Long-term study of extreme giant pulses from PSR B0950+08 with AARTFAAC

We report on the detection of extreme giant pulses (GPs) from one of the oldest-known pulsars, the highly variable PSR B0950+08, with the Amsterdam-ASTRON Radio Transient Facility And Analysis Centre (AARTFAAC), a parallel transient detection instrument operating as a subsystem of the LOw Frequency ARray (LOFAR). During processing of our Northern Hemisphere survey for low-frequency radio transients, a sample of 275 pulses with fluences ranging from 42 to 177 kJy ms were detected in one-second snapshot images. The brightest pulses are an order of magnitude brighter than those previously reported at 42 and 74 MHz, on par with the levels observed in a previous long-term study at 103 MHz. Both their rate and fluence distribution differ between and within the various studies done to date. The GP rate is highly variable, from 0 to 30 per hour, with only two three-hour observations accounting for nearly half of the pulses detected in the 96 h surveyed. It does not vary significantly within a few-hour observation, but can vary strongly one from day to the next. The spectra appear strongly and variably structured, with emission sometimes confined to a single 195.3 kHz subband, and the pulse spectra changing on a timescale of order 10 min.Comment: 10 pages, 9 figures, 1 table. Submitted to MNRA

LOFAR early-time search for coherent radio emission from GRB 180706A

© 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society.The nature of the central engines of gamma-ray bursts (GRBs) and the composition of their relativistic jets are still under debate. If the jets are Poynting flux dominated rather than baryon dominated, a coherent radio flare from magnetic re-connection events might be expected with the prompt gamma-ray emission. There are two competing models for the central engines of GRBs; a black hole or a newly formed milli-second magnetar. If the central engine is a magnetar it is predicted to produce coherent radio emission as persistent or flaring activity. In this paper, we present the deepest limits to date for this emission following LOFAR rapid response observations of GRB 180706A. No emission is detected to a 3$\sigma$ limit of 1.7 mJy beam$^{-1}$ at 144 MHz in a two-hour LOFAR observation starting 4.5 minutes after the gamma-ray trigger. A forced source extraction at the position of GRB 180706A provides a marginally positive (1 sigma) peak flux density of $1.1 \pm 0.9$ mJy. The data were time-sliced into different sets of snapshot durations to search for FRB like emission. No short duration emission was detected at the location of the GRB. We compare these results to theoretical models and discuss the implications of a non-detection.Peer reviewedFinal Accepted Versio

The AARTFAAC All-Sky Monitor: System Design and Implementation

The Amsterdam-ASTRON Radio Transients Facility And Analysis Center (AARTFAAC) all sky monitor is a sensitive, real time transient detector based on the Low Frequency Array (LOFAR). It generates images of the low frequency radio sky with spatial resolution of 10s of arcmin, MHz bandwidths, and a time cadence of a few seconds, while simultaneously but independently observing with LOFAR. The image timeseries is then monitored for short and bright radio transients. On detection of a transient, a low latency trigger will be generated for LOFAR, which can interrupt its schedule to carry out follow-up observations of the trigger location at high sensitivity and resolutions. In this paper, we describe our heterogeneous, hierarchical design to manage the 240 Gbps raw data rate, and large scale computing to produce real-time images with minimum latency. We discuss the implementation of the instrumentation, its performance, and scalability.Comment: Submitted to Journal of Astronomical Instrumentation, Special issue on 'Digital Signal Processing (DSP) in Radio Astronomy

The AARTFAAC Cosmic Explorer:Observations of the 21-cm power spectrum in the EDGES absorption trough

The 21-cm absorption feature reported by the EDGES collaboration is several times stronger than that predicted by traditional astrophysical models. If genuine, a deeper absorption may lead to stronger fluctuations on the 21-cm signal on degree scales (up to 1~Kelvin in rms), allowing these fluctuations to be detectable in nearly 50~times shorter integration times compared to previous predictions. We commenced the "AARTFAAC Cosmic Explorer" (ACE) program, that employs the AARTFAAC wide-field imager, to measure or set limits on the power spectrum of the 21-cm fluctuations in the redshift range $z = 17.9-18.6$ ($\Delta\nu = 72.36-75.09$~MHz) corresponding to the deep part of the EDGES absorption feature. Here, we present first results from two LST bins: 23.5-23.75h and 23.5-23.75h, each with 2~h of data, recorded in `semi drift-scan' mode. We demonstrate the application of the new ACE data-processing pipeline (adapted from the LOFAR-EoR pipeline) on the AARTFAAC data. We observe that noise estimates from the channel and time-differenced Stokes~$V$ visibilities agree with each other. After 2~h of integration and subtraction of bright foregrounds, we obtain $2\sigma$ upper limits on the 21-cm power spectrum of $\Delta_{21}^2 < (8139~\textrm{mK})^2$ and $\Delta_{21}^2 < (8549~\textrm{mK})^2$ at $k = 0.144~h\,\textrm{cMpc}^{-1}$ for the two LST bins. Incoherently averaging the noise bias-corrected power spectra for the two LST bins yields an upper limit of $\Delta_{21}^2 < (7388~\textrm{mK})^2$ at $k = 0.144~h\,\textrm{cMpc}^{-1}$. These are the deepest upper limits thus far at these redshifts.Comment: 16 pages, 10 figures, accepted for publication in MNRA

The AARTFAAC All-Sky Monitor:System Design and Implementation

The Amsterdam–ASTRON Radio Transients Facility and Analysis Center (AARTFAAC) all-sky monitor is a sensitive, real-time transient detector based on the Low Frequency Array (LOFAR). It generates images of the low frequency radio sky with spatial resolution of tens of arcmin, MHz bandwidths, and a time cadence of a few seconds, while simultaneously but independently observing with LOFAR. The image timeseries is then monitored for short and bright radio transients. On detection of a transient, a low latency trigger will be generated for LOFAR, which can interrupt its schedule to carry out follow-up observations of the trigger location at high sensitivity and resolutions. In this paper, we describe our heterogeneous, hierarchical design to manage the 259Gbps raw data rate and large scale computing to produce real-time images with minimum latency. We discuss the implementation of the instrumentation, its performance and scalability

Degree-scale galactic radio emission at 122 MHz around the North Celestial Pole with LOFAR-AARTFAAC

International audience Aims: Contamination from bright diffuse Galactic thermal and non-thermal radio emission poses crucial challenges in experiments aiming to measure the 21-cm signal of neutral hydrogen from the cosmic dawn (CD) and Epoch of Reionisation (EoR). If not included in calibration, this diffuse emission can severely impact the analysis and signal extraction in 21-cm experiments. We examine large-scale diffuse Galactic emission at 122 MHz around the North Celestial Pole, using the Amsterdam-ASTRON Radio Transient Facility and Analysis Centre (AARTFAAC-) High Band Antenna (HBA) system. Methods: In this pilot project, we present the first-ever wide-field image produced with a single sub-band of the data recorded with the AARTFAAC-HBA system. We demonstrate two methods, multi-scale CLEAN and shapelet decomposition, to model the diffuse emission revealed in the image. We used angular power spectrum metrics to quantify different components of the emission and compared the performance of the two diffuse structure modelling approaches. Results: We observed that the point sources dominate the angular power spectrum (ℓ(ℓ + 1)Cℓ/2π≡Δ2(ℓ)) of the emission in the field on scales of ℓ ≳ 60 (≲3 degree). The angular power spectrum after subtraction of compact sources is flat within the 20 ≲ ℓ ≲ 200 range, suggesting that the residual power is dominated by the diffuse emission on scales of ℓ ≲ 200. The residual diffuse emission has a brightness temperature variance of Δℓ=1802 = (145.64 ± 13.61) K2 at 122 MHz on angular scales of 1 degree, and it is consistent with a power law following Cℓ ∝ ℓ−2.0 in the 20 ≲ ℓ ≲ 200 range. We also find that, in the current set-up, multi-scale CLEAN is suitable to model the compact and diffuse structures on a wide range of angular scales, whereas the shapelet decomposition method better models the large scales, which are of the order of a few degrees and wider