Analysing the impact of far-out sidelobes on the imaging performance of the SKA-LOW telescope

The Square Kilometre Array’s Low Frequency instrument (SKA-LOW) will operate in the undersampled regime for most of the frequency band where grating lobes pose particular challenges. To achieve the expected level of sensitivity for SKA-LOW, it is particularly important to understand how interfering sources in both near and far side-lobes of the station beam affect the imaging performance. In this study, we discuss options for station designs, and adopting a random element layout, we assess its effectiveness by investigating how sources far from the main lobe of the station beam degrade images of the target field. These sources have the effect of introducing a noise-like corruption to images, which is called the far sidelobe confusion noise (FSCN). Using $\tiny{OSKAR}$, a software simulator accelerated using graphics processing units, we carried out end-to-end simulations using an all-sky model and telescope configuration representative of the SKA-LOW instrument. The FSCN is a function of both the station beam and the interferometric point spread function, and decreases with increasing observation time until the coverage of the aperture plane no longer improves. Using apodization to reduce the level of near-in sidelobes of the station beam had a notable improvement on the level of the FSCN at low frequencies. Our results indicate that the effects of picking up sources in the sidelobes are worse at low frequencies, where the array is less sparse.This work used the Wilkes GPU cluster at the University of Cambridge High Performance Computing Service (http://www.hpc.cam.ac.uk/), provided by Dell Inc., NVIDIA and Mellanox, and part funded by STFC with industrial sponsorship from Rolls Royce and Mitsubishi Heavy Industries

AMI-LA Observations of the SuperCLASS Super-cluster

We present a deep survey of the SuperCLASS super-cluster - a region of sky known to contain five Abell clusters at redshift $z\sim0.2$ - performed using the Arcminute Microkelvin Imager (AMI) Large Array (LA) at 15.5$~$GHz. Our survey covers an area of approximately 0.9 square degrees. We achieve a nominal sensitivity of $32.0~\mu$Jy beam$^{-1}$ toward the field centre, finding 80 sources above a $5\sigma$ threshold. We derive the radio colour-colour distribution for sources common to three surveys that cover the field and identify three sources with strongly curved spectra - a high-frequency-peaked source and two GHz-peaked-spectrum sources. The differential source count (i) agrees well with previous deep radio source count, (ii) exhibits no evidence of an emerging population of star-forming galaxies, down to a limit of 0.24$~$mJy, and (iii) disagrees with some models of the 15$~$GHz source population. However, our source count is in agreement with recent work that provides an analytical correction to the source count from the SKADS Simulated Sky, supporting the suggestion that this discrepancy is caused by an abundance of flat-spectrum galaxy cores as-yet not included in source population models.Comment: 17 pages, 14 figures, 3 tables. Accepted for publication in MNRA

AMI-CL J0300+2613: A Galactic anomalous-microwave-emission ring masquerading as a galaxy cluster

The Arcminute Microkelvin Imager (AMI) carried out a blind survey for galaxy clusters via their Sunyaev-Zel'dovich effect decrements between 2008 and 2011. The first detection, known as AMI-CL J0300+2613, has been reobserved with AMI equipped with a new digital correlator with high dynamic range. The combination of the new AMI data and more recent high-resolution sub-mm and infra-red maps now shows the feature in fact to be a ring of positive dust-correlated Galactic emission, which is likely to be anomalous microwave emission (AME). If so, this is the first completely blind detection of AME at arcminute scales

A Multi-telescope Campaign on FRB 121102: Implications for the FRB Population

We present results of the coordinated observing campaign that made the first subarcsecond localization of a Fast Radio Burst, FRB 121102. During this campaign, we made the first simultaneous detection of an FRB burst by multiple telescopes: the VLA at 3 GHz and the Arecibo Observatory at 1.4 GHz. Of the nine bursts detected by the Very Large Array at 3 GHz, four had simultaneous observing coverage at other observatories. We use multi-observatory constraints and modeling of bursts seen only at 3 GHz to confirm earlier results showing that burst spectra are not well modeled by a power law. We find that burst spectra are characterized by a ~500 MHz envelope and apparent radio energy as high as $10^{40}$ erg. We measure significant changes in the apparent dispersion between bursts that can be attributed to frequency-dependent profiles or some other intrinsic burst structure that adds a systematic error to the estimate of DM by up to 1%. We use FRB 121102 as a prototype of the FRB class to estimate a volumetric birth rate of FRB sources $R_{FRB} \approx 5x10^{-5}/N_r$ Mpc$^{-3}$ yr$^{-1}$, where $N_r$ is the number of bursts per source over its lifetime. This rate is broadly consistent with models of FRBs from young pulsars or magnetars born in superluminous supernovae or long gamma-ray bursts, if the typical FRB repeats on the order of thousands of times during its lifetime.Comment: 17 pages, 7 figures. Submitted to AAS Journal

Rapid radio flaring during an anomalous outburst of SS Cyg

The connection between accretion and jet production in accreting white dwarf binary systems, especially dwarf novae, is not well understood. Radio wavelengths provide key insights into the mechanisms responsible for accelerating electrons, including jets and outflows. Here, we present densely sampled radio coverage, obtained with the Arcminute MicroKelvin Imager Large Array, of the dwarf nova SS Cyg during its 2016 February anomalous outburst. The outburst displayed a slower rise (3 dmag-1) in the optical than typical ones and lasted for more than three weeks. Rapid radio flaring on time-scales <1 h was seen throughout the outburst. The most intriguing behaviour in the radio was towards the end of the outburst where a fast, luminous ('giant'), flare peaking at ~20 mJy and lasting for 15 min was observed. This is the first time that such a flare has been observed in SS Cyg and insufficient coverage could explain its non-detection in previous outbursts. These data, together with past radio observations, are consistent with synchrotron emission from plasma ejection events as being the origin of the radio flares. However, the production of the giant flare during the declining accretion rate phase remains unexplained within the standard accretion-jet framework and appears to be markedly different to similar patterns of behaviour in X-ray binaries

AMI-LA observations of the SuperCLASS supercluster

We present a deep survey of the SuperCLASS super-cluster - a region of sky known to contain five Abell clusters at redshift $z\sim0.2$ - performed using the Arcminute Microkelvin Imager (AMI) Large Array (LA) at 15.5$~$GHz. Our survey covers an area of approximately 0.9 square degrees. We achieve a nominal sensitivity of $32.0~\mu$Jy beam$^{-1}$ toward the field centre, finding 80 sources above a $5\sigma$ threshold. We derive the radio colour-colour distribution for sources common to three surveys that cover the field and identify three sources with strongly curved spectra - a high-frequency-peaked source and two GHz-peaked-spectrum sources. The differential source count (i) agrees well with previous deep radio source count, (ii) exhibits no evidence of an emerging population of star-forming galaxies, down to a limit of 0.24$~$mJy, and (iii) disagrees with some models of the 15$~$GHz source population. However, our source count is in agreement with recent work that provides an analytical correction to the source count from the SKADS Simulated Sky, supporting the suggestion that this discrepancy is caused by an abundance of flat-spectrum galaxy cores as-yet not included in source population models

Flares, wind and nebulae: the 2015 December mini-outburst of V404 Cygni

After more than 26 years in quiescence, the black hole transient V404 Cyg went into a luminous outburst in June 2015, and additional activity was detected in late December of the same year. Here, we present an optical spectroscopic follow-up of the December mini-outburst, together with X-ray, optical and radio monitoring that spanned more than a month. Strong flares with gradually increasing intensity are detected in the three spectral ranges during the ∼ 10 days following the Swift trigger. Our optical spectra reveal the presence of a fast outflowing wind, as implied by the detection of a P-Cyg profile (He i–5876 ˚A) with a terminal velocity of ∼ 2500 kms−1 . Nebularlike spectra – with an H α equivalent width of ∼ 500 ˚A – are also observed. All these features are similar to those seen during the main June 2015 outburst. Thus, the fast optical wind simultaneous with the radio jet is most likely present in every V404 Cyg outburst. Finally, we report on the detection of a strong radio flare in late January 2016, when X-ray and optical monitoring had stopped due to Sun constraints

Hydrogen Epoch of Reionization Array (HERA)

The Hydrogen Epoch of Reionization Array (HERA) is a staged experiment to measure 21 cm emission from the primordial intergalactic medium (IGM) throughout cosmic reionization ($z=6-12$), and to explore earlier epochs of our Cosmic Dawn ($z\sim30$). During these epochs, early stars and black holes heated and ionized the IGM, introducing fluctuations in 21 cm emission. HERA is designed to characterize the evolution of the 21 cm power spectrum to constrain the timing and morphology of reionization, the properties of the first galaxies, the evolution of large-scale structure, and the early sources of heating. The full HERA instrument will be a 350-element interferometer in South Africa consisting of 14-m parabolic dishes observing from 50 to 250 MHz. Currently, 19 dishes have been deployed on site and the next 18 are under construction. HERA has been designated as an SKA Precursor instrument. In this paper, we summarize HERA's scientific context and provide forecasts for its key science results. After reviewing the current state of the art in foreground mitigation, we use the delay-spectrum technique to motivate high-level performance requirements for the HERA instrument. Next, we present the HERA instrument design, along with the subsystem specifications that ensure that HERA meets its performance requirements. Finally, we summarize the schedule and status of the project. We conclude by suggesting that, given the realities of foreground contamination, current-generation 21 cm instruments are approaching their sensitivity limits. HERA is designed to bring both the sensitivity and the precision to deliver its primary science on the basis of proven foreground filtering techniques, while developing new subtraction techniques to unlock new capabilities. The result will be a major step toward realizing the widely recognized scientific potential of 21 cm cosmology.Comment: 26 pages, 24 figures, 2 table