G. Leloudas

Improved cosmological constraints from a joint analysis of the SDSS-II and SNLS supernova samples

Aims. We present cosmological constraints from a joint analysis of type Ia supernova (SN Ia) observations obtained by the SDSS-II and SNLS collaborations. The dataset includes several low-redshift samples (z< 0.1), all three seasons from the SDSS-II (0.05 <z< 0.4), and three years from SNLS (0.2 <z< 1), and it totals 740 spectroscopically confirmed type Ia supernovae with high-quality light curves. Methods. We followed the methods and assumptions of the SNLS three-year data analysis except for the following important improvements: 1) the addition of the full SDSS-II spectroscopically-confirmed SN Ia sample in both the training of the SALT2 light-curve model and in the Hubble diagram analysis (374 SNe); 2) intercalibration of the SNLS and SDSS surveys and reduced systematic uncertainties in the photometric calibration, performed blindly with respect to the cosmology analysis; and 3) a thorough investigation of systematic errors associated with the SALT2 modeling of SN Ia light curves. Results. We produce recalibrated SN Ia light curves and associated distances for the SDSS-II and SNLS samples. The large SDSS-II sample provides an effective, independent, low-z anchor for the Hubble diagram and reduces the systematic error from calibration systematics in the low-z SN sample. For a flat ΛCDM cosmology, we find Ωm =0.295 ± 0.034 (stat+sys), a value consistent with the most recent cosmic microwave background (CMB) measurement from the Planck and WMAP experiments. Our result is 1.8σ (stat+sys) different than the previously published result of SNLS three-year data. The change is due primarily to improvements in the SNLS photometric calibration. When combined with CMB constraints, we measure a constant dark-energy equation of state parameter w =−1.018 ± 0.057 (stat+sys) for a flat universe. Adding baryon acoustic oscillation distance measurements gives similar constraints: w =−1.027 ± 0.055. Our supernova measurements provide the most stringent constraints to date on the nature of dark energy.

The Sloan Digital Sky Survey - II:supernova survey: technical summary

The Sloan Digital Sky Survey-II (SDSS-II) has embarked on a multi-year project to identify and measure light curves for intermediate-redshift (0.05 < z < 0.35) Type Ia supernovae (SNe Ia) using repeated five-band (ugriz) imaging over an area of 300 sq. deg. The survey region is a stripe 2.5° wide centered on the celestial equator in the Southern Galactic Cap that has been imaged numerous times in earlier years, enabling construction of a deep reference image for the discovery of new objects. Supernova imaging observations are being acquired between September 1 and November 30 of 2005-7. During the first two seasons, each region was imaged on average every five nights. Spectroscopic follow-up observations to determine supernova type and redshift are carried out on a large number of telescopes. In its first two three-month seasons, the survey has discovered and measured light curves for 327 spectroscopically confirmed SNe Ia, 30 probable SNe Ia, 14 confirmed SNe Ib/c, 32 confirmed SNe II, plus a large number of photometrically identified SNe Ia, 94 of which have host-galaxy spectra taken so far. This paper provides an overview of the project and briefly describes the observations completed during the first two seasons of operation.

An Extremely Luminous Panchromatic Outburst from the Nucleus of a Distant Galaxy

A recent bright emission observed by the Swift satellite is due to the sudden accretion of a star onto a massive black hole. Variable x-ray and γ-ray emission is characteristic of the most extreme physical processes in the universe. We present multiwavelength observations of a unique γ-ray–selected transient detected by the Swift satellite, accompanied by bright emission across the electromagnetic spectrum, and whose properties are unlike any previously observed source. We pinpoint the event to the center of a small, star-forming galaxy at redshift z = 0.3534. Its high-energy emission has lasted much longer than any γ-ray burst, whereas its peak luminosity was ∼100 times higher than bright active galactic nuclei. The association of the outburst with the center of its host galaxy suggests that this phenomenon has its origin in a rare mechanism involving the massive black hole in the nucleus of that galaxy.

Super-luminous type Ic supernovae : catching a magnetar by the tail.

We report extensive observational data for five of the lowest redshift Super-Luminous Type Ic Supernovae (SL-SNe Ic) discovered to date, namely PTF10hgi, SN2011ke, PTF11rks, SN2011kf and SN2012il. Photometric imaging of the transients at +50 to +230 days after peak combined with host galaxy subtraction reveals a luminous tail phase for four of these SL-SNe. A high resolution, optical and near infrared spectrum from xshooter provides detection of a broad He I �10830 emission line in the spectrum (+50d) of SN2012il, revealing that at least some SL-SNe Ic are not completely helium free. At first sight, the tail luminosity decline rates that we measure are consistent with the radioactive decay of 56 Co, and would require 1-4 M⊙ of 56 Ni to produce the luminosity. These 56 Ni masses cannot be made consistent with the short diffusion times at peak, and indeed are insufficient to power the peak luminosity. We instead favour energy deposition by newborn magnetars as the power source for these objects. A semi-analytical diffusion model with energy input from the spindown of a magnetar reproduces the extensive lightcurve data well. The model predictions of ejecta velocities and temperatures which are required are in reasonable agreement with those determined from our observations. We derive magnetar energies of 0.4 . E(10 51 erg) . 6.9 and ejecta masses of 2.3 . Mej(M⊙) . 8.6. The sample of five SL-SNe Ic presented here, combined with SN 2010gx - the best sampled SL-SNe Ic so far - point toward an explosion driven by a magnetar as a viable explanation for all SL-SNe Ic. Subject headings: supernovae: general - supernovae: individual (PTF10hgi, SN 2011ke, PTF11rks, SN 2011kf, SN 2012il) - stars: magnetars

An asymmetric explosion as the origin of spectral evolution diversity in type Ia supernovae

Type Ia supernovae form an observationally uniform class of stellar explosions, in that more luminous objects have smaller decline-rates. This one-parameter behaviour allows type Ia supernovae to be calibrated as cosmological ‘standard candles’, and led to the discovery of an accelerating Universe. Recent investigations, however, have revealed that the true nature of type Ia supernovae is more complicated. Theoretically, it has been suggested that the initial thermonuclear sparks are ignited at an offset from the centre of the white-dwarf progenitor, possibly as a result of convection before the explosion. Observationally, the diversity seen in the spectral evolution of type Ia supernovae beyond the luminosity–decline-rate relation is an unresolved issue. Here we report that the spectral diversity is a consequence of random directions from which an asymmetric explosion is viewed. Our findings suggest that the spectral evolution diversity is no longer a concern when using type Ia supernovae as cosmological standard candles. Furthermore, this indicates that ignition at an offset from the centre is a generic feature of type Ia supernovae.

Slowly fading super-luminous supernovae that are not pair-instability explosions

Super-luminous supernovae that radiate more than 1044 ergs per second at their peak luminosity have recently been discovered in faint galaxies at redshifts of 0.1–4. Some evolve slowly, resembling models of ‘pair-instability’ supernovae. Such models involve stars with original masses 140–260 times that of the Sun that now have carbon–oxygen cores of 65–130 solar masses. In these stars, the photons that prevent gravitational collapse are converted to electron–positron pairs, causing rapid contraction and thermonuclear explosions. Many solar masses of 56Ni are synthesized; this isotope decays to 56Fe via 56Co, powering bright light curves. Such massive progenitors are expected to have formed from metal-poor gas in the early Universe. Recently, supernova 2007bi in a galaxy at redshift 0.127 (about 12 billion years after the Big Bang) with a metallicity one-third that of the Sun was observed to look like a fading pair-instability supernova. Here we report observations of two slow-to-fade super-luminous supernovae that show relatively fast rise times and blue colours, which are incompatible with pair-instability models. Their late-time light-curve and spectral similarities to supernova 2007bi call the nature of that event into question. Our early spectra closely resemble typical fast-declining super-luminous supernovae, which are not powered by radioactivity. Modelling our observations with 10–16 solar masses of magnetar-energized ejecta demonstrates the possibility of a common explosion mechanism. The lack of unambiguous nearby pair-instability events suggests that their local rate of occurrence is less than 6 × 10−6 times that of the core-collapse rate.

A real-time fast radio burst: polarization detection and multiwavelength follow-up

Fast radio bursts (FRBs) are one of the most tantalizing mysteries of the radio sky; their progenitors and origins remain unknown and until now no rapid multiwavelength follow-up of an FRB has been possible. New instrumentation has decreased the time between observation and discovery from years to seconds, and enables polarimetry to be performed on FRBs for thefirst time. We have discovered an FRB (FRB 140514) in real-time on 2014 May 14 at 17:14:11.06 UTCattheParkesradiotelescopeandtriggeredfollow-upatotherwavelengthswithinhoursof theevent.FRB140514wasfoundwithadispersionmeasure(DM)of562.7(6)cm −3 pc,giving an upper limit on source redshift of z 0.5. FRB 140514 was found to be 21 ± 7 per cent (3σ) circularly polarized on the leading edge with a 1σ upper limit on linear polarization <10 per cent. We conclude that this polarization is intrinsic to the FRB. If there was any intrinsic linear polarization, as might be expected from coherent emission, then it may have been depolarized by Faraday rotation caused by passing through strong magnetic fields and/or high-density environments. FRB 140514 was discovered during a campaign to re-observe known FRB fields, and lies close to a previous discovery, FRB 110220; based on the difference in DMs of these bursts and time-on-sky arguments, we attribute the proximity to sampling bias and conclude that they are distinct objects. Follow-up conducted by 12 telescopes observing from X-ray to radio wavelengths was unable to identify a variable multiwavelength counterpart, allowing us to rule out models in which FRBs originate from nearby ( z< 0.3) supernovae and long duration gamma-ray bursts.

A PANCHROMATIC VIEW OF THE RESTLESS SN 2009ip REVEALS THE EXPLOSIVE EJECTION OF A MASSIVE STAR ENVELOPE

The double explosion of SN 2009ip in 2012 raises questions about our understanding of the late stages of massive star evolution. Here we present a comprehensive study of SN 2009ip during its remarkable rebrightenings. High-cadence photometric and spectroscopic observations from the GeV to the radio band obtained from a variety of ground-based and space facilities (including the Very Large Array, Swift, Fermi, Hubble Space Telescope, and XMM) constrain SN 2009ip to be a low energy (E similar to 1050 erg for an ejecta mass similar to 0.5 M-circle dot) and asymmetric explosion in a complex medium shaped by multiple eruptions of the restless progenitor star. Most of the energy is radiated as a result of the shock breaking out through a dense shell of material located at similar to 5 x 10(14) cm with M similar to 0.1 M-circle dot, ejected by the precursor outburst similar to 40 days before the major explosion. We interpret the NIR excess of emission as signature of material located further out, the origin of which has to be connected with documented mass-loss episodes in the previous years. Our modeling predicts bright neutrino emission associated with the shock break-out if the cosmic-ray energy is comparable to the radiated energy. We connect this phenomenology with the explosive ejection of the outer layers of the massive progenitor star, which later interacted with material deposited in the surroundings by previous eruptions. Future observations will reveal if the massive luminous progenitor star survived. Irrespective of whether the explosion was terminal, SN 2009ip brought to light the existence of new channels for sustained episodic mass loss, the physical origin of which has yet to be identified.

A very luminous magnetar-powered supernova associated with an ultra-long gamma-ray burst

A new class of ultra-long-duration (more than 10,000 seconds) γ-ray bursts has recently been suggested. They may originate in the explosion of stars with much larger radii than those producing normal long-duration γ-ray bursts or in the tidal disruption of a star. No clear supernova has yet been associated with an ultra-long-duration γ-ray burst. Here we report that a supernova (SN 2011kl) was associated with the ultra-long-duration γ-ray burst GRB 111209A, at a redshift z of 0.677. This supernova is more than three times more luminous than type Ic supernovae associated with long-duration γ-ray bursts, and its spectrum is distinctly different. The slope of the continuum resembles those of super-luminous supernovae, but extends further down into the rest-frame ultraviolet implying a low metal content. The light curve evolves much more rapidly than those of super-luminous supernovae. This combination of high luminosity and low metal-line opacity cannot be reconciled with typical type Ic supernovae, but can be reproduced by a model where extra energy is injected by a strongly magnetized neutron star (a magnetar), which has also been proposed as the explanation for super-luminous supernovae.

Spectroscopy of superluminous supernova host galaxies: A preference of hydrogen-poor events for extreme emission line galaxies

Superluminous supernovae (SLSNe) were only discovered recently due to their preference for occurring in faint dwarf galaxies. Understanding why stellar evolution yields dierent types of stellar explosions in these environments is fundamental in order to both uncover the elusive progenitors of SLSNe and to study star formation in dwarf galaxies. In this paper, we present the rst results of our project to study SUperluminous Supernova Host galaxIES (SUSHIES), focusing on the sample for which we have obtained spectroscopy. We show that SLSNe-I and SLSNe-R (hydrogen-poor) often ( 50% in our sample) occur in a class of galaxies that is known as Extreme Emission Line Galaxies (EELGs). The probability of this happening by chance is negligible and we therefore conclude that the extreme environmental conditions and the SLSN phenomenon are related. In contrast, SLSNe-II (hydrogen-rich) occur in more massive, more metal-rich galaxies with softer radiation elds. Therefore, if SLSNe-II constitute a uniform class, their progenitor systems must be dierent from those of H-poor SLSNe. Gamma-ray bursts (GRBs) are, on average, not found in as extreme environments as H-poor SLSNe. We propose that H-poor SLSNe result from the very rst stars exploding in a starburst, even earlier than GRBs. This might indicate a bottom-light initial mass function in these systems. SLSNe present a novel method of selecting candidate EELGs independent of their luminosity.