Aris Karastergiou

Abstract:

We have observed a complex and continuous change in the integrated pulse profile of PSR B2217+47, manifested as additional components trailing the main peak. These transient components are detected over 6 yr at 150 MHz using the LOw Frequency ARray (LOFAR), but they are not seen in contemporaneous Lovell observations at 1.5 GHz. We argue that propagation effects in the ionized interstellar medium (IISM) are the most likely cause. The putative structures in the IISM causing the profile variation are roughly half-way between the pulsar and the Earth and have transverse radii R ∼ 30 au. We consider different models for the structures. Under the assumption of spherical symmetry, their implied average electron density is n¯¯¯e∼100 cm−3. Since PSR B2217+47 is more than an order of magnitude brighter than the average pulsar population visible to LOFAR, similar profile variations would not have been identified in most pulsars, suggesting that subtle profile variations in low-frequency profiles might be more common than we have observed to date. Systematic studies of these variations at low frequencies can provide a new tool to investigate the proprieties of the IISM and the limits to the precision of pulsar timing.

Abstract:

Copyright © 2018 by the Editors.All rights reserved. Fast Radio Bursts (FRBs) are apparently one-time, relatively bright radio pulses that have been observed in recent years. The origin of FRBs is currently unknown and many instruments are being built to detect more of these bursts to better characterize their physical properties and identify the source population. ALFABURST is one such instrument. ALFABURST takes advantage of the 7-beam Arecibo L-band Feed Array (ALFA) receiver on the 305-m Arecibo Radio Telescope in Puerto Rico, to detect FRBs in real-time at L-band (1.4 GHz). We present the results of recent on-sky tests and observations undertaken during the commissioning phase of the instrument. ALFABURST is now available for commensal observations with other ALFA projects.

Abstract:

© 2017 The Authors Published by Oxford University Press. We measure the effects of interstellar scattering on average pulse profiles from 13 radio pulsars with simple pulse shapes. We use data from the LOFAR High Band Antennas, at frequencies between 110 and 190 MHz.We apply a forward fitting technique, and simultaneously determine the intrinsic pulse shape, assuming single Gaussian component profiles. We find that the constant τ , associated with scattering by a single thin screen, has a power-law dependence on frequency τ ∝ ν -a , with indices ranging from α = 1.50 to 4.0, despite simplest theoretical models predicting α =4.0 or 4.4. Modelling the screen as an isotropic or extremely anisotropic scatterer, we find anisotropic scattering fits lead to larger power-law indices, often in better agreement with theoretically expected values.We compare the scattering models based on the inferred, frequency-dependent parameters of the intrinsic pulse, and the resulting correction to the dispersion measure (DM). We highlight the cases in which fits of extreme anisotropic scattering are appealing, while stressing that the data do not strictly favour either model for any of the 13 pulsars. The pulsars show anomalous scattering properties that are consistent with finite scattering screens and/or anisotropy, but these data alone do not provide the means for an unambiguous characterization of the screens. We revisit the empirical t versus DM relation and consider how our results support a frequency dependence of a. Very long baseline interferometry, and observations of the scattering and scintillation properties of these sources at higher frequencies, will provide further evidence.

Abstract:

Copyright © International Astronomical Union 2018. The Low Frequency Array (LOFAR) is ideally suited to pulsar scattering studies, providing broad bands at low frequencies where the imprints of the ionized Interstellar Medium (IISM) are exaggerated. We analyse a set of sources at 110-190 MHz, and find unexpectedly shallow dependencies of pulse scatter broadening on frequency. These anomalous scattering values are discussed by considering evidence for anisotropic scattering and small scattering clouds.