The ESSENCE / w Project: Strategies and Initial Observations

1. The ESSENCE / w Project: Strategies and Initial Observations R. Chris Smith, C. Aguilera, K. Krisciunas, N. B. Suntzeff (NOAO/CTIO), A. Becker, R. Covarrubias, A. Miceli, G. Miknaitis, A. Rest, C. Stubbs (U. Washington), P. M. Garnavich, S. T. Holland (Notre Dame), B. P. Schmidt (MSSSO), A. V. Filippenko, S. Jha, W. Li (U.C. Berkeley), P. Challis, R. P. Kirshner, T. Matheson (CfA), B. Barris, J. L. Tonry (U. Hawaii), A. Riess (STScI), B. Leibundgut, J. Sollerman, J. Spyromilio (ESO), A. Clocchiatti (PUC), S. Pompea (NOAO) http://www.ctio.noao.edu/essence DATA FLOW (& OTHER POSTERS): As the data are taken, they are immediately shipped down to La Serena for processing in the SuperMacho+SuperNova (SM+SN) pipeline (see Miceli et al. poster 55.01) and the results flow into the SM+SN time-domain database (see Hiriart et al. Poster 78.06). INTRODUCTION: The ESSENCE project seeks to constrain the properties of the dark energy by discovering and following more than 200 Type Ia supernovae (SNe) over the redshift range 0.2 < z < 0.8 – see Garnavich et al. poster (78.09) for more details. Here we present the strategy designed to achieve this result over the five year lifetime of the project, and the preliminary results of our first year of observations. DATA ACQUISITION: In order to obtain the balance of wide field of view and depth, we chose to base our supernova search on the CTIO Blanco 4m telescope combined with the 8Kx8K Mosaic imager, which provides a field of view of 0.36 deg2. Through the NOAO Survey program, we have been allocated half nights every other night in dark and grey time for three months (Oct, Nov, Dec) each year over a five year period (2002 -- 2006). Observations are performed in three filters, V, R, and I, so that the search itself provides the data for multi-color light curves on all SNe discovered. Exposure times from 60s to 900s in R are used to balance the discovery of supernovae across the desired range in redshift. FIELDS: We have selected 22 fields covering ~8 deg2 to achieve the discovery rate necessary to achieve our goals . Fields were selected such that they could be observed for the whole three month period during the first half of the night while also having minimal galactic extinction and bright stars. Each field is observed every fourth night, providing adequate sampling of the resulting SN light curves. Based on published SN rates, these observations should provide 30 to 50 Type Ia SNe per three-month observing season, well distributed in redshift from z of 0.2 to 0.8. ESSENCE 2002B SUPERNOVAE: We began the scheduled observations in October 2002. The first nights were used to build up template images to allow for subtraction from 2nd epoch images. Observations continued in November and December 2002. A total of 19 SNe have been confirmed to date, and several more remain to be spectroscopically observed. Figure 3: Preliminary reduction of a spectrum of SN 2002jq. This spectrum was taken at Gemini-N with GMOS in Nod & Shuffle mode, allowing for very clean sky subtraction. SPECTROSCOPY: Soon after the SNe are discovered, we obtain spectroscopy both to type the SNe and determine their ages. Spectra are obtained using Gemini + GMOS, Keck + LRIS & ESI, Magellan + LDSS2, and the VLT + FORS1. These large apertures are needed in order to obtain good signal-to-noise on the faint SNe in our sample. The spectra will also be used to look for evolutionary differences as a function of redshift. Figure 4: Preliminary light curves of SN 2002iv (B4), a Type Ia SN at z=0.2 discovered before maximum. The OBSERVER frame filters are shown. At this redshift, the VRI light curves correspond roughly to BVR in the SN rest frame. At higher redshifts (z~0.5) we rely mainly on RI, which correspond to BV in the SN rest frame. PRELIMINARY LIGHT CURVES: As we repeat observations of the same fields every four nights, we automatically build up a three-color light curve for each of the SNe we discover. We also have multiple pre-discovery images which will be valuable in studying the rise time of these objects as a function of redshift. A preliminary set of light curves of SN 2002iv (B4) is shown to the right. DATA PUBLICALLY AVAILABLE: In order to ensure maximal use of the unique time-domain dataset we are collecting, we have waived the proprietary period on the raw and flat-fielded data. The raw data can be downloaded from an NOAO FTP site linked off of our project web page, and the reduced data will soon be available in the NOAO Science Archive. Figure 1: A sample ESSENCE field, wxv2. This particular field produced 3 SNe this search season. SUMMARY: In the first 3-month season of operation, the ESSENCE project (a.k.a. “w project”) has obtained ~0.7 Terabytes of raw data in 28 half nights of observing. The images were processed in near-real-time, and SN candidates were posted to our public website within hours of the observations. We have discovered more than 18 SNe ranging in redshift from z = 0.1 to 0.6 despite less than ideal conditions at CTIO this year (el Niño). The raw and reduced data have been sent to NOAO/Tucson to provide for public access to the dataset. For access to this data and to obtain more information about our project, please visit our web site. Figure 2: An ESSENCE SN detection. This detection turned out to be a Type Ia SN at z=0.4 (SN 2002jw). This project is funded in part by a grant from the National Science Foundation, AST-0206329.