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Register a new userDetecting highly-dispersed bursts with next-generation radio telescopes
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Recently, there have been reports of six bright, dispersed bursts of coherent radio emission found in pulsar surveys with the Parkes Multi-beam Receiver. Not much is known about the progenitors of these bursts, but they are highly-energetic, and probably of extragalactic origin. Their properties suggest extreme environments and interesting physics, but in order to understand and study these events, more examples need to be found. Fortunately, the recent boom in radio astronomy means many next-generation radio telescopes are set to begin observing in the near future. In this paper we discuss the prospects of detecting short extragalactic bursts, in both beamformed and imaging data, using these instruments. We find that often the volume of space probed by radio surveys of fast transients is limited by the dispersion measure (DM) of the source, rather than its physical distance (although the two quantities are related). This effect is larger for low-frequency telescopes, where propagation effects are more prominent, but, their larger fields-of-view are often enough to compensate for this. Our simulations suggest that the low-frequency component of SKA1 could find an extragalactic burst every hour. We also show that if the sensitivity of the telescope is above a certain threshold, imaging surveys may prove more fruitful than beamformed surveys in finding these sorts of transients.
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We have searched three Parkes multibeam 1.4 GHz surveys for the presence of fast radio bursts (FRBs) out to a dispersion measure (DM) of 5000 pc cm$^{-3}$. These surveys originally targeted the Magellanic Clouds (in two cases) and unidentified gamma-ray sources at mid-Galactic latitudes (in the third case) for new radio pulsars. In previous processing, none of these surveys were searched to such a high DM limit. The surveys had a combined total of 719 hr of Parkes multibeam on-sky time. One known FRB, 010724, was present in our data and was detected in our analysis but no new FRBs were found. After adding in the on-sky Parkes time from these three surveys to the on-sky time (7512 hr) from the five Parkes surveys analysed by Rane et al., all of which have now been searched to high DM limits, we improve the constraint on the all-sky rate of FRBs above a fluence level of 3.8 Jy ms at 1.4 GHz to $R = 3.3^{+3.7}_{-2.2} times 10^{3}$ events per day per sky (at the 99% confidence level). Future Parkes surveys that accumulate additional multibeam on-sky time (such as the ongoing high-resolution Parkes survey of the LMC) can be combined with these results to further constrain the all-sky FRB rate.
The detection of gravitational waves from a neutron star merger, GW170817, marked the dawn of a new era in time-domain astronomy. Monitoring of the radio emission produced by the merger, including high-resolution radio imaging, enabled measurements of merger properties including the energetics and inclination angle. In this work we compare the capabilities of current and future gravitational wave facilities to the sensitivity of radio facilities to quantify the prospects for detecting the radio afterglows of gravitational wave events. We consider three observing strategies to identify future mergers -- widefield follow-up, targeting galaxies within the merger localisation and deep monitoring of known counterparts. We find that while planned radio facilities like the Square Kilometre Array will be capable of detecting mergers at gigaparsec distances, no facilities are sufficiently sensitive to detect mergers at the range of proposed third-generation gravitational wave detectors that would operate starting in the 2030s.
Intergalactic space is believed to contain non-zero magnetic fields (the Intergalactic Magnetic Field: IGMF) which at scales of Mpc would have intensities below $10^{-9}$ G. Very high energy (VHE $>$100 GeV) gamma rays coming from blazars can produce e$^+$e$^-$ pairs when interacting with the Extragalactic Background Light (EBL) and the Cosmic Microwave Background, generating an electromagnetic cascade of Mpc scale. The IGMF may produce a detectable broadening of the emission beam that could lead to important constrains both on the IGMF intensity and its coherence length. Using the Monte Carlo-based Elmag code, we simulate the electromagnetic cascade corresponding to two detected TeV sources: PKS 2155-304 visible from the South and H1426+428 visible from the North. Assuming an EBL model and intrinsic spectral properties of the sources we obtain the spectral and angular distribution of photons when they arrive at Earth. We include the response of the next generation Cherenkov telescopes by using simplified models for CTA (Cherenkov Telescope Array)-south and CTA-north based on a full simulation of each array performance. Combining the instrument properties with the simulated source fluxes, we calculate the telescope point spread function for null and non-null IGMF intensities and develop a method to test the statistical feasibility of detecting IGMF imprints by comparing the resulting angular distributions. Our results show that for the analysed source PKS 2155-304 corresponding to the southern site, CTA should be able to detect IGMF with intensities stronger than 10$^{-14.5}$G within an observation time of $sim$100 hours.
The next generation of X-ray telescopes have the potential to detect faint quasars at very high redshift and probe the early growth of massive black holes (BHs). We present modelling of the evolution of the optical and X-ray AGN luminosity function at 2 < z < 6 based on a CDM merger-driven model for the triggering of nuclear activity combined with a variety of fading laws. We extrapolate the merger-driven models to z > 6 for a range of BH growth scenarios. We predict significant numbers of sources at z ~ 6 with fluxes just an order of magnitude below the current detection limits and thus detectable with XEUS and Constellation-X, relatively independently of the fading law chosen. The predicted number of sources at even higher redshift depends sensitively on the early growth history of BHs. For passive evolution models in which BHs grow constantly at their Eddington limit, detectable BHs may be rare beyond z ~ 10 even with Generation-X. However, in the more probable scenario that BH growth at z > 6 can be described by passive evolution with a small duty cycle, or by our merger driven accretion model, then we predict that XEUS and Generation-X will detect significant numbers of black holes out to z ~ 10 and perhaps beyond.
LOFAR (the Low Frequency Array), a distributed digital radio telescope with stations in the Netherlands, Germany, France, Sweden, and the United Kingdom, is designed to enable full-sky monitoring of transient radio sources. These capabilities are ideal for the detection of broadband radio pulses generated in cosmic ray air showers. The core of LOFAR consists of 24 stations within 4 square kilometers, and each station contains 96 low-band antennas and 48 high-band antennas. This dense instrumentation will allow detailed studies of the lateral distribution of the radio signal in a frequency range of 10-250 MHz. Such studies are key to understanding the various radio emission mechanisms within the air shower, as well as for determining the potential of the radio technique for primary particle identification. We present the status of the LOFAR cosmic ray program, including the station design and hardware, the triggering and filtering schemes, and our initial observations of cosmic-ray-induced radio pulses.
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