Session Y4: Pulsars, Magnetars and Others

Pulsar magnetosphere: a new view from PIC simulations

Pulsar emission is produced by charged particles that are accelerated as they flow in the star's magnetosphere. The magnetosphere is populated by electrons and positrons while the physical conditions are characterized by the so called force-free regime. However, the magnetospheric plasma configuration is still unknown, besides some general features, which inhibits the understanding of the emission generation. Here we show the closest to force-free solution ever obtained with a particle-in-cell (PIC) code. The importance of obtaining a force-free solution with PIC is that we can understand how the different particle species support the corresponding magnetosphere structure. Moreover, some aspects of the emission generation are captured. These are the necessary steps to go toward a self consistent modeling of the magnetosphere, connecting the microphysics of the pair plasma to its macroscopic quantities. Understanding the pulsar magnetosphere is essential for interpreting the broad neutron star phenomenology (young pulsars, magnetars, millisecond pulsars, etc.). The study of these plasma physics processes is also crucial for putting limits on the ability of these objects to accelerate particles.   

Inclined Pulsar Magnetospheres: Analytic Results at Realistic Compaction, Rotation, and Magnetization.

Most previous studies of the pulsar magnetosphere have made three unrealistic assumptions: rapid rotation, pure magnetic dipole, and low stellar compaction (i.e. flat spacetime). We relax all three assumptions with a combined numerical-analytical technique that leverages the rotation rate as a small parameter. We consider a perfectly conducting, nearly spherical star with a force-free magnetosphere. We derive a general approach and then provide definite results for magnetic fields that are symmetric about an axis inclined relative to the rotation axis. We discuss polar cap shapes and pair production regions for a variety of magnetic field configurations. These results are relevant for X-ray pulsations as well as coherent radio emission.   

Pulsars in the Mid-Energy Gamma-Ray Band - Implications for ComPair

The investigation of the high-energy gamma-ray band by Fermi has revolutionized our understanding of the populations of pulsars - and by extension neutron starts - in the Galactic field. However, there exist a number of pulsars with energy output that peaks below 500 GeV, and whose gamma-ray characteristics are not well constrained by Fermi. The Compton-Pair Telescope (ComPair) is a proposed wide-field medium-energy gamma-ray mission (0.2 keV to > 500 MeV), re-opening an energy regime that was last investigated by COMPTEL on the Compton Gamma-Ray Observatory. The increased sensitivity and spatial resolution of the proposed instrument may lead to a similar knowledge revolution for these MeV-peaked pulsars. Here we discuss the properties of the MeV-peaked pulsar population, and speculate on the potential new science that ComPair may provide.   

Measuring the radius of PSR J0437$-$4715 using NICER observations of X-ray oscillations

The Neutron Star Interior Composition Explorer (NICER) will launch early in 2017. Its first scientific objective is to precisely and reliably measure the radius of several neutron stars, thereby constraining the properties of cold matter at supranuclear densities. This will be done by fitting energy-dependent waveform models to the observed thermal X-ray waveforms of selected rotation-powered millisecond pulsars. A key target is the 174-Hz pulsar PSR J0437$-$4715. Using synthetic waveform data and Bayesian methods, we have estimated the precisions with which its mass $M$ and radius $R$ can be measured by NICER. When generating the synthetic data, we assumed $M = 1.4 M_\odot$ and $R = 13$ km. When generating the data and when analyzing it, we assumed the X-ray spectrum and radiation beaming pattern given by models with cool hydrogen atmospheres and two hot spots. Assuming NICER observations lasting a total of 1.0 Msec, current knowledge of $M$ and the distance, and knowledge of the pulsar's spin axis to within 1$^{\circ}$, the $1\sigma$ credible region in $R$ extends from 11.83 to 13.73~km (7.4\%) and in $M$, from 1.307 to 1.567~$M_\odot$ (9.1\%). Marginalizing over $M$, we find the $1\sigma$ credible interval for $R$ alone extends from 12.62 to 13.68~km (4\%).   

Observations of pulsar microstructure with the Giant Metrewave Radio Telescope

Microstructure emission, involving short time scale intensity fluctuations in subpulse emission, is well known in normal pulsars. However, the high time resolution and sensitivity required to detect these features has limited such studies to only few pulsars, mostly in the northern sky. The Giant Metrewave Radio Telescope (GMRT), owing to its high sensitivity, extensive sky coverage and frequency coverage at low frequencies is an attractive prospect for high time resolution single pulse studies of pulsars. In this paper, we present results from an extensive statistical analysis of the polarization (with single frequency observations) and spectral (with simultaneous dual-frequency observations) properties of microstructure emission in pulsars observed with the GMRT. We further present the first detections of quasi-periodic microstructure emission from millisecond pulsars (MSPs), in GMRT observations of two MSPs at 325 and 610 MHz. We thus extend the microstructure timescale - rotation period relationship by more than an order of magnitude, down to a rotation period of $\sim$ 5 ms. We discuss the physical implications of our results, pointing to a radial / temporal modulation origin of microstructure emission as a likely explanation for the observed characteristics.   

Torsional Oscillations Of A Magnetar With A Tangled Magnetic Field

Motivated by stability considerations and observational evidence, we argue that magnetars possess highly-tangled internal magnetic fields. We propose that the quasi-periodic oscillations (QPOs) seen to accompany giant flares can be explained as torsional modes supported by a tangled magnetic field, and we present a simple model that supports this hypothesis for SGR 1900+14. Taking the strength of the tangle as a free parameter, we find that the magnetic energy in the tangle must dominate that in the dipolar component by a factor of $\sim 14$ to accommodate the observed 28 Hz QPO. Our simple model provides useful scaling relations for how the QPO spectrum depends on the bulk properties of the neutron star and the tangle strength. The energy density in the tangled field inferred for SGR 1900+14 renders the crust nearly dynamically irrelevant, a significant simplification for study of the QPO problem. The predicted spectrum is about three times denser than observed, which could be explained by preferential mode excitation or beamed emission. We emphasize that field tangling is needed to stabilize the magnetic field, so should not be ignored in treatment of the QPO problem.   

Preferential Excitation of Stellar Oscillations of a Magnetar with a Tangled Magnetic Field

Magnetars are strongly magnetized ($ \sim 10^{15}$ Gauss) neutron stars. Some of them produce giant flares that exhibit quasi-periodic oscillations which have been attributed to stellar oscillations that modulate the emission. A tangled magnetic field model introduces a spectrum of magnetic normal modes that can explain the observed quasi-periodic oscillations, as expected from stability considerations. We show that reasonable initial conditions preferentially excite stellar oscillations, and find promising agreement with data.   

Global crustal dynamics of Magnetars in relation to their bright X-ray outbursts

We consider the yielding response of a neutron star crust to smooth, unbalanced Maxwell stresses imposed at the core-crust boundary, and the coupling of the dynamic crust to the external magnetic field. Stress buildup and yielding in a magnetar crust is a global phenomenon: an elastic distortion radiating from one plastically deforming zone is shown to dramatically increase the creep rate in distant zones. Runaway creep to dynamical rates is shown to be possible, being enhanced by in situ heating and suppressed by thermal conduction and shearing of an embedded magnetic field. A global and time-dependent model of elastic, plastic, magnetic, and thermal evolution is developed. Fault-like structures develop naturally, and a range of outburst timescales is observed. Transient events with time profiles similar to giant magnetar flares (millisecond rise, about 0.1 s duration, and decaying power-law tails) result from runaway creep that starts in localized sub-km-sized patches and spreads across the crust. We also discuss the key role of crust's plastic and elastic motion in Magnetar short bursts and after-flare quasi-periodic oscillations.   

Evidence of high-frequency/small-scale turbulence in the Cygnus region and anomalous Faraday rotation

Faraday effect --- a common and useful probe of cosmic magnetic fields --- is the result of magnetically-induced birefringence in plasmas causing rotation of the polarization plane of a linearly polarized electromagnetic wave. Classically, the rotation angle scales with the wavelength as $\Delta\phi=\text{RM}\, \lambda^2$, where $\text{RM}$ is the rotation measure. Although a typical $\text{RM}$ in the Milky Way is of the order of a few hundred to a few thousand, a famous Cygnus region shows anomalously small, even negative rotation measures. Moreover, Faraday rotation measurements seem to be inconsistent with the standard $\lambda^2$-law. We argue that fast micro-turbulence can cause this anomaly. We demonstrate that electromagnetic high-frequency and/or small-scale fluctuations can lead to effective plasma collisionality by scattering electrons over pitch-angle. We show that such quasi-collisionality radically alters Faraday rotation and other radiative transport properties, e.g., absorption, transmission and reflection. Thus, we explain the Cygnus puzzle by anomalous Faraday rotation in a thin ``blanket" of highly turbulent plasma at the front of an interstellar bubble/shock.