1,721,202 research outputs found
Pulsars – Extreme Cosmic Lighthouses / Christo Venter
Full text linkPulsars are ancient, fast-rotating, highly-magnetised neutron stars that radiate across the electromagnetic spectrum. New discoveries by the Fermi Large Area Telescope (LAT) in the gamma-ray band since 2008, combined with the quality of new multi-frequency data, have caused a revolution in the field of gamma-ray rotation-powered pulsars. There are still many unsolved mysteries regarding the magnetospheric conditions in these stars – even after 50 years of research! This paper will relate several thoughts surrounding this field from a personal perspective that has taken shape over the past 15 years
New Advances in the Modelling of Pulsar Magnetospheres
Full text linkThe wealth of high-energy (E &50MeV) and very-high-energy (E >100 GeV) data accumulated
over the past few years have provided unprecedented opportunities to probe pulsar emission models.
The Fermi Large Area Telescope (LAT) has now detected over 200 g -ray pulsars of vastly
different ages, providing an extensive dataset of spectra and light curves compared to a mere
decade ago. Ground-based Cherenkov telescopes deepened our appreciation of the mysterious
richness of the pulsar mechanism by a surprise detection of pulsed emission up to 1 TeV from the
Crab pulsar. These discoveries position us to make real progress in pulsar theory. A number of
studies have developed new and enhanced existing dissipative magnetohydrodynamical (MHD)
and particle-in-cell (PIC) codes to solve the global electrodynamics and pursue fundamental questions
about magnetospheric particle injection, acceleration, and radiation. While MHD models
capture the global aspects of pulsar magnetospheres, the microphysics need to be probed selfconsistently
by PIC simulations. Some dissipative MHD models consider the current sheet (CS)
as an important site for high-energy curvature radiation (CR), while early PIC results point to
energy dissipation taking place in the CS, where particles are accelerated by magnetic reconnection
and may possibly emit g rays via synchrotron radiation (SR). This is in marked contrast to
some older local emission models that studied CR g rays produced in gaps nearer to the spinning
neutron star. The universality of these results should become clearer as current computational
restrictions are overcome and boundary conditions are refined. Continued and combined polarimetric,
spectral, and temporal measurements should aid us in scrutinising these new emission
models in our persistent pursuit of a deeper understanding of the pulsar marve
High-energy emission properties of pulsars
Full text linkThe sheer number of new γ-ray pulsar discoveries by the Fermi Large Area Telescope since 2008,
combined with the quality of new multi-frequency data, has caused a revolution in the field of
high-energy rotation-powered pulsars. These rapidly rotating neutron stars exhibit rich spectral
and temporal phenomenology, indicating that there are still many unsolved mysteries regarding
the magnetospheric conditions in these stars – even after 50 years of research! Indeed, 2017marks
the golden anniversary of the discovery of the first radio pulsar, and theorists and observers alike
are looking forward to another half-century of discovery, with many new experiments coming
online in the next decades. In this review paper, we will briefly summarise recent HE pulsar observations,
mention some theoretical models that provide a basic framework within which to make
sense of the varied measurements, and finally review some of the latest theoretical developments
in pulsar emission modellin
Simultaneous Fitting of the Spectral Energy Density, Energy-dependent Size, and X-ray Spectral Index vs.Radius of The Young Pulsar Wind Nebula PWNG0.9+0.1
Full text linkWe have constructed and calibrated a spherically-symmetric, spatially-dependent particle
transport and emission code for young pulsar wind nebulae (PWNe). This code predicts
the spectral energy distribution (SED) of the radiation spectrum at different positions
in a PWN, thus yielding the surface brightness vs. radius and hence the nebular size as
function of energy. It also predicts the X-ray spectral index at different radii from the
central pulsar, depending on the nebular B-field profile and particle transport properties.
We apply the code to PWN G0.9+0.1 and fit these three functions concurrently, thus
maximizing the constraining power of the data. We use a Markov-chain-Monte-Carlo
(MCMC) method to find best-fit parameters with accompanying errors. This approach
should allow us to better probe the spatial behaviour of the bulk-particle motion, the Bfield and diffusion coefficient, and break degeneracies between different model parameters.
Our model will contribute to interpreting results by the future Cherenkov Telescope Array
(CTA) that will yield many more discoveries plus morphological details of very-highenergy Galactic PWN
Multi-TeV emission from the Vela pulsar
Full text linkPulsed emission from the Vela pulsar at energies above 3 TeV has recently been detected by the High Energy Stereoscopic System (H.E.S.S.) Collaboration. We present a model for the broadband spectrum of Vela from infrared (IR) to beyond 10 TeV. Recent simulations of the global pulsar magnetosphere have shown that most of the particle acceleration occurs in the equatorial current sheet outside of the light cylinder and that the magnetic field structure is nearly force-free for younger pulsars. We adopt this picture to compute the radiation from both electron–positron pairs produced in polar cap cascades and from primary particles accelerated in the separatrix and current sheet. The synchrotron spectrum from pairs resonantly absorbing radio photons at relatively low altitude can account for the observed IR-optical emission. We set the parallel electric field in the current sheet to produce the Fermi GeV emission through curvature radiation, producing particles with energies of 30–60 TeV. These particles then produce very-high-energy emission up to around 30 TeV through inverse-Compton scattering of the IR-optical emission. We present model spectra and light curves that can match the IR-optical through GeV spectrum and make predictions for the multi-TeV emissio
Pulsations from the Vela pulsar down to 20 GeV with H.E.S.S. II
Full text linkThe Vela pulsar (PSR J0835 − 4510) is the brightest persistent source in the high-energy γ-ray
sky. It is a relatively near, young and energetic rotation-powered pulsar. Vela was a key target for
the High Energy Stereoscopic System phase II array (H.E.S.S. II). Observations were carried out
following a hint of pulsed emission above 20GeV observed with the Fermi-LAT data. In this talk
we present the results of the analysis of H.E.S.S. II data obtained with the new 28m telescope
in monoscopic mode which yielded the detected of pulsed emission from the Vela pulsar at a
high statistical significance level. The low-energy performance of the H.E.S.S. II instrument in
monoscopic mode is clearly demonstrated given a distinct pulsed excess down to energies of
20GeV. The H.E.S.S. II data provide a thorough insight into the general phase profile of the Vela
pulsar and reveal the specific pulse shape at these energie
A NICER view of spectral and profile evolution for three x-ray-emitting millisecond pulsars
No full textWe present two years of Neutron star Interior Composition Explorer (NICER) X-ray observations of three energetic rotation-powered millisecond pulsars (MSPs): PSRs B1937+21, B1821−24, and J0218+4232. We fit Gaussians and Lorentzians to the pulse profiles for different energy sub-bands of the soft X-ray regime to measure the energy dependence of pulse separation and width. We find that the separation between pulse components of PSR J0218+4232 decreases with increasing energy at >3σ confidence. The 95% upper limit on pulse separation evolution for PSRs B1937+21 and B1821−24 is less than 2 milliperiods per keV. Our phase-resolved spectral results provide updated constraints on the non-thermal X-ray emission of these three pulsars. The photon indices of the modeled X-ray emission spectra for each pulse component of PSR B1937+21 are inconsistent with each other at the 90% confidence level, suggesting different emission origins for each pulse. We find that the PSR B1821−24 and PSR J0218+4232 emission spectra are invariant with phase at the 90% confidence level. We describe the implications of our profile and spectral results in the context of equatorial current sheet emission models for these three MSPs with non-thermal, magnetospheric X-ray emissio
The aid of optical studies in understanding millisecond pulsar binaries
Full text linkA large number of new “black widow” and “redback” energetic millisecond pulsars with irradiated stellar companions have been discovered through radio and optical searches of unidentified
Fermi sources. Synchrotron emission, from particles accelerated up to several TeV in the intrabinary shock, exhibits modulation at the binary orbital period. Our simulated double-peaked X-ray
light curves modulated at the orbital period, produced by relativistic Doppler-boosting along the
intrabinary shock, are found to qualitatively match those observed in many sources. In this model,
redbacks and transitional pulsar systems where the double-peaked X-ray light curve is observed
at inferior conjunction have intrinsically different shock geometry than other millisecond pulsar
binaries where the light curve is centered at superior conjunction. We discuss, and advocate, how
current and future optical observations may aid in constraining the emission geometry, intrabinary
shock and the unknown physics of pulsar wind
Constraining the emission geometry and mass of the white dwarf pulsar AR Sco using the rotating vector model
Full text linkWe apply the standard radio pulsar rotating vector model to the white dwarf (WD) pulsar AR Sco's optical polarization position angle swings folded at the WD's spin period as obtained by Buckley et al. Owing to the long duty cycle of spin pulsations with a good signal-to-noise ratio over the entire spin phase, in contrast to neutron star radio pulsars, we find well-constrained values for the magnetic obliquity α and observer viewing direction ζ with respect to the spin axis. We find and , implying an orthogonal rotator with an observer angle \zeta ={60\buildrel{\circ}\over{.} 4}_{-6\buildrel{\circ}\over{.} \,0}^{+5\buildrel{\circ}\over{.} \,3}. This orthogonal nature of the rotator is consistent with the optical light curve consisting of two pulses per spin period, separated by 180° in phase. Under the assumption that ζ ≈ i, where i is the orbital inclination, and that the companion M star is Roche-lobe-filling, we obtain for the WD mass. These polarization modeling results suggest the that nonthermal emission arises from a dipolar WD magnetosphere and close to the star, with synchrotron radiation (if nonzero pitch angles can be maintained) being the plausible loss mechanism, marking AR Sco as an exceptional system for future theoretical and observational stud
High-energy emission from Black Widows and Redbacks
Full text linkA large number of new black widow and redback energetic millisecond pulsars with irradiated
stellar companions have been discovered through radio searches of unidentified Fermi sources.
We construct a 3D emission model of these systems to predict the high-energy emission components from particles accelerated to several TeV in the intrabinary shocks, and its predicted
modulation at the binary orbital period. Synchrotron emission is expected at X-ray energies and
such modulated emission has already been detected by Chandra and XMM-Newton in some systems. Synchrotron and inverse Compton emission from relativistic particles in the pulsar wind
and intrabinary shock can probe the unknown physics of pulsar winds and relativistic shock acceleration in these compact binaries. Orbitally-modulated emission in the GeV and TeV bands
may be detectable under some favorable condition
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