119161876
The hunt for new pulsars with the Green Bank Telescope
The Green Bank Telescope (GBT) is the largest fully steerable radio telescope in the world and is one of our greatest tools for discovering and studying radio pulsars. Over the last decade, the GBT has successfully found over 100 new pulsars through large-area surveys. Here I discuss the two most recent—the GBT 350 MHz Drift-scan survey and the Green Bank North Celestial Cap survey. The primary science goal of both surveys is to find interesting individual pulsars, including young pulsars, rotating radio transients, exotic binary systems, and especially bright millisecond pulsars (MSPs) suitable for inclusion in Pulsar Timing Arrays, which are trying to directly detect gravitational waves. These two surveys have combined to discover 85 pulsars to date, among which are 14 MSPs and many unique and fascinating systems. I present highlights from these surveys and discuss future plans. I also discuss recent results from targeted GBT pulsar searches of globular clusters and Fermi sources.
54070997
The LOFAR radio environment
Aims: This paper discusses the spectral occupancy for performing radio astronomy with the Low-Frequency Array (LOFAR), with a focus on imaging observations. Methods: We have analysed the radio-frequency interference (RFI) situation in two 24-h surveys with Dutch LOFAR stations, covering 30-78 MHz with low-band antennas and 115-163 MHz with high-band antennas. This is a subset of the full frequency range of LOFAR. The surveys have been observed with a 0.76 kHz / 1 s resolution. Results: We measured the RFI occupancy in the low and high frequency sets to be 1.8% and 3.2% respectively. These values are found to be representative values for the LOFAR radio environment. Between day and night, there is no significant difference in the radio environment. We find that lowering the current observational time and frequency resolutions of LOFAR results in a slight loss of flagging accuracy. At LOFAR's nominal resolution of 0.76 kHz and 1 s, the false-positives rate is about 0.5%. This rate increases approximately linearly when decreasing the data frequency resolution. Conclusions: Currently, by using an automated RFI detection strategy, the LOFAR radio environment poses no perceivable problems for sensitive observing. It remains to be seen if this is still true for very deep observations that integrate over tens of nights, but the situation looks promising. Reasons for the low impact of RFI are the high spectral and time resolution of LOFAR; accurate detection methods; strong filters and high receiver linearity; and the proximity of the antennas to the ground. We discuss some strategies that can be used once low-level RFI starts to become apparent. It is important that the frequency range of LOFAR remains free of broadband interference, such as DAB stations and windmills.
3353965
Calibrating high-precision Faraday rotation measurements for LOFAR and the next generation of low-frequency radio telescopes
Faraday rotation measurements using the current and next generation of low-frequency radio telescopes will provide a powerful probe of astronomical magnetic fields. However, achieving the full potential of these measurements requires accurate removal of the time-variable ionospheric Faraday rotation contribution. We present ionFR, a code that calculates the amount of ionospheric Faraday rotation for a specific epoch, geographic location, and line-of-sight. ionFR uses a number of publicly available, GPS-derived total electron content maps and the most recent release of the International Geomagnetic Reference Field. We describe applications of this code for the calibration of radio polarimetric observations, and demonstrate the high accuracy of its modeled ionospheric Faraday rotations using LOFAR pulsar observations. These show that we can accurately determine some of the highest-precision pulsar rotation measures ever achieved. Precision rotation measures can be used to monitor rotation measure variations - either intrinsic or due to the changing line-of-sight through the interstellar medium. This calibration is particularly important for nearby sources, where the ionosphere can contribute a significant fraction of the observed rotation measure. We also discuss planned improvements to ionFR, as well as the importance of ionospheric Faraday rotation calibration for the emerging generation of low-frequency radio telescopes, such as the SKA and its pathfinders.
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.
13221938
Multifrequency Study of Giant Radio Pulses from the Crab Pulsar with the K5 VLBI Recording Terminal
AbstractSimultaneous multifrequency observations of the Crab pulsar giant pulses (GPs)were performed with the 64-m Kalyazin radio telescope at four frequencies ν = 0.6,1.4, 2.2 and 8.3 GHz, using the K5 VLBI recording terminal. The K5 terminal pro-vided continuous recording in 16 4-MHz wide frequency channels distributed over 4frequency bands. Several thousand GPs were detected during about 6 hours of obser-vations in two successive days in July 2005. Radio spectra of single GPs were analysedat separate frequencies and over the whole frequency range. These spectra mani-fest notable modulation over frequency ranges, ∆ν, both on large (∆ν/ν ≈ 0.5) andsmall (∆ν/ν ≈ 0.01) frequency scales. Cross-correlation analysis of GPs at 2.2 GHzshowed that their pulse shapes can be interpreted as an ensemble of unresolved burstsgrouped together at time scales of ≈1 µs being well-correlated over a 60-MHz band.The corresponding GP cross-correlation functions do not obey the predictions of theamplitude-modulated noise model of Rickett (1975), thus indicating that unresolvedcomponents represent a small number of elementary emitters.Key words: Pulsars — giant pulses, Crab pulsar, PSR B0531+21 — e-VLBI1
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.
119799578
200 and 300 MeV/nucleon nuclear reactions responsible for single-event effects in microelectronics
An experimental study of nuclear reactions between Si-28 nuclei at 200 and 300 MeV/nucleon and hydrogen or deuterium target nuclei was performed at the CELSIUS storage ring in Uppsala, Sweden, to collect information about the reactions responsible for single-event effects in microelectronics. Inclusive data on Si-28 fragmentation, as well as data on correlations between recoils and spectator protons or alpha particles are compared to predictions from the Dubna cascade model and the Japan Atomic Energy Research Institute version of the quantum molecular dynamics model. The comparison shows satisfactory agreement for inclusive data except for He fragments where low-energy sub-barrier fragments and recoiling fragments with very large momenta are produced much more frequently than predicted. The yield of exclusive data are also severely underestimated by the models whereas the charge distributions of recoils in these correlations compare well. The observed enhancement in He emission, which may well be important for the description of single-event effects, is most likely to be attributed to alpha clustering in Si-28 nuclei.
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