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We present ab initio global general-relativistic Particle-in-cell (GR-PIC) simulations of compact millisecond neutron star magnetospheres in the axisymmetric aligned rotator configuration. We investigate the role of GR and plasma supply on the polar cap particle acceleration efficiency – the precursor of coherent radio emission – employing a new module for the PIC code OSIRIS, designed to model plasma dynamics around compact objects with fully self-consistent GR effects. We provide a detailed description of the main sub-algorithms of the novel PIC algorithm, including a charge-conserving current deposit scheme for curvilinear coordinates. We demonstrate efficient particle acceleration in the polar caps of compact neutron stars with denser magnetospheres, numerically validating the spacelike current extension provided by force-free models. We show that GR relaxes the minimum required poloidal magnetospheric current for the transition of the polar cap to the accelerator regime, thus justifying the observation of weak pulsars beyond the expected death line. We denote that spin-down luminosity intermittency and radio pulse nullings for older pulsars might arise from the interplay between the polar and outer gaps. Also, narrower radio beams are expected for weaker low-obliquity pulsars. • We present an axisymmetric general-relativistic Particle-in-cell code, OSIRIS-GR. • Charge-conserving GR-PIC code designed to model compact object magnetospheres. • GR-PIC simulations of compact neutron star magnetospheres are presented. • We show that spacelike polar angle deviates from GR-FF estimates for weak pulsars. • Older pulsars may be observable beyond the expected death line due to GR. [ABSTRACT FROM AUTHOR]

Abstract: We present spectral and directional properties of gamma radiation obtained by numerical modeling of two millisecond pulsars, PSR J0218+4232 and PSR J0437-4715. The general relativistic space charge limited flow model of Muslimov and Harding [Toward the quasi-steady state electrodynamics of a neutron star. Astrophys. J. 485, 735–746, 1997] in its version of an unscreened accelerating electric field was used. The results give both general ideas and constraints on the specific character of magnetospheric activity in each of these two pulsars. [Copyright &y& Elsevier]

We report on high-resolution spatial and timing observations of the millisecond pulsar PSR J0218+4232 performed with the Chandra X-ray Observatory (CXO) and the Rossi X-ray Timing Explorer (RXTE). With these observations we were able to study: (a) the possible spatial extent at X-ray energies of the DC source coincident with PSR J0218+4232 in detail (CXO); (b) the relative phasing between the X-ray, radio and γ-ray profiles (CXO and RXTE); and (c) the spectral properties at energies beyond 10 keV (RXTE). We found no indications for extended emission at X-ray energies down to ∼1″ scales and confirmed the presence of a point-like DC-component. The two non-thermal pulses in the X-ray profile are found to be aligned with two of the three pulses visible at radio-frequencies and more importantly with the two γ-ray pulses seen in the EGRET 100–1000 MeV pulse profile. The latter reduces now the random occurrence probability for the detected γ-ray signal to ∼10−6, which corresponds to a 4.9σ detection significance. This strengthens the credibility of our earlier claimed detection of pulsed high-energy γ-ray emission from this millisecond pulsar. The pulsed RXTE spectrum appears to extend to ∼20 keV and can be described by a power-law with a photon index of 1.14, slightly softer than measured by BeppoSAX and XMM for energies below 10 keV. We compare the high-energy characteristics of PSR J0218+4232 with those of two other X-ray emitting millisecond pulsars PSR B1821-24 and PSR B1937+21. [Copyright &y& Elsevier]