supernovae of all types, active galactic nuclei, cataclysmic variables, and rare transients such as superluminous supernovae and tidal disruption events. These observations will provide the community with benchmark spectrophotometry to constrain the next generation of hydrodynamic and radiative transfer models., Comment: 12 pages, 9 figures, 2 tables. Accepted for publication in to PASP

Massive black holes (BHs) at the centres of massive galaxies are ubiquitous. The population of BHs within dwarf galaxies, on the other hand, is evasive. Dwarf galaxies are thought to harbour BHs with proportionally small masses, including intermediate mass BHs, with masses $10^{2} < M_{BH} < 10^{6} M_{\odot}$. Identification of these systems has historically relied upon the detection of light emitted from accreting gaseous discs close to the BHs. Without this light, they are difficult to detect. Tidal disruption events (TDEs), the luminous flares produced when a star strays close to a BH and is shredded, are a direct way to probe massive BHs. The rise times of these flares theoretically correlate with the BH mass. Here we present AT2020neh, a fast rising TDE candidate, hosted by a dwarf galaxy. AT2020neh can be described by the tidal disruption of a main sequence star by a 10$^{4.7} - 10^{5.9} M_{\odot}$ BH. We find the observable rate of fast rising nuclear transients like AT2020neh to be rare, at $\lesssim 2 \times 10^{-8}$ events Mpc$^{-3}$ yr$^{-1}$. Finding non-accreting BHs in dwarf galaxies is important to determine how prevalent BHs are within these galaxies, and constrain models of BH formation. AT2020neh-like events may provide a galaxy-independent method of measuring IMBH masses., Comment: Accepted for publication in Nature Astronomy

The Keck Wide-Field Imager (KWFI) is a proposed 1-degree diameter field of view UV-sensitive optical camera for Keck prime focus. KWFI will be the most powerful optical wide-field camera in the world and the only such 8m-class camera sensitive down to ~3000 A for the foreseeable future. Twenty science cases are described for KWFI compiled largely during 2019-2021, preceded by a brief discussion of the instrument, components, and capabilities for context., Comment: 56 pages, 31 figures

AT2019pev is a nuclear transient in a narrow-line Seyfert 1 galaxy at $z=0.096$. The archival ultraviolet, optical and infrared data showed features of both tidal disruption events (TDEs) and active galactic nuclei (AGNs), and its nature is not fully understood. We present detailed X-ray observations of AT2019pev taken with Swift, Chandra and NICER over 173 days of its evolution since the first Swift XRT epoch. The X-ray luminosity increases by a factor of five in five days from the first Swift XRT epoch to the lightcurve peak. The lightcurve decays by a factor of ten over $\sim$75 days and then flattens with a weak re-brightening trend at late times. The X-ray spectra show a "harder-when-brighter" trend before peak and a "harder-when-fainter" trend after peak, which may indicate a transition of accretion states. The archival ground-based optical observations show similar time evolution as the X-ray lightcurves. Beyond the seasonal limit of the ground-based observations, the Gaia lightcurve is rising toward an equally bright or brighter peak 223 days after the optical discovery. Combining our X-ray analysis and archival multi-wavelength data, AT2019pev more closely resembles an AGN transient., Comment: 14 pages, 6 figures; submitted to MNRAS

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 report the discovery of a stripped giant + lower giant binary, 2M04123153+6738486 (2M0412), identified during a search for non-interacting compact object-star binaries. 2M0412 is an evolved ($T_{\rm eff, giant}\simeq4000$ K), luminous ($L_{\rm giant}\simeq150~L_\odot$) red giant in a circular $P=81.2$ day binary. 2M0412 is a known variable star previously classified as a semi-regular variable. The cross-correlation functions of follow-up Keck/HIRES and LBT/PEPSI spectra show an RV-variable second component with implied mass ratio $q=M_{\rm giant}/M_{\rm comp}\simeq0.20\pm0.01$. The ASAS-SN, ATLAS, TESS and ZTF light curves show that the giant is a Roche lobe filling ellipsoidal variable with an inclination of $49.4^\circ{}\pm{0.3^{\circ}}$, and a giant mass of $M_{\rm giant}=0.38\pm0.01~ M_\odot$ for a distance of $\simeq3.7$ kpc. The mass of the giant indicates that its envelope has been stripped. The giant companion on the lower red giant branch has a mass of $M_{\rm comp}=1.91\pm0.03~M_\odot$ with $T_{\rm eff, comp}\simeq5000$ K, $L_{\rm comp}\simeq60~L_\odot$ and $R_{\rm comp}\simeq11~R_\odot$. We also identify an orbital phase dependent, broad $\rm H\alpha$ emission line which could indicate ongoing accretion from the stripped red giant onto the companion., Comment: Revised version of the paper following comments from the referee, submitted to MNRAS. arXiv admin note: text overlap with arXiv:2101.02212

We present multi-wavelength follow-up observations of the ATLAS discovered faint Iax supernova SN 2020kyg that peaked at an absolute magnitude of $M_g \approx -14.9 \pm 0.2$, making it another member of the faint Iax supernova population. The bolometric light curve requires only $\approx 7 \times 10^{-3}$ M$_{\odot}$ of radioactive $^{56}$Ni, with an ejected mass of $M_{\rm ej} \sim 0.4$ M$_{\odot}$ and a low kinetic energy of $E \approx 0.05 \pm 0.02 \times 10^{51}$ erg. We construct a homogeneous volume-limited sample of 902 transients observed by ATLAS within 100 Mpc during a 3.5 year span. Using this sample, we constrain the rates of faint Iax ($M_r \gtrsim -16$) events within 60 Mpc at $12^{+14}_{-8}\%$ of the SN Ia rate. The overall Iax rate, at $15^{+17}_{-9}\%$ of the Ia rate, is dominated by the low-luminosity events, with luminous SNe Iax ($M_r \lesssim -17.5$) like 2002cx and 2005hk accounting for only $0.9^{+1.1}_{-0.5}\%$ of the Ia rate (a 2$\sigma$ upper limit of approximately 3\%). We favour the hybrid CONe WD + He star progenitor channel involving a failed deflagration of a near Chandrasekhar mass white dwarf, expected to leave a bound remnant and a surviving secondary companion, as a candidate explanation for faint Iax explosions. This scenario requires short delay times, consistent with the observed environments of SNe Iax. Furthermore, binary population synthesis calculations have suggested rates of $1-18\%$ of the SN Ia rate for this channel, consistent with our rate estimates., Comment: Accepted to MNRAS after minor revision

SN 2017jgh is a type IIb supernova discovered by Pan-STARRS during the C16/C17 campaigns of the Kepler/K2 mission. Here we present the Kepler/K2 and ground based observations of SN 2017jgh, which captured the shock cooling of the progenitor shock breakout with an unprecedented cadence. This event presents a unique opportunity to investigate the progenitors of stripped envelope supernovae. By fitting analytical models to the SN 2017jgh lightcurve, we find that the progenitor of SN 2017jgh was likely a yellow supergiant with an envelope radius of $\sim50-290~R_{\odot}$, and an envelope mass of $\sim0-1.7~M_{\odot}$. SN 2017jgh likely had a shock velocity of $\sim7500-10300$ km s$^{-1}$. Additionally, we use the lightcurve of SN 2017jgh to investigate how early observations of the rise contribute to constraints on progenitor models. Fitting just the ground based observations, we find an envelope radius of $\sim50-330~R_{\odot}$, an envelope mass of $\sim0.3-1.7~M_{\odot}$ and a shock velocity of $\sim9,000-15,000$ km s$^{-1}$. Without the rise, the explosion time can not be well constrained which leads to a systematic offset in the velocity parameter and larger uncertainties in the mass and radius. Therefore, it is likely that progenitor property estimates through these models may have larger systematic uncertainties than previously calculated., Comment: 15 pages, 9 figures. Accepted to MNRAS

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

We characterize the extreme heartbeat star system MACHO 80.7443.1718 in the LMC using TESS photometry and spectroscopic observations from the Magellan Inamori Kyocera Echelle (MIKE) and SOAR Goodman spectographs. MACHO 80.7443.1718 was first identified as a heartbeat star system in the All-Sky Automated Survey for SuperNovae (ASAS-SN) with $P_{\rm orb}=32.836\pm0.008\,{\rm d}$. MACHO 80.7443.1718 is a young (${\sim}6$~Myr), massive binary, composed of a B0 Iae supergiant with $M_1 \simeq 35 M_\odot$ and an O9.5V secondary with $M_2 \simeq 16 M_\odot$ on an eccentric ($e=0.51\pm0.03$) orbit. In addition to having the largest variability amplitude amongst all known heartbeats stars, MACHO 80.7443.1718 is also one of the most massive heartbeat stars yet discovered. The B[e] supergiant has Balmer emission lines and permitted/forbidden metallic emission lines associated with a circumstellar disk. The disk rapidly dissipates at periastron which could indicate mass transfer to the secondary, but re-emerges immediately following periastron passage. MACHO 80.7443.1718 also shows tidally excited oscillations at the $N=25$ and $N=41$ orbital harmonics and has a rotational period of 4.4 d., Comment: 21 pages, 16 figures. Submitted to MNRAS

Observational astronomy of tidal disruption events (TDEs) began with the detection of X-ray flares from quiescent galaxies during the ROSAT all-sky survey of 1990-1991. The flares complied with theoretical expectations, having high peak luminosities ($L_{\rm x}$ up to $\ge4\times 10^{44}$ erg/s), a thermal spectrum with $kT\sim$few$\times10^5$ K, and a decline on timescales of months to years, consistent with a diminishing return of stellar debris to a black hole of mass $10^{6-8}$ solar masses. These measurements gave solid proof that the nuclei of quiescent galaxies are habitually populated by a super-massive black hole. Beginning in 2000, XMM-Newton, Chandra and Swift have discovered further TDEs which have been monitored closely at multiple wavelengths. A general picture has emerged of, initially near-Eddington accretion, powering outflows of highly-ionised material, giving way to a calmer sub-Eddington phase, where the flux decays monotonically, and finally a low accretion rate phase with a harder X-ray spectrum indicative of the formation of a disk corona. There are exceptions to this rule though which at the moment are not well understood. A few bright X-ray TDEs have been discovered in optical surveys but in general X-ray TDEs show little excess emission in the optical band, at least at times coincident with the X-ray flare. X-ray TDEs are powerful new probes of accretion physics down to the last stable orbit, revealing the conditions necessary for launching jets and winds. Finally we see that evidence is mounting for nuclear and non-nuclear intermediate mass black holes based on TDE flares which are relatively hot and/or fast., Comment: 54 pages, 16 figures. Accepted for publication in Springer Space Science Reviews. Chapter in ISSI review "The Tidal Disruption of Stars by Massive Black Holes" vol. 79. The article was originally published as Space Sci. Rev., 216, 85 (2020) with a full corrected version published as Space Sci. Rev., 217, 18 (2021)

We report the discovery of the closest known black hole candidate as a binary companion to V723 Mon. V723 Mon is a nearby ($d\sim460\,\rm pc$), bright ($V\simeq8.3$~mag), evolved ($T_{\rm eff, giant}\simeq4440$~K, and $L_{\rm giant}\simeq173~L_\odot$) red giant in a high mass function, $f(M)=1.72\pm 0.01~M_\odot$, nearly circular binary ($P=59.9$ d, $e\simeq 0$). V723 Mon is a known variable star, previously classified as an eclipsing binary, but its All-Sky Automated Survey (ASAS), Kilodegree Extremely Little Telescope (KELT), and Transiting Exoplanet Survey Satellite (TESS) light curves are those of a nearly edge-on ellipsoidal variable. Detailed models of the light curves constrained by the period, radial velocities and stellar temperature give an inclination of $87.0^\circ{}^{+1.7^{\circ}}_{-1.4^{\circ}} $, a mass ratio of $q\simeq0.33\pm0.02$, a companion mass of $M_{\rm comp}=3.04\pm0.06~M_\odot$, a stellar radius of $R_{\rm giant}=24.9\pm0.7~R_\odot$, and a giant mass of $M_{\rm giant}=1.00\pm0.07~ M_\odot$. We identify a likely non-stellar, diffuse veiling component with contributions in the $B$ and $V$-band of ${\sim}63\%$ and ${\sim}24\%$, respectively. The SED and the absence of continuum eclipses imply that the companion mass must be dominated by a compact object. We do observe eclipses of the Balmer lines when the dark companion passes behind the giant, but their velocity spreads are low compared to observed accretion disks. The X-ray luminosity of the system is $L_{\rm X}\simeq7.6\times10^{29}~\rm ergs~s^{-1}$, corresponding to $L/L_{\rm edd}{\sim}10^{-9}$. The simplest explanation for the massive companion is a single compact object, most likely a black hole in the "mass gap"., Comment: 28 pages, 16 figures. Accepted for publication in MNRAS

We present Multi-Unit Spectroscopic Explorer (MUSE) integral-field spectroscopy of ESO 253$-$G003, which hosts a known Active Galactic Nucleus (AGN) and the periodic nuclear transient ASASSN-14ko, observed as part of the All-weather MUse Supernova Integral-field of Nearby Galaxies (AMUSING) survey. The MUSE observations reveal that the inner region hosts two AGN separated by $1.4\pm0.1~\rm{kpc}$ ($\approx 1.\!\!^{\prime\prime}7$). The brighter nucleus has asymmetric broad, permitted emission-line profiles and is associated with the archival AGN designation. The fainter nucleus does not have a broad emission-line component but exhibits other AGN characteristics, including $v_{\rm{FWHM}}\approx 700~\rm{km}~\rm{s}^{-1}$ forbidden line emission, $\log_{10}(\rm{[OIII]}/\rm{H}\beta) \approx 1.1$, and high excitation potential emission lines such as [Fe$~$VII]$~\lambda6086$ and He$~$II$~\lambda4686$. The host galaxy exhibits a disturbed morphology with large kpc-scale tidal features, potential outflows from both nuclei, and a likely superbubble. A circular relativistic disk model cannot reproduce the asymmetric broad emission-line profiles in the brighter nucleus, but two non-axisymmetric disk models provide good fits to the broad emission-line profiles: an elliptical disk model and a circular disk + spiral arm model. Implications for the periodic nuclear transient ASASSN-14ko are discussed., Comment: 20 pages, 16 figures, and 2 tables. Accepted by MNRAS

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 observations of ASASSN-19dj, a nearby tidal disruption event (TDE) discovered in the post-starburst galaxy KUG 0810+227 by the All-Sky Automated Survey for Supernovae (ASAS-SN) at a distance of d $\simeq98$ Mpc. We observed ASASSN-19dj from $-$21 to 392 d relative to peak ultraviolet (UV)/optical emission using high-cadence, multiwavelength spectroscopy and photometry. From the ASAS-SN $g$-band data, we determine that the TDE began to brighten on 2019 February 6.8 and for the first 16 d the rise was consistent with a flux $\propto t^2$ power-law. ASASSN-19dj peaked in the UV/optical on 2019 March 6.5 (MJD = 58548.5) at a bolometric luminosity of $L = (6.2 \pm 0.2) \times 10^{44} \text{ erg s}^{-1}$. Initially remaining roughly constant in X-rays and slowly fading in the UV/optical, the X-ray flux increased by over an order of magnitude $\sim$225 d after peak, resulting from the expansion of the X-ray emitting region. The late-time X-ray emission is well fitted by a blackbody with an effective radius of $\sim1 \times 10^{12} \text{ cm}$ and a temperature of $\sim6 \times 10^{5} \text{ K}$. The X-ray hardness ratio becomes softer after brightening and then returns to a harder state as the X-rays fade. Analysis of Catalina Real-Time Transient Survey images reveals a nuclear outburst roughly 14.5 yr earlier with a smooth decline and a luminosity of $L_V\geq1.4 \times 10^{43}$ erg s$^{-1}$, although the nature of the flare is unknown. ASASSN-19dj occurred in the most extreme post-starburst galaxy yet to host a TDE, with Lick H$\delta_{A}$ = $7.67\pm0.17$ \AA., Comment: Updated to match accepted MNRAS version

Long-duration gamma-ray bursts (GRBs) are almost unequivocally associated with very energetic, broad-lined supernovae (SNe) of Type Ic-BL. While the gamma-ray emission is emitted in narrow jets, the SN emits radiation isotropically. Therefore, some SN Ic-BL not associated with GRBs have been hypothesized to arise from events with inner engines such as off-axis GRBs or choked jets. Here we present observations of the nearby ($d = 120$ Mpc) SN 2020bvc (ASAS-SN 20bs) which support this scenario. \textit{Swift} UVOT observations reveal an early decline (up to two days after explosion) while optical spectra classify it as a SN Ic-BL with very high expansion velocities ($\approx$ 70,000 km/s), similar to that found for the jet-cocoon emission in SN 2017iuk associated with GRB 171205A. Moreover, \textit{Swift} X-Ray Telescope and \textit{CXO} X-ray Observatory detected X-ray emission only three days after the SN and decaying onwards, which can be ascribed to an afterglow component. Cocoon and X-ray emission are both signatures of jet-powered GRBs. In the case of SN 2020bvc, we find that the jet is off axis (by $\approx$ 23 degrees), as also indicated by the lack of early ($\approx 1$ day) X-ray emission which explains why no coincident GRB was detected promptly or in archival data. These observations suggest that SN 2020bvc is the first orphan GRB detected through its associated SN emission., Comment: 9 pages, 6 figures, 5 tables. Accepted for publication in A&A

We present the discovery of ASASSN-18jd (AT 2018bcb), a luminous optical/UV/X-ray transient located in the nucleus of the galaxy 2MASX J22434289--1659083 at $z=0.1192$. Over the year after discovery, Swift UVOT photometry shows the UV SED of the transient to be well modeled by a slowly shrinking blackbody with temperature $T \sim 2.5 \times 10^{4} \rm ~K$, a maximum observed luminosity of $L_\text{max} = 4.5^{+0.6}_{-0.3} \times 10^{44} \rm ~erg ~s^{-1}$, and a radiated energy of $E = 9.6^{+1.1}_{-0.6} \times 10^{51} \rm ~erg$. X-ray data from Swift XRT and XMM-Newton show a transient, variable X-ray flux with blackbody and power-law components that fade by nearly an order of magnitude over the following year. Optical spectra show strong, roughly constant broad Balmer emission as well as transient features attributable to He II, N III-V, O III, and coronal Fe. While ASASSN-18jd shares similarities with Tidal Disruption Events (TDEs), it is also similar to the newly-discovered nuclear transients seen in quiescent galaxies and faint Active Galactic Nuclei (AGNs)., Comment: 24 pages, 16 figures, 2 machine-readable tables (included as ancillary txt files)

We present and analyse a new tidal disruption event (TDE), AT2017eqx at redshift z=0.1089, discovered by Pan-STARRS and ATLAS. The position of the transient is consistent with the nucleus of its host galaxy; it peaks at a luminosity of $L \approx 10^{44}$ erg s$^{-1}$; and the spectrum shows a persistent blackbody temperature $T \gtrsim 20,000$ K with broad H I and He II emission. The lines are initially centered at zero velocity, but by 100 days the H I lines disappear while the He II develops a blueshift of $\gtrsim 5,000$ km s$^{-1}$. Both the early- and late-time morphologies have been seen in other TDEs, but the complete transition between them is unprecedented. The evolution can be explained by combining an extended atmosphere, undergoing slow contraction, with a wind in the polar direction becoming visible at late times. Our observations confirm that a lack of hydrogen a TDE spectrum does not indicate a stripped star, while the proposed model implies that much of the diversity in TDEs may be due to the observer viewing angle. Modelling the light curve suggests AT2017eqx resulted from the complete disruption of a solar-mass star by a black hole of $\sim 10^{6.3} M_\odot$. The host is another quiescent, Balmer-strong galaxy, though fainter and less centrally concentrated than most TDE hosts. Radio limits rule out a relativistic jet, while X-ray limits at 500 days are among the deepest for a TDE at this phase., Comment: Updated to match published version

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

Supernova (SN) 2018oh (ASASSN-18bt) is the first spectroscopically-confirmed type Ia supernova (SN Ia) observed in the $Kepler$ field. The $Kepler$ data revealed an excess emission in its early light curve, allowing to place interesting constraints on its progenitor system (Dimitriadis et al. 2018, Shappee et al. 2018b). Here, we present extensive optical, ultraviolet, and near-infrared photometry, as well as dense sampling of optical spectra, for this object. SN 2018oh is relatively normal in its photometric evolution, with a rise time of 18.3$\pm$0.3 days and $\Delta$m$_{15}(B)=0.96\pm$0.03 mag, but it seems to have bluer $B - V$ colors. We construct the "uvoir" bolometric light curve having peak luminosity as 1.49$\times$10$^{43}$erg s$^{-1}$, from which we derive a nickel mass as 0.55$\pm$0.04M$_{\odot}$ by fitting radiation diffusion models powered by centrally located $^{56}$Ni. Note that the moment when nickel-powered luminosity starts to emerge is +3.85 days after the first light in the Kepler data, suggesting other origins of the early-time emission, e.g., mixing of $^{56}$Ni to outer layers of the ejecta or interaction between the ejecta and nearby circumstellar material or a non-degenerate companion star. The spectral evolution of SN 2018oh is similar to that of a normal SN Ia, but is characterized by prominent and persistent carbon absorption features. The C II features can be detected from the early phases to about 3 weeks after the maximum light, representing the latest detection of carbon ever recorded in a SN Ia. This indicates that a considerable amount of unburned carbon exists in the ejecta of SN 2018oh and may mix into deeper layers., Comment: 48 pages, 23 figures. This paper is part of a coordinated effort between groups. Accepted for publication in ApJ

The number of binaries containing black holes or neutron stars depends critically on the fraction of binaries that survive supernova explosions. We searched for surviving star plus remnant binaries in a sample of 49 supernova remnants (SNR) containing 23 previously identified compact remnants and three high mass X-ray binaries (HMXB), finding no new interacting or non-interacting binaries. The upper limits on any main sequence stellar companion are typically <0.2Msun and are at worst <3Msun. This implies that f<0.1 of core collapse SNRs contain a non-interacting binary, and f=0.083 (0.032

We present the discovery of ASASSN-18ey (MAXI J1820+070), a new black hole low-mass X-ray binary discovered by the All-Sky Automated Survey for SuperNovae (ASAS-SN). A week after ASAS-SN discovered ASASSN-18ey as an optical transient, it was detected as an X-ray transient by MAXI/GCS. Here, we analyze ASAS-SN and Asteroid Terrestrial-impact Last Alert System (ATLAS) pre-outburst optical light curves, finding evidence of intrinsic variability for several years prior to the outburst. While there was no long-term rise leading to outburst, as has been seen in several other systems, the start of the outburst in the optical preceded that in the X-rays by $7.20\pm0.97~\rm days$. We analyze the spectroscopic evolution of ASASSN-18ey from pre-maximum to $> 100~\rm days$ post-maximum. The spectra of ASASSN-18ey exhibit broad, asymmetric, double-peaked H$\alpha$ emission. The Bowen blend ($\lambda \approx 4650$\AA) in the post-maximum spectra shows highly variable double-peaked profiles, likely arising from irradiation of the companion by the accretion disk, typical of low-mass X-ray binaries. The optical and X-ray luminosities of ASASSN-18ey are consistent with black hole low-mass X-ray binaries, both in outburst and quiescence., Comment: 10 pages, 5 figures, accepted by ApJL. A summary video describing this publication can be found at https://youtu.be/YbM_koBfRSI

We present the discovery of PS18kh, a tidal disruption event (TDE) discovered at the center of SDSS J075654.53+341543.6 ($d\simeq322$ Mpc) by the Pan-STARRS Survey for Transients. Our dataset includes pre-discovery survey data from Pan-STARRS, the All-Sky Automated Survey for Supernovae (ASAS-SN), and the Asteroid Terrestrial-impact Last Alert System (ATLAS) as well as high-cadence, multi-wavelength follow-up data from ground-based telescopes and Swift, spanning from 56 days before peak light until 75 days after. The optical/UV emission from PS18kh is well-fit as a blackbody with temperatures ranging from $T\simeq12000$ K to $T\simeq25000$ K and it peaked at a luminosity of $L\simeq8.8\times10^{43}$ ergs s$^{-1}$. PS18kh radiated $E=(3.45\pm0.22)\times10^{50}$ ergs over the period of observation, with $(1.42\pm0.20)\times10^{50}$ ergs being released during the rise to peak. Spectra of PS18kh show a changing, boxy/double-peaked H$\alpha$ emission feature, which becomes more prominent over time. We use models of non-axisymmetric accretion disks to describe the profile of the H$\alpha$ line and its evolution. We find that at early times the high accretion rate leads the disk to emit a wind which modifies the shape of the line profile and makes it bell-shaped. At late times, the wind becomes optically thin, allowing the non-axisymmetric perturbations to show up in the line profile. The line-emitting portion of the disk extends from $r_{\rm in}\sim60r_{\rm g}$ to an outer radius of $r_{\rm out}\sim1400r_{\rm g}$ and the perturbations can be represented either as an eccentricity in the outer rings of the disk or as a spiral arm in the inner disk., Comment: 27 pages, 13 figures, 8 tables. Updated to reflect changes made in the published version. A table containing the host-subtracted photometry presented in this manuscript is included in machine-readable format as an ancillary file

On 2018 Feb. 4.41, the All-Sky Automated Survey for SuperNovae (ASAS-SN) discovered ASASSN-18bt in the K2 Campaign 16 field. With a redshift of z=0.01098 and a peak apparent magnitude of B_{max}=14.31, ASASSN-18bt is the nearest and brightest SNe Ia yet observed by the Kepler spacecraft. Here we present the discovery of ASASSN-18bt, the K2 light curve, and pre-discovery data from ASAS-SN and the Asteroid Terrestrial-impact Last Alert System (ATLAS). The K2 early-time light curve has an unprecedented 30-minute cadence and photometric precision for an SN~Ia light curve, and it unambiguously shows a ~4 day nearly linear phase followed by a steeper rise. Thus, ASASSN-18bt joins a growing list of SNe Ia whose early light curves are not well described by a single power law. We show that a double-power-law model fits the data reasonably well, hinting that two physical processes must be responsible for the observed rise. However, we find that current models of the interaction with a non-degenerate companion predict an abrupt rise and cannot adequately explain the initial, slower linear phase. Instead, we find that existing, published models with shallow 56Ni are able to span the observed behavior and, with tuning, may be able to reproduce the ASASSN-18bt light curve. Regardless, more theoretical work is needed to satisfactorily model this and other early-time SNe~Ia light curves. Finally, we use Swift X-ray non-detections to constrain the presence of circumstellar material (CSM) at much larger distances and lower densities than possible with the optical light curve. For a constant density CSM these non-detections constrain rho<4.5 * 10^5 cm^-3 at a radius of 4 *10^15 cm from the progenitor star. Assuming a wind-like environment, we place mass-loss limits of Mdot< 8 * 10^-6 M_sun yr^-1 for v_w=100 km s^-1, ruling out some symbiotic progenitor systems., Comment: 17 pages, 8 figures, 3 Tables. Accepted to ApJ. This work is part of a number of papers analyzing ASASSN-18bt, with coordinated papers from Dimitriadis et al. (2018) and Li et al. (2018)

We present late-time optical spectroscopy and X-ray, UV, and optical photometry of the nearby ($d=214$ Mpc, $z=0.0479$) tidal disruption event (TDE) ASASSN-15oi. The optical spectra span 450 days after discovery and show little remaining transient emission or evolution after roughly 3 months. In contrast, the Swift and XMM-Newton observations indicate the presence of evolving X-ray emission and lingering thermal UV emission that is still present 600 days after discovery. The thermal component of the X-ray emission shows a unique, slow brightening by roughly an order of magnitude to become the dominant source of emission from the TDE at later times, while the hard component of the X-ray emission remains weak and relatively constant throughout the flare. The TDE radiated $(1.32\pm0.06)\times10^{51}$ ergs across all wavelengths, and the UV and optical emission is consistent with a power law decline and potentially indicative of a late-time shift in the power-law index that could be caused by a transition in the dominant emission mechanism., Comment: 16 pages, 9 figures, 4 tables. Updated to reflect changes made in the published version

We present the ultraviolet (UV) spectroscopic evolution of a tidal disruption event (TDE) for the first time. After the discovery of the nearby TDE iPTF16fnl, we obtained a series of observations with the Space Telescope Imaging Spectrograph (STIS) onboard the Hubble Space Telescope (HST). The dominant emission features closely resemble those seen in the UV spectra of the TDE ASASSN-14li and are also similar to those of N-rich quasars. However, there is significant evolution in the shape and central wavelength of the line profiles over the course of our observations, such that at early times the lines are broad and redshifted, while at later times the lines are significantly narrower and peak near the wavelengths of their corresponding atomic transitions. Like ASASSN-14li, but unlike N-rich quasars, iPTF16fnl shows neither MgII$\lambda 2798$\AA\ nor CIII]$\lambda 1909$\AA\ emission features. We also present optical photometry and spectroscopy, which suggest that the complex HeII profiles observed in the optical spectra of many TDEs are in part due to the presence of NIII and CIII Wolf-Rayet features, which can potentially serve as probes of the far-UV when space-based observations are not possible. Finally, we use Swift XRT and UVOT observations to place strong limits on the X-ray emission and determine the characteristic temperature, radius, and luminosity of the emitting material. We find that iPTF16fnl is subluminous and evolves more rapidly than other optically discovered TDEs., Comment: 15 pages, 8 figures, for a brief video overview of this paper see https://www.youtube.com/watch?v=Uh8zHAW4Xxc

Fomalhaut b is a directly imaged object in the debris disk of the star Fomalhaut. It has been hypothesized to be a planet, however there are issues with the observed colours of the object that do not fit planetary models. An alternative hypothesis is that the object is a neutron star in the near fore- or background of Fomalhaut's disk. We test if Fomalhaut b could be a neutron star using X-ray observations with Chandra's HRC-I instrument in the energy range of 0.08-10 keV. We do not detect X-ray emission from either Fomalhaut b or the star Fomalhaut itself. Our nondetection corresponds to an upper limit on the X-ray flux of Fomalhaut b of F_X < 1.3e-14 erg/cm/s^2 in the energy range 0.08-10 keV. For the A-type central star Fomalhaut, we derive an X-ray upper limit of L_X < 2e25 erg/s in the energy range 0.08-10 keV. Fomalhaut b's X-ray non-detection constrains the parameter space for a possible neutron star significantly, implying surface temperatures lower than 91000 K and distances closer than 13.3 pc to the solar system. In addition we find that reflected starlight from the central star fits the available optical detections of Fomalhaut b; a smaller planet with a large ring system might explain such a scenario., Comment: 7 pages

We present late-time optical spectroscopy taken with the Large Binocular Telescope's Multi-Object Double Spectrograph, late-time SWIFT UVOT and XRT observations, as well as improved ASAS-SN pre-discovery limits on the nearby (d=90.3 Mpc, z=0.0206) tidal disruption event (TDE) ASASSN-14li. The late-time optical spectra show H$\alpha$ emission well in excess of that seen in the SDSS host galaxy spectrum, indicating that the processes powering the luminous flares associated with TDEs can operate for several hundreds of days. The SWIFT observations reveal the presence of lingering apparently thermal UV (T$_{\rm UV} \sim 3.5\times10^4$ K) and X-ray (T$_{\rm X} \sim 7\times10^5$ K) emission. The characteristic temperatures evolve by, at most, a factor of $\sim2$ over the 600 day follow-up campaign. The X-ray, UV, and H$\alpha$ luminosities evolve roughly in tandem and at a rate that is consistent with a power-law decay at late times. This behavior is in stark contrast with the majority of optically discovered TDEs, which are X-ray faint and evolve on shorter timescales. Finally we address how the unique properties of ASASSN-14li can be used to probe the relationship between the TDE rate and host galaxy properties., Comment: 15 pages, 8 figures. For a brief video summarizing the paper, see https://www.youtube.com/watch?v=DsNaCUeqr_g