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Classical novae are shock-powered multi-wavelength transients triggered by a thermonuclear runaway on an accreting white dwarf. V1674 Her is the fastest nova ever recorded (time to declined by two magnitudes is t_2=1.1 d) that challenges our understanding of shock formation in novae. We investigate the physical mechanisms behind nova emission from GeV gamma-rays to cm-band radio using coordinated Fermi-LAT, NuSTAR, Swift and VLA observations supported by optical photometry. Fermi-LAT detected short-lived (18 h) 0.1-100 GeV emission from V1674 Her that appeared 6 h after the eruption began; this was at a level of (1.6 +/- 0.4)x10^-6 photons cm^-2 s^-1. Eleven days later, simultaneous NuSTAR and Swift X-ray observations revealed optically thin thermal plasma shock-heated to kT_shock = 4 keV. The lack of a detectable 6.7 keV Fe K_alpha emission suggests super-solar CNO abundances. The radio emission from V1674 Her was consistent with thermal emission at early times and synchrotron at late times. The radio spectrum steeply rising with frequency may be a result of either free-free absorption of synchrotron and thermal emission by unshocked outer regions of the nova shell or the Razin-Tsytovich effect attenuating synchrotron emission in dense plasma. The development of the shock inside the ejecta is unaffected by the extraordinarily rapid evolution and the intermediate polar host of this nova., 20 pages, 9 figures, 3 tables. Accepted to MNRAS

The Andromeda Galaxy recurrent nova M31N 2008-12a had been observed in eruption ten times, including yearly eruptions from 2008-2014. With a measured recurrence period of $P_\mathrm{rec}=351\pm13$ days (we believe the true value to be half of this) and a white dwarf very close to the Chandrasekhar limit, M31N 2008-12a has become the leading pre-explosion supernova type Ia progenitor candidate. Following multi-wavelength follow-up observations of the 2013 and 2014 eruptions, we initiated a campaign to ensure early detection of the predicted 2015 eruption, which triggered ambitious ground and space-based follow-up programs. In this paper we present the 2015 detection; visible to near-infrared photometry and visible spectroscopy; and ultraviolet and X-ray observations from the Swift observatory. The LCOGT 2m (Hawaii) discovered the 2015 eruption, estimated to have commenced at Aug. $28.28\pm0.12$ UT. The 2013-2015 eruptions are remarkably similar at all wavelengths. New early spectroscopic observations reveal short-lived emission from material with velocities $\sim13000$ km s$^{-1}$, possibly collimated outflows. Photometric and spectroscopic observations of the eruption provide strong evidence supporting a red giant donor. An apparently stochastic variability during the early super-soft X-ray phase was comparable in amplitude and duration to past eruptions, but the 2013 and 2015 eruptions show evidence of a brief flux dip during this phase. The multi-eruption Swift/XRT spectra show tentative evidence of high-ionization emission lines above a high-temperature continuum. Following Henze et al. (2015a), the updated recurrence period based on all known eruptions is $P_\mathrm{rec}=174\pm10$ d, and we expect the next eruption of M31N 2008-12a to occur around mid-Sep. 2016., 46 pages, 19 figures, 14 tables, accepted for publication in ApJ (accepted version, minor changes made during the refereeing process)

PTF09dav is a peculiar subluminous type Ia supernova (SN) discovered by the Palomar Transient Factory (PTF). Spectroscopically, it appears superficially similar to the class of subluminous SN1991bg-like SNe, but it has several unusual features which make it stand out from this population. Its peak luminosity is fainter than any previously discovered SN1991bg-like SN Ia (M_B -15.5), but without the unusually red optical colors expected if the faint luminosity were due to extinction. The photospheric optical spectra have very unusual strong lines of Sc II and Mg I, with possible Sr II, together with stronger than average Ti II and low velocities of ~6000 km/s. The host galaxy of PTF09dav is ambiguous. The SN lies either on the extreme outskirts (~41kpc) of a spiral galaxy, or in an very faint (M_R>-12.8) dwarf galaxy, unlike other 1991bg-like SNe which are invariably associated with massive, old stellar populations. PTF09dav is also an outlier on the light-curve-width--luminosity and color--luminosity relations derived for other sub-luminous SNe Ia. The inferred 56Ni mass is small (0.019+/-0.003Msun), as is the estimated ejecta mass of 0.36Msun. Taken together, these properties make PTF09dav a remarkable event. We discuss various physical models that could explain PTF09dav. Helium shell detonation or deflagration on the surface of a CO white-dwarf can explain some of the features of PTF09dav, including the presence of Sc and the low photospheric velocities, but the observed Si and Mg are not predicted to be very abundant in these models. We conclude that no single model is currently capable of explaining all of the observed signatures of PTF09dav., Accepted for publication in ApJ

We report on follow-up observations of the GRB 060927 using the ROTSE-IIIa telescope and a suite of larger aperture ground-based telescopes. An optical afterglow was detected 20 s after the burst, the earliest rest-frame detection of optical emission from any GRB. Spectroscopy performed with the VLT about 13 hours after the trigger shows a continuum break at lambda ~ 8070 A produced by neutral hydrogen absorption at z~5.6. We also detect an absorption line at 8158 A which we interpret as SiII at z=5.467. Hence, GRB 060927 is the second most distant GRB with a spectroscopically measured redshift. The shape of the red wing of the spectral break can be fitted by a damped Lyalpha profile with a column density with log(N_HI/cm^-2) ~ 22.5. We discuss the implications of this work for the use of GRBs as probes of the end of the dark ages and draw three main conclusions: i) GRB afterglows originating from z>6 should be relatively easy to detect from the ground, but rapid NIR monitoring is necessary to ensure that they are found; ii) The presence of large HI column densities in some GRBs host galaxies at z>5 makes the use of GRBs to probe the reionization epoch via spectroscopy of the red damping wing challenging; iii) GRBs appear crucial to locate typical star-forming galaxies at z>5 and therefore the type of galaxies responsible for the reionization of the universe., 15 pages, 6 figures, 2 tables, accepted for publication in ApJ, uses emulateapj

Interaction of supernova (SN) ejecta with the optically thick circumstellar medium (CSM) of a progenitor star can result in a bright, long-lived shock breakout event. Candidates for such SNe include Type IIn and superluminous SNe. If some of these SNe are powered by interaction, then there should be a relation between their peak luminosity, bolometric light-curve rise time, and shock-breakout velocity. Given that the shock velocity during shock breakout is not measured, we expect a correlation, with a significant spread, between the rise time and the peak luminosity of these SNe. Here, we present a sample of 15 SNe IIn for which we have good constraints on their rise time and peak luminosity from observations obtained using the Palomar Transient Factory. We report on a possible correlation between the R-band rise time and peak luminosity of these SNe, with a false-alarm probability of 3%. Assuming that these SNe are powered by interaction, combining these observables and theory allows us to deduce lower limits on the shock-breakout velocity. The lower limits on the shock velocity we find are consistent with what is expected for SNe (i.e., ~10^4 km/s). This supports the suggestion that the early-time light curves of SNe IIn are caused by shock breakout in a dense CSM. We note that such a correlation can arise from other physical mechanisms. Performing such a test on other classes of SNe (e.g., superluminous SNe) can be used to rule out the interaction model for a class of events., Accepted to ApJ, 6 pages

Type Ia supernovae are key tools for measuring distances on a cosmic scale. They are generally thought to be the thermonuclear explosion of an accreting white dwarf in a close binary system. The nature of the mass donor is still uncertain. In the single-degenerate model it is a main-sequence star or an evolved star, whereas in the double-degenerate model it is another white dwarf. We show that the velocity structure of absorbing material along the line of sight to 35 type Ia supernovae tends to be blueshifted. These structures are likely signatures of gas outflows from the supernova progenitor systems. Thus many type Ia supernovae in nearby spiral galaxies may originate in single-degenerate systems., Accepted for publication in Science 5 July 2011

Supernovae (SNe) are stellar explosions driven by gravitational or thermonuclear energy, observed as electromagnetic radiation emitted over weeks or more. In all known SNe, this radiation comes from internal energy deposited in the outflowing ejecta by either radioactive decay of freshly-synthesized elements (typically 56Ni), stored heat deposited by the explosion shock in the envelope of a supergiant star, or interaction between the SN debris and slowly-moving, hydrogen-rich circumstellar material. Here we report on a new class of luminous SNe whose observed properties cannot be explained by any of these known processes. These include four new SNe we have discovered, and two previously unexplained events (SN 2005ap; SCP 06F6) that we can now identify as members. These SNe are all ~10 times brighter than SNe Ia, do not show any trace of hydrogen, emit significant ultra-violet (UV) flux for extended periods of time, and have late-time decay rates which are inconsistent with radioactivity. Our data require that the observed radiation is emitted by hydrogen-free material distributed over a large radius (~10^15 cm) and expanding at high velocities (>10^4 km s^-1). These long-lived, UV-luminous events can be observed out to redshifts z>4 and offer an excellent opportunity to study star formation in, and the interstellar medium of, primitive distant galaxies., Accepted to Nature. Press embargoed until 2011 June 8, 18:00 UT

Stars with initial masses 10 M_{solar} < M_{initial} < 100 M_{solar} fuse progressively heavier elements in their centres, up to inert iron. The core then gravitationally collapses to a neutron star or a black hole, leading to an explosion -- an iron-core-collapse supernova (SN). In contrast, extremely massive stars (M_{initial} > 140 M_{solar}), if such exist, have oxygen cores which exceed M_{core} = 50 M_{solar}. There, high temperatures are reached at relatively low densities. Conversion of energetic, pressure-supporting photons into electron-positron pairs occurs prior to oxygen ignition, and leads to a violent contraction that triggers a catastrophic nuclear explosion. Tremendous energies (>~ 10^{52} erg) are released, completely unbinding the star in a pair-instability SN (PISN), with no compact remnant. Transitional objects with 100 M_{solar} < M_{initial} < 140 M_{solar}, which end up as iron-core-collapse supernovae following violent mass ejections, perhaps due to short instances of the pair instability, may have been identified. However, genuine PISNe, perhaps common in the early Universe, have not been observed to date. Here, we present our discovery of SN 2007bi, a luminous, slowly evolving supernova located within a dwarf galaxy (~1% the size of the Milky Way). We measure the exploding core mass to be likely ~100 M_{solar}, in which case theory unambiguously predicts a PISN outcome. We show that >3 M_{solar} of radioactive 56Ni were synthesized, and that our observations are well fit by PISN models. A PISN explosion in the local Universe indicates that nearby dwarf galaxies probably host extremely massive stars, above the apparent Galactic limit, perhaps resulting from star formation processes similar to those that created the first stars in the Universe., Accepted version of the paper appearing in Nature, 462, 624 (2009), including all supplementary information

WOS: 000274848100054, We present a multi-wavelength study of GRB 081008, at redshift 1.967, by Swift, ROTSE-III, and Gamma-Ray Burst Optical/NearInfrared Detector. Compared to other Swift GRBs, GRB 081008 has a typical gamma-ray isotropic equivalent energy output (similar to 10(53) erg) during the prompt phase, and displayed two temporally separated clusters of pulses. The early X-ray emission seen by the Swift X-Ray Telescope was dominated by the softening tail of the prompt emission, producing multiple flares during and after the Swift Burst Alert Telescope detections. Optical observations that started shortly after the first active phase of gamma-ray emission showed two consecutive peaks. We interpret the first optical peak as the onset of the afterglow associated with the early burst activities. A second optical peak, coincident with the later gamma-ray pulses, imposes a small modification to the otherwise smooth light curve and thus suggests a minimal contribution from a probable internal component. We suggest the early optical variability may be from continuous energy injection into the forward shock front by later shells producing the second epoch of burst activities. These early observations thus provide a potential probe for the transition from the prompt phase to the afterglow phase. The later light curve of GRB 081008 displays a smooth steepening in all optical bands and X-ray. The temporal break is consistent with being achromatic at the observed wavelengths. Our broad energy coverage shortly after the break constrains a spectral break within optical. However, the evolution of the break frequency is not observed. We discuss the plausible interpretations of this behavior., NASA [NNX-08AN25G, NAS5-00136, NNX-08AV63G, PHY-0801007]; STFC; DFG; Australian Research Council; University of New South Wales; University of Texas; University of Michigan; Leibniz-Prize [HA 1850/28-1]; Science and Technology Facilities Council [ST/H001972/1], F.Y. gratefully acknowledges useful discussions with S. B. Pandey. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester. F. Y. is supported by the NASA Swift Guest Investigator grants NNX-08AN25G. J.L.R. acknowledges support for this work from NASA contract NAS5-00136. R. W. and P.T.O. gratefully acknowledge funding from the STFC for Swift at the University of Leicester. T. K. acknowledges support by the DFG cluster of excellence "Origin and Structure of the Universe." ROTSE-III has been supported by NASA grant NNX-08AV63G, NSF grant PHY-0801007, the Australian Research Council, the University of New South Wales, the University of Texas, and the University of Michigan. Special thanks to the H. E. S. S. staff, especially Toni Hanke. Part of the funding for GROND (both hardware as well as personnel) was generously granted from the Leibniz-Prize (DFG grant HA 1850/28-1) to Professor G. Hasinger (IPP).

PILOT (the Pathfinder for an International Large Optical Telescope) is a proposed 2.5 m optical/infrared telescope to be located at Dome C on the Antarctic plateau. Conditions at Dome C are known to be exceptional for astronomy. The seeing (above ~30 m height), coherence time, and isoplanatic angle are all twice s good as at typical mid-latitude sites, while the water-vapour column, and the atmosphere and telescope thermal emission are all an order of magnitude better. These conditions enable a unique scientific capability for PILOT, which is addressed in this series of papers. The current paper presents an overview of the optical and instrumentation suite for PILO and its expected performance, a summary of the key science goals and observational approach for the facility, a discussion of the synergies between the science goals for PILOT and other telescopes, and a discussion of the future of Antarctic astronomy. Paper II and Paper III present details of the science projects divided, respectively, between the distant Universe (i.e., studies of first light, and the assembly and evolution of structure) and the nearby Universe (i.e., studies of Local Group galaxies, the Milky Way, and the Solar System)., 19 pages, 8 figures, accepted for publication in PASA

We present lightcurves of the afterglow of GRB050502A, including very early data at t-t_{GRB} < 60s. The lightcurve is composed of unfiltered ROTSE-IIIb optical observations from 44s to 6h post-burst, R-band MDM observations from 1.6 to 8.4h post-burst, and PAIRITEL J H K_s observations from 0.6 to 2.6h post-burst. The optical lightcurve is fit by a broken power law, where t^{alpha} steepens from alpha = -1.13 +- 0.02 to alpha = -1.44 +- 0.02 at \~5700s. This steepening is consistent with the evolution expected for the passage of the cooling frequency nu_c through the optical band. Even in our earliest observation at 44s post-burst, there is no evidence that the optical flux is brighter than a backward extrapolation of the later power law would suggest. The observed decay indices and spectral index are consistent with either an ISM or a Wind fireball model, but slightly favor the ISM interpretation. The expected spectral index in the ISM interpretation is consistent within 1 sigma with the observed spectral index beta = -0.8 +- 0.1; the Wind interpretation would imply a slightly (~2 sigma) shallower spectral index than observed. A small amount of dust extinction at the source redshift could steepen an intrinsic spectrum sufficiently to account for the observed value of beta. In this picture, the early optical decay, with the peak at or below 4.7e14 Hz at 44s, requires very small electron and magnetic energy partitions from the fireball., Comment: 22 pages, including 3 tables and 1 figure, Accepted by ApJ

Aims. We study PTF11mnb, a He-poor supernova (SN) whose light curves resemble those of SN 2005bf, a peculiar double-peaked stripped-envelope (SE) SN, until the declining phase after the main peak. We investigate the mechanism powering its light curve and the nature of its progenitor star. Methods. Optical photometry and spectroscopy of PTF11mnb are presented. We compared light curves, colors and spectral properties to those of SN 2005bf and normal SE SNe. We built a bolometric light curve and modeled this light curve with the SuperNova Explosion Code (SNEC) hydrodynamical code explosion of a MESA progenitor star and semi-analytic models. Results. The light curve of PTF11mnb turns out to be similar to that of SN 2005bf until ~50 d when the main (secondary) peaks occur at -18.5 mag. The early peak occurs at ~20 d and is about 1.0 mag fainter. After the main peak, the decline rate of PTF11mnb is remarkably slower than what was observed in SN 2005bf, and it traces well the 56Co decay rate. The spectra of PTF11mnb reveal a SN Ic and have no traces of He unlike in the case of SN Ib 2005bf, although they have velocities comparable to those of SN 2005bf. The whole evolution of the bolometric light curve is well reproduced by the explosion of a massive (Mej = 7.8 MȮ), He-poor star characterized by a double-peaked 56Ni distribution, a total 56Ni mass of 0.59 MȮ, and an explosion energy of 2.2 × 1051 erg. Alternatively, a normal SN Ib/c explosion (M(56Ni) = 0.11 MȮ, EK = 0.2 × 1051 erg, Mej = 1 MȮ) can power the first peak while a magnetar, with a magnetic field characterized by B = 5.0 × 1014 G, and a rotation period of P = 18.1 ms, provides energy for the main peak. The early g-band light curve can be fit with a shock-breakout cooling tail or an extended envelope model from which a radius of at least 30 RȮ is obtained. Conclusions. We presented a scenario where PTF11mnb was the explosion of a massive, He-poor star, characterized by a double-peaked 56Ni distribution. In this case, the ejecta mass and the absence of He imply a large ZAMS mass (~85 MȮ) for the progenitor, which most likely was a Wolf-Rayet star, surrounded by an extended envelope formed either by a pre-SN eruption or due to a binary configuration. Alternatively, PTF11mnb could be powered by a SE SN with a less massive progenitor during the first peak and by a magnetar afterward.