First detection of frequency-dependent, time-variable dispersion measures

Donner J, Verbiest J, Tiburzi C, et al. First detection of frequency-dependent, time-variable dispersion measures. Astronomy & Astrophysics. 2019;624: A22.Context. High-precision pulsar-timing experiments are affected by temporal variations of the dispersion measure (DM), which are related to spatial variations in the interstellar electron content and the varying line of sight to the source. Correcting for DM variations relies on the cold-plasma dispersion law which states that the dispersive delay varies with the squared inverse of the observing frequency. This may, however, give incorrect measurements if the probed electron content (and therefore the DM) varies with observing frequency, as is predicted theoretically due to the different refraction angles at different frequencies. Aims. We study small-scale density variations in the ionised interstellar medium. These structures may lead to frequency-dependent DMs in pulsar signals. Such an effect could inhibit the use of lower-frequency pulsar observations as tools to correct time-variable interstellar dispersion in higher-frequency pulsar-timing data. Methods. We used high-cadence, low-frequency observations with three stations from the German LOng-Wavelength (GLOW) consortium, which are part of the LOw-Frequency ARray (LOFAR). Specifically, 3.5 yr of weekly observations of PSR J2219+4754 are presented. Results. We present the first detection of frequency-dependent DMs towards any interstellar object and a precise multi-year time-series of the time- and frequency-dependence of the measured DMs. The observed DM variability is significant and may be caused by extreme scattering events. Potential causes for frequency-dependent DMs are quantified and evaluated. Conclusions. We conclude that frequency dependence of DMs has been reliably detected and is indeed caused by small-scale (up to tens of AUs) but steep density variations in the interstellar electron content. We find that long-term trends in DM variability equally affect DMs measured at both ends of our frequency band and hence the negative impact on long-term high-precision timing projects is expected to be limited

A Gaussian-processes approach to fitting for time-variable spherical solar wind in pulsar timing data

Propagation effects are one of the main sources of noise in high-precision pulsar timing. For pulsars below an ecliptic latitude of 5°, the ionized plasma in the solar wind can introduce dispersive delays of order 100 µs around solar conjunction at an observing frequency of 300 MHz. A common approach to mitigate this assumes a spherical solar wind with a time-constant amplitude. However, this has been shown to be insufficient to describe the solar wind. We present a linear, Gaussian-process piecewise Bayesian approach to fit a spherical solar wind of time-variable amplitude, which has been implemented in the pulsar software RUN_ENTERPRISE. Through simulations, we find that the current EPTA+InPTA data combination is not sensitive to such variations; however, solar wind variations will become important in the near future with the addition of new InPTA data and data collected with the low-frequency LOFAR telescope. We also compare our results for different high-precision timing data sets (EPTA+InPTA, PPTA, and LOFAR) of 3 ms pulsars (J0030+0451, J1022+1001, J2145−0450), and find that the solar-wind amplitudes are generally consistent for any individual pulsar, but they can vary from pulsar to pulsar. Finally, we compare our results with those of an independent method on the same LOFAR data of the three millisecond pulsars. We find that differences between the results of the two methods can be mainly attributed to the modelling of dispersion variations in the interstellar medium, rather than the solar wind modelling

CELLMICROCOSMOS 4.1, An Interactive Approach to Integrating Spatially Localized Metabolic Networks into a Virtual 3D Cell Environment.

Sommer B, Künsemöller J, Sand N, Husemann A, Rumming M, Kormeier B. CELLMICROCOSMOS 4.1, An Interactive Approach to Integrating Spatially Localized Metabolic Networks into a Virtual 3D Cell Environment. Presented at the BIOINFORMATICS 2010, Valencia, Spain.The high potential of Bioinformatics research concerning quantitative and qualitative data acquisition such as data warehouses, spatial structure prediction and 3D microscopy conveys the vision of generating a computational virtual cell. This paper discusses an approach which allows the creation and exploration of an abstract compartmented cell environment, which can be used for (semi-)automatic, species- and organelle-specific mapping and the comparison of metabolic data

JVideoGazer - Towards an Automatic Annotation of Gaze Videos from Natural Scenes

Essig K, Pfeiffer T, Sand N, Künsemöller J, Ritter H, Schack T. JVideoGazer - Towards an Automatic Annotation of Gaze Videos from Natural Scenes. Journal of Eye Movement Research. 2011;4(3):48

Fully-automatic annotation of scene videos. Establishing eye tracking effectively in various industrial applications

Essig K, Sand N, Schack T, Künsemöller J, Weigelt M, Ritter H. Fully-automatic annotation of scene videos. Establishing eye tracking effectively in various industrial applications. In: Society of Instrument and Control Engineers (SICE), Japan, ed. Proceedings of SICE annual conference 2010. Piscataway, NJ: IEEE; 2010: 3304-3307

Multifrequency behaviour of the anomalous events of PSR J0922+0638

Shaifullah G, Tiburzi C, Oslowski S, et al. Multifrequency behaviour of the anomalous events of PSR J0922+0638. MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY. 2018;477(1):L25-L29.PSR J0922+0638 (B0919+06) shows unexplained anomalous variations in the on-pulse phase, where the pulse appears to episodically move to an earlier longitude for a few tens of rotations before reverting to the usual phase for approximately several hundred to more than a thousand rotations. These events, where the pulse moves in phase by up to 5 degrees, have been previously detected in observations from similar to 300 to 2000 MHz. We present simultaneous observations from the Effelsberg 100-m radio telescope at 1350 MHz and the Bornim (Potsdam) station of the LOw Frequency ARray at 150 MHz. Our observations present the first evidence for an absence of the anomalous phase-shifting behaviour at 150 MHz. Instead, the observed intensity at the usual pulse-phase typically decreases, often showing a pseudo-nulling feature corresponding to the times when phase shifts are observed at 1350 MHz. The presence of weak emission at the usual pulse-phase supports the theory that these shifts may result from processes similar to the 'profile-absorption' expected to operate for PSR J0814+7429 (B0809+74). A possible mechanism for this could be intrinsic variations of the emission within the pulsar's beam combined with absorption by expanding shells of electrons in the line of sight

Pulsar scintillation studies with LOFAR. I. The census

Context. Interstellar scintillation (ISS) of pulsar emission can be used both as a probe of the ionised interstellar medium (IISM) and cause corruptions in pulsar timing experiments. Of particular interest are so-called scintillation arcs which can be used to measure time-variable interstellar scattering delays directly, potentially allowing high-precision improvements to timing precision. Aims. The primary aim of this study is to carry out the first sizeable and self-consistent census of diffractive pulsar scintillation and scintillation-arc detectability at low frequencies, as a primer for larger-scale IISM studies and pulsar-timing related propagation studies with the LOw-Frequency ARray (LOFAR) High Band Antennae (HBA). Results. In this initial set of 31 sources, 15 allow full determination of the scintillation properties; nine of these show detectable scintillation arcs at 120-180 MHz. Eight of the observed sources show unresolved scintillation; and the final eight don't display diffractive scintillation. Some correlation between scintillation detectability and pulsar brightness and dispersion measure is apparent, although no clear cut-off values can be determined. Our measurements across a large fractional bandwidth allow a meaningful test of the frequency scaling of scintillation parameters, uncorrupted by influences from refractive scintillation variations. Conclusions. Our results indicate the powerful advantage and great potential of ISS studies at low frequencies and the complex dependence of scintillation detectability on parameters like pulsar brightness and interstellar dispersion. This work provides the first installment of a larger-scale census and longer-term monitoring of interstellar scintillation effects at low frequencies

Pulsar scintillation studies with LOFAR, I. The census

Wu Z, Verbiest J, Main RA, et al. Pulsar scintillation studies with LOFAR, I. The census. Astronomy & Astrophysics. 2022;663: A116.Context.Interstellar scintillation (ISS) of pulsar emission can be used both as a probe of the ionized interstellar medium (IISM) and cause corruptions in pulsar timing experiments. Of particular interest are so-called scintillation arcs which can be used to measure time-variable interstellar scattering delays directly, potentially allowing high-precision improvements to timing precision.Aims.The primary aim of this study is to carry out the first sizeable and self-consistent census of diffractive pulsar scintillation and scintillation-arc detectability at low frequencies, as a primer for larger-scale IISM studies and pulsar-timing related propagation studies with the LOw-Frequency ARray (LOFAR) High Band Antennae (HBA).Methods.We use observations from five international LOFAR stations and the LOFAR core in the Netherlands. We analyze the 2D auto-covariance function of the dynamic spectra of these observations to determine the characteristic bandwidth and timescale of the ISS toward the pulsars in our sample and investigate the 2D power spectra of the dynamic spectra to determine the presence of scintillation arcs.Results.In this initial set of 31 sources, 15 allow for the full determination of the scintillation properties; nine of these show detectable scintillation arcs at 120–180 MHz. Eight of the observed sources show unresolved scintillation; and the final eight do not display diffractive scintillation. Some correlation between scintillation detectability and pulsar brightness and a dispersion measure is apparent, although no clear cut-off values can be determined. Our measurements across a large fractional bandwidth allow a meaningful test of the frequency scaling of scintillation parameters, uncorrupted by influences from refractive scintillation variations.Conclusions.Our results indicate the powerful advantage and great potential of ISS studies at low frequencies and the complex dependence of scintillation detectability on parameters such as pulsar brightness and interstellar dispersion. This work provides the first installment of a larger-scale census and longer-term monitoring of ISS effects at low frequencies

Pulsar scintillation studies with LOFAR. I. The census

Wu Z, Verbiest J, Main RA, et al. Pulsar scintillation studies with LOFAR. I. The census. arXiv:2203.10409. 2022.Context. Interstellar scintillation (ISS) of pulsar emission can be used both as a probe of the ionised interstellar medium (IISM) and cause corruptions in pulsar timing experiments. Of particular interest are so-called scintillation arcs which can be used to measure time-variable interstellar scattering delays directly, potentially allowing high-precision improvements to timing precision. Aims. The primary aim of this study is to carry out the first sizeable and self-consistent census of diffractive pulsar scintillation and scintillation-arc detectability at low frequencies, as a primer for larger-scale IISM studies and pulsar-timing related propagation studies with the LOw-Frequency ARray (LOFAR) High Band Antennae (HBA). Results. In this initial set of 31 sources, 15 allow full determination of the scintillation properties; nine of these show detectable scintillation arcs at 120-180 MHz. Eight of the observed sources show unresolved scintillation; and the final eight don't display diffractive scintillation. Some correlation between scintillation detectability and pulsar brightness and dispersion measure is apparent, although no clear cut-off values can be determined. Our measurements across a large fractional bandwidth allow a meaningful test of the frequency scaling of scintillation parameters, uncorrupted by influences from refractive scintillation variations. Conclusions. Our results indicate the powerful advantage and great potential of ISS studies at low frequencies and the complex dependence of scintillation detectability on parameters like pulsar brightness and interstellar dispersion. This work provides the first installment of a larger-scale census and longer-term monitoring of interstellar scintillation effects at low frequencies

Testing the accuracy of the ionospheric Faraday rotation corrections through LOFAR observations of bright northern pulsars

Porayko NK, Noutsos A, Tiburzi C, et al. Testing the accuracy of the ionospheric Faraday rotation corrections through LOFAR observations of bright northern pulsars. MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY. 2019;483(3):4100-4113.Faraday rotation of polarized emission from pulsars measured at radio frequencies provides a powerful tool to investigate the interstellar and interplanetary magnetic fields. However, besides being sensitive to the astrophysical media, pulsar observations in radio are affected by the highly time-variable ionosphere. In this article, the amount of ionospheric Faraday rotation has been computed by assuming a thin layer model. For this aim, ionospheric maps of the free electron density (based on Global Positioning System data) and semi-empirical geomagnetic models are needed. Through the data of five highly polarized pulsars observed with the individual German LOw-Frequency ARray stations, we investigate the performances of the ionospheric modelling. In addition, we estimate the parameters of the systematics and the correlated noise generated by the residual unmodelled ionospheric effects, and show the comparison of the different free-electron density maps. For the best ionospheric maps, we have found that the rotation measure corrections on 1 yr time-scales after subtraction of diurnal periodicity are accurate to similar to 0.06-0.07 rad m(-2)