THE CALTECH-NRAO STRIPE 82 SURVEY (CNSS) PAPER. I. THE PILOT RADIO TRANSIENT SURVEY IN 50 DEG²

Abstract

We have commenced a multi-year program, the Caltech-NRAO Stripe 82 Survey (CNSS), to search for radio transients with the Jansky VLA in the Sloan Digital Sky Survey Stripe 82 region. The CNSS will deliver five epochs over the entire ∼270 deg 2 of Stripe 82, an eventual deep combined map with a rms noise of ∼40 µJy and catalogs at a frequency of 3 GHz, and having a spatial resolution of 3. This first paper presents the results from an initial pilot survey of a 50 deg 2 region of Stripe 82, involving four epochs spanning logarithmic timescales between one week and 1.5 years, with the combined map having a median rms noise of 35 µJy. This pilot survey enabled the development of the hardware and software for rapid data processing, as well as transient detection and follow-up, necessary for the full 270 deg 2 survey. Data editing, calibration, imaging, source extraction, cataloging and transient identification were completed in a semi-automated fashion within six hours of completion of each epoch of observations, using dedicated computational hardware at the NRAO in Socorro, and custom-developed data reduction and transient detection pipelines. Classification of variable and transient sources relied heavily on the wealth of multi-wavelength legacy survey data in the Stripe 82 region, supplemented by repeated mapping of the region by the Palomar Transient Factory (PTF). 3.9 +0.5 −0.9 % of the few thousand detected point sources were found to vary by greater than 30%, consistent with similar studies at 1.4 GHz and 5 GHz. Multi-wavelength photometric data and light curves suggest that the variability is mostly due to shock-induced flaring in the jets of AGN. Although this was only a pilot survey, we detected two bona fide transients, associated with an RS CVn binary and a dKe star. Comparison with existing legacy survey data (FIRST, VLA-Stripe 82) revealed additional highly variable and transient sources on timescales between 5-20 years, largely associated with renewed AGN activity. The rates of such AGN possibly imply episodes of enhanced accretion and jet activity occurring once every ∼40,000 years in these galaxies. We compile the revised radio transient rates and make recommendations for future transient surveys and joint radio-optical experiments.

Radio Counterparts of Compact Binary Mergers Detectable in Gravitational Waves: A Simulation for an Optimized Survey

“…While many of them have not detected any radio transients, Bannister et al (2011) have found 15 in 3000 deg 2 at 10 mJy. Mooley et al (2016) also have found a few radio transients in 50 deg 2 at 0.2 mJy. Two of them are arising from Galactic flaring stars and the others likely from variable AGNs.…”

“…We estimate the areal densities of radio transients based on the known radio bright astrophysical phenomena(see also Metzger et al 2015b;Mooley et al 2016). Table 5 summarizes relevant values of the radio transients used for the estimates of false positives.…”

“…If the variability of these sources are large enough(30%) on timescales between days and a few years, they will be detected as false positives. According to radio variable studies(e.g., Thyagarajan et al 2011;Mooley et al 2013Mooley et al , 2016, the population of radio variables on these timescales with flux densities at 1.4 GHz between 0.3 mJy and 100 mJy is about 1% or less of the total persistent radio sources and these variables are mainly AGNs. The areal density of persistent radio point sources with flux densities larger than 0.1 mJy is~-1000 deg 2 (Huynh et al 2005) and roughly half of them are AGNs, therefore hundreds to thousands of radio variables are expected to be in GW localization areas.…”

“…In fact, radio afterglows have been already detected for four sGRBs(e.g., Fong et al 2015) and they will be detectable counterparts of GW merger events (Feng et al 2014). Recent observations suggest that the radio transient sky is far quieter compared to the optical one, so we expect far fewer false positives by a factor of a hundred or more depending on the sky location than in the optical-IR (e.g., Frail et al 2012;Mooley et al 2013Mooley et al , 2016. This paper focuses on strategies for detecting and identifying, following the detection of a GW event, two different radio post-merger counterparts: (i) the longer duration radio remnants that may last from months to years and (ii) the faster ultrarelativistic radio afterglow components that last weeks to months (Nakar & Piran 2011;Piran et al 2013;Margalit & Piran 2015).…”

The MASIV Survey – IV. Relationship between intra-day scintillation and intrinsic variability of radio AGNs

“…While single-field studies are improving our understanding of the sky density of variable sources and its dependence on the flux-limits of these surveys (e.g., Mooley et al 2016), there is only a cursory understanding of the origin of the observed AGN variability in these studies. In the meantime, new surveys have been planned for upcoming radio telescopes, including the Variable and Slow Transients (VAST) Survey (Murphy et al 2013) on the Australia Square Kilometre Array Pathfinder (ASKAP), and eventually the Square Kilometre Array (Bignall et al 2015).…”