We report the discovery and classification of SDSS J053341.43+001434.1 (SDSS0533), an early-L dwarf first discovered during a powerful Delta V < -11 magnitude flare observed as part of the ASAS-SN survey. Optical and infrared spectroscopy indicate a spectral type of L0 with strong H alpha emission and a blue NIR spectral slope. Combining the photometric distance, proper motion, and radial velocity of SDSS0533 yields three-dimensional velocities of (U, V, W) = (14 +/- 13, -35 +/- 14, -94 +/- 22) km s(-1), indicating that it is most likely part of the thick disk population and probably old. The three detections of SDSS0533 obtained during the flare are consistent with a total V-band flare energy of at least 4.9 x 10(33) erg (corresponding to a total thermal energy of at least E-tot > 3.7 x 10(34) erg), placing it among the strongest detected M dwarf flares. The presence of this powerful flare on an old L0 dwarf may indicate that stellar-type magnetic activity persists down to the end of the main sequence and on older ML transition dwarfs.

We present spatially and spectrally resolved Atacama Large Millimeter/submillimeter Array (ALMA) observations of gas and dust orbiting the pre-main-sequence hierarchical triple-star system GW Ori. A forward modeling of the (CO)-C-13 and (CO)-O-18 J = 2-1 transitions permits a measurement of the total stellar mass in this system, 5.29 +/-. 0.09 M-circle dot, and the circumtriple disk inclination, 137 degrees.6 +/- 2 degrees.0. Optical spectra spanning a 35 yr period were used to derive new radial velocities and, coupled with a spectroscopic disentangling technique, revealed that the A and B components of GWOri form a double-lined spectroscopic binary with a period of 241.50 +/- 0.05 days; a tertiary companion orbits that inner pair with a period of 4218 +/- 50 days. Combining the results from the ALMA data and the optical spectra with three epochs of astrometry in the literature, we constrain the individual stellar masses in the system (M-A approximate to 2.7M(circle dot), M-B approximate to 1.7M(circle dot), M-C approximate to 0.9M(circle dot)) and find strong evidence that at least one of the stellar orbital planes (and likely both) is misaligned with the disk plane by as much as 45 degrees. A V-band light curve spanning 30 yr reveals several new similar to 30-day eclipse events 0.1-0.7. mag in depth and a 0.2 mag sinusoidal oscillation that is clearly phased with the AB-C orbital period. Taken together, these features suggest that the A-B pair may be partially obscured by material in the inner disk as the pair approaches apoastron in the hierarchical orbit. Lastly, we conclude that stellar evolutionary models are consistent with our measurements of the masses and basic photospheric properties if the GWOri system is similar to 1Myr old.

We present similar to 800 days of photometric monitoring of Boyajian's Star (KIC 8462852) from the All-Sky Automated Survey for Supernovae (ASAS-SN) and similar to 4000 days of monitoring from the All Sky Automated Survey (ASAS). We show that from 2015 to the present the brightness of Boyajian's Star has steadily decreased at a rate of 6.3 +/- 1.4 mmag yr(-1), such that the star is now 1.5%. fainter than it was in 2015 February. Moreover, the longer time baseline afforded by ASAS suggests that Boyajian's Star has also undergone two brightening episodes in the past 11 years, rather than only exhibiting a monotonic decline. We analyze a sample of similar to 1000 comparison stars of similar brightness located in the same ASAS-SN field and demonstrate that the recent fading is significant at. 99.4%. confidence. The 2015-2017 dimming rate is consistent with that measured with Kepler data for the time period from 2009 to 2013. This long-term variability is difficult to explain with any of the physical models for the star's behavior proposed to date.

Context. Accretion outbursts are key elements in star formation. ASASSN-13db is a M5-type star with a protoplanetary disk, the lowest-mass star known to experience accretion outbursts. Since its discovery in 2013, it has experienced two outbursts, the second of which started in November 2014 and lasted until February 2017.

We present the first results from a reverberation-mapping campaign undertaken during the first half of 2012, with additional data on one active galactic nucleus (AGN) (NGC 3227) from a 2014 campaign. Our main goals are (1) to determine the black hole masses from continuum-H beta reverberation signatures, and (2) to look for velocity-dependent time delays that might be indicators of the gross kinematics of the broad-line region. We successfully measure H beta time delays and black hole masses for five AGNs, four of which have previous reverberation mass measurements. The values measured here are in agreement with earlier estimates, though there is some intrinsic scatter beyond the formal measurement errors. We observe velocity-dependent H beta lags in each case, and find that the patterns have changed in the intervening five years for three AGNs that were also observed in 2007.

We present the ATLAS discovery and initial analysis of the first 18 days of the unusual transient event, ATLAS18qqn/AT2018cow. It is characterized by a high peak luminosity (similar to 1.7 x 10(44) erg s(-1)),rapidly evolving light curves (>5 mag rise to peak in similar to 3.5 days), and hot blackbody spectra, peaking at similar to 27,000 K that are relatively featureless and unchanging over the first two weeks. The bolometric light curve cannot be powered by radioactive decay under realistic assumptions. The detection of high-energy emission may suggest a central engine as the powering source. Using a magnetar model, we estimated an ejected mass of 0.1-0.4 M-circle dot, which lies between that of low-energy core-collapse events and the kilonova, AT2017gfo. The spectra cooled rapidly from 27,000 to 15,000 K in just over two weeks but remained smooth and featureless. Broad and shallow emission lines appear after about 20 days, and we tentatively identify them as He I although they would be redshifted from their rest wavelengths. We rule out that there are any features in the spectra due to intermediate mass elements up to and including the Fe group. The presence of r-process elements cannot be ruled out. If these lines are due to He, then we suggest a low-mass star with residual He as a potential progenitor. Alternatively, models of magnetars formed in neutron star mergers, or accretion onto a central compact object, give plausible matches to the data.

We present the discovery of ASASSN-18ey (MAXI J1820+070), a new black hole low-mass X-ray binary (LMXB) 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 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 the 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 +/- 0.97 days. We analyze the spectroscopic evolution of ASASSN-18ey from pre-maximum to >100 days post-maximum. The spectra of ASASSN-18ey exhibit broad, asymmetric, double-peaked Ha emission. The Bowen blend (lambda approximate to 4650 angstrom) 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.

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 us to place interesting constraints on its progenitor system. Here we present extensive optical, ultraviolet, and nearinfrared 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 +/- 0.3 days and Delta m(15)(B) = 0.96 +/- 0.03 mag, but it seems to have bluer B - V colors. We construct the "UVOIR" bolometric light curve having a peak luminosity of 1.49. x. 10(43) erg s(-1), from which we derive a nickel mass as 0.55 +/- 0.04M(circle dot) by fitting radiation diffusion models powered by centrally located Ni-56. 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 Ni-56 to outer layers of the ejecta or interaction between the ejecta and nearby circumstellar material or a nondegenerate 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 an SN Ia. This indicates that a considerable amount of unburned carbon exists in the ejecta of SN 2018oh and may mix into deeper layers.

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.