Magnetospheric Gap Research Articles

What Drove the Carrington Event? An Analysis of Currents and Geospace Regions

AbstractThe 1859 Carrington event is the most intense geomagnetic storm in recorded history, and the literature provides numerous explanations for what drove the negative H perturbation on the Earth. There is debate on what dominated the event. Our analysis shows a combination of causes of similar orders of magnitude. Previous analyses generally rely upon the observed H perturbation at Colaba, India; historic newspaper reports; and empirical models. We expand the analysis using two Space Weather Modeling Framework simulations to examine what drove the event. We compute contributions from currents and geospace regions to the northward B field on Earth's surface, BN. We examine magnetospheric currents parallel and perpendicular to the local B field, ionospheric currents, and gap region field–aligned currents (FACs). We also evaluate contributions from the magnetosheath, near–Earth, and neutral sheet regions. A combination of currents and geospace regions significantly contribute to BN on the Earth's surface, changing as the storm evolves. At storm onset, magnetospheric currents and gap–region FACs dominate in the equatorial region. At auroral latitudes, gap–region FACs and ionospheric currents are the largest contributors. At storm peak, azimuthal magnetospheric currents and gap–region FACs dominate at equatorial latitudes. Gap–region FACs and ionospheric currents dominate in the auroral zone, down to mid‐latitudes. Both the magnetosheath and FACs contribute at storm peak, but are less significant than that from the near–Earth ring current. During recovery, the near–Earth ring current is the largest contributor at equatorial latitudes. Ionospheric currents and gap–region FACs dominate in the auroral zone.

The radio shut-off, glitch, and X-ray burst in 1E 1547.0−5408 interpreted through magnetic reconfiguration

ABSTRACT A short X-ray burst was observed from the radio-loud magnetar 1E 1547.0–5408 in 2022 April. Unusually however, the source stopped showing radio pulsations $\gtrsim 3\,$ weeks prior to the burst. After recovery, radio timing revealed that the object had also undergone a modest glitch. A model for the overall event is constructed where an initially mild perturbation adjusts the magnetic geometry near the polar caps, leading to shallow fractures. Crustal ejecta or particles leaking from a pair-plasma fireball pollute the magnetospheric gaps, shutting off the pulsar mechanism, but the energy release is not yet large enough to noticeably enhance the X-ray flux. This perturbation gradually ramps, eventuating in a large-scale energy redistribution which fuels the burst. The star’s mass quadrupole moment changes in tandem, issuing a glitch. Some quantitative estimates for the magnetic reconfiguration under this interpretation are provided, based on a quasi-static model where the fluid evolves through a sequence of hydromagnetic equilibria.

Radio Emission of Pulsars. II. Coherence Catalyzed by Cerenkov-unstable Shear Alfvén Waves

This paper explores small-scale departures from force-free electrodynamics around a rotating neutron star, extending our treatment of resistive instability in a quantizing magnetic field. A secondary, Cerenkov instability is identified: relativistic particles flowing through thin current sheets excite propagating charge perturbations that are localized near the sheets. Growth is rapid at wavenumbers below the inverse ambient skin depth k p,ex. Small-scale Alfvénic wavepackets are promising sources of coherent curvature radiation. When the group Lorentz factor , where R c is the magnetic curvature radius, a fraction ∼10−3–10−2 of the particle kinetic energy is radiated into the extraordinary mode at a peak frequency ∼10−2 ck p,ex. Consistency with observations requires a high pair multiplicity (∼103–5) in the pulsar magnetosphere. Neither the primary, slow resistive instability nor the secondary, Alfvénic instability depend directly on the presence of magnetospheric gaps, and may activate where the mean current is fully supplied by outward drift of the corotation charge. The resistive mode is overstable and grows at a rate comparable to the stellar spin frequency; the model directly accommodates strong pulse-to-pulse radio flux variations and coordinated subpulse drift. Alfvén mode growth can track the local plasma conditions, allowing for lower-frequency emission from the outer magnetosphere. Beamed radio emission from charged packets with γ gr ∼ 50–100 also varies on submillisecond timescales. The modes identified here will be excited inside the magnetosphere of a magnetar, and may mediate Taylor relaxation of the magnetic twist.

The Life Cycle of Magnetars: A Novel Approach to Estimate Their Ages

Abstract Anomalous X-ray pulsars and soft gamma repeaters are slowly rotating, young, and isolated neutron stars exhibiting sporadic outbursts and high X-ray quiescent luminosities. They are believed to be powered by ultrastrong magnetic fields, B ∼ 1014−1015 G, associated with “magnetars.” In the peculiar case of SGR 0418+5729, timing parameters imply a dipolar B-field of 6.1 × 1012 G. This discovery has challenged the traditional picture of magnetars in terms of B-field strengths, evolutionary stages, and ages. Here we provide a novel approach to estimate a magnetar’s age by considering the self-consistent time evolution of a plasma-filled oblique pulsar with the state-of-the-art magnetospheric particle acceleration gaps. The rotational period of magnetars increases over time due to angular momentum extraction by gravitational-wave radiations, magnetic dipole radiations, and particle winds. These torques also change the obliquity angle between the magnetic and rotation axes. For SGR 0418+5729, we obtain a dipolar B-field of 1.0 × 1014 G, and a realistic age of ∼18 kyr, consistent within the magnetar paradigm.

Electron-beam interaction with emission-line clouds in blazars

Context. An electron-positron beam escaping from the magnetospheric vacuum gap of an accreting black hole interacts with recombination-line photons from surrounding gas clouds. Inverse-Compton scattering and subsequent pair production initiate unsaturated electromagnetic cascades exhibiting a characteristic spectral energy distribution. Aims. By modelling the interactions of beam electrons (positrons) with hydrogen and helium recombination-line photons, we seek to describe the spectral signature of beam-driven cascades in the broad emission-line region of blazar jets. Methods. Employing coupled kinetic equations for electrons (positrons) and photons including an escape term, we numerically obtained their steady-state distributions and the escaping photon spectrum. Results. We find that cascade emission resulting from beam interactions can produce a narrow spectral feature at TeV energies. Indications of such an intermittent feature, which defies an explanation in the standard shock-in-jet scenario, have been found at ≈ 4σ confidence level at an energy of ≈ 3 TeV in the spectrum of the blazar Mrk 501. Conclusions. The energetic requirements for explaining the intermittent 3 TeV bump with the beam-interaction model are plausible: Gap discharges that lead to multi-TeV beam electrons (positrons) carrying ≈ 0.1% of the Blandford-Znajek luminosity, which interact with recombination-line photons from gas clouds that reprocess ≈ 1% of the similar accretion luminosity are required.

Study of the variable broadband emission of Markarian 501 during the most extreme Swift X-ray activity

Context. Markarian 501 (Mrk 501) is a very high-energy (VHE) gamma-ray blazar located at z = 0.034, which is regularly monitored by a wide range of multi-wavelength instruments, from radio to VHE gamma rays. During a period of almost two weeks in July 2014, the highest X-ray activity of Mrk 501 was observed in ∼14 years of operation of the Neil Gehrels Swift Gamma-ray Burst Observatory. Aims. We characterize the broadband variability of Mrk 501 from radio to VHE gamma rays during the most extreme X-ray activity measured in the last 14 years, and evaluate whether it can be interpreted within theoretical scenarios widely used to explain the broadband emission from blazars. Methods. The emission of Mrk 501 was measured at radio with Metsähovi, at optical–UV with KVA and Swift/UVOT, at X-ray with Swift/XRT and Swift/BAT, at gamma ray with Fermi-LAT, and at VHE gamma rays with the FACT and MAGIC telescopes. The multi-band variability and correlations were quantified, and the broadband spectral energy distributions (SEDs) were compared with predictions from theoretical models. Results. The VHE emission of Mrk 501 was found to be elevated during the X-ray outburst, with a gamma-ray flux above 0.15 TeV varying from ∼0.5 to ∼2 times the Crab nebula flux. The X-ray and VHE emission both varied on timescales of 1 day and were found to be correlated. We measured a general increase in the fractional variability with energy, with the VHE variability being twice as large as the X-ray variability. The temporal evolution of the most prominent and variable segments of the SED, characterized on a day-by-day basis from 2014 July 16 to 2014 July 31, is described with a one-zone synchrotron self-Compton model with variations in the break energy of the electron energy distribution (EED), and with some adjustments in the magnetic field strength and spectral shape of the EED. These results suggest that the main flux variations during this extreme X-ray outburst are produced by the acceleration and the cooling of the high-energy electrons. A narrow feature at ∼3 TeV was observed in the VHE spectrum measured on 2014 July 19 (MJD 56857.98), which is the day with the highest X-ray flux (>0.3 keV) measured during the entire Swift mission. This feature is inconsistent with the classical analytic functions to describe the measured VHE spectra (power law, log-parabola, and log-parabola with exponential cutoff) at more than 3σ. A fit with a log-parabola plus a narrow component is preferred over the fit with a single log-parabola at more than 4σ, and a dedicated Monte Carlo simulation estimated the significance of this extra component to be larger than 3σ. Under the assumption that this VHE spectral feature is real, we show that it can be reproduced with three distinct theoretical scenarios: (a) a pileup in the EED due to stochastic acceleration; (b) a structured jet with two-SSC emitting regions, with one region dominated by an extremely narrow EED; and (c) an emission from an IC pair cascade induced by electrons accelerated in a magnetospheric vacuum gap, in addition to the SSC emission from a more conventional region along the jet of Mrk 501.

The magnetic field and accretion regime of CI Tau

ABSTRACT This paper exploits spectropolarimetric data of the classical T Tauri star CI Tau collected with ESPaDOnS at the Canada–France–Hawaii Telescope, with the aims of detecting and characterizing the large-scale magnetic field that the star hosts, and of investigating how the star interacts with the inner regions of its accretion disc through this field. Our data unambiguously show that CI Tau has a rotation period of 9.0 d, and that it hosts a strong, mainly poloidal large-scale field. Accretion at the surface of the star concentrates within a bright high-latitude chromospheric region that spatially overlaps with a large dark photospheric spot, in which the radial magnetic field reaches −3.7 kG. With a polar strength of −1.7 kG, the dipole component of the large-scale field is able to evacuate the central regions of the disc up to about 50 per cent of the co-rotation radius (at which the Keplerian orbital period equals the stellar rotation period) throughout our observations, during which the average accretion rate was found to be unusually high. We speculate that the magnetic field of CI Tau is strong enough to sustain most of the time a magnetospheric gap extending to at least 70 per cent of the co-rotation radius, which would explain why the rotation period of CI Tau is as long as 9 d. Our results also imply that the 9 d radial velocity (RV) modulation that CI Tau exhibits is attributable to stellar activity, and thus that the existence of the candidate close-in massive planet CI Tau b to which these RV fluctuations were first attributed needs to be reassessed with new evidence.

Insights into the inner regions of the FU Orionis disc

Context. We investigate small-amplitude light variations in FU Ori occurring in timescales of days and weeks. Aims. We seek to determine the mechanisms that lead to these light changes. Methods. The visual light curve of FU Ori gathered by the MOST satellite continuously for 55 d in the 2013–2014 winter season and simultaneously obtained ground-based multi-colour data were compared with the results from a disc and star light synthesis model. Results. Hotspots on the star are not responsible for the majority of observed light variations. Instead, we found that the long periodic family of 10.5–11.4 d (presumably) quasi-periods showing light variations up to 0.07 mag may arise owing to the rotational revolution of disc inhomogeneities located between 16 and 20 R⊙. The same distance is obtained by assuming that these light variations arise because of a purely Keplerian revolution of these inhomogeneities for a stellar mass of 0.7 M⊙. The short-periodic (∼3 – 1.38 d) small amplitude (∼0.01 mag) light variations show a clear sign of period shortening, similar to what was discovered in the first MOST observations of FU Ori. Our data indicate that these short-periodic oscillations may arise because of changing visibility of plasma tongues (not included in our model), revolving in the magnetospheric gap and/or likely related hotspots as well. Conclusions. Results obtained for the long-periodic 10–11 d family of light variations appear to be roughly in line with the colour-period relation, which assumes that longer periods are produced by more external and cooler parts of the disc. Coordinated observations in a broad spectral range are still necessary to fully understand the nature of the short-periodic 1–3 d family of light variations and their period changes.

Gamma-ray flaring activity of NGC1275 in 2016–2017 measured by MAGIC

We report on the detection of flaring activity from the Fanaroff-Riley I radio galaxy NGC 1275 in very-high-energy (VHE, E > 100 GeV) gamma rays with the Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescopes. The observations were performed between 2016 September and 2017 February, as part of a monitoring programme. The brightest outburst, with ∼1.5 times the Crab Nebula flux above 100 GeV (C.U.), was observed during the night between 2016 December 31 and 2017 January 1. The flux is fifty times higher than the mean flux previously measured in two observational campaigns between 2009 October and 2010 February and between 2010 August and 2011 February. Significant variability of the day-by-day light curve was measured. The shortest flux-doubling timescale was found to be of (611 ± 101) min. The spectra calculated for this period are harder and show a significant curvature with respect to the ones obtained in the previous campaigns. The combined spectrum of the MAGIC data during the strongest flare state and simultaneous data from the Fermi-LAT around 2017 January 1 follows a power law with an exponential cutoff at the energy (492 ± 35) GeV. We further present simultaneous optical flux density measurements in the R-band obtained with the Kungliga Vetenskaps Akademien (KVA) telescope and investigate the correlation between the optical and gamma-ray emission. Due to possible internal pair-production, the fast flux variability constrains the Doppler factor to values that are inconsistent with a large viewing angle as observed in the radio band. We investigate different scenarios for the explanation of fast gamma-ray variability, namely emission from magnetospheric gaps, relativistic blobs propagating in the jet (mini-jets), or an external cloud (or star) entering the jet. We find that the only plausible model to account for the luminosities here observed would be the production of gamma rays in a magnetospheric gap around the central black hole, only in the eventuality of an enhancement of the magnetic field threading the hole from its equipartition value with the gas pressure in the accretion flow. The observed gamma-ray flare therefore challenges all the discussed models for fast variability of VHE gamma-ray emission in active galactic nuclei.

Existence of steady gap solutions in rotating black hole magnetospheres

Under conditions prevailing in certain classes of compact astrophysical systems, the active magnetosphere of a rotating black hole becomes charge-starved, giving rise to formation of a spark gap in which plasma is continuously produced. The plasma production process is accompanied by curvature and inverse Compton emission of gamma rays in the GeV-TeV band, that may be detectable by current and future experiments. The properties of the gap emission have been studied recently using a fully general relativistic model of a local steady gap. However, this model requires artificial adjustment of the electric current which is determined, in reality, by the global properties of the magnetosphere. In this paper we map the parameter regime in which steady gap solutions exist, using a steady-state gap model in Kerr geometry, and show that such solutions are allowed only under restrictive conditions that may not apply to most astrophysical systems. We further argue that even the allowed solutions are inconsistent with the global magnetospheric structure. We conclude that magnetospheric gaps are inherently intermittent, and point out that this may drastically change their emission properties.

Black hole physics. Black hole lightning due to particle acceleration at subhorizon scales.

Supermassive black holes with masses of millions to billions of solar masses are commonly found in the centers of galaxies. Astronomers seek to image jet formation using radio interferometry but still suffer from insufficient angular resolution. An alternative method to resolve small structures is to measure the time variability of their emission. Here we report on gamma-ray observations of the radio galaxy IC 310 obtained with the MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov) telescopes, revealing variability with doubling time scales faster than 4.8 min. Causality constrains the size of the emission region to be smaller than 20% of the gravitational radius of its central black hole. We suggest that the emission is associated with pulsar-like particle acceleration by the electric field across a magnetospheric gap at the base of the radio jet.

HIGH-ENERGY EMISSIONS FROM THE GAMMA-RAY BINARY LS 5039

We study mechanisms of multi-wavelength emissions (X-ray, GeV and TeV gamma-rays) from the gamma-ray binary LS~5039. This paper is composed of two parts. In the first part, we report on results of observational analysis using four year data of \fermi\ Large Area Telescope. Due to the improvement of instrumental response function and increase of the statistics, the observational uncertainties of the spectrum in $\sim$100-300 MeV bands and $>10$GeV bands are significantly improved. The present data analysis suggests that the 0.1-100GeV emissions from LS~5039 contain three different components; (i) the first component contributes to $<$1GeV emissions around superior conjunction, (ii) the second component dominates in 1-10GeV energy bands and (iii) the third component is compatible to lower energy tail of the TeV emissions. In the second part, we develop an emission model to explain the properties of the phase-resolved emissions in multi-wavelength observations. Assuming that LS~5039 includes a pulsar, we argue that both emissions from magnetospheric outer gap and inverse-Compton scattering process of cold-relativistic pulsar wind contribute to the observed GeV emissions. We assume that the pulsar is wrapped by two kinds of termination shock; Shock-I due to the interaction between the pulsar wind and the stellar wind and Shock-II due to the effect of the orbital motion. We propose that the X-rays are produced by the synchrotron radiation at Shock-I region and the TeV gamma-rays are produced by the inverse-Compton scattering process at Shock-II region.

Plasma‐producing gaps in the global structure of the pulsar force‐free magnetosphere

AbstractThe pulsar high‐energy emission is usually associated with the magnetospheric gaps which are traditionally considered within the framework of the vacuum dipole model of the magnetosphere. Actually, the presence of an abundant plasma should modify the magnetospheric structure, and the context of a force‐free magnetosphere where the electromagnetic forces are balanced over most part of the volume is more appropriate. The gaps included into the structure of the force‐free magnetosphere should play an important role in sustaining and closing the pulsar current circuit. Here we examine the vacuum polar gap adjustment to the stationary force‐free magnetosphere in the presence of a closing current sheet. Our simplified model incorporates the regimes of the force‐free and vacuum monopole with the transition region amidst, where the bulk of pair plasma is formed and the poloidal force‐free current is closed. The analysis of the force and energy balance in the transition region shows that the current sheet cannot be regarded as infinitesimal and the closing process may be dissipation‐free. The implications of our results for the pulsar emission are outlined. In particular, the interpretation of a peculiar radio–X‐ray correlation in the pulsar PSR B0943+10 is suggested. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Non-thermal emissions from accreting X-ray binary pulsars

We study non-thermal emissions from cascade processes in accreting X-ray binary pulsars. In the framework of the magnetospheric gap model, we consider three photon fields, which are respectively from the polar cap of a pulsar, its surrounding accretion disk and a massive companion star with a circumstellar disk, to shield the gap. The gap-accelerated ultra-relativistic electrons emit high-energy photons via curvature radiation and an inverse Compton scattering process, in which part of these high-energy photons absorbed by interactions with the surrounding photon fields can facilitate the following electromagnetic cascades. We first carry out numerical calculations of the cascade processes in order to obtain the predicted emission spectra. As an example, we subsequently apply this model to reproduce observations of LS I +61° 303. We find that the results can fit observations ranging from hard X-ray to γ-ray bands. In particular, they can explain the spectral cutoff feature at a few GeV. Finally, we suggest that the emissions detected by the Fermi Large Area Telescope from X-ray binary pulsars originate in the magnetosphere region of the pulsar.

Pulsed high-energyγ-rays from thermal populations in the current sheets of pulsar winds

Context. More than one hundred GeV pulsars have been detected up to now by the LAT telescope on the Fermi gamma-ray observatory, showing peak energies around a few GeV. Current modelling proposes that the high energy emission comes from outer magnetospheric gaps, however radiation from the equatorial current sheet which separates the two magnetic hemispheres outside the light cylinder has also been investigated. Aims. In this paper we discuss the region right outside the light cylinder, or "near wind" zone. We investigate the possibility that synchrotron radiation emitted by thermal populations in the equatorial current sheet of the pulsar wind in this region can explain the lightcurves and spectra observed by Fermi/LAT. Methods. We use analytical estimates as well as detailed numerical computation to calculate the gamma-ray luminosities, lightcurves and spectra of gamma-ray pulsars. Results. Many of the characteristics of the gamma-ray pulsars observed by Fermi/LAT can be reproduced by our model, most notably the position of these objects in the P - Pdot diagram, and the range of gamma-ray luminosities. A testable result is a sub-exponential cutoff with an index b = 0.35. We also predict the existence of a population of pulsars with cutoff energies in the MeV range. These have systematically lower spindown luminosities than the Fermi/LAT detected pulsars. Conclusions. It is possible for relativistic populations of electrons and positrons in the current sheet of a pulsar's wind right outside the light cylinder to emit synchrotron radiation that peaks in the sub-GeV to GeV regime, with gamma-ray efficiencies similar to those observed for the Fermi/LAT pulsars.

Particle acceleration and non-thermal emission in the pulsar outer magnetospheric gap

A two-dimensional electrodynamic model is used to study particle acceleration and non-thermal emission mechanisms in the pulsar magnetospheres. We solve distribution of the accelerating electric field with the emission process and the pair-creation process in meridional plane, which includes the rotational axis and the magnetic axis. By solving the evolutions of the Lorentz factor, and of the pitch angle, we calculate spectrum in optical through $\gamma$-ray bands with the curvature radiation, synchrotron radiation, and inverse-Compton process not only for outgoing particles, but also for ingoing particles, which were ignored in previous studies. We apply the theory to the Vela pulsar. We find that the curvature radiation from the outgoing particles is the major emission process above 10 MeV bands. In soft $\gamma$-ray to hard X-ray bands, the synchrotron radiation from the ingoing primary particles in the gap dominates in the spectrum. Below hard X-ray bands, the synchrotron emissions from both outgoing and ingoing particles contribute to the calculated spectrum. The calculated spectrum is consistent with the observed phase-averaged spectrum of the Vela pulsar. We show that the observed five-peak pulse profile in the X-ray bands of the Vela pulsar is reproduced by the inward and outward emissions, and the observed double-peak pulse profile in $\gamma$-ray bands is explained by the outward emissions.

Three-dimensional simulations of accretion to stars with complex magnetic fields

Disc accretion to rotating stars with complex magnetic fields is investigated using full 3D magnetohydrodynamic (MHD) simulations. The studied magnetic configurations include superpositions of misaligned dipole and quadrupole fields and off-centre dipoles. The simulations show that when the quadrupole component is comparable to the dipole component, the magnetic field has a complex structure with three major magnetic poles on the surface of the star and three sets of loops of field lines connecting them. A significant amount of matter flows to the quadrupole ‘belt’, forming a ring-like hotspot on the star. If the maximum strength of the magnetic field on the star is fixed, then we observe that the mass accretion rate, the torque on the star and the area covered by hotspots are several times smaller in the quadrupole-dominant cases than in the pure dipole cases. The influence of the quadrupole component on the shape of the hotspots becomes noticeable when the ratio of the quadrupole and dipole field strengths Bq/Bd≳ 0.5. It becomes dominant in determining the shape of the hotspots when Bq/Bd≳ 1. We conclude that if the quadrupole component is larger than the dipole one, then the shape of the hotspots is determined by the quadrupole field component. In the case of an off-centre dipole field, most of the matter flows through a one-armed accretion stream, forming a large hotspot on the surface, with a second much smaller secondary spot. The light curves may have simple, sinusoidal shapes, thus mimicking stars with pure dipole fields. Or, they may be complex and unusual. In some cases, the light curves may be indicators of a complex field, in particular if the inclination angle is known independently. We also note that in the case of complex fields, magnetospheric gaps are often not empty, and this may be important for the survival of close-in exosolar planets.

Production of TeV Gamma Radiation in the Vicinity of the Supermassive Black Hole in the Giant Radio Galaxy M87

Although the giant radio galaxy M87 harbors many distinct regions of broadband nonthermal emission, the recently reported fast variability of TeV γ-rays from M87, on a timescale of days, strongly constrains the range of speculations concerning the possible sites and scenarios of particle acceleration responsible for the observed TeV emission. A natural production site of this radiation is the immediate vicinity of the central supermassive black hole (BH). Because of its low bolometric luminosity, the nucleus of M87 can be effectively transparent for γ-rays up to an energy of 10 TeV, which makes this source an ideal laboratory for the study of particle acceleration processes close to the BH event horizon. We critically analyze different possible radiation mechanisms in this region and argue that the observed very high energy γ-ray emission can be explained as the inverse Compton emission of ultrarelativistic electron-positron pairs produced through the development of an electromagnetic cascade in the BH magnetosphere. We demonstrate, through detailed numerical calculations of acceleration and radiation of electrons in the magnetospheric vacuum gap, that this pulsar magnetosphere-like scenario can satisfactorily explain the main properties of the TeV γ-ray emission from M87.

MHD simulations of disk-star interaction

AbstractWe discuss a number of topics relevant to disk-magnetosphere interaction and how numerical simulations illuminate them. The topics include: (1) disk-magnetosphere interaction and the problem of disk-locking; (2) the wind problem; (3) structure of the magnetospheric flow, hot spots at the star's surface, and the inner disk region; (4) modeling of spectra from 3D funnel streams; (5) accretion to a star with a complex magnetic field; (6) accretion through 3D instabilities; (7) magnetospheric gap and survival of protoplanets. Results of both 2D and 3D simulations are discussed.

On the Nature of Part-Time Radio Pulsars

The recent discovery of rotating radio transients and the quasi-periodicity of pulsar activity in the radio pulsar PSR B1931$+$24 has challenged the conventional theory of radio pulsar emission. Here we suggest that these phenomena could be due to the interaction between the neutron star magnetosphere and the surrounding debris disk. The pattern of pulsar emission depends on whether the disk can penetrate the light cylinder and efficiently quench the processes of particle production and acceleration inside the magnetospheric gap. A precessing disk may naturally account for the switch-on/off behavior in PSR B1931$+$24.