Evolution Of Pulsar Wind Nebulae Research Articles

Reverberation of pulsar wind nebulae – III. Modelling of the plasma interface empowering a long term radiative evolution

ABSTRACT The vast majority of pulsar wind nebulae (PWNe) present in the Galaxy is formed by middle-aged systems characterized by a strong interaction of the PWN itself with the supernova remnant (SNR). Unfortunately, modelling these systems can be quite complex and numerically expensive, due to the non-linearity of the PWN–SNR evolution even in the simple one-dimensional (1D)/one-zone case when the reverse shock of the SNR reaches the PWN, and the two begin to interact (and reverberation starts). Here, we introduce a new numerical technique that couples the numerical efficiency of the one-zone thin shell approach with the reliability of a full ‘Lagrangian’ evolution, able to correctly reproduce the PWN–SNR interaction during the reverberation, and to consistently evolve the particle spectrum beyond. Based on our previous findings, we show that our novel strategy resolves many of the uncertainties present in previous approaches, as the arbitrariness in the SNR structure, and ensure a robust evolution, compatible with results that can be obtained with more complex 1D dynamical approaches. Our approach enable us for the first time to provide reliable spectral models of the later compression phases in the evolution of PWNe. While in general, we found that the compression is less extreme than that obtained without such detailed dynamical considerations, leading to the formation of less structured spectral energy distributions, we still find that a non-negligible fraction of PWNe might experience a super-efficient phase, with the optical and/or X-ray luminosity exceeding the spin-down one.

Generation of cosmic rays in pulsar wind nebulae

AbstractThe overall observations of plerions from radio to the very high energy gamma‐rays could provide information about the evolution of pulsar wind nebulae (PWN) from the young Crab‐like stage to the older plerion stages. Geminga is a nearby (250 pc) and middle‐aged pulsar of years. The Geminga pulsar wind nebula was detected in X‐rays, and the detailed studies reveal the morphological and spectral variability, which may raise a high energy emission. The extended MeV‐TeV emission from Geminga in SHALON, Milagro, HAWC observations, and Fermi‐LAT detection and upper limits could arise from the PWN associated with the Geminga SNR. 3C58 is similar to the Crab Nebula (explosion of the 1054 year) on many parameters, but these two objects differ significantly in luminosity and size at X‐rays and radio‐emission. The 3C 58 estimated age varies from to years. MeV‐GeV gamma‐ray emission from the nebula has been detected by Fermi‐LAT. The γ‐ray source associated with the 3C 58 was detected above 800 GeV for the first time by SHALON in observations of 2011 year and systematically studied since then. The overall spectral energy distribution and information about the extension of PWN from radio to GeV‐TeV energies can contribute to particle transport models and also to the understanding of the mechanisms of PWN expanding, which is, in turn, can shed light on the age of 3C 58 and the history of progenitor SN explosion.

Reverberation of pulsar wind nebulae (I): impact of the medium properties and other parameters upon the extent of the compression

ABSTRACT The standard approach to the long-term evolution of pulsar wind nebulae (PWNe) is based on one-zone models treating the nebula as a uniform system. In particular for the late phase of evolved systems, many of the generally used prescriptions are based on educated guesses for which a proper assessment lacks. Using an advanced radiative code, we evaluate the systematic impact of various parameters, like the properties of the supernova ejecta, of the inner pulsar, as well of the ambient medium, upon the extent of the reverberation phase of PWNe. We investigate how different prescriptions shift the starting time of the reverberation phase, how this affects the amount of the compression, and how much of this can be ascribable to the radiation processes. Some critical aspects are the description of the reverse shock evolution, the efficiency by which at later times material from the ejecta accretes on to the swept-up shell around the PWN, and finally the density, velocity, and pressure profiles in the surrounding supernova remnant. We have explicitly treated the cases of the Crab Nebula, and of J1834.9−0846, taken to be representatives of the more and the less energetic pulsars, respectively. Especially for the latter object, the prediction of large compression factors is confirmed, even larger in the presence of radiative losses, also confirming our former prediction of periods of superefficiency during the reverberation phase of some PWNe.

The population of TeV pulsar wind nebulae in the H.E.S.S. Galactic Plane Survey

The nine-year H.E.S.S. Galactic Plane Survey (HGPS) has yielded the most uniform observation scan of the inner Milky Way in the TeV gamma-ray band to date. The sky maps and source catalogue of the HGPS allow for a systematic study of the population of TeV pulsar wind nebulae found throughout the last decade. To investigate the nature and evolution of pulsar wind nebulae, for the first time we also present several upper limits for regions around pulsars without a detected TeV wind nebula. Our data exhibit a correlation of TeV surface brightness with pulsar spin-down power Ė. This seems to be caused both by an increase of extension with decreasing Ė, and hence with time, compatible with a power law RPWN(Ė) ~Ė−0.65±0.20, and by a mild decrease of TeV gamma-ray luminosity with decreasing Ė, compatible with L1−10 TeV ~Ė0.59±0.21. We also find that the offsets of pulsars with respect to the wind nebula centre with ages around 10 kyr are frequently larger than can be plausibly explained by pulsar proper motion and could be due to an asymmetric environment. In the present data, it seems that a large pulsar offset is correlated with a high apparent TeV efficiency L1−10 TeV∕Ė. In addition to 14 HGPS sources considered firmly identified pulsar wind nebulae and 5 additional pulsar wind nebulae taken from literature, we find 10 HGPS sources that are likely TeV pulsar wind nebula candidates. Using a model that subsumes the present common understanding of the very high-energy radiative evolution of pulsar wind nebulae, we find that the trends and variations of the TeV observables and limits can be reproduced to a good level, drawing a consistent picture of present-day TeV data and theory.

Ancient Pulsar Wind Nebulae as a natural explanation for unidentified gamma-ray sources

A large part of the Galactic sources emitting very high energy (VHE; > 1011 eV) gamma-rays are currently still unidentified. The evolution of Pulsar Wind Nebulae (PWNe) plays a crucial role in interpreting these sources. The time-dependent modeling of PWNe has been tested on a sample of well-known young and intermediate age PWNe; and it is currently applied to the full-sample of unidentified VHE Galactic sources. The consequences of this interpretation go far beyond the interpretation of “dark sources” (i.e. VHE gamma-ray sources without lower energies, radio or X-ray, counterparts): e.g. there could be strong implication in the origin of cosmic rays and (when considering a leptonic origin of the gamma-ray signal) they can be important for reinterpreting the detection of starburst galaxies in the TeV gamma-ray band. Moreover, the number of Galactic VHE sources is currently increasing with further observation by Imaging Atmospheric Cherenkov Telescopes (IACTs) and by the advent of more sensitive water Cherenkov telescopes such as HAWC (High-Altitude Water Cherenkov Observatory); therefore the physical interpretation of unidentified sources becomes more and more crucial.

Multi-D magnetohydrodynamic modelling of pulsar wind nebulae: recent progress and open questions

In the last decade, the relativistic magnetohydrodynamic (MHD) modelling of pulsar wind nebulae, and of the Crab nebula in particular, has been highly successful, with many of the observed dynamical and emission properties reproduced down to the finest detail. Here, we critically discuss the results of some of the most recent studies: namely the investigation of the origin of the radio emitting particles and the quest for the acceleration sites of particles of different energies along the termination shock, by using wisp motions as a diagnostic tool; the study of the magnetic dissipation process in high magnetization nebulae by means of new long-term three-dimensional simulations of the pulsar wind nebula evolution; the investigation of the relativistic tearing instability in thinning current sheets, leading to fast reconnection events that might be at the origin of the Crab nebula gamma-ray flares.

Modelling spectral evolution of pulsar wind nebulae inside supernova remnants

We present a new model for the spectral evolution of Pulsar Wind Nebulae inside Supernova Remnants. The model couples the long-term dynamics of these systems, as derived in the 1-D approximation, with a 1-zone description of the spectral evolution of the emitting plasma. Our goal is to provide a simplified theoretical description that can be used as a tool to put constraints on unknown properties of PWN-SNR systems: a piece of work that is preliminary to any more accurate and sophisticated modeling. In the present paper we apply the newly developed model to a few objects of different ages and luminosities. We find that an injection spectrum in the form of a broken-power law gives a satisfactory description of the emission for all the systems we consider. More surprisingly, we also find that the intrinsic spectral break turns out to be at a similar energy for all sources, in spite of the differences mentioned above. We discuss the implications of our findings on the workings of pulsar magnetospheres, pair multiplicity and on the particle acceleration mechanism(s) that might be at work at the pulsar wind termination shock.

Magnetar-driven bubbles and the origin of collimated outflows in gamma-ray bursts

We model the interaction between the wind from a newly formed rapidly rotating magnetar and the surrounding supernova shock and host star. The dynamics is modelled using the two-dimensional, axisymmetric thin-shell equations. In the first ∼10-100 s after core-collapse the magnetar inflates a bubble of plasma and magnetic fields behind the supernova shock. The bubble expands asymmetrically because of the pinching effect of the toroidal magnetic field, even if the host star is spherically symmetric, just as in the analogous problem of the evolution of pulsar wind nebulae. The degree of asymmetry depends on E mag /E tot , the ratio of the magnetic energy to the total energy in the bubble. The correct value of E mag /E tot is uncertain because of uncertainties in the conversion of magnetic energy into kinetic energy at large radii in relativistic winds; we argue, however, that bubbles inflated by newly formed magnetars are likely to be significantly more magnetized than their pulsar counterparts. We show that for a ratio of magnetic to total power supplied by the central magnetar E mag /E tot ≤0.1 the bubble expands relatively spherically. For E mag /E tot ≥0.3, however, most ofthe pressure in the bubble is exerted close to the rotation axis, driving a collimated outflow out through the host star. This can account for the collimation inferred from observations of long-duration gamma-ray bursts (GRBs). Outflows from magnetars become increasingly magnetically dominated at late times, due to the decrease in neutrino-driven mass loss as the young neutron star cools. We thus suggest that the magnetar-driven bubble initially expands relatively spherically, enhancing the energy of the associated supernova, while at late times it becomes progressively more collimated, producing the GRB. The same physical processes may operate in more modestly rotating neutron stars to produce asymmetric supemovae and lower energy transients such as X-ray flashes.

Axially symmetric relativistic MHD simulations of Pulsar Wind Nebulae in Supernova Remnants

The structure and the evolution of Pulsar Wind Nebulae (PWNe) are studied by means of two-dimensional axisymmetric relativistic magnetohydrodynamic (RMHD) simulations. After the first imaging of the Crab Nebula with Chandra, a growing number of objects has been found to show in the X-rays spatial features such as rings and jets, that clearly cannot be accounted for within the standard framework of one-dimensional semi-analytical models. The most promising explanation suggested so far is based on the combined effects of the latitude dependence of the pulsar wind energy flux, shaping the wind termination shock and naturally providing a higher equatorial emission, and of the wind magnetization, likely responsible for the jet collimation by hoop stresses downstream of the shock. This scenario is investigated here by following the evolution of a PWN interacting with the confining Supernova Remnant (SNR), from the free expansion to the beginning of the reverberation phase. Our results confirm the oblate shape of the wind termination shock and the formation of a polar jet with supersonic velocities () for high enough values of the equatorial wind magnetization parameter ().

An evolutionary model for pulsar-driven supernova remnants

We present a model of a pulsar wind nebula evolving inside its associated supernova remnant. The model uses a hydrodynamics code to simulate the evolution of this system when the pulsar has a high velocity. The simulation distinguishes four different stages of pulsar wind nebula evolution: the supersonic expansion stage, the reverse shock interaction stage, the subsonic expansion stage and ultimately the bow shock stage. The simulation bears out that, due to the high velocity of the pulsar, the position of the pulsar is off-centered with respect to its pulsar wind nebula, after the passage of the reverse shock. Subsequently the pulsar wind nebula expands subsonically untill the event of the bow shock formation, when the motion of the pulsar becomes supersonic. The bow shock formation event occurs at roughly half the crossing time, when the pulsar is positioned at 0.677 times the radius of the supernova remnant blastwave, in complete agreement with analytical predictions. The crossing time is defined by the age of the supernova remnant, when the pulsar overtakes the blastwave bounding the supernova remnant. The results of the model are applied to three supernova remnants: N157B, G327.1-1.1 and W44. We argue that the head of the pulsar wind nebula, containing the active pulsar, inside the first two systems are not bounded by a bow shock. However, in the case of W44 we argue for a scenario in which the pulsar wind nebula is bounded by a bow shock, due to the supersonic motion of the pulsar.