On the future of Gamma-Ray Burst Cosmology

With the understanding that the enigmatic Gamma-Ray Burts (GRBs) are beamed explosions, and with the recently discovered ``Ghirlanda-relation'', the dream of using GRBs as cosmological yardsticks may have come a few steps closer to reality. Assuming the Ghirlanda-relation is real, we have investigated possible constraints on cosmological parameters using a simulated future sample of a large number of GRBs inspired by the ongoing SWIFT mission. Comparing with constraints from a future sample of Type Ia supernovae, we find that GRBs are not efficient in constraining the amount of dark energy or its equation of state. The main reason for this is that very few bursts are available at low redshifts.Comment: 5 pages, 2 figures, matches version accepted for publication in JCA

New UltraCool and Halo White Dwarf Candidates in SDSS Stripe 82

A 2.5 x 100 degree region along the celestial equator (Stripe 82) has been imaged repeatedly from 1998 to 2005 by the Sloan Digital Sky Survey. A new catalogue of ~4 million light-motion curves, together with over 200 derived statistical quantities, for objects in Stripe 82 brighter than r~21.5 has been constructed by combining these data by Bramich et al. (2007). This catalogue is at present the deepest catalogue of its kind. Extracting the ~130000 objects with highest signal-to-noise ratio proper motions, we build a reduced proper motion diagram to illustrate the scientific promise of the catalogue. In this diagram disk and halo subdwarfs are well-separated from the cool white dwarf sequence. Our sample of 1049 cool white dwarf candidates includes at least 8 and possibly 21 new ultracool H-rich white dwarfs (T_eff < 4000K) and one new ultracool He-rich white dwarf candidate identified from their SDSS optical and UKIDSS infrared photometry. At least 10 new halo white dwarfs are also identified from their kinematics.Comment: 10 pages, 5 figures, published in MNRAS, minor text changes, final versio

The SuperMACHO Microlensing Survey

We present the first results from our next-generation microlensing survey, the SuperMACHO project. We are using the CTIO 4m Blanco telescope and the MOSAIC imager to carry out a search for microlensing toward the Large Magellanic Cloud (LMC). We plan to ascertain the nature of the population responsible for the excess microlensing rate seen by the MACHO project. Our observing strategy is optimized to measure the differential microlensing rate across the face of the LMC. We find this derivative to be relatively insensitive to the details of the LMC's internal structure but a strong discriminant between Galactic halo and LMC self lensing. In December 2003 we completed our third year of survey operations. 2003 also marked the first year of real-time microlensing alerts and photometric and spectroscopic followup. We have extracted several dozen microlensing candidates, and we present some preliminary light curves and related information. Similar to the MACHO project, we find SNe behind the LMC to be a significant contaminant - this background has not been completely removed from our current single-color candidate sample. Our follow-up strategy is optimized to discriminate between SNe and true microlensing.Comment: To appear in Proceedings of IAU Symposium 225: Impact of Gravitational Lensing on Cosmology, 6 page

Light and Motion in SDSS Stripe 82: The Catalogues

We present a new public archive of light-motion curves in Sloan Digital Sky Survey (SDSS) Stripe 82, covering 99 deg in right ascension from RA = 20.7 h to 3.3 h and spanning 2.52 deg in declination from Dec = -1.26 to 1.26 deg, for a total sky area of ~249 sq deg. Stripe 82 has been repeatedly monitored in the u, g, r, i and z bands over a seven-year baseline. Objects are cross-matched between runs, taking into account the effects of any proper motion. The resulting catalogue contains almost 4 million light-motion curves of stellar objects and galaxies. The photometry are recalibrated to correct for varying photometric zeropoints, achieving ~20 mmag and ~30 mmag root-mean-square (RMS) accuracy down to 18 mag in the g, r, i and z bands for point sources and extended sources, respectively. The astrometry are recalibrated to correct for inherent systematic errors in the SDSS astrometric solutions, achieving ~32 mas and ~35 mas RMS accuracy down to 18 mag for point sources and extended sources, respectively. For each light-motion curve, 229 photometric and astrometric quantities are derived and stored in a higher-level catalogue. On the photometric side, these include mean exponential and PSF magnitudes along with uncertainties, RMS scatter, chi^2 per degree of freedom, various magnitude distribution percentiles, object type (stellar or galaxy), and eclipse, Stetson and Vidrih variability indices. On the astrometric side, these quantities include mean positions, proper motions as well as their uncertainties and chi^2 per degree of freedom. The here presented light-motion curve catalogue is complete down to r~21.5 and is at present the deepest large-area photometric and astrometric variability catalogue available.Comment: MNRAS accepte

Pushing the Boundaries of Conventional Core-Collapse Supernovae: The Extremely Energetic Supernova SN 2003ma

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 M_R = -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 10^51 ergs, 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 ten times more luminous at all epochs. The fast velocity measured for the intermediate-width H_alpha component (~6000 km/s) points towards an extremely energetic explosion (> 10^52 ergs), 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.Comment: 21 pages, 15 figures, accepted for publication in Ap

23 High Redshift Supernovae from the IfA Deep Survey: Doubling the SN Sample at z>0.7

We present photometric and spectroscopic observations of 23 high redshift supernovae spanning a range of z=0.34-1.03, 9 of which are unambiguously classified as Type Ia. These supernovae were discovered during the IfA Deep Survey, which began in September 2001 and observed a total of 2.5 square degrees to a depth of approximately m=25-26 in RIZ over 9-17 visits, typically every 1-3 weeks for nearly 5 months, with additional observations continuing until April 2002. We give a brief description of the survey motivations, observational strategy, and reduction process. This sample of 23 high-redshift supernovae includes 15 at z>0.7, doubling the published number of objects at these redshifts, and indicates that the evidence for acceleration of the universe is not due to a systematic effect proportional to redshift. In combination with the recent compilation of Tonry et al. (2003), we calculate cosmological parameter density contours which are consistent with the flat universe indicated by the CMB (Spergel et al. 2003). Adopting the constraint that Omega_total = 1.0, we obtain best-fit values of (Omega_m, Omega_Lambda)=(0.33, 0.67) using 22 SNe from this survey augmented by the literature compilation. We show that using the empty-beam model for gravitational lensing does not eliminate the need for Omega_Lambda > 0. Experience from this survey indicates great potential for similar large-scale surveys while also revealing the limitations of performing surveys for z>1 SNe from the ground.Comment: 67 pages, 12 figures, 12 tables, accepted for publication in the Astrophysical Journa

The ESSENCE Supernova Survey: Survey Optimization, Observations, and Supernova Photometry

We describe the implementation and optimization of the ESSENCE supernova survey, which we have undertaken to measure the equation of state parameter of the dark energy. We present a method for optimizing the survey exposure times and cadence to maximize our sensitivity to the dark energy equation of state parameter w=P/rho c^2 for a given fixed amount of telescope time. For our survey on the CTIO 4m telescope, measuring the luminosity distances and redshifts for supernovae at modest redshifts (z~0.5 +- 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 near real-time. Since making cosmological inferences with supernovae relies crucially on accurate measurement of their brightnesses, we describe our efforts to establish a thorough calibration of the CTIO 4m natural photometric system. In its first four years, ESSENCE has discovered and spectroscopically confirmed 102 type Ia SNe, 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 the all type Ia supernovae found by ESSENCE and used in our measurement of w, presented in Wood-Vasey et al, 2007.Comment: Submitted to ApJ. Companion paper to Wood-Vasey et al (2007). Electronic tables available at http://www.ctio.noao.edu/essence/wresult

Probing for cosmological parameters with LAMOST measurement

In this paper we study the sensitivity of the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) project to the determination of cosmological parameters, employing the Monte Carlo Markov Chains (MCMC) method. For comparison, we first analyze the constraints on cosmological parameters from current observational data, including WMAP, SDSS and SN Ia. We then simulate the 3D matter power spectrum data expected from LAMOST, together with the simulated CMB data for PLANCK and the SN Ia from 5-year Supernovae Legacy Survey (SNLS). With the simulated data, we investigate the future improvement on cosmological parameter constraints, emphasizing the role of LAMOST. Our results show the potential of LAMOST in probing for the cosmological parameters, especially in constraining the equation-of-state (EoS) of the dark energy and the neutrino mass.Comment: 7 pages and 3 figures. Replaced with version accepted for publication in JCA

Observational signatures of Jordan-Brans-Dicke theories of gravity

We analyze the Jordan-Brans-Dicke model (JBD) of gravity, where deviations from General Relativity (GR) are described by a scalar field non-minimally coupled to gravity. The theory is characterized by a constant coupling parameter, $\omega_{\rm JBD}$; GR is recovered in the limit $\omega_{\rm JBD} \to \infty$. In such theories, gravity modifications manifest at early times, so that one cannot rely on the usual approach of looking for inconsistencies in the expansion history and perturbations growth in order to discriminate between JBD and GR. However, we show that a similar technique can be successfully applied to early and late times observables instead. Cosmological parameters inferred extrapolating early-time observations to the present will match those recovered from direct late-time observations only if the correct gravity theory is used. We use the primary CMB, as will be seen by the Planck satellite, as the early-time observable; and forthcoming and planned Supernov{\ae}, Baryonic Acoustic Oscillations and Weak Lensing experiments as late-time observables. We find that detection of values of $\omega_{\rm JBD}$ as large as 500 and 1000 is within reach of the upcoming (2010) and next-generation (2020) experiments, respectively.Comment: minor revision, references added, matching version published in JCA