Janusz Gil

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 E × B drift of spark plasma filaments, provides a natural and plausible physical mechanism of the subpulse drift phenomenon. Moreover, this is the only model with quantitative predictions that can be compared with observa- tions. 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 pul- sars demonstrate significant time variations of the drift rate, including a change of the apparent drift direction, which is obviously inconsistent with the E × B drift scenario in a pure vacuum gap. We attempted to resolve these discrepancies by considering a partial flow of iron ions from the positively charged polar cap, co- existing with the production of outflowing electron-positron plasmas. The model of such charge-depleted acceleration region is highly sensitive to both the critical ion temperature Ti � 10 6 K (above which ions flow freely with the corotational charge density) and the actual surface temperature Ts of the polar cap, heated by the bombardment of ultra-relativistic charged particles. By fitting the obser- vationally deduced drift-rates to the theoretical values, we managed to estimate polar cap surface temperatures in a number of pulsars. The estimated surface temperatures Ts correspond to a small charge depletion of the order of a few percent of the Goldreich-Julian corotational charge density. Nevertheless, the re-

CURVATURE RADIATION IN PULSAR MAGNETOSPHERIC PLASMA

We consider the curvature radiation of a pointlike charge moving relativistically along curved magnetic field lines through a pulsar magnetospheric electron-positron plasma. We demonstrate that the radiation power is largely suppressed compared with the vacuum case, but still at a considerable level, high enough to explain the observed pulsar luminosities. The outgoing waves are polarized perpendicularly to the plane of the dipolar magnetic field lines. Our results strongly support coherent curvature radiation by the spark-associated solitons as a plausible mechanism of pulsar radio emission.

Vacuum gaps in pulsars and PSR J2144-3933

In this paper we revisit the radio pulsar death line problem within the framework of curvature radiation and/or the inverse Compton scattering-induced vacuum gap model above neutron star polar caps. Our special interest is in the recently detected pulsar PSR J2144-3933 with extremal period 8.5 s, which lies far beyond conventional death lines. We argue that the formation of vacuum gaps requires a complicated multipolar surface magnetic field, with a strength Bs typically much higher than the surface dipolar component Bd and a radii of curvature much smaller than the neutron star radius R = 106 cm. Such a multipolar surface field is also consistent with death lines including the extremal pulsar PSR J2144-3933. Since vacuum gap models produce sparks, our paper naturally supports the spark related models of subpulse drift phenomenon as well as the spark-associated models of coherent pulsar radio emission.

On the origins of part-time radio pulsars

Growing evidence suggests that some radio pulsars only act sporadically. These ‘part-time’ pulsars include long-term nulls, quasi-periodic radio flares in PSR B1931+24, as well as the so-called Rotating Radio Transients (RRATs). Based on the assumption that these objects are isolated neutron stars similar to conventional radio pulsars, we discuss two possible interpretations to the phenomenon. The first interpretation suggests that these objects are pulsars slightly below the radio emission ‘death line’, which become occasionally active only when the conditions for pair production and coherent emission are satisfied. The second interpretation invokes a radio emission direction reversal in conventional pulsars, as has been introduced to interpret the peculiar mode changing phenomenon in PSR B1822−09. In this picture, our line of sight misses the main radio emission beam of the pulsar but happens to sweep the emission beam when the radio emission direction is reversed. These part-time pulsars are therefore the other half of ‘nulling’ pulsars. We suggest that X-ray observations may provide clues to differentiate between these two possibilities.

XMM-Newton Observations of Radio Pulsars B0834+06 and B0826–34 and Implications for the Pulsar Inner Accelerator

We report the X-ray observations of two radio pulsars with drifting subpulses, B0834+06 and B0826?34, using XMM-Newton. PSR B0834+06 was detected with a total of 70 counts from the three EPIC instruments over 50 ks exposure time. Its spectrum is best described as that of a blackbody (BB), with temperature -->Ts = (2.0+ 2.0?0.9) ? 106 K and bolometric luminosity -->Lb = (8.6+ 14.2?4.4) ? 1028 erg s?1. As is typical in pulsars with BB thermal components in their X-ray spectra, the hot-spot surface area is much smaller than that of the canonical polar cap, implying a non-dipolar surface magnetic field much stronger than the dipolar component derived from the pulsar spin-down (in this case about 50 times smaller and stronger, respectively). The second pulsar, PSR B0826?34, was not detected over the 50 ks exposure time, giving an upper limit for the bolometric luminosity -->Lb ? 1.4 ? 1029 erg s?1. We use these data, as well as the radio emission data concerned with drifting subpulses, to test the partially screened gap (PSG) model of the inner accelerator in pulsars. This model predicts a simple and very intuitive relationship between the polar cap thermal X-ray luminosity ( -->Lb) and the carousel period ( -->P4) for drifting subpulses detected in the radio band. The PSG model has been previously successfully tested with four radio pulsars whose -->Lb and -->P4 were both measured: PSR B0943+10, PSR B1133+16, PSR B0656+14, and PSR B0628?28. The XMM-Newton X-ray data of PSR B0834+16 reported here are also in agreement with the model prediction, and the upper limit derived from the PSR B0826?34 observation does not contradict it. We also include two other pulsars, PSR B1929+10 and B1055?52, whose -->Lb and/or -->P4 data became available just recently. These pulsars also follow the prediction of the PSG model. The clear prediction of the PSG model is now supported by all pulsars whose -->Lb and -->P4 are measured and/or estimated.

Convection in protoneutron stars and the structure of surface magnetic fields in pulsars

We consider generation and evolution of small-scale magnetic fields in neutron stars. These fields can be generated by small-scale turbulent dynamo action soon after the collapse when the protoneutron star is subject to convective and neutron finger instabilities. After instabilities stop, small-scale fields should be frozen into the crust that forms initially at high density ∼10 14 g/cm 3 and then spreads to the surface. Because of high crustal conductivity, magnetic fields with the lengthscale ∼1-3 km can survive in the crust as long as 10-100 Myr and form a sunspot-like structure at the surface of radiopulsars.We consider generation and evolution of small-scale magnetic fields in neutron stars. These fields can be generated by small-scale turbulent dynamo action soon after the collapse when the proto-neutron star is subject to convective and neutron finger instabilities. After instabilities stop, small-scale fields should be frozen into the crust that forms initially at high density about 10^{14} g/cm^{3} and then spreads to the surface. Because of high crustal conductivity, magnetic fields with the lengthscale about 1-3 km can survive in the crust as long as 10-100 Myr and form a sunspot-like structure at the surface of radiopulsars.

UNRAVELING THE NATURE OF COHERENT PULSAR RADIO EMISSION

Forty years have passed since the discovery of pulsars, yet the physical mechanism of their coherent radio emission is a mystery. Recent observational and theoretical studies strongly suggest that the radiation coming out from the pulsar magnetosphere mainly consists of extraordinary waves polarized perpendicular to the planes of pulsar dipolar magnetic field. However, the fundamental question of whether these waves are excited by maser or coherent curvature radiation, remains open. High-quality single-pulse polarimetry is required to distinguish between these two possible mechanisms. Here we showcase such decisive, strong single pulses from 10 pulsars observed with the Giant Meterwave Radio Telescope, showing extremely high linear polarization with the position angle following locally the mean position angle traverse. These pulses, which are relatively free from depolarization, must consist exclusively of a single polarization mode. We associate this mode with the extraordinary wave excited by the coherent curvature radiation. This crucial observational signature enables us to argue, for the first time, in favor of the coherent curvature emission mechanism, excluding the maser mechanism.

Formation of a partially-screened inner acceleration region in radio pulsars: drifting subpulses and thermal x-ray emission from polar cap surface

The subpulse drifting phenomenon in pulsar radio emission is considered within the partially screened inner gap model, in which the sub-Goldreich-Julian thermionic flow of iron ions or electrons coexists with the spark-associated electron-positron plasma flow. We derive a simple formula that relates the thermal X-ray luminosity LX from the spark-heated polar cap and the × subpulse periodicity 3 (polar cap carousel time). For PSRs B0943+10 and B1133+16, the only two pulsars for which both 3 and LX are known observationally, this formula holds well. For a few other pulsars, for which only one quantity is measured observationally, we predict the value of the other quantity and propose relevant observations that can confirm or discard the model. Then we further study the detailed physical conditions that allow such partially screened inner gap to form. By means of the condition Tc/Ts > 1 (where Tc is the critical temperature above which the surface delivers a thermal flow to adequately supply the corotation charge density, and Ts is the actual surface temperature), it is found that a partially screened gap (PSG) can be formed given that the near surface magnetic fields are very strong and curved. We consider both curvature radiation (CR) and resonant inverse Compton scattering (ICS) to produce seed photons for pair production, and find that the former is the main agency to produce gamma rays to discharge the PSG.

The braking indices in pulsar emission models

Using the method proposed in a previous paper, we calculate pulsar braking indices in the models with torque contributions from both inner and outer accelerating regions, assuming that the interaction between them is negligible. We suggest that it is likely that the inverse Compton scattering induced polar vacuum gap and the outer gap coexist in the pulsar magnetosphere. We include the new near threshold vacuum gap models with curvature-radiation and inverse Compton scattering induced cascades, respectively; and find that these models can well reproduce the measured values of the braking indices.

Two modes of partially screened gap

The analysis of X-ray observations suggest an ultrastrong (