16224154
The brightness and spatial distributions of terrestrial radio sources
Faint undetected sources of radio-frequency interference (RFI) might become visible in long radio observations when they are consistently present over time. Thereby, they might obstruct the detection of the weak astronomical signals of interest. This issue is especially important for Epoch of Reionization (EoR) projects that try to detect the faint redshifted H i signals from the time of the earliest structures in the Universe. We explore the RFI situation at 30–163 MHz by studying brightness histograms of visibility data observed with Low-Frequency Array (LOFAR), similar to radio-source-count analyses that are used in cosmology. An empirical RFI distribution model is derived that allows the simulation of RFI in radio observations. The brightness histograms show an RFI distribution that follows a power-law distribution with an estimated exponent around −1.5. With several assumptions, this can be explained with a uniform distribution of terrestrial radio sources whose radiation follows existing propagation models. Extrapolation of the power law implies that the current LOFAR EoR observations should be severely RFI limited if the strength of RFI sources remains strong after time integration. This is in contrast with actual observations, which almost reach the thermal noise and are thought not to be limited by RFI. Therefore, we conclude that it is unlikely that there are undetected RFI sources that will become visible in long observations. Consequently, there is no indication that RFI will prevent an EoR detection with LOFAR.
12768338
Observing pulsars and fast transients with LOFAR
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.
118528110
The Pulsar Search Collaboratory.
The Pulsar Search Collaboratory [PSC, NSF #0737641] is a joint project between the National Radio Astronomy Observatory (NRAO) and West Virginia University (WVU) designed to interest high school students in science, technology, engineering, and mathematics [STEM] related career paths by helping them to conduct authentic scientific research. The 3- year PSC program, which began in summer 2008, teaches students to analyze astronomical radio data acquired with the 100-m Robert C. Byrd Green Bank Telescope for the purpose of discovering new pulsars. We present the results of the first complete year of the PSC, which includes two astronomical discoveries.
13852209
Arecibo pulsar survey using ALFA: probing radio pulsar intermittency and transients
We present radio transient search algorithms, results, and statistics from the ongoing Arecibo Pulsar ALFA (PALFA) survey of the Galactic plane. We have discovered seven objects through a search for isolated dispersed pulses. All of these objects are Galactic and have measured periods between 0.4 and 4.7 s. One of the new discoveries has a duty cycle of 0.01%, smaller than that of any other radio pulsar. We discuss the impact of selection effects on the detectability and classification of intermittent sources, and compare the efficiencies of periodicity and single-pulse (SP) searches for various pulsar classes. For some cases we find that the apparent intermittency is likely to be caused by off-axis detection or a short time window that selects only a few bright pulses and favors detection with our SP algorithm. In other cases, the intermittency appears to be intrinsic to the source. No transients were found with DMs large enough to require that they originate from sources outside our Galaxy. Accounting for the on-axis gain of the ALFA system, as well as the low gain but large solid-angle coverage of far-out sidelobes, we use the results of the survey so far to place limits on the amplitudes and event rates of transients of arbitrary origin.
16206012
Giant pulses from PSR B1937+21 with widths ≤15 nanoseconds and Tb ≥ 5 × 1039 K, the highest brightness temperature observed in the universe
Giant radio pulses of the millisecond pulsar B1937+21 were recorded with the S2 VLBI system at 1.65 GHz with NASA/JPL's 70 m radio telescope at Tidbinbilla, Australia. These pulses have been observed as strong as 65,000 Jy with widths ≤15 ns, corresponding to a brightness temperature of Tb ≥ 5 × 1039 K, the highest observed in the universe. The vast majority of these pulses occur in 5.8 and 8.2 μs windows at the very trailing edges of the regular main pulse and interpulse profiles, respectively. Giant pulses occur, in general, with a single spike. Only in one case of 309 was the structure clearly more complex. The cumulative distribution is fitted by a power law with index -1.40 ± 0.01 with a low-energy but no high-energy cutoff. We estimate that giant pulses occur frequently but are only rarely detected. When corrected for the directivity factor, 25 giant pulses are estimated to be generated in one neutron star revolution alone. The intensities of the giant pulses of the main pulses and interpulses are not correlated with each other nor with the intensities or energies of the main pulses and interpulses themselves. Their radiation energy density can exceed 300 times the plasma energy density at the surface of the neutron star and can even exceed the magnetic field energy density at that surface. We therefore do not think that the generation of giant pulses is linked to the plasma mechanisms in the magnetosphere. Instead we suggest that it is directly related to discharges in the polar cap region of the pulsar.
7001742
Detecting cosmic rays with the LOFAR radio telescope
The low frequency array (LOFAR), is the first radio telescope designed with the capability to measure radio emission from cosmic-ray induced air showers in parallel with interferometric observations. In the first ~2 years of observing, 405 cosmic-ray events in the energy range of 1016−1018 eV have been detected in the band from 30−80 MHz. Each of these air showers is registered with up to ~1000 independent antennas resulting in measurements of the radio emission with unprecedented detail. This article describes the dataset, as well as the analysis pipeline, and serves as a reference for future papers based on these data. All steps necessary to achieve a full reconstruction of the electric field at every antenna position are explained, including removal of radio frequency interference, correcting for the antenna response and identification of the pulsed signal.
7628453
Pulsar microstructure and its quasi-periodicities with the S2 VLBI system at a resolution of 62.5 nanoseconds
We report a study of microstructure and its quasi-periodicities of three pulsars at 1.65 GHz with the S2 VLBI system at a resolution of 62.5 ns, by far the highest for any such statistical study yet. For PSR B1929+10 we found in the average cross-correlation function (CCF) broad microstructure with a characteristic timescale of 95 ± 10 µs and confirmed microstructure with characteristic timescales between 100 and 450 µs for PSRs B0950+08 and B1133+16. On a finer scale PSRs B0950+08, B1133+16 (component II) and B1929+10 show narrow microstructure with a characteristic timescale in the CCFs of ∼10 µs, the shortest found in the average CCF or autocorrelation function (ACF) for any pulsar, apart perhaps for the Crab pulsar. Histograms of microstructure widths are skewed heavily toward shorter timescales but display a sharp cutoff. The shortest micropulses have widths between 2 and 7 µs. There is some indication that the timescales of the broad, narrow, and shortest micropulses are, at least partly, related to the widths of the components of the integrated profiles and the subpulse widths. If the shortest micropulses observed are indeed due to beaming then the ratio, γ, of the relativistic energy of the emitting particles to the rest energy is about 20000, independent of the pulsar period. We predict an observable lower limit for the width of micropulses from these pulsars at 1.65 GHz of 0.5 µs. If the short micropulses are instead interpreted as a radial modulation of the radiation pattern, then the associated emitting sources have dimensions of about 3 km in the observer's frame. For PSRs B0950+08 and B1133+16 (both components) the micropulses had a residual dispersion delay over a 16-MHz frequency difference of ∼2 µs when compared to that of average pulse profiles over a much larger relative and absolute frequency range. This residual delay is likely the result of propagation effects in the pulsar magnetosphere that contribute to limiting the width of micropulses. No nanopulses or unresolved pulse spikes were detected. Cross-power spectra of single pulses show a large range of complexity with single spectral features representing classic quasi-periodicities and broad and overlapping features with essentially no periodicities at all. Significant differences were found for the two components of PSR B1133+16 in every aspect of our statistical analysis of micropulses and their quasi-periodicities. Asymmetries in the magnetosphere and the hollow cone of emission above the polar cap of the neutron star may be responsible for these differences. the observed intensity fluctuations can be either caused by a longitudinal modulation of the radiation pattern over the cross section of the polar magnetic field lines or by a radial modula- tion of the radiation pattern along the opening polar magnetic field lines, or, again, by a combination of both. The longitudi- nal modulation is most likely related to the stationary geome- try of the emission beam fixed to each of the poles of the ro- tating neutron star. The radial modulation is likely related to plasma bunching and linked to the elementary emission mech- anism. In this model the spectrum of the radio emission is a function of the radial distance from the neutron star, and the beam width is frequency dependent. High frequency radiation is emitted closer to the neutron star and the beam is narrower, low frequency radiation is emitted further out and the beam is broader, reflecting the opening of the polar magnetic field
121572324
Results of in-beam tests of an MCP-based vacuum sector prototype of the T0/centrality detector for ALICE
AbstractWereportonresultsofeﬃciencytestsforMIPsandtimingresolutionstudiesobtainedwithdiﬀerentlayoutsofmicro-channelplate(MCP)detectorprototypesatCERNPStestbeams.ThedetectoriscomposedofanMCPstackand multipad readout anodes integrated with a passive isochronal summator. Several types of setups were tested,includingMCPswithdiﬀerentgain.Also,averyﬁrsttestoftheMicroSpherePlatesdetectorwasperformed.TheperformanceoffastelectronicsincludingDoubleThresholdDiscriminatorsandafastmultiplicitydiscriminatorforshort2nssignalswasstudied.Currently,thebestexperimentalresultsforMCPdetectorsshow75pstimingresolutionobtainedat75%eﬃciencyfortheregistrationofasinglemuon.Weexpectfurtherenhancements,mostlythroughimprovementsinthefastelectronics.r 2002ElsevierScienceB.V.Allrightsreserved. 1. IntroductionA Micro-Channel Plate (MCP) disk detectorsystem with integrated isochronal signal readoutandprecisetimingcapabilitiesisconsideredasoneof the options for the ALICE pre-trigger T0/centralitydetector.MCPsareawell-knowndevicefor the registration of charged particles due totheir excellent timing properties (20–50ps), highinternalgain(10