Steep Power Research Articles

Observation and theoretical modeling of electron scale solar wind turbulence

Abstract Turbulence at MagnetoHydroDynamics (MHD) scales in the solar wind has been studied for more than three decades, using data analysis, theoretical and numerical modeling. However, smaller scales have not been explored until very recently. Here, we review recent results on the first observation of cascade and dissipation of the solar wind turbulence at the electron scales. Thanks to the high resolution magnetic and electric field data of the Cluster spacecraft, we computed the spectra of turbulence up to ∼ 100 Hz (in the spacecraft reference frame) and found evidence of energy dissipation around the Doppler-shifted electron gyroscale f ρ e . Before its dissipation, the energy is shown to undergo two cascades: a Kolmogorov-like cascade with a scaling f − 1.6 above the proton gyroscale, and a new f − 2.3 cascade at the sub-proton and electron gyroscales. Above f ρ e the spectrum has a steeper power law ∼ f − 4.1 down to the noise level of the instrument. Solving numerically the linear Maxwell–Vlasov equations combined with recent theoretical predictions of the Gyro-Kinetic theory, we show that the present results are consistent with a scenario of a quasi-two-dimensional cascade into Kinetic Alfven modes (KAW). New analyses of other data sets, where the Cluster separation (of about ∼ 200 km ) allowed us to explore the sub-proton scales using the k-filtering technique, and to confirm the 2D nature of the turbulence at those scales.

Relationship Between Corneal Topographic Indices and Scleral Lens Base Curve

To determine whether parameters of anterior corneal contour as identified by topographic analysis (steep and flat simulated keratometry, reference sphere) predict the base curve of Jupiter scleral lenses in patients with dry eye syndrome and keratoconus. We identified 33 eyes with dry eye syndrome and 21 eyes with keratoconus that were fit with Jupiter scleral lenses of standard design between June 2006 and July 2009. Steep and flat simulated keratometry powers and shape factor from axial topographic maps, reference sphere from elevation maps, and base curve of the scleral lens prescribed for each eye were recorded. Correlations between topographic indices and base curve were evaluated by using the Pearson correlation coefficient, and significances were completed by using generalized estimating equation models. In dry eye syndrome, the base curve of the final scleral lens prescribed correlated with the steep keratometric power (r = 0.70, P = 0.05, n = 33), the flat keratometric power (r = 0.71, P<0.001, n = 33), and the reference sphere (r = 0.73, P = 0.002, n = 33). In eyes with keratoconus, base curve also correlated with the steep keratometric power (r = 0.72, P<0.001, n = 19), the flat keratometric power (r = 0.70, P<0.001, n = 19), and the reference sphere (r = 0.68, P<0.001, n = 21). There were no correlations between base curve and shape factor. In eyes with normal and abnormal ocular contour, base curve of scleral lenses correlates with reference sphere and steep and flat keratometric powers, but the predictive relationship is weak (r ∼0.50). Diagnostic fitting may be the most efficient method of fitting scleral lenses at present.

THE COMPLEX X-RAY SPECTRUM OF THE SEFYERT 1.5 SOURCE NGC 6860

The X-ray spectrum of the Seyfert 1.5 source NGC 6860 is among the most complex of the sources detected in the Swift Burst Alert Telescope all-sky survey. A short XMM-Newton follow-up observation of the source revealed a flat spectrum both above and below 2 keV. To uncover the complexity of the source, in this paper we analyze both a 40 ks Suzaku and a 100 ks XMM-Newton observation of NGC 6860. While the spectral state of the source changed between the newer observations presented here and the earlier short XMM-Newton spectrum - showing a higher flux and steeper power law component - the spectrum of NGC 6860 is still complex with clearly detected warm absorption signatures. We find that a two component warm ionized absorber is present in the soft spectrum, with column densities of about 10^20 and 10^21 cm$^-2, ionization parameters of xi = 180 and 45 ergs cm s^-1, and outflow velocities for each component in the range of 0-300 km s^-1. Additionally, in the hard spectrum we find a broad (approx 11000 km s^-1) Fe K-alpha emission line, redshifted by approx 2800 km s^-1.

Dark energy from large-scale structure lensing information

Wide area Large-Scale Structure (LSS) surveys are planning to map a substantial fraction of the visible universe to quantify dark energy through Baryon Acoustic Oscillations (BAO). At increasing redshift, for example that probed by proposed 21-cm intensity mapping surveys, gravitational lensing potentially limits the fidelity (Hui et al., 2007) because it distorts the apparent matter distribution. In this paper we show that these distortions can be reconstructed, and actually used to map the distribution of intervening dark matter. The lensing information for sources at z=1-3 allows accurate reconstruction of the gravitational potential on large scales, l <~ 100, which is well matched for Integrated Sachs-Wolfe (ISW) effect measurements of dark energy and its sound speed, and a strong constraint for modified gravity models of dark energy. We built an optimal quadratic lensing estimator for non-Gaussian sources, which is necessary for LSS. The phenomenon of "information saturation" (Rimes & Hamilton, 2005) saturates reconstruction at mildly non-linear scales, where the linear source power spectrum Delta^2 ~ 0.2-0.5. We find that steeper power spectra, saturate more quickly. We compute the effective number densities of independent lensing sources for LSS lensing, and find that they increase rapidly with redshifts. For LSS/21-cm sources at z ~ 2-4, the lensing reconstruction is limited by cosmic variance at l<~ 100.

Relativistic disc reflection in the extreme NLS1 IRAS13224−3809

We present a spectral variability study of the XMM-Newton and Suzaku observations of one of the most extreme Narrow Line Seyfert 1 galaxies, IRAS13224-3809. The X-ray spectrum is characterized by two main peculiar features, i) a strong soft excess with a steep rise below about 1.3 keV and ii) a deep drop in flux above 8.2 keV. We focus here on a reflection-based interpretation which interprets both features, as well as the large soft excess, in terms of partially ionized reflection off the inner accretion disc. We show that the two peculiar spectral features mentioned above can be reproduced by two relativistic emission lines due to Fe K and Fe L. The lines are produced in the inner accretion disc and independently yield consistent disc parameters. We argue that the high L/K intensity ratio is broadly consistent with expectations from an ionized accretion disc reflection, indicating that they belong to a single ionized reflection component. The spectral shape, X-ray flux, and variability properties are very similar in the XMM-Newton and Suzaku observations, performed about 5 years apart. The overall X-ray spectrum and variability can be described by a simple two-component model comprising a steep power law continuum plus its ionised reflection off the inner accretion disc. In this model, a rapidly rotating Kerr black hole and a steep emissivity profile are required to describe the data. The simultaneous detection of broad relativistic Fe L and K lines in IRAS 13224-3809 follows that in another extreme NLS1 galaxy, 1H0707-495. Although the data quality for IRAS13224-3809 does not allow us to rule out competing models as in 1H0707-495, we show here that our reflection-based interpretation describes in a self-consistent manner the available data and points towards IRAS13224-3809 being a very close relative of 1H0707-495 in terms of both spectral and variability properties.

The Parkes Galactic Meridian Survey: observations and CMB polarization foreground analysis

We present observations and CMB foreground analysis of the Parkes Galactic Meridian Survey (PGMS), an investigation of the Galactic latitude behaviour of the polarized synchrotron emission at 2.3 GHz with the Parkes Radio Telescope. The survey consists of a 5-deg wide strip along the Galactic meridian l=254-deg extending from Galactic plane to South Galactic pole. We identify three zones distinguished by polarized emission properties: the disc, the halo, and a transition region connecting them. The halo section lies at latitudes |b| > 40-deg and has weak and smooth polarized emission mostly at large scale with steep angular power spectra of median slope $\beta_{\rm med} \sim -2.6$. The disc region covers the latitudes |b|<20-deg and has a brighter, more complex emission dominated by the small scales with flatter spectra of median slope $\beta_{\rm med} = -1.8$. The transition region has steep spectra as in the halo, but the emission increases toward the Galactic plane from halo to disc levels. The change of slope and emission structure at $b \sim -20\degr$ is sudden, indicating a sharp disc-halo transition. The whole halo section is just one environment extended over 50-deg with very low emission which, once scaled to 70GHz, is equivalent to the CMB B-Mode emission for a tensor-to-scalar perturbation power ratio r_halo = 3.3 +/- 0.4 x 10^{-3}. Applying a conservative cleaning procedure, we estimate an r detection limit of $\delta r \sim 2\times 10^{-3}$ at 70~GHz (3-sigma C.L.) and, assuming a dust polariztion fraction <12%, $\delta r \sim 1\times 10^{-2}$ at 150~GHz. The 150-GHz limit matches the goals of planned sub-orbital experiments, which can therefore be conducted at this high frequency. The 70-GHz limit is close to the goal of proposed next generation space missions, which thus might not strictly require space-based platforms.

Effects of morphology on swimming performance in wild and laboratory crosses of brook trout ecotypes

Summary1. Trophic polymorphism is common in fish and is manifested by individual differences in habitat use, food preference, and morphology. The significance of morphological variations among ecotypes is thought to reflect individual specializations in habitat use, however, their functional relationship has rarely been examined.2. The objectives of this study were to determine if littoral and pelagic ecotypes of brook trout differ in their swimming performances, and if so to investigate the functional significance of morphological variations on their swimming performances.3. One‐year‐old laboratory‐raised littoral and pelagic individuals (and hybrids), as well as wild individuals were forced to swim in respirometers at 10, 15, and 20 °C, and 16 morphological characters were measured on each fish.4. The critical swimming test (Ucrit) revealed that pelagic trout reared in the wild are better swimmers than littoral ones. In addition, both laboratory‐raised littoral and pelagic individuals had higherUcritperformances than their hybrids. HighUcrit.was associated with short pectoral, anal, caudal and dorsal fins (wild fish) as well as with high body width (both wild and laboratory‐raised) and body height (laboratory‐raised). Wild littoral individuals had a steeper power curve (i.e. higher energy expenditure due to swimming) than pelagic ones; however, this was compensated by a lower standard metabolic rate for the former, resulting in comparable minimum transport cost for ecotypes. The swimming performances of laboratory‐raised ecotypes were more homogenous than those of wild fish, suggesting an important environmental effect on the performances themselves. Water temperature seemly had little influence on the difference between swimming performances of ecotypes.5. The results of the present study only partially support our hypothesis that critical swimming speed of pelagic individuals will be higher than that of littoral individuals due to their shorter pectoral and dorsal fins and streamlined body shape; highUcritwas associated with short fins but not with streamlined body shape. Because brook trout is not considered a fast swimmer, it is possible that a higher muscular mass provides higher benefits related to thrust for swimming in the pelagic zone than costs due to drag, usually associated with a high aspect ratio.6. Our results also support the cost reduction strategy hypothesis (that higher energy expenditure due to swimming is compensated by a lower standard metabolic rate). This implies that the lower performance, due to the higher energy expenditure, of a given ecotype can be compensated by a low standard metabolic rate, a phenomenon that could apply to all species exhibiting morphological plasticity.7. While previous studies developed conceptual models and predictions relating morphological differences to swimming performance in fishes, this study is one of the first to test such a model. Collectively, our results show that morphological differentiation, associated with trophic polymorphism, have important functional consequences on the swimming energetics of ecotypes.

Moral Experience of Canadian Healthcare Professionals in Humanitarian Work

Expatriate healthcare professionals frequently participate in international relief operations that are initiated in response to disasters due to natural hazards or humanitarian emergencies in low resource settings. This practice environment is significantly different from the healthcare delivery environment in the home countries of expatriate healthcare professionals. Human rights, public health, medicine, and ethics intersect in distinct ways as healthcare professionals provide care and services in communities affected by crisis. The purpose of this study was to explore the moral experience of Canadian healthcare professionals during humanitarian relief work. This is a qualitative study with 18 semi-structured individual interviews based on Interpretive Description methodology. There are two groups of participants: (1) 15 healthcare professionals (nine doctors, five nurses, and one midwife) with more than three months experience in humanitarian work; and (2) three individuals who have experience as human resource or field coordination officers for humanitarian, non-governmental organizations. Participants were recruited by contacting non-governmental organizations, advertisement at the global health interest group of a national medical society, word of mouth, and a snowball sampling approach in which participants identified healthcare professionals with experience practicing in humanitarian settings who might be interested in the research. Five central themes were identified during the analysis: (1) examination of motivations and expectations; (2) the relational nature of humanitarian work; (3) attending to steep power imbalances; (4) acknowledging and confronting the limits of what is possible in a particular setting; and (5) recognition of how organizational forms and structures shape everyday moral experience. Humanitarian relief work is a morally complex activity. Healthcare professionals who participate in humanitarian relief activities, or who are contemplating embarking on a humanitarian project, will benefit from carefully considering the moral dimensions of this work. Humanitarian organizations should address the moral experiences of healthcare professionals in staff recruitment, as they implement training prior to departure, and in supporting healthcare professionals in the field.

THE COLLISIONAL DIVOT IN THE KUIPER BELT SIZE DISTRIBUTION

This paper presents the results of collisional evolution calculations for the Kuiper Belt starting from an initial size distribution similar to that produced by accretion simulations of that region—a steep power-law large object size distribution that breaks to a shallower slope at r ∼ 1–2 km, with collisional equilibrium achieved for objects r ≲ 0.5 km. We find that the break from the steep large object power law causes a divot, or depletion of objects at r ∼ 10–20 km, which, in turn, greatly reduces the disruption rate of objects with r ≳ 25–50 km, preserving the steep power-law behavior for objects at this size. Our calculations demonstrate that the roll-over observed in the Kuiper Belt size distribution is naturally explained as an edge of a divot in the size distribution; the radius at which the size distribution transitions away from the power law, and the shape of the divot from our simulations are consistent with the size of the observed roll-over, and size distribution for smaller bodies. Both the kink radius and the radius of the divot center depend on the strength scaling law in the gravity regime for Kuiper Belt objects. These simulations suggest that the sky density of r ∼ 1 km objects is ∼106–107 objects per square degree. A detection of the divot in the size distribution would provide a measure of the strength of large Kuiper Belt objects, and constrain the shape of the size distribution at the end of accretion in the Kuiper Belt.

Discrete self-similarity in ultrarelativistic type-II strong explosions

A solution to the ultrarelativistic strong explosion problem with a nonpower law density gradient is delineated. We consider a blast wave expanding into a density profile falling off as a steep radial power law with small, spherically symmetric, and log-periodic density perturbations. We find discretely self-similar solutions to the perturbation equations and compare them to numerical simulations. These results are then generalized to encompass small spherically symmetric perturbations with arbitrary profiles.

Kinetic Alfvén waves turbulence in the Earth’s magnetosphere

The numerical simulations of the model equation governing the nonlinear dynamics of kinetic Alfvén waves in the intermediate-β plasmas are performed. When the nonlinearity arises due to the ponderomotive force driven density perturbations of kinetic Alfvén waves, the model equation turns out to be a modified nonlinear Schrödinger equation. This has been solved numerically by using appropriate boundary conditions. The coherent, damped magnetic filaments with turbulent spectra have been observed. Our results reveal the interesting change in spectral index because of the damping effect. The steeper power spectra follow ∼k−3.4 scaling. Using the Fokker–Planck equation with the new velocity space diffusion coefficient, we find the distribution function of energetic electrons in these turbulent structures. These turbulent structures can be responsible for plasma heating in Earth’s magnetosphere.

MAGNETIC INVERSION AS A MECHANISM FOR THE SPECTRAL TRANSITION OF BLACK HOLE BINARIES

A mechanism for the transition between low/hard, high/soft, and steep power law (SPL) spectral states in black hole X-ray binaries is proposed. The low/hard state is explained by the development of a magnetically arrested accretion disk attributable to the accumulation of a vertical magnetic field in a central bundle. This disk forms powerful jets and consists of thin spiral accretion streams of a dense optically thick plasma surrounded by hot, magnetized, optically thin corona, which emits most of the energy in hard X-rays. State transition occurs because of the quasi-periodic or random inversion of poloidal magnetic fields in the accretion flow supplied by the secondary star. The inward advection of the inverted field results in a temporal disappearance of the central bundle caused by the annihilation of the opposed fields and restoration of the optically thick disk in the innermost region. This disk represents the high/soft state. The SPL state develops at the period of intensive field annihilation and precedes the high/soft state. The continuous supply of the inverted field leads to a new low/hard state because of the formation of another magnetically arrested disk.

A truncated accretion disk in the galactic black hole candidate source H1743–322

To investigate the geometry of the accretion disk in the source H1743–322, we have carried out a detailed X-ray temporal and spectral study using RXTE pointed observations. We have selected all data pertaining to the Steep Power Law (SPL) state during the 2003 outburst of this source. We find anti-correlated hard X-ray lags in three of the observations and the changes in the spectral and timing parameters (like the QPO frequency) confirm the idea of a truncated accretion disk in this source. Compiling data from similar observations of other sources, we find a correlation between the fractional change in the QPO frequency and the observed delay. We suggest that these observations indicate a definite size scale in the inner accretion disk (the radius of the truncated disk) and we explain the observed correlation using various disk parameters like Compton cooling time scale, viscous time scale etc.

Evidence of a Cascade and Dissipation of Solar-Wind Turbulence at the Electron Gyroscale

We report the first direct determination of the dissipation range of magnetofluid turbulence in the solar wind at the electron scales. Combining high resolution magnetic and electric field data of the Cluster spacecraft, we computed the spectrum of turbulence and found two distinct breakpoints in the magnetic spectrum at 0.4 and 35 Hz, which correspond, respectively, to the Doppler-shifted proton and electron gyroscales, f(rho p) and f(rho e). Below f(rho p), the spectrum follows a Kolmogorov scaling f (-1.62), typical of spectra observed at 1 AU. Above f (rho p), a second inertial range is formed with a scaling f;{-2.3} down to f (rho e). Above f (rho e), the spectrum has a steeper power law approximately f (-4.1) down to the noise level of the instrument. We interpret this as the dissipation range and show a remarkable agreement with theoretical predictions of a quasi-two-dimensional cascade into Kinetic Alfvén Waves (KAW).

Composition of the galactic center star cluster

<i>Context. <i/>The GC is the closest galactic nucleus, offering the unique possibility of studying the population of a dense stellar cluster surrounding an SMBH.<i>Aims. <i/>The goals of this work are to develop a new method of separating early and late type stellar components of a dense stellar cluster based on narrow band filters, applying it to the central parsec of the GC, and conducting a population analysis of this area.<i>Methods. <i/>We use AO assisted observations obtained at the ESO VLT in the NIR H-band and 7 intermediate bands covering the NIR <i>K<i/>-band. A comparison of the resulting SEDs with a blackbody of variable extinction then allows us to determine the presence and strength of a CO absorption feature to distinguish between early and late type stars.<i>Results. <i/>This new method is suitable for classifying K giants (and later), as well as B2 main sequence (and earlier) stars that are brighter than 15.5 mag in the <i>K<i/> band in the central parsec. Compared to previous spectroscopic investigations that are limited to 13–14 mag, this represents a major improvement in the depth of the observations and reduces the needed observation time. Extremely red objects and foreground sources can also be reliably removed from the sample. Comparison to sources of known classification indicates that the method has an accuracy of better than ~87%. We classify 312 stars as early type candidates out of a sample of 5914 sources. Several results, such as the shape of the KLF and the spatial distribution of both early and late type stars, confirm and extend previous works. The distribution of the early type stars can be fitted with a steep power law ( = -1.49 <i>±<i/> 0.12), alternatively with a broken power law, = -1.08 <i>±<i/> 0.12, = -3.46 <i>±<i/> 0.58, since we find a drop in the early type density at ~10″. We also detect early type candidates outside of 0.5 pc in significant numbers for the first time. The late type density function shows an inversion in the inner 6″, with a power-law slope of = 0.17 <i>±<i/> 0.09. The late type KLF has a power-law slope of 0.30 <i>±<i/> 0.01, closely resembling the KLF obtained for the bulge of the Milky Way. The early type KLF has a much flatter slope of (0.14 <i>±<i/> 0.02). Our results agree best with an in-situ star formation scenario.

A COMPREHENSIVE X-RAY SPECTRAL ANALYSIS OF THE SEYFERT 1.5 NGC 3227

We present results of a 100 ks XMM observation of the Seyfert 1.5 NGC 3227. Our best-fit broadband model to the pn spectrum consists of a moderately flat (photon index 1.57) hard X-ray power-law absorbed by cold gas with N_H = 3 * 10^21 cm^-2, plus a strong soft excess, modeled as a steep power law with a photon index of 3.35, absorbed by cold gas with N_H = 9 * 10^20 cm^-2. The soft excess normalization increases by ~20% in ~20 ks, independently of the hard X-ray component, and the UV continuum, tracked via the OM, also shows a strong increasing trend over the observation, consistent with reprocessing of soft X-ray emission. Warm absorber signatures are evident in both the EPIC and RGS; we model two layers, with log(xi) = 1.2 and 2.9 erg cm s^-1, and with similar column densities (~1-2 * 10^21 cm^-2). The outflow velocities relative to systemic of the high- and low-ionization absorbers are estimated to be -(2060(+240,-170)) km/s and -(420(+430,-190)) km/s, respectively. The Fe K alpha line width FWHM is 7000 +/- 1500 km/s; its inferred radius is consistent with the BLR and with the inner radius of the dust reverberation-mapped by Suganuma et al. An emission feature near 6.0 keV is modeled equally well as a narrow redshifted Fe K line, possibly associated with a disk "hot-spot," or as the red wing to a relativistically broadened Fe line profile. Swift-BAT and archival RXTE data suggest at most weak Compton reflection (R <~ 0.5), and a high-energy cutoff near 100 keV. From RXTE monitoring, we find tentative evidence for a significant fraction of the Fe line flux to track continuum variations on time scales < 700 days.

Chemical Composition of Cosmic Rays around the Knee Observed by the Tibet Air-Shower-Core Detector

Recent results and future prospects of the Tibet air-shower experiment are reported on the all-particle energy spectrum and the chemical composition of cosmic rays around 10 15 eV energy range. The knee of the all-particle energy spectrum has been clearly observed at the energy around 4 ×10 15 eV by Tibet III air-shower array located at Yangbajing, Tibet, China (atmospheric depth 606 g/cm 2 , area 37,000 m 2 ). The energy spectrum of light components (protons and helium) measured by air-shower-core detector exhibits steeper power index than that of direct measurements below 10 14 eV, leading to decreasing fraction of the light component toward the knee. Hence, it is strongly suggested that the origin of the knee is related with the change of the chemical composition of cosmic rays. A new air-shower-core detector called `YAC' is under the preparation for explicit measurement of the heavy component.

Development of anisotropy in incompressible magnetohydrodynamic turbulence

We present a set of three-dimensional direct numerical simulations of incompressible decaying magnetohydrodynamic turbulence in which we investigate the influence of an external uniform magnetic field B0 . A parametric study in terms of B0 intensity is made where, in particular, we distinguish the shear-from the pseudo-Alfvén waves dynamics. The initial kinetic and magnetic energies are equal with a negligible cross correlation. Both the temporal and spectral effects of B0 are discussed. A subcritical balance is found between the Alfvén and nonlinear times with both a global and a spectral definition. The nonlinear dynamics of strongly magnetized flows is characterized by a different k_{ perpendicular} spectrum (where B0 defines the parallel direction) if it is plotted at a fixed k_{ parallel} (two-dimensional spectrum) or if it is integrated (averaged) over all k_{ parallel} (one-dimensional spectrum). In the former case a much wider inertial range is found with a steep power law, closer to the wave turbulence prediction than the Kolmogorov one such as in the latter case. It is believed that the averaging effect may be a source of difficulty to detect the transition towards wave turbulence in natural plasmas. Another important result of this paper is the formation of filaments reported within current and vorticity sheets in strongly magnetized flows, which modifies our classical picture of dissipative sheets in conductive flows.

INTEGRAL and Swift/XRT observations of the SFXT IGR J16479–4514: from quiescence to fast flaring activity

Context: IGR J16479-4514 is a fast X-ray transient known to display flares lasting typically a few hours. Recently, its counterpart has been identified with a supergiant star, therefore the source can be classified as member of the newly discovered class of Supergiant Fast X-ray Transients (SFXTs), specifically it is the one with the highest duty cycle. Aims: to characterize the quiescent X-ray behaviour of the source and to compare its broad band spectrum to that during fast X-ray flares. Methods: we performed an analysis of IBIS and JEM-X data with OSA 5.1 as well as an analysis of archival Swift/XRT data. Results: we present results from a long term monitoring of IGR J16479-4514 with detailed spectral and timing informations on 19 bright fast X-ray flares, 10 of which newly discovered. We also report for the first time results on the quiescent X-ray emission; the typical luminosity value (about 10^34 erg s^-1) is about 2 orders of magnitude greater than that typical of SFXTs while its broad band X-ray spectrum has a shape very similar to that during fast X-ray transient activity, i.e. a rather steep power law with Gamma=2.6. Conclusions: IGR J16479$-$4514 is characterized by a quiescent X-ray luminosity higher than that typical of other known SFXTs but lower than persistent emission from classical SGXBs. We suggest that such source is a kind of transition object between these two systems, supporting the idea that there is a continuum of behaviours between the class of SFXTs and that of classical persistent SGXBs.

Non-thermal emission from relativistic MHD simulations of pulsar wind nebulae: from synchrotron to inverse Compton

Aims. The goal of this paper is twofold. In the first place we complet e the set of diagnostic tools for synchrotron emitting sourc es presented by Del Zanna et al. (Astron. Astrophys. 453, 621, 2006 ) with the computation of inverse Compton radiation from the same relativistic particles. Moreover we investigate, for the fi rst time, the gamma-ray emission properties of Pulsar Wind Nebulae in the light of the axisymmetric jet-torus scenario. Methods. The proposed method consists in evolving the relativistic MHD equations and the maximum energy of the emitting particles including adiabatic and synchrotron losses along streamlines. The particle energy distribution function is split in t wo components: one corresponding to the radio emitting electrons interpreted as a relic population born at the outburst of the supernova and the other associated with the wind population continuously accelerated at the termination shock and emitting up to the gamma-ray band. The inverse Compton emissivity is calculated using the general Klein-Nishina differential cross-section and three different photon targets for the relativistic particles are considered: the nebular synchrotron photons, photons associated with the far-infrared thermal excess and the cosmic microwave background. Results. When the method is applied to the simulations that better reproduce the optical and X-ray morphology of the Crab Nebula, the overall synchrotron spectrum can only be fitted assuming an excess of injected particles and a steeper power law (E −2.7 ) with respect to previous models. The resulting TeV emission has then the correct shape but is in excess of the data. This is rela ted to the magnetic field structure in the nebula as obtained by the simu lations, in particular the field is strongly compressed near the termination shock but with a lower than expected volume average. The jet-torus structure is found to be clearly visible in high-resolution gammaray synthetic maps too. We also present a preliminary exploration of time variability in the X and gamma-ray bands. We find variations with time-scales of about 2 years in both bands. The variability observed originates from the strongly time-dependent MHD motions inside the nebula.