We describe the implementation and optimization of the ESSENCE supernova survey, which we have undertaken to measure the dark energy equation-of-state parameter, w = P/(pc super(2)). We present a ...method for optimizing the survey exposure times and cadence to maximize our sensitivity to w for a given fixed amount of telescope time. For our survey on the CTIO 4 m telescope, measuring the luminosity distances and redshifts for supernovae at modest redshifts (z approximately 0.5 plus or minus 0.2) is optimal for determining w. We describe the data analysis pipeline based on using reliable and robust image subtraction to find supernovae automatically and in nearly real time. Since making cosmological inferences with supernovae relies crucially on accurate measurement of their apparent brightnesses, we describe our efforts to establish a thorough calibration of the CTIO 4 m telescope's natural photometric system. In its first four years, ESSENCE has discovered and spectroscopically confirmed 102 Type Ia supernovae, at redshifts from 0.10 to 0.78, identified through an impartial, effective methodology for spectroscopic classification and redshift determination. We present the resulting light curves for all of the Type Ia supernovae found by ESSENCE and used in our measurement of w, presented in a companion paper by Wood-Vasey and coworkers.
We report the discovery of a supernova (SN) with the highest apparent energy output to date and conclude that it represents an extreme example of the Type IIn subclass. The SN, which was discovered ...behind the Large Magellanic Cloud at z = 0.289 by the SuperMACHO microlensing survey, peaked at MR = --21.5 mag and only declined by 2.9 mag over 4.7 years after the peak. Over this period, SN 2003ma had an integrated bolometric luminosity of 4 X 1051 erg, more than any other SN to date. The radiated energy is close to the limit allowed by conventional core-collapse explosions. Optical spectra reveal that SN 2003ma has persistent single-peaked intermediate-width hydrogen lines, a signature of interaction between the SN and a dense circumstellar medium. The light curves show further evidence for circumstellar interaction, including a long plateau with a shape very similar to the classic SN IIn 1988Z--however, SN 2003ma is 10 times more luminous at all epochs. The fast velocity measured for the intermediate-width H Delta *a component (~6000 km s--1) points toward an extremely energetic explosion (>1052 erg), which imparts a faster blast-wave speed to the post-shock material and a higher luminosity from the interaction than is observed in typical SNe IIn. Mid-infrared observations of SN 2003ma suggest an infrared light echo is produced by normal interstellar dust at a distance ~0.5 pc from the SN.
Characterizing the nature and spatial distribution of the lensing objects that produce the previously measured microlensing optical depth toward the Large Magellanic Cloud (LMC) remains an open ...problem. We present an appraisal of the ability of the SuperMACHO Project, a next-generation microlensing survey directed toward the LMC, to discriminate between various proposed lensing populations. We consider two scenarios: lensing by a uniform foreground screen of objects and self-lensing by LMC stars. The optical depth for "screen lensing" is essentially constant across the face of the LMC, whereas the optical depth for self-lensing shows a strong spatial dependence. We have carried out extensive simulations, based on data obtained during the first year of the project, to assess the SuperMACHO survey's ability to discriminate between these two scenarios. In our simulations we predict the expected number of observed microlensing events for various LMC models for each of our fields by adding artificial stars to the images and estimating the spatial and temporal efficiency of detecting microlensing events using Monte Carlo methods. We find that the event rate itself shows significant sensitivity to the choice of the LMC luminosity function, limiting the conclusions that can be drawn from the absolute rate. If instead we determine the differential event rate across the LMC, we will decrease the impact of these systematic biases and render our conclusions more robust. With this approach the SuperMACHO Project should be able to distinguish between the two categories of lens populations. This will provide important constraints on the nature of the lensing objects and their contributions to the Galactic dark matter halo.
ABSTRACT The ESSENCE survey discovered 213 Type Ia supernovae at redshifts between 2002 and 2008. We present their R- and I-band photometry, measured from images obtained using the MOSAIC II camera ...at the CTIO Blanco, along with rapid-response spectroscopy for each object. We use our spectroscopic follow-up observations to determine an accurate, quantitative classification, and precise redshift. Through an extensive calibration program we have improved the precision of the CTIO Blanco natural photometric system. We use several empirical metrics to measure our internal photometric consistency and our absolute calibration of the survey. We assess the effect of various potential sources of systematic bias on our measured fluxes, and estimate the dominant term in the systematic error budget from the photometric calibration on our absolute fluxes is ∼1%.
We present late-time spectropolarimetric observations of SN 2007sr, obtained with the Very Large Telescope at the ESO Paranal Observatory when the object was 63 days after maximum light. The ...late-time spectrum displays strong line polarization in the Ca II absorption features. SN 2007sr adds to the case of some normal Type Ia supernovae that show high line polarization or repolarization at late times, a fact that might be connected with the presence of high-velocity features at early times.
We present a large-scale effort of creating composite spectra of high- redshift SNe Ia and comparing them to low-redshift counterparts in an attempt to understand possible cosmic evolution of SNe Ia, ...which has major implications for studies of dark energy. Through the ESSENCE project, we have obtained 107 spectra of 88 high-redshift SNe Ia with excellent light-curve information. In addition, we have obtained 397 spectra of low-redshift SNe Ia through a multiple-decade effort at the Lick and Keck Observatories, and we have used 45 UV spectra obtained by HST and IUE. The low-redshift spectra act as a control sample when comparing to the ESSENCE spectra. In all instances, the ESSENCE and Lick composite spectra appear very similar. The addition of galaxy light to the Lick composite spectra allows an excellent match of the overall SED with the ESSENCE composite spectra, indicating that the high-redshift SNe are more contaminated with host galaxy light than their low-redshift counterparts. This is caused by observing objects at all redshifts with similar angular slit widths, which corresponds to different projected physical distances. After correcting for the galaxy light contamination, a few marginally significant differences in the spectra remain. We have estimated the systematic errors when using current spectral templates for K-corrections to be image0.02 mag. The variance in the composite spectra gives an estimate of the intrinsic variance in low-redshift maximum light SN spectra of image3% relative flux in the optical and growing toward the UV. The difference between the maximum light low- and high-redshift spectra constrains the evolution of SN spectral features between our samples to be <10% relative flux in the rest-frame optical. Currently, galaxy contamination and the small samples of rest-frame UV spectra at low and high redshifts are the limiting factors for future studies.
PDF
