Emission Radius Research Articles

Marginally fast cooling synchrotron models for prompt GRBs

Previous studies have considered synchrotron as the emission mechanism for prompt Gamma-Ray Bursts (GRBs). These works have shown that the electrons must cool on a timescale comparable to the dynamic time at the source in order to satisfy spectral constraints while maintaining high radiative efficiency. We focus on conditions where synchrotron cooling is balanced by a continuous source of heating, and in which these constraints are naturally satisfied. Assuming that a majority of the electrons in the emitting region are contributing to the observed peak, we find that the energy per electron has to be $E\gtrsim 20$ GeV and that the Lorentz factor of the emitting material has to be very large $10^3\lesssim \Gamma_{\rm em} \lesssim 10^4$, well in excess of the bulk Lorentz factor of the jet inferred from GRB afterglows. A number of independent constraints then indicate that the emitters must be moving relativistically, with $\Gamma'\approx 10$, relative to the bulk frame of the jet and that the jet must be highly magnetized upstream of the emission region, $\sigma_{\rm up}\gtrsim 30$. The emission radius is also strongly constrained in this model to $R\gtrsim 10^{16}$cm. These values are consistent with magnetic jet models where the dissipation is driven by magnetic reconnection that takes place far away from the base of the jet.

Detection of the thermal component in GRB 160107A

Abstract We present the detection of a blackbody component in gamma-ray burst GRB 160107A emission by using the combined spectral data of the CALET Gamma-ray Burst Monitor (CGBM) and the MAXI Gas Slit Camera (GSC). MAXI/GSC detected the emission ∼45 s prior to the main burst episode observed by the CGBM. The MAXI/GSC and the CGBM spectrum of this prior emission period is fitted well by a blackbody with temperature $1.0^{+0.3}_{-0.2}$ keV plus a power law with a photon index of −1.6 ± 0.3. We discuss the radius of the photospheric emission and the main burst emission based on the observational properties. We stress the importance of coordinated observations via various instruments collecting high-quality data over a broad energy coverage in order to understand the GRB prompt emission mechanism.

SN 2014C: VLBI images of a supernova interacting with a circumstellar shell

We report on VLBI measurements of supernova 2014C at several epochs between $t = 384$ and 1057 days after the explosion. SN 2014C was an unusual supernova that initially had Type Ib optical spectrum, but after $t = 130$ d it developed a Type IIn spectrum with prominent H$\alpha$ lines, suggesting the onset of strong circumstellar interaction. Our first VLBI observation was at $t = 384$ d, and we find that the outer radius of SN 2014C was $(6.40 \pm 0.26) \times 10^{16}$ cm (for a distance of 15.1 Mpc), implying an average expansion velocity of $19300 \pm 790$ \kms\ up to that time. At our last epoch, SN 2014C was moderately resolved and shows an approximately circular outline but with an enhancement of the brightness on the W side. The outer radius of the radio emission at $t = 1057$ d is $(14.9 \pm 0.6) \times 10^{16}$ cm. We find that the expansion between $t = 384$ and 1057 d is well described by a constant velocity expansion with $v = 13600 \pm 650$ \kms. SN 2014C had clearly been substantially decelerated by $t = 384$ d. Our measurements are compatible with a scenario where the expanding shock impacted upon a shell of dense circumstellar material during the first year, as suggested by the observations at other wavelengths, but had progressed through the dense shell by the time of the VLBI observations.

Lessons from the Short GRB 170817A: The First Gravitational-wave Detection of a Binary Neutron Star Merger

Abstract The first, long-awaited, detection of a gravitational-wave (GW) signal from the merger of a binary neutron star (NS–NS) system was finally achieved (GW170817) and was also accompanied by an electromagnetic counterpart—the short-duration gamma-ray burst (GRB) 170817A. It occurred in the nearby ( Mpc) elliptical galaxy NGC 4993 and showed optical, IR, and UV emission from half a day up to weeks after the event, as well as late-time X-ray (at days) and radio (at days) emission. There was a delay of between the GW merger chirp signal and the prompt GRB emission onset, and an upper limit of was set on the viewing angle w.r.t the jet’s symmetry axis from the GW signal. In this letter we examine some of the implications of these groundbreaking observations. The delay sets an upper limit on the prompt GRB emission radius, , for a jet with sharp edges at an angle . GRB 170817A’s relatively low isotropic equivalent γ-ray energy output may suggest a viewing angle slightly outside the jet’s sharp edge, , but its peak photon energy and afterglow emission suggest instead that the jet does not have sharp edges and the prompt emission was dominated by less energetic material along our line of sight, at . Finally, we consider the type of remnant that is produced by the NS–NS merger and find that a relatively long-lived ( s) massive NS is strongly disfavored, while a hyper-massive NS of lifetime appears to be somewhat favored over the direct formation of a black hole.

A Privacy Protection Strategy for Source Location in WSN Based on Angle and Dynamical Adjustment of Node Emission Radius

We propose two novel methods to improve the source location privacy security protection and the node energy utilization in Wireless sensor networks (WSN). A privacy preservation protocol for source location in WSN based on angles(APS) and an Enhanced protocol for source location (EAPS), which dynamically adjusts emission radius during routing. The APS protocol produces geographically dispersed phantom source nodes and utilizes the energy from the energy-abundant regions to make the routing path versatile among the entire network. In the EAPS protocol, according to the number of its own adjacent nodes, residual energy and the distance to the base station, a node adjusts its radius adaptively. Experiments show that the two novel protocols can improve the security and take advantage of the residual energy in the network balance the network life and energy consumption in comparison with the existing routing protocols based on the phantom sources.

The Structure of the Radio Recombination Line Maser Emission in the Envelope of MWC349A

AbstractThe Submillimeter Array (SMA) has been used to image the emission from radio recombination lines of hydrogen at subarcsecond angular resolution from the young high-mass star MWC349A in the H26α, H30α, and H31α transitions at 353, 232, and 211 GHz, respectively. Emission was seen over a range of 80 km s−1 in velocity and 50 mas (corresponding to 60 AU for a distance of 1200 pc). The emission at each frequency has two distinct components, one from gas in a nearly edge-on annular disk structure in Keplerian motion, and another from gas lifted off the disk at distances of up to about 25 AU from the star. The slopes of the position-velocity (PV) curves for the disk emission show a monotonic progression of the emission radius with frequency with relative radii of 0.85 ± 0.04, 1, and 1.02 ± 0.01 for the H26α, H30α, and H31α transitions, respectively. This trend is consistent with theoretical excitation models of maser emission from a region where the density decreases with radius and the lower transitions are preferentially excited at higher densities. The mass is difficult to estimate from the PV diagrams because the wind components dominate the emission at the disk edges. The mass estimate is constrained to be only in the range of 10–30 solar masses. The distribution of the wind emission among the transitions is surprisingly different, which reflects its sensitivity to excitation conditions. The wind probably extracts significant angular momentum from the system.

Time Domain Filtering of Resolved Images of Sgr A∗

Abstract The goal of the Event Horizon Telescope (EHT) is to provide spatially resolved images of Sgr A*, the source associated with the Galactic Center black hole. Because Sgr A* varies on timescales that are short compared to an EHT observing campaign, it is interesting to ask whether variability contains information about the structure and dynamics of the accretion flow. In this paper, we introduce “time-domain filtering,” a technique to filter time fluctuating images with specific temporal frequency ranges and to demonstrate the power and usage of the technique by applying it to mock millimeter wavelength images of Sgr A*. The mock image data is generated from the General Relativistic Magnetohydrodynamic (GRMHD) simulation and the general relativistic ray-tracing method. We show that the variability on each line of sight is tightly correlated with a typical radius of emission. This is because disk emissivity fluctuates on a timescale of the order of the local orbital period. Time-domain filtered images therefore reflect the model dependent emission radius distribution, which is not accessible in time-averaged images. We show that, in principle, filtered data have the power to distinguish between models with different black-hole spins, different disk viewing angles, and different disk orientations in the sky.

Black hole masses of tidal disruption event host galaxies

The mass of the central black hole in a galaxy that hosted a tidal disruption event (TDE) is an important parameter in understanding its energetics and dynamics. We present the first homogeneously measured black hole masses of a complete sample of 12 optically/UV selected TDE host galaxies (down to $g_{host}$$\leq$22 mag and $z$=0.37) in the Northern sky. The mass estimates are based on velocity dispersion measurements, performed on late time optical spectroscopic observations. We find black hole masses in the range 3$\times$10$^5$ M$_{\odot}$$\leq$M$_{\rm BH}$$\leq$2$\times$10$^7$ M$_{\odot}$. The TDE host galaxy sample is dominated by low mass black holes ($\sim$10$^6$ M$_{\odot}$), as expected from theoretical predictions. The blackbody peak luminosity of TDEs with M$_{\rm BH}$$\leq$10$^{7.1}$ M$_{\odot}$ is consistent with the Eddington limit of the SMBH, whereas the two TDEs with M$_{\rm BH}$$\geq$10$^{7.1}$ M$_{\odot}$ have peak luminosities below their SMBH Eddington luminosity, in line with the theoretical expectation that the fallback rate for M$_{\rm BH}$$\geq$10$^{7.1}$ M$_{\odot}$ is sub-Eddington. In addition, our observations suggest that TDEs around lower mass black holes evolve faster. These findings corroborate the standard TDE picture in 10$^6$ M$_{\odot}$ black holes. Our results imply an increased tension between observational and theoretical TDE rates. By comparing the blackbody emission radius with theoretical predictions, we conclude that the optical/UV emission is produced in a region consistent with the stream self-intersection radius of shallow encounters, ruling out a compact accretion disk as the direct origin of the blackbody radiation at peak brightness.

Multiwavelength Polarization of Rotation-powered Pulsars

Abstract Polarization measurements provide strong constraints on models for emission from rotation-powered pulsars. We present multiwavelength polarization predictions showing that measurements over a range of frequencies can be particularly important for constraining the emission location, radiation mechanisms, and system geometry. The results assume a generic model for emission from the outer magnetosphere and current sheet in which optical to hard X-ray emission is produced by synchrotron radiation (SR) from electron–positron pairs and γ-ray emission is produced by curvature radiation (CR) or SR from accelerating primary electrons. The magnetic field structure of a force-free magnetosphere is assumed and the phase-resolved and phase-averaged polarization is calculated in the frame of an inertial observer. We find that large position angle (PA) swings and deep depolarization dips occur during the light-curve peaks in all energy bands. For synchrotron emission, the polarization characteristics are strongly dependent on photon emission radius with larger, nearly 180°, PA swings for emission outside the light cylinder (LC) as the line of sight crosses the current sheet. The phase-averaged polarization degree for SR is less that 10% and around 20% for emission starting inside and outside the LC, respectively, while the polarization degree for CR is much larger, up to 40%–60%. Observing a sharp increase in polarization degree and a change in PA at the transition between X-ray and γ-ray spectral components would indicate that CR is the γ-ray emission mechanism.

Simulations of branched carbon-chain chemistry in star-forming regions

AbstractThe detection of iso-propyl cyanide (i-C3H7CN) toward the Galactic Center hot-core source Sgr B2(N) (by Belloche et al. 2014) marked the first interstellar detection of an aliphatic molecule with a branched carbon-chain structure. Surprisingly, this branched form was found to have an almost equal abundance with its straight-chain homologue, normal-propyl cyanide (i:n = 0.40 ± 0.06). The detection of this first example of an interstellar molecule with a side-chain raises the question as to how prominent such structures may be in interstellar chemistry, and whether the large branched-to-straight chain ratio is maintained for even larger molecules.Here are presented recently published models that simulate the chemistry occurring in Sgr B2(N) using a chemical network that explicitly includes the straight-chain and branched forms of propyl cyanide (normal/iso) and butyl cyanide (normal/iso/sec/tert), as well as butane (n/i) and pentane (n/i/neo). Formation is assumed to occur on dust-grain surfaces, but a full complement of destruction mechanisms is included both on the grains and in the gas phase.The models suggest that branched structures become increasingly dominant as molecular sizes increase. In the case of butyl cyanide, the sec form is at least ∼2 times more abundant than the straight-chain normal form, and together the branched forms dominate normal-butyl cyanide by a factor of at least 3. The results for the larger alkanes suggest similarly large ratios of branched to straight-chain molecules. A key set of reactions in the surface/ice chemistry of interstellar nitriles is found to be the addition of the CN radical to unsaturated hydrocarbons, especially acetylene and ethene. The models also predict that the dominant, sec form of butyl cyanide reaches a peak abundance equal to that of n-propyl cyanide, albeit with a smaller emission radius. This makes s-C4H9CN a good candidate for detection. New ALMA observations to search for this molecule are ongoing.

THE DEPLETION OF WATER DURING DISPERSAL OF PLANET-FORMING DISK REGIONS

ABSTRACT We present a new velocity-resolved survey of 2.9 μm spectra of hot H2O and OH gas emission from protoplanetary disks, obtained with the Cryogenic Infrared Echelle Spectrometer at the VLT (R ∼ 96,000). With the addition of archival Spitzer-IRS spectra, this is the most comprehensive spectral data set of water vapor emission from disks ever assembled. We provide line fluxes at 2.9–33 μm that probe from the dust sublimation radius at ∼0.05 au out to the region of the water snow line. With a combined data set for 55 disks, we find a new correlation between H2O line fluxes and the radius of CO gas emission, as measured in velocity-resolved 4.7 μm spectra (R ), which probes molecular gaps in inner disks. We find that H2O emission disappears from 2.9 μm (hotter water) to 33 μm (colder water) as increases and expands out to the snow line radius. These results suggest that the infrared water spectrum is a tracer of inside-out water depletion within the snow line. It also helps clarify an unsolved discrepancy between water observations and models by finding that disks around stars of generally have inner gaps with depleted molecular gas content. We measure radial trends in H2O, OH, and CO line fluxes that can be used as benchmarks for models to study the chemical composition and evolution of planet-forming disk regions at 0.05–20 au. We propose that JWST spectroscopy of molecular gas may be used as a probe of inner disk gas depletion, complementary to the larger gaps and holes detected by direct imaging and by ALMA.

Structural and EELS studies on Doped Carbon Nanostructures for Cold Field Emission

Along with the development of cold field emissi on technology, more and more microscopes equipped with cold field emission gun (C-FEG ) are applied in scien tific research. Thanks to the lower working temperature, the service time of emitter has been ex tended. In the meantime, this technology allows to emit electrons from smaller emission radius compared to thermionic emission and Schottky emission [1]. Based on this property, higher curre nt density, higher brightness, be tter special coherence and lower energy distribution of the emission can be expected, wh ich also means that a better electron beam can be obtained [2]. For the further improvement of cold field emission, new emitter materials have been explored. As a rolled-up graphe ne sheet, carbon nanotubes (CNTs) have the similar property, for instance, low work function, high mechanic performan ce, high melting point, high carrier mobility [3-4]. In addition, CNTs have small apex radius curvature for its quasi-one -dimensional character at nanoscale. Therefore, CNTs are capable of working under the operating condition of cold field emission [5]. Furthermore, carbon nanocones are considered as candi date as well [6]. Significant improvements of such kind of carbon nanocones are demonstrated by repl acing the regular tungste n tip in transmission electron microscope [7]. For a higher performance of the cold field emission application, the electronic band structure of carbon nano-object can be modulat ed through the introduction of heteroatoms and a lower work function can be expected [8,9]. Our work is devoted to th e doping of these carbon nanostructures by nitrogen and/or boron, and to the evaluation of these doped carbon nano-objects by a characterization for the cold field emission application.

GAMMA-RAY BURSTS FROM MAGNETIC RECONNECTION: VARIABILITY AND ROBUSTNESS OF LIGHT CURVES

ABSTRACT The dissipation mechanism that powers gamma-ray bursts (GRBs) remains uncertain almost half a century after their discovery. The two main competing mechanisms are the extensively studied internal shocks and the less studied magnetic reconnection. Here we consider GRB emission from magnetic reconnection accounting for the relativistic bulk motions that it produces in the jet's bulk rest frame. Far from the source the magnetic field is almost exactly normal to the radial direction, suggesting locally quasi-spherical thin reconnection layers between regions of oppositely directed magnetic field. We show that if the relativistic motions in the jet's frame are confined to such a quasi-spherical uniform layer, then the resulting GRB light curves are independent of their direction distribution within this layer. This renders previous results for a delta-function velocity-direction distribution applicable to a much more general class of reconnection models, which are suggested by numerical simulations. Such models that vary in their velocity-direction distribution differ mainly in the size of the bright region that contributes most of the observed flux at a given emission radius or observed time. The more sharply peaked this distribution, the smaller this bright region, and the stronger the light curve variability that may be induced by deviations from a uniform emission over the thin reconnection layer, which may be expected in a realistic GRB outflow. This is reflected both in the observed image at a given observed time and in the observer-frame emissivity map at a given emission radius, which are calculated here for three simple velocity-direction distributions.

Imaging the expanding shell of SN 2011dh

We report on third epoch VLBI observations of the radio-bright supernova SN 2011dh located in the nearby (7.8 Mpc) galaxy M51. The observations took place at $t=453$ d after the explosion and at a frequency of 8.4 GHz. We obtained a fairly well resolved image of the shell of SN 2011dh, making it one of only six recent supernovae for which resolved images of the ejecta are available. SN 2011dh has a relatively clear shell morphology, being almost circular in outline, although there may be some asymmetry in brightness around the ridge. By fitting a spherical shell model directly to the visibility measurements we determine the angular radius of SN 2011dh's radio emission to be $636 \pm 29$ $\mu$as. At a distance of 7.8 Mpc, this angular radius corresponds to a linear radius of $(7.4 \pm 0.3) \times 10^{16}$ cm and an average expansion velocity since the explosion of $19000^{+2800}_{-2400}$ kms$^{-1}$. We combine our VLBI measurements of SN 2011dh's radius with values determined from the radio spectral energy distribution under the assumption of a synchrotron-self-absorbed spectrum, and find all the radii are consistent with a power-law evolution, with $R \sim t^{0.97\pm0.01}$, implying almost free expansion over the period $t=4$ d to 453 d.

An anisotropic minijets model for the GRB prompt emission

Abstract In order to explain rapid light-curve variability without invoking a variable source, several authors have proposed ‘minijets’ that move relativistically relative to the main flow of the jet. Here, we consider the possibility that these minijets, instead of being isotropically distributed in the comoving frame of the jet, form primarily perpendicular to the direction of the flow, as the jet dissipates its energy at a large emission radius. This yields two robust features. First, the emission is significantly delayed compared with the isotropic case. This delay allows for the peak of the afterglow emission to appear while the source is still active, in contrast to the simplest isotropic model. Secondly, the flux decline after the source turns off is steeper than the isotropic case. We find that these two features are realized in gamma-ray bursts (GRBs). (1) The peak of most GeV light curves (ascribed to the external shock) appears during the prompt emission phase. (2) Many X-ray light curves exhibit a period of steep decay, which is faster than that predicted by the standard isotropic case. The gamma-ray generation mechanism in GRBs, and possibly in other relativistic flows, may therefore be anisotropic.

THE DUST SUBLIMATION RADIUS AS AN OUTER ENVELOPE TO THE BULK OF THE NARROW Fe KαLINE EMISSION IN TYPE 1 AGNs

The Fe Kalpha emission line is the most ubiquitous feature in the X-ray spectra of active galactic nuclei (AGN), but the origin of its narrow core remains uncertain. Here, we investigate the connection between the sizes of the Fe core emission regions and the measured sizes of the dusty tori in 13 local Type 1 AGN. The observed Fe K emission radii (R_fe) are determined from spectrally resolved line widths in X-ray grating spectra, and the dust sublimation radii (R_dust) are measured either from optical/near-infrared reverberation time lags or from resolved near-infrared interferometric data. This direct comparison shows, on an object-by-object basis, that the dust sublimation radius forms an outer envelope to the bulk of the Fe K emission. R_fe matches R_dust well in the AGN with the best constrained line widths currently. In a significant fraction of objects without a clear narrow line core, R_fe is similar to, or smaller than the radius of the optical broad line region. These facts place important constraints on the torus geometries for our sample. Extended tori in which the solid angle of fluorescing gas peaks at well beyond the dust sublimation radius can be ruled out. We also test for luminosity scalings of R_fe, finding that Eddington ratio is not a prime driver in determining the line location in our sample. We discuss in detail potential caveats due to data analysis and instrumental limitations, simplistic line modeling, uncertain black hole masses, as well as sample selection, showing that none of these is likely to bias our core result. The calorimeter on board Astro-H will soon vastly increase the parameter space over which line measurements can be made, overcoming many of these limitations.

DISK FORMATION VERSUS DISK ACCRETION—WHAT POWERS TIDAL DISRUPTION EVENTS?

A tidal disruption event (TDE) takes place when a star passes near enough to a massive black hole to be disrupted. About half the star’s matter is given elliptical trajectories with large apocenter distances, and the other half is unbound. To form an accretion flow, the bound matter must lose a significant amount of energy, with the actual amount depending on the characteristic scale of the flow measured in units of the black hole’s gravitational radius ( erg). Recent numerical simulations have revealed that the accretion flow scale is close to the scale of the most bound initial orbits, cm from the black hole, and the corresponding energy dissipation rate is erg s−1. We suggest that the energy liberated during the formation of the accretion disk, rather than the energy liberated by subsequent accretion onto the black hole, powers the observed optical TDE candidates. The observed rise times, luminosities, temperatures, emission radii, and line widths seen in these TDEs are all more readily explained in terms of heating associated with disk formation rather than in terms of accretion.

Space charge limited current emission for a sharp tip

In this paper, we formulate a self-consistent model to study the space charge limited current emission from a sharp tip in a dc gap. The tip is assumed to have a radius in the order of 10s nanometer. The electrons are emitted from the tip due to field emission process. It is found that the localized current density J at the apex of the tip can be much higher than the classical Child Langmuir law (flat surface). A scaling of J ∝ Vg3/2/Dm, where Vg is the gap bias, D is the gap size, and m = 1.1–1.2 (depending on the emission area or radius) is proposed. The effects of non-uniform emission and the spatial dependence of work function are presented.

X-RAY ANALYSIS OF THE PROPER MOTION AND PULSAR WIND NEBULA FOR PSR J1741-2054

We obtained six observations of PSR J1741-2054 using the $Chandra$ ACIS-S detector totaling $\sim$300 ks. By registering this new epoch of observations to an archival observation taken 3.2 years earlier using X-ray point sources in the field of view, we have measured the pulsar proper motion at $\mu =109 \pm 10 {\rm mas\ yr}^{-1}$ in a direction consistent with the symmetry axis of the observed H$\alpha$ nebula. We investigated the inferred past trajectory of the pulsar but find no compelling association with OB associations in which the progenitor may have originated. We confirm previous measurements of the pulsar spectrum as an absorbed power law with photon index $\Gamma$=2.68$\pm$0.04, plus a blackbody with an emission radius of (4.5$^{+3.2}_{-2.5})d_{0.38}$ km, for a DM-estimated distance of $0.38d_{0.38}$ kpc and a temperature of $61.7\pm3.0$ eV. Emission from the compact nebula is well described by an absorbed power law model with a photon index of $\Gamma$ = 1.67$\pm$0.06, while the diffuse emission seen as a trail extending northeast of the pulsar shows no evidence of synchrotron cooling. We also applied image deconvolution techniques to search for small-scale structures in the immediate vicinity of the pulsar, but found no conclusive evidence for such structures.

The nature of ULX source M101 X-1: optically thick outflow from a stellar mass black hole

Abstract The nature of ultraluminous X-ray sources (ULXs) has long been plagued by an ambiguity about whether the central compact objects are intermediate-mass (IMBH, ≳103 M⊙) or stellar-mass (a few tens M⊙) black holes (BHs). The high-luminosity (≃1039 erg s−1) and supersoft spectrum (T ≃ 0.1 keV) during the high state of the ULX source X-1 in the galaxy M101 suggest a large emission radius (≳109 cm), consistent with being an IMBH accreting at a sub-Eddington rate. However, recent kinematic measurement of the binary orbit of this source and identification of the secondary as a Wolf–Rayet star suggest a stellar-mass BH primary with a super-Eddington accretion. If that is the case, a hot, optically thick outflow from the BH can account for the large emission radius and the soft spectrum. By considering the interplay of photons’ absorption and scattering opacities, we determine the radius and mass density of the emission region of the outflow and constrain the outflow mass-loss rate. The analysis presented here can be potentially applied to other ULXs with thermally dominated spectra, and to other super-Eddington accreting sources.