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Register a new userNew Limits on the Low-frequency Radio Transient Sky Using 31 hr of All-sky Data with the OVRO-LWA
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We present the results of the first transient survey from the Owens Valley Radio Observatory Long Wavelength Array (OVRO-LWA) using 31 hr of data, in which we place the most constraining limits on the instantaneous transient surface density at timescales of 13 s to a few minutes and at frequencies below 100 MHz. The OVRO-LWA is a dipole array that images the entire viewable hemisphere with 58 MHz of bandwidth from 27 to 84 MHz at 13 s cadence. No transients are detected above a 6.5$sigma$ flux density limit of 10.5 Jy, implying an upper limit to the transient surface density of $2.5times10^{-8}$ deg$^{-2}$ at the shortest timescales probed, which is orders of magnitude deeper than has been achieved at sub-100 MHz frequencies and comparable flux densities to date. The nondetection of transients in the OVRO-LWA survey, particularly at minutes-long timescales, allows us to place further constraints on the rate of the potential population of transients uncovered by Stewart et al. (2016). From their transient rate, we expect a detection of $8.4^{+31.8}_{-8.0}$ events, and the probability of our null detection is $1.9^{+644}_{-1.9}times10^{-3}$, ruling out a transient rate $>1.4times10^{-4} text{days}^{-1} text{deg}^{-2}$ with 95% confidence at a flux density limit of 18.1 Jy, under the assumption of a flat spectrum and wide bandwidth. We discuss the implications of our nondetection for this population and further constraints that can be made on the source spectral index, intrinsic emission bandwidth, and resulting luminosity distribution.
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We have conducted the most sensitive low frequency (below 100 MHz) search to date for prompt, low-frequency radio emission associated with short-duration gamma-ray bursts (GRBs), using the Owens Valley Radio Observatory Long Wavelength Array (OVRO-LWA). The OVRO-LWAs nearly full-hemisphere field-of-view ($sim20$,$000$ square degrees) allows us to search for low-frequency (sub-$100$ MHz) counterparts for a large sample of the subset of GRB events for which prompt radio emission has been predicted. Following the detection of short GRB 170112A by Swift, we used all-sky OVRO-LWA images spanning one hour prior to and two hours following the GRB event to search for a transient source coincident with the position of GRB 170112A. We detect no transient source, with our most constraining $1sigma$ flux density limit of $650~text{mJy}$ for frequencies spanning $27~text{MHz}-84~text{MHz}$. We place constraints on a number of models predicting prompt, low-frequency radio emission accompanying short GRBs and their potential binary neutron star merger progenitors, and place an upper limit of $L_text{radio}/L_gamma lesssim 7times10^{-16}$ on the fraction of energy released in the prompt radio emission. These observations serve as a pilot effort for a program targeting a wider sample of both short and long GRBs with the OVRO-LWA, including bursts with confirmed redshift measurements which are critical to placing the most constraining limits on prompt radio emission models, as well as a program for the follow-up of gravitational wave compact binary coalescence events detected by advanced LIGO and Virgo.
We report the detection of a potential cosmic radio transient source using the two stations of the Long Wavelength Array. The transient was detected on 18 October 2017 08:47 UTC near the celestial equator while reducing 10,240 hours of archival all-sky images from the LWA1 and LWA-SV stations. The detected transient at 34 MHz has a duration of 15 - 20 seconds and a flux density of 842 +/- 116 Jy at LWA1 and 830 +/- 92 Jy at LWA-SV. The transient source has not repeated, and its nature is not well understood. The Pan-STARRS optical telescope has detected a supernova that occurred on the edge of the position error circle of the transient on the same day.
We report on a search for radio transients at 340 MHz with the Jansky Very Large Array (VLA) Low band Ionospheric and Transient Experiment (VLITE). Between 2015 July 29 and 2015 September 27, operating in commensal mode, VLITE imaged approximately 2800 pointings covering 12,000 deg$^2$ on the sky, sampling timescales ranging from tens of seconds to several hours on a daily basis. In addition, between 2015 February 25 and 2015 May 9, VLITE observed 55 epochs of roughly 2-4 hours each toward the COSMOS field. Using existing radio source catalogs we have searched all the daily VLITE images for transients, while for the COSMOS field we compared individual images and the summed image to search for new sources in repeated observations of the same field. The wide range of timescales makes VLITE sensitive to both coherent and incoherent transient source classes. No new transients are found, allowing us to set stringent upper limits on transients at milliJansky levels and at low frequencies where comparatively few such surveys have been carried out to date. An all-sky isotropic rate of bursting radio transients with similar rates, duration, and intensity as the unusual transient GCRT J1745$-$3009, discovered in wide-field monitoring toward the Galactic center, is ruled out with high confidence. The resulting non-detections allows us to argue that this is a coherent source, whose properties most resemble the growing class of nulling pulsars. We end with a discussion of the future prospects for the detection of transients by VLITE and other experiments.
We describe the parameters of a low-frequency all-sky survey of compact radio sources using Interplanetary Scintillation (IPS), undertaken with the Murchison Widefield Array (MWA). While this survey gives important complementary information to low-resolution survey such as the MWA GLEAM survey, providing information on the subarsecond structure of every source, a survey of this kind has not been attempted in the era of low-frequency imaging arrays such as the MWA and LOFAR. Here we set out the capabilities of such a survey, describing the limitations imposed by the heliocentric observing geometry and by the instrument itself. We demonstrate the potential for IPS measurements at any point on the celestial sphere and we show that at 160MHz, reasonable results can be obtained within 30deg of the ecliptic (2{pi} str: half the sky). We also suggest some observational strategies and describe the first such survey, the MWA Phase I IPS survey. Finally we analyse the potential of the recently-upgraded MWA and discuss the potential of the SKA-low to use IPS to probe sub-mJy flux density levels at sub-arcsecond angular resolution.
We present the first southern-hemisphere all-sky imager and radio-transient monitoring system implemented on two prototype stations of the low-frequency component of the Square Kilometre Array. Since its deployment the system has been used for real-time monitoring of the recorded commissioning data. Additionally, a transient searching algorithm has been executed on the resulting all-sky images. It uses a difference imaging technique, and has enabled identification of a wide variety of transient classes, ranging from human-made radio-frequency interference to genuine astrophysical events. Observations at the frequency 159.4 MHz and higher in a single coarse channel (0.926 MHz) were made with 2s time resolution, and multiple nights were analysed. Despite having modest sensitivity (~few Jy/beam), using a single coarse channel and 2-s imaging, the system detected bright transients from PSR B0950+08, proving that it can be used to detect bright transients of an astrophysical origin. The unusual, extreme activity of the pulsar PSR B0950+08 (up to ~155 Jy/beam) was initially detected in a blind search in the 2020-04-10/11 data and later assigned to this specific pulsar. The limitations of our data, however, prevent use from making firm conclusions of the effect being due to a combination of refractive and diffractive scintillation or intrinsic emission mechanisms. The system can routinely collect data over many days without interruptions; the large amount of recorded data at 159.4 and 229.7 MHz allowed us to determine a preliminary transient surface density upper limit of $1.32 times 10^{-9} text{deg}^{-2}$ for a timescale and limiting flux density of 2s and 42 Jy, respectively. We plan to extend the observing bandwidth to tens of MHz and improve time resolution to tens of milliseconds in order to increase the sensitivity and enable detections of Fast Radio Bursts below 300 MHz.
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