Source-linked AI summary
First Results from the Catalina Real-time Transient Survey
A. J. Drake, S. G. Djorgovski, A. Mahabal, E. Beshore, S. Larson, M. J. Graham, R. Williams, E. Christensen, M. Catelan, A. Boattini, A. Gibbs, R. Hill, R. Kowalski
TL;DR
CRTS addresses the need to find and characterize high-amplitude optical transients across timescales from minutes to years. It analyzes repeatedly observed Catalina Sky Survey data and distributes detections publicly in real time, reporting a diverse transient population and evidence for supernovae associated with faint hosts. The survey also identifies selection and cadence boundaries that constrain which objects it can detect.
Problem
CRTS seeks to discover and characterize high-amplitude variable populations across the timescales probed by Catalina Sky Survey data.
Method
CRTS analyzes Catalina Sky Survey observations, identifies objects varying by more than two magnitudes, and distributes transient data publicly through real-time alerts.
Results
Over 350 optical transients passed visual selection in the first six months, including cataclysmic variables, supernovae, flaring events, Blazars, and high-proper-motion stars.
Takeaways & Limitations
CRTS results suggest that many supernovae missed by current surveys are associated with faint host galaxies and that bright CRTS outbursts are suitable for small-telescope follow-up.
Takeaways & Limitations
Selecting transients more than two magnitudes brighter than associated sources biases supernova discoveries toward faint galaxies and misses some intermediate-host events without image subtraction.
Abstract
from arXiv · showhide
We report on the results from the first six months of the Catalina Real-time Transient Survey (CRTS). In order to search for optical transients with timescales of minutes to years, the CRTS analyses data from the Catalina Sky Survey which repeatedly covers twenty six thousand of square degrees on the sky. The CRTS provides a public stream of transients that are bright enough to be followed up using small telescopes. Since the beginning of the survey, all CRTS transients have been made available to astronomers around the world in real-time using HTML tables, RSS feeds and VOEvents. As part of our public outreach program the detections are now also available in KML through Google Sky. The initial discoveries include over 350 unique optical transients rising more than two magnitudes from past measurements. Sixty two of these are classified as supernovae, based on light curves, prior deep imaging and spectroscopic data. Seventy seven are due to cataclysmic variables (only 13 previously known), while an additional 100 transients were too infrequently sampled to distinguish between faint CVs and SNe. The remaining optical transients include AGN, Blazars, high proper motions stars, highly variable stars (such as UV Ceti stars) and transients of an unknown nature. Our results suggest that there is a large population of SNe missed by many current supernova surveys because of selection biases. These objects appear to be associated with faint host galaxies. We also discuss the unexpected discovery of white dwarf binary systems through dramatic eclipses.
1. INTRODUCTION
CRTS occupies an intermediate position between targeted or small-area variability surveys and future wide, deep synoptic surveys. It combines broad transient searches with real-time public distribution to support follow-up and future transient-rate estimates.
- Survey landscape: Past surveys typically covered tens to hundreds of square degrees or targeted specific transient classes, while deep variability surveys covered less than 25 deg2.Large-area surveys such as SDSS, 2MASS, and GALEX were generally not synoptic.
- Survey landscape: CRTS and Palomar-Quest search tens of thousands of square degrees between earlier targeted surveys and future wide, deep transient surveys.Palomar-Quest began real-time optical-transient analysis in August 2006, while CRTS began in November 2007.
- CRTS role: CRTS uses a purpose-built pipeline for real-time transient detection, analysis, and distribution.The surveys aim to estimate the rates and types of optical transients expected from future synoptic surveys.
- CRTS role: CRTS and related experiments make results public within minutes through VOEvent and VOEventNet, including a dedicated real-time layer in Google Sky.This distribution includes transients from CRTS, Palomar-Quest, microlensing surveys, and gamma-ray-burst alerts.
- Paper scope: The paper reports the first six months of CRTS optical-transient searches using Catalina Sky Survey Schmidt Telescope data.It presents observations and data analysis, current results, transient types, and a summary of findings.
2. OBSERVATIONS AND DATA REDUCTION
CRTS processes Catalina Sky Survey observations with catalog-based matching, artifact and moving-object filtering, and rapid VOEvent distribution. Its design supports searches for transients from minute-scale variability while reducing asteroid and artifact contamination.
- Observations: The Catalina Schmidt Telescope covers approximately 1200 square degrees per clear night using sequences of four 30-second exposures.Its 0.7m telescope uses a single unfiltered 4k x 4k CCD with an eight-square-degree field of view.
- Transient detection: CRTS defines optical transients as objects varying by more than two magnitudes relative to past high-signal-to-noise co-added-image catalogs.The characterization process covers known variable-object populations and searches for new transient types.
- Transient detection: New detections are compared with deep clean source catalogs and higher-resolution catalogs rather than relying only on prior observations.This reduces unmatched detections caused by image artifacts and varying image depth.
- Transient detection: Catalog-based processing avoids some image-subtraction problems, including atmospheric-differential-refraction residuals and artifacts from poor observing conditions.Image subtraction remains effective for transients blended with constant sources in dense stellar fields.
- Moving objects and short timescales: CSS image sequences help identify asteroids through motion while also enabling searches for objects varying on minute timescales.Positional coincidence between images aids removal of image artifacts.
- Real-time distribution: The pipeline filters artifact and moving-object detections, then posts transient cutouts, light curves, and metadata to VOEventNet.VOEvent alerts are intended to enable rapid automated follow-up by robotic telescopes.
- Real-time distribution: Approximately 1 source in 200,000 becomes a significant optical-transient candidate after filtering, and approximately 50% of candidates are genuine optical transients.All candidates are sent as VOEvents before remaining candidates are screened and the transient list is updated.
3. RESULTS
CRTS uncovered a diverse population of optical transients, including supernovae associated with very faint or undetected hosts, cataclysmic variables, stellar flares, and eclipsing white-dwarf binaries. Its discoveries also reveal selection effects in supernova searches and provide targets relevant to distance-scale calibration.
- Over 350 optical transients were found after searching approximately 450,000 square degrees from about 2,000 fields during CRTS’s first six months.The sample included cataclysmic variables, flares, high-proper-motion stars, blazars, asteroids, comets, and supernovae.
- Supernovae with Faint Galaxy Hosts?: Spectroscopy confirmed type Ia and type II supernovae at z ∼0.03 whose host galaxies were intrinsically very faint.The host limits were Mr ∼−13.9 for SN 2008ba and Mr > −12.9 for SN 2008bb.
- Supernovae with Faint Galaxy Hosts?: CRTS discovered dozens of long-timescale transients associated with faint galaxies or lacking detectable sources in CSS, Palomar Quest, and SDSS images.Seventeen had no visible source to approximately magnitude 21–23, while 24 matched galaxies more than two magnitudes fainter than their transients.
- Supernovae with Faint Galaxy Hosts?: CRTS selection favors supernovae more than two magnitudes above faint hosts, while intermediate-brightness hosts may require image subtraction for detection.This selection differs from surveys that preferentially follow supernovae in bright galaxies.
- Supernovae: A CRTS supernova in the Antennae can support independent calibration of the cosmological distance scale because its host has a Cepheid distance.SN 2007sr has well-sampled light curves and occurred in a galaxy at 22 ± 3 Mpc.
- Eclipses: Two white-dwarf–M-dwarf binaries exhibited eclipses deeper than 1.7 magnitudes, including one newly discovered system.Archival CSS data gave the known system a period of 3.58652 hours; its light curve and Hα emission suggest a pre-cataclysmic-variable system.
4. SUMMARY AND DISCUSSION
CRTS’s first six months revealed a transient population dominated by dwarf novae and supernovae, while survey design limited sensitivity to several other variable-star classes. The survey emphasizes rapid public dissemination and follow-up strategies needed to classify the broader transient sample.
- Results: Dwarf novae and supernovae dominated the first six months of CRTS discoveries.Miras were among the few other highly variable stars detected.
- Selection effects: A 2-magnitude detection threshold, co-added-image catalogs, and observing only fields with |b| > 10° constrained discovery of many highly variable stars.Co-added images can average out cyclic variability on short timescales, while the latitude cut limits the number of observed stars.
- Follow-up and classification: Spectroscopic follow-up classified only a small number of candidates, leaving photometric follow-up and probabilistic classification as important next steps.The authors also describe machine-learning methods intended to optimize discovery and enable unsupervised rapid follow-up.
- Public dissemination: CRTS made discoveries and associated metadata public quickly, while continuing announcements through astronomical networks and expanding searches with two additional CSS telescopes.The added telescopes were expected to find similar numbers of transients in the Southern sky and fainter transients in the North.
- Follow-up and classification: Broad characterization of transients requires follow-up observations, which are easiest when events are accessible to the astronomical community and bright enough for small telescopes.