Pulsars steadily dissipate their rotational energy via relativistic winds. Confinement of these outflows generates luminous pulsar wind nebulae, seen across the electromagnetic spectrum in ...synchrotron and inverse Compton emission and in optical emission lines when they shock the surrounding medium. These sources act as important probes of relativistic shocks, particle acceleration, and interstellar gas. We review the many recent advances in the study of pulsar wind nebulae, with particular focus on the evolutionary stages through which these objects progress as they expand into their surroundings, and on morphological structures within these nebulae that directly trace the physical processes of particle acceleration and outflow. We conclude by considering some exciting new probes of pulsar wind nebulae, including the study of TeV gamma-ray emission from these sources, and observations of pulsar winds in close binary systems.
ABSTRACT We present a comprehensive study of interstellar X-ray extinction using the extensive Chandra supernova remnant (SNR) archive and use our results to refine the empirical relation between the ...hydrogen column density and optical extinction. In our analysis, we make use of the large, uniform data sample to assess various systematic uncertainties in the measurement of the interstellar X-ray absorption. Specifically, we address systematic uncertainties that originate from (i) the emission models used to fit SNR spectra; (ii) the spatial variations within individual remnants; (iii) the physical conditions of the remnant such as composition, temperature, and non-equilibrium regions; and (iv) the model used for the absorption of X-rays in the interstellar medium. Using a Bayesian framework to quantify these systematic uncertainties, and combining the resulting hydrogen column density measurements with the measurements of optical extinction toward the same remnants, we find the empirical relation NH = (2.87 0.12) × 1021 AV cm−2, which is significantly higher than the previous measurements.
Abstract In this paper, we present updated estimates of the velocity of the neutron star (NS) in the supernova remnant Cassiopeia A using over two decades of Chandra observations. We use two methods: ...(1) recording the NS positions from dozens of Chandra observations, including the astrometric uncertainty estimates on the data points, but not correcting the astrometry of the observations; and (2) correcting the astrometry of the 13 Chandra observations that have a sufficient number of point sources with identified Gaia counterparts. For method #1, we observe a heliocentric velocity of 275 ± 121 km s −1 , with an angle of 177° ± 22° east of north. For method #2, we observe a heliocentric velocity of 436 ± 89 km s −1 at an angle of 158° ± 12°. Correcting for galactic rotation and the Sun’s peculiar motion decreases these estimates to 256 km s −1 at 167° and 433 km s −1 at 151°, respectively. Both of our estimates match with the explosion-center-estimated velocity of ∼350 km s −1 and the previous 10 yr baseline proper-motion measurement of 570 ± 260 km s −1 , but our use of additional data over a longer baseline has led to a smaller uncertainty by a factor of 2–3. Our estimates rule out velocities ≳600 km s −1 and better match with simulations of Cassiopeia A that include NS kick mechanisms.
A pulsar wind nebula (PWN) inside a supernova remnant provides a unique insight into the properties of the central neutron star, the relativistic wind powered by its loss of rotational energy, its ...progenitor supernova, and the surrounding environment. In this paper, we present a new semianalytic model for the evolution of such a PWN throughout its lifetime. This model couples the dynamical and radiative evolution of the PWNe, and predicts both the dynamical (e.g., radius and expansion velocity) and radiative (radio to TeV Delta *g-ray spectrum) properties of the PWN during this period. As a result, it is well suited for using the observed properties of a PWN to constrain the physical characteristics of the neutron star, pulsar wind, progenitor supernova, and surrounding environment. We also discuss the expected evolution for a particular set of these parameters, and show that it reproduces the large spectral break inferred from the radio and X-ray spectrum of many young PWNe, and the low break frequency, low radio luminosity, high TeV Delta *g-ray luminosity, and high magnetization observed for several older PWNe. The predicted spectrum of this PWN also contains spectral features which appear during different evolutionary phases detectable with new radio and Delta *g-ray observing facilities such as the Extended Very Large Array and the Fermi Gamma-ray Space Telescope. Finally, this model has implications for determining if PWNe can inject a sufficient number of energetic electrons and positrons into their surroundings to explain the recent measurements of the cosmic-ray positron fraction by PAMELA and the cosmic-ray lepton spectrum by ATIC and HESS.
ABSTRACT
We outline a model of the Crab pulsar wind nebula with two different populations of synchrotron emitting particles, arising from two different acceleration mechanisms: (i) Component-I due to ...Fermi-I acceleration at the equatorial portion of the termination shock, with particle spectral index pI ≈ 2.2 above the injection break corresponding to γwindσwind ∼ 105, peaking in the ultraviolet (UV, γwind ∼ 102 is the bulk Lorentz factor of the wind, σwind ∼ 103 is wind magnetization); and (ii) Component-II due to acceleration at reconnection layers in the bulk of the turbulent Nebula, with particle index pII ≈ 1.6. The model requires relatively slow but highly magnetized wind. For both components, the overall cooling break is in the infrared at ∼0.01 eV, so that the Component-I is in the fast cooling regime (cooling frequency below the peak frequency). In the optical band, Component-I produces emission with the cooling spectral index of αo ≈ 0.5, softening towards the edges due to radiative losses. Above the cooling break, in the optical, UV, and X-rays, Component-I mostly overwhelms Component-II. We hypothesize that acceleration at large-scale current sheets in the turbulent nebula (Component-II) extends to the synchrotron burn-off limit of ϵs ∼ 100 MeV. Thus in our model acceleration in turbulent reconnection (Component-II) can produce both hard radio spectra and occasional gamma-ray flares. This model may be applicable to a broader class of high-energy astrophysical objects, like active galactic nuclei and gamma-ray burst jets, where often radio electrons form a different population from the high-energy electrons.
Abstract
The evolution of a pulsar wind nebula (PWN) depends on properties of the progenitor star, supernova, and surrounding environment. As some of these quantities are difficult to measure, ...reproducing the observed dynamical properties and spectral energy distribution (SED) with an evolutionary model is often the best approach to estimating their values. G21.5−0.9, powered by the pulsar J1833−1034, is a well observed PWN for which previous modeling efforts have struggled to reproduce the observed SED. In this study, we reanalyze archival infrared (IR; Herschel, Spitzer) and X-ray (Chandra, NuSTAR, Hitomi) observations. The similar morphology observed between IR line and continuum images of this source indicates that a significant portion of this emission is generated by surrounding dust and gas, and not synchrotron radiation from the PWN. Furthermore, we find that the broadband X-ray spectrum of this source is best described by a series of power laws fit over distinct energy bands. For all X-ray detectors, we find significant softening and decreasing unabsorbed flux in higher energy bands. Our model for the evolution of a PWN is able to reproduce the properties of this source when the supernova ejecta has a low initial kinetic energy
E
sn
≈ 1.2 × 10
50
erg and the spectrum of particles injected into the PWN at the termination shock is softer at low energies. Lastly, our hydrodynamical modeling of the supernova remnant can reproduce its morphology if there is a significant increase in the density of the ambient medium ∼1.8 pc north of the explosion center.
The mixed morphology class of supernova remnants has centrally peaked X-ray emission along with a shell-like morphology in radio emission. White & Long proposed that these remnants are evolving in a ...cloudy medium wherein the clouds are evaporated via thermal conduction once being overrun by the expanding shock. Their analytical model made detailed predictions regarding temperature, density, and emission profiles as well as shock evolution. We present numerical hydrodynamical models in 2D and 3D including thermal conduction, testing the White & Long model and presenting results for the evolution and emission from remnants evolving in a cloudy medium. We find that, while certain general results of the White & Long model hold, such as the way the remnants expand and the flattening of the X-ray surface brightness distribution, in detail there are substantial differences. In particular we find that the X-ray luminosity is dominated by emission from shocked cloud gas early on, leading to a bright peak, which then declines and flattens as evaporation becomes more important. In addition, the effects of thermal conduction on the intercloud gas, which is not included in the White & Long model, are important and lead to further flattening of the X-ray brightness profile as well as lower X-ray emission temperatures.
The soft γ-ray repeater (SGR) 0526-66 is the first-identified magnetar, and is projected within the supernova remnant N49 in the Large Magellanic Cloud. Based on our ∼50 ks NuSTAR observation, we ...detect the quiescent-state 0526-66 for the first time in the 10-40 keV band. Based on the joint analysis of our NuSTAR and the archival Chandra ACIS data, we firmly establish the presence of the nonthermal component in the X-ray spectrum of 0526-66 in addition to the thermal emission. In the best-fit blackbody (BB) plus power-law (PL) model, the slope of the PL component (photon index Γ = 2.1) is steeper than those (Γ 1.5) for other magnetars. The soft part of the X-ray spectrum can be described with a BB component with the temperature of kT = 0.43 keV. The best-fit radius (R = 6.5 km) of the X-ray-emitting area is smaller than the canonical size of a neutron star. If we assume an underlying cool BB component with the canonical radius of R = 10 km for the neutron star in addition to the hot BB component (2BB + PL model), a lower BB temperature of kT = 0.24 keV is obtained for the passively cooling neutron star's surface, while the hot spot emission with kT = 0.46 keV dominates the thermal spectrum (∼85% of the thermal luminosity in the 0.5-5 keV band). The nonthermal component (Γ ∼ 1.8) is still required.
ABSTRACT The evolution of a pulsar wind nebula (PWN) inside a supernova remnant (SNR) is sensitive to the properties of the central neutron star, pulsar wind, progenitor supernova, and interstellar ...medium. These properties are both difficult to measure directly and critical for understanding the formation of neutron stars and their interaction with the surrounding medium. In this paper, we determine these properties for PWN G54.1+0.3 by fitting its observed properties with a model for the dynamical and radiative evolution of a PWN inside an SNR. Our modeling suggests that the progenitor of G54.1+0.3 was an isolated ∼15-20 star which exploded inside a massive star cluster, creating a neutron star initially spinning with a period of ∼ 30-80 ms. We also find that 99.9% of the pulsar's rotational energy is injected into the PWN as relativistic electrons and positrons whose energy spectrum is well characterized by a broken power law. Finally, we propose future observations which can both test the validity of this model and better determine the properties of this source-in particular, its distance and the initial spin period of the central pulsar.
Abstract
The exact origins of many Type Ia supernovae—progenitor scenarios and explosive mechanisms—remain uncertain. In this work, we analyze the global Suzaku X-ray spectrum of Kepler’s supernova ...remnant (SNR) in order to constrain the mass ratios of various ejecta species synthesized during explosion. Critically, we account for the Suzaku telescope effective-area calibration uncertainties of 5%–20% by generating 100 mock effective-area curves and using Markov Chain Monte Carlo–based spectral fitting to produce 100 sets of best-fit parameter values. Additionally, we characterize the uncertainties from assumptions made about the emitting volumes of each model plasma component and find that these uncertainties can be the dominant source of error. We then compare our calculated mass ratios to previous observational studies of Kepler’s SNR and to the predictions of Type Ia simulations. Our mass ratio estimates require a 90% attenuated
12
C+
16
O reaction rate and are potentially consistent with both near- and sub-
M
Ch
progenitors, but are inconsistent with the dynamically stable double-detonation origin scenario and only marginally consistent with the dynamically unstable dynamically driven double-degenerate double-detonation (D
6
) scenario.
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