Interferometric Observations Research Articles

Characterizing beam errors for radio interferometric observations of reionization

ABSTRACT A limiting systematic effect in 21-cm interferometric experiments is the chromaticity due to the coupling between the sky and the instrument. This coupling is sourced by the instrument primary beam; therefore it is important to know the beam to extremely high precision. Here, we demonstrate how known beam uncertainties can be characterized using data bases of beam models. In this introductory work, we focus on beam errors arising from physically offset and/or broken antennas within a station. We use the public code oskar to generate an ‘ideal’ SKA beam formed from 256 antennas regularly spaced in a 35-m circle, as well as a large data base of ‘perturbed’ beams sampling distributions of broken/offset antennas. We decompose the beam errors (‘ideal’ minus ‘perturbed’) using principal component analysis (PCA) and Kernel PCA (KPCA). Using 20 components, we find that PCA/KPCA can reduce the residual of the beam in our data sets by $60\!-\!90{{\ \rm per\ cent}}$ compared with the assumption of an ideal beam. Using a simulated observation of the cosmic signal plus foregrounds, we find that assuming the ideal beam can result in $1{{\ \rm per\ cent}}$ error in the epoch of reionization (EoR) window and $10{{\ \rm per\ cent}}$ in the wedge of the 2D power spectrum. When PCA/KPCA is used to characterize the beam uncertainties, the error in the power spectrum shrinks to below $0.01{{\ \rm per\ cent}}$ in the EoR window and $\le 1{{\ \rm per\ cent}}$ in the wedge. Our framework can be used to characterize and then marginalize over uncertainties in the beam for robust next-generation 21-cm parameter estimation.

LMT/AzTEC observations of Vega

ABSTRACT Vega is the prototypical debris disc system. Its architecture has been extensively studied at optical to millimetre wavelengths, revealing a near face-on, broad, and smooth disc with multiple distinct components. Recent millimetre-wavelength observations from ALMA spatially resolved the inner edge of the outer, cold planetesimal belt from the star for the first time. Here we present early science imaging observations of the Vega system with the AzTEC instrument on the 32-m LMT, tracing extended emission from the disc out to 150 au from the star. We compare the observations to three models of the planetesimal belt architecture to better determine the profile of the outer belt. A comparison of these potential architectures for the disc does not significantly differentiate between them with the modelling results being similar in many respects to the previous ALMA analysis, but differing in the slope of the outer region of the disc. The measured flux densities are consistent between the LMT (single dish) and ALMA (interferometric) observations after accounting for the differences in wavelength of observation. The LMT observations suggest the outer slope of the planetesimal belt is steeper than was suggested in the ALMA analysis. This would be consistent with the interferometric observations being mostly blind to structure at the disc outer edges, but the overall low signal to noise of the LMT observations does not definitively resolve the structure of the outer planetesimal belt.

Synthetic observations using POLARIS: an application to simulations of massive prestellar cores

Young massive stars are usually found embedded in dense and massive molecular clumps which are known for being highly obscured and distant. During their formation process, the degree of deuteration can be used as a potential indicator of the very early formation stages. This is particularly effective when employing the abundance of H2D+. However, its low abundances and large distances make detections in massive sources hard to achieve. We present an application of the radiative transfer code POLARIS, with the goal to test the observability of the ortho-H2D+ transition 1_{10}-1_{11} (∼372.42 GHz) using simulations of high-mass collapsing cores that include deuteration chemistry. We analyzed an early and a late stage of the collapse of a 60 M⊙ core, testing different source distances. For all cases, we generated synthetic single-dish and interferometric observations and studied the differences in both techniques. The column densities we derive are comparable to values reported for similar sources. These estimates depend on the extent over which they are averaged, and sources with compact emission they can be highly affected by beam dilution. Combined ALMA-ACA observations improve in signal-to-noise ratio and lead to better column density estimates as compared to ALMA alone. We confirm the feasibility to study ortho-H2D+ emission up to distances of ∼7 kpc. We provide a proof-of-concept of our framework for synthetic observations and highlight its importance when comparing numerical simulations with real observations. This work also proves how relevant it is to combine single-dish and interferometric measurements to derive appropriate source column densities.

Unraveling the Innermost Jet Structure of OJ 287 with the First GMVA + ALMA Observations

We present the first very long baseline interferometric (VLBI) observations of the blazar OJ 287 carried out jointly with the Global Millimeter VLBI Array (GMVA) and the phased Atacama Large Millimeter/submillimeter Array (ALMA) at 3.5 mm on 2017 April 2. The participation of phased ALMA has not only improved the GMVA north–south resolution by a factor of ∼3, but has also enabled fringe detections with signal-to-noise ratios up to 300 at baselines longer than 2 Gλ. The high sensitivity has motivated us to image the data with newly developed regularized maximum likelihood imaging methods, revealing the innermost jet structure with unprecedentedly high angular resolution. Our images reveal a compact and twisted jet extending along the northwest direction, with two bends within the inner 200 μas, resembling a precessing jet in projection. The component at the southeastern end shows a compact morphology and high brightness temperature, and is identified as the VLBI core. An extended jet feature that lies at ∼200 μas northwest of the core shows a conical shape, in both total and linearly polarized intensity, and a bimodal distribution of the linear polarization electric vector position angle. We discuss the nature of this feature by comparing our observations with models and simulations of oblique and recollimation shocks with various magnetic field configurations. Our high-fidelity images also enabled us to search for possible jet features from the secondary supermassive black hole (SMBH) and test the SMBH binary hypothesis proposed for this source.

Imaging of the Quiet Sun in the Frequency Range of 20–80 MHz

Radio emission of the quiet Sun is considered to be due to thermal bremsstrahlung emission of the hot solar atmosphere. The properties of the quiet Sun in the microwave band have been well studied, and they can be well described by the spectrum of bremsstrahlung emission. In the meter-wave and decameter-wave bands, properties of the quiet Sun have rarely been studied due to the instrumental limitations. In this work, we use the LOw Frequency ARray telescope to perform high quality interferometric imaging spectroscopy observations of quiet Sun coronal emission at frequencies below 90 MHz. We present the brightness temperature spectrum and the size of the Sun in the frequency range of 20–80 MHz. We report on dark coronal regions with low brightness temperatures that persist with frequency. The brightness temperature spectrum of the quiet Sun is discussed and compared with the bremsstrahlung emission of a coronal model and previous quiet Sun observations.

A Gap at 1 au in the Disk of DI Cha A Revealed by Infrared Interferometry* *Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programs 077.C-0662, 079.C-0101, and 081.C-0020

DI Cha A is K0-type pre-main-sequence star, the brightest component of a quadruple stellar system. Here we report on a detailed study of this star based on archival VLTI/MIDI and VLTI/PIONIER infrared interferometric observations, as well as optical-infrared photometric monitoring from ground-based and space-born instruments. We determined the structure of the circumstellar disk by fitting simultaneously the interferometric visibilities and the spectral energy distribution, using both analytical models and the radiative transfer code RADMC-3D. The modeling revealed that the radial density distribution of the disk appears to have a gap between 0.21 and 3.0 au. The inner ring, whose inner size coincides with the sublimation radius, is devoid of small, submicrometer-sized dust grains. The inner edge of the outer disk features a puffed-up rim, typically seen in intermediate-mass stars. Grain growth, although less progressed, was also detected in the outer disk. The inner ring is variable at mid-infrared wavelengths on both daily and annual timescales, while the star stays remarkably constant in the optical, pointing to geometrical or accretion changes in the disk as possible explanations for the flux variations.

Speckle Interferometric Observations With the Gemini 8-m Telescopes: Signal-to-Noise Calculations and Observational Results

Using 3 years of observations with the Zorro and ’Alopeke speckle interferometric instruments at Gemini South and North, respectively, we present an analysis of the sensitivity of the data taken in two narrow-band optical filters centered at 562 and 832 nm (widths of 54 and 40 nm, respectively). In this paper we focus on model calculations of the predicted signal-to-noise values achievable and the results of over 2500 actual observations. We find that S/N values of several 100 are easily achieved, but that the sky background during full moon is a very limiting factor in the observations, especially those performed in the short-wavelength (blue) optical spectral range and for targets fainter than R ∼14. A comparison of our Gemini speckle observations over six observing semesters reveals that red band-pass observations provide more robust results in general, likely due to better atmospheric performance at these wavelengths. Using the identical instruments on Gemini North and South, we find that similar results are obtained, yielding typical contrast limits of 5-9 magnitudes from the diffraction limit out to 1.2″ for a range of target brightness (optical magnitudes from ∼ 3 to >16). Using our S/N model along with the observational results, an estimation of the contrast limits achievable for a given observation can be predicted based on the target brightness, sky illumination and seeing conditions, and the total integration time.

Transtensional coseismic fault slip of the 2021 Mw 6.7 Turt Earthquake and heterogeneous tectonic stress surrounding the Hovsgol Basin, Northwest Mongolia

Knowledge of the regional crustal deformation and stress field is fundamental to understanding and constrain the ongoing evolution of Hovsgol basin, northwest Mongolia. The 2021 Mw 6.7 Turt earthquake provides an unprecedented opportunity to probe the local tectonic stress field and upper crust deformation. We investigate the coseismic surface displacements and invert fault slip models using Interferometric Synthetic Aperture Radar observations and teleseismic data. The mainshock occurred as a result of normal faulting with a right-lateral strike-slip component on an NW striking plane, which is consistent with the transtensive local stress field inverted from regional focal mechanisms. It is surrounded by transpressive and strike-slip stress fields proposed by previous studies indicating regional stress heterogeneity probably controlled by the distinct structure geometries in and around Hovsgol basin. Our results also suggest that the current deformation of the Hovsgol basin is dominated by half-graben forming. Seven historical strong earthquakes (M 7) may advance the 2021 Turt earthquake by ~12% recurrence interval, based on the assumption of equal stress release from recurrence earthquakes, meanwhile, the 2021 Turt earthquake may increase the potential seismic hazard on the neighbor Mondy and South Hovsgol faults, which deserves more attention.

Two Rings and a Marginally Resolved, 5 au Disk around LkCa 15 Identified via Near-infrared Sparse Aperture Masking Interferometry

Sparse aperture masking interferometry (SAM) is a high-resolution observing technique that allows for imaging at and beyond a telescope’s diffraction limit. The technique is ideal for searching for stellar companions at small separations from their host star; however, previous analyses of SAM observations of young stars surrounded by dusty disks have had difficulties disentangling planet and extended disk emission. We analyze VLT/SPHERE-IRDIS SAM observations of the transition disk LkCa 15, model the extended disk emission, probe for planets at small separations, and improve contrast limits for planets. We fit geometrical models directly to the interferometric observables and recover previously observed extended disk emission. We use dynamic nested sampling to estimate uncertainties on our model parameters and to calculate evidences to perform model comparison. We compare our extended disk emission models against point-source models to robustly conclude that the system is dominated by extended emission within 50 au. We report detections of two previously observed asymmetric rings at ∼17 and ∼45 au. The peak brightness location of each model ring is consistent with the previous observations. We also, for the first time with imaging, robustly recover an elliptical Gaussian inner disk, previously inferred via SED fitting. This inner disk has an FWHM of 5 au and a similar inclination and orientation to the outer rings. Finally, we recover no clear evidence for candidate planets. By modeling the extended disk emission, we are able to place a lower limit on the near-infrared companion contrast of at least 1000.

Analyzing Interferometric CO (3-2) Observations of NGC 4039

Starburst merging galaxies are important in the history of galaxy evolution timeline. For this work, we have chosen the Antennae galaxy (NGC4039), which is one of the most famous starburst merging galaxies. We analyzed the CO (3–2) interferometric observations for the southern mosaic of the NGC 4039, together with the CO (2–1) data taken from Atacama Large Millimeter/Sub-millimeter Array (ALMA). Using the galactic CO luminosity to H2 mass conversion factor, we found molecular gas mass range in this galaxy to be (0.8–2.92) × 108 Mʘ. Line emissions at CO (2–1) and CO (3–2) were detected at selected regions in the nucleus of NGC 4039. The CO (3–2) / CO (2–1) ratio for this galaxy was calculated to be approximately 0.62. In addition, we found a significant correlation between the brightness temperature ratio and IR luminosity for this galaxy. We used a new model to interpret the rotation curve and found that the most important factor is related to gas mass distribution. The disturbance in the gas distribution may be caused by the merging process. We have also analyzed the spatially resolved star formation law in this galaxy up to 345 parsec. We found a breakdown of the Kennicutt–Schmidt law at this scale. The results are consistent with the previous findings that there is a possibility of sub-thermally excited widespread gas in the neighborhood of denser regions, which causes the flatter star formation law.

Interferometric SAR Observation of Permafrost Status in the Northern Qinghai-Tibet Plateau by ALOS, ALOS-2 and Sentinel-1 between 2007 and 2021

With global warming, permafrost is undergoing degradation, which may cause thawing subsidence, collapse, and emission of greenhouse gases preserved in previously frozen permafrost, change the local hydrology and ecology system, and threaten infrastructure and indigenous communities. The Qinghai-Tibet Plateau (QTP) is the world’s largest permafrost region in the middle and low latitudes. Permafrost status monitoring in the QTP is of great significance to global change and local economic development. In this study, we used 66 scenes of ALOS data (2007–2009), 73 scenes of ALOS-2 data (2015–2020) and 284 scenes of Sentinel-1 data (2017–2021) to evaluate the spatial and temporal permafrost deformation over the 83,000 km2 in the northern QTP, passing through the Tuotuohe, Beiluhe, Wudaoliang and Xidatan regions. We use the SBAS-InSAR method and present a coherence weighted least squares estimator without any hypothetical model to calculate long-term deformation velocity (LTDV) and maximum seasonal deformation (MSD) without any prior knowledge. Analysis of the ALOS results shows that the LTDV ranged from −20 to +20 mm/year during 2007–2009. For the ALOS-2 and Sentinel-1 results, the LTDV ranged from −30 to 30 mm/year during 2015–2021. Further study shows that the expansion areas of permafrost subsidence are concentrated on braided stream plains and thermokarst lakes. In these areas, due to glacial erosion, surface runoff and river alluvium, the contents of water and ground ice are sufficient, which could accelerate permafrost subsidence. In addition, by analyzing LTDV and MSD for the different periods, we found that the L-band ALOS-2 is more sensitive to the thermal collapse of permafrost than the C-band sensor and the detected collapse areas (LTDV < −10 mm/year) are consistent with the GF-1/2 thermal collapse dataset. This research indicates that the InSAR technique could be crucial for monitoring the evolution of permafrost and freeze-thaw disasters.

Implications of Multiple Corona Bursts in Lightning Processes for Radio Frequency Interferometer Observations

AbstractRecent observations from LOFAR indicate that multiple, spatially distributed corona bursts can occur in lightning processes with a timescale of 10 microseconds. The close proximity of the corona bursts in space and time poses a great observation challenge for short‐baseline (typically ≤100 m) radio interferometers. This paper reports simulations to show the interferometry results that would be obtained with such an interferometer. In particular, spatially‐separated corona bursts at fixed locations may be seen as a fast (>107 m/s) propagating source with large power variation if the resolution of the instrument is greater than the spatial separation of the bursts. The implications and suggestions for lightning interferometry studies are discussed in the paper.

The Energetics of the Central Engine in the Powerful Quasar 3C 298

Abstract The compact steep-spectrum radio source 3C 298 (redshift of 1.44) has the largest 178 MHz luminosity in the Third Cambridge Revised Catalogue (3CR); its radio lobes are among the most luminous in the universe. The plasma state of the radio lobes is modeled with the aid of interferometric radio observations (in particular, the new Low Frequency Array observation and archival MERLIN data) and archival single-station data. It is estimated that the long-term time-averaged jet power required to fill these lobes with leptonic plasma is Q ¯ ≈ 1.28 ± 0.51 × 10 47 erg s − 1 , rivaling the largest time-averaged jet powers from any quasar. Supporting this notion of extraordinary jet power is a 0.5–10 keV luminosity of ≈ 5.2 × 1046 erg s−1, comparable to luminous blazars, yet there is no other indication of strong relativistic beaming. We combine two new high signal-to-noise ratio optical spectroscopic observations from the Hobby-Eberly Telescope with archival Hubble Space Telescope, Two Micron All Sky Survey, and Galaxy Evolutionary Explorer data to compute a bolometric luminosity from the accretion flow of L bol ≈ 1.55 ± 0.15 × 1047 erg s−1. The ratio, Q ¯ / L bol ≈ 1 , is the approximate upper limit for quasars. Characteristic of a large Q ¯ / L bol , we find an extreme-ultraviolet (EUV) spectrum that is very steep (the “EUV deficit” of powerful radio quasars relative to radio-quiet quasars), and this weak ionizing continuum is likely a contributing factor to the relatively small equivalent widths of the broad emission lines in this quasar.

Constraints on the abundance of supermassive primordial black holes from lensing of compact radio sources

ABSTRACT The possibility that primordial black holes (PBHs) form a part of dark matter has been considered over a wide mass range from the Planck mass ($10^{-5}~\rm g$) to the level of the supermassive black hole in the center of the galaxy. Primordial origin might be one of the most important formation channel of supermassive black holes. We use the non-detection of lensing effect of very long baseline interferometer observations of compact radio sources with extremely high angular resolution as a promising probe to constrain the abundance of intergalactic PBHs in the mass range ∼104–109 M⊙. For a sample of well-measured 543 flat-spectrum compact radio sources, no milli-lensed images are found with angular separations between 1.5 and 50 milli-arcsec. From this null search result, we derive that the fraction of dark matter made up of supermassive PBHs in the mass range ∼106–108 M⊙ is $\lesssim 1.48{{\ \rm per\, cent}}$ at $95{{\ \rm per\, cent}}$ confidence level. This constraints would be significantly improved due to the rapid increase of the number of measured compact radio sources. For instance, on the basis of none confirmed milli-lensing candidate in the latest ∼14 000 sources, we derive the abundance of supermassive PBHs and obtain that it is $\lesssim 0.06{{\ \rm per\, cent}}$ at $95{{\ \rm per\, cent}}$ confidence level.

Analysis of orbital configurations for Millimetron space observatory

In this contribution a primary feasibility study of different orbital configurations for Millimetron space observatory is presented. Priority factors and limitations were considered by which it is possible to assess the capabilities of a particular orbit. It included technical and scientific capabilities of each orbit regarding the fuel costs, satellite observability, the quality of very long baseline interferometric (VLBI) imaging observations and source visibilities.

Probing the low-mass end of the companion mass function for O-type stars

Context. Past observations of O-type stars in the Galaxy have shown that almost all massive stars are part of a binary or higher-order multiple system. Given the wide range of separations at which these companions are found, several observational techniques have been adopted to characterize them. Despite the recent advancements in interferometric and adaptive optics observations, contrasts greater than 4 in the H band have never been reached between 100 and 1000 mas. Aims. Using new adaptive optics (AO) assisted coronagraphic observations, we aim to study the multiplicity properties of a sample of 18 dwarf (or sub-giant) O stars in the galactic field and in OB associations to probe the existence of stellar companions in the angular separation range from 0.″15 to 6″ down to very low mass ratios. Methods. We used VLT/SPHERE to observe simultaneously with the IRDIS and IFS sub-systems 18 O-type stars within 6 kpc and ages between 1 and 5 Myr. The IFS YJH band observations have allowed us to probe the presence of sub-solar companions in a 1.7″ × 1.7″ field-of-view down to magnitude limits of ΔH = 10 at 0.″4. In the wider 12″ × 12″ IRDIS field-of-view, we reached contrasts of ΔK = 12 at 1″, enabling us to look for even fainter companions at larger angular separations and to probe the source density of the surrounding portion of the sky. Results. This paper presents five newly discovered intermediate (< 1″) separation companions, three of which are smaller than 0.2 M⊙. If confirmed by future analyses of proper motions, these new detections represent the lowest-mass companions ever found around O-type stars. Additionally, 29 other sources are found in the IRDIS field-of-view with spurious association probabilities smaller than 5%. Assuming that all sources detected within 1″ are physically bound companions, the observed (uncorrected for bias) fraction of companions for O-type stars between 150 and 900 mas is 0.39 ± 0.15, whereas it increases to 1.6 ± 0.3 in the separation range from 0.″9 to 6″. Conclusions. These findings clearly support the notion that massive stars form almost exclusively in multiple systems, serving as proof of concept that supports the application of larger AO-assisted coronagraphic surveys as a crucial step in placing constraints on the multiplicity properties of massive star companions in regions of the parameter space that have previously gone unexplored. These results also demonstrate that the companion mass function is populated down to the lowest stellar masses.

Source Parameters and Slip Distribution of the 2019 Mw 5.8 Mirpur (Pakistan) Earthquake Inferred from the Corrected InSAR Observations

AbstractOn 24 September 2019, an Mw 5.8 earthquake occurred on the Mangla-Samwal anticline near Mirpur city, Pakistan. Because the seismogenic fault is hidden and the near-field seismic data are scarce, the magnitude and slip distribution of this earthquake are not determined. Furthermore, due to small deformation and significant atmospheric noise in the coseismic interferograms, it is difficult to accurately determine source parameters and slip distribution using the original single ascending and descending Interferometric Synthetic Aperture Radar observations. In this article, we used Sentinel-1A satellite Synthetic Aperture Radar data to generate 60 ascending and 56 descending coseismic interferograms and performed atmospheric error correction including Kriging interpolation correction and displacement stacking to obtain more accurate coseismic displacement fields for the 2019 Mw 5.8 Mirpur earthquake. Considering the local geological structures, focal mechanism solutions, and the optimal fault parameters obtained from the coseismic displacement inversion, we determine that the strike and dip of the fault of the 2019 Mirpur earthquake are 296.3° and 4.0°, respectively. Based on this fault model, the fault plane was extended to 12.0 km long and 11.0 km wide and divided into subfaults of 1.0×1.0 km2. We inverted the coseismic displacement fields to obtain slip distribution on the fault plane. Slip is mainly distributed at a depth of 4.7 ∼ 5.0 km, with a maximum of 0.95 m at a depth of 4.88 km. The slip distribution shows that this earthquake was a thrust event with right-lateral strike-slip components, which is consistent with the focal mechanism solution from the U.S. Geological Survey. The estimated geodetic moment is about 7.14×1017 N·m, equivalent to Mw 5.8.

Misalignments in NGC 1068

NGC 1068 is a nearby Active Galactic Nuclei (AGN) of type 2, meaning that its accretion disk is hidden behind a large amount of foreground extinction. Observations at several wavelengths have revealed various disk-like structures around the nucleus, all possibly part of the putative torus responsible for the obscuration of the AGN. I present results based on a GRAVITY/VLTI interferometric observation in the near-infrared, which provides very high angular resolution and gives insights into the geometry of the innermost region of this torus. The 3D orientation of the structure is surprising in several aspects, as it is misaligned with other disks present around the nucleus, and leaves a clear line of sight toward the central source.

Dark photon bursts from compact binary systems and constraints

In this work, we consider the burst signal of the dark photon, the hypothetical vector boson of the $U(1)_B$ or $U(1)_{B-L}$ gauge group, generated by a compact binary star system. The absence of the signal in the laser interferometer puts bounds on the coupling constant $\epsilon$ to the ordinary matter. It turns out that if the dark photon is massless, $\epsilon^2$ is on the order of $10^{-37}-10^{-33}$ at most; in the massive case, the upper bound of $\epsilon^2$ is about $10^{-38}-10^{-31}$ in the mass range from $10^{-19}$ eV to $10^{-11}$ eV. These are the first bounds derived from the interferometer observations independent of the assumption of dark photons being dark matter.

First high resolution interferometric observation of a solar prominence with ALMA

ABSTRACT We present the first observation of a solar prominence at 84 − 116 GHz using the high resolution interferometric imaging of ALMA. Simultaneous observations in Hα from Białkaw Observatory and with SDO/AIA reveal similar prominence morphology to the ALMA observation. The contribution functions of 3 mm and Hα emission are shown to have significant overlap across a range of gas pressures. We estimate the maximum millimetre-continuum optical thickness to be τ3mm ≈ 2, and the brightness temperature from the observed Hα intensity. The brightness temperature measured by ALMA is ∼6000 – 7000 K in the prominence spine, which correlates well with the estimated brightness temperature for a kinetic temperature of 8000 K.