On 2018 February 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 prediscovery data from ASAS-SN and the Asteroid Terrestrial-impact Last Alert System. 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 similar to 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 nondegenerate companion predict an abrupt rise and cannot adequately explain the initial, slower linear phase. Instead, we find that existing published models with shallow Ni-56 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 nondetections 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 nondetections constrain rho < 4.5 x 10(5) cm(-3) at a radius of 4 x 10(15) cm from the progenitor star. Assuming a wind-like environment, we place mass loss limits of <(M)over dot> < 8 x 10(-6) M-circle dot yr(-1). for v(w) =. 100 km s(-1), ruling out some symbiotic progenitor systems. This work highlights the power of well-sampled early-time data and the need for immediate multiband, high-cadence follow-up for progress in understanding SNe Ia.

We employ Very Large Telescope Interferometer GRAVITY to resolve, for the first time, the two images generated by a gravitational microlens. The measurements of the image separation Delta theta(-,+) = 3.78 +/- 0.05 mas, and hence the Einstein radius theta(E) = 1.87 +/- 0.03 mas, are precise. This demonstrates the robustness of the method, provided that the source is bright enough for GRAVITY (K less than or similar to 10.5) and the image separation is of order of or larger than the fringe spacing. When theta(E) is combined with a measurement of the "microlens parallax" pi(E), the two will together yield the lens mass and lens-source relative parallax and proper motion. Because the source parallax and proper motion are well measured by Gaia, this means that the lens characteristics will be fully determined, whether or not it proves to be luminous. This method can be a powerful probe of dark, isolated objects, which are otherwise quite difficult to identify, much less characterize. Our measurement contradicts Einstein's prediction that "the luminous circle [i.e., microlensed image] cannot be distinguished" from a star.

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 three 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 +/- 0.06) x 10(51) erg 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.

The All-sky Automated Survey for Supernovae (ASAS-SN) is the only project in existence to scan the entire sky in optical light approximately every day, reaching a depth of g similar to 18 mag. Over the course of its first 4 yr of transient alerts (2013-2016), ASAS-SN observed 53 events classified as likely M dwarf flares. We present follow-up photometry and spectroscopy of all 53 candidates, confirming flare events on 47 M dwarfs, one K dwarf, and one L dwarf. The remaining four objects include a previously identified T Tauri star, a young star with outbursts, and two objects too faint to confirm. A detailed examination of the 49 flare star light curves revealed an additional six flares on five stars, resulting in a total of 55 flares on 49 objects ranging in V-band contrast from Delta V = -1 to -10.2. mag. Using an empirical flare model to estimate the unobserved portions of the flare light curve, we obtain lower limits on the V-band energy emitted during each flare, spanning log(E-V/erg) = 32-35, which are among the most energetic flares detected on M dwarfs. The ASAS-SN M dwarf flare stars show a higher fraction of Ha emission, as well as stronger Ha emission, compared to M dwarfs selected without reference to activity, consistent with belonging to a population of more magnetically active stars. We also examined the distribution of tangential velocities, finding that the ASAS-SN flaring M dwarfs are likely to be members of the thin disk and are neither particularly young nor old.

We present the discovery of PS18kh, a tidal disruption event discovered at the center of SDSS J075654.53 +341543.6 (d similar or equal to 322 Mpc) by the Pan-STARRS Survey for Transients. Our data set includes pre-discovery survey data from Pan-STARRS, the All-sky Automated Survey for Supernovae, and the Asteroid Terrestrial-impact Last Alert System as well as high-cadence, multiwavelength 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 similar or equal to 12,000 K to T similar or equal to 25,000 K and it peaked at a luminosity of L similar or equal to 8.8 x 10(43) erg s(-1). PS18kh radiated E = (3.45 +/- 0.22) x 10(50) erg over the period of observation, with (1.42 +/- 0.20) x 10(50) erg being released during the rise to peak. Spectra of PS18kh show a changing, boxy/double-peaked Ha emission feature, which becomes more prominent over time. We use models of non-axisymmetric accretion disks to describe the profile of the Ha 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(in) similar to 60r(g) to an outer radius of r(out) similar to 1400r(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.

We study the sudden optical and ultraviolet (UV) brightening of 1ES 1927+ 654, which until now was known as a narrow-line active galactic nucleus (AGN). 1ES 1927+ 654 was part of the small and peculiar class of "true Type2" AGNs that lack broad emission lines and line-of-sight obscuration. Our high-cadence spectroscopic monitoring captures the appearance of a blue, featureless continuum, followed several weeks later by the appearance of broad Balmer emission lines. This timescale is generally consistent with the expected light travel time between the central engine and the broadline emission region in (persistent) broadline AGN. Hubble Space Telescope spectroscopy reveals no evidence for broad UV emission lines (e. g., C IV lambda 1549, C III] lambda 1909, Mg II lambda 2798), probably owing to dust in the broadline emission region. To the best of our knowledge, this is the first case where the lag between the change in continuum and in broadline emission of a "changing look" AGN has been temporally resolved. The nature and timescales of the photometric and spectral evolution disfavor both a change in line-of-sight obscuration and a change of the overall rate of gas inflow as driving the drastic spectral transformations seen in this AGN. Although the peak luminosity and timescales are consistent with those of tidal disruption events seen in inactive galaxies, the spectral properties are not. The X-ray emission displays a markedly different behavior, with frequent flares on timescales of hours to days, and will be presented in a companion publication.

Black hole binary systems with companion stars are typically found via their x-ray emission, generated by interaction and accretion. Noninteracting binaries are expected to be plentiful in the Galaxy but must be observed using other methods. We combine radial velocity and photometric variability data to show that the bright, rapidly rotating giant star 2MASS J05215658+4359220 is in a binary system with a massive unseen companion. The system has an orbital period of similar to 83 days and near-zero eccentricity. The photometric variability period of the giant is consistent with the orbital period, indicating star spots and tidal synchronization. Constraints on the giant's mass and radius imply that the unseen companion is 3.3(-0.7)(+2.8) solar masses, indicating that it is a noninteracting low-mass black hole or an unexpectedly massive neutron star.

We analyzed the light curves of 1376 early-to-late, nearby M dwarfs to search for white-light flares using photometry from the All-Sky Automated Survey for Supernovae. We identified 480 M dwarfs with at least one potential flare employing a simple statistical algorithm that searches for sudden increases in V-band flux. After more detailed evaluation, we identified 62 individual flares on 62 stars. The event amplitudes range from. Using classical flare models, we place lower limits on the flare energies and obtain V-band energies spanning. The fraction of flaring stars increases with spectral type, and most flaring stars show moderate to strong H alpha emission. Additionally, we find that 14 of the 62 flaring stars are rotational variables, and they have shorter rotation periods and stronger H alpha emission than nonflaring rotational variable M dwarfs.

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 log10(Lpeak) = ( -)44.1+ 0.10.1.+.()(D +) 1.6+ L 0.5 0.20.4 40 in cgs, where Delta L-40 = log(10)(L-40/L-peak).

We present the discovery of ASASSN-18jd (AT 2018bcb), a luminous optical/ultraviolet(UV)/X-ray transient located in the nucleus of the galaxy 2MASX J22434289-1659083 at z = 0.1192. Over the year after discovery, Swift UltraViolet and Optical Telescope (UVOT) photometry shows the UV spectral energy distribution of the transient to be well modelled by a slowly shrinking blackbody with temperature T similar to 2.5 x 10(4) K, a maximum observed luminosity of L-max = 4.5(-0.3)(+0.6) x 10(44) erg s(-1), and a radiated energy of E = 9.6(-0.6)(+1.1) x 10(51) erg. X-ray data from Swift X-Ray Telescope (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 and 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).