Strong Flickering Research Articles

Simulation model of a Permanent Magnet Synchronous Generator for grid studies

Permanent Magnet Synchronous Generators are an emerging technology in wind power applications. A simplified Thevenin equivalent simulation model of a wind generator has been developed in the ATPDraw environment and validated by comparing its dynamics with that of a Matlab/Simulink detailed model. The simplified model includes also power electronic circuits, for interfacing the generator to a MV primary distribution grid, and their controls. A 2,5 MVA wind generator was simulated in ATPDraw in order to assess flicker severity dependence upon different grid and circuit parameters. Simulation results show a strong flicker severity dependence on grid short circuit power and X/R line characteristics, in particular when generator supplies reactive power to the grid, as required by some National standards.

Methods to Reduce Flicker and Light Pollution of Low-Mounting-Height Luminaires in Urban Road Lighting

Current low-mounting-height luminaires (LMHLs) have low effective luminous flux, high surface brightness, and many stray lights, resulting in high energy consumption and light interference, and the flicker effect may threaten driver safety. Currently, the flicker effect in LMHL lighting has been relatively poorly studied. However, in prior research, a Flicker Index (FI) quantification model was developed. In this study, we have modified the FI model to incorporate visual persistence, visual masking, and driving speed of the driver, and proposed a dynamic energy ratio model for quantifying flicker effects. The notion of effective luminous flux in road lighting was introduced, revealing that the reasons for high energy consumption, severe light pollution, and strong flicker effects in regular LMHLs were low effective luminous flux and high surface brightness. We recommend a low-mounting-height lighting technique that functions in both forward and reverse directions, which can significantly enhance the effective luminous flux of lighting fixtures, reducing stray light. The lighting fixtures created were tested on a road. The findings suggest that the energy consumption of both forward and reverse lighting fixtures is only 40% of the international standard requirements, resulting in 81% more energy savings than in existing LMHLs. The surface brightness of forward and reverse lighting fixtures is only 1.18 cd/m2, which is equivalent to 0.001% of the surface brightness of existing lighting fixtures. This indicates that 99.89% less invalid light is projected into the environment, indirectly reducing light pollution. Additionally, the flicker index of these fixtures is only 2% of that of existing lighting fixtures, reducing the impact of the flicker effect on drivers. This study provides insights into quantifying and mitigating the flicker effect in LMHLs, as well as presenting novel approaches and strategies for enhancing the efficiency and reducing light pollution of streetlamps.

A correlation between H α trough depth and inclination in quiescent X-ray transients: evidence for a low-mass black hole in GRO J0422+32

ABSTRACT We present a new method to derive binary inclinations in quiescent black hole (BH) X-ray transients (XRTs), based on the depth of the trough (T) from double-peaked H α emission profiles arising in accretion discs. We find that the inclination angle (i) is linearly correlated with T in phase-averaged spectra with sufficient orbital coverage (≳50 per cent) and spectral resolution, following i(deg) = 93.5 × T + 23.7. The correlation is caused by a combination of line opacity and local broadening, where a leading (excess broadening) component scales with the deprojected velocity of the outer disc. Interestingly, such scaling allows to estimate the fundamental ratio M1/Porb by simply resolving the intrinsic width of the double-peak profile. We apply the T–i correlation to derive binary inclinations for GRO J0422+32 and Swift J1357−0933, two BH XRTs where strong flickering activity has hindered determining their values through ellipsoidal fits to photometric light curves. Remarkably, the inclination derived for GRO J0422+32 (i = 55.6 ± 4.1○) implies a BH mass of $2.7^{+0.7}_{-0.5}$ M⊙ thus placing it within the gap that separates BHs from neutron stars. This result proves that low-mass BHs exist in nature and strongly suggests that the so-called ‘mass gap’ is mainly produced by low number statistics and possibly observational biases. On the other hand, we find that Swift J1357−0933 contains a $10.9^{+1.7}_{-1.6}$ M⊙ BH, seen nearly edge on ($i=87.4^{+2.6}_{-5.6}$ deg). Such extreme inclination, however, should be treated with caution since it relies on extrapolating the T–i correlation beyond i ≳ 75○, where it has not yet been tested.

(Invited) Axial Silicon-Germanium Nanowires: Properties and Device Applications

Dense nanowire films can be considered as quasi-porous structures with a high surface-to-volume ratio. From this perspective, structural, optical and electrical properties of axial Si-Ge nanowire heterojunctions produced by the vapor-liquid-solid growth method using Au nanoclusters as catalysts are analyzed. The lattice mismatch induced strain is partially relieved due to spontaneous SiGe intermixing at the heterointerface and lateral expansion of the Ge segment of the nanowire. The mismatch in Ge and Si coefficients of thermal expansion and low thermal conductivity of Si-Ge nanowire heterojunctions are found to be responsible for the thermally induced stress detected under intense laser radiation in photoluminescence and Raman scattering measurements. In electrical measurements, the observed non-linear current-voltage characteristics, strong flicker noise, and damped current oscillations with frequencies of 20-30 MHz are explained using a proposed Si-Ge nanowire heterojunction energy band diagram that includes energy states associated with structural imperfections at the nanowire surface. It is also shown that the nanowire surface states act as shunt resistors and considerably degrade the Si-Ge nanowire device characteristics.

(Invited) Axial Silicon-Germanium Nanowires: Properties and Device Applications

Dense nanowire films can be considered as quasi-porous structures with a high surface-to-volume ratio. From this perspective, structural, optical and electrical properties of axial Si-Ge nanowire heterojunctions produced by the vapor-liquid-solid growth method using Au nanoclusters as catalysts are analyzed. The lattice mismatch induced strain is partially relieved due to spontaneous SiGe intermixing at the heterointerface and lateral expansion of the Ge segment of the nanowire. The mismatch in Ge and Si coefficients of thermal expansion and low thermal conductivity of Si-Ge nanowire heterojunctions are found to be responsible for the thermally induced stress detected under intense laser radiation in photoluminescence and Raman scattering measurements. In electrical measurements, the observed non-linear current-voltage characteristics, strong flicker noise, and damped current oscillations with frequencies of 20-30 MHz are explained using a proposed Si-Ge nanowire heterojunction energy band diagram that includes energy states associated with structural imperfections at the nanowire surface.

Adaptive Temporal Sampling for Volumetric Path Tracing of Medical Data

AbstractMonte‐Carlo path tracing techniques can generate stunning visualizations of medical volumetric data. In a clinical context, such renderings turned out to be valuable for communication, education, and diagnosis. Because a large number of computationally expensive lighting samples is required to converge to a smooth result, progressive rendering is the only option for interactive settings: Low‐sampled, noisy images are shown while the user explores the data, and as soon as the camera is at rest the view is progressively refined. During interaction, the visual quality is low, which strongly impedes the user's experience. Even worse, when a data set is explored in virtual reality, the camera is never at rest, leading to constantly low image quality and strong flickering. In this work we present an approach to bring volumetric Monte‐Carlo path tracing to the interactive domain by reusing samples over time. To this end, we transfer the idea of temporal antialiasing from surface rendering to volume rendering. We show how to reproject volumetric ray samples even though they cannot be pinned to a particular 3D position, present an improved weighting scheme that makes longer history trails possible, and define an error accumulation method that downweights less appropriate older samples. Furthermore, we exploit reprojection information to adaptively determine the number of newly generated path tracing samples for each individual pixel. Our approach is designed for static, medical data with both volumetric and surface‐like structures. It achieves good‐quality volumetric Monte‐Carlo renderings with only little noise, and is also usable in a VR context.

Axial silicon-germanium nanowire heterojunctions: Structural properties and carrier transport

We analyzed structural and electrical properties of axial Si-Ge nanowire heterojunctions produced by the vapor-liquid-solid growth method using Au nanoclusters as catalysts. The observed nonlinear current-voltage characteristics, strong flicker noise, and damped current oscillations with frequencies of 20–30 MHz are explained using a proposed Si-Ge nanowire heterojunction energy band diagram that includes energy states associated with structural imperfections, as revealed by transmission electron microscopy.

(Invited) Structural Properties and Carrier Transport in Axial Silicon-Germanium Nanowire Heterojunctions

Recent advances in forming semiconductor heterojunctions within spatially confined nanoscale objects, including nanowires (NWs), show that the traditional limitations in the lattice-mismatched hetero-growth can be challenged. In various III-V semiconductor NWs, abrupt heterojunctions have been successfully demonstrated using the vapor-liquid-solid (VLS) growth for GaAs/InAs (7% mismatch) and InAs/InP (3% mismatch) heterojunctions. In group IV semiconductors, the approach is complicated not only by the 4.2% lattice mismatch between Si and Ge but also because Si and Ge both have a quite high solubility in the Au-Si catalyst. During chemical vapor deposition (CVD) based VLS growth using SiH4 and GeH4 (or similar gases), a supply of Si remains effectively “on” in the catalyst, and Si effectively intermixes with the arriving Ge even if the SiH4 flow is already switched “off”. One way to address this problem is to choose a catalyst with a lower Si solubility, e.g. AlAu2 and AgAu. Another possibility is to significantly reduce growth temperature before turning a GeH4 source “on”. Using the latter technique, we fabricated Si-Ge heterojunction NWs with nearly ideal interface and only an 8 nm thick SiGe transition layer between straight and nearly micron-long Si and Ge NW segments and analyzed their structural and electrical properties. The observed changes in the photoluminescence and Raman spectra as function of temperature, non-linear and rectifying current-voltage characteristics, strong flicker noise and damped current oscillations with frequencies of 20-30 MHz are explained using the proposed SiGe heterojunction NW energy band diagram including the energy states associated with the NW surface (and near-surface) structural imperfections revealed by transmission electron microscopy and a mismatch in Si and Ge coefficients of thermal expansion.

Binarity and Accretion in AGB Stars: HST/STIS Observations of UV Flickering in Y Gem.

Binarity is believed to dramatically affect the history and geometry of mass loss in AGB and post-AGB stars, but observational evidence of binarity is sorely lacking. As part of a project to search for hot binary companions to cool AGB stars using the GALEX archive, we discovered a late-M star, Y Gem, to be a source of strong and variable UV and X-ray emission. Here we report UV spectroscopic observations of Y Gem obtained with the Hubble Space Telescope that show strong flickering in the UV continuum on time-scales of ≲ 20 s, characteristic of an active accretion disk. Several UV lines with P-Cygni-type profiles from species such as Si IV and C IV are also observed, with emission and absorption features that are red- and blue-shifted by velocities of ~500 km s-1 from the systemic velocity. Our model for these (and previous) observations is that material from the primary star is gravitationally captured by a companion, producing a hot accretion disk. The latter powers a fast outflow that produces blue-shifted features due to absorption of UV continuum emited by the disk, whereas the red-shifted emission features arise in heated infalling material from the primary. The outflow velocities support a previous inference by Sahai et al. (2015) that Y Gem's companion is a low-mass main-sequence star. Blackbody fitting of the UV continuum implies an accretion luminosity of about 13 L⊙, and thus a mass-accretion rate > 5 × 10-7M⊙ yr-1; we infer that Roche lobe overflow is the most likely binary accretion mode for Y Gem.

Accretion disk coronae of intermediate polar cataclysmic variables

Context. Intermediate polar cataclysmic variables (IPCV) contain a magnetic, rotating white dwarf surrounded by a magnetically truncated accretion disk. To explain their strong flickering X-ray emission, accretion has been successfully taken into account. Nevertheless, observations suggest that accretion phenomena might not be the only process behind it. An intense flaring activity occurring on the surface of the disk may generate a corona, contribute to the thermal X-ray emission, and influence the system stability.

Photometry of some neglected bright cataclysmic variables and candidates

As part of an effort to better characterize bright cataclysmic variables (CVs) which have received little attention in the past light curves of four confirmed systems (CZ Aql, BO Cet, V380 Oph and EF Tuc) and one candidate (Lib 3) are analyzed. For none of these stars time resolved photometry has been published previously. While no variability was found in the case of Lib 3, which thus cannot be confirmed as a CV, the light curves of all other targets are dominated by strong flickering. Modulations on hourly time scales superimposed on the flickering can probably be related to orbital variations in BO Cet and V380 Oph, but not in CZ Aql and EF Tuc. Variations on the time scale of 10 min in CZ Aql, while not yet constituting convincing evidence, together with previous suspicions of a magnetically channeled accretion flow may point at an intermediate polar nature of this star. Some properties of the flickering are quantified in an effort to enlarge the data base for future comparative flickering studies in CVs and to refine the classification of the target stars.

Novel Measurement Technique for Time Dependant Intensity Variations of AC-LEDs

LEDs have surpassed incandescent and compact florescent lamps for general lighting during the last 10 years.1 Nowadays, they show very high wall plug efficiency while providing a good colour rendering index (CRI). However, flicker is still a perceivable drawback to these light sources.2 The impact and perception of flicker is increasingly investigated and its consequences for human beings range from headaches over migraines to severe neurological problems, including epileptic seizure.3 Evaluating these effects requires a number of factors to be taken into account, for instance duration of exposure, stimulated area of the retina and brightness and colour of the flickering LED source. Flicker is defined as “the rapid variation in light source intensity”.4 To describe the flicker of a light source, two metrics are currently used, flicker percentage and flicker index. Both are mathematical approaches which use the overall emission intensity and compare the minimum and the maximum intensity within a single cycle or curve integrals to gain a key figure. They do not take the wavelength, the frequency, or the duty cycle into account. The presented, novel measurement technique processes the spectrally resolved emission intensity during an adjustable amount of duty cycles, shapes and frequencies. In order to verify this measurement technique, standard phosphors such as Y3Al5O12:Ce3+ (YAG:Ce), BaMgAl10O17:Eu2+,Mn2+(BAM:Eu,Mn) have been investigated with a rectangular pulsed 50 or 100 Hz LED as excitation source. YAG:Ce showed strong flicker while BAM:Eu,Mn exhibits a flicker reducing effect. The spectrally resolved measurement technique allows us to correlate the measurement data with the eye sensitivity curve resulting in a new kind of flicker index.

Investigation of the flickering of La2O3 and ThO2 doped tungsten cathodes

Short-arc lamps are equipped with tungsten electrodes due to their ability to withstand a high thermal load during operation. Nominal currents of more than one hundred amperes lead to a cathode tip temperature near the melting point of tungsten. To reduce the electrode temperature and, thereby, to increase the maintenance of such lamps, ThO2 or tentatively La2O3 are added to the electrode material. They generate a reduced work function by establishing a monolayer of emitter atoms on the tungsten surface. Emitter enrichments on the lateral surface of doped cathodes are formed. They are traced back to transport mechanisms of emitter oxides in the interior of the electrode and on the electrode surface in dependence of the electrode temperature and to the redeposition of vaporized and ionized emitter atoms onto the cathode tip by the electric field in front. The investigation is undertaken by means of glow discharge mass spectrometry, scanning electron microscope images, energy dispersive x-ray spectroscopy, and through measurements of the optical surface emissivity. The effect of emitter enrichments on the stability of the arc attachment is presented by means of temporally resolved electrode temperature measurements and by measurements of the luminous flux from the cathode-near plasma. They show that the emitter enrichments on the lateral surface of the cathode are attractive for the arc attachment if the emitter at the cathode tip is depleted. In this case, it moves along the lateral surface from the cathode tip to sections of the cathode with a reduced work function. It induces a temporary variation of the cathode tip temperature and of the light intensity from the cathode-near plasma, a so-called flickering. In particular, in case of lanthanated cathodes, strong flickering is observed.

Electrical flicker-noise generated by filling and emptying of impurity states in injectors of quantum-cascade lasers

Free running line-widths (>100 kHz), much broader than intrinsic line-widths ∼100 Hz, of existing quantum-cascade lasers are governed by strong flicker frequency-noise originating from electrical flicker noise. Understanding of microscopic origins of the electrical flicker noises in quantum-cascade lasers is crucially important for the reduction of strength of flicker frequency-noise without assistances of any type of feedback schemes. In this article, an ad hoc model that is based on fluctuating charge-dipoles induced by electron trappings and de-trappings at indispensable impurity states in injector super-lattices of a quantum-cascade laser is proposed, developing theoretical framework based on the model. The validity of the present model is evaluated by comparing theoretical voltage-noise power spectral densities based on the model with experimental ones obtained by using mid-infrared quantum-cascade lasers with designed impurity-positioning. The obtained experimental results on flicker noises, in comparison with the theoretical ones, shed light on physical mechanisms, such as the inherent one due to impurity states in their injectors and extrinsic ones due to surface states on the ridge-walls and due to residual deep traps, for electrical flicker-noise generation in existing mid-infrared quantum-cascade lasers. It is shown theoretically that quasi-delta doping of impurities in their injectors leads to strong suppression of electrical flicker noise by minimization of the dipole length at a certain temperature, for instance ∼300 K and, in turn, is expected to result in substantial narrowing of the free running line-width down below 10 kHz.

A photometric study of the nova‐like variable TT Arietis with the MOST satellite

AbstractVariability on all time scales between seconds and decades is typical for cataclysmic variables (CVs). One of the brightest and best studied CVs is TT Ari, a nova‐like variable which belongs to the VY Scl subclass, characterized by occasional low states in their light curves. It is also known as a permanent superhumper at high state, revealing “positive” (PS > P0) as well as “negative” (PS < P0) superhumps, where PS is the period of the superhump and P0 the orbital period. TT Ari was observed by the Canadian space telescope MOST for about 230 hours nearly continuously in 2007, with a time resolution of 48 seconds. Here we analyze these data, obtaining a dominant “negative” superhump signal with a period PS = 0.1331 days and a mean amplitude of 0.09 mag. Strong flickering with amplitudes up to 0.2 mag and peak‐to‐peak time scales of 15–20 minutes is superimposed on the periodic variations. We found no indications for significant quasi‐periodic oscillations with periods around 15 minutes, reported by other authors. We discuss the known superhump behaviour of TTAri during the last five decades and conclude that our period value is at the upper limit of all hitherto determined “negative” superhump periods of TTAri, before and after the MOST run. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

RR Pictoris: an old nova showing superhumps and QPOs

We present time-resolved V photometry of the old nova RR Pictoris (RR Pic). Apart from the hump-like variability, the light curves show the strong flickering and random variation typical for RR Pic. We do not find any convincing evidence for the previously reported eclipse. The extrapolated eclipse phase coincides with a broad minimum, but comparing the overall shape of the light curve suggests that the eclipse should actually be located around phase 0.2. The orbital period which we derive from these data agrees well with the old one, any uncertainty is too small to account for the possible phase shift. Apart from the 3.48 h period, which is usually interpreted as the orbital one, we find an additional period at P = 3.78 h, which we interpret as the superhump period of the system; the corresponding precession period at 1.79 d is also present in the data. We also find indications for the presence of a 13 min quasi-periodic oscillation.

Keck infrared observations of GRO J0422+32 in quiescence

We present Keck K-band photometry and low-resolution H- and K-band spectroscopy of the X-ray nova GRO J0422+32 obtained while the system was in the quiescent state. No clear ellipsoidal modulation is present in the light curve, which is instead dominated by a strong flickering component. In the K band, we observe strong Brγ emission, with an equivalent width of 38 ± 5 Å. From this, we conclude that the accretion disc is the most likely source of the observed photometric contamination, and that previous infrared-based attempts to constrain the mass of the putative black hole in this system are prone to considerable uncertainty. We finally proceed to show how it is possible to place meaningful constraints on some of the binary parameters of this system, even in the presence of a relatively high level of contamination from the disc.

Photometry of the SW Sextantis-type nova-like BH Lyncis in high state

Aims. We present a photometric study of the deeply eclipsing SW Sex-type nova-like cataclysmic variable star BH Lyn. Methods. Time-resolved V -band CCD photometry was obtained for seven nights between 1999 and 2004. Results. We determined 11 new eclipse timings of BH Lyn and derived a refined orbital ephemeris with an orbital period of 0$\fd$155875577(14). During the observations, BH Lyn was in high-state with $V\simeq15.5$ mag. The star presents ~1.5 mag deep eclipses with mean full-width at half-flux of $0.0683(\pm0.0054)P_{\rm orb}$. The eclipse shape is highly variable, even changing form cycle to cycle. This is most likely due to accretion disc surface brightness distribution variations, most probably caused by strong flickering. Time-dependent accretion disc self-occultation or variations of the hot spot(s) intensity are also possible explanations. Negative superhumps with period of ${\sim}0\fd145$ are detected in two long runs in 2000. A possible connection between SW Sex and negative superhump phenomena through the presence of tilted accretion disc is discussed, and a way to observationally test this is suggested.

High-speed photometry of faint cataclysmic variables -- V. V433 Ara, OQ Car, V591 Cen, V1039 Cen, Cal 86 (Dor), UY Mic, LB 9963 (Oct), V367 Peg, Sgr (MACHO peculiar variable), RX J0403+044 (Tau), V382 Vel and SY Vol

High-speed photometry is presented for 12 faint cataclysmic variable (CV) stars that have not previously been investigated. V433 Ari, a dwarf nova, has deep eclipses, an orbital period Porb= 4.698 h and evidence that it is a Z Cam star. OQ Car, another dwarf nova, shows flickering but no orbital modulation. The candidate for V591 Cen, a suspected dwarf nova, is found not to flicker in quiescence, nor does any other star in the vicinity. V1039 Cen (Nova Cen 2001) has characteristics of an intermediate polar, with a rotation period of 719 s and Porb= 5.92 h; there is also a strong brightness modulation at 8.98 h or its 1–d alias of 14.34 h. CAL 86, a dwarf nova, has a double-humped modulation at 1.587 h which is probably the orbital period, but has another modulation 4.5 per cent shorter in period that appears to be a rare example of a negative superhump observed in quiescence, and yet another periodicity at 4.74 h that is probably an example of the still unexplained GW Lib phenomenon. UY Mic, a suspected CV, has no flickering but has a periodic modulation at 4.856 h, which may be from a reflection effect. LB 9963, another suspected CV, is found to have strong flickering but no observable periodicity. V367 Peg, a dwarf nova, has a double-humped light curve with an eclipse at one of the minima and Porb= 3.89 h, strongly resembling BD Pav with an evolved secondary. Sgr, a possible nova type, discovered by the massive compact halo object survey, has a 2.808-h brightness modulation with occasional apparent secondary eclipses of unknown cause. RX J0403+044, a suspected CV, has low- and high-luminosity states, with a conspicuous 1012.7-s oscillation in the high state and an oscillation with a period near to twice that in the low state. V382 Vel (Nova Vel 1999) has a clear modulation with amplitude 0.12 mag and period 3.795 h, and also a quasi-periodic oscillation (QPO) at 2570 s and a longer period dwarf nova oscillation at 725 s. SY Vol, a dwarf nova, has no indication of any orbital modulation.

High-speed photometry of faint cataclysmic variables - IV. V356 Aql, Aqr1, FIRST J1023+0038, Hα 0242-2802, GI Mon, AO Oct, V972 Oph, SDSS 0155+00, SDSS 0233+00, SDSS 1240−01, SDSS 1556−00, SDSS 2050−05, FH Ser

We present results from high-speed photometry of a further 13 faint cataclysmic variables. V356 Aql (Nova Aql 1936) shows flare-like outbursts with recurrence time-scales ∼3000 s, but no coherent periodicities. Aqrl is an intermediate polar with a spin period of 6.7284 min and a probable orbital period P orb = 3.226 h derived from orbital sidebands. Its orbital sideband frequencies show very variable amplitudes. The published spectroscopic period of 2.0 h suggests that Aqrl is similar to GW Lib and FS Aur in having an additional periodicity of unknown origin. FIRST J1023+0038 has P orb = 4.7548 h with an orbital modulation of range 0.45 mag, probably caused by reflection effect from a hot white dwarf primary; as such it may have been a nova sometime in the past few decades. Ha 0242 is a deeply eclipsing very low mass-transfer rate system, probably a dwarf nova of very long outburst interval, with P orb = 1.792 h. GI Mon, an old nova, has optical modulations at 4.325 h and possibly also at 48.6 min and is thus a candidate intermediate polar. AO Oct, an SU UMa type dwarf nova, shows orbital modulation at quiescence with P orb = 94.097 min. V972 Oph (Nova Ophiuchi 1957) showed no flickering activity during one set of observations, but did so at a later time, confirming the correctness of the identification of this object, but it shows no orbital modulation. SDSS 0155 is a deeply eclipsing polar with an orbital period of 87.13 min. SDSS 0233 shows flaring activity but no discernible periodicity. No periodicity was found in the new AM CVn candidate SDSS 1240. SDSS 1556 shows a periodic modulation at 1.778 h, which is possibly due to orbital motion. SDSS 2050 is an eclipsing polar with P orb = 1.5702 h. FH Ser (Nova Serpentis 1970) has strong flickering activity but no detectable orbital modulation.