Northern Sky Variability Survey (nsvs): Public Data Release

The Northern Sky Variability Survey (NSVS) is a temporal record of the sky over the optical magnitude range from 8 to 15.5. It was conducted in the course of the first generation Robotic Optical Transient Search Experiment (ROTSE-I) using a robotic system of four co-mounted unfiltered telephoto lenses equipped with CCD cameras. The survey was conducted from Los Alamos, NM, and primarily covers the entire northern sky. Some data in southern fields between declinations 0 • and −38 • is also available, although with fewer epochs and noticeably lesser quality. The NSVS contains light curves for approximately 14 million objects. With a one year baseline and typically 100–500 measurements per object, the NSVS is the most extensive record of stellar variability across the bright sky available today. In a median field, bright unsaturated stars attain a point to point photometric scatter of ∼0.02 mag and position errors within 2 ′′. At Galactic latitudes |b| < 20 • the data quality is limited by severe blending due to ∼14 ′′ pixel size. We present basic characteristics of the data set and describe data collection, analysis, and distribution. All NSVS photometric measurements are available for on-line public access from the Sky Database for Objects in Time-Domain (SkyDOT; http://skydot.lanl.gov) at LANL. Copies of the full survey photometry may also be requested on tape. An amazing fact about modern astronomy is that the global time variability of the optical sky is largely un-explored for objects fainter than those observable with the naked eye (Paczy´nski 1997). As a result, the existing samples of known variables are quite incomplete. The commonly accepted standard catalog of variable stars is the General Catalog of Variable Stars (GCVS; Kholopov 1998). However, the GCVS catalog was compiled from a multitude of heterogeneous observational sources—in many cases based on analysis of photographic plates— and does not present any light curves. Spatial coverage in the GCVS is strikingly patchy at magnitude 10 and fainter. With the data currently available for most stars, we cannot answer the simple question of whether or not the star is variable. This is unfortunate because temporal flux changes not only carry useful physical information about the star but they are of concern for experiments where variability could degrade the quality of the comparison star grid (e.g. Space Interferometry Mission, Frink et al. 2001). The reasonably sensitive, inexpensive CCDs that have become available in the last few years, …