Reverberation Phase Research Articles

Study of maximum electron energy of sub-PeV pulsar wind nebulae by multiwavelength modelling

ABSTRACTRecently, the Large High Altitude Air Shower Observatory (LHAASO) reported the discovery of 12 ultrahigh-energy (UHE; ε ≥ 100 TeV) gamma-ray sources located in the Galactic plane. A few of these UHE gamma-ray emitting regions are in spatial coincidence with pulsar wind nebulae (PWNe). We consider a sample of five sources: two of them are LHAASO sources (LHAASO J1908+0621 and LHAASO J2226+6057) and the remaining three are GeV–TeV gamma-ray emitters. In addition, X-rays, radio observations, or upper limits are also available for these objects. We study multiwavelength radiation from these sources by considering a PWN origin, where the emission is powered by spin-down luminosity of the associated pulsars. In this Leptonic emission model, the electron population is calculated at different times under the radiative (synchrotron and inverse-Compton) and adiabatic cooling. We also include the onset of the reverberation phase for the PWN, by assuming radially symmetric expansion. However, in this work, we find that multiwavelength emission can be interpreted before the onset of this phase. The maximum energy of the electrons based on the spectral fit is found to be above 0.1 PeV and close to 1 PeV. For LHAASO J2226+6057, using its observations in radio to UHE gamma-rays, we find that UHE gamma-rays can be interpreted using electrons with maximum energy of 1 PeV. We estimate the upper limits on the minimum Lorentz factor of the electrons and it also infers the minimum value of the pair-multiplicity of charged pairs.

LHAASO J2226+6057 as a pulsar wind nebula

Context. The Large High Altitude Air Shower Observatory has reported the detection of cosmic-ray sources in the Milky Way that can accelerate particles up to PeV (=1015 eV) energies. These sources, so-called “PeVatrons”, are mostly unidentified. Several classes of sources, such as supernova remnants, pulsar wind nebula, or young stellar clusters can potentially be the counterparts of these PeVatrons. Aims. The aim of this work is to study a pulsar wind nebula interpretation of one of these PeVatrons, LHAASO J2226+6057, which has a relatively well covered multifrequency spectrum. Methods. We have performed a leptonic, time-dependent modeling of the pulsar wind nebula (PWN) associated with PSR J2229+6114 considering a time-energy-dependent diffusion-loss equation. Injection, energy losses, as well as the escape of particles were considered to balance the time-dependent lepton population. We have also included the dynamics of the PWN and the associated supernova remnant and their interaction via the reverse shock to study the reverberation phase of the system. Results. We have considered different values of the braking index (n) and true age (tage) for the fitting of the MultiWaveLength (MWL) spectral energy distribution (SED) of LHAASO J2226+6057. The best-fit PWN model parameters and their 1σ confidence intervals have been evaluated. We have also demonstrated the impact of reverberation on the MWL SED with increasing time. Additionally, we have discussed the resultant large radius and low magnetic field associated with the PWN in question, as caveats for the possible physical connection of the pulsar as the origin of this high energy source.

Modelling the γ-ray pulsar wind nebulae population in our galaxy

ABSTRACT Pulsar wind nebulae (PWNe) represent the largest class of sources that upcoming γ-ray surveys will detect. Therefore, accurate modelling of their global emission properties is one of the most urgent problems in high-energy astrophysics. Correctly characterizing these dominant objects is a needed step to allow γ-ray surveys to detect fainter sources, investigate the signatures of cosmic ray propagation, and estimate the diffuse emission in the Galaxy. In this paper, we present an observationally motivated construction of the Galactic PWNe population. We made use of a modified one-zone model to evolve for a long period of time the entire population. The model provides, for every source, at any age, a simplified description of the dynamical and spectral evolution. The long-term effects of the reverberation phase on the spectral evolution are described, for the first time, based on physically motivated prescriptions for the evolution of the nebular radius supported by numerical studies. This effort tries to solve one of the most critical aspects of one-zone modelling, namely the typical overcompression of the nebula during the reverberation phase, resulting in a strong modification of its spectral properties at all frequencies. We compare the emission properties of our synthetic PWNe population with the most updated catalogues of TeV Galactic sources. We find that the firmly identified and candidate PWNe sum up to about 50 per cent of the expected objects in this class above threshold for detection. Finally, we estimate that Cherenkov Telescope Array will increase the number of TeV-detected PWNe by a factor of ≳3.

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.

Oscillatory brain dynamics of pronoun processing in native Spanish speakers and in late second language learners of Spanish

AbstractA longstanding question in the second language acquisition literature is whether late second language (L2) learners process grammatical structures in a native-like manner. Here, we use Time Frequency Representation (TFR) analysis to test morpho-syntactic processing of clitic pronouns in native and late L2 learners of Spanish. The TFR results show overall similar power decreases in theta, alpha, and beta frequencies in both groups. Critically, the observed oscillatory effects persisted in time for native Spanish speakers, but declined earlier for L2 learners. We discuss the results using cell-assembly theory models for language processing (e.g., Pulvermüller, 1999) that posit a biphasic time-course for neural assemblies consisting of an early ignition (recognition) and a later reverberation (re-processing) phase. We propose a working hypothesis for L2 processing in tune with a cell-assembly theory suggesting that the length of the reverberation phase could be a distinguishing feature between native and L2 processing.

On the Gamma-Ray Nebula of Vela Pulsar. II. The Soft Spectrum of the Extended Radio Nebula

Abstract The Vela X pulsar wind nebula (PWN) is characterized by the extended radio nebula (ERN) and the central X-ray “cocoon.” We have interpreted the γ-ray spectral properties of the cocoon in the first paper; here, we account for the broadband photon spectrum of the ERN. Since the diffusive escape of the electrons from the TeV emitting region is expected to play an insignificant role in shaping the spectrum of the ERN, we attribute the GeV cutoff of the ERN to the reverse shock–PWN interaction. Due to the disruption of the reverse shock, most of the plasma of the PWN is driven into the ERN. During the subsequent reverberation phase, the ERN could be compressed by a large factor in radius, and the magnetic field in the ERN is thus significantly enhanced, burning off the high-energy electrons. We thus obtain the electron spectrum of the ERN, and the broadband spectrum of the ERN is explained satisfactorily.

Towards observing reverberating and superefficient pulsar wind nebulae

In a recent work, we numerically studied the radiative properties of the reverberation phase of pulsar wind nebulae (PWNe), i.e., when the reverse shock created by the supernova explosion travels back towards the pulsar, compressing the wind bubble. We focused on several well-characterized PWNe and used them as examples for introducing the concept of superefficiency. The latter is a period of the PWN evolution, happening within reverberation, where the luminosity in a given band exceeds the spin-down power at the time. Here, we explore a broad range of PWN models to study their reverberation and superefficiency phases in a systematic way. Armed with these models we consider two aspects: On the one hand, we analyze via Monte Carlo simulations how many Galactic PWNe are expected to be reverberating or in a superefficiency stage at any given time, providing the first such estimations. On the other hand, we focus on searching for observational signatures of such periods. We analyze archival observations and check for the existence of possible candidates for superefficient PWNe. We also provide predictions for the future evolution of the magnetar nebula J1834.9-0846 (which we consider to be starting its reverberation period) along the next 50 years. Using our simulations as input we study how sensitive current and future X-ray satellites (like eXTP or Athena) will be to observe such evolution, concluding that they will be able to track it in detail.

Discovery and Characterization of Superefficiency in Pulsar Wind Nebulae

Abstract We numerically study the radiative properties of the reverberation phase of pulsar wind nebulae. Reverberation brings a significant evolution in a short period of time. We show that even the Crab Nebula, associated with the more energetic pulsar of the sample that we consider, has a period in its future time evolution where the X-ray luminosity will exceed the spin-down power at the time. In fact, all of the nebulae in our sample are expected to have a period of radio, X-ray, and GeV superefficiency, and most will also have a period of TeV superefficiency. We analyze and characterize these superefficient phases.

Modeling statistical properties of the X‐ray emission from aged pulsar wind nebulae

AbstractThe number of known pulsar wind nebulae (PWNe) has recently increased considerably, and the majority of them are now middle‐age objects. Recent studies have shown a clear correlation of both X‐ray luminosity and size with the PWN age, but fail in providing a thorough explanation of the observed trends. Here I propose a different approach to these effects, based on the hypothesis that the observed trends do not simply reproduce the evolution of a “typical” PWN, but are a combined effect of PWNe evolving under different ambient conditions, the leading parameter being the ambient medium density. Using a simple analytic approach, I show that most middle‐aged PWNe are more likely observable during the reverberation phase, and I succeed in reproducing trends consistent with those observed, provided that the evolution of the X‐ray emitting electrons remains adiabatic over the whole reverberation phase. As a direct consequence, I show that the X‐ray spectra of older PWNe should be harder, also consistent with observations. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Phase response of transfer functions and coherent field in a reverberation room

AbstractThis paper analyzes the properties of sound field transfer functions that change with the sound source distance (SSD), by measuring reverberant room impulse responses from a point of view of the phase characteristics. We analyzed the propagation phase (PP) according to the SSD in a reverberant field that R. Lyon investigated, by using a narrow‐band linear regression analysis of the impulse responses and considered the relation to the zero distribution of the transfer function. As a result, we clarified that the SSD information is included in the phase frequency characteristics of the minimum‐phase component and the variances from the PP. The distance from the source where the PP could be observed corresponds to the coherent field based on the wave theory or the critical distance defined by the energy ratio of direct to reverberation sound. Therefore, the field where the PP could be observed in the minimum‐phase component of the transfer function can be treated as the direct sound field. In addition, we also present that the reverberation phase (RP) separated and extracted along with PP from the phase frequency characteristics is according to the estimated value from the number of non‐minimum‐phase zeros. © 2006 Wiley Periodicals, Inc. Electron Comm Jpn Pt 3, 90(4): 1–8, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjc.20293

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 ().