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

Detection of two new RRATs at 111 MHz

A search for pulse signals in a area with declinations of +52\degr <\delta <+55\degr was carried out on the LPA LPI radio telescope. When processing ten months of observations recorded in six frequency channels with a channel width of 415 kHz and a total bandwidth of 2.5 MHz, 22 thousand events were found with a pronounced dispersion delay of signals over frequency channels, i.e. having signs of pulsar pulses. It turned out that the found pulses belong to four known pulsars and two new rotating radio transients (RRATs). An additional pulse search conducted in 32-channel data with a channel width of 78 kHz revealed 8 pulses for the transient J0249+52 and 7 pulses for the transient J0744+55. Periodic radiation of transients was not detected. The analysis of observations shows that the found RRATs are most likely pulsars with nullings, where the proportion of nulling is greater than 99.9\%.Comment: published in Astronomy Reports, translated by Yandex translator with correction of scientific lexis, 5 pages, 2 figures, 1 tabl

A Census of Rotating Radio Transients at 150 MHz with the Irish LOFAR Station

Rotating radio transients (RRATs) are neutron stars that emit detectable radio bursts sporadically. They are statistically distinct in the neutron star population, in many observable properties, but by their nature are practically difficult to study in depth. In this paper, we present the results from 1408 h of observations of RRAT candidates using the Irish station of the Low Frequency Array (LOFAR) at 150 MHz. As of October 2022, this census involved observing 113 sources, leading to 29 detections which were then followed up systematically. Single-pulse emission was detected from 25 sources, and periodic emission from 14 sources. 18 sources were found to have emission behaviour that is not discussed in prior works using LOFAR instruments. Four novel or modified source periods have been determined, ranging from 1.5-3.9 s, and 8 new or updated phase-coherent pulsar timing ephemerides have been produced using detected bursts. One unexpected single-pulse with a clearly-Galactic dispersion measure was detected as a part of this work, but has not been re-detected in follow-up observations. Observations are ongoing to expand the number of observed sources and further characterise and improve ephemerides for the detected sources. This census has demonstrated the capability for international LOFAR stations to detect, monitor and characterise a significant fraction of these unique sources.Comment: Accepted by MNRAS. 24 pages, 7 figures, 8 table

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

Interplanetary scintillation observation and space weather modelling

Interplanetary scintillation (IPS) refers to random fluctuations in radio intensity of distant small-diameter celestial object, over time periods of the order of 1 s. The scattering and scintillation of emergent radio waves are ascribed to turbulent density irregularities transported by the ubiquitous solar wind streams. The spatial correlation length of density irregularities and the Fresnel radius of radio diffraction are two key parameters in determining the scintillation pattern. Such a scintillation pattern can be measured and correlated between multi-station radio telescopes on the Earth. Using the “phase-changing screen” scenario based on the Born approximation, the bulk-flow speed and turbulent spectrum of the solar wind streams can be extracted from the single-station power spectra fitting and the multi-station cross-correlation analysis. Moreover, a numerical computer-assisted tomography (CAT) model, iteratively fit to a large number of IPS measurements over one Carrington rotation, can be used to reconstruct the global velocity and density structures in the inner heliosphere for the purpose of space weather modelling and prediction. In this review, we interpret the underlying physics governing the IPS phenomenon caused by the solar wind turbulence, describe the power spectrum and cross correlation of IPS signals, highlight the space weather application of IPS-CAT models, and emphasize the significant benefits from international cooperation within the Worldwide IPS Stations (WIPSS) network

Report on active and planned spacecraft and experiments

Information is presented, concerning active and planned spacecraft and experiments known to the National Space Science Data Center. The information included a wide range of disciplines: astronomy, earth sciences, meteorology, planetary sciences, aeronomy, particles and fields, solar physics, life sciences, and material sciences. These spacecraft projects represented the efforts and funding of individual countries as well as cooperative arrangements among different countries

Discovery and characterisation of fast radio transients

Many types of astronomical objects are detectable through the radio waves that they produce. The observed properties of sources in the 'radio sky' can vary on a wide range of timescales, both for intrinsic and extrinsic reasons. Transients are those sources whose observed properties change drastically within a timescale that a human astronomer can measure. Traditionally, the qualifier 'fast' refers to transients whose emission properties change significantly over less than a second, and most commonly on millisecond timescales. The two classes of fast radio transients studied in this thesis are pulsars and fast radio bursts. We are motivated to study these sources because they probe extreme astrophysical environments as well as the intervening magnetised and ionised medium between observer and source. Furthermore, the origin of the relatively recently discovered fast radio bursts remains enigmatic, though the properties of these signals share traits of pulsar emission. Thus, we aim to better understand the physical nature of fast radio bursts and their possible link to pulsar emission. We discuss the characteristics of these source classes and outline some of the outstanding scientific questions we can address through observations with a radio telescope and other complementary, multi-wavelength information

EFFECTS OF PULSED ELECTRIC FIELDS ON MAMMALIAN CELL MEMBRANES

The effect of pulsed electric fields on cell membranes were studied to investigate the effects of an electric field, per se, on biological systems, without the densometric and other technical problems associated with other forms of nonionizing radiation. Exposure of mouse splenocytes to a high voltage pulse resulted in an increase in membrane permeability to K+ that was dependent on both the electric field strength and the pulse duration. Exposure to a 2 μsec, 3.0 kV/cm pulse elicited a 50% loss of intracellular K+ indicating that the critical transmembrane potential (Vm) at breakdown was 1.26 volts for the membrane of mouse spleen cells. These results agreed with previous studies on erythrocytes and micro-organisms. Effects of a pulsed electric field on a cell\u27s functional integrity were assessed by measuring 3H-thymidine incorporation by lymphocytes cultured in the presence and absence of various mitogens following exposure to an electrical pulse. No statistically significant effects on the response of mouse spleen lymphocytes to Con-A, PHA, or LPS were observed following exposure to a 2 usec electric pulse. Exposure to 10 μsec pulses ≥ 2.4 kV/cm produced a statistically significant reduction in the response of lymphocytes to LPS stimulation. While not statistically significant, results from both 2 and 10 usec experiments consistently indicated that exposure to pulses at sublethal electric field strengths may have a stimulatory effect on mouse spleen lymphocytes. This result is discussed and an experiment to resolve this issue is presented. Exposure of Ehrlich ascites tumor cells to 2 μsec electrical pulses produced no statistically significant alterations in the tumorigenicity of these cells. K+ release data indicated these cells are unusually resistant to the effects of pulsed electric fields having a high breakdown potential, Vm = 2.37 volts

Solar flare gamma-ray spectroscopy with CGRO-COMPTEL

The X-ray and gamma-ray emission from solar flares provides important information about high-energy particles in solar flares. Energetic protons and ions interact with the solar atmosphere, giving rise to nuclear line emission at MeV energies and higher energy photons from the decay of neutral and charged pions. Electrons interact with the solar atmosphere producing a bremsstrahlung continuum. The solar flare spectrum is generally a superposition of these spectra with nuclear line emission dominating from ∼1--8 MeV and the bremsstrahlung at lower and higher energies. The main goal of this thesis has been to explain a small part of a gamma-ray flare observed by COMPTEL in June 1991. A difficult interval to explain in the 11 June 1991 solar flare is the Intermediate (Rank 1997) or Interphase (Murphy and Share 1999; Dunphy et al. 1999) immediately following the peak of the impulsive phase. All three analyses of this flare using COMPTEL, OSSE and EGRET data yielded a hard proton spectrum with a power law index around 2 using the 2.2 to 4.44 MeV fluence ratio. This hard of a spectrum would indicate the presence of a high-energy component above eight MeV and emission due to spallation products. However, none of the three instruments observed such a component. We discuss the standard techniques used in solar flare spectral deconvolution and introduce a new technique we use with the COMPTEL observations. This work presented the explanation that the proton spectrum is soft during this interval of the 11 June 1991 solar flare based on this new analysis of the COMPTEL observations. This means that the region of 2.223/4--7 MeV fluence space is largely unexplored for soft proton spectra. The use of this ratio must be reexamined for proton spectra with indices greater than 5 or 6. We then applied a model we developed for the transport of neutrons created from a soft proton spectrum to determine the photospheric 3He abundance during this flare. We calculated a 3He/H ratio of 8.7e-05 with a 1 a range of 1.96e-04 to 1.75e-05 for this flare using this new model. This is larger than all previous values reported. In addition, we presented an additional flare observation from COMPTEL. In response to a BACODINE cosmic gamma-ray burst alert, COMPTEL on the CGRO recorded gamma rays above 1 MeV from the C4 flare at 0221 UT 20 January 2000. This event, though at the limits of COMPTEL\u27s sensitivity, clearly shows a nuclear line excess above the continuum. Using new spectroscopy techniques we were able to resolve individual lines. This allowed us to make a basic comparison of this event with the GRL (gamma ray line) flare distribution from SMM and also compare this flare with a well-observed large GRL flare seen by OSSE. We showed this flare is normal, i.e., it is a natural extension of the SMM distribution of flares. The analysis of this flare means there is no evidence for a lower flare size for proton acceleration. Protons even in small flares contain a large part of the accelerated particle energy