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Emission-line variability studies have the potential to provide key information about the structures of the near-nuclear outflow regions of AGN. Here we present a VLT/Xshooter spectrum of the nucleus of the ULIRG F01004-2237 that was taken in August 2018, about 8 yr after a major flare in its integrated optical emission. Compared with our WHT/ISIS spectrum from September 2015, the broad, red wings of the emission lines most closely associated with the flaring event, including HeII(4686), NIII(4100,4640) and HeI(5876), have substantially declined in flux. In contrast, the broad, blue wings that dominate the [OIII], [NeIII], [NeV] and [OI] forbidden lines have increased in flux by a factor 1.4 - 4.4 (depending on the line). Moreover, the [FeVII](6087) line is detected in the new spectrum for the first time. We interpret these results in terms of a light echo from the outflowing warm gas: the direct emission from the flaring event is continuing to fade, but due to light travel time effects we are only now observing the impact of the flare on the emission from the extended outflow region. Unless the outflow is confined to a small range of angles close to our line of sight, these observations imply that the outflow must be relatively compact (r, 12 pages, 8 figures, accepted for publication in MNRAS

The detection by Rest et al. (2012) of light echoes from $\eta$ Carinae has provided important new observational constraints on the nature of its 1840's era giant eruption. Spectra of the echoes suggest a relatively cool spectral temperature of about 5500K, lower than the lower limit of about 7000K suggested in the optically thick wind outflow analysis of Davidson (1987). This has lead to a debate about the viability of this steady wind model relative to alternative, explosive scenarios. Here we present an updated analysis of the wind outflow model using newer low-temperature opacity tabulations and accounting for the stronger mass loss implied by the $>$10 Msun mass now inferred for the Homunculus. A major conclusion is that, because of the sharp drop in opacity due to free electron recombination for $T, Comment: 7 pages, 6 figures, submitted to MNRAS

We consider a model for the low-luminosity gamma-ray burst GRB 060218 that plausibly accounts for multiwavelength observations to day 20. The model components are: (1) a long-lived ($t_j \sim 3000$ s) central engine and accompanying low-luminosity ($L_j \sim 10^{47}$ erg s$^{-1}$), semirelativistic ($\gamma \sim 10$) jet; (2) a low-mass ($ \sim 10^{-2} M_\odot$) envelope surrounding the progenitor star; and (3) a modest amount of dust ($A_V \sim 0.1$ mag) in the interstellar environment. Blackbody emission from the transparency radius in a low-power jet outflow can fit the prompt thermal X-ray emission, and the nonthermal X-rays and gamma-rays may be produced via Compton scattering of thermal photons from hot leptons in the jet interior or the external shocks. The later mildly relativistic phase of this outflow can produce the radio emission via synchrotron radiation from the forward shock. Meanwhile, interaction of the associated SN 2006aj with a circumstellar envelope extending to $\sim10^{13}$ cm can explain the early optical emission. The X-ray afterglow can be interpreted as a light echo of the prompt emission from dust at $\sim 30$ pc. Our model is a plausible alternative to that of Nakar, who recently proposed shock breakout of a jet smothered by an extended envelope as the source of prompt emission. Both our results and Nakar's suggest that bursts such as GRB 060218 may originate from unusual progenitors with extended circumstellar envelopes, and that a jet is necessary to decouple the prompt emission from the supernova., Comment: 28 pages, 8 figures, submitted to MNRAS

SN 2006gy was the most luminous SN ever observed at the time of its discovery and the first of the newly defined class of superluminous supernovae (SLSNe). The extraordinary energetics of SN 2006gy and all SLSNe (>10^51 erg) require either atypically large explosion energies (e.g., pair-instability explosion) or the efficient conversion of kinetic into radiative energy (e.g., shock interaction). The mass-loss characteristics can therefore offer important clues regarding the progenitor system. For the case of SN 2006gy, both a scattered and thermal light echo from circumstellar material (CSM) have been reported at later epochs (day ~800), ruling out the likelihood of a pair-instability event and leading to constraints on the characteristics of the CSM. Owing to the proximity of the SN to the bright host-galaxy nucleus, continued monitoring of the light echo has not been trivial, requiring the high resolution offered by the Hubble Space Telescope (HST) or ground-based adaptive optics (AO). Here we report detections of SN 2006gy using HST and Keck AO at ~3000 days post-explosion and consider the emission mechanism for the very late-time light curve. While the optical light curve and optical spectral energy distribution are consistent with a continued scattered-light echo, a thermal echo is insufficient to power the K'-band emission by day 3000. Instead, we present evidence for late-time infrared emission from dust that is radiatively heated by CSM interaction within an extremely dense dust shell, and we consider the implications on the CSM characteristics and progenitor system., Submitted to MNRAS, 14 pages

We present the results of a search for nova shells around 101 cataclysmic variables (CVs), using Halpha images taken with the 4.2-m William Herschel Telescope (WHT) and the 2.5-m Isaac Newton Telescope Photometric Halpha Survey of the Northern Galactic Plane (IPHAS). Both telescopes are located on La Palma. We concentrated our WHT search on nova-like variables, whilst our IPHAS search covered all CVs in the IPHAS footprint. We found one shell out of the 24 nova-like variables we examined. The newly discovered shell is around V1315 Aql and has a radius of approx.2.5 arcmin, indicative of a nova eruption approximately 120 years ago. This result is consistent with the idea that the high mass-transfer rate exhibited by nova-like variables is due to enhanced irradiation of the secondary by the hot white dwarf following a recent nova eruption. The implications of our observations for the lifetime of the nova-like variable phase are discussed. We also examined 4 asynchronous polars, but found no new shells around any of them, so we are unable to confirm that a recent nova eruption is the cause of the asynchronicity in the white dwarf spin. We find tentative evidence of a faint shell around the dwarf nova V1363 Cyg. In addition, we find evidence for a light echo around the nova V2275 Cyg, which erupted in 2001, indicative of an earlier nova eruption approx.300 years ago, making V2275 Cyg a possible recurrent nova., 14 pages, 50 figures, 3 Tables

In this letter we present an optical spectrum of SN 2011fe taken 1034 d after the explosion, several hundred days later than any other spectrum of a Type Ia supernova (disregarding light-echo spectra and local-group remnants). The spectrum is still dominated by broad emission features, with no trace of a light echo or interaction of the supernova ejecta with surrounding interstellar material. Comparing this extremely late spectrum to an earlier one taken 331 d after the explosion, we find that the most prominent feature at 331 d - [Fe III] emission around 4700 A - has entirely faded away, suggesting a significant change in the ionisation state. Instead, [Fe II] lines are probably responsible for most of the emission at 1034 d. An emission feature at 6300-6400 A has newly developed at 1034 d, which we tentatively identify with Fe I {\lambda}6359, [Fe I] {\lambda}{\lambda}6231, 6394 or [O I] {\lambda}{\lambda}6300, 6364. Interestingly, the features in the 1034-d spectrum seem to be collectively redshifted, a phenomenon that we currently have no convincing explanation for. We discuss the implications of our findings for explosion models, but conclude that sophisticated spectral modelling is required for any firm statement., Comment: 6 pages, 3 figures, MNRAS Letters accepted

We examine a binary merger model for the formation of the mysterious triple-ring nebula surrounding Supernova 1987A, which still has not been convincingly explained in detailed hydrodynamical calculations. During the merger of 15 and 5 M⊙ binary systems, mass is ejected primarily at mid-latitudes for a sufficiently evolved primary, as demonstrated by Morris & Podsiadlowski. This material is swept up by the fast wind of the central star during its post-merger blue supergiant phase, leading to a density contrast of ∼150 in the outer rings at the time of the supernova. The equatorial ring probably formed later when the star contracted to become a blue supergiant. The asymmetry between the northern and southern outer rings can be explained by a 10 per cent asymmetry during the merger, perhaps due to a pulsational instability in the common envelope. We present a parameter study from which we determine a mass-loss rate in the blue supergiant wind in the range 1.5–3 × 10−7 M⊙ yr−1 in agreement with previous estimates. The morphology of the best model is consistent with the well-known Hubble Space Telescope image at better than 5 per cent and is also in broad agreement with light-echo observations. The circumstellar environment on larger scales (up to 3 pc) is also investigated. We conclude with a brief discussion of the bipolar nebulae surrounding the Galactic stars, Sheridan 25, HD 168625 and η Carinae.

We report the discovery of an unusual object near the spiral galaxy IC 2497, discovered by visual inspection of the Sloan Digital Sky Survey (SDSS) as part of the Galaxy Zoo project. The object, known as Hanny's Voorwerp, is bright in the SDSS g band due to unusually strong OIII 4959-5007 emission lines. We present the results of the first targeted observations of the object in the optical, UV and X-ray, which show that the object contains highly ionized gas. Although the line ratios are similar to extended emission-line regions near luminous AGN, the source of this ionization is not apparent. The emission-line properties, and lack of x-ray emission from IC 2497, suggest either a highly obscured AGN with a novel geometry arranged to allow photoionization of the object but not the galaxy's own circumnuclear gas, or, as we argue, the first detection of a quasar light echo. In this case, either the luminosity of the central source has decreased dramatically or else the obscuration in the system has increased within 10^5 years. This object may thus represent the first direct probe of quasar history on these timescales., Comment: 13 pages, 10 figures, Accepted by MNRAS

We report on a Suzaku observation of the nearby Seyfert 2 galaxy NGC 4388. The overall spectrum is well described by the combination of strongly absorbed power-law plus thin thermal plasma emission, as well as previous observations. Thanks to well-calibrated instruments and a low background level, Suzaku measured the broad-band X-ray spectra more precisely than ever, and enabled us to probe the physical structure of the nuclear region. In addition, a time variability having a factor of 1.5 with a half-day time scale was found up to above 10 keV. The center energy and the Kˇ/K ratio of the narrow Fe-K line, together with less time variability, indicate that the fluorescent material is far from the black hole. A spectral hump around the 30–40 keV band, together with a Compton shoulder of the Fe-K line, requires a significant contribution of the reflection component. This is also consistent with a smaller amplitude of time variability in a higher energy band. A large contribution of the reflection component was not observed by the CGRO/OSSE and BeppoSAX, suggesting that NGC 4388 had been in a higher flux state, and that Suzaku observed its light echo. A He-like Fe-K absorption line was also detected, indicating the existence of highly ionized plasma.