Stappers B.W.,University of Manchester |
Hessels J.W.T.,Netherlands Institute for Radio Astronomy |
Hessels J.W.T.,University of Amsterdam |
Alexov A.,University of Amsterdam |
And 104 more authors.
Astronomy and Astrophysics | Year: 2011
Low frequency radio waves, while challenging to observe, are a rich source of information about pulsars. The LOw Frequency ARray (LOFAR) is a new radio interferometer operating in the lowest 4 octaves of the ionospheric "radio window": 10-240 MHz, that will greatly facilitate observing pulsars at low radio frequencies. Through the huge collecting area, long baselines, and flexible digital hardware, it is expected that LOFAR will revolutionize radio astronomy at the lowest frequencies visible from Earth. LOFAR is a next-generation radio telescope and a pathfinder to the Square Kilometre Array (SKA), in that it incorporates advanced multi-beaming techniques between thousands of individual elements. We discuss the motivation for low-frequency pulsar observations in general and the potential of LOFAR in addressing these science goals. We present LOFAR as it is designed to perform high-time-resolution observations of pulsars and other fast transients, and outline the various relevant observing modes and data reduction pipelines that are already or will soon be implemented to facilitate these observations. A number of results obtained from commissioning observations are presented to demonstrate the exciting potential of the telescope. This paper outlines the case for low frequency pulsar observations and is also intended to serve as a reference for upcoming pulsar/fast transient science papers with LOFAR. © 2011 ESO.
Royall B.,Hicks Building |
Royall B.,University of Essex |
Khalil H.,University of Essex |
Mazzucato S.,University of Essex |
And 3 more authors.
Nanoscale Research Letters | Year: 2014
Photocurrent oscillations, observed at low temperatures in lattice-matched Ga1-xInxNyAs1-y/GaAs multiple quantum well (MQW) p-i-n samples, are investigated as a function of applied bias and excitation wavelength and are modelled with the aid of semiconductor simulation software. The oscillations appear only at low temperatures and have the highest amplitude when the optical excitation energy is in resonance with the GaInNAs bandgap. They are explained in terms of electron accumulation and the formation of high-field domains in the GaInNAs QWs as a result of the disparity between the photoexcited electron and hole escape rates from the QWs. The application of the external bias results in the motion of the high-field domain towards the anode where the excess charge dissipates from the well adjacent to anode via tunnelling. © 2014 Royall et al.
Buckley A.,Hicks Building |
Buckley A.,MicroEmissive Displays Ltd. |
Pickup D.,MicroEmissive Displays Ltd. |
Yates C.,MicroEmissive Displays Ltd. |
And 3 more authors.
Journal of Applied Physics | Year: 2011
We report spectroscopic and electrical measurements to explore hole injection and conduction in devices comprising a molybdenum sub-oxide (MoOx) hole injection layers and poly[(9,9-dioctylfluorenyl-2, 7-diyl)-co-(4,4'(N-(4- sec-butylphenyl))) diphenylamine](TFB) hole transporting polymer. We report improvements in device conductivity over benchmark structures incorporating an ITO electrode and polyethylenedioxythiophene polystyrene sulfonate (PEDOT:PSS) hole injection layers and furthermore achieve injection from MoOx to TFB that is efficient even with an underlying low workfunction Al electrode. XPS spectroscopy has been used to investigate the electronic structure of the interfaces and we find discrete energy alignment regimes consistent with recent surface science studies by Tengstedt [Appl. Phys. Lett. 88, 053502 (2006)], corresponding to Fermi level pinning for MoOx/TFB and vacuum level pinning in the case of Al/TFB. While the energetic alignment regime is measured to be independent of MoOx thickness, the device conductivity continuously varies with MoOx thickness; an observation that can be qualitatively explained by considering two independent charge injection mechanisms from molybdenum oxide sites having different stoicheometry. © 2011 American Institute of Physics.
Villar-Martin M.,Institute Astrofisica Of Andalucia Csic |
Villar-Martin M.,CSIC - National Institute of Aerospace Technology |
Cabrera Lavers A.,Institute of Astrophysics of Canarias |
Bessiere P.,Hicks Building |
And 3 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012
We present a compilation of Hubble Space Telescope (HST) images of 58 luminous Sloan Digital Sky Survey (SDSS) type 2 active galactic nuclei (AGNs) at Of these, 42 are type 2 quasars, which is a good representation of all optically selected SDSS type 2 quasars in this z range. We find that the majority of the host galaxies are ellipticals (30/42 or 71 per cent). This is consistent with studies of radio-loud and radio-quiet type 1 quasars, which show that their host galaxies are, in general, ellipticals. A significant fraction of type 2 quasars (≥25/42 or ≥59 per cent) show clear signatures of morphological disturbance. In most cases, these are clearly identified with merger/interaction processes. We discuss this in the context of related works on type 2 quasars and powerful radio galaxies. We study in detail the particular case of the radio-quiet type 2 quasar SDSS J143027.66-005614.8 at z= 0.318 based on imaging and spectroscopic data from the Very Large Telescope, HST and SDSS. The system shows highly complex morphology, similar to that found in many ultraluminous infrared galaxies, which suggests that it is in the late pre-coalescence stage of a major galaxy merger. The optical continuum spectrum is dominated by a young stellar population of age <80 Myr, probably formed as a consequence of the merger-induced starburst. Ionized gas is confirmed up to a maximum total extension of r∼ 13 kpc from the quasar, although gas is also possibly detected at up to r∼ 32 kpc. The ionizing mechanism, AGN versus stellar photoionization, varies depending on the spatial location. There is a nuclear (r≤ few kpc) ionized outflow, which is blueshifted by ∼520 km s -1 relative to the systemic redshift and has FWHM ∼ 1600 km s -1. Several × 10 5M ⊙ at most are expected to participate in the outflow. We discuss the global properties of the object in the context of theoretical and observational studies of galaxy mergers/interactions and their role in the triggering of the nuclear and star formation activities in the most luminous active galaxies. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.