Small Angular Distances Research Articles

Probing Lorentz Invariance Violation with Absorption of Astrophysical γ-Rays by Solar Photons

We compute in detail the absorption optical depth for astrophysical γ-ray photons interacting with solar photons to produce electron–positron pairs. This effect is greatest for γ-ray sources at small angular distances from the Sun, reaching optical depths as high as τ γ γ ∼ 10−2. We also calculate this effect including modifications to the absorption cross-section threshold from subluminal Lorentz invariance violation (LIV). We show for the first time that subluminal LIV can lead to increases or decreases in τ γ γ compared to the non-LIV case. We show that, at least in principle, LIV can be probed with this effect with observations of γ-ray sources near the Sun at ≳20 TeV by HAWC or LHAASO, although a measurement will be extremely difficult due to the small size of the effect.

The Lomonossov arc: refraction and scattering in Venus atmosphere during solar transits

The main observations of 1761 by M. Lomonossov and those that followed are recalled by extending the discussion to other remarkable visual observations of the passages, then with more and more powerful imagers producing images in profusion. The modern treatment of parasitic effects is briefly recalled by focusing on the expert observation of 1761 which has recently been widely commented on and criticized. It included a spurious effect called the “black drop effect”. The shell or aureole or atmospheric ring of Venus observed outside the solar disk is considered with reference to the today parameters of the Venus atmosphere. The contacts during the transit are discussed taking into account effects of scattering, absorption, and the dominant effects of the refraction at the small angular distances found to be comparable to a fraction of the angular dimension of the planet. Modern observations of the 2004 and the 2012 transit are tentatively discussed to elucidate what is the arc of Lomonossov.

Astrometry via close approach events: applications to main-belt asteroid (702) Alauda

ABSTRACT The release of Gaia catalogue is revolutionary to the astronomy of Solar system objects. After some effects such as atmospheric refraction and CCD geometric distortion have been taken into account, the astrometric precision for ground-based telescopes can reach the level of tens of milli-arcseconds (mas). If an object approaches a reference star in a small relative angular distance (less than 100 arcsec), which is called close approach event in this work, the relative positional precision between the object and reference star will be further improved since the systematic effects of atmospheric turbulence and local telescope optics can be reduced. To obtain the precise position of a main-belt asteroid in an close approach event, a second-order angular velocity model with time is supposed in the sky plane. By fitting the relationship between the relative angular distance and observed time, we can derive the time of maximum approximation and calculate the corresponding position of the asteroid. In practice, five nights’ CCD observations including 15 close approach events of main-belt asteroid (702) Alauda are taken for testing by the 1 m telescope at Yunnan Observatory, China. Compared with conventional solutions, our results show that the positional precision significantly improves, which reaches better than 4 mas, and 1 mas in the best case when referenced for the Jet Propulsion Laboratory (JPL) ephemeris in both right ascension and declination.

Flares, Warps, Truncations, and Satellite: The Ultra-thin Galaxy UGC 11859

The structure of the outskirts of galaxies provides valuable information about their past and evolution. Due to their projected orientation, edge-on isolated galaxies effectively serve as test labs in which to study the three-dimensional structures of galaxies, including warps and flares, and to explore the possible sources of such distortions. We analyzed the structure of the apparently isolated edge-on ultra-thin galaxy UGC 11859 to look for the presence of distortions. The deep optical imaging observations ( and 30.0 in the g- and r-bands, respectively) we acquired with the 10.4 m Gran Telescopio Canarias are used to derive the radial and vertical surface brightness profiles and g − r color radial profile. We find that UGC 11859’s disk displays a significant gravitational distortion. A warp is clearly detected on one side of the disk, and the galactic plane on both sides of the center shows increasing scale height with an increasing galactocentric radius, indicating the presence of a flare in the stellar distribution. The surface brightness profile of the disk shows a sharp break at 24 kpc galactocentric radius, and a steep decline to larger radii, an “edge-on truncation,” which we associate with the presence of the flare. The present study is the first observational support for a connection between truncations and flares. Just beyond the warped side of the disk, a faint galaxy is observed within a small angular distance, identified as a potential interacting companion. Based on ultradeep g and r photometry we estimate that if the potential companion is at the same distance as UGC 11859, the stellar mass of the satellite galaxy is log10(M ⊙) = .

Utility of the ganglion cell complex thickness map in glaucoma: the presence of raphe sign

Macular ganglion cell complex (GCC) thickness map is a functional ancillary test to detect early structural changes in glaucomatous optic neuropathy (GON). The temporal raphe sign is present in almost all patients with narrow retinal nerve fiber layer (RNFL) defect, especially when there is a small angular distance between the fovea and the RNFL defect. A case series of patients with GCC thickness map and a positive temporal raphe sign is presented.

P-velocity of the upper mantle

The authors have constructed models featuring seismic P-wave velocity distribution in the upper mantle beneath oceanic, continental and transition regions, such as mid-ocean ridges, basins, trenches, island arcs, and back-arc troughs, Atlantic transitional zones, flanking plateaus of mid-ocean ridges, platforms, geosynclines, rifts, recent activation zones. The models are in agreement with the deep-seated processes in the tectonosphere as predicted in terms of the advection-polymorphism hypothesis. The models for areas of island arcs and coastal ridges are similar to those for alpine geosynclines disturbed by recent activation. The models for areas of mid-ocean ridges and back-arc troughs are identical. They fit the pattern of recent heat-and-mass transfer in the case of rifting, which, given the basic crust with continental thickness, leads to oceanization. The model for the basin reflects the effect of thermal anomalies smoothing beneath mid-ocean ridges or back-arc troughs about 60 million years later. The model for the trench and flanking plateau reflects the result of lateral heating of the mantle’s upper layers beneath the quiescent block from the direction of the island arc and basin (trench) and mid-ocean ridge and basin (flanking plateau).
 A detailed bibliography on regions covered by studies was presented in the authors’ earlier publications over past eight years. There are quite significant differences between models for regions of the same type that are described in publications of other authors. This is largely due to the fact that individual authors adopt a priori concepts on the velocity structure of the upper mantle.
 High variability of seismic P-wave velocities within the subsurface depth interval has been detected as a result of all sufficiently detailed studies. This variability is responsible for the sharp increase in the scatter of arrival times of waves from earthquakes at small angular distances. The corresponding segments of travel-time graphs were simply ignored, and the graphs started from about 3° after which the scatter of arrival time acquired a stable character. Accordingly, velocity profiles were constructed, as a rule, starting from depths of about 50 km. The constructed velocity profiles vary little from region to region with the same type of endogenous regimes. This enables us to maintain that the models represent standard (typical) VP distributions in the mantle beneath the regions, just as presumed in terms of the theory.

INS/CNS navigation system based on multi-star pseudo measurements

In the integrated navigation of Inertial Navigation Systems (INS) and Celestial Navigation Systems (CNS), small stellar angular distances in a single field-of-view (FOV) star sensor lead to inaccurate navigation parameter estimations. A novel, INS/CNS integrated navigation system based on multi-star pseudo measurements is proposed in this paper. First, according to the identified navigation star's observation information, five candidate stars are directly selected from the Smithsonian Astrophysical Observatory (SAO) star catalog as those most favorable for estimating the probe position. Then, in addition to the identified navigation star's measurement, the five candidate stars' pseudo observation information are estimated to supplement the measurements. Finally, the INS/CNS integrated navigation system in the tightly coupled mode estimates the probe position with high accuracy utilizing the available measurement and pseudo measurements. The simulation results illustrate that, compared to the traditional INS/CNS integrated navigation system which uses the infrared Earth sensor to assist in measuring the celestial measurements of navigation stars in the star image, and the improved method which directly supplements the position measurement by the infrared Earth sensor, the proposed method markedly improves the positioning accuracy, while the three methods' total computational efficiencies including measurement acquisition and filtering are effectively the same.

Polarization Channel Estimation for Circular and Non-Circular Signals in Massive MIMO Systems

The polarization millimeter wave (mmWave) massive multiple-input multiple-output (MIMO) system has been deployed in next-generation wireless communication since it can provide a high-data stream and high space efficiency simultaneously. Polarization channel parameter estimation for polarized mmWave massive MIMO systems is extremely important for directional beamforming with data transmissions. In this paper, the base station (BS) equipped with a large polarization-sensitive array is considered in massive MIMO systems. The polarization channel consists of direction-of-arrival (DOA) and polarization parameters that are estimated from the coexistence of circular and non-circular signals. Based on the unconjugated covariance matrix, the initialized polarization channel estimation is achieved by multiple signal classification (MUSIC). Then, the high-accuracy polarization channel estimation for general non-circular rate signals is performed by reconstructing the corresponding noise matrix. The high-accuracy polarization channel estimation for circular signals is obtained based on covariance matrix differencing. Moreover, the dimension of parameter search is reduced based on the partial derivative of the spectrum function with respect to the non-circular phase. The high-accuracy polarization channel estimation for the maximum non-circular rate signal is finally achieved based on the initialized parameter estimation of the polarization channel. The proposed algorithm estimates different kinds of signals separately. The effect of different kinds of signals is reduced significantly, which means that the resolution probability of different kinds of signals can be dramatically improved. Numerical examples are provided to demonstrate the performance of the proposed algorithm, especially in small angular distances.

Orbits through polytropes

We describe how orbital tunnels could be used to transport payloads through the Earth. If you use a brachistochrone for the tunnel, the body forces in the tunnel become overwhelmingly large for small angular distances traveled. Projectiles move along an orbital tunnel faster than they would along a brachistochrone connecting the same points but the body force components cancel. We describe how parabolic Keplerian orbits outside the object merge onto quasi-Keplerian orbits inside the object. We use models of the interior of the Earth with three values of the polytropic index (n) to calculate interior orbits that travel between surface points. The n = 3 results are also scaled to the Sun. Numerical integrations of the equations describing polytropes were used to generate the initial models. Numerical integration of the equations of motion are then used to calculate the angular distance you can travel along the surface and the traversal time as a function of the parabolic periapsis distance for each model. Trajectories through objects of low central condensation show a focussing effect that decreases as the central condensation increases. Analytic solutions for the trajectories in a homogeneous sphere are derived and compared to the numeric results. The results can be scaled to other planets, stars, or even globular clusters.

Scattered moonlight observations with X-shooter

Estimating the sky background is critical for ground-based astronomical research. In the optical, scattered moonlight dominates the sky background, when the moon is above the horizon. The most uncertain component of a scattered moonlight model is the aerosol scattering. The current, official sky background model for Cerro Paranal uses an extrapolated aerosol extinction curve. With a set of X-shooter sky observations, we have tested the current sky background model as well as determined the aerosol extinction from the ultra-violet (UV) to near-infrared (NIR). To our knowledge, this is the first time that a scattered moonlight model has been used for this purpose. These observations were taken of blank sky, during three different lunar phases, and at six different angular distances from the moon for each lunar phase. Overall, the current model does reproduce the observations for average conditions quite well. Using a set of sky background models with varying aerosol distributions to compare with the observations, we found the most likely aerosol extinction curves, phase functions, and volume densities for the three nights of observations and compare them with the current model. While there are some degeneracies in the aerosol scattering properties, the extinction curves tend to flatten towards redder wavelengths and are overall less steep compared to the extrapolated curve used in the current model. Also, the current model had significantly less coarse particles compared to the favored volume densities from the X-shooter data. Having more coarse particles affects the phase function by being more peaked at small angular distances. For the three nights of sky observations, the aerosol size distributions differed, most likely reflecting the changes in atmospheric conditions and aerosol content, which is expected. In short, the current sky background model is in fair agreement with the observations, and we have determined better aerosol extinction curves and phase functions for Cerro Paranal. Using nighttime sky observations of scattered moonlight and a set of sky background models is a new method to probe the aerosol content of the atmosphere.

Relationship of Halo CMEs and Solar Proton Events

Coronal Mass Ejections (CMEs) are important sources of Solar Proton Events (SPEs). Their speeds and source region locations have significant effects on the occurrence of SPEs. In this paper, all the halo CMEs observed in recent five years are statistically analyzed. The results show that the fast halo CMEs with small angular distances are more likely to produce SPEs, especially, those halo CMEs with a speed greater than 1200kms−1 and an angular distance less than 60°. Three fast halo CMEs with no SPEs caused are elaborately studied. The results show that the ejection direction of the CME's main body and the variation of interplanetary magnetic field also have important impacts on the occurrence of SPEs. Consequently, in the practical daily space environment forecasts, an accurate forecast for SPEs must take various factors into account, such as the eruption speed, source region location, the main-body ejection direction of CMEs, and the interplanetary environment, etc.

Search for long distance correlations between extensive air showers detected by the EEE network

A search for long distance correlations between individual Extensive Air Showers (EAS) detected by pairs of MRPC telescopes of the Extreme Energy Events (EEE) network was carried out. The search for an anomaly in these events is the purpose of our work. A dataset obtained by all the possible 45 pairs between 10 EEE cluster sites (hosting at least two telescopes), located at relative distances between 86 and 1200km, was analyzed, corresponding to an overall period of 3968 days time exposure. To estimate the possible event excess with respect to the spurious rate, the number of coincidence events was extracted as a function of the time difference between the arrival of the showers in the two sites, from $\pm 10$ s to the smallest time interval where events are still observed. The analysis was done taking into account both the time and orientation correlation between the showers detected by the telescope pairs. A few candidate events with unusually small time difference and angular distance were observed, with a p-value sensibly smaller than a confidence level of 0.05.

Modelling of mid-infrared interferometric signature of hot exozodiacal dust emission

Hot exozodiacal dust emission was detected in recent surveys around two dozen main sequence stars at distances of less than $1\,\text{au}$ using H and K band interferometry. Due to the high contrast as well as the small angular distance between the circumstellar dust and the star, direct observation of this dust component is challenging. An alternative way to explore the hot exozodiacal dust is provided by mid-infrared interferometry. We analyze the L, M and N band interferometric signature of this emission in order to find stronger constraints for the properties and the origin of the hot exozodiacal dust. Considering the parameters of nine debris disc systems derived previously, we model the discs in each of these bands. We find that the M band possesses the best conditions to detect hot dust emission, closely followed by L and N bands. The hot dust in three systems - HD 22484 (10 Tau), HD 102647 ($\beta$ Leo) and HD 177724 ($\zeta$ Aql) - shows a strong signal in the visibility functions which may even allow one to constrain the dust location. In particular, observations in the mid-infrared could help to determine whether the dust piles up at the sublimation radius or is located at radii up to $1\,\text{au}$. In addition, we explore observations of the hot exozodiacal dust with the upcoming mid-infrared interferometer MATISSE at the Very Large Telescope Interferometer (VLTI).

Probing the possibility of hotspots on the central neutron star in HESS J1731−347

The X-ray spectra of the neutron stars located in the centers of supernova remnants Cas A and HESS J1731-347 are well fit with carbon atmosphere models. These fits yield plausible neutron star sizes for the known or estimated distances to these supernova remnants. The evidence in favor of the presence of a pure carbon envelope at the neutron star surface is rather indirect and is based on the assumption that the emission is generated uniformly by the entire stellar surface. Although this assumption is supported by the absence of pulsations, the observational upper limit on the pulsed fraction is not very stringent. In an attempt to quantify this evidence, we investigate the possibility that the observed spectrum of the neutron star in HESS J1731-347 is a combination of the spectra produced in a hydrogen atmosphere of the hotspots and of the cooler remaining part of the neutron star surface. The lack of pulsations in this case has to be explained either by a sufficiently small angle between the neutron star spin axis and the line of sight, or by a sufficiently small angular distance between the hotspots and the neutron star rotation poles. As the observed flux from a non-uniformly emitting neutron star depends on the angular distribution of the radiation emerging from the atmosphere, we have computed two new grids of pure carbon and pure hydrogen atmosphere model spectra accounting for Compton scattering. Using new hydrogen models, we have evaluated the probability of a geometry that leads to a pulsed fraction below the observed upper limit to be about 8.2 %. Such a geometry thus seems to be rather improbable but cannot be excluded at this stage.

Advanced weight graph transformation matching algorithm

An efficient and accurate point matching algorithm named advanced weight graph transformation matching (AWGTM) is proposed in this study. Instead of relying only on the elimination of dubious matches, the method iteratively reserve correspondences which have a small angular distance between two nearest‐neighbour graphs. The proposed algorithm is compared against weight graph transformation matching (WGTM) and graph transformation matching (GTM). Experimental results demonstrate the superior performance in eliminating outliers and reserving inliers of AWGTM algorithm under various conditions for images, such as duplication of patterns and non‐rigid deformation of objects. An execution time comparison is also presented, where AWGTM shows the best results for high outlier rates.

SECULAR DECREASE THE FLUX OF SUPERNOVA REMNANT CAS A ON MONITORING RESULTS TO RADIOTELESCOPE "URAN-4" IRA NASU

This work is dedicated to the study of secular decrease of the flux of young supernova remnant Cas A according to observations by radio- telescope "URAN-4» of Odessa Observatory IRA NASU from 1987 to 2001 years on frequency of 25 MHz. On the investigation base there is a relationship analysis of flux CasA to the "stable" source – radio-galaxy Cyg A (CasA/Cyg A) which is located on a small angular distance. Results of the observations held on RT «URAN- 4» show that there is no noticeable decrease of fluxes in the period 1987-1993, with the relationship ratio (CasA/Cyg A) = 1.5. While considering data from 1987 to 2001 manifested a slight decrease trend in flux equal to 8.4% for the all period. At the same time, according to various investigations the average value flux of Cas A in the interval of frequencies 38-2924 MHz is 0.8% per year. At the meantime in this frequency the range ratio (CasA/Cyg A) has become less than one. Thus, there is a noticable contradiction of secular decrease of the flux Cas A on this radio frequencies in comparison with the predictions of the theory in 1.7% per year.

M 101 group galaxies

Based on archival Hubble Space Telescope images, we have performed stellar photometry for the galaxy M 101 and other neighboring galaxies located at a small angular distance from M 101 and having radial velocities similar to that of M 101: M 51, M 63, NGC 5474, NGC 5477, UGC 9405, Ho IV, KUG1413+573, and others. Based on the TRGB method, we have determined the distances to these galaxies. We have found that the M 101 group lies at a distance of 6.8 Mpc and is a small compact galaxy group consisting of four galaxies: NGC 5474, NGC 5477, UGC 9405, and Ho IV. The bright massive galaxies M 51 and M 63 are considerably farther (D = 9.0 and 9.3 Mpc, respectively) than the M 101 group and do not belong to it. Applying the virial theorem to 27 objects (H II regions and galaxies),M 101 satellites located at different distances from the galaxy, has revealed an increase in the dynamical mass of M 101 with increasing sizes of the system of satellites used in calculating the mass. The maximum calculated mass of M 101 is 7.5 × 1011M⊙. The dynamical mass of M 101 calculated on the basis of the four galaxies constituting the group is 6.2 × 1011M⊙. The mass-to-light ratio for this mass is M/L = 18 (at the adopted luminosity of M 101, MB = −20.8).

Achromatic interfero-coronagraph with variable rotation shearing for studying extrasolar planets

Direct observation of exoplanets will make it possible to clarify many principal questions connected both with extrasolar planets and planetary systems and to measure atmospheric spectra of the planets. Obtaining an exoplanet image not distorted by the light from a star is at the cutting edge of present-day optical technologies owing to the combination of tremendous brightness contrasts and small angular distances between the planet and star. To observe the exo-Earth, it is necessary to weaken the brightness of the parent star image by 9–10 orders of magnitude (in the optical and near-IR ranges). To compensate the influence of the atmosphere, ground-based (e.g., 8–10 m) telescopes intended for observing exoplanets are equipped with adaptive optics systems, the spatial and temporal resolutions of which are not yet sufficient. A meter-class space telescope equipped with a star coronagraph will make it possible to observe the nearest exoplanets. In this paper, an improved tool for star coronagraphy is considered, namely, the achromatic interferometric coronagraph with a variable rotational shear. It is fabricated according to the optical scheme of the common path interferometer for studying extrasolar planets by direct observations. Theoretical and experimental estimations for the main characteristics of the scheme were carried out. Laboratory experimental measurements were carried out on a coronagraph model.

On the detectability of H i 21 cm in Mg ii absorption systems

We investigate the effect of two important, but oft neglected, factors which can affect the detectability of HI 21-cm absorption in MgII absorption systems: The effect of line-of-sight geometry of the coverage of the background radio flux and any possible correlation between the 21-cm line strength and the rest frame equivalent width of the MgII line. Regarding the former, while the observed detection rate at small angular diameter distance ratios is a near certainty, for an unbiased sample, where either a detection or a non-detection are equally likely, at ratios > 0.8 the observed detection rate has an 8 sigma significance, suggesting that the mix of ratios values at z < 1 is correlated with the mix of detections and non-detections at low redshift, while the exclusively high values of the ratio at z > 1 contribute to the low detection rates at high redshift. In DLAs, the correlation between the 21-cm line strength and the MgII equivalent width is dominated by the velocity spread of the 21-cm line. This has recently been shown not to hold for MgII systems in general. However, we do find the significance of the correlation to increase when the MgII absorbers with MgI equivalent widths of >0.5 A are added to the DLA sample. Large values of the angular diameter distance ratio may explain why the absorbers which have similar equivalent widths to the detections remain undetected. We do, however, also find the neutral hydrogen column densities of the non-detections to be significantly lower. Applying the 21-cm line strength/equivalent width correlation to yield column densities for the MgII absorbers in which this is unmeasured, we find no evidence of a cosmological evolution in the neutral hydrogen column density.

NEUTRAL HYDROGEN CLOUDS IN THE M81/M82 GROUP

We have observed a 3° × 3° area centered on the M81/M82 group of galaxies using the Robert C. Byrd Green Bank Telescope in a search for analogs to the high-velocity clouds of neutral hydrogen found around our Galaxy. The velocity range from −605 to −85 km s−1 and from 25 to 1970 km s−1 was searched for H i clouds. Over the inner 2° × 2° the 7σ detection threshold was 9.6 × 105 M☉. We detect five previously unknown H i clouds associated with the group, as well as numerous associated filamentary H i structures, all lying in the range −105 ⩽ Vhelio ⩽ +280 km s−1. From the small angular distance of the clouds to group members, and the small velocity difference between group members and clouds, we conclude that the clouds are most likely relics of ongoing interactions between galaxies in the group.