Band Continuum Research Articles

Cygnus survey with the Giant Metrewave Radio Telescope at 325 and 610 MHz: the catalog

Context.Observations at the radio continuum band below the gigahertz band are key when the nature and properties of nonthermal sources are investigated because their radio radiation is strongest at these frequencies. The low radio frequency range is therefore the best to spot possible counterparts to very high-energy (VHE) sources: relativistic particles of the same population are likely to be involved in radio and high-energy radiation processes. Some of these counterparts to VHE sources can be stellar sources.Aims.The Cygnus region in the northern sky is one of the richest in this type of sources that are potential counterparts to VHE sources. We surveyed the central ∼15 sq deg of the Cygnus constellation at the 325 and 610 MHz bands with angular resolutions and sensitivities of 10″ and 6″, and 0.5 and 0.2 mJy beam−1, respectively.Methods.The data were collected during 172 h in 2013–2017, using the Giant Metrewave Radio Telescope with 32 MHz bandwidth, and were calibrated using the SPAM routines. The source extraction was carried out with the PyBDSF tool, followed by verification through visual inspection of every putative catalog candidate source in order to determine its reliability.Results.In this first paper we present the catalog of sources, consisting of 1048 sources at 325 MHz and 2796 sources at 610 MHz. By cross-matching the sources from both frequencies with the objects of the SIMBAD database, we found possible counterparts for 143 of them. Most of the sources from the 325-MHz catalog (993) were detected at the 610 MHz band, and their spectral indexαwas computed adoptingS(ν) ∝να. The maximum of the spectral index distribution is atα = −1, which is characteristic of nonthermal emitters and might indicate an extragalactic population.

Quantum breathers and intrinsic localized excitations in an isotropic ferromagnet with octupole–dipole interaction

A numerical and analytical work on quantum breathers and intrinsic localized excitations in an isotropic ferromagnet with octupole–dipole interaction are reported. For the case with two bosons, we find many interesting and unusual features. For examples, the energy spectrum is composed of two symmetric continuums interconnected by two isolated bands that are sharply merged in a double continuum bands. These continuum bands are surrounded by two symmetric isolated bands and are also symmetric with respect to the degenerated point localized at the center of the Brillouin zone. When the value of octupole–dipole parameter increases, the shape of the continuum band undergoes grave changes and looks like a superposition of two continuum bands, but merged and becomes non degenerated at the center of the Brillouin zone. The eigenfunctions of the corresponding bound states are analyzed and their existences as quantum breathers are proved. The intrinsic localized excitation analysis, show that the bright and dark type intrinsic localized modes (ILMs) can exist in the system. The accuracy of the analytical solutions is tested by numerical simulations. Further more, we have proved that the increasing value of the coupling strength can lead the decreasing intensity of localization of the bright ILMs whereas the opposite behavior appears for the dark ILMs case.

First-principles study of the topological surface states of α-Sn(111)

α-Sn is on the boundary of a couple of distinct topological phases. It will transform into a topological insulator under a suitable strain. However, a clear picture of its topological surface states (TSSs) is still lacking. Here we perform first-principles calculations on the electronic structure of α-Sn(111) surface to identify its TSSs and reveal their properties. The results show that the presence of valence band reorganizes the TSSs in the inverted sp gap into two Dirac cones. The lower one is in the valence band continuum; the upper one resides in the gap between the valence and conduction bands. We also demonstrate the transformation of the surface states by switching on or off of strain and/or spin-orbit coupling. Without spin-orbit coupling, only the TSSs associated with the lower Dirac cone survive, and they are spin unpolarized. The results are useful for understanding and engineering the topological properties of α-Sn.

Diffuse Lyα Halos around 300 Spectroscopically Confirmed Lyα Emitters at z ∼ 5.7

Abstract We report the detection of diffuse halos (LAHs) around star-forming galaxies at z ∼ 5.7 by stacking 310 spectroscopically confirmed emitters (LAEs). The majority of the LAEs are identified from our spectroscopic survey of galaxies at z > 5.5. They are all located in well-studied fields with deep narrowband and broadband imaging data. We combine the LAE sample and its subsamples in the narrowband NB816 (i.e., the band) and z band (i.e., the continuum band). By comparing the stacked objects with their corresponding point-spread functions, we clearly detect extended LAHs around these LAEs. We perform sophisticated simulations and analyses on statistical and systematic errors, and confirm that the detected halos are not caused by errors. The scale lengths of the LAHs, when described by a double-component model, range from 1.2 to 5.3 kpc with a typical value of ∼2 kpc. The LAH sizes from our sample are in agreement with those of individual LAEs at the similar redshift measured by Very Large Telescope/MUSE but are relatively smaller than those of photometrically selected LAEs in previous studies. We also find that LAEs with higher luminosities, higher UV-continuum luminosities, or smaller equivalent widths tend to have larger LAH sizes. Our results are consistent with a scenario that LAHs originate from the scattered light of the central galaxies by H i gas in the circumgalactic medium.

Defect states in monolayer hexagonal BN: A comparative DFT and DFT-1/2 study

Hexagonal boron nitride (h-BN) acts like a semiconductor vacuum to point defects enabling stable and controllable spin states at room temperature which qualifies them for quantum technological applications. To characterize their properties first-principles techniques constitute indispensable tools. The currently established paradigm for such solid-state electronic structure calculations is the density functional theory (DFT). Recently its variant, so-called DFT-1/2 method was introduced with the promise of accurate band gaps without a computational overhead with respect to ordinary DFT. Here, for the monolayer h-BN we contrast DFT and DFT-1/2 results for carbon substitutional impurities (CB, CN), boron and nitrogen single vacancies (VB, VN), divacancy, and Stone-Wales defects. Comparisons with more sophisticated, yet computationally costly techniques namely, hybrid functional DFT and the GW are also made, where available. From the standpoint of defect states embedded in the band gap region we demonstrate a clear advantage of DFT-1/2 in revealing the localized states otherwise buried within either the valence or conduction band continuum due to well-known gap underestimation syndrome of the standard DFT implementations. Thus, DFT-1/2 can serve for the rapid screening of candidate defect systems before more demanding considerations.

Multiwavelength consensus of large-scale linear bias

ABSTRACT We model the large-scale linear galaxy bias bg(x, z) as a function of redshift z and observed absolute magnitude threshold x for broad-band continuum emission from the far-infrared to ultraviolet, as well as for prominent emission lines, such as the H α, H β, Ly a, and [O ii] lines. The modelling relies on the semi-analytic galaxy formation model galform, run on the state-of-the-art N-body simulation surfs with the Planck 2015 cosmology. We find that both the differential bias at observed absolute magnitude x and the cumulative bias for magnitudes brighter than x can be fitted with a five-parameter model: bg(x, z) = a + b(1 + z)e(1 + exp [(x − c)d]). We also find that the bias for the continuum bands follows a very similar form regardless of wavelength due to the mixing of star-forming and quiescent galaxies in a magnitude-limited survey. Differences in bias only become apparent when an additional colour separation is included, which suggest extensions to this work could look at different colours at fixed magnitude limits. We test our fitting formula against observations, finding reasonable agreement with some measurements within 1σ statistical uncertainties, and highlighting areas of improvement. We provide the fitting parameters for various continuum bands, emission lines, and intrinsic galaxy properties, enabling a quick estimation of the linear bias in any typical survey of large-scale structure.

Ultrafast Transient Spectroscopy of Trans-Polyacetylene in the Midinfrared Spectral Range.

Trans-polyacetylene [t-(CH)_{x}] possesses twofold ground state degeneracy. Using the Su-Schrieffer-Heeger Hamiltonian, scientists predicted charged solitons to be the primary photoexcitations in t-(CH)_{x}; this prediction, however, has led to sharp debate. To resolve this saga, we use subpicosecond transient photomodulation spectroscopy in the mid-IR spectral range (0.1-1.5eV) in neat t-(CH)_{x} thin films. We show that odd-parity singlet excitons are the primary photoexcitations in t-(CH)_{x}, similar to many other nondegenerate π-conjugated polymers. The exciton transitions are characterized by two photoinduced absorption (PA) bands at 0.38 and 0.6eV, and an associated photoluminescence band at ∼1.5 eV having similar polarization memory. The primary excitons undergo internal conversion within ∼100 fs to an even-parity (dark) singlet exciton with a PA band at ∼1.4 eV. We also find ultrafast photogeneration of charge polarons when pumping deep into the polymer continuum band, which are characterized by two other PA bands in the mid-IR and associated photoinduced IR vibrational modes.

Near-infrared monitoring of the accretion outburst in the massive young stellar object S255-NIRS3

Abstract We followed up the massive young stellar object S255-NIRS3 (= S255-IRS1b) during its recent accretion outburst event in the $K_{\rm s}$ band with Kanata/HONIR for four years after its burst and obtained a long-term light curve. This is the most complete near-infrared light curve of the S255-NIRS3 burst event that has ever been presented. The light curve showed a steep increase reaching a peak flux that was 3.4 mag brighter than the quiescent phase and then a relatively moderate year-scale fading until the last observation, similar to that of the accretion burst events such as EXors found in lower-mass young stellar objects. The behavior of the $K_{\rm s}$-band light curve is similar to that observed in 6.7 GHz class II methanol maser emission, with a sudden increase followed by moderate year-scale fading. However, the maser emission peaks appear 30–50 d earlier than that of the $K_{\rm s}$ band emission. The similarities confirmed that the origins of the maser emission and the $K_{\rm s}$-band continuum emission are common, as previously shown from other infrared and radio observations by Stecklum et al. (2016, Astronomer’s Telegram, 8732), Caratti o Garatti et al. (2017b, Nature Phys., 13, 276), and Moscadelli et al. (2017, A&A, 600, L8). However, the differences in energy transfer paths, such as the exciting/emitting/scattering structures, may cause the delay in the flux-peak dates.

TNT photometric reverberation mapping analysis of high-redshift quasars

Supermassive black hole (SMBH) mass determination is essential for understanding the galaxy-SMBH co-evolution. Photometric reverberation mapping (PRM) has been proposed as an alternative to the traditional method, spectroscopic reverberation mapping (SRM), which has limitation to only relatively low redshift, z, and bright objects. However, the most common and important sample of high-z active galaxies known as quasar or QSO have its populations peak at around z ≈ 2-3 thus out of reach for the SRM. We carried out a proof-of-concept campaign of quasar PRM using the 2.4-m Thai National Telescope (TNT) between 2015-2018. Such a study is important to inform a future wide-field high-cadence survey such as the Large Synoptic Survey Telescope (LSST). Our main sample contains 10 quasars at redshift z ≈ 0.7-1.2 with rSDSS = 19.7-20.7 mag, selected from the SDSS data release 10. The processed data and light curves were analysed using the discrete cross-correlation function (DCF). We used Monte Carlo (MC) simulations to model the noise characteristics and non-uniform coverage of our data as well as to verify robustness of the DCF results. Our analyses show a significant detection of lag time between continuum and broad-line emission bands of the quasar SDSS J081506.93+254124.7 (z = 1.18, rSDSS = 20.5 mag). The estimated broad line region (BLR) distance is 125±20 light-day which equate to the estimated SMBH mass of (4.3±2.0)×108M#x2299;.

Detection of Hα emission from PZ Telescopii B using SPHERE/ZIMPOL

Hα is a powerful tracer of accretion and chromospheric activity, which has been detected in the case of young brown dwarfs and even recently in planetary mass companions (e.g. PDS70 b and c). Hα detections and characterisation of brown dwarf and planet companions can further our knowledge of their formation and evolution, and expanding such a sample is therefore our primary goal. We used the Zurich Imaging POLarimeter (ZIMPOL) of the SPHERE instrument at the Very Large Telescope (VLT) to observe the known 38−72 MJ companion orbiting PZ Tel, obtaining simultaneous angular differential imaging observations in both continuum and narrow Hα band. We detect Hα emission from the companion, making this only the second Hα detection of a companion using the SPHERE instrument. We used our newly added astrometric measurements to update the orbital analysis of PZ Tel B, and we used our photometric measurements to evaluate the Hα line flux. Given the estimated bolometric luminosity, we obtained an Hα activity (log(LHα/Lbol)) between −4.16 and −4.31. The Hα activity of PZ Tel B is consistent with known average activity levels for M dwarf of the same spectral type. Given the absence of a known gaseous disk and the relatively old age of the system (24 Myr), we conclude that the Hα emission around PZ Tel B is likely due to chromospheric activity.

Photon avalanche effect in quantum wells: controlling light with light

A new mechanism of controlling the absorption of high-intensity signal light with frequency $\omega $ω and intensity ${j_\omega }$jω by complementary control light with frequency $\Omega \gt {2}\omega $Ω>2ω and 2 or 3 orders of magnitude lower intensity ${j_\Omega }$jΩ is proposed. The mechanism is based on the photon avalanche effect in quantum wells. The electron concentrations in size-quantization subbands and conduction band continuum and the absorbed power of signal light are obtained as functions of the intensities ${j_\omega }$jω and ${j_\Omega }$jΩ and the time of exposure to laser radiation $t$t. It is established that the resulting dependences exhibit well-pronounced threshold behavior. For example, at ${j_\Omega }\;\sim\;{0.2 - 1}\;{\text{kW/cm}^2}$jΩ∼0.2−1kW/cm2 and $t\;\sim\;{0.1 - 5}\;\text{ns}$t∼0.1−5ns, it is possible to control the absorption of signal light, so that the absorption of this light is practically lacking at intensities ${j_\omega }$jω below the threshold intensity (${40 - 200}\;{\text{kW/cm}^2}$40−200kW/cm2) and becomes very strong at intensities above the threshold intensity. The case when control light $\Omega $Ω is the third harmonic of high-intensity signal light is also considered.

Non-Bloch Band Theory of Non-Hermitian Systems.

In spatially periodic Hermitian systems, such as electronic systems in crystals, the band structure is described by the band theory in terms of the Bloch wave functions, which reproduce energy levels for large systems with open boundaries. In this paper, we establish a generalized Bloch band theory in one-dimensional spatially periodic tight-binding models. We show how to define the Brillouin zone in non-Hermitian systems. From this Brillouin zone, one can calculate continuum bands, which reproduce the band structure in an open chain. As an example, we apply our theory to the non-Hermitian Su-Schrieffer-Heeger model. We also show the bulk-edge correspondence between the winding number and existence of the topological edge states.

Multi-wavelength observations of the 2014 June 11 M3.9 flare: temporal and spatial characteristics

We present multi-wavelength observations of an M-class flare (M3.9) that occurred on 2014 June 11. Our observations were conducted with the Dunn Solar Telescope (DST), employing adaptive optics, the multi-camera system Rapid Oscillations in Solar Atmosphere (ROSA), the new Hydrogen-Alpha Rapid Dynamics camera (HARDcam) in various wavelengths, such as Ca II K, Mg I b2 (at 5172.7 Å), and Hα narrow band and G-band continuum filters. Images were re-constructed using the Kiepenheuer-Institut Speckle Interferometry Package (KISIP) code, to improve our image resolution. We observed intensity increases of ≈120%–150% in the Mg, Ca K and Hα narrow band filters during the flare. Intensity increases for the flare observed in the SDO EUV channels were several times larger, and the X-rays, as recorded by GOES, increased over a factor of 30 for the harder band. Only a modest delay was found between the onset of flare ribbons of a nearby sympathetic flare and the main flare ribbons observed in these narrow band filters. The peak flare emission occurred within a few seconds for the Ca K, Mg and Hα bands. Time-distance techniques indicate propagation velocities of ≈60 km s−1 for the main flare ribbon and as high as 300 km s−1 for smaller regions, which we attribute to filament eruptions. This result and delays and velocities observed with SDO (≈100 km s−1) for different coronal heights agree well with the simple model of energy propagation versus height, although a more detailed model for the flaring solar atmosphere is needed. Finally, we detected marginal quasi-periodic pulsations (QPPs) in the 40–60 s range for the Ca K, Mg and Hα bands, and such measurements are important for disentangling the detailed flare-physics.

On the visible continuum and bands in the interstellar extinction curve

This work purports to help understand the InterStellar Extinction Curve in and near the visible range. In this range, crystalline materials are known to be transparent, so amorphous dust is needed. Molecular modelling experiments are used to compute the electronic spectra of various, relatively large, carbon and silicate structures. Hardly any transition shows up beyond 0.4 μm when the structure is in its ground state (the lowest, most stable state, usually crystalline). This is no longer the case as soon as the structure is distorted in any way. Examples of simulated distortions (or ‘defects') are: angular or linear bond alteration, insertion of free radicals near the main structure, dangling bonds; their cumulative effects lead to the amorphous state. It is shown that, in this state, a structure bears a majority of weak transitions and a minority of strong ones. As the structure grows in size, the former ultimately form a weak continuum already detected experimentally, in the visible, on amorphous carbons and silicates. The stronger transitions will manage to emerge above the continuum, especially when they bunch together by chance near the same wavelength. Parallels are drawn between several properties of the computed continua and transitions and the observed continuum and Diffuse Interstellar Bands. .

Effect of point defects on electronic and magnetic properties of single-layer SiO

ABSTRACTBased on spin-polarised density functional theory calculations, we investigated the effect of point defects on electronic and magnetic properties of the single-layer (SL) asymmetric washboard silicon oxide (aw-SiO). The SL-aw-SiO is a counterpart of black phosphorene, and a new candidate of two-dimensional material family. This structure is dynamically and thermally stable and is a nonmagnetic semiconductor with a direct band gap. We found that single vacancy and divacancy give rise to significant change in the electronic and magnetic properties of SL-aw-SiO. The band gap of aw-SiO can be tuned by the substitution of Si atom instead of O atom, the antisite defect, the O atom vacancy and two atom vacancies. In addition, impurity states due to the defects can occur in the band continua and hence the band gap of aw-SiO is reduced. Having an integer magnetic moment, SL-aw-SiO upon Si vacancy and by substitution of O atom instead of Si atom may display half-metallic features.

Anomalous behaviour of the UV–optical continuum bands in NGC 5548

During the 2014 HST/Swift and ground-based multi-wavelength monitoring campaign of NGC 5548 (AGN STORM), the UV-optical broad emission lines exhibited anomalous, decorrelated behaviour relative to the far-UV continuum flux variability. Here, we use key diagnostic emission lines (Ly-alpha and He II) for this campaign to infer a proxy for the all important, variable driving EUV continuum incident upon BLR clouds. The inferred driving continuum provides a crucial step towards the recovery of the broad emission line response functions in this AGN. In particular, the ionising continuum seen by the BLR was weaker and softer during the anomalous period than during the first third of the campaign, and apparently less variable than exhibited by the far-UV continuum. We also report the first evidence for anomalous behaviour in the longer wavelength (relative to 1157A) continuum bands. This is corroborative evidence that a significant contribution to the variable UV-optical continuum emission arises from a diffuse continuum emanating from the same gas that emits the broad emission lines.

InAs/InP quantum dots stacking: Impact of spacer layer on optical properties

The optical properties of a vertical multilayer stack of semiconductor self-assembled quantum dots (QDs) depend on the coupling status. We present an improved modeling method for analysis of QD stacks, consisting of several separate steps in the elastic strain analysis, to reproduce the realistic unidirectional strain accumulation along the QD growth direction from lower and upper dots. Based on this modeling method, by varying the spacer thickness, we systematically study a large variety of QD stack structures, for better understanding the influence of strain and electronic coupling mechanisms. A “quasi continuum band” or highly degenerate excited state (ES) is found in the closely multi-stacked QDs, probably accounting for the occurrence of distinct blueshifting of the photoluminescence peak wavelength for the decreasing spacer thickness. The enhanced ES emission from this type of stacking QD modeled by using rate equations may pave a way for designing potential high-performance laser devices operating on ES.

The excitation mechanisms of X-ray oxygen emission-lines

AbstractWe present the Catalogue of High REsolution Spectra of Obscured Sources (CHRESOS) from the XMM-Newton Science Archive. It comprises soft X-ray emission-lines from C to Si and the Fe 3C and Fe 3G L-shell transitions. Here, we concentrate on the oxygen emission-lines O VII(f) and O VIII Lyα to shed light onto the physical processes with which their formation can be related to: active galactic nucleus vs. star-forming regions. We are analysing the relationships between the oxygen lines and the luminosities of: [OIII]λ5007, [OIV]25.89μm, MIR-12μm, FIR-60μm, FIR-100μm, and hard X-rays continuum bands.

On the nature of trapped states in an MoS2 two-dimensional semiconductor with sulfur vacancies

ABSTRACTMolybdenum disulfide (MoS2) is a common two-dimensional semiconductor that has been highly studied as an emerging material for catalysis and electronics. The most common material defects in MoS2 are sulfur vacancies. In order to reveal the nature of the trapped states induced by sulfur vacancies, we perform Density Functional Theory (DFT) combined with quantum dynamics calculations. According to our model, we find that the sulfur vacancies create trap states in the original band gap of monolayer MoS2 that disrupt charge transmission through the monolayer. In addition, we did not find any resonance states among the shallow states in the conduction band continuum.

ALMA High Angular Resolution Polarization Study: An Extremely Young Class 0 Source, OMC-3/MMS 6

Abstract Using the ≈16 km long baseline data obtained with the Atacama Large Millimeter/submillimeter Array (ALMA), we imaged the Stokes I emission and linearly polarized intensity (PI) in the 1.1 mm continuum band of a very young intermediate-mass protostellar source, MMS 6, in the Orion Molecular Cloud-3. The achieved angular resolution, 0.″02 × 0.″03 (≈10 au), shows for the first time a wealth of data on the dust emission polarization in the central 200 au of a protostar. The PI peak is offset to the southeast (SE) by ≈20 au with respect to the Stokes I peak. Its polarization degree is 11% with its E-vector orientation of the position angle ≈135°. A partial ringlike structure with a radius of ≈80 au is detected in PI but not in the Stokes I. Northwest (NW) and SE parts of the ring are bright, with a high polarization degree of ≳10%, and their E-vector orientations are roughly orthogonal to those observed near the center. We also detected an armlike polarized structure, extending to 1000 au scale to the north, with the E-vectors aligned along the minor axis of the structure. We explored possible origins of the polarized emission by comparing them with magnetohydrodynamical simulations of the toroidal wrapping of the magnetic field. The simulations are consistent with the PI emission in the ringlike and the extended armlike structures observed with ALMA. However, the current simulations do not completely reproduce observed polarization characteristics in the central 50 au. Although the self-scattering model can explain the polarization pattern and positional offset between the Stokes I and PI in the central 50 au, this model is not able to reproduce the observed high degree of polarization.