Giant radio pulses from the millisecond pulsar PSR B1937+21 at 327 MHz

Seven giant radio pulses were recorded from the millisecond pulsar PSR B1937+21 during ≈8.1 min observation by the Ooty Radio Telescope (ORT) at 326.5 MHz. Although sparse, these observations support most of the giant pulse behaviour reported at higher radio frequencies (430 to 2380 MHz). Within the main component of the integrated profile, they are emitted only in a narrow (≲47 μs) window of pulse phase, close to its peak. This has important implications for doing super-high precision timing of PSR B1937+21 at low radio frequencies.     

Optimal filters for the construction of the ensemble pulsar time

An algorithm of the ensemble pulsar time based on the optimal Wiener filtration method has been constructed. This algorithm allows the separation of the contributions to the post-fit pulsar timing residuals of the atomic clock and the pulsar itself. Filters were designed using the cross- and auto-covariance functions of the timing residuals. The method has been applied to the timing data of millisecond pulsars PSR B1855+09 and B1937+21 and allowed the filtering out of the atomic-scale component from the pulsar data. Direct comparison of the terrestrial time TT(BIPM06) and the ensemble pulsar time PT_ens revealed that the fractional instability of TT(BIPM06)−PT_ens is equal to  σ _z = (0.8 ± 1.9) × 10⁻¹⁵  . Based on the  σ _z   statistics of TT(BIPM06)−PT_ens, a new limit of the energy density of the gravitational wave background was calculated to be equal to  Ω_g h ²∼ 3 × 10⁻⁹  .     

The effect of pulse profile evolution on pulsar dispersion measure

In an earlier paper, based on simultaneous multifrequency observations with the Giant Metrewave Radio Telescope (GMRT), we reported the variation of pulsar dispersion measures (DMs) with frequency. A few different explanations are possible for such frequency dependence, and a possible candidate is the effect of pulse shape evolution on the DM estimation technique. In this paper we describe extensive simulations we have done to investigate the effect of pulse profile evolution on pulsar DM estimates. We find that it is only for asymmetric pulse shapes that the DM estimate is significantly affected due to profile evolution with frequency. Using multifrequency data sets from our earlier observations, we have carried out systematic analyses of PSR B0329+54 and PSR B1642−03. Both these pulsars have central core-dominated emission which does not show significant asymmetric profile evolution with frequency. Even so, we find that the estimated DM shows significant variation with frequency for these pulsars. We also report results from new, simultaneous multifrequency observations of PSR B1133+16 carried out using the GMRT in phased array mode. This pulsar has an asymmetric pulse profile with significant evolution with frequency. We show that in such a case, amplitude of the observed DM variations can be attributed to profile evolution with frequency. We suggest that genuine DM variations with frequency could arise due to propagation effects through the interstellar medium and/or the pulsar magnetosphere.     

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 PWN associated with the pulsar. If these pulsars power unseen, compact radio PWNe, upper limits on the radio flux indicate that a fraction of less than 10⁻⁶ of their spin-down energy is used to power this emission. Alternatively, PSRs B1046−58 and B1610−50 may have relativistic winds similar to other young pulsars and the unseen PWN may be resolved and fainter than our surface brightness sensitivity threshold. We can then determine upper limits on the local interstellar medium (ISM) density of 2.2×10⁻³ and 1×10⁻² cm⁻³, respectively. Furthermore, we derive the spatial velocities of these pulsars to be ∼450 km s⁻¹ and thus rule out the association of PSR B1610−50 with supernova remnant (SNR) G332.4+00.1 (Kes 32). Strong limits on the ratio of unpulsed to pulsed emission are also determined for three pulsars.     

An empirical model for the beams of radio pulsars

Motivated by recent results on the location of the radio emission in pulsar magnetospheres, we have developed a model which can account for the large diversity found in the average profile shapes of pulsars. At the centre of our model lies the idea that radio emission at a particular frequency arises from a wide range of altitudes above the surface of the star, and that it is confined to a region close to the last open field lines. We assert that the radial height range over which emission occurs is responsible for the complex average pulse shapes rather than the transverse (longitudinal) range proposed in most current models. By implementing an abrupt change in the height range to discriminate between young, short-period, highly energetic pulsars and their older counterparts, we obtain the observed transition between the simple and complex average pulse profiles observed in each group respectively. Monte Carlo simulations are used to demonstrate the match of our model to real observations.     

Optical polarization of the Crab pulsar: precision measurements and comparison to the radio emission

The linear polarization of the Crab pulsar and its close environment was derived from observations with the high-speed photopolarimeter Optical Pulsar TIMing Analyser at the 2.56-m Nordic Optical Telescope in the optical spectral range (400–750 nm). Time resolution as short as 11 μs, which corresponds to a phase interval of 1/3000 of the pulsar rotation, and high statistics allow the derivation of polarization details never achieved before. The degree of optical polarization and the position angle correlate in surprising details with the light curves at optical wavelengths and at radio frequencies of 610 and 1400 MHz. Our observations show that there exists a subtle connection between presumed non-coherent (optical) and coherent (radio) emissions. This finding supports previously detected correlations between the optical intensity of the Crab and the occurrence of giant radio pulses. Interpretation of our observations requires more elaborate theoretical models than those currently available in the literature.     

A new technique for timing the double pulsar system

In 2004, McLaughlin et al. discovered a phenomenon in the radio emission of PSR J0737−3039B (B) that resembles drifting subpulses. The repeat rate of the subpulses is equal to the spin frequency of PSR J0737−3039A (A); this led to the suggestion that they are caused by incidence upon B's magnetosphere of electromagnetic radiation from A. Here, we describe a geometrical model which predicts the delay of B's subpulses relative to A's radio pulses. We show that measuring these delays is equivalent to tracking A's rotation from the point of view of a hypothetical observer located near B. This has three main astrophysical applications: (i) to determine the sense of rotation of A relative to its orbital plane, (ii) to estimate where in B's magnetosphere the radio subpulses are modulated and (iii) to provide an independent estimate of the mass ratio of A and B. The latter might improve existing tests of gravitational theories using this system.     

利用南山数据对PSR B0329+54的长时标轮廓稳定性的研究

利用新疆天文台25 m射电望远镜2003—2009年对PSR B0329+54长达453 h的观测数据,研究了这颗源在1540 MHz上正常模式和反常模式的轮廓稳定性时标.通过不同时间的积分脉冲轮廓与参考轮廓交叉相关系数分析,发现随积分时间的增加,两种模式的轮廓先是以较快的速度趋于稳定,在积分时间增加到约(4±1) min时,轮廓趋稳速率放缓,当积分时间达到(140±60) min (正常模式)或达到(65±15) min (反常模式)时,轮廓趋稳速度再次变快.相关系数的结果显示两种模式的轮廓在绝大多数积分时标上都不是完全随机的涨落.如果以发生轮廓趋稳速率从快到慢转变所需的积分时间作为轮廓稳定时标的判据,两种模式的稳定时标均大约为5 min.轮廓中不同成分强度比例的涨落随积分时间的变化关系各不相同,由此可以推断3个成分强度涨落性质有差异.造成轮廓趋稳行为在不同时标上有明显差异的原因有观测噪声和星际闪烁等.     

The topology and polarization of subbeams associated with the 'drifting' subpulse emission of pulsar B0943+10 – V. A new look at the low-frequency burst-mode emission

This paper reports new observations of pulsar B0943+10 carried out at the Pushchino Radio Astronomy Observatory (PRAO) at the low radio frequencies of 42, 62 and 112 MHz. B0943+10 is well known for its exquisitely regular burst-mode (B-mode) drifting subpulses as well as its weaker and chaotic quiescent mode. Earlier Arecibo investigations at 327 MHz have identified remarkable, continuous changes in its B-mode subpulse drift rate and integrated-profile shape with durations of several hours. These PRAO observations reveal that the changes in profile shape during the B-mode lifetime are strongly frequency dependent – namely the measured changes in the component amplitude ratio are more dramatic at 327 and 112 MHz as compared with those at 62 and 42 MHz. The differences, however, are most marked during the first several tens of minutes after B-mode onset; after an hour or so the profile shape changes tend to be more similar at all four frequencies. We also have found that the linear polarization of the integrated profile increases continuously throughout the lifetime of the B mode, going from hardly 10 per cent just after onset to some 40–50 per cent after several hours. Pulsar B0943+10's B mode thus provides a unique new opportunity to investigate continuous systematic changes in the plasma flow within the polar flux tube. While refraction in the pulsar's magnetosphere may well play some role, we find that the various frequency-dependent effects, both between and within the two modes, can largely be understood geometrically. If the modes and B-mode decay reflect systematic variations in the carousel-'spark' radius and emission height then a specific set of profile and linear polarization changes would be expected.     

The topology and polarization of sub-beams associated with the 'drifting' sub-pulse emission of pulsar B0943+10 – II. Analysis of Gauribidanur 35-MHz observations

In the previous paper of this series, Deshpande & Rankin reported results regarding the sub-pulse drift phenomenon in pulsar B0943+10 at 430 and 111 MHz. This study has led to the identification of a stable system of sub-beams circulating around the magnetic axis of this star. Here, we present a single-pulse analysis of our observations of this pulsar at 35 MHz. The fluctuation properties seen at this low frequency, as well as our independent estimates of the number of sub-beams required and their circulation time, agree remarkably well with the reported behaviour at higher frequencies. We use the 'cartographic' transform mapping technique developed by Deshpande & Rankin in Paper I to study the emission pattern in the polar region of this pulsar. The significance of our results in the context of radio emission mechanisms is also discussed.     

Timing models for the long orbital period binary pulsar PSR B1259–63

The pulsar PSR B1259–63 is in a highly eccentric 3.4-yr orbit with the Be star SS 2883. Timing observations of this pulsar, made over a 7-yr period using the Parkes 64-m radio telescope, cover two periastron passages, in 1990 August and 1994 January. The timing data cannot be fitted by the normal pulsar and Keplerian binary parameters. A timing solution including a (non-precessing) Keplerian orbit and timing noise (represented as a polynomial of fifth order in time) provides a satisfactory fit to the data. However, because the Be star probably has a significant quadrupole moment, we prefer to interpret the data by a combination of timing noise, dominated by a cubic phase term, and ω^. and x ^. terms. We show that the ω^. and x ^. terms are likely to be a result of a precessing orbit caused by the quadrupole moment of the tilted companion star. We further rule out a number of possible physical effects which could contribute to the timing data of PSR B1259–63 on a measurable level.     

PSR J1453+1902 and the radio luminosities of solitary versus binary millisecond pulsars

We present 3 yr of timing observations for PSR J1453+1902, a 5.79-ms pulsar discovered during a 430-MHz drift-scan survey with the Arecibo telescope. Our observations show that PSR J1453+1902 is solitary and has a proper motion of  8 ±  2  mas yr⁻¹. At the nominal distance of 1.2 kpc estimated from the pulsar's dispersion measure, this corresponds to a transverse speed of  46 ± 11   km s⁻¹  , typical of the millisecond pulsar population. We analyse the current sample of 55 millisecond pulsars in the Galactic disc and revisit the question of whether the luminosities of isolated millisecond pulsars are different from their binary counterparts. We demonstrate that the apparent differences in the luminosity distributions seen in samples selected from 430-MHz surveys can be explained by small-number statistics and observational selection biases. An examination of the sample from 1400-MHz surveys shows no differences in the distributions. The simplest conclusion from the current data is that the spin, kinematic, spatial and luminosity distributions of isolated and binary millisecond pulsars are consistent with a single homogeneous population.     

The possible companions of young radio pulsars

We discuss the formation of pulsars with massive companions in eccentric orbits. We demonstrate that the probability for a non-recycled radio pulsar to have a white dwarf as a companion is comparable to that of having an old neutron star as a companion. Special emphasis is given to PSR B1820−11 and PSR B2303+46. Based on population synthesis calculations we argue that PSR B1820−11 and PSR B2303+46 could very well be accompanied by white dwarfs with mass ≳1.1 M_⊙. For PSR B1820−11, however, we cannot exclude the possibility that its companion is a main-sequence star with a mass between ∼0.7 M_⊙ and ∼5 M_⊙.