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R. Dekany

Publications and source records attributed to R. Dekany.

At least 37 records · Page 2Linked to original sources
arXiv2021

The luminous and rapidly evolving SN 2018bcc: Clues toward the origin of Type Ibn SNe from the Zwicky Transient Facility

Supernovae (SNe) Type Ibn are rapidly evolving and bright (M$_\text{R,peak}$ $\sim-19$) transients interacting with He-rich circumstellar material (CSM). SN 2018bcc, detected by the ZTF shortly after explosion, provides the best constraints on the shape of the rising light curve (LC) of a fast Type Ibn. Aims: We used the high-quality data set of SN 2018bcc to study observational signatures of the class. Additionally, the powering mechanism of SN 2018bcc offers insights into the debated progenitor connection of Type Ibn SNe. Methods: We compared well-constrained LC properties obtained from empirical models are compared with the literature. We fit the pseudo-bolometric LC with semi-analytical models powered by radioactive decay and CSM interaction. Finally, we modeled the line profiles and emissivity of the prominent He I lines, in order to study the formation of P-Cygni profiles and estimate CSM properties. Results: SN 2018bcc had a rise time to peak of $5.6^{+0.2}_{-0.1}$ days in the restframe with a rising shape power-law index close to 2, and seems to be a typical rapidly evolving Type Ibn SN. The spectrum lacked signatures of SN-like ejecta and was dominated by over 15 He emission features at 20 days past peak, alongside Ca and Mg, all with V$_{\text{FWHM}} \sim 2000~\text{km}~\text{s}^{-1}$. The luminous and rapidly evolving LC could be powered by CSM interaction but not by the decay of radioactive $^{56}$Ni. Modeling of the He I lines indicated a dense and optically thick CSM that can explain the P-Cygni profiles. Conclusions: Like other rapidly-evolving Type Ibn SNe, SN 2018bcc is a luminous transient with a rapid rise to peak powered by shock interaction inside a dense and He-rich CSM. Its spectra do not support the existence of two Type Ibn spectral classes. We also note the remarkable observational match to pulsational pair instability (PPI) SN models.

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arXiv2020

ZTF Early Observations of Type Ia Supernovae II: First Light, the Initial Rise, and Time to Reach Maximum Brightness

While it is clear that Type Ia supernovae (SNe) are the result of thermonuclear explosions in C/O white dwarfs (WDs), a great deal remains uncertain about the binary companion that facilitates the explosive disruption of the WD. Here, we present a comprehensive analysis of a large, unique data set of 127 SNe$\,$Ia with exquisite coverage by the Zwicky Transient Facility (ZTF). High-cadence (six observations per night) ZTF observations allow us to measure the SN rise time and examine its initial evolution. We develop a Bayesian framework to model the early rise as a power law in time, which enables the inclusion of priors in our model. For a volume-limited subset of normal SNe$\,$Ia, we find that the mean power-law index is consistent with 2 in the $r_\mathrm{ZTF}$-band ($α_r = 2.01\pm0.02$), as expected in the expanding fireball model. There are, however, individual SNe that are clearly inconsistent with $α_r=2$. We estimate a mean rise time of 18.9$\,$d (with a range extending from $\sim$15 to 22$\,$d), though this is subject to the adopted prior. We identify an important, previously unknown, bias whereby the rise times for higher-redshift SNe within a flux-limited survey are systematically underestimated. This effect can be partially alleviated if the power-law index is fixed to $α=2$, in which case we estimate a mean rise time of 21.7$\,$d (with a range from $\sim$18 to 23$\,$d). The sample includes a handful of rare and peculiar SNe$\,$Ia. Finally, we conclude with a discussion of lessons learned from the ZTF sample that can eventually be applied to observations from the Vera C. Rubin Observatory.

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arXiv2020

Helium-rich Superluminous Supernovae From the Zwicky Transient Facility

Helium is expected to be present in the ejecta of some hydrogen-poor superluminous supernovae (SLSN-I). However, so far only one event, PTF10hgi has been identified with He features in its photospheric spectra (Quimby et al. 2018). We present the discovery of a new He-rich SLSN-I, ZTF19aawfbtg (SN2019hge) at $z=0.0866$. This event has more than 10 optical spectra at phases from $-41$ to $+103$\,days relative to the peak, most of which match well with that of PTF10hgi. Confirmation comes from a near-IR spectrum taken at $+34$ days, revealing He I features with P-Cygni profiles at 1.083 and 2.058$μ$m. Using the optical spectra of PTF10hgi and SN2019hge as templates, we examine 70 SLSN-I discovered by ZTF in the first two years of operation and found additional five SLSN-I with distinct He-features. The excitation of He\,I atoms in normal core collapse supernovae requires non-thermal radiation, as proposed by previous studies. These He-rich events can not be explained by traditional $^{56}$Ni mixing model because of their blue spectra, high peak luminosity and slow rising time scales. Magnetar models offer a possible solution since pulsar winds naturally generate high energy particles as sources of non-thermal excitation. An alternative model is ejecta interaction with H-poor CSM which may be supported by the observed light curve undulations. These six SLSN-Ib appear to have relatively low-peak luminosities (rest-frame $M_g = -20.06\pm0.16$ mag).

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arXiv2020

SN2020bvc: a Broad-lined Type Ic Supernova with a Double-peaked Optical Light Curve and a Luminous X-ray and Radio Counterpart

We present optical, radio, and X-ray observations of SN2020bvc (=ASASSN20bs; ZTF20aalxlis), a nearby ($z=0.0252$; $d$=114 Mpc) broad-lined (BL) Type Ic supernova (SN). Our observations show that SN2020bvc shares several properties in common with the Ic-BL SN2006aj, which was associated with the low-luminosity gamma-ray burst (LLGRB) 060218. First, the 10 GHz radio light curve is on the faint end of LLGRB-SNe ($L_\mathrm{radio} \approx 10^{37}$erg/s): we model our VLA observations (spanning 13-43 d) as synchrotron emission from a mildly relativistic ($v \gtrsim 0.3c$) forward shock. Second, with Swift and Chandra we detect X-ray emission ($L_X \approx 10^{41}$erg/s) that is not naturally explained as inverse Compton emission or as part of the same synchrotron spectrum as the radio emission. Third, high-cadence ($6\times$/night) data from the Zwicky Transient Facility (ZTF) shows a double-peaked optical light curve, the first peak from shock-cooling emission from extended low-mass material (mass $M<10^{-2} M_\odot$ at radius $R>10^{12}$cm) and the second peak from the radioactive decay of Ni-56. SN2020bvc is the first confirmed double-peaked Ic-BL SN discovered without a GRB trigger, and shows X-ray and radio emission similar to LLGRB-SNe: this is consistent with models in which the same mechanism produces both the LLGRB and the shock-cooling emission. For four of the five other nearby ($z\lesssim0.05$) Ic-BL SNe with ZTF high-cadence data, we rule out a first peak like that seen in SN2006aj and SN2020bvc, i.e. that lasts $\approx 1$d and reaches a peak luminosity $M \approx -18$. X-ray and radio follow-up observations of future such events will establish whether double-peaked optical light curves are indeed predictive of LLGRB-like X-ray and radio emission.

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arXiv2020

The Spectacular Ultraviolet Flash From the Peculiar Type Ia Supernova 2019yvq

Early observations of Type Ia supernovae (SNe$\,$Ia) provide essential clues for understanding the progenitor system that gave rise to the terminal thermonuclear explosion. We present exquisite observations of SN$\,$2019yvq, the second observed SN$\,$Ia, after iPTF$\,$14atg, to display an early flash of emission in the ultraviolet (UV) and optical. Our analysis finds that SN$\,$2019yvq was unusual, even when ignoring the initial flash, in that it was moderately underluminous for an SN$\,$Ia ($M_g \approx -18.5\,$mag at peak) yet featured very high absorption velocities ($v \approx 15,000\,\mathrm{km\,s}^{-1}$ for Si II $λ$6355 at peak). We find that many of the observational features of SN$\,$2019yvq, aside from the flash, can be explained if the explosive yield of radioactive $^{56}\mathrm{Ni}$ is relatively low (we measure $M_{^{56}\mathrm{Ni}} = 0.31 \pm 0.05\,M_\odot$) and it and other iron-group elements are concentrated in the innermost layers of the ejecta. To explain both the UV/optical flash and peak properties of SN$\,$2019yvq we consider four different models: interaction between the SN ejecta and a nondegenerate companion, extended clumps of $^{56}\mathrm{Ni}$ in the outer ejecta, a double-detonation explosion, and the violent merger of two white dwarfs. Each of these models has shortcomings when compared to the observations; it is clear additional tuning is required to better match SN$\,$2019yvq. In closing, we predict that the nebular spectra of SN$\,$2019yvq will feature either H or He emission, if the ejecta collided with a companion, strong [Ca II] emission, if it was a double detonation, or narrow [O I] emission, if it was due to a violent merger.

astro-ph.HE↗
arXiv2020

The Koala: A Fast Blue Optical Transient with Luminous Radio Emission from a Starburst Dwarf Galaxy at $z=0.27$

We present ZTF18abvkwla (the "Koala"), a fast blue optical transient discovered in the Zwicky Transient Facility (ZTF) One-Day Cadence (1DC) Survey. ZTF18abvkwla has a number of features in common with the groundbreaking transient AT2018cow: blue colors at peak ($g-r\approx-0.5$ mag), a short rise time from half-max of under two days, a decay time to half-max of only three days, a high optical luminosity ($M_{g,\mathrm{peak}}\approx-20.6$mag), a hot ($\gtrsim 40,000$K) featureless spectrum at peak light, and a luminous radio counterpart. At late times ($Δt>80$d) the radio luminosity of ZTF18abvkwla ($νL_ν\gtrsim 10^{40}$erg/s at 10 GHz, observer-frame) is most similar to that of long-duration gamma-ray bursts (GRBs). The host galaxy is a dwarf starburst galaxy ($M\approx5\times10^{8}M_\odot$, $\mathrm{SFR}\approx7 M_\odot$/yr) that is moderately metal-enriched ($\log\mathrm{[O/H]} \approx 8.5$), similar to the hosts of GRBs and superluminous supernovae. As in AT2018cow, the radio and optical emission in ZTF18abvkwla likely arise from two separate components: the radio from fast-moving ejecta ($Γβc >0.38c$) and the optical from shock-interaction with confined dense material ($<0.07M_\odot$ in $\sim 10^{15}$cm). Compiling transients in the literature with $t_\mathrm{rise} <5$d and $M_\mathrm{peak}<-20$mag, we find that a significant number are engine-powered, and suggest that the high peak optical luminosity is directly related to the presence of this engine. From 18 months of the 1DC survey, we find that transients in this rise-luminosity phase space are at least two to three orders of magnitude less common than CC SNe. Finally, we discuss strategies for identifying such events with future facilities like the Large Synoptic Survey Telescope, and prospects for detecting accompanying X-ray and radio emission.

astro-ph.HE↗
arXiv2019

The Zwicky Transient Facility Bright Transient Survey I: Spectroscopic Classification and the Redshift Completeness of Local Galaxy Catalogs

The Zwicky Transient Facility (ZTF) is performing a three-day cadence survey of the visible Northern sky (~3$π$). The transient candidates found in this survey are announced via public alerts. As a supplementary product ZTF is also conducting a large spectroscopic campaign: the ZTF Bright Transient Survey (BTS). The goal of the BTS is to spectroscopically classify all extragalactic transients brighter than 18.5 mag at peak brightness and immediately announce those classifications to the public. Extragalactic discoveries from ZTF are predominantly Supernovae (SNe). The BTS is the largest flux-limited SN survey to date. Here we present a catalog of the761 SNe that were classified during the first nine months of the survey (2018 Apr. 1 to 2018 Dec. 31). The BTS SN catalog contains redshifts based on SN template matching and spectroscopic host galaxy redshifts when available. Based on this data we perform an analysis of the redshift completeness of local galaxy catalogs, dubbed as the Redshift Completeness Fraction (RCF; the number of SN host galaxies with known spectroscopic redshift prior to SN discovery divided by the total number of SN hosts). In total, we identify the host galaxies of 512 Type Ia supernovae, 227 of which have known spectroscopic redshifts, yielding an RCF estimate of $44\% \pm1\%$. We find a steady decrease in the RCF with increasing distance in the local universe. For z<0.05, or ~200 Mpc, we find RCF=0.6, which has important ramifications when searching for multimessenger astronomical events. Prospects for dramatically increasing the RCF are limited to new multi-fiber spectroscopic instruments, or wide-field narrowband surveys. We find that existing galaxy redshift catalogs are only $50\%$ complete at $r\approx16.9$ mag. Pushing this limit several magnitudes deeper will pay huge dividends when searching for electromagnetic counterparts to gravitational wave events.

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arXiv2019

Directly Imaging Rocky Planets from the Ground

Over the past three decades instruments on the ground and in space have discovered thousands of planets outside the solar system. These observations have given rise to an astonishingly detailed picture of the demographics of short-period planets, but are incomplete at longer periods where both the sensitivity of transit surveys and radial velocity signals plummet. Even more glaring is that the spectra of planets discovered with these indirect methods are either inaccessible (radial velocity detections) or only available for a small subclass of transiting planets with thick, clear atmospheres. Direct detection can be used to discover and characterize the atmospheres of planets at intermediate and wide separations, including non-transiting exoplanets. Today, a small number of exoplanets have been directly imaged, but they represent only a rare class of young, self-luminous super-Jovian-mass objects orbiting tens to hundreds of AU from their host stars. Atmospheric characterization of planets in the <5 AU regime, where radial velocity (RV) surveys have revealed an abundance of other worlds, is technically feasible with 30-m class apertures in combination with an advanced AO system, coronagraph, and suite of spectrometers and imagers. There is a vast range of unexplored science accessible through astrometry, photometry, and spectroscopy of rocky planets, ice giants, and gas giants. In this whitepaper we will focus on one of the most ambitious science goals --- detecting for the first time habitable-zone rocky (<1.6 R_Earth) exoplanets in reflected light around nearby M-dwarfs

astro-ph.EP↗
arXiv2019

ZTF18aalrxas: A Type IIb Supernova from a very extended low-mass progenitor

We investigate ZTF18aalrxas, a double-peaked Type IIb core-collapse supernova (SN) discovered during science validation of the Zwicky Transient Facility (ZTF). ZTF18aalrxas was discovered while the optical emission was still rising towards the initial cooling peak (0.7 mag over 2 days). Our observations consist of multi-band (UV, optical) light-curves, and optical spectra spanning from $\approx0.7$ d to $\approx180$ d past the explosion. We use a Monte-Carlo based non-local thermodynamic equilibrium (NLTE) model, that simultanously reproduces both the $\rm ^{56}Ni$ powered bolometric light curve and our nebular spectrum. This model is used to constrain the synthesized radioactive nickel mass (0.17 $\mathrm{M}_{\odot}$) and the total ejecta mass (1.7 $\mathrm{M}_{\odot}$) of the SN. The cooling emission is modeled using semi-analytical extended envelope models to constrain the progenitor radius ($790-1050$ $\mathrm{R}_{\odot}$) at the time of explosion. Our nebular spectrum shows signs of interaction with a dense circumstellar medium (CSM), and this spetrum is modeled and analysed to constrain the amount of ejected oxygen ($0.3-0.5$ $\mathrm{M}_{\odot}$) and the total hydrogen mass ($\approx0.15$ $\mathrm{M}_{\odot}$) in the envelope of the progenitor. The oxygen mass of ZTF18aalrxas is consistent with a low ($12-13$ $\mathrm{M}_{\odot}$) Zero Age Main Sequence mass progenitor. The light curves and spectra of ZTF18aalrxas are not consistent with massive single star SN Type IIb progenitor models. The presence of an extended hydrogen envelope of low mass, the presence of a dense CSM, the derived ejecta mass, and the late-time oxygen emission can all be explained in a binary model scenario.

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arXiv2018

Ground Layer Adaptive Optics for the W. M. Keck Observatory: Feasibility Study

Ground-layer adaptive optics (GLAO) systems offer the possibility of improving the "seeing" of large ground-based telescopes and increasing the efficiency and sensitivity of observations over a wide field-of-view. We explore the utility and feasibility of deploying a GLAO system at the W. M. Keck Observatory in order to feed existing and future multi-object spectrographs and wide-field imagers. We also briefly summarize science cases spanning exoplanets to high-redshift galaxy evolution that would benefit from a Keck GLAO system. Initial simulations indicate that a Keck GLAO system would deliver a 1.5x and 2x improvement in FWHM at optical (500 nm) and infrared (1.5 micron), respectively. The infrared instrument, MOSFIRE, is ideally suited for a Keck GLAO feed in that it has excellent image quality and is on the telescope's optical axis. However, it lacks an atmospheric dispersion compensator, which would limit the minimum usable slit size for long-exposure science cases. Similarly, while LRIS and DEIMOS may be able to accept a GLAO feed based on their internal image quality, they lack either an atmospheric dispersion compensator (DEIMOS) or flexure compensation (LRIS) to utilize narrower slits matched to the GLAO image quality. However, some science cases needing shorter exposures may still benefit from Keck GLAO and we will investigate the possibility of installing an ADC.

astro-ph.IM↗
arXiv2017

Project 1640 Observations of the White Dwarf HD 114174 B

We present the first near infra-red spectrum of the faint white dwarf companion HD 114174 B, obtained with Project 1640. Our spectrum, covering the Y, J and H bands, combined with previous TRENDS photometry measurements, allows us to place further constraints on this companion. We suggest two possible scenarios; either this object is an old, low mass, cool H atmosphere white dwarf with Teff ~ 3800 K or a high mass white dwarf with Teff > 6000 K, potentially with an associated cool (Teff ~ 700 K) brown dwarf or debris disk resulting in an infra-red excess in the L band. We also provide an additional astrometry point for 2014 June 12 and use the modelled companion mass combined with the RV and direct imaging data to place constraints on the orbital parameters for this companion.

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arXiv2017

Project 1640 Observations of Brown Dwarf GJ 758 B: Near-Infrared Spectrum and Atmospheric Modeling

The nearby Sun-like star GJ 758 hosts a cold substellar companion, GJ 758 B, at a projected separation of $\lesssim$30 AU, previously detected in high-contrast multi-band photometric observations. In order to better constrain the companion's physical characteristics, we acquired the first low-resolution ($R \sim 50$) near-infrared spectrum of it using the high-contrast hyperspectral imaging instrument Project 1640 on Palomar Observatory's 5-m Hale telescope. We obtained simultaneous images in 32 wavelength channels covering the $Y$, $J$, and $H$ bands ($\sim$952-1770 nm), and used data processing techniques based on principal component analysis to efficiently subtract chromatic background speckle-noise. GJ 758 B was detected in four epochs during 2013 and 2014. Basic astrometric measurements confirm its apparent northwest trajectory relative to the primary star, with no clear signs of orbital curvature. Spectra of SpeX/IRTF observed T dwarfs were compared to the combined spectrum of GJ 758 B, with $χ^2$ minimization suggesting a best fit for spectral type T7.0$\pm$1.0, but with a shallow minimum over T5-T8. Fitting of synthetic spectra from the BT-Settl13 model atmospheres gives an effective temperature $T_{\text{eff}}=741 \pm 25$ K and surface gravity $\log g = 4.3 \pm 0.5$ dex (cgs). Our derived best-fit spectral type and effective temperature from modeling of the low-resolution spectrum suggest a slightly earlier and hotter companion than previous findings from photometric data, but do not rule out current results, and confirm GJ 758 B as one of the coolest sub-stellar companions to a Sun-like star to date.

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arXiv2014

Reconnaissance of the HR 8799 Exosolar System II: Astrometry and Orbital Motion

We present an analysis of the orbital motion of the four sub-stellar objects orbiting HR8799. Our study relies on the published astrometric history of this system augmented with an epoch obtained with the Project 1640 coronagraph + Integral Field Spectrograph (IFS) installed at the Palomar Hale telescope. We first focus on the intricacies associated with astrometric estimation using the combination of an Extreme Adaptive Optics system (PALM-3000), a coronagraph and an IFS. We introduce two new algorithms. The first one retrieves the stellar focal plane position when the star is occulted by a coronagraphic stop. The second one yields precise astrometric and spectro-photometric estimates of faint point sources even when they are initially buried in the speckle noise. The second part of our paper is devoted to studying orbital motion in this system. In order to complement the orbital architectures discussed in the literature, we determine an ensemble of likely Keplerian orbits for HR8799bcde, using a Bayesian analysis with maximally vague priors regarding the overall configuration of the system. While the astrometric history is currently too scarce to formally rule out coplanarity, HR8799d appears to be misaligned with respect to the most likely planes of HR8799bce orbits. This misalignment is sufficient to question the strictly coplanar assumption made by various authors when identifying a Laplace resonance as a potential architecture. Finally, we establish a high likelihood that HR8799de have dynamical masses below 13 M_Jup using a loose dynamical survival argument based on geometric close encounters. We illustrate how future dynamical analyses will further constrain dynamical masses in the entire system.

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arXiv2013

PALM-3000: Exoplanet Adaptive Optics for the 5-meter Hale Telescope

We describe and report first results from PALM-3000, the second-generation astronomical adaptive optics facility for the 5.1-m Hale telescope at Palomar Observatory. PALM-3000 has been engineered for high-contrast imaging and emission spectroscopy of brown dwarfs and large planetary mass bodies at near-infrared wavelengths around bright stars, but also supports general natural guide star use to V ~ 17. Using its unique 66 x 66 actuator deformable mirror, PALM-3000 has thus far demonstrated residual wavefront errors of 141 nm RMS under 1 arcsecond seeing conditions. PALM-3000 can provide phase conjugation correction over a 6.4 x 6.4 arcsecond working region at an observing wavelength of 2.2 microns, or full electric field (amplitude and phase) correction over approximately one half of this field. With optimized back-end instrumentation, PALM-3000 is designed to enable as high as 10e-7 contrast at ~1 arc second angular separation, after including post-observation speckle suppression processing. While optimization of the adaptive optics system is ongoing, we have already successfully commissioned five back-end science instruments and begun a major exoplanet characterization survey, Project 1640, with our partners at American Museum of Natural History and Jet Propulsion Laboratory.

astro-ph.IM↗
arXiv2013

Reconnaissance of the HR 8799 Exosolar System I: Near IR Spectroscopy

We obtained spectra, in the wavelength range λ= 995 - 1769 nm, of all four known planets orbiting the star HR 8799. Using the suite of instrumentation known as Project 1640 on the Palomar 5-m Hale Telescope, we acquired data at two epochs. This allowed for multiple imaging detections of the companions and multiple extractions of low-resolution (R ~ 35) spectra. Data reduction employed two different methods of speckle suppression and spectrum extraction, both yielding results that agree. The spectra do not directly correspond to those of any known objects, although similarities with L and T-dwarfs are present, as well as some characteristics similar to planets such as Saturn. We tentatively identify the presence of CH_4 along with NH_3 and/or C_2H_2, and possibly CO_2 or HCN in varying amounts in each component of the system. Other studies suggested red colors for these faint companions, and our data confirm those observations. Cloudy models, based on previous photometric observations, may provide the best explanation for the new data presented here. Notable in our data is that these presumably co-eval objects of similar luminosity have significantly different spectra; the diversity of planets may be greater than previously thought. The techniques and methods employed in this paper represent a new capability to observe and rapidly characterize exoplanetary systems in a routine manner over a broad range of planet masses and separations. These are the first simultaneous spectroscopic observations of multiple planets in a planetary system other than our own.

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arXiv2012

Robo-AO: autonomous and replicable laser-adaptive-optics and science system

We have created a new autonomous laser-guide-star adaptive-optics (AO) instrument on the 60-inch (1.5-m) telescope at Palomar Observatory called Robo-AO. The instrument enables diffraction-limited resolution observing in the visible and near-infrared with the ability to observe well over one-hundred targets per night due to its fully robotic operation. Robo- AO is being used for AO surveys of targets numbering in the thousands, rapid AO imaging of transient events and longterm AO monitoring not feasible on large diameter telescope systems. We have taken advantage of cost-effective advances in deformable mirror and laser technology while engineering Robo-AO with the intention of cloning the system for other few-meter class telescopes around the world.

astro-ph.IM↗
arXiv2011

Hydrogen-poor superluminous stellar explosions

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.

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