Aris Karastergiou

Abstract:

We have conducted observations of 22 pulsars at frequencies of 0.7, 1.4 and 3.1 GHz and present their polarization profiles. The observations were carried out for two main purposes. First, we compare the orientation of the spin and velocity vectors to verify the proposed alignment of these vectors by Johnston et al. We find, for the 14 pulsars for which we were able to determine both vectors, that seven are plausibly aligned, a fraction which is lower than, but consistent with, earlier measurements. Secondly, we use profiles obtained simultaneously at widely spaced frequencies to compute the radio emission heights. We find, similar to other workers in the field, that radiation from the centre of the profile originates from lower in the magnetosphere than the radiation from the outer parts of the profile. © 2007 The Authors. Journal compilation © 2007 RAS.

Abstract:

Aims. In this paper we revisit the well-known phenomenon of pulse nulling using high-quality single-pulse data of PSR B1133+16 from simultaneous multifrequency observations. Methods. Observations were made at 325, 610, 1400 and 4850 MHz as part of a joint program between the European Pulsar Network (EPN) and the Giant Metrewave Radio Telescope (GMRT). The pulse energy time series are analysed to derive improved statistics of nulling pulses as well as to investigate the frequency dependence of the phenomenon. Results. The pulsar is observed to be in null state for approximately 15% of the time; however, we find that nulling does not always occur simultaneously at all four frequencies of observation. We characterise the statistics of such "selective nulling" as a function of frequency, separation in frequency, and combination of frequencies. The most remarkable case of such selective nulling seen in our data is a significantly large number of nulls (≈6%) at lower frequencies, that are marked by the presence of a fairly narrow emission feature at the highest frequency of 4850 MHz. We refer to these as "low frequency (LF) nulls". We characterise the properties of high frequency (HF) emission at the occurrence of LF nulls, and compare and contrast them with that of "normal emission" at 4850 MHz. Our analysis shows that this high frequency emission tends to occur preferentially over a narrow range in longitude and with pulse widths typically of the order of a few milliseconds. We discuss the implications of our results for the pulsar emission mechanism in general and for the broadbandness of nulling phenomenon in particular. Our results signify the presence of an additional process of emission which does not turn off when the pulsar nulls at low frequencies, and becomes more prominent at higher frequencies. Our analysis also hints at a possible outer gap origin for this new population of pulses, and thus a likely connection to some high-energy emission processes that occur in the outer parts of the pulsar magnetosphere. © ESO 2007.

Abstract:

We were recently faced with the following problem: The T-shaped Plateau de Bure Interferometer is expanding its tracks to achieve higher spatial resolution in astronomical images at mm wavelengths. Two more stations for positioning the antennas during observations are being built at the ends of the extended tracks. Which of the given stations should the remaining four antennas occupy to accommodate the new stations? What is the optimal set of antenna configurations, given the new extended one, to achieve necessary coverage of the uv-plane at a variety of spatial resolutions? We present in this paper the solutions to the above questions, resulting from a novel method we have recently developed. The method is based on identifying which placement of elements provides the most appropriate uv-plane sampling. © 2006 American Institute of Physics.

Abstract:

We revisit the phenomenon of pulse nulling using high-quality single-pulse data of PSR B1133+16 from simultaneous multifrequency observations. Observations were made at 325, 610, 1400 and 4850 MHz as part of a joint program between the European Pulsar Network and the Giant Metrewave Radio Telescope. The pulse energy time series are analysed to derive improved statistics of nulling pulses as well as to investigate the frequency dependence of the phenomenon. The pulsar is observed to be in null state for approximately 15% of the time; however, we find that nulling does not always occur simultaneously at all four frequencies of observation. We characterise this "selective nulling'' as a function of frequency, separation in frequency, and combination of frequencies. The most remarkable case is a significantly large number of nulls ($\approx$6%) at lower frequencies, that are marked by the presence of a fairly narrow emission feature at the highest frequency of 4850 MHz. We refer to these as "low frequency (LF) nulls." Our analysis shows that this high frequency emission tends to occur preferentially over a narrow range in longitude and with pulse widths typically of the order of a few milliseconds. We discuss the implications of our results for the pulsar emission mechanism in general and for the broadbandness of nulling phenomenon in particular. Our results signify the presence of an additional process of emission which does not turn off when the pulsar nulls at low frequencies, and becomes more prominent at higher frequencies. Our analysis also hints at a possible outer gap origin for this new population of pulses, and thus a likely connection to some high-energy emission processes that occur in the outer parts of the pulsar magnetosphere.

Abstract:

We outline here a simple yet efficient method for finding optimized configurations of the elements of radio-astronomical interferometers with fixed pad locations. The method can be successfully applied, as we demonstrate, to define new configurations when changes to the array take place. This may include the addition of new pads or new antennas, or the loss of pads or antennas. Our method is based on identifying which placement of elements provides the most appropriate u-v plane sampling for astronomical imaging. © 2006. The American Astronomical Society. All rights reserved.