[Paper Review] Exploring the Time Domain with the Palomar-QUEST Sky Survey

A. Mahabal, S. G. Djorgovski+17 more|arXiv (Cornell University)|Aug 3, 2004

Astronomy and Astrophysical Research3 citations

TL;DR

This paper introduces the Palomar-QUEST (PQ) sky survey, a wide-field, synoptic optical survey using a 162-megapixel camera on the Palomar 48-inch Schmidt telescope to detect variable and transient sources in real time. It demonstrates that the survey can detect up to tens of highly variable or transient objects per night, with a focus on real-time alerts using machine learning and NVO-compliant pipelines, enabling rapid spectroscopic follow-up of rare or bright optical transients.

ABSTRACT

Exploration of the time variability on the sky over a broad range of flux levels and wavelengths is rapidly becoming a new frontier of astronomical research. We describe here briefly the Palomar-QUEST survey being carried out from the Samuel Oschin 48-inch Schmidt telescope at Palomar. The following features make the survey an attractive candidate for studying time variability: anticipated survey area of 12,000 - 15,000 sq. degrees in the drift scan mode, point source depth of 21st mag. in I under good conditions, near simultaneous observations in four filters, and at least four passes per year at each location covered. The survey will yield a large number of transients and highly variable sources in the near future and in that sense is a prototype of LSST and Pan-STARRS. We briefly outline our strategy for searching such objects and the proposed pipeline for detecting transients in real-time.

Motivation & Objective

• To develop a real-time, wide-area synoptic survey for detecting optical transients and variable sources in the time domain.
• To enable rapid detection and alerting of transient phenomena, including rapidly fading sources, within minutes of observation.
• To reduce false positives and contamination in transient detection using statistical and machine learning techniques.
• To serve as a technological and scientific precursor for future large-scale surveys like LSST and Pan-STARRS.
• To characterize the faint variable sky, including AGN, flaring M-dwarfs, CVs, and unknown transients.

Proposed method

• The survey uses a 112-CCD, 162-megapixel camera on the Palomar 48-inch Schmidt telescope in Point-and-Stare and Drift Scan modes.
• Data are collected in near-simultaneous UBRI and r'i'z'z' filter sets, with typical limiting magnitudes of R ≈ 22 mag and I ≈ 21 mag for point sources.
• The pipeline processes data within hours, with a real-time component under development to enable sub-minute detection of transients.
• Real-time alerts are generated by comparing nightly catalogs with historical data and external archives using NVO-compliant protocols.
• Statistical and machine learning techniques are applied to filter spurious or uninteresting candidates and maintain high completeness.
• Spectroscopic follow-up is prioritized for high-confidence transient candidates at the Palomar 200-inch telescope.

Experimental results

Research questions

• RQ1 What is the rate and diversity of highly variable and transient sources detectable in a wide-field, deep, synoptic survey at optical wavelengths?
• RQ2 How effective can real-time data processing pipelines be in detecting rare, rapidly fading transients with low false alarm rates?
• RQ3 What fraction of variable sources at r ≈ 19–22 mag are associated with AGN, flaring stars, or other known classes?
• RQ4 How do the properties of optical transients detected in single-epoch surveys compare to known transient classes?
• RQ5 To what extent can a pilot survey like DPOSS inform the design and sensitivity of future large-scale time-domain surveys?

Key findings

• The survey detects up to several tens of highly variable or transient sources per night, based on extrapolation from the DPOSS pilot study.
• Approximately one-third to one-half of all highly variable objects down to r ≈ 19 mag at moderate to high Galactic latitudes are associated with AGN.
• The faint variable sky exhibits a rich and diverse phenomenology, including flaring M-dwarfs, OVV QSOs, BL Lacs, and rare magnetic cataclysmic variables.
• A single-epoch survey down to r ≈ 21 mag can contain up to ~1000 transients per square degree, indicating a high transient density.
• Deep follow-up imaging of two optical transients revealed faint host galaxies, suggesting a possible association with distant or obscured sources.
• The PQ survey is expected to achieve a limiting magnitude of r ≈ 22 mag, with multiple passes per year and time baselines extending to multi-year scales.