Bright Arc Research Articles

Discovery of an exceptionally bright giant arc atz= 2.369, gravitationally lensed by thePlanckcluster PSZ1 G311.65−18.48

As part of an all-sky follow-up of the Planck catalogue of Sunyaev-Zeldovich (SZ) cluster candidates detected in the first 14 months of data, we are observing cluster candidates in the southern sky in the optical imaging and spectroscopy through an ESO Large Programme. Inspection of ESO New Technology Telescope (NTT) R -and z -band imaging data from our programme has revealed an unusually large and bright arc in the field of PSZ1 G311.65−18.48. We establish the basic photometric and morphological properties of the arc and provide conclusive evidence for the gravitational lensing nature of this object. Guided by the NTT images, we have obtained a long-slit spectrum with IMACS on the Magellan-I Baade Telescope, covering a part of the arc and the brightest cluster galaxy of PSZ1 G311.65−18.48. Our imaging data confirm the presence of a galaxy cluster coinciding (within 0.6) with the position of the Planck SZ source. The arc is separated by ~30″ from the brightest cluster galaxy, which closely coincides with the center of curvature of the arc. A photometric analysis yields integrated (Vega) magnitudes of ( R,z,J, K s ) = (17.82,17.38,16.75,15.43) for the arc, more than one magnitude brighter than any previously known lensed arc at z ~ 2–3. The arc is a vigorously star-forming galaxy at z = 2.369, while the Planck SZ cluster lens is at z = 0.443.Even when allowing for lensing magnifications as high as μ = 100 still leads to the conclusion that the source galaxy is among the intrinsically most luminous normal (i.e., non-AGN) galaxies known at z ~ 2–3.

Relationship between auroral oval poleward boundary intensifications and magnetic field variations in the solar wind

As a rule, bright auroral arcs evolve near the poleward boundary of the auroral oval at the growth phase of a substorm, a phenomenon that is known to occur near the poleward edge of the auroral oval. The closeness of these arcs to the projection of the magnetic separatrix on the night side suggests that their generation is related to magnetic reconnection in the magnetospheric tail in a particular way. In this study this suggestion is confirmed by the fact that integral brightness of the auroral oval at the poleward edge correlates with magnetic field structures in the solar wind that are observed by ACE and Wind satellites at distances of 50–300 R E upstream and are shifted towards the magnetospheric tail with time delays of ~ 10–80 min, consistent with measurements of the solar wind velocity. About 50 examples of this correlation have been found. The possible physical mechanisms of the effect observed are discussed.

Detecting stellar-wind bubbles through infrared arcs in H ii regions

Mid-infrared arcs of dust emission are often seen near ionizing stars within HII regions. A possible explanations for these arcs is that they could show the outer edges of asymmetric stellar wind bubbles. We use two-dimensional, radiation-hydrodynamics simulations of wind bubbles within HII regions around individual stars to predict the infrared emission properties of the dust within the HII region. We assume that dust and gas are dynamically well-coupled and that dust properties (composition, size distribution) are the same in the HII region as outside it, and that the wind bubble contains no dust. We post-process the simulations to make synthetic intensity maps at infrared wavebands using the TORUS code. We find that the outer edge of a wind bubble emits brightly at 24um through starlight absorbed by dust grains and re-radiated thermally in the infrared. This produces a bright arc of emission for slowly moving stars that have asymmetric wind bubbles, even for cases where there is no bow shock or any corresponding feature in tracers of gas emission. The 24um intensity decreases exponentially from the arc with increasing distance from the star because the dust temperature decreases with distance. The size distribution and composition of the dust grains has quantitative but not qualitative effects on our results. Despite the simplifications of our model, we find good qualitative agreement with observations of the HII region RCW120, and can provide physical explanations for any quantitative differences. Our model produces an infrared arc with the same shape and size as the arc around CD -38 11636 in RCW120, and with comparable brightness. This suggests that infrared arcs around O stars in HII regions may be revealing the extent of stellar wind bubbles, although we have not excluded other explanations.

EUV FLICKERING OF SOLAR CORONAL LOOPS: A NEW DIAGNOSTIC OF CORONAL HEATING

ABSTRACT A previous work of ours found the best agreement between EUV light curves observed in an active region core (with evidence of super-hot plasma) and those predicted from a model with a random combination of many pulse-heated strands with a power-law energy distribution. We extend that work by including spatially resolved strand modeling and by studying the evolution of emission along the loops in the EUV 94 Å and 335 Å channels of the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory. Using the best parameters of the previous work as the input of the present one, we find that the amplitude of the random fluctuations driven by the random heat pulses increases from the bottom to the top of the loop in the 94 Å channel and from the top to the bottom in the 335 Å channel. This prediction is confirmed by the observation of a set of aligned neighboring pixels along a bright arc of an active region core. Maps of pixel fluctuations may therefore provide easy diagnostics of nanoflaring regions.

Relationship between wave-like auroral arcs and Pi2 disturbances in plasma sheet prior to substorm onset.

Wave-like substorm arc features in the aurora and Pi2 magnetic disturbances observed in the near-Earth plasma sheet are frequently, and sometimes simultaneously, observed around the substorm onset time. We perform statistical analyses of the THEMIS ASI auroral observations that show wave-like bright spot structure along the arc prior to substorm onset. The azimuthal mode number values of the wave-like substorm arcs are found to be in the range of ~100–240 and decrease with increasing geomagnetic latitude of the substorm auroral arc location. We suggest that the azimuthal mode number is likely related to the ion gyroradius and azimuthal wave number. We also perform correlation study of the pre-onset wave-like substorm arc features and Pi2 magnetic disturbances for substorm dipolarization events observed by THEMIS satellites during 2008–2009. The wave-like arc brightness structures on the substorm auroral arcs tend to move azimuthally westward, but with a few exceptions of eastward movement, during tens of seconds prior to the substorm onset. The movement of the wave-like arc brightness structure is linearly correlated with the phase velocity of the Pi2 δBy disturbances in the near-Earth plasma sheet region. The result suggests that the Pi2 transverse δBy disturbances are related to the intensifying wave-like substorm onset arcs. One plausible explanation of the observations is the kinetic ballooning instability, which has high azimuthal mode number due to the ion gyroradius effect and finite parallel electric field that accelerates electrons into the ionosphere to produce the wave-like arc structure.

Vacuum arc behavior and its voltage characteristics in drawing process controlled by composite magnetic fields along axial and transverse directions

In this research, drawing vacuum arc (VA) experiments were conducted using composite contacts under currents ranging from 5 kA to 20 kA root mean square (rms). The new type of contact comprised an axial magnetic field (AMF) configuration and a transverse magnetic field (TMF) configuration. The TMF plate was in the center, surrounded by the AMF plate. The contact generated both AMFs and TMFs simultaneously. VA appearances and arc voltages were recorded, and the VA was modeled as a conductor for electromagnetic force analysis in ANSYS software. The results showed that the coaxiality of operating mechanisms significantly influenced arc behavior just as the arc was ignited. When arc brightness did not increase after ignition, there was a voltage drop accompanied with diffusion of the VA. As to VA development, when an arc was ignited on an AMF plate, it spread on the plate and rotated. Over time the arc current increased, the constricting arc forms, and the arc column rotated on the TMF plate under the action of Ampere's force. With regard to the influence of a magnetic field on a VA at different stages, in the initial drawing arc stage the TMF was dominant, and the arc started to rotate under the action of Ampere's force. Afterwards, the AMF was dominant, with a steadily burning arc. As for contact melting, in the initial arcing period, a contracted short arc caused severe melting and erosion of the contact plate. When the ignition spot or root was close to the slot of plate, the electromagnetic force pushed the arc toward slot and contact edge, resulting in local erosion of the slot region.

Saturn’s aurora observed by the Cassini camera at visible wavelengths

The first observations of Saturn’s visible-wavelength aurora were made by the Cassini camera. The aurora was observed between 2006 and 2013 in the northern and southern hemispheres. The color of the aurora changes from pink at a few hundred km above the horizon to purple at 1000–1500km above the horizon. The spectrum observed in 9 filters spanning wavelengths from 250nm to 1000nm has a prominent H-alpha line and roughly agrees with laboratory simulated auroras. Auroras in both hemispheres vary dramatically with longitude. Auroras form bright arcs between 70° and 80° latitude north and between 65° and 80° latitude south, which sometimes spiral around the pole, and sometimes form double arcs. A large 10,000-km-scale longitudinal brightness structure persists for more than 100h. This structure rotates approximately together with Saturn. On top of the large steady structure, the auroras brighten suddenly on the timescales of a few minutes. These brightenings repeat with a period of ∼1h. Smaller, 1000-km-scale structures may move faster or lag behind Saturn’s rotation on timescales of tens of minutes. The persistence of nearly-corotating large bright longitudinal structure in the auroral oval seen in two movies spanning 8 and 11 rotations gives an estimate on the period of 10.65±0.15h for 2009 in the northern oval and 10.8±0.1h for 2012 in the southern oval. The 2009 north aurora period is close to the north branch of Saturn Kilometric Radiation (SKR) detected at that time.

Flow bursts, breakup arc, and substorm current wedge

AbstractEnergy liberated by the reconnection process in the near‐Earth tail is transported via flow bursts toward the dipolar magnetosphere during substorms. The breakup arc is a manifestation of the arrival of the bursts under flow braking and energy deposition. Its structure and behavior is analyzed on the basis of five striking spatial, temporal, and energetic properties, qualitatively and in part also quantitatively. A key element is the formation of stop layers. They are thin layers, of the width of an ion gyro radius, in which the magnetic field makes a transition from tail to near‐dipolar magnetosphere configurations and in which the kinetic energy of fast flows is converted into electromagnetic energy of kinetic Alfvén waves. The flows arise from the relaxation of the strong magnetic shear stresses in the leading part of the flow bursts. The bright narrow arcs of less than 10 km width inside the broad poleward expanding breakup arc, Alfvénic in nature and visually characterized by erratic short‐lived rays, are seen as traces of the stop layers. The gaps between two narrow and highly structured arcs are filled with more diffuse emissions. They are attributed to the relaxation of the less strained magnetic field of the flow bursts. Eastward flows along the arcs are linked to the shrinking gaps between two successive arcs and the entry of auroral streamers into the dipolar magnetosphere in the midnight sector. Flow braking in the stop layers forms multiple pairs of narrow balanced currents and cannot be behind the formation of the substorm current wedge. Instead, its origin is attributed to the force exerted by the dipolarized magnetic field of the flow bursts on the high‐beta plasma, after the high magnetic shears have relaxed and the fast flows and stop layer process have subsided, in other words, to the “dying flow bursts.”

Saturn’s auroral morphology and field-aligned currents during a solar wind compression

On 21–22 April 2013, during a coordinated auroral observing campaign, instruments onboard Cassini and the Hubble Space Telescope observed Saturn’s aurora while Cassini traversed Saturn’s high latitude auroral field lines. Signatures of upward and downward field-aligned currents were detected in the nightside magnetosphere in the magnetic field and plasma measurements. The location of the upward current corresponded to the bright ultraviolet auroral arc seen in the auroral images, and the downward current region was located poleward of the upward current in an aurorally dark region. Within the polar cap magnetic field and plasma fluctuations were identified with periods of ∼20 and ∼60min. The northern and southern auroral ovals were observed to rock in latitude in phase with the respective northern and southern planetary period oscillations. A solar wind compression impacted Saturn’s magnetosphere at the start of 22 April 2013, identified by an intensification and extension to lower frequencies of the Saturn kilometric radiation, with the following sequence of effects: (1) intensification of the auroral field-aligned currents; (2) appearance of a localised, intense bulge in the dawnside (04–06 LT) aurora while the midnight sector aurora remained fainter and narrow; and (3) latitudinal broadening and poleward contraction of the nightside aurora, where the poleward motion in this sector is opposite to that expected from a model of the auroral oval’s usual oscillation. These observations are interpreted as the response to tail reconnection events, initially involving Vasyliunas-type reconnection of closed mass-loaded magnetotail field lines, and then proceeding onto open lobe field lines, causing the contraction of the polar cap region on the night side.

The physical structure of planetary nebulae around sdO stars: Abell 36, DeHt 2, and RWT 152★

We present narrow-band Halpha and [OIII] images, and high-resolution, long-slit spectra of the planetary nebulae (PNe) Abell36, DeHt2, and RWT152 aimed at studying their morphology and internal kinematics. These data are complemented with intermediate-resolution, long-slit spectra to describe the spectral properties of the central stars and nebulae. The morphokinematical analysis shows that Abell36 consists of an inner spheroid and two bright point-symmetric arcs; DeHt2 is elliptical with protruding polar regions and a bright non-equatorial ring; and RWT152 is bipolar. The formation of Abell36 and DeHt2 requires several ejection events including collimated bipolar outflows that probably are younger than and have disrupted the main shell. The nebular spectra of the three PNe show a high excitation and also suggest a possible deficiency in heavy elements in DeHt2 and RWT152. The spectra of the central stars strongly suggest an sdO nature and their association with PNe points out that they have most probably evolved through the asymptotic giant branch. We analyze general properties of the few known sdOs associated to PNe and find that most of them are relatively or very evolved PNe, show complex morphologies, host binary central stars, and are located at relatively high Galactic latitudes.

Connection between the discrete auroral structures drifting equatorward during the substorm expansion phase and the energetic particle injections at the geostationary orbit

The formation of an auroral bulge with a bright dynamical arc at its polar boundary is one of the main manifestations of the magnetospheric substorm expansion phase at the ionospheric level. At the same time, the region of discrete aurora broadens not only polewards but equatorwards as well. The discrete forms of auroras moving equatorwards form a dynamical equatorial boundary of the auroral bulge shifting together with them. The paper presents a spatial-time comparison of the drifting discrete auroras to the injection of energetic particles at the geostationary orbit. It is shown that bursts in the fluxes of energetic particles at the LANL geostationary satellites located in the same sector of MLT correspond to the majority of drifting discrete auroral structures observed by the all-sky camera. In the cases when the bursts in the fluxes are absent, the minimum latitude reached by the auroral structures at the equatorward drift is higher than the ionospheric projection of the geostationary orbit. A possible relation of the drifting discrete auroras to the plasma stream jets in the plasma sheet is discussed.

Arc Behavior of Dry Hyperbaric Gas Metal Arc Welding

The arc behavior in dry hyperbaric Gas Metal Arc Welding (GMAW) process was investigated by using a high speed camera system and welding electric signal acquisition system. The arc shape in hyperbaric argon environment of 0.1-2MPa shows quite different characteristic from that at normal pressure. With the increase of ambient pressure, the arc length turns shorter, arc column is contracted, and the arc brightness increases. At elevated ambient pressure, the arc length increases with increasing welding voltage. Arc voltage has a good linear relation with arc length. The sum of the fall voltages at ambient pressure of 0.4MPa, 0.8MPa, and 2MPa is nearly constant which is about 20.2-21.7V. The values of electric field strength of arc column at different ambient pressure were gained through the linear fit, which are increased with increasing ambient pressure. The arc static characteristics at elevated ambient pressure are raising characteristics, and it is shifted upward with increasing ambient pressure.

The architecture of the LkCa 15 transitional disk revealed by high-contrast imaging

We present four new epochs of Ks-band images of the young pre-transitional disk around LkCa 15, and perform extensive forward modeling to derive the physical parameters of the disk. We find indications of strongly anisotropic scattering (Henyey-Greenstein g = 0.67 [-0.11,+0.18]) and a significantly tapered gap edge ('round wall'), but see no evidence that the inner disk, whose existence is predicted by the spectral energy distribution, shadows the outer regions of the disk visible in our images. We marginally confirm the existence of an offset between the disk center and the star along the line of nodes; however, the magnitude of this offset (x = 27 [-20,+19] mas) is notably lower than that found in our earlier H-band images (Thalmann et al. 2010). Intriguingly, we also find, at high significance, an offset of y = 69 [-25, +49] mas perpendicular to the line of nodes. If confirmed by future observations, this would imply a highly elliptical -- or otherwise asymmetric -- disk gap with an effective eccentricity of e = ~0.3. Such asymmetry would most likely be the result of dynamical sculpting by one or more unseen planets in the system. Finally, we find that the bright arc of scattered light we see in direct imaging observations originates from the near side of the disk, and appears brighter than the far side because of strong forward scattering.

Westward traveling surges: Sliding along boundary arcs and distinction from onset arc brightening

After onset, auroral brightening rapidly spreads azimuthally along the onset arc, typically, but not always, extending substantially farther westward than eastward. Expansion‐phase auroras also expand poleward and can contact an arc along the auroral poleward boundary. For such events, we find that poleward expansion of the region of active aurora accelerates after contact, forming a bulge region that protrudes into the preexisting polar cap and expands westward as the westward traveling surge. This westward motion is slower than, and initiates later than, the azimuthal spreading of the onset arc brightening. The poleward boundary arc remains connected to the surge as the surge slides along the equatorward moving edge of the boundary arc, and this connection appears to set off the westward traveling, poleward protruding activity of the surge via localized polar cap flow adjacent to the boundary arc. Ground magnetic signatures show a relatively weak H component depression moving westward with the onset arc brightening, and a delayed substantially larger depression moving with the westward propagating surge that must be associated with a substantially larger westward ionospheric current. The clear distinction between onset arc brightening and the westward traveling surge seen in auroral and magnetometer current signatures implies that their underlying processes are very different. Evidence suggests that flow channels adjacent to the boundary arc possibly feed surge development. These results are consistent with current wedge formation being more associated with post‐onset activity than with onset, and with flow channels from the polar cap being important for surge/bulge development.

Large flow shears around auroral beads at substorm onset

We present a fine‐scale electrodynamic structure behind the auroral beads observed immediately before substorm onset, as inferred from high spatial and temporal resolution measurements of substorm with an all‐sky TV camera (ATV) and a coherent high frequency radar of Super Dual Auroral Radar Network in Iceland. On the night of 24 September 2006, the ATV observed eastward propagating auroral beads in the initial brightening arc of a substorm just prior to the poleward expansion. During the sequential passage of the beads across the radar beams, the radar detected large velocity flow shears whose magnitude was in excess of 0.27 s−1. The observations suggest that flow shears were located very close to the center of the beads; thus, they corresponded to electric fields converging toward the beads, which is consistent with the existence of upward field‐aligned currents (FACs) flowing out of the beads. The temporal and spatial resolutions of the current radar measurement were still insufficient for fully resolving the detailed electrodynamic structure behind the fast moving auroral beads. At least, however, we can suggest the existence of highly localized filamentary FAC structures behind the beads, which would be an important consequence of possible plasma instabilities operating in the M‐I coupling region or near the equatorial plane of the magnetosphere.

The puzzling radio source in the cool core cluster A2626

Abstract We report on new Very Large Array (VLA) radio observations performed at 1.4 and 4.8 GHz with unprecedented sensitivity and angular resolution (∼1 arcsec) of the cool core cluster A2626, which is known to possess a radio mini-halo at its centre. The most unusual features of A2626 are two elongated radio features detected in previous observations to the north and south, having morphologies not common to the typical jet-lobe structures in cool cores. In our new sensitive images the two elongated features appears clearly as bright radio arcs, and we discover the presence of a new arc to the west. These radio arcs are not detected at 4.8 GHz, implying a steep (α > 1) spectrum, and their origin is puzzling. After subtracting the flux density contributed by these discrete features from the total flux measured at low resolution, we estimate a residual 18.0 ± 1.8 mJy flux density of diffuse radio emission at 1.4 GHz. We therefore confirm the detection of diffuse radio emission, which appears distinct from the discrete radio arcs embedded in it. Although its radio power is lower (1.4 × 1023 W Hz−1) than previously known, the diffuse emission may still be classified as a radio mini-halo.

SDSS J2222+2745: A GRAVITATIONALLY LENSED SEXTUPLE QUASAR WITH A MAXIMUM IMAGE SEPARATION OF 15.″1 DISCOVERED IN THE SLOAN GIANT ARCS SURVEY

We report the discovery of a unique gravitational lens system, SDSSJ2222+2745, producing five spectroscopically confirmed images of a z_s=2.82 quasar lensed by a foreground galaxy cluster at z_l=0.49. We also present photometric and spectroscopic evidence for a sixth lensed image of the same quasar. The maximum separation between the quasar images is 15.1". Both the large image separations and the high image multiplicity of the lensed quasar are in themselves exceptionally rare, and observing the combination of these two factors is an exceptionally unlikely occurrence in present datasets. This is only the third known case of a quasar lensed by a cluster, and the only one with six images. The lens system was discovered in the course of the Sloan Giant Arcs Survey, in which we identify candidate lenses in the Sloan Digital Sky Survey and target these for follow up and verification with the 2.56m Nordic Optical Telescope. Multi-band photometry obtained over multiple epochs from September 2011 to September 2012 reveal significant variability at the ~10-30% level in some of the quasar images, indicating that measurements of the relative time delay between quasar images will be feasible. In this lens system we also identify a bright (g = 21.5) giant arc corresponding to a strongly lensed background galaxy at z_s=2.30. We fit parametric models of the lens system, constrained by the redshift and positions of the quasar images and the redshift and position of the giant arc. The predicted time delays between different pairs of quasar images range from ~100 days to ~6 years.

Estimating energy spectra of electron precipitation above auroral arcs from ground‐based observations with radar and optics

In 2008, coordinated radar‐optical auroral observations were organized in Northern Scandinavia using the European Incoherent Scatter Radar (EISCAT) and the Auroral Large Imaging System (ALIS). A bright auroral arc was imaged on 5 March 2008 from four ground‐based stations, remaining stable between 18:41 and 18:44 UT and coinciding with increased electron densities. This work presents a unified inversion framework deriving the electron energy spectrum from either optical ( 1NG(0,1) emission at 4278 Å) or radar (electron density) observations. An updated forward model of the ionosphere based on a 1‐D kinetic Monte Carlo model is described, characterizing the linear system to invert. The 3‐D blue volume emission rate is first estimated with an iterative reconstruction technique. Also presented is a novel way to calculate an accurate initial guess for auroral tomography taking into account the horizontal/vertical nonuniformity of the emission region. This technique supersedes the often‐used Chapman profile as initial guess when high spatial resolution is needed. The second step, performed for the first time with ALIS optical observations, uses the forward model to retrieve from the emission rates the 2‐D latitude/longitude map of the electron energy spectrum. The same model and inversion methods are finally applied to the EISCAT electron density profiles to derive the temporal energy spectrum of precipitating electrons along the magnetic zenith. Energy spectra from radar and optics are in good agreement. Results suggest that the arc is generated by a complex energy spectrum reminiscent of dispersive Alfvén waves with two main peaks (2.5, 6 keV) and a typical latitudinal width of 7.5 km.

Análise da oscilação da poça de solda em P-GMAW por meio de processamento de imagens obtidas por perfilografia

Este artigo apresenta uma metodologia de estudo da oscilação da poça de solda que permite obter as frequências de oscilação da poça tanto por sinais de tensão quanto por análise das imagens. O intenso brilho do arco é reduzido utilizando-se a técnica de perfilografia, e as imagens são obtidas a 1000 fotos por segundo por uma câmera CMOS de alta velocidade. A geometria da poça é visualizada em perfil e são obtidos espectros de frequência por transformada rápida de Fourier por meio de processamento das imagens em MATLAB. O processo de soldagem experimentado foi o GMAW pulsado com e sem a chamada "corrente de destacamento". Os resultados mostram que é possível medir a oscilação da poça por esta metodologia e até mesmo pode ser mais confiável a quantificação da oscilação pelas imagens do que pela a análise do sinal de tensão.

Supernova remnant G296.7–0.9 in X-rays

Aims: We present a detailed study of the supernova remnant (SNR) G296.7-0.9 in the 0.2-12 keV X-ray band. Methods: Using data from XMM-Newton we performed a spectro-imaging analysis of G296.7-0.9 in order to deduce the basic parameters of the remnant and to search for evidence of a young neutron star associated with it. Results: In X-rays the remnant is characterized by a bright arc located in the south-west direction. Its X-ray spectrum can best be described by an absorbed non-equilibrium collisional plasma model with a hydrogen density of N_H=1.24_{-0.05}^{+0.07} x 10^{22} cm^{-2} and a plasma temperature of 6.2^{+0.9}_{-0.8} million Kelvin. The analysis revealed a remnant age of 5800 to 7600 years and a distance of 9.8_{-0.7}^{+1.1} kpc. The latter suggests a spatial connection with a close-by HII region. We did not find evidence for a young neutron star associated with the remnant.