Radiative Luminosity Research Articles

On the Structure and Properties of Differentially Rotating, Main‐Sequence Stars in the 1–2M⊙Range

We present models for chemically homogeneous, differentially rotating, main-sequence stars with masses in the range 1-2 M☉. The models were constructed using a code based on a reformulation of the self-consistent field method of computing the equilibrium stellar structure for a specified conservative internal rotation law. Relative to nonrotating stars of the same mass, these models all have reduced luminosities and effective temperatures, and flattened photospheric shapes (i.e., decreased polar radii) with equatorial radii that can be larger or smaller, depending on the degree of differential rotation. For a fixed ratio of the axial rotation rate to the surface equatorial rotation rate, increasingly rapid rotation generally deepens convective envelopes, shrinks convective cores, and can lead to the presence of a convective core (envelope) in a 1 (2) M☉ model, a feature that is absent in a nonrotating star of the same mass. The positions of differentially rotating models for a given mass in the H-R diagram can be shifted in such a way as to approximate the nonrotating ZAMS for lower mass stars. Implications of these results include (1) possible ambiguities arising from similarities between the properties of rotating and nonrotating models of different masses, (2) a reduced radiative luminosity for a young, rapidly rotating Sun, (3) modified rates of lithium destruction by nuclear processes in the layers beneath an outer convective envelope, and (4) the excitation of solar-like oscillations and the operation of a solar-like hydromagnetic dynamo in some 1.5-2 M☉ stars.

The CMS Silicon Strip Tracker

The CMS strip tracker is the first large scale tracker entirely based on silicon detector technology. It consists of 198 m 2 of detector sensitive area instrumenting the inner region of the experiment with a pseudo-rapidity coverage of | η | < 2.5 . This instrument, together with a silicon pixel system, is expected to perform robust tracking and detailed vertex reconstruction while embedded in the LHC high radiation and high luminosity environment. The project is in a well advanced construction phase: the detector module production is completed, the integration of the single components into large sub-detector units is underway and the full tracker commissioning is about to start. In this paper, after a description of the tracker layout, detector modules production overview and a summary of the integration procedures for the inner barrel part of the tracker will be reported.

Off‐Axis Emission from Relativistic Plasma Flows

We show that there is no universal law describing how the spectra and luminosity of synchrotron and inverse Compton radiation from relativistic jets change with increasing observation angle. Instead, the physics of particle acceleration leaves pronounced imprints in the observed spectra and allows for a freedom in numerous modifications of them. The impact of these effects is the strongest for high-energy radiation and depends on the details of the particle acceleration mechanism(s). Thus, the observed impact is sensitive to the models' details, and one can use that sensitivity to discriminate between various (but maybe rather similar) models. Generally, the beam patterns of relativistic jets in the GeV-TeV spectral domain are much wider than the inverse Lorentz factor. The off-axis emission in this energy range appears to be brighter and has a much harder spectrum and a much higher cutoff frequency compared to the values derived from Doppler-boosting considerations alone. The implications include the possibility of explaining high-latitude unidentified EGRET sources as off-axis but otherwise typical relativistic jet sources, such as blazars, and the prediction of GeV-TeV afterglow from transient jet sources, such as gamma-ray bursts. We also discuss the phenomenon of beam pattern broadening as applied to neutrino emission.

ChandraObservations of Magnetic Fields and Relativistic Beaming in Four Quasar Jets

We discuss the physical properties of four quasar jets imaged with the Chandra X-Ray Observatory in the course of a survey for X-ray emission from radio jets (Marshall et al.). These objects have sufficient counts to study their spatially resolved properties, even in the 5 ks survey observations. We have acquired Australia Telescope Compact Array data with resolution matching Chandra. We have searched for optical emission with Magellan, with subarcsecond resolution. The radio to X-ray spectral energy distribution for most of the individual regions indicates against synchrotron radiation from a single-component electron spectrum. We therefore explore the consequences of assuming that the X-ray emission is the result of inverse Compton scattering on the cosmic microwave background. If particles and magnetic fields are near minimum energy density in the jet rest frames, then the emitting regions must be relativistically beamed, even at distances of order 500 kpc from the quasar. We estimate the magnetic field strengths, relativistic Doppler factors, and kinetic energy flux as a function of distance from the quasar core for two or three distinct regions along each jet. We develop, for the first time, estimates in the uncertainties in these parameters, recognizing that they are dominated by our assumptions in applying the standard synchrotron minimum energy conditions. The kinetic power is comparable with, or exceeds, the quasar radiative luminosity, implying that the jets are a significant factor in the energetics of the accretion process powering the central black hole. The measured radiative efficiencies of the jets are of order 10-4.

Galactic Centre stellar winds and Sgr A* accretion

We present a detailed discussion of our new 3D numerical models for the accretion of stellar winds on to Sgr A*. In our most sophisticated models, we put stellar wind sources on realistic orbits around Sgr A*, we include recently discovered ‘slow’ winds (vw∼ 300 km s−1), and we account for optically thin radiative cooling. We test our approach by first modelling only one-phase ‘fast’ stellar winds (vw∼ 1000 km s−1). For stellar wind sources fixed in space, the accretion rate is of the order of , fluctuates by ≲10 per cent, and is in good agreement with previous models. In contrast, decreases by an order of magnitude for wind sources following circular orbits, and fluctuates by ∼50 per cent. Then we allow a fraction of stars to produce slow winds. Much of these winds cool radiatively after being shocked, forming cold clumps and filaments immersed into the X-ray-emitting gas. We investigate two orbital configurations for the stars in this scenario, an isotropic distribution and two rotating discs with perpendicular orientation. The morphology of cold gas is quite sensitive to the orbital distribution of the stars. In both cases, however, most of the accreted gas is hot, producing a quasi-steady ‘floor’ in the accretion rate, of the order of ∼3 × 10−6 M⊙ yr−1, consistent with the values deduced from Chandra observations. The cold gas accretes in intermittent, short but powerful accretion episodes, which may give rise to large-amplitude variability in the luminosity of Sgr A* on time-scales of tens to hundreds of years. The circularization radii for the flows are about 103 and 104 Schwarzschild radii, for the one- and two-phase wind simulations, respectively, never forming the quasi-spherical accretion flows suggested in some previous work. Our work suggests that, averaged over time-scales of hundreds to thousands of years, the radiative and mechanical luminosity of Sgr A* may be substantially higher than it is in its current state. Further improvements of the wind accretion modelling of Sgr A* will rely on improved observational constraints for the wind velocities, mass-loss rates and stellar orbits.

Outflows in Active Galactic Nucleus/Starburst‐Composite Ultraluminous Infrared Galaxies

Galactic superwinds occur in almost all infrared-luminous galaxies with star formation rates (SFRs) above 10 M☉ yr-1, as shown by studies of the Na I D interstellar absorption line. We demonstrate that this result also applies to ultraluminous infrared galaxies (ULIRGs) that host an active galactic nucleus (AGN) embedded in a strong starburst (SFR 100 M☉ yr-1) by studying a sample of 26 Seyfert ULIRGs in Na I D. The infrared luminosity of these galaxies is powered jointly by the AGN and starburst. We find that there are hints of the influence of the AGN on outflows in Seyfert 2/starburst composites, but the evidence is not yet statistically conclusive. The evidence we find is lower wind detection rates (i.e., wind opening angles) in Seyfert 2 ULIRGs than in galaxies of comparable LIR, higher velocities than in galaxies of comparable SFR, and correlations between the neutral gas and the ionized gas in the extended narrow-line region. Although the AGN probably contributes to the outflows in Seyfert 2 ULIRGs, its momentum and energy injection is equal to or less than that of the starburst. Similarly, the outflow mechanical luminosity (energy outflow rate) per unit radiative luminosity is the same for starburst and Seyfert 2 ULIRGs. In the nuclei of Seyfert 1 ULIRGs, we observe small-scale outflows that are powered solely by the AGN. However, in Mrk 231, we observe both a high-velocity, small-scale and low-velocity, extended outflow. The latter may be powered by a starburst or radio jet. These large-scale, lower velocity outflows certainly exist in other Seyfert 1 ULIRGs, but they are washed out by the light of the nucleus.

The low-power nucleus of PKS 1246-410 in the Centaurus cluster

We present Chandra, Very Large Array (VLA), and Very Long Baseline Array (VLBA) observations of the nucleus of NGC 4696, a giant elliptical in the Centaurus cluster of galaxies. Like M87 in the Virgo cluster, PKS 1246-410 in the Centaurus cluster is a nearby example of a radio galaxy in a dense cluster environment. In analyzing the new X-ray data we have found a compact X-ray feature coincident with the optical and radio core. While nuclear emission from the X-ray source is expected, its luminosity is low, <10^{40} erg/s. We estimate the Bondi accretion radius to be 30 pc and the accretion rate to be 0.01 M_sun/yr which under the canonical radiative efficiency of 10% would overproduce by 3.5 orders of magnitude the radiative luminosity. Much of this energy can be directed into the kinetic energy of the jet, which over time inflates the observed cavities seen in the thermal gas. The VLBA observations reveal a weak nucleus and a broad, one-sided jet extending over 25 parsecs in position angle -150 degrees. This jet is deflected on the kpc-scale to a more east-west orientation (position angle of -80 degrees).

On the Extreme Positive Feedback Star-forming Mode from Massive and Compact Super Star Clusters

The force of gravity acting within the volume occupied by young, compact, and massive super star clusters is here shown to drive in situ all the matter deposited by winds and supernovae into several generations of star formation. These events are promoted by radiative cooling, which drains the thermal energy of the ejected gas causing its accumulation to then rapidly exceed the gravitational instability criterion. A detailed account of the integrated ionizing radiation and mechanical luminosity as a function of time is here shown to lead to a new stationary solution. In this, the mass deposition rate , instead of causing a wind as in the adiabatic solution, turns into a positive feedback star-forming mode equal to the star formation rate. Some of the implications of this extreme positive feedback mode are discussed.

ABOUT THE GENERALIZED UNCERTAINTY PRINCIPLE

Some consequences of the generalized uncertainty principle (GUP) are investigated, including the deformations of the Wein's law and the state equations of black body radiation. The effects of the GUP on the thermodynamics of black holes are investigated by a heuristic method. A bound on the luminosity of the black hole radiation is obtained. The logarithmic corrections to the Bekenstein–Hawking entropy are obtained in three cases. The potential relation between the GUP and the holographic principle is also briefly discussed.

High-energy emission from pulsars: the polar cap scenario

The study of pulsars in the three and a half decades since their discovery has highlighted a handful of issues critical to their understanding. To date there is no consensus on the physical mechanism for their radio emission, despite a rapid increase in the observed population due to the Parkes multi-beam survey and prospects for similar growth in the radio population database in the near future. The small subset of pulsars that emit at X-ray to γ-ray wavelengths is critical to refining the pulsar paradigm, since this energy band (i) is where the vast majority of radiative luminosity is observed and (ii) is intimately connected to the pair winds that form the dominant mode of energy deposition in the circum-pulsar environment. The most crucial point of contention pertaining to the high energy astrophysics of pulsars is the location of the acceleration region in their magnetospheres: is an outer gap model or a polar cap scenario (or both) the most appropriate picture. Radiative signatures provide the clues to this current enigma. This review focuses on salient characteristics of the polar cap scenario; these form the basis for discriminating observational diagnostics that should drive pulsars studies in the GLAST era just three years away.

Robert Wilson

Robert Wilson

Time-dependent modelling of the Markarian 501 X-ray and TeV gamma-ray data taken during 1997 March and April

If the high-energy emission from TeV blazars is produced by the Synchrotron Self-Compton (SSC) mechanism, then simultaneous X-ray and gamma-ray observations of these objects are a powerful probe of the electron (and positron) populations responsible for this emission. Understanding the emitting particle distributions and their temporal evolution in turn allows us to probe physical conditions in the inner blazar jet and test, for example, various acceleration scenarios. Furthermore, by constraining the SSC model parameters, such observations enable us to predict the intrinsic (unabsorbed) gamma-ray energy spectra of these sources, a major uncertainty in current attempts to use gamma-ray observations to constrain the intensity of the Diffuse Extragalactic Background Radiation (DEBRA) at optical/infrared wavelengths. As a next step in testing the SSC model and as a demonstration of the potential power of coordinated X-ray and gamma-ray observations, we model in detail the X-ray and gamma-ray light curves of the TeV blazar Mrk 501 during its 1997 April-May outburst with a time-dependent SSC model. Extensive, quasi-simultaneous X-ray and gamma-ray coverage exists for this period. We discuss and explore quantitatively several of the flare scenarios presented in the literature. We show that simple two-component models (with a soft, steady X-ray component plus a variable SSC component) involving substantial pre-acceleration of electrons to Lorentz factors on the order of γ m i n = 10 5 describe the data train surprisingly well. All considered models imply an emission region that is strongly out of equipartition and low radiative efficiencies (ratio between kinetic jet luminosity and comoving radiative luminosity) of 1 per-mill and less. Degeneracy in both, model variant and jet parameters, prevents us from using the time-resolved SSC calculations to tighten substantially the constraints on the amount of extragalactic gamma-ray extinction by the DEBRA in the relevant 0.5-50 μm wavelength range, compared with earlier work.

Solar opacities

Solar opacities are presented from the center of the Sun to the photosphere. The temperatures, densities and hydrogen mass fractions are taken from the standard solar model. For the heavy element abundance the Grevesse mixture is used. In the solar interior photoabsorption is dominated by free–free absorption and we compare two sets of opacities based on two different models for the inverse bremsstrahlung. The radiative luminosity calculated from the two sets of opacities are compared with those predicted by previous models of the standard solar model and also with the known luminosity of the Sun. Pressures, specific heats and the speed of sound in the solar plasma are also presented.

Newtonian hydrodynamics of the coalescence of black holes with neutron stars — III. Irrotational binaries with a stiff equation of state

We present a numerical study of the hydrodynamics in the final stages of inspiral of a black hole-neutron star binary, when the binary separation becomes comparable to the stellar radius. We use a Newtonian three-dimensional Smooth Particle Hydrodynamics (SPH) code, and model the neutron star with a soft (adiabatic index Gamma=5/3) polytropic equation of state and the black hole as a Newtonian point mass which accretes matter via an absorbing boundary at the Schwarzschild radius. Our initial conditions correspond to tidally locked binaries in equilibrium, and we have explored configurations with different values of the mass ratio q=Mns/Mbh, ranging from q=1 to q=0.1. The dynamical evolution is followed for approximately 23 ms, and in every case studied here we find that the neutron star is tidally disrupted on a dynamical timescale, forming a dense torus around the black hole that contains a few tenths of a solar mass. A nearly baryon-free axis is present in the system throughout the coalescence, and only modest beaming of a fireball that could give rise to a gamma-ray burst would be sufficient to avoid excessive baryon contamination. We find that some mass (on the order of 0.01 to 0.001 solar masses) may be dynamically ejected from the system, and could thus contribute substantially to the amount of observed r-process material in the galaxy. We calculate the gravitational radiation waveforms and luminosity in the quadrupole approximation.

Rapid photometry of supernova 1987A: a 2.14 ms pulsar?

We have monitored Supernova 1987A in optical/near-infrared bands using various high-speed photometers from a few weeks following its birth until early 1996 in order to search for a pulsar remnant. While we have found no clear evidence of any pulsar of constant intensity and stable timing, we have found emission with a complex period modulation near the frequency of 467.5 Hz – a 2.14 ms pulsar candidate. We first detected this signal in data taken on the remnant at the Las Campanas Observatory (LCO) 2.5-m Dupont telescope during 14–16 Feb. 1992 UT. We detected further signals near the 2.14 ms period on numerous occasions over the next four years in data taken with a variety of telescopes, data systems and detectors, at a number of ground- and space-based observatories. In particular, an effort during mid-1993 to monitor this signal with the U. of Tasmania 1-m telescope, when SN1987A was inaccessible to nearly all other observing sites due to high airmass, clearly detected the 2.14 ms signal in the first three nights' observations. The sequence of detections of this signal from Feb. `92 through August `93, prior to its apparent subsequent fading, is highly improbable (<10 −10 for any noise source). In addition, the frequency of the signals followed a consistent and predictable spin-down (∼2–3×10 −10 Hz/s) over the several year timespan (`92–`96). We also find evidence in data, again taken by more than one telescope and recording system, for modulation of the 2.14 ms period with a ∼1,000 s period which complicates its detection. The 1,000 s modulation was clearly detected in the first two observations with the U. Tas. 1-m during mid-1993. The characteristics of the 2.14 ms signature and its ∼1,000 s modulation are consistent with precession and spindown via gravitational radiation of a neutron star with an effective non-axisymmetric oblateness of ∼10 −6. The implied luminosity of the gravitational radiation exceeds the spindown luminosity of the Crab Nebula pulsar by an order of magnitude. Due to the nature of the 2.14 ms signature and its modulation, and the analysis techniques necessary for detection, it is difficult to determine the overall probability that all aspects of the signal are real, though it has remained consistent with an astrophysical origin throughout the several year timespan of our study.

Young Circumstellar Disks near Evolved Massive Stars and Supernovae

There is increasing evidence that low-mass stars with circumstellar disks can be born close to massive stars, in some cases within tenths of a parsec. If the disks have lifetimes greater than those of the more massive stars, they are exposed to radiation fields and gas flows from the late evolutionary phases and supernovae of the massive stars. The fast flows from supernovae are likely to give some stripping of mass from the disks but do not typically lead to complete disruption of the disks. In the slow wind from a red supergiant star, there is the possibility of gravitational accretion of wind matter onto the circumstellar disk. In the supernova explosion of a red supergiant, the radiative flux at the time of shock breakout can heat and ionize a nearby disk, leading to transient, narrow-line emission. There are consequences for the solar nebula if it was born ~0.2 pc from a massive star that became a red supergiant. Accretion from the wind could supply a substantial amount of 26Al to the disk. The high radiative luminosity of the eventual supernova could lead to the melting of dust grains and the formation of chondrules. The passage of the supernova ejecta could drive a shock wave in the disk, heating it.

Newtonian Hydrodynamics of the Coalescence of Black Holes with Neutron Stars. I. Tidally Locked Binaries with a Stiff Equation of State

ABSTRA C T We present a numerical study of the hydrodynamics in the final stages of inspiral of a black hole‐neutron star binary, when the binary separation becomes comparable to the stellar radius. We use a Newtonian three-dimensional smooth particle hydrodynamics (SPH) code, and model the neutron star with a soft (adiabatic index Ga 5=3) polytropic equation of state, and the black hole as a Newtonian point mass that accretes matter via an absorbing boundary at the Schwarzschild radius. Our initial conditions correspond to tidally locked binaries in equilibrium, and we have explored configurations with different values of the mass ratio qa MNS=MBH, ranging from qa 1 to 0.1. The dynamical evolution is followed for approximately 23 ms, and in every case studied here we find that the neutron star is tidally disrupted on a dynamical time-scale, forming a dense torus around the black hole that contains a few tenths of a solar mass. A nearly baryon-free axis is present in the system throughout the coalescence, and only modest beaming of a fireball that could give rise to a gamma-ray burst would be sufficient to avoid excessive baryon contamination. We find that some mass (of the order of 10 23 ‐10 22 M() may be dynamically ejected from the system, and could thus contribute substantially to the amount of observed r-process material in the galaxy. We calculate the gravitational radiation waveforms and luminosity emitted during the coalescence in the quadrupole approximation.

The CMS silicon strip tracker

Abstract The CMS strip tracker is the first large scale tracker entirely based on silicon detector technology. It consists of 198 m 2 of detector sensitive area instrumenting the inner region of the experiment with a pseudo-rapidity coverage of | η | 2.5 . This instrument, together with a silicon pixel system, is expected to perform robust tracking and detailed vertex reconstruction while embedded in the LHC high radiation and high luminosity environment. The project is in a well advanced construction phase: the detector module production is completed, the integration of the single components into large sub-detector units is underway and the full tracker commissioning is about to start. In this paper, after a description of the tracker layout, detector modules production overview and a summary of the integration procedures for the inner barrel part of the tracker will be reported.

Electron energization by lower-hybrid waves as a possible source for x-ray emission from comets

A goal of this presentation is to demonstrate that recently observed x-ray emission from comets can be interpreted as a combination of bremsstrahlung and line k-shell radiation produced by interaction of energetic electrons with the cometary atmosphere. Electron energization inside the cometary bow-shock is due to mass-loading of the shocked solar wind flow by newly born heavy cometary ions created by photoionization of the neutral gas outflow. The new ion component in the crossed E × B fields of the host (solar wind) plasma moves in cycloidal orbits, forming an unstable ring distribution, which drives the modified two stream instability and excites lower hybrid waves. These waves have the phase velocity along the magnetic field much larger than across it. For this reason, they can be in simultaneous Landau resonance with both slow unmagnetized ions and fast, but magnetized (field aligned), electrons efficiently transferring ion energy to the electrons and leading to their energization. We have investigated this process in a quasilinear approximation. By solving the Fokker-Planck equation for the field aligned electron energy diffusion, we have been able to calculate fluxes and energy spectra of electrons accelerated by the lower hybrid waves and to determine spectrum and total luminosity of x-ray radiation. Results of theoretical analysis are compared with ROSAT and Beppo-Sax observations of x-ray emission from the different comets.

Non-pulsed gamma radiation from a binary system with a pulsar

Consider binary system with a millisecond pulsar ejecting relativistic particles and an optical star emitting soft photons with the energy $\omega \simeq 1-10$ eV. These low-energy photons are scattered by the relativistic electrons and positrons of the pulsar's wind. The scattered photons forms a wide spectrum from hard X-ray band up to gamma band $\epsilon \simeq 1-1000$GeV >.When the pulsar wind is isotropic the luminosity of gamma radiation $L_\gamma =L_\gamma (\psi)$ depends heavily on the angle $\psi $ between the directions to the optical star and to the observer from the pulsar. During the orbital motion this angle varies periodically giving rise to the periodical change of the observed intensity of the gamma radiation and its spectrum. We calculated the spectral shape of the scattered hard photons. Under the assumption that the energy losses of the relativistic particles are small we receive analytical formulas. We apply our results to the binary system PSR B1259-63 and show that if the wind from the Be star is accounted for then it is possible to reproduce the observed spectrum.