We present ground-based and Swift photometric and spectroscopic observations of the candidate tidal disruption event (TDE) ASASSN-14li, found at the center of PGC 043234 (d 90 Mpc) by the All-Sky Automated Survey for SuperNovae (ASAS-SN). The source had a peak bolometric luminosity of L 10 44 ergs s −1 and a total integrated energy of E 7 × 10 50 ergs radiated over the ∼ 6 months of observations presented. The UV/optical emission of the source is well-fit by a blackbody with roughly constant temperature of T ∼ 35, 000 K, while the luminosity declines by roughly a factor of 16 over this time. The optical/UV luminosity decline is broadly consistent with an exponential decline, L ∝ e −t/t0 , with t 0 60 days. ASASSN14li also exhibits soft X-ray emission comparable in luminosity to the optical and UV emission but declining at a slower rate, and the X-ray emission now dominates. Spectra of the source show broad Balmer and helium lines in emission as well as strong blue continuum emission at all epochs. We use the discoveries of ASASSN14li and ASASSN-14ae to estimate the TDE rate implied by ASAS-SN, finding an average rate of r 4.1 × 10 −5 yr −1 per galaxy with a 90% confidence interval of (2.2 − 17.0) × 10 −5 yr −1 per galaxy. ASAS-SN found roughly 1 TDE for every 70 Type Ia supernovae in 2014, a rate that is much higher than that of other surveys.

We present the light curves obtained during an eight-year program of optical spectroscopic monitoring of nine Seyfert 1 galaxies: 3C 120, Akn 120, Mrk 79, Mrk 110, Mrk 335, Mrk 509, Mrk 590, Mrk 704, and Mrk 817. All objects show significant variability in both the continuum and emission-line fluxes. We use cross-correlation analysis to derive the sizes of the broad Hβ-emitting regions based on emission-line time delays, or lags. We successfully measure time delays for eight of the nine sources, and find values ranging from about two weeks to a little over two months. Combining the measured lags and widths of the variable parts of the emission lines allows us to make virial mass estimates for the active nucleus in each galaxy. The virial masses are in the range 10 7−8 M ⊙ .

Some classes of stars, including supernovae and novae, undergo explosive outbursts that eject stellar material into space. In 2002, the previously unknown variable star V838 Monocerotis brightened suddenly by a factor of about 10^4. Unlike a supernova or nova, V838 Mon did not explosively eject its outer layers; rather, it simply expanded to become a cool supergiant with a moderate-velocity stellar wind. Superluminal light echoes were discovered as light from the outburst propagated into surrounding, pre-existing circumstellar dust. Here we report high-resolution imaging and polarimetry of the light echoes, which allow us to set direct geometric distance limits to the object. At a distance of >6 kpc, V838 Mon at its maximum brightness was temporarily the brightest star in the Milky Way. The presence of the circumstellar dust implies that previous eruptions have occurred, and spectra show it to be a binary system. When combined with the high luminosity and unusual outburst behavior, these characteristics indicate that V838 Mon represents a hitherto unknown type of stellar outburst, for which we have no completely satisfactory physical explanation.Comment: To appear in Nature, March 27, 2003. 9 pages, 6 figure

We present results of an intensive 2 month campaign of ground-based spectrophotometric monitoring of the Seyfert 1 galaxy NGC 7469, with a temporal resolution day. The broad Ha and Hb emission [ 1 lines respond to D35% ultraviolet continuum variations with an amplitude of D10% and time delays of 5.6^1.3 days and 5.4^0.8 days, respectively. We interpret this as evidence of variable Balmer line gas D5È6 light days from the central source in this object, widely believed to be a supermassive black hole. The virial mass of the central source implied by line widths and time delays is D106È107 Concomi-M _. tantly, we Ðnd evidence for wavelength-dependent continuum time delays : optical continuum variations lag those at 1315 by 1.0^0.3 days at 4865 to 1.5^0.7 days at 6962 This suggests a stratiÐed A A A. continuum reprocessing region extending several light days from the central source, possibly an accretion disk.

We report photometry and spectroscopy of the evolution of Nova Sagittarii 1994 #1 (V4332 Sagittarii) during outburst. We compare the photometric and spectral evolution of this outburst to known classes of outbursts -including classical novae and outbursts occurring on symbiotic stars -and find this object does not conform to any known class of outburst. The closest match to the behavior of this unusual object is M31 RV, an extremely luminous and red variable object discovered in the bulge of M31 in 1988. However, the temporal behavior and maximum luminosity of the two events differ by several orders of magnitude, requiring substantial intrinsic variation if these two events are members the same type of outburst.Our model of the spectroscopic evolution of this outburst shows that the effective temperature cooled from 4400 K to 2300 K over the three month span of our observations. In combination with line diagnostics in our later spectra, including [OI] λ5577 and the dramatic increase in the Hα to Hβ ratio, we infer the existence of a 1 Postal Address: Steward Observatory, University of Arizona, Tucson, AZ 85721 cool, dense (N e ∼ 10 8−9 cm −3 ) envelope that is optically thick in the Hydrogen Balmer recombination lines (case C). We suggest that a nuclear event in a single star, in which a slow shock drove the photosphere outwards, can power the observed luminosity evolution and the emission spectrum.

We report on the results of a three-year program of coordinated X-ray and optical monitoring of the narrow-line Seyfert 1 galaxy NGC 4051. The rapid continuum variations observed in the X-ray spectra are not detected in the optical, although the time-averaged X-ray and optical continuum fluxes are well-correlated. Variations in the flux of the broad Hβ line are found to lag behind the optical continuum variations by 6 days (with an uncertainty of 2-3 days), and combining this with the line width yields a virial mass estimate of ∼ 1.1 × 10 6 M ⊙ , at the very low end of the distribution of AGN masses measured by line reverberation. Strong variability of He ii λ4686 is also detected, and the response time measured is similar to that of Hβ, but with a much larger uncertainty. The He ii λ4686 line is almost five times broader than Hβ, and it is strongly blueward asymmetric, as are the high-ionization UV lines recorded in archival spectra of NGC 4051. The data are consistent with the Balmer lines arising in a low to moderate inclination disk-like configuration, and the high-ionization lines arising in an outflowing wind, of which we observe preferentially the near side. Previous observations of the narrow-line region morphology of this source suggest that the system is inclined by ∼ 50 o , and if this is applicable to the broad Hβ-emitting region, a central mass of ∼ 1.4 × 10 6 M ⊙ can be inferred. During the third year of monitoring, both the X-ray continuum and the He ii λ4686 line went into extremely low states, although the optical continuum and the Hβ broad line were both still present and 6 Deceased.-2variable. We suggest that the inner part of the accretion disk may have gone into an advection-dominated state, yielding little radiation from the hotter inner disk.2. NLS1s are low-inclination (i.e., nearly face-on) systems (Osterbrock & Pogge 1985). In this model, the line widths are again due to orbital motion around the central black hole, and the bulk of the broad-line region (BLR) gas orbits in a common plane that is almost perpendicular to the line of sight, leading to relatively small Doppler widths.3. NLS1s have relatively low black-hole masses, but high accretion rates. Again, the basic assumption is that the BLR motions are virial, but the central source has a lower mass. The luminosity can be kept relatively high by supposing that the accretion rate (relative to the Eddington rate) is correspondingly high in these sources.

We report on studies of classical nova (CN) explosions where we follow the evolution of thermonuclear runaways (TNRs) on carbon-oxygen (CO) white dwarfs (WDs). We vary both the mass of the WD (from 0.6 M e to 1.35 M e ) and the composition of the accreted material. Our simulations are guided by the results of multidimensional studies of TNRs in WDs, which find that sufficient mixing with WD core material occurs after the TNR is well underway, and levels of enrichment are reached that agree with observations of CN ejecta abundances. We use NOVA (our one-dimensional hydrodynamic code) to accrete solar matter until the TNR is ongoing and then switch to a mixed composition (either 25% WD material and 75% solar or 50% WD material and 50% solar). Because the amount of accreted material is inversely proportional to the initial 12 C abundance, by first accreting solar matter the amount of material taking part in the outburst is larger than in those simulations where we assume a mixed composition from the beginning. Our results show large enrichments of 7 Be in the ejected gases, implying that CO CNe may be responsible for a significant fraction (∼100 M e ) of the 7 Li in the galaxy (∼1000 M e ). Although the ejected gases are enriched in WD material, the WDs in these simulations eject less material than they accrete. We predict that the WD is growing in mass as a consequence of the accretion-outburst-accretion cycle, and CO CNe may be an important channel for SN Ia progenitors.

V4743 Sgr (Nova Sgr 2002 No. 3) was discovered on 20 September 2002. We obtained a 5 ks ACIS-S spectrum in November 2002 and found that the nova was faint in X-rays. We then obtained a 25 ks CHANDRA LETGS observation on 19 March 2003. By this time, it had evolved into the Super Soft X-ray phase exhibiting a continuous spectrum with deep absorption features. The light curve from the observation showed large amplitude oscillations with a period of 1325 s (22 min) followed by a decline in total count rate after ∼ 13 ks of observations. The count rate dropped from ∼ 40 cts s −1 to practically zero within ∼ 6 ks and stayed low for the rest of the observation (∼ 6 ks. The spectral hardness ratio changed from maxima to minima in correlation with the oscillations, and then became significantly softer during the decay. Strong H-like and He-like lines of oxygen, nitrogen, and carbon were found in absorption during the bright phase, indicating temperatures between 1-2 MK, but they were shifted in wavelength corresponding to a Doppler velocity of -2400 km s −1 . The spectrum obtained after the decline in count rate showed emission lines of C vi, N vi, and N vii suggesting that we were seeing expanding gas ejected during the outburst, probably originating from CNO-cycled material. An XMM-Newton ToO observation, obtained on 4 April 2003 and a later LETGS observation from 18 July 2003 also showed oscillations, but with smaller amplitudes.