Bulk Comptonization spectra in blazars

We study the time dependent spectra produced via the bulk Compton process by a cold, relativistic shell of plasma moving (and accelerating) along the jet of a blazar, scattering on external photons emitted by the accretion disc and reprocessed in the broad line region. Bulk Comptonization of disc photons is shown to yield a spectral component contributing in the far UV band, and would then be currently unobservable. On the contrary, the bulk Comptonization of broad line photons may yield a significant feature in the soft X-ray band. Such a feature is time-dependent and transient, and dominates over the non thermal continuum only when: a) the dissipation occurs close to, but within, the broad line region; b) other competing processes, like the synchrotron self-Compton emission, yield a negligible flux in the X-ray band. The presence of a bulk Compton component may account for the X-ray properties of high redshift blazars that show a flattening (and possibly a hump) in the soft X-rays, previously interpreted as due to intrinsic absorption. We discuss why the conditions leading to a detectable bulk Compton feature might be met only occasionally in high redshift blazars, concluding that the absence of such a feature in the spectra of most blazars should not be taken as evidence against matter--dominated relativistic jets. The detection of such a component carries key information on the bulk Lorentz factor and kinetic energy associated to (cold) leptons.Comment: 8 pages; 4 figures; MNRAS, accepte

Wall Effects in Cavity Flows and their Correction Rules

The wall effects in cavity flows have been long recognized to be more important and more difficult to determine than those in single-phase, nonseparated flows. Earlier theoretical investigations of this problem have been limited largely to simple body forms in plane flows, based on some commonly used cavity-flow models, such as the Riabouchinsky, the reentrant jet, or the linearized flow model, to represent a finite cavity. Although not meant to be exhaustive, references may be made to Cisotti (1922), Birkhoff, Plesset and Simmons (1950, 1952), Gurevich (1953), Cohen et al. (1957, 1958), and Fabula (1964). The wall effects in axisymmetric flows with a finite cavity has been evaluated numerically by Brennen (1969) for a disk and a sphere. Some intricate features of the wall effects have been noted in experimental studies by Morgan (1966) and Dobay (1967). Also, an empirical method for correcting the wall effect has been proposed by Meijer (1967). The presence of lateral flow boundaries in a closed water tunnel introduces the following physical effects: (i) First, in dealing with the part of irrotational flow outside the viscous region, these flow boundaries will impose a condition on the flow direction at the rigid tunnel walls. This "streamline-blocking" effect will produce extraneous forces and modifications of cavity shape. (ii) The boundary layer built up at the tunnel walls may effectively reduce the tunnel cross-sectional area, and generate a longitudinal pressure gradient in the working section, giving rise to an additional drag force known as the "horizontal buoyancy." (iii) The lateral constraint of tunnel walls results in a higher velocity outside the boundary layer, and hence a greater skin friction at the wetted body surface. (iv) The lateral constraint also affects the spreading of the viscous wake behind the cavity, an effect known as the "wake-blocking." (v) It may modify the location of the "smooth detachment" of cavity boundary from a continuously curved body. In the present paper, the aforementioned effect (i) will be investigated for the pure-drag flows so that this primary effect can be clarified first. Two cavity flow models, namely, the Riabouchinsky and the open-wake (the latter has been attributed, independently, to Joukowsky, Roshko, and Eppler) models, are adopted for detailed examination. The asymptotic representations of these theoretical solutions, with the wall effect treated as a small correction to the unbounded-flow limit, have yielded two different wall-correction rules, both of which can be applied very effectively in practice. It is of interest to note that the most critical range for comparison of these results lies in the case when the cavitating body is slender, rather than blunt ones, and when the cavity is short, instead of very long ones in the nearly choked-flow state. Only in this critical range do these flow models deviate significantly from each other, thereby permitting a refined differentiation and a critical examination of the accuracy of these flow models in representing physical flows. A series of experiments carefully planned for this purpose has provided conclusive evidences, which seem to be beyond possible experimental uncertainties, that the Riabouchinsky model gives a very satisfactory agreement with the experimental results, and is superior to other models, even in the most critical range when the wall effects are especially significant and the differences between these theoretical flow models become noticeably large. These outstanding features are effectively demonstrated by the relatively simple case of a symmetric wedge held in a non-lifting flow within a closed tunnel, which we discuss in the sequel

A Massive Protostar Embedded in the Scuba Core JCMT 18354-0649S

We report the discovery of an extremely red object embedded in the massive SCUBA core JCMT 18354-0649S. This object is not associated with any known radio or far-IR source, though it appears in Spitzer IRAC data obtained as part of the GLIMPSE survey. At shorter wavelengths, this embedded source exhibits an extreme color, K – L' = 6.7. At an assumed distance of 5.7 kpc, this source has a near-IR luminosity of ~1000 L_☉. Its spectral energy distribution (SED) rises sharply from 2.1 μm to 8 μm, similar to that of a Class 0 young stellar object. Theoretical modeling of the SED indicates that the central star has a mass of 6-12 M_☉, with an optical extinction of more than 30. As both inflow and outflow motions are present in JCMT 18354-0649S, we suggest that this deeply embedded source is (1) a massive protostar in the early stages of accretion, and (2) the driving source of a massive molecular outflow evident in HCN J = 3-2 profiles observed toward this region

Predicting QSO Continua in the Ly Alpha Forest

We present a method to make predictions with sets of correlated data values, in this case QSO flux spectra. We predict the continuum in the Lyman-Alpha forest of a QSO, from 1020 -- 1216 A, using the spectrum of that QSO from 1216 -- 1600 A . We find correlations between the unabsorbed flux in these two wavelengths regions in the HST spectra of 50 QSOs. We use principal component analysis (PCA) to summarize the variety of these spectra and we relate the weights of the principal components for 1020 -- 1600 A to the weights for 1216 -- 1600 A, and we apply this relation to make predictions. We test the method on the HST spectra, and we find an average absolute flux error of 9%, with a range 3 -- 30%, where individual predictions are systematically too low or too high. We mention several ways in which the predictions might be improved.Comment: 17 pages, 4 figures, submitted to Ap

Comparative study of design: application to Engineering Design

A recent exploratory study examines design processes across domains and compares them. This is achieved through a series of interdisciplinary, participative workshops. A systematic framework is used to collect data from expert witnesses who are practising designers across domains from engineering through architecture to product design and fashion, including film production, pharmaceutical drugs, food, packaging, graphics and multimedia and software. Similarities and differences across domains are described which indicate the types of comparative analysis we have been able to do from our data. The paper goes further and speculates on possible lessons for selected areas of engineering design which can be drawn from comparison with processes in other domains. As such this comparative design study offers the potential for improving engineering design processes. More generally it is a first step in creating a discipline of comparative design which aims to provide a new rich picture of design processes

2-D Radiative Transfer in Protostellar Envelopes: II. An Evolutionary Sequence

We present model spectral energy distributions, colors, polarization, and images for an evolutionary sequence of a low-mass protostar from the early collapse stage (Class 0) to the remnant disk stage (Class III). We find a substantial overlap in colors and SEDs between protostars embedded in envelopes (Class 0-I) and T Tauri disks (Class II), especially at mid-IR wavelengths. Edge-on Class I-II sources show double-peaked spectral energy distributions, with a short-wavelength hump due to scattered light and the long-wavelength hump due to thermal emission. These are the bluest sources in mid-IR color-color diagrams. Since Class 0 and I sources are diffuse, the size of the aperture over which fluxes are integrated has a substantial effect on the computed colors, with larger aperture results showing significantly bluer colors. This causes overlap in color-color diagrams between all evolutionary states, especially in the mid-IR. However the near-IR polarization of the Class 0 sources is much higher than the Class I-II sources, providing a means to separate these evolutionary states. We varied the grain properties in the circumstellar envelope, allowing for larger grains in the disk midplane and smaller in the envelope. We find that grain growth in disks of Class I sources can be detected at wavelengths greater than 100 $\mu$m. Our image calculations predict that the diffuse emission from edge-on Class I and II sources should be detectable in the mid-IR with the Space Infrared Telescope Facility (SIRTF) in nearby star forming regions (out to several hundred parsecs).Comment: A version with high-resolution images is available at http://www.astro.wisc.edu/glimpse/glimpsepubs.htm

Herschel Observations of a Newly Discovered UX Ori Star in the Large Magellanic Cloud

The LMC star, SSTISAGE1C J050756.44-703453.9, was first noticed during a survey of EROS-2 lightcurves for stars with large irregular brightness variations typical of the R Coronae Borealis (RCB) class. However, the visible spectrum showing emission lines including the Balmer and Paschen series as well as many Fe II lines is emphatically not that of an RCB star. This star has all of the characteristics of a typical UX Ori star. It has a spectral type of approximately A2 and has excited an H II region in its vicinity. However, if it is an LMC member, then it is very luminous for a Herbig Ae/Be star. It shows irregular drops in brightness of up to 2 mag, and displays the reddening and "blueing" typical of this class of stars. Its spectrum, showing a combination of emission and absorption lines, is typical of a UX Ori star that is in a decline caused by obscuration from the circumstellar dust. SSTISAGE1C J050756.44-703453.9 has a strong IR excess and significant emission is present out to 500 micron. Monte Carlo radiative transfer modeling of the SED requires that SSTISAGE1C J050756.44-703453.9 has both a dusty disk as well as a large extended diffuse envelope to fit both the mid- and far-IR dust emission. This star is a new member of the UX Ori subclass of the Herbig Ae/Be stars and only the second such star to be discovered in the LMC.Comment: ApJ, in press. 9 pages, 5 figure

Electrical Control of Linear Dichroism in Black Phosphorus from the Visible to Mid-Infrared

The incorporation of electrically tunable materials into photonic structures such as waveguides and metasurfaces enables dynamic control of light propagation by an applied potential. While many materials have been shown to exhibit electrically tunable permittivity and dispersion, including transparent conducting oxides (TCOs) and III-V semiconductors and quantum wells, these materials are all optically isotropic in the propagation plane. In this work, we report the first known example of electrically tunable linear dichroism, observed here in few-layer black phosphorus (BP), which is a promising candidate for multi-functional, broadband, tunable photonic elements. We measure active modulation of the linear dichroism from the mid-infrared to visible frequency range, which is driven by anisotropic quantum-confined Stark and Burstein-Moss effects, and field-induced forbidden-to-allowed optical transitions. Moreover, we observe high BP absorption modulation strengths, approaching unity for certain thicknesses and photon energies

Near-Infrared Imaging Polarimetry of Young Stellar Objects in rho-Ophiuchi

The results of a near-infrared (J H K LP) imaging linear polarimetry survey of 20 young stellar objects (YSOs) in rho Ophiuchi are presented. The majority of the sources are unresolved, with K-band polarizations, P_K < 6 per cent. Several objects are associated with extended reflection nebulae. These objects have centrosymmetric vector patterns with polarization discs over their cores; maximum polarizations of P_K > 20 per cent are seen over their envelopes. Correlations are observed between the degree of core polarization and the evolutionary status inferred from the spectral energy distribution. K-band core polarizations >6 per cent are only observed in Class I YSOs. A 3D Monte Carlo model with oblate grains aligned with a magnetic field is used to investigate the flux distributions and polarization structures of three of the rho Oph YSOs with extended nebulae. A rho proportional to r^(-1.5) power law for the density is applied throughout the envelopes. The large-scale centrosymmetric polarization structures are due to scattering. However, the polarization structure in the bright core of the nebula appears to require dichroic extinction by aligned non-spherical dust grains. The position angle indicates a toroidal magnetic field in the inner part of the envelope. Since the measured polarizations attributed to dichroic extinction are usually <10 per cent, the grains must either be nearly spherical or very weakly aligned. The higher polarizations observed in the outer parts of the reflection nebulae require that the dust grains responsible for scattering have maximum grain sizes <=1.05 microns.Comment: 26 pages. Accepted by MNRAS. Available as online early versio