Asgard: A Large Survey for Slow Galactic Radio Transients. I. Overview and First Results

Williams, Peter K. G.; Bower, Geoffrey C.; Croft, Steve; Keating, Garrett K.; Law, Casey J.; Wright, M. C. H.

doi:10.1088/0004-637x/762/2/85

Cited by 22 publications

(23 citation statements)

References 133 publications

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“…Among the Ofek et al radio sources, only 0.3% have modulation indices greater than 0.2 on timescales less than two years. The variability fraction that we find on week-month timescales in our pilot survey is similar ( 1%), and agrees with the variability fraction found in narrow-deep surveys by Frail et al (1994), Carilli et al (2003), and Mooley et al (2013), wide-field surveys such as Williams et al (2013), and other studies between 1-5 GHz on week-month-year timescales. On 1.5 year timescale we find the variability fraction to be less than 4%.…”

Section: Comparison Of Variability With Previous Surveyssupporting

confidence: 90%

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

Mooley

Hallinan

Bourke

et al. 2016

ApJ

133

213

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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.

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Section: Comparison Of Variability With Previous Surveyssupporting

confidence: 90%

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

Mooley

Hallinan

Bourke

et al. 2016

ApJ

133

213

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“…This data point is not displayed in Figure 6 and Figure 5. c The transient surface density for this survey is obtained from the calculation performed by Williams et al (2013) which takes into account results from Hyman et al (2002Hyman et al ( , 2006Hyman et al ( , 2009).…”

Section: Discussionmentioning

confidence: 99%

New methods to constrain the radio transient rate: results from a survey of four fields with LOFAR

Carbone

Horst

Wijers

et al. 2016

Mon. Not. R. Astron. Soc.

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We report on the results of a search for radio transients between 115 and 190 MHz with the LOw-Frequency ARray (LOFAR). Four fields have been monitored with cadences between 15 min and several months. A total of 151 images were obtained, giving a total survey area of 2275 deg. We analysed our data using standard LOFAR tools and searched for radio transients using the LOFAR Transients Pipeline. No credible radio transient candidate has been detected; however, we are able to set upper limits on the surface density of radio transient sources at low radio frequencies. We also show that low-frequency radio surveys are more sensitive to steep-spectrum coherent transient sources than GHz radio surveys. We used two new statistical methods to determine the upper limits on the transient surface density. One is free of assumptions on the flux distribution of the sources, while the other assumes a power-law distribution in flux and sets more stringent constraints on the transient surface density. Both of these methods provide better constraints than the approach used in previous works. The best value for the upper limit we can set for the transient surface density, using the method assuming a power-law flux distribution, is 1.3 × 10 deg for transients brighter than 0.3 Jy with a time-scale of 15 min, at a frequency of 150 MHz. We also calculated for the first time upper limits for the transient surface density for transients of different time-scales. We find that the results can differ by orders of magnitude from previously reported, simplified estimates.

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“…Stellar flares from active M dwarfs and close binaries are expected to be among the most common types of radio transients at decimetric wavelengths (Osten 2008;Williams et al 2013;Mooley et al 2016). This expectation applies to ongoing and upcoming transient surveys by telescopes including ASKAP, MeerKAT, and the VLA.…”

Section: Implications For Transient Surveysmentioning

confidence: 99%

“…The right panel of Figure 6 shows our predicted transient areal density as a function of frequency for the frequency bands of these three surveys. The figure also shows 95% confidence upper limits from a selection of published surveys at frequencies from 0.3-4 GHz (Williams et al 2013;Mooley et al 2016;Polisensky et al 2016;Bhandari et al 2018), and predicted upper limits for VAST-Wide and VLASS single epochs, assuming no detections were made. Another relevant transient search, Thyagarajan et al (2011), used data from the VLA FIRST survey at 1.4 GHz to search for transient/variable sources, detecting 5 radio-variable stars, but we do not plot that detection since they do not distinguish between types of stars.…”

Section: Application To Current Transient Surveysmentioning

confidence: 99%

Ultra-wideband Detection of 22 Coherent Radio Bursts on M Dwarfs

Villadsen

Hallinan

2019

ApJ

106

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Coherent radio bursts detected from M dwarfs have some analogy with solar radio bursts, but reach orders of magnitude higher luminosities. These events trace particle acceleration, powered by magnetic reconnection, shock fronts (such as formed by coronal mass ejections, CMEs), and magnetospheric currents, in some cases offering the only window into these processes in stellar atmospheres. We conducted a 58-hour, ultra-wideband survey for coherent radio bursts on 5 active M dwarfs. We used the Karl G. Jansky Very Large Array (VLA) to observe simultaneously in three frequency bands covering a subset of 224-482 MHz and 1-6 GHz, achieving the widest fractional bandwidth to date for any observations of stellar radio bursts. We detected 22 bursts across 13 epochs, providing the first large sample of wideband dynamic spectra of stellar coherent radio bursts. The observed bursts have diverse morphology, with durations ranging from seconds to hours, but all share strong (40-100%) circular polarization. No events resemble solar Type II bursts (often associated with CMEs), but we cannot rule out the occurrence of radio-quiet stellar CMEs. The hours-long bursts are all polarized in the sense of the x-mode of the star's large-scale magnetic field, suggesting they are cyclotron maser emission from electrons accelerated in the large-scale field, analogous to auroral processes on ultracool dwarfs. The duty cycle of luminous coherent bursts peaks at 25% at 1-1.4 GHz, declining at lower and higher frequencies, indicating source regions in the low corona. At these frequencies, active M dwarfs should be the most common galactic transient source.

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Asgard: A Large Survey for Slow Galactic Radio Transients. I. Overview and First Results

Cited by 22 publications

References 133 publications

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

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

New methods to constrain the radio transient rate: results from a survey of four fields with LOFAR

Ultra-wideband Detection of 22 Coherent Radio Bursts on M Dwarfs

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Asgard: A Large Survey for Slow Galactic Radio Transients. I. Overview and First Results

Cited by 22 publications

References 133 publications

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

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

New methods to constrain the radio transient rate: results from a survey of four fields with LOFAR

Ultra-wideband Detection of 22 Coherent Radio Bursts on M Dwarfs

Contact Info

Product

Resources

About

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

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