Nebular spectra of 111 Type Ia supernovae disfavour single-degenerate progenitors

We place statistical constraints on Type Ia supernova (SN Ia) progenitors using 227 nebular-phase spectra of 111 SNe Ia. We find no evidence of stripped companion emission in any of the nebular-phase spectra. Upper limits are placed on the amount of mass that could go undetected in each spectrum using recent hydrodynamic simulations. With these null detections, we place an observational 3 sigma upper limit on the fraction of SNe Ia that are produced through the classical H-rich non-degenerate companion scenario of < 5.5 per cent. Additionally, we set a tentative 3 sigma upper limit otan He star progenitor scenarios of < 6.4 per cent, although further theoretical modelling is required. These limits refer to our most representative sample including normal, 91bg-like, 91T-like, and 'super-Chandrasekhar' SNe Ia but excluding SNe Iax and SNe Ia-CSM. As part of our analysis, we also derive a Nebular Phase Phillips Relation, which approximates the brightness of an SN Ia from 150 to 500 d after maximum using the peak magnitude and decline rate parameter Delta m(15)(B)

The extraplanar type II supernova ASASSN-14jb in the nearby edge-on galaxy ESO 467-G051

We present optical photometry and spectroscopy of the Type II supernova ASASSN-14jb, together with Very Large Telescope (VLT) Multi Unit Spectroscopic Explorer (MUSE) integral field observations of its host galaxy and a nebular-phase spectrum. This supernova, in the nearby galaxy ESO 467-G051 (z = 0.006), was discovered and followed-up by the all-sky automated survey for supernovae (ASAS-SN). We obtained well-sampled las cumbres network (LCOGTN) BVgri and Swift omega 2m1 omega 1ubv optical, near-UV/optical light curves, and several optical spectra in the early photospheric phases. The transient ASASSN-14jb exploded similar to 2 kpc above the star-forming disk of ESO 467-G051, an edge-on disk galaxy. The large projected distance from the disk of the supernova position and the non-detection of any H II region in a 1.4 kpc radius in projection are in conflict with the standard environment of core-collapse supernova progenitors and suggests the possible scenario that the progenitor received a kick in a binary interaction. We present analysis of the optical light curves and spectra, from which we derived a distance of 25 +/- 2 Mpc using state-of-the-art empirical methods for Type II SNe, physical properties of the SN explosion (Ni-56 mass, explosion energy, and ejected mass), and properties of the progenitor; namely the progenitor radius, mass, and metallicity. Our analysis yields a Ni-56 mass of 0.0210 +/- 0.0025 M-circle dot, an explosion energy of approximate to 0.25 x 10(51) ergs, and an ejected mass of approximate to 6 M-circle dot. We also constrained the progenitor radius to be R-* = 580 +/- 28 R-circle dot which seems to be consistent with the sub-Solar metallicity of 0.3 +/- 0.1 Z(circle dot) derived from the supernova Fe II lambda 5018 line. The nebular spectrum constrains strongly the progenitor mass to be in the range 10-12 M-circle dot. From the Spitzer data archive we detect ASASSN-14jb approximate to 330 days past explosion and we derived a total dust mass of 10(-4) M-circle dot from the 3.6 mu m and 4.5 mu m photometry. Using the FUV, NUV, BVgri,K-s, 3.6 mu m, and 4.5 mu m total magnitudes for the host galaxy, we fit stellar population synthesis models, which give an estimate of M-* approximate to 1 x 10(9) M-circle dot, an age of 3.2 Gyr, and a SFR approximate to 0.07 M-circle dot yr(-1). We also discuss the low oxygen abundance of the host galaxy derived from the MUSE data, having an average of 12 + log (O/H) = 8.27(-0.20)(+0.16) using the O3N2 diagnostic with strong line methods. We compared it with the supernova spectra, which is also consistent with a sub-Solar metallicity progenitor. Following recent observations of extraplanar H II regions in nearby edge-on galaxies, we derived the metallicity offset from the disk, being positive, but consistent with zero at 2 sigma, suggesting enrichment from disk outflows. We finally discuss the possible scenarios for the unusual environment for ASASSN-14jb and conclude that either the in-situ star formation or runaway scenario would imply a low-mass progenitor, agreeing with our estimate from the supernova nebular spectrum. Regardless of the true origin of ASASSN-14jb, we show that the detailed study of the environment roughly agree with the stronger constraints from the observation of the transient

A characterization of ASAS-SN core-collapse supernova environments with VLT+MUSE I. Sample selection, analysis of local environments, and correlations with light curve properties

The analysis of core-collapse supernova (CCSN) environments can provide important information on the life cycle of massive stars and constrain the progenitor properties of these powerful explosions. The MUSE instrument at the Very Large Telescope (VLT) enables detailed local environment constraints of the progenitors of large samples of CCSNe. Using a homogeneous SN sample from the All-Sky Automated Survey for Supernovae (ASAS-SN) survey, an untargeted and spectroscopically complete transient survey, has enabled us to perform a minimally biased statistical analysis of CCSN environments. Aims. We analyze 111 galaxies observed by MUSE that hosted 112 CCSNe-78 II, nine IIn, seven IIb, four Ic, seven Ib, three Ibn, two Ic-BL, one ambiguous Ibc, and one superluminous SN-detected or discovered by the ASAS-SN survey between 2014 and 2018. The majority of the galaxies were observed by the All-weather MUse Supernova Integral field Nearby Galaxies (AMUSING) survey. Here we analyze the immediate environment around the SN locations and compare the properties between the different CCSN types and their light curves. Methods. We used stellar population synthesis and spectral fitting techniques to derive physical parameters for all H ¯II regions detected within each galaxy, including the star formation rate (SFR), Hα equivalent width (EW), oxygen abundance, and extinction. Results. We found that stripped-envelope supernovae (SESNe) occur in environments with a higher median SFR, Hα EW, and oxygen abundances than SNe II and SNe IIn/Ibn. Most of the distributions have no statistically significant differences, except between oxygen abundance distributions of SESNe and SNe II, and between Hα EW distributions of SESNe and SNe II. The distributions of SNe II and IIn are very similar, indicating that these events explode in similar environments. For the SESNe, SNe Ic have higher median SFRs, Hα EWs, and oxygen abundances than SNe Ib. SNe IIb have environments with similar SFRs and Hα EWs to SNe Ib, and similar oxygen abundances to SNe Ic. We also show that the postmaximum decline rate, s, of SNe II correlates with the Hα EW, and that the luminosity and the Δ m15 parameter of SESNe correlate with the oxygen abundance, Hα EW, and SFR at their environments. This suggests a connection between the explosion mechanisms of these events to their environment properties

PS18kh: A New Tidal Disruption Event with a Non-axisymmetric Accretion Disk

We present the discovery of PS18kh, a tidal disruption event discovered at the center of SDSS J075654.53+341543.6 (d ≃ 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 ≃ 12,000 K to T ≃ 25,000 K and it peaked at a luminosity of L ≃ 8.8 × 10 erg s . PS18kh radiated E = (3.45 ± 0.22) × 10 erg over the period of observation, with (1.42 ± 0.20) × 10 erg being released during the rise to peak. Spectra of PS18kh show a changing, boxy/double-peaked Hα emission feature, which becomes more prominent over time. We use models of non-axisymmetric accretion disks to describe the profile of the Hα 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 ∼ 60r to an outer radius of r ∼ 1400r 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. 43 -1 50 50 in g out

SN 2015bn: A DETAILED MULTI-WAVELENGTH VIEW of A NEARBY SUPERLUMINOUS SUPERNOVA

We present observations of SN 2015bn (=PS15ae = CSS141223-113342+004332 = MLS150211-113342+004333), a Type I superluminous supernova (SLSN) at redshift z = 0.1136. As well as being one of the closest SLSNe I yet discovered, it is intrinsically brighter (${M}_{U}\approx -23.1$) and in a fainter galaxy (${M}_{B}\approx -16.0$) than other SLSNe at $z\sim 0.1$. We used this opportunity to collect the most extensive data set for any SLSN I to date, including densely sampled spectroscopy and photometry, from the UV to the NIR, spanning −50 to +250 days from optical maximum. SN 2015bn fades slowly, but exhibits surprising undulations in the light curve on a timescale of 30–50 days, especially in the UV. The spectrum shows extraordinarily slow evolution except for a rapid transformation between +7 and +20–30 days. No narrow emission lines from slow-moving material are observed at any phase. We derive physical properties including the bolometric luminosity, and find slow velocity evolution and non-monotonic temperature and radial evolution. A deep radio limit rules out a healthy off-axis gamma-ray burst, and places constraints on the pre-explosion mass loss. The data can be consistently explained by a $\gtrsim 10$ M ${}_{\odot }$ stripped progenitor exploding with $\sim {10}^{51}$ erg kinetic energy, forming a magnetar with a spin-down timescale of ~20 days (thus avoiding a gamma-ray burst) that reheats the ejecta and drives ionization fronts. The most likely alternative scenario—interaction with ~20 M ${}_{\odot }$ of dense, inhomogeneous circumstellar material—can be tested with continuing radio follow-up.S.J.S. acknowledges funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC Grant agreement no [291222] and STFC grants ST/I001123/1 and ST/L000709/1. This work is based (in part) on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile as part of PESSTO, (the Public ESO Spectroscopic Survey for Transient Objects Survey) ESO program 188.D-3003, 191.D-0935. The Pan-STARRS1 Surveys (PS1) have been made possible through contributions of the Institute for Astronomy, the University of Hawaii, the Pan-STARRS Project Office, the Max-Planck Society and its participating institutes, the Max Planck Institute for Astronomy, Heidelberg and the Max Planck Institute for Extraterrestrial Physics, Garching, The Johns Hopkins University, Durham University, the University of Edinburgh, Queen's University Belfast, the Harvard-Smithsonian Center for Astrophysics, the Las Cumbres Observatory Global Telescope Network Incorporated, the National Central University of Taiwan, the Space Telescope Science Institute, the National Aeronautics and Space Administration under Grant No. NNX08AR22G issued through the Planetary Science Division of the NASA Science Mission Directorate, the National Science Foundation under Grant No. AST-1238877, the University of Maryland, and Eotvos Lorand University (ELTE). Operation of the Pan-STARRS1 telescope is supported by the National Aeronautics and Space Administration under Grant No. NNX12AR65G and Grant No. NNX14AM74G issued through the NEO Observation Program. Based on observations made with the Nordic Optical Telescope, operated by the Nordic Optical Telescope Scientific Association at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofisica de Canarias. A.G.-Y. is supported by the EU/FP7 via ERC grant No. 307260, the Quantum universe I-Core programme by the Israeli Committee for Planning and Budgeting and the ISF; by Minerva and ISF grants; by the Weizmann-UK "making connections" programme; and by the Kimmel and YeS awards. B.D.M. is supported by NSF grant AST-1410950 and the Alfred P. Sloan Foundation. Support for L.G. is provided by the Ministry of Economy, Development, and Tourism's Millennium Science Initiative through grant IC120009 awarded to The Millennium Institute of Astrophysics (MAS), and CONICYT through FONDECYT grant 3140566. This work was partly supported by the European Union FP7 programme through ERC grant number 320360. K.M. acknowledges support from the STFC through an Ernest Rutherford Fellowship. A.M. acknowledges funding from CNRS. Development of ASAS-SN has been supported by NSF grant AST-0908816 and CCAPP at the Ohio State University. ASAS-SN is supported by NSF grant AST-1515927, the Center for Cosmology and AstroParticle Physics (CCAPP) at OSU, the Mt. Cuba Astronomical Foundation, George Skestos, and the Robert Martin Ayers Sciences Fund. B.S. 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. C.S.K. is supported by NSF grants AST-1515876 and AST-1515927. T.W.-S.H. is supported by the DOE Computational Science Graduate Fellowship, grant number DE-FG02-97ER25308. V.A.V. is supported by a NSF Graduate Research Fellowship. P.S.C. is grateful for support provided by the NSF through the Graduate Research Fellowship Program, grant DGE1144152. P.B. is supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. DGE1144152. D.A.H., C.M., and G.H. are supported by NSF grant 1313484.This is the author accepted manuscript. The final version is available from the Institute of Physics via http://dx.doi.org/10.3847/0004-637X/826/1/3

The Curious Case of ASASSN-20hx: A Slowly Evolving, UV- and X-Ray-Luminous, Ambiguous Nuclear Transient

We present observations of ASASSN-20hx, a nearby ambiguous nuclear transient (ANT) discovered in NGC 6297 by the All-Sky Automated Survey for Supernovae (ASAS-SN). We observed ASASSN-20hx from -30 to 275 days relative to the peak UV/optical emission using high-cadence, multiwavelength spectroscopy and photometry. From Transiting Exoplanet Survey Satellite data, we determine that the ANT began to brighten on 2020 June 22.8 with a linear rise in flux for at least the first week. ASASSN-20hx peaked in the UV/optical 30 days later on 2020 July 22.8 (MJD = 59052.8) at a bolometric luminosity of L = (3.15 ± 0.04) × 1043 erg s-1. The subsequent decline is slower than any TDE observed to date and consistent with many other ANTs. Compared to an archival X-ray detection, the X-ray luminosity of ASASSN-20hx increased by an order of magnitude to L x ∼1.5 × 1042 erg s-1 and then slowly declined over time. The X-ray emission is well fit by a power law with a photon index of "∼2.3-2.6. Both the optical and near-infrared spectra of ASASSN-20hx lack emission lines, unusual for any known class of nuclear transient. While ASASSN-20hx has some characteristics seen in both tidal disruption events and active galactic nuclei, it cannot be definitively classified with current data

ASASSN-18am/SN 2018gk: an overluminous Type IIb supernova from a massive progenitor

ASASSN-18am/SN 2018gk is a newly discovered member of the rare group of luminous, hydrogen-rich supernovae (SNe) with a peak absolute magnitude of MV ≈ -20 mag that is in between normal core-collapse SNe and superluminous SNe. These SNe show no prominent spectroscopic signatures of ejecta interacting with circumstellar material (CSM), and their powering mechanism is debated. ASASSN-18am declines extremely rapidly for a Type II SN, with a photospheric-phase decline rate of ∼6.0 mag (100 d)-1. Owing to the weakening of H I and the appearance of He I in its later phases, ASASSN-18am is spectroscopically a Type IIb SN with a partially stripped envelope. However, its photometric and spectroscopic evolution shows significant differences from typical SNe IIb. Using a radiative diffusion model, we find that the light curve requires a high synthesized 56Ni mass MNi ∼0.4 M⊙ and ejecta with high kinetic energy Ekin = (7-10) x 1051 erg. Introducing a magnetar central engine still requires MNi ∼0.3 M⊙ and E-kin = 3 x 1051 erg. The high 56Ni mass is consistent with strong iron-group nebular lines in its spectra, which are also similar to several SNe Ic-BL with high 56Ni yields. The earliest spectrum shows 'flash ionization' features, from which we estimate a mass-loss rate of Ṁ ≈ 2 x 10-4 M⊙ yr-1. This wind density is too low to power the luminous light curve by ejecta-CSM interaction. We measure expansion velocities as high as 17 000 km s-1 for Hα, which is remarkably high compared to other SNe II. We estimate an oxygen core mass of 1.8-3.4 M⊙ using the [O I] luminosity measured from a nebular-phase spectrum, implying a progenitor with a zero-age main-sequence mass of 19-26 M⊙.</p

Space Telescope and Optical Reverberation Mapping Project. V. Optical Spectroscopic Campaign and Emission-line Analysis for NGC 5548

We present the results of an optical spectroscopic monitoring program targeting NGC 5548 as part of a larger multiwavelength reverberation mapping campaign. The campaign spanned 6 months and achieved an almost daily cadence with observations from five ground-based telescopes. The Hβ and He ii λ4686 broad emission-line light curves lag that of the 5100 +-optical continuum by 4.17+0.36-0.36 and 0.79+0.35-0.34 days, respectively. The Hβ lag relative to the 1158 ultraviolet continuum light curve measured by the Hubble Space Telescope is ∼50% longer than that measured against the optical continuum, and the lag difference is consistent with the observed lag between the optical and ultraviolet continua. This suggests that the characteristic radius of the broad-line region is ∼50% larger than the value inferred from optical data alone. We also measured velocity-resolved emission-line lags for Hβ and found a complex velocity-lag structure with shorter lags in the line wings, indicative of a broad-line region dominated by Keplerian motion. The responses of both the Hβ and He ii emission lines to the driving continuum changed significantly halfway through the campaign, a phenomenon also observed for C iv, Lyα, He ii(+O iii]), and Si iv(+O iv]) during the same monitoring period. Finally, given the optical luminosity of NGC 5548 during our campaign, the measured Hβ lag is a factor of five shorter than the expected value implied by the R BLR-L AGN relation based on the past behavior of NGC 5548

ASASSN-15hy: An Underluminous, Red 03fg-like Type Ia Supernova

We present photometric and spectroscopic observations of the 03fg-like Type Ia supernova (SN Ia) ASASSN-15hy from the ultraviolet (UV) to the near-infrared (NIR). ASASSN-15hy shares many of the hallmark characteristics of 03fg-like SNe Ia, previously referred to as "super-Chandrasekhar" SNe Ia. It is bright in the UV and NIR, lacks a clear i-band secondary maximum, shows a strong and persistent C ii feature, and has a low Si ii lambda 6355 velocity. However, some of its properties are also extreme among the subgroup. ASASSN-15hy is underluminous (M (B,peak) = -19.14(-0.16)(+0.11) mag), red ((B-V)(Bmax)= 0.18(-0.03)(+0.01) mag), yet slowly declining (Delta m (15)(B) = 0.72 +/- 0.04 mag). It has the most delayed onset of the i-band maximum of any 03fg-like SN. ASASSN-15hy lacks the prominent H-band break emission feature that is typically present during the first month past maximum in normal SNe Ia. Such events may be a potential problem for high-redshift SN Ia cosmology. ASASSN-15hy may be explained in the context of an explosion of a degenerate core inside a nondegenerate envelope. The explosion impacting the nondegenerate envelope with a large mass provides additional luminosity and low ejecta velocities. An initial deflagration burning phase is critical in reproducing the low Ni-56 mass and luminosity, while the large core mass is essential in providing the large diffusion timescales required to produce the broad light curves. The model consists of a rapidly rotating 1.47 M-circle dot degenerate core and a 0.8 M-circle dot nondegenerate envelope. This "deflagration core-degenerate" scenario may result from the merger between a white dwarf and the degenerate core of an asymptotic giant branch star

Space Telescope and Optical Reverberation Mapping Project. VII. Understanding the Ultraviolet Anomaly in NGC 5548 with X-Ray Spectroscopy

During the Space Telescope and Optical Reverberation Mapping Project observations of NGC 5548, the continuum and emission-line variability became decorrelated during the second half of the six-month-long observing campaign. Here we present Swift and Chandra X-ray spectra of NGC 5548 obtained as part of the campaign. The Swift spectra show that excess flux (relative to a power-law continuum) in the soft X-ray band appears before the start of the anomalous emission-line behavior, peaks during the period of the anomaly, and then declines. This is a model-independent result suggesting that the soft excess is related to the anomaly. We divide the Swift data into on- and off-anomaly spectra to characterize the soft excess via spectral fitting. The cause of the spectral differences is likely due to a change in the intrinsic spectrum rather than to variable obscuration or partial covering. The Chandra spectra have lower signal-to-noise ratios, but are consistent with the Swift data. Our preferred model of the soft excess is emission from an optically thick, warm Comptonizing corona, the effective optical depth of which increases during the anomaly. This model simultaneously explains all three observations: the UV emission-line flux decrease, the soft-excess increase, and the emission-line anomaly