Curvature radiation in pulsar magnetospheric plasma

We consider the curvature radiation of the point-like charge moving relativistically along curved magnetic field lines through a pulsar magnetospheric electron-positron plasma. We demonstrate that the radiation power is largely suppressed as compared with the vacuum case, but still at a considerable level, high enough to explain the observed pulsar luminosities. The emitted radiation is polarized perpendicularly to the plane of the curved magnetic filed lines coincides with $ which can freely escape from the magnetospheric plasma. Our results strongly support the coherent curvature radiation by the spark-associated solitons as a plausible mechanism of pulsar radio emission

X-ray pulsar radiation from polar cap heated by back-flow bombardment

We consider the problem of the thermal X-ray radiation from the hot polar cap of radio pulsars showing evidence of \EB subpulse drift in radio band. In our recent Paper I, using the partially screened gap (PSG) model of inner acceleration region we derived a simple relationship between the drift rate of subpulses observed in a radio-band and the thermal X-ray luminosity from polar caps heated by the back-flow particle bombardment. This relationship can be tested for pulsars in which the so-called carousel rotation time $P_4$, reflecting the \EB plasma drift, and the thermal X-ray luminosity $L_x$ from the hot polar cap are known. To test the model we used only two available pulsars: PSRs B0943+10 and B1133+16. They both satisfied the model prediction, although due to low photon statistics the thermal component could not be firmly identified from the X-ray data. Nevertheless, these pulsars were at least consistent with PSG pulsar model. In the present paper we consider two more pulsars: PSRs B0656+14 and B0628-28, whose data have recently become available. In PSR B0656+14 the thermal radiation from the hot polar cap was clearly detected, and PSR B0628-28 also seems to have such a component. In all cases for which both $P_4$ and $L_x$ are presently known, the PSG pulsar model seems to be fully confirmed. Other available models of inner acceleration region fail to explain the observed relationship between radio and X-ray data. The pure vacuum gap model predicts too high $L_x$ and too low $P_4$, while the space charge limited model predicts too low $L_x$ and the origin of the subpulse drift has no natural explanation

Nature of eclipsing pulsars

We present a model for pulsar radio eclipses in some binary systems, and test this model for PSRs B1957+20 and J2051-0827. We suggest that in these binaries the companion stars are degenerate dwarfs with strong surface magnetic fields. The magnetospheres of these stars are permanently infused by the relativistic particles of the pulsar wind. We argue that the radio waves emitted by the pulsar split into the eigenmodes of the electron-positron plasma as they enter the companion's magnetosphere and are then strongly damped due to cyclotron resonance with the ambient plasma particles. Our model explains in a natural way the anomalous duration and behavior of radio eclipses observed in such systems. In particular, it provides stable, continuous, and frequency-dependent eclipses, in agreement with the observations. We predict a significant variation of linear polarization both at eclipse ingress and egress. In this paper we also suggest several possible mechanisms of generation of the optical and $X$-ray emission observed from these binary systems.Comment: 12 pages, 5 figures, submitted to Ap

The spark-associated soliton model for pulsar radio emission

We propose a new, self-consistent theory of coherent pulsar radio emission based on the non-stationary sparking model of Ruderman & Sutherland (1975), modified by Gil & Sendyk (2000) in the accompanying Paper I. According to these authors, the polar cap is populated as densely as possible by a number of sparks with a characteristic perpendicular dimension D approximately equal to the polar gap height scale h, separated from each other also by about h. Each spark reappears in approximately the same place on the polar cap for a time scale much longer than its life-time and delivers to the open magnetosphere a sequence of electron-positron clouds which flow orderly along a flux tube of dipolar magnetic field lines. The overlapping of particles with different momenta from consecutive clouds leads to effective two-stream instability, which triggers electrostatic Langmuir waves at the altitudes of about 50 stellar radii. The electrostatic oscillations are modulationally unstable and their nonlinear evolution results in formation of ``bunch-like'' charged solitons. A characteristic soliton length along magnetic field lines is about 30 cm, so they are capable of emitting coherent curvature radiation at radio wavelengths. The net soliton charge is about 10^21 fundamental charges, contained within a volume of about 10^14 cm^3. For a typical pulsar, there are about 10^5 solitons associated with each of about 25 sparks operating on the polar cap at any instant. One soliton moving relativisticaly along dipolar field lines with a Lorentz factor of the order of 100 generates a power of about 10^21 erg/s by means of curvature radiation. Then the total power of a typical radio pulsar can be estimated as being about 10^(27-28) erg/s.Comment: 27 pages, 5 figures, accepted by Ap

Vacuum Gap Model for PSR B0943+10

PSR B0943+10 is known to show remarkably stable drifting subpulses, which can be interpreted in terms of a circumferential motion of 20 sparks, each completing one circulation around the periphery of the polar cap in 37 pulsar periods. We use this observational constraint and argue that the vacuum gap model can adequately describe the observed drift patterns. Further we demonstrate that {\em only} the presence of strong non-dipolar surface magnetic field can favor such vacuum gap formation. Subsequently, for the first time we are able to constrain the parameters of the surface magnetic field, and model the expected magnetic structure on the polar cap of PSR B0943+10 considering the inverse Compton scattering photon dominated vacuum gap.Comment: 6 pages, 3 figures, acccepted for publication in Astronomy and Astrophysic

Drifting subpulses and inner acceleration regions in radio pulsars

The classical vacuum gap model of Ruderman & Sutherland, in which spark-associated subbeams of subpulse emission circulate around the magnetic axis due to the EB drift, provides a natural and plausible physical mechanism of the subpulse drift phenomenon. Recent progress in the analysis of drifting subpulses in pulsars has provided a strong support to this model by revealing a number of subbeams circulating around the magnetic axis in a manner compatible with theoretical predictions. However, a more detailed analysis revealed that the circulation speed in a pure vacuum gap is too high when compared with observations. Moreover, some pulsars demonstrate significant time variations of the drift rate, including a change of the apparent drift direction, which is obviously inconsistent with the EB drift scenario in a pure vacuum gap. We resolved these discrepancies by considering a partial flow of iron ions from the positively charged polar cap, coexisting with the production of outflowing electron-positron plasmas. By fitting the observationally deduced drift-rates to the theoretical values, we managed to estimate polar cap surface temperatures in a number of pulsars. The estimated surface temperatures correspond to a small charge depletion of the order of a few percent of the corotational charge density. We also argue that if the thermionic electron outflow from the surface of a negatively charged polar cap is slightly below the Goldreich-Julian density, then the resulting small charge depletion will have similar consequences as in the case of the ions outflow. We thus believe that the sparking discharge of a partially shielded acceleration potential drop occurs in all pulsars, with both positively (``pulsars'') and negatively (``anti-pulsars'') charged polar caps

On the formation of inner vacuum gaps in radio pulsars

The problem of formation of the Ruderman-Sutherland type inner vacuum gap in neutron stars with ${\bf\Omega}\cdot{\bf B}<0$ is considered. It is argued by means of the condition $T_i/T_s>1$ (where $T_i$ is the critical temperature above which $^{56}_{26}$Fe ions will not be bound at the surface and $T_s$ is the actual temperature of the polar cap surface heated by the back-flow of relativistic electrons) that the inner vacuum gap can form, provided that the actual surface magnetic field is extremaly strong ($B_s\gtrsim 10^{13}$ G) and curved (${\cal R}<10^6$ cm), irrespective of the value of dipolar component measured from the pulsar spin down rate. Calculations are carried out for pulsars with drifting subpulses and/or periodic intensity modulations, in which the existence of the quasi steady vacuum gap discharging via ${\bf E}\times{\bf B}$ drifting sparks is almost unavoidable. Using different pair-production mechanisms and different estimates of the cohesive energies of surface iron ions, we show that it is easier to form the vacuum gap controlled by the resonant inverse Compton scaterring seed photons than by the curvature radiation seed photons.Comment: 14 pages, 3 figure

Radio Properties of Rotating Radio Transients I: searches for periodicities and randomness in pulse arrival times

We have analysed the long- and short-term time dependence of the pulse arrival times and the pulse detection rates for eight Rotating Radio Transient (RRAT) sources from the Parkes Multi-beam Pulsar Survey (PMPS). We find significant periodicities in the individual pulse arrival times from six RRATs. These periodicities range from 30 minutes to 2100 days and from one to 16 independent (i.e. non-harmonically related) periodicities are detected for each RRAT. In addition, we find that pulse emission is a random (i.e. Poisson) process on short (hour-long) time scales but that most of the objects exhibit longer term (months-years) non-random behaviour. We find that PSRs J1819-1458 and J1317-5759 emit more doublets (two consecutive pulses) and triplets (three consecutive pulses) than is expected in random pulse distributions. No evidence for such an excess is found for the other RRATs. There are several different models for RRAT emission depending on both extrinsic and intrinsic factors which are consistent with these properties.Comment: Accepted by MNRAS on 2011 July 2. Contains 11 pages, 4 figures, 4 table

Modelling of surface magnetic field in neutron stars: application to radio pulsars

We propose a vacuum gap (VG) model which can be applied uniformly for normal and high magnetic field pulsars. The model requires strong and non-dipolar surface magnetic field near the pulsar polar cap. We assume that the actual surface magnetic field in pulsars results from a superposition of global dipole field and crust-anchored small scale magnetic anomaly. We provide a numerical formalism for modelling such structures of surface magnetic field and explore it within the framework of VG model, which requires strong surface fields more than 10^{13} G.Comment: Submitted to A&A, 11 pages, 9 figure