arXiv subjects

S. Johnston

Publications and source records attributed to S. Johnston.

At least 325 records · Page 18Linked to original sources

arXiv2003

Imprints of neutron stars in the interstellar medium

We have carried out an HI survey towards X-ray central compact objects (CCOs) inside supernova remnants (SNRs) which shows that many of them are placed within local HI minima. The nature of these minima is not clear, but the most likely explanation is that the CCOs have evacuated the neighboring gas. This survey also allowed us to detect a weak, diffuse radio nebula inside the SNR Vela Jr, probably created by the winds of its associated CCO.

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arXiv2003

Influence of the neutron star 1E 161348-5055 in RCW 103 on the surrounding medium

We have carried out a study of the neutral hydrogen in the direction of the X-ray source 1E 161348-5055, a compact central object (CCO) located in the interior of the supernova remnant (SNR) RCW 103. The HI 21 cm line observations were carried out using the Australia Telescope Compact Array, complemented with single dish data from the Parkes radio telescope to recover information at all spatial scales. We derive a distance to RCW 103 of 3.3 kpc, in agreement with previous distance measurements. We have also detected a small hole in the HI emission which is positionally and kinematically coincident with the location of the CCO which confirms the association between the SNR and the CCO. This is the third case of a depression in HI emission seemingly associated with CCOs in SNRs. The characteristic parameters of the holes such as their size, eccentricity and evacuated mass are similar in all three cases. We estimate the absorbing HI column density towards 1E 161348-5055 to be ~6 x 10^{21} cm^{-2}, a value compatible with a blackbody solution for the CCO X-ray emission. However, the implied brightness temperature is very high comparedto most neutron stars. Moreover, the strong long-term variability in X-rays favours the hypothesis that 1E 161348-5055 is an accreting binary sourcerather than an isolated, cooling neutron star. An analysis of the continuum image obtained at 1.4 GHz from these observations shows no trace of a pulsar wind nebula around 1E 161348-5055, in spite of it being a young object.

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arXiv2003

|V|: New insight into the circular polarization of radio pulsars

We present a study of single pulses from nine bright northern pulsars to investigate the behaviour of circular polarisation, V. The observations were conducted with the Effelsberg 100-m radio telescope at 1.41 GHz and 4.85 GHz and the Westerbork radio telescope at 352 MHz. For the first time, we present the average profile of the absolute circular polarisation |V| in the single pulses. We demonstrate that the average profile of |V| is the distinguishing feature between pulse components that exhibit low V in the single pulses and components that exhibit high V of either handedness, despite both cases resulting in a low mean. We also show that the |V| average profile remains virtually constant with frequency, which is not generally the case for V, leading us to the conclusion that |V| is a key quantity in the pulsar emission problem.

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arXiv2003

Observations of the neutral hydrogen surrounding the radio quiet neutron star RX J0822-4300 in Puppis A

We have observed the HI distribution in an area 40' x 40' around the neutron star candidate RX J0822-4300, which is located in the supernova remnant Puppis A. The observations of the 21 cm line were obtained with the Australia Telescope Compact Array (ATCA) and were combined with single dish data from the Southern Galactic Plane Survey. The spatial resolution is 90", and the velocity resolution, 1 km/s . A sensitivity of ~ 0.7 K was achieved. The results revealed a double lobed feature of reduced emission at +16 km/s, centered on the central compact object (CCO), and aligned with an HI hole blueshifted by 13 km/s. The HI depressions have probably been created by the sweeping up of ~ 2 solar masses. The alignement between the lobes and the optical expansion centre of Puppis A suggests that the CCO could be ejecting two opposite jets. The velocity at which the two lobes are best defined allowed us to confirm that the distance to Puppis A is 2.2 kpc, based on a systemic velocity of +16 km/s. The hydrogen column density computed using this systemic velocity is consistent with estimates from models for X-ray spectra, thus reinforcing our conclusion that the kinematic distance is 2.2 k pc.

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arXiv2003

Simultaneous single-pulse observations of radio pulsars - IV. Flux density spectra of individual pulses

In this paper we demonstrate that a large, unexplored reservoir of information about pulsar emission exists, that is directly linked to the radiating particles and their radiation process: We present a study of flux density measurements of individual pulses simultaneously observed at four different frequencies. Correcting for effects caused by the interstellar medium, we derive intrinsic flux density spectra of individual radio pulses observed at several frequencies for the first time. Pulsar B0329+54 was observed at 238, 626, 1412 and 4850 MHz, while observations of PSR B1133+16 were made at 341, 626, 1412 and 4850 MHz. We derive intrinsic pulse-to-pulse modulation indices which show a minimum around 1 GHz. Correlations between the flux densities of different frequency pairs worsen as the frequency separation widens and also tend to be worse for outer profile components. The single pulse spectra of PSR B0329+54 resemble the spectra of the integrated profile. However, the spectral index distributions for the single pulses of PSR B1133+16 show significant deviations from a Gaussian. This asymmetry is caused by very strong pulses with flux densities exceeding the mean value by more than a factor of ten. These strong pulses occur preferentially at the trailing edge of the leading component and appear to be broadband in most cases. Their properties are similar to those of so-called giant pulses, suggesting that these phenomena are related.

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arXiv2003

Intrinsic Variability and Field Statistics for the Vela Pulsar: 2. Systematics and Single-Component Fits

Individual pulses from pulsars have intensity-phase profiles that differ widely from pulse to pulse, from the average profile, and from phase to phase within a pulse. Widely accepted explanations do not exist for this variability or for the mechanism producing the radiation. The variability corresponds to the field statistics, particularly the distribution of wave field amplitudes, which are predicted by theories for wave growth in inhomogeneous media. This paper shows that the field statistics of the Vela pulsar (PSR B0833-45) are well-defined and vary as a function of pulse phase, evolving from Gaussian intensity statistics off-pulse to approximately power-law and then lognormal distributions near the pulse peak to approximately power-law and eventually Gaussian statistics off-pulse again. Detailed single-component fits confirm that the variability corresponds to lognormal statistics near the peak of the pulse profile and Gaussian intensity statistics off-pulse. The lognormal field statistics observed are consistent with the prediction of stochastic growth theory (SGT) for a purely linear system close to marginal stability. The simplest interpretations are that the pulsar's variability is a direct manifestation of an SGT state and the emission mechanism is linear (either direct or indirect), with no evidence for nonlinear mechanisms like modulational instability and wave collapse which produce power-law field statistics. Stringent constraints are placed on nonlinear mechanisms: they must produce lognormal statistics when suitably ensemble-averaged. Field statistics are thus a powerful, potentially widely applicable tool for understanding variability and constraining mechanisms and source characteristics of coherent astrophysical and space emissions.

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arXiv2003

Intrinsic Variability and Field Statistics for the Vela Pulsar: 3. Two-Component Fits and Detailed Assessment of Stochastic Growth Theory

The variability of the Vela pulsar (PSR B0833-45) corresponds to well-defined field statistics that vary with pulsar phase, ranging from Gaussian intensity statistics off-pulse to approximately power-law statistics in a transition region and then lognormal statistics on-pulse, excluding giant micropulses. These data are analyzed here in terms of two superposed wave populations, using a new calculation for the amplitude statistics of two vectorially-combined transverse fields. Detailed analyses show that the approximately power-law and lognormal distributions observed are fitted well at essentially all on-pulse phases by Gaussian-lognormal and double-lognormal combinations, respectively. These good fits, plus the smooth but significant variations in fit parameters across the source, provide strong evidence that the approximately power-law statistics observed in the transition region are not intrinsic. Instead, the data are consistent with normal pulsar emission having lognormal statistics at all phases. This is consistent with generation in an inhomogeneous source obeying stochastic growth theory (SGT) and with the emission mechanism being purely linear (either direct or indirect). A nonlinear mechanism is viable only if it produces lognormal statistics when suitably ensemble-averaged. Variations in the SGT fit parameters with phase imply that the radiation is relatively more variable near the pulse edges than near the center, as found in earlier work. In contrast, Vela's giant micropulses come from a very restricted phase range and have power-law statistics with indices ($6.7 \pm 0.6$) not inconsistent with nonlinear wave collapse. These results imply that normal pulses have a different source and generation mechanism than giant micropulses, as suggested previously on other grounds.

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arXiv2003

Simultaneous single-pulse observations of radio pulsars III. The behaviour of circular polarization

We investigate circular polarization in pulsar radio emission through simultaneous observations of PSR B1133+16 at two frequencies. In particular, we investigate the association of the handedness of circular polarization with the orthogonal polarization mode phenomenon at two different frequencies. We find the association to be significant across the pulse for PSR B1133+16, making a strong case for orthogonal polarization modes determining the observed circular polarization. The association however is not perfect and decreases with frequency. Based on these results and assuming emission occurs in superposed orthogonal polarization modes, we present a technique of mode decomposition based on single pulses. Average profiles of the polarization of each mode can then be computed by adding the individual mode-separated single pulses. We show that decomposing single pulses produces different average profiles for the orthogonal polarization modes from decomposing average profiles. Finally, we show sample single pulses and discuss the implications of the frequency dependence of the correlation of the circular polarization with the orthogonal polarization mode phenomenon.

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arXiv2002

A search for the radio counterpart of the unidentified gamma-ray source 3EG J1410-6147

We have made radio continuum, HI and X-ray observations in the direction of the unidentified EGRET source 3EG J1410-6147, using the Australia Telescope Compact Array and the Chandra X-ray Observatory. The observations encompass the supernova remnant (SNR) G312.4-0.4 and the two young pulsars PSRs J1412-6145 and J1413-6141. We derive a lower distance limit of 6 kpc to the SNR, although interpretation of positive velocity features in the HI spectrum may imply the SNR is more distant than 14 kpc. PSR J1412-6145, with an age of 50 kyr, is the pulsar most likely associated with SNR G312.4-0.4. X-rays are not detected from either pulsar and diffuse X-ray emission near the bright western edge of the SNR is weak. Although there is circumstantial evidence that this western region is a pulsar wind nebula (PWN), the embedded pulsar PSR J1412-6145 is apparently not sufficiently powerful to explain the radio enhancement. The origin of the electron acceleration in this region and of the gamma-rays remain unidentified, unless the distance to PSR J1413-6141 is at least a factor of 3 lower than its dispersion measure distance.

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arXiv2002

The 2000 Periastron Passage of PSR B1259-63

We report here on a sequence of 28 observations of the binary pulsar system PSR B1259-63/SS2883 at four radio frequencies made with the Australia Telescope Compact Array around the time of the 2000 periastron passage. Observations made on 2000 Sep 1 show that the pulsar's apparent rotation measure (RM) reached a maximum of $-14800 \pm 1800$ rad m$^{-2}$, some 700 times the value measured away from periastron, and is the largest astrophysical RM measured. This value, combined with the dispersion measure implies a magnetic field in the Be star's wind of 6 mG. We find that the light curve of the unpulsed emission is similar to that obtained during the 1997 periastron but that differences in detail imply that the emission disc of the Be star is thicker and/or of higher density. The behaviour of the light curve at late times is best modelled by the adiabatic expansion of a synchrotron bubble formed in the pulsar/disc interaction. The expansion rate of the bubble $\sim 12$ km s$^{-1}$ is surprisingly low but the derived magnetic field of 1.6 G close to that expected.

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arXiv2002

Simultaneous single-pulse observations of radio pulsars: II. Orthogonal polarization modes in PSR B1133+16

In this paper, we present a study of orthogonal polarization modes in the radio emission of PSR B1133+16, conducted within the frame of simultaneous, multi-frequency, single-pulse observations. Simultaneously observing at two frequencies (1.41 GHz and 4.85 GHz) provides the means to study the bandwidth of polarization features such as the polarization position angle. We find two main results. First, that there is a high degree of correlation between the polarization modes at the two frequencies. Secondly, the modes occur more equally and the fractional linear polarization decreases towards higher frequencies. We discuss this frequency evolution and propose propagation effects in the pulsar magnetosphere as its origin.

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arXiv2002

High-resolution single-pulse studies of the Vela Pulsar

We present high-resolution multi-frequency single-pulse observations of the Vela pulsar, PSR B0833-45, aimed at studying micro-structure, phase-resolved intensity fluctuations and energy distributions at 1.41 and 2.30 GHz. We show that the micro-pulse width in pulsars has a period dependence. Like individual pulses, Vela's micro-pulses are highly elliptically polarized. There is a strong correlation between Stokes parameters V and I in the micro-structure. We show that the V/I distribution is Gaussian with a narrow width and that this width appears to be constant as a function of pulse phase. The phase-resolved intensity distributions of I are best fitted with log-normal statistics. Extra emission components, i.e.``bump'' and ``giant micro-pulses'', discovered by Johnston et al.(2001) are also present at 2.3 GHz. The bump component seems to be an extra component superposed on the main pulse profile but does not appear periodically. The giant micro-pulses are time-resolved and have significant jitter in their arrival times. Their flux density distribution is best fitted by a power-law, indicating a link between these features and ``classical'' giant pulses as observed for the Crab pulsar, (PSR B0531+21), PSR B1937+21 and PSR B1821-24. We find that Vela contains a mixture of emission properties representing both ``classical'' properties of radio pulsars (e.g. micro-structure, high degree of polarization, S-like position angle swing, orthogonal modes) and features which are most likely related to high-energy emission (e.g. extra profile components, giant micro-pulses). It hence represents an ideal test case to study the relationship between radio and high-energy emission in significant detail.

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arXiv2001

Intrinsic Variability of the Vela Pulsar: Lognormal Statistics and Theoretical Implications

Individual pulses from pulsars have intensity-phase profiles that differ widely from pulse to pulse, from the average profile, and from phase to phase within a pulse. Widely accepted explanations for pulsar radio emission and its time variability do not exist. Here, by analysing data near the peak of the Vela pulsar's average profile, we show that Vela's variability corresponds to lognormal field statistics, consistent with the prediction of stochastic growth theory (SGT) for a purely linear system close to marginal stability. Vela's variability is therefore a direct manifestation of an SGT state and the field statistics constrain the emission mechanism to be linear (either direct or indirect), ruling out nonlinear mechanisms like wave collapse. Field statistics are thus a powerful, potentially widely applicable tool for understanding variability and constraining mechanisms and source characteristics of coherent astrophysical and space emissions.

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arXiv2001

Scintillation measurements of the millisecond pulsar PSR J0030+0451 and pulsar space velocities

Scintillation observations of the nearby single millisecond pulsar (MSP) PSR J0030+0451 were carried out with the Parkes 64m radiotelescope at three different epochs in 1999. From analysis of the dynamic spectrum we obtained the amplitude of the electron density power spectrum log C_N^2 ~= -3.33$ and a scintillation velocity V_iss <~ 15 km s^-1. This result shows that the Shklovskii effect on the spin-down rate \dot P is negligible. We also performed a correlation analysis between pulsar proper motions (V_pm) and scintillation velocities (V_iss) using updated measurements for a sample of 77 objects, 17 of which are MSPs. The full sample shows a correlation coefficient r_s ~= 80% at an extremely high significance level, while for the MSP sub-sample (excluding 2 outliers) we obtain r_s ~= 90%.

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arXiv2000

Limits on radio emission from pulsar wind nebulae

We report on a sensitive survey for radio pulsar wind nebulae (PWN) towards 27 energetic and/or high velocity pulsars. Observations were carried out at 1.4 GHz using the Very Large Array and the Australia Telescope Compact Array, and utilised pulsar-gating to search for off-pulse emission. These observing parameters resulted in a considerably more sensitive search than previous surveys, and could detect PWN over a much wider range of spatial scales (and hence ambient densities and pulsar velocities). However, no emission clearly corresponding to a PWN was discovered. Based on these non-detections we argue that the young and energetic pulsars in our sample have winds typical of young pulsars, but produce unobservable PWN because they reside in low density (n approx 0.003 cm^-3) regions of the ISM. However, non-detections of PWN around older and less energetic pulsars can only be explained if the radio luminosity of their winds is less than 1e-5 of their spin-down luminosity, implying an efficiency at least an order of magnitude smaller than that seen for young pulsars.

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arXiv2000

Nanoarcsecond single-dish imaging of the Vela pulsar

We have measured the properties of the diffractive scintillation toward the Vela pulsar under the extremely strong scattering conditions encountered at 660 MHz. We obtain a decorrelation bandwidth of $ν_d = 244 \pm 4$ Hz and diffractive decorrelation timescale of $t_{\rm diff} = 3.3\pm 0.3$ s. Our measurement of the modulation indices $m=0.87\pm 0.003\pm 0.05$ and $m=0.93\pm 0.03 \pm 0.05$ (one for each polarization stream), are at variance with the modulation index of the Vela pulsar obtained at 2.3 GHz by Gwinn et al. (1997) {\it if} the deviation from a modulation index of unity is ascribed to a source size effect.

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arXiv1999

A deep search for pulsar wind nebulae using pulsar gating

Using the Australia Telescope Compact Array (ATCA) we have imaged the fields around five promising pulsar candidates to search for radio pulsar wind nebulae (PWNe). We have used the ATCA in its pulsar gating mode; this enables an image to be formed containing only off-pulse visibilities, thereby dramatically improving the sensitivity to any underlying PWN. Data from the Molonglo Observatory Synthesis Telescope were also used to provide sensitivity on larger spatial scales. This survey found a faint new PWN around PSR B0906-49; here we report on non-detections of PWNe towards PSRs B1046-58, B1055-52, B1610-50 and J1105-6107. Our radio observations of the field around PSR B1055-52 argue against previous claims of an extended X-ray and radio PWNe associated with the pulsar. If these pulsars power unseen, compact radio PWN, upper limits on the radio flux indicate that less than 1e-6 of their spin-down energy is used to power this emission. Alternatively PSR B1046-58 and PSR B1610-50 may have relativistic winds similar to other young pulsars and the unseen PWN is resolved and fainter than our surface brightness sensitivity threshold. We can then determine upper limits on the local ISM density of 2.2e-3 cm^-3 and 1e-2 cm^-3, respectively. Furthermore we constrain the spatial velocities of these pulsars to be less than ~450 km/s and thus rule out the association of PSR B1610-50 with SNR G332.4+00.1 (Kes 32). Strong limits on the ratio of unpulsed to pulsed emission are also determined for three pulsars.

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arXiv1999

SNR G320.4-01.2 and PSR B1509-58: New radio observations of a complex interacting system

We describe radio continuum and spectral line observations of the source G320.4-01.2 (MSH 15-52) and the coincident young pulsar B1509-58 made with the Australia Telescope Compact Array. Based on a comparison between X-ray and radio observations, we argue that the two main radio components of G320.4-01.2 are a single supernova remnant (SNR), which HI absorption indicates is at a distance of 5.2+/-1.4 kpc. A high-resolution correspondence between radio and X-rays argues that the pulsar is interacting with the SNR via an opposed pair of collimated outflows. The outflow itself is seen as an elongated X-ray feature surrounded by a highly polarized radio sheath, while the interaction with the SNR manifests itself as a ring of radio/X-ray knots within the optical nebula RCW 89. We reject the hypothesis that the pulsar outflow powers the entire RCW 89 region. SNR G320.4-01.2 and PSR B1509-58 agree in distance and in rotation measure, and appear to be interacting. We thus conclude that the two objects are associated and have an age of <~1700 yr. We propose that the SNR resulted from a high-energy or low-mass supernova which occurred near the edge of an elongated cavity. Such a model can account for the SNR's bilateral appearance, its large apparent age, the significant offset of the pulsar from the SNR's centre and the faintness of the pulsar-powered nebula at radio wavelengths.

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