The Rise and Peak of the Luminous Type IIn SN 2017hcc/ATLAS17lsn from ASAS-SN and Swift UVOT Data

We present observations of the rise and peak of the Type IIn supernova SN 2017hcc/ATLAS17lsn obtained by the All-Sky Automated Survey for Supernovae (ASAS-SN) and Swift UVOT. The light curve of SN 2017hcc/ATLAS17lsn peaks at $V\simeq 13.7$ mag, which from the estimated redshift of the host galaxy ($z=0.0168$, $D\simeq 73$ Mpc) implies an absolute peak magnitude $M_{V,peak} \simeq -20.7$ mag. The near-UV to optical spectral energy distribution of SN 2017hcc/ATLAS17lsn from Swift UVOT is consistent with a hot, but cooling blackbody with $\rm T_{bb}\simeq 16500$ K on Oct. 28.4 and $\rm T_{bb} \simeq 11700$ K on Nov. 19.6. The estimated peak bolometric luminosity $L_{bol, peak}\simeq 1.3\times 10^{44}$ erg s$^{-1}$ makes SN2017hcc/ATLAS17lsn one of the most luminous Type IIn supernovae studied to date. From the bolometric light curve we constrain the risetime to be $\sim 27$ days and the total radiated energy of the event to date is $4\times 10^{50}$ erg

ASASSN-14ae: a tidal disruption event at 200 Mpc

ASASSN-14ae is a candidate tidal disruption event (TDE) found at the centre of SDSS J110840.11+340552.2 (d ≃ 200 Mpc) by the All-Sky Automated Survey for Supernovae (ASAS-SN). We present ground-based and Swift follow-up photometric and spectroscopic observations of the source, finding that the transient had a peak luminosity of L ≃ 8 × 1043 erg s−1 and a total integrated energy of E ≃ 1.7 × 1050 erg radiated over the ∼5 months of observations presented. The blackbody temperature of the transient remains roughly constant at T ∼ 20 000 K while the luminosity declines by nearly 1.5 orders of magnitude during this time, a drop that is most consistent with an exponential, L ∝ e-t/t 0 with t0 ≃ 39 d. The source has broad Balmer lines in emission at all epochs as well as a broad He ii feature emerging in later epochs. We compare the colour and spectral evolution to both supernovae and normal AGN to show that ASASSN-14ae does not resemble either type of object and conclude that a TDE is the most likely explanation for our observations. At z = 0.0436, ASASSN-14ae is the lowest-redshift TDE candidate discovered at optical/UV wavelengths to date, and we estimate that ASAS-SN may discover 0.1–3 of these events every year in the future

Supernova progenitors, their variability and the Type IIP Supernova ASASSN-16fq in M66

We identify a pre-explosion counterpart to the nearby Type IIP supernova ASASSN-16fq (SN 2016cok) in archival $\textit{Hubble Space Telescope}$ data. The source appears to be a blend of several stars that prevents obtaining accurate photometry. However, with reasonable assumptions about the stellar temperature and extinction, the progenitor almost certainly had an initial mass $M_*$ $\lesssim$ 17 M$_\odot$, and was most likely in the mass range of $M_*$ = 8–12 M$_\odot$. Observations once ASASSN-16fq has faded will have no difficulty accurately determining the properties of the progenitor. In 8 yr of Large Binocular Telescope (LBT) data, no significant progenitor variability is detected to rms limits of roughly 0.03 mag. Of the six nearby supernova (SN) with constraints on the low-level variability, SN 1987A, SN 1993J, SN 2008cn, SN 2011dh, SN 2013ej and ASASSN-16fq, only the slowly fading progenitor of SN 2011dh showed clear evidence of variability. Excluding SN 1987A, the 90 per cent confidence limit implied by these sources on the number of outbursts over the last decade before the SN that last longer than 0.1 yr (full width at half-maximum) and are brighter than $M_R$ < −8 mag is approximately $N_\text{out}$ $\lesssim$ 3. Our continuing LBT monitoring programme will steadily improve constraints on pre-SN progenitor variability at amplitudes far lower than achievable by SN surveys.CSK, KZS, JSB, SMA and TWSH are supported by NSF grants AST-1515876 and AST-1515927. BJS is supported by NASA through Hubble Fellowship grant HF-51348.001 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS 5-26555. TW-SH is supported by the DOE Computational Science Graduate Fellowship, grant number DE-FG02- 97ER25308. TS is partly supported by NSF grant PHY-1404311 to J. Beacom. This work was partly supported by the European Union FP7 programme through ERC grant number 320360. Support for JLP is provided in part by FONDECYT through the grant 1151445 and by the Ministry of Economy, Development, and Tourism’s Millennium Science Initiative through grant IC120009, awarded to The Millennium Institute of Astrophysics, MAS. SD is supported by the Strategic Priority Research Program ‘The Emergence of Cosmological Structures’ of the Chinese Academy of Sciences (Grant No. XDB09000000) and NSFC project 11573003. Some of the observations were carried out using the LBT at Mt Graham, AZ. The LBT is an international collaboration among institutions in the United States, Italy and Germany. LBT Corporation partners are the University of Arizona on behalf of the Arizona university system; Istituto Nazionale di Astrofisica, Italy; LBT Beteiligungsgesellschaft, Germany, representing the Max–Planck Society, the Astrophysical Institute Potsdam and Heidelberg University; the Ohio State University; and The Research Corporation, on behalf of the University of Notre Dame, University of Minnesota and University of Virginia. This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA, and in part on observations made with the NASA/ESA HST obtained at the Space Telescope Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. Some observations were obtained from the Hubble Legacy Archive, which is a collaboration between the Space Telescope Science Institute (STScI/NASA), the Space Telescope European Coordinating Facility (ST-ECF/ESA) and the Canadian Astronomy Data Centre (CADC/NRC/CSA)

ASASSN-15oi: A Rapidly Evolving, Luminous Tidal Disruption Event at 216 Mpc

We present ground-based and Swift photometric and spectroscopic observations of the tidal disruption event (TDE) ASASSN-15oi, discovered at the center of 2MASX J20390918-3045201 ($d\simeq216$ Mpc) by the All-Sky Automated Survey for SuperNovae (ASAS-SN). The source peaked at a bolometric luminosity of $L\simeq1.9\times10^{44}$ ergs s$^{-1}$ and radiated a total energy of $E\simeq5.0\times10^{50}$ ergs over the $\sim3.5$ months of observations. The early optical/UV emission of the source can be fit by a blackbody with temperature increasing from $T\sim2\times10^4$ K to $T\sim6\times10^4$ K while the luminosity declines from $L\simeq1.9\times10^{44}$ ergs s$^{-1}$ to $L\simeq2.8\times10^{43}$ ergs s$^{-1}$, requiring the photosphere to be shrinking rapidly. The optical/UV luminosity decline is broadly consistent with an exponential decline, $L\propto e^{-t/t_0}$, with $t_0\simeq35$ days. ASASSN-15oi also exhibits roughly constant soft X-ray emission that is significantly weaker than the optical/UV emission. Spectra of the source show broad helium emission lines and strong blue continuum emission in early epochs, although these features fade rapidly and are not present $\sim3$ months after discovery. The early spectroscopic features and color evolution of ASASSN-15oi are consistent with a TDE, but the rapid spectral evolution is unique among optically-selected TDEs

1ES 1927+654: An AGN Caught Changing Look on a Timescale of Months

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 Type-2" 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 λ1549, C iii] λ1909, Mg ii λ2798), 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

Constraints on the Progenitor System of the Type Ia Supernova SN 2011fe/PTF11kly

Type Ia supernovae (SNe) serve as a fundamental pillar of modern cosmology, owing to their large luminosity and a well-defined relationship between light-curve shape and peak brightness. The precision distance measurements enabled by SNe Ia first revealed the accelerating expansion of the universe, now widely believed (though hardly understood) to require the presence of a mysterious "dark" energy. General consensus holds that Type Ia SNe result from thermonuclear explosions of a white dwarf (WD) in a binary system; however, little is known of the precise nature of the companion star and the physical properties of the progenitor system. Here we make use of extensive historical imaging obtained at the location of SN 2011fe/PTF11kly, the closest SN Ia discovered in the digital imaging era, to constrain the visible-light luminosity of the progenitor to be 10-100 times fainter than previous limits on other SN Ia progenitors. This directly rules out luminous red giants and the vast majority of helium stars as the mass-donating companion to the exploding white dwarf. Any evolved red companion must have been born with mass less than 3.5 times the mass of the Sun. These observations favour a scenario where the exploding WD of SN 2011fe/PTF11kly, accreted matter either from another WD, or by Roche-lobe overflow from a subgiant or main-sequence companion star.Comment: 22 pages, 6 figures, submitte

The Cow: Discovery of a Luminous, Hot, and Rapidly Evolving Transient

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 (~1.7 × 1044 erg s−1), rapidly evolving light curves (>5 mag rise to peak in ~3.5 days), and hot blackbody spectra, peaking at ~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 ${}_{\odot }$, 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

Periodic eclipses of the young star PDS 110 discovered with WASP and KELT photometry

We report the discovery of eclipses by circumstellar disc material associated with the young star PDS 110 in the Ori OB1a association using the SuperWASP and Kilodegree Extremely Little Telescope surveys. PDS 110 (HD 290380, IRAS 05209-0107) is a rare Fe/Ge-type star, an similar to 10 Myr-old accreting intermediate-mass star showing strong infrared excess (L-IR/L-bol similar or equal to 0.25). Two extremely similar eclipses with a depth of 30 per cent and duration similar to 25 d were observed in 2008 November and 2011 January. We interpret the eclipses as caused by the same structure with an orbital period of 808 +/- 2 d. Shearing over a single orbit rules out diffuse dust clumps as the cause, favouring the hypothesis of a companion at similar to 2 au. The characteristics of the eclipses are consistent with transits by an unseen low-mass (1.8-70M(Jup)) planet or brown dwarf with a circumsecondary disc of diameter similar to 0.3 au. The next eclipse event is predicted to take place in 2017 September and could be monitored by amateur and professional observatories across the world

Six months of multiwavelength follow-up of the tidal disruption candidate ASASSN-14li and implied TDE rates from ASAS-SN

We present ground-based and Swift photometric and spectroscopic observations of the candidate tidal disruption event (TDE) ASASSN-14li, found at the center of PGC043234 (d ' 90 Mpc) by the All-Sky Automated Survey for SuperNovae (ASAS-SN). The source had a peak bolometric luminosity of L ' 1044 ergs

ASASSN-15lh: The Most Luminous Supernova Ever Discovered

We report the discovery and early evolution of ASASSN-15lh, the most luminous supernova ever found. At redshift z=0.2326, ASASSN-15lh reached an absolute magnitude of M_{u,AB} ~ -23.5 and bolometric luminosity L_bol ~ 2.2x10^45 ergs/s, which is >~ 2 times more luminous than any previously known supernova. Its spectra match the hydrogen-poor sub-class of super-luminous supernovae (SLSNe-I), whose energy sources and progenitors are poorly understood. In contrast to known SLSNe-I, most of which reside in star-forming, dwarf galaxies, its host appears to be a luminous galaxy (M_V ~ -22; M_K ~ -25.1) with little star formation. In the two months since its first detection, ASASSN-15lh has radiated ~7.5x10^51 ergs, challenging the popular magnetar model for the engine of SLSNe-I