Investigating Anisotropies in the Local Universe with Type Ia Supernovae from the Nearby Supernova Factory

The dipole anisotropy of the cosmic microwave background temperature is two orders of magnitude larger than its other multipole components, which is attributed to the Doppler shift due to the motion of our Local Group of galaxies with a velocity of 627±22 km s-1. While velocities of this amplitude are possible within the standard model of cosmology, ΛDM, gravitational attraction by large mass concentrations is required to produce them. Even though the peculiar velocity field has been studied extensively, the source of this motion has still not been identified with certainty. If no sufficiently large mass concentrations can be found that cause the motion, it may stem from a fundamental anisotropy in universe, which would contradict the cosmological principle. In this context, one must investigate the "dark flow", i.e. the reported coherent motion of galaxy clusters at ∼ 1000 km s-1 on distance scales, at which their velocities are expected to average out. This work presents an analysis of large-scale anisotropy using 279 type Ia supernovae (SNe Ia) at low-redshifts (0.015 The SN Ia dataset was analysed by determining the bulk flow velocity, i.e. the coherent dipole motion, for SNe binned in redshift shells. Furthermore a method of spatially smoothing the Hubble residuals was used to verify the bulk flow results. Additionally, a simple model for the infall towards a potential attractor (e.g. Shapley) was studied to constrain the mass responsible for the coherent peculiar velocities. The measured bulk flow is found to be compatible with the direction of the Local Group motion up to z ∼ 0.06, showing no evidence for a reversal of the dipole direction behind Shapley that would indicate a backside infall into it. At redshifts 0.06 -1 (68% confidence level), ruling out the reported large-amplitude flow. Mass estimates from SN data for an attractor in the proximity of Shapley are found to exceed those of independent studies. Instead, the data favour an additional attractor at greater distance, likely near the Sloan Great Wall. As a final step, the prospects for future surveys, e.g. the Zwicky Transient Facility (ZTF), are simulated. The bulk flow model is expanded by a shear (quadrupole) term that is shown to constrain the location of the main attractor in a less model-dependent way than using a matter overdensity as attractor. The simulation shows that ∼ 1000 additional SNe will be required to measure the shear due to Shapley and the Sloan Great Wall at a significance < 2σ. This number of SNe will become accessible with ZTF

The Zwicky Transient Facility: Surveys and Scheduler

We present a novel algorithm for scheduling the observations of time-domain imaging surveys. Our Integer Linear Programming approach optimizes an observing plan for an entire night by assigning targets to temporal blocks, enabling strict control of the number of exposures obtained per field and minimizing filter changes. A subsequent optimization step minimizes slew times between each observation. Our optimization metric self-consistently weights contributions from time-varying airmass, seeing, and sky brightness to maximize the transient discovery rate. We describe the implementation of this algorithm on the surveys of the Zwicky Transient Facility and present its on-sky performance.Comment: Published in PASP Focus Issue on the Zwicky Transient Facility (https://dx.doi.org/10.1088/1538-3873/ab0c2a). 13 Pages, 11 Figure

The Self-Calibrating Hubble Diagram

As an increasing number of well measured type Ia supernovae (SNe Ia) become available, the statistical uncertainty on w has been reduced to the same size as the systematic uncertainty. The statistical error will decrease further in the near future, and hence the improvement of systematic uncertainties needs to be addressed, if further progress is to be made. We study how uncertainties in the primary reference spectrum - which are a main contribution to the systematic uncertainty budget - affect the measurement of the Dark Energy equation of state parameter w from SNe Ia. The increasing number of SN observations can be used to reduce the uncertainties by including perturbations of the reference spectrum as nuisance parameters in a cosmology fit, thus "self-calibrating" the Hubble diagram. We employ this method to real SNe data for the first time and find the perturbations of the reference spectrum consistent with zero at the 1%-level. For future surveys we estimate that ~3500 SNe will be required for our method to outperform the standard method of deriving the cosmological parameters.Comment: 17 pages, 8 figures, 1 table. Update to revised version accepted for publication in JCA

A New Class of Changing-Look LINERs

We report the discovery of six active galactic nuclei (AGN) caught "turning on" during the first nine months of the Zwicky Transient Facility (ZTF) survey. The host galaxies were classified as LINERs by weak narrow forbidden line emission in their archival SDSS spectra, and detected by ZTF as nuclear transients. In five of the cases, we found via follow-up spectroscopy that they had transformed into broad-line AGN, reminiscent of the changing-look LINER iPTF 16bco. In one case, ZTF18aajupnt/AT2018dyk, follow-up HST UV and ground-based optical spectra revealed the transformation into a narrow-line Seyfert 1 (NLS1) with strong [Fe VII, X, XIV] and He II 4686 coronal lines. Swift monitoring observations of this source reveal bright UV emission that tracks the optical flare, accompanied by a luminous soft X-ray flare that peaks ~60 days later. Spitzer follow-up observations also detect a luminous mid-infrared flare implying a large covering fraction of dust. Archival light curves of the entire sample from CRTS, ATLAS, and ASAS-SN constrain the onset of the optical nuclear flaring from a prolonged quiescent state. Here we present the systematic selection and follow-up of this new class of changing-look LINERs, compare their properties to previously reported changing-look Seyfert galaxies, and conclude that they are a unique class of transients well-suited to test the uncertain physical processes associated with the LINER accretion state.Comment: Submitted to ApJ, 31 pages, 17 Figures (excluding Appendix due to file size constraints but will be available in electronic version

The Zwicky Transient Facility: Surveys and Scheduler

We present a novel algorithm for scheduling the observations of time-domain imaging surveys. Our integer linear programming approach optimizes an observing plan for an entire night by assigning targets to temporal blocks, enabling strict control of the number of exposures obtained per field and minimizing filter changes. A subsequent optimization step minimizes slew times between each observation. Our optimization metric self-consistently weights contributions from time-varying airmass, seeing, and sky brightness to maximize the transient discovery rate. We describe the implementation of this algorithm on the surveys of the Zwicky Transient Facility and present its on-sky performance

ZTF Early Observations of Type Ia Supernovae. I. Properties of the 2018 Sample

Early-time observations of Type Ia supernovae (SNe Ia) are essential to constrain the properties of their progenitors. In this paper, we present high-quality light curves of 127 SNe Ia discovered by the Zwicky Transient Facility (ZTF) in 2018. We describe our method to perform forced point-spread function photometry, which can be applied to other types of extragalactic transients. With a planned cadence of six observations per night (three g + three r), all of the 127 SNe Ia are detected in both g and r bands more than 10 days (in the rest frame) prior to the epoch of g-band maximum light. The redshifts of these objects range from z = 0.0181 to 0.165; the median redshift is 0.074. Among the 127 SNe, 50 are detected at least 14 days prior to maximum light (in the rest frame), with a subset of nine objects being detected more than 17 days before g-band peak. This is the largest sample of young SNe Ia collected to date; it can be used to study the shape and color evolution of the rising light curves in unprecedented detail. We discuss six peculiar events in this sample: one 02cx-like event ZTF18abclfee (SN 2018crl), one Ia-CSM SN ZTF18aaykjei (SN 2018cxk), and four objects with possible super-Chandrasekhar mass progenitors: ZTF18abhpgje (SN 2018eul), ZTF18abdpvnd (SN 2018dvf), ZTF18aawpcel (SN 2018cir), and ZTF18abddmrf (SN 2018dsx). © 2019. The American Astronomical Society