- 标题
- 摘要
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- 实验方案
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Denoising, deconvolving, and decomposing multi-domain photon observations
摘要： Astronomical imaging based on photon count data is a non-trivial task. In this context we show how to denoise, deconvolve, and decompose multi-domain photon observations. The primary objective is to incorporate accurate and well motivated likelihood and prior models in order to give reliable estimates about morphologically different but superimposed photon flux components present in the data set. Thereby we denoise and deconvolve photon counts, while simultaneously decomposing them into diffuse, point-like and uninteresting background radiation fluxes. The decomposition is based on a probabilistic hierarchical Bayesian parameter model within the framework of information field theory (IFT). In contrast to its predecessor D3PO, D4PO reconstructs multi-domain components. Thereby each component is defined over its own direct product of multiple independent domains, for example location and energy. D4PO has the capability to reconstruct correlation structures over each of the sub-domains of a component separately. Thereby the inferred correlations implicitly define the morphologically different source components, except for the spatial correlations of the point-like flux. Point-like source fluxes are spatially uncorrelated by definition. The capabilities of the algorithm are demonstrated by means of a synthetic, but realistic, mock data set, providing spectral and spatial information about each detected photon. D4PO successfully denoised, deconvolved, and decomposed a photon count image into diffuse, point-like and background flux, each being functions of location as well as energy. Moreover, uncertainty estimates of the reconstructed fields as well as of their correlation structure are provided employing their posterior density function and accounting for the manifolds the domains reside on.
关键词： gamma rays: general,methods: data analysis,methods: statistical,X-rays: general,methods: numerical,techniques: image processing
更新于2025-09-23 15:23:52
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An image reconstruction method (IRBis) for optical/infrared interferometry
摘要： Aims.We present an image reconstruction method for optical/infrared long-baseline interferometry called IRBis (image reconstruction software using the bispectrum). We describe the theory and present applications to computer-simulated interferograms. Methods. The IRBis method can reconstruct an image from measured visibilities and closure phases. The applied optimization routine ASA_CG is based on conjugate gradients. The method allows the user to implement different regularizers, apply residual ratios as an additional metric for goodness-of-fit, and use previous iteration results as a prior to force convergence. Results. We present the theory of the IRBis method and several applications of the method to computer-simulated interferograms. The image reconstruction results show the dependence of the reconstructed image on the noise in the interferograms (e.g., for ten electron read-out noise and 139 to 1219 detected photons per interferogram), the regularization method, the angular resolution, and the reconstruction parameters applied. Furthermore, we present the IRBis reconstructions submitted to the interferometric imaging beauty contest 2012 initiated by the IAU Working Group on Optical/IR Interferometry and describe the performed data processing steps.
关键词： techniques: high angular resolution,methods: data analysis,instrumentation: interferometers,techniques: interferometric,methods: numerical,techniques: image processing
更新于2025-09-23 15:23:52
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Optical polarisation variability of radio-loud narrow-line Seyfert 1 galaxies
摘要： Context. Narrow-line Seyfert 1 galaxies (NLSy1s) constitute the active galactic nuclei subclass associated with systematically lower black hole masses. A few radio-loud NLSy1s have been detected in MeV–GeV energy bands by Fermi, and evidence that blazar-like jets are operating also in radio-loud NLSy1s, has been accumulated. Aims. We wish to quantify the temporal behaviour of the optical polarisation, fraction, and angle for a selected sample of radio-loud NLSy1s. We also search for rotations of the polarisation plane similar to those commonly observed in blazars. Methods. We have conducted R-band optical linear polarisation monitoring of a sample of ten radio-loud NLSy1 galaxies; five of them have previously been detected by Fermi. The dataset obtained with our pivoting instrument, the RoboPol polarimeter of the Skinakas observatory, has been complemented with observations from the KANATA, Perkins, and Steward observatories. When evidence for long rotations of the polarisation plane was found (at least three consecutive measurements covering at least 90?), we carried out numerical simulations to assess the probability that they are caused by intrinsically evolving electric vector position angles (EVPAs) instead of observational noise. Results. Even our moderately sampled sources show clear indications of variability in both polarisation fraction and angle. For the four best-sampled objects in our sample we find multiple periods of significant polarisation angle variability. Several of these events qualify as long rotations. In the two best-sampled cases, namely J1505+0326 and J0324+3410, we find indications for three long rotations of the polarisation angle. We show that although noise can induce the observed behaviour, it is much more likely that the apparent rotation is indeed caused by intrinsic evolution of the EVPA. To our knowledge, this is the very first detection of such events in this class of sources. In the largest dataset (J0324+3410), we find that the EVPA concentrates around a direction that is at 49.3? to the 15 GHz radio jet, implying a projected magnetic field at an angle of 40.7? to that axis. Conclusions. We assess the probability that pure measurement uncertainties are the reason behind the observed long rotations of the polarisation plane. We conclude that although this is not improbable, it is much more likely that intrinsic rotations are responsible for the observed phenomenology. We conclude, however, that much better sampled and larger datasets of larger source samples are necessary to constrain the physical mechanism(s) that generate long EVPA rotations in NLSy1s.
关键词： galaxies: active,methods: statistical,galaxies: Seyfert,methods: numerical,polarization,techniques: polarimetric
更新于2025-09-23 15:23:52
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Rotationally resolved spectroscopy of (20000) Varuna in the near-infrared
摘要： Context. Models of the escape and retention of volatiles by minor icy objects exclude any presence of volatile ices on the surface of trans-Neptunian objects (TNOs) smaller than ～1000 km in diameter at the typical temperature in this region of the solar system, whereas the same models show that water ice is stable on the surface of objects over a wide range of diameters. Collisions and cometary activity have been used to explain the process of surface refreshing of TNOs and Centaurs. These processes can produce surface heterogeneity that can be studied by collecting information at di?erent rotational phases. Aims. The aims of this work are to study the surface composition of (20000) Varuna, a TNO with a diameter 668+154 ?86 km and to search for indications of rotational variability. Methods. We observed (20000) Varuna during two consecutive nights in January 2011 with the near-infrared camera and spectrometer NICS at the Telescopio Nazionale Galileo, La Palma, Spain. We used the low resolution mode with the AMICI prism to obtain a set of spectra covering the whole rotation period of the Varuna (Pr = 6.34 h). We ?t the resulting relative re?ectance with radiative transfer models of the surface of atmosphereless bodies. Results. After studying the spectra corresponding to di?erent rotational phases of Varuna, we did not ?nd any indication of surface variability at 2σ level. In all the spectra, we detect an absorption at 2.0 μm, suggesting the presence of water ice on the surface. We do not detect any other volatiles on the surface, although the signal-to-noise ratio is not high enough to discard their presence in small quantities. Based on scattering models, we present two possible compositions compatible with our set of data and discuss their implications in the framework of the collisional history of the trans-Neptunian belt. Conclusions. We ?nd that the most probable composition for the surface of Varuna is a mixture of amorphous silicates, complex organics, and water ice. This composition is compatible with all the materials being primordial, so no replenishment mechanism is needed in the equation. However, our data can also be ?tted by models containing up to a 10% of methane ice. For an object with the characteristics of Varuna, this volatile could not be primordial, so an event, such as an energetic impact, would be needed to explain its presence on the surface.
关键词： Kuiper belt objects: individual: (2000) Varuna,planets and satellites: composition,methods: numerical,techniques: spectroscopic,methods: observational
更新于2025-09-23 15:23:52
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Beyond the diffraction limit of optical/IR interferometers
摘要： Context. As previously demonstrated on Achernar, one can derive the angular radius, rotational velocity, axis tilt, and orientation of a fast-rotating star from the differential phases obtained by spectrally resolved long baseline interferometry using earth-rotation synthesis. Aims. We applied this method on a small sample of stars for different spectral types and classes, in order to generalize the technique to other rotating stars across the H-R diagram and determine their fundamental parameters. Methods. We used differential phase data from the AMBER/VLTI instrument obtained prior to refurbishing its spectrometer in 2010. With the exception of Fomalhaut, which has been observed in the medium-resolution mode of AMBER (λ/δλ ≈ 1500), our three other targets, Achernar, Altair, and δ Aquilae offered high-resolution (λ/δλ ≈ 12 000) spectro-interferometric data around the Brγ absorption line in K band. These data were used to constrain the input parameters of an analytical, still realistic model to interpret the observations with a systematic approach for the error budget analysis in order to robustly conclude on the physics of our 4 targets. We applied the super resolution provided by differential phases φdiff to measure the size (equatorial radius Req and angular diameter /(cid:13)eq), the equatorial rotation velocity (Veq), the inclination angle (i), and the rotation axis position angle (PArot) of 4 fast-rotating stars: Achernar, Altair, δ Aquilae, and Fomalhaut. The stellar parameters of the targets were constrained using a semi-analytical algorithm dedicated to fast rotators SCIROCCO. Results. The derived parameters for each star were Req = 11.2 ± 0.5 R(cid:12), Veq sin i = 290 ± 17 km s?1, PArot = 35.4? ± 1.4?, for Achernar; Req = 2.0 ± 0.2 R(cid:12), Veq sin i = 226 ± 34 km s?1, PArot = ?65.5? ± 5.5?, for Altair; Req = 2.2 ± 0.3 R(cid:12), Veq sin i = 74 ± 35 km s?1, PArot = ?101.2? ± 14?, for δ Aquilae; and Req = 1.8 ± 0.2 R(cid:12), Veq sin i = 93 ± 16 km s?1, PArot = 65.6? ± 5?, for Fomalhaut. They were found to be compatible with previously published values from differential phase and visibility measurements, while we were able to determine, for the first time, the inclination angle i of Fomalhaut (i = 90? ± 9?) and δ Aquilae (i = 81? ± 13?), and the rotation-axis position angle PArot of δ Aquilae. Conclusions. Beyond the theoretical diffraction limit of an interferometer (ratio of the wavelength to the baseline), spatial super resolution is well suited to systematically estimating the angular diameters of rotating stars and their fundamental parameters with a few sets of baselines and the Earth-rotation synthesis provided a high enough spectral resolution.
关键词： stars: rotation,stars: general,techniques: high angular resolution,techniques: interferometric,methods: numerical,methods: observational
更新于2025-09-23 15:23:52
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Estimating the mirror seeing for a large optical telescope with a numerical method
摘要： It is widely accepted that mirror seeing is caused by turbulent fluctuations in the index of air refraction in the vicinity of a telescope mirror. Computational Fluid Dynamics (CFD) is a useful tool to evaluate the effects of mirror seeing. In this paper, we present a numerical method to estimate the mirror seeing for a large optical telescope (～ 4 m) in cases of natural convection with the ANSYS ICEPAK software. We get the FWHM of the image for different inclination angles (i) of the mirror and different temperature differences (?T ) between the mirror and ambient air. Our results show that the mirror seeing depends very weakly on i, which agrees with observational data from the Canada-France-Hawaii Telescope. The numerical model can be used to estimate mirror seeing in the case of natural convection although with some limitations. We can determine ?T for thermal control of the primary mirror according to the simulation, empirical data and site seeing.
关键词： astronomical instrumentation,methods: numerical,telescopes
更新于2025-09-23 15:23:52
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Photometric redshifts for the Kilo-Degree Survey
摘要： We present a machine-learning photometric redshift (ML photo-z) analysis of the Kilo-Degree Survey Data Release 3 (KiDS DR3), using two neural-network based techniques: ANNz2 and MLPQNA. Despite limited coverage of spectroscopic training sets, these ML codes provide photo-zs of quality comparable to, if not better than, those from the Bayesian Photometric Redshift (BPZ) code, at least up to zphot ? 0.9 and r ? 23.5. At the bright end of r ? 20, where very complete spectroscopic data overlapping with KiDS are available, the performance of the ML photo-zs clearly surpasses that of BPZ, currently the primary photo-z method for KiDS. Using the Galaxy And Mass Assembly (GAMA) spectroscopic survey as calibration, we furthermore study how photo-zs improve for bright sources when photometric parameters additional to magnitudes are included in the photo-z derivation, as well as when VIKING and WISE infrared (IR) bands are added. While the fiducial four-band ugri setup gives a photo-z bias ?δz/(1+z)? = ?2×10?? and scatter σδz/(1+z) < 0.022 at mean z ～ 0.23, combining magnitudes, colours, and galaxy sizes reduces the scatter by ～7% and the bias by an order of magnitude. Once the ugri and IR magnitudes are joined into 12-band photometry spanning up to 12 μm, the scatter decreases by more than 10% over the fiducial case. Finally, using the 12 bands together with optical colours and linear sizes gives ?δz/(1+z)? < 4×10?? and σδz/(1+z) < 0.019. This paper also serves as a reference for two public photo-z catalogues accompanying KiDS DR3, both obtained using the ANNz2 code. The first one, of general purpose, includes all the 39 million KiDS sources with four-band ugri measurements in DR3. The second dataset, optimised for low-redshift studies such as galaxy-galaxy lensing, is limited to r ? 20, and provides photo-zs of much better quality than in the full-depth case thanks to incorporating optical magnitudes, colours, and sizes in the GAMA-calibrated photo-z derivation.
关键词： methods: statistical,large-scale structure of Universe,galaxies: distances and redshifts,catalogs,methods: data analysis,methods: numerical
更新于2025-09-23 15:22:29
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Simulating charge transport to understand the spectral response of Swept Charge Devices
摘要： Context. Swept Charge Devices (SCD) are novel X-ray detectors optimized for improved spectral performance without any demand for active cooling. The Chandrayaan-1 X-ray Spectrometer (C1XS) experiment onboard the Chandrayaan-1 spacecraft used an array of SCDs to map the global surface elemental abundances on the Moon using the X-ray fluorescence (XRF) technique. The successful demonstration of SCDs in C1XS spurred an enhanced version of the spectrometer on Chandrayaan-2 using the next-generation SCD sensors. Aims. The objective of this paper is to demonstrate validation of a physical model developed to simulate X-ray photon interaction and charge transportation in a SCD. The model helps to understand and identify the origin of individual components that collectively contribute to the energy-dependent spectral response of the SCD. Furthermore, the model provides completeness to various calibration tasks, such as generating spectral matrices (RMFs – redistribution matrix files), estimating efficiency, optimizing event selection logic, and maximizing event recovery to improve photon-collection efficiency in SCDs. Methods. Charge generation and transportation in the SCD at different layers related to channel stops, field zones, and field-free zones due to photon interaction were computed using standard drift and diffusion equations. Charge collected in the buried channel due to photon interaction in different volumes of the detector was computed by assuming a Gaussian radial profile of the charge cloud. The collected charge was processed further to simulate both diagonal clocking read-out, which is a novel design exclusive for SCDs, and event selection logic to construct the energy spectrum. Results. We compare simulation results of the SCD CCD54 with measurements obtained during the ground calibration of C1XS and clearly demonstrate that our model reproduces all the major spectral features seen in calibration data. We also describe our understanding of interactions at different layers of SCD that contribute to the observed spectrum. Using simulation results, we identify the origin of different spectral features and quantify their contributions.
关键词： methods: numerical,X-rays: general,instrumentation: detectors
更新于2025-09-23 15:22:29
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Numerical problems in coupling photon momentum (radiation pressure) to gas
摘要： Radiation pressure (RP; or photon momentum absorbed by gas) is important in a tremendous range of astrophysical systems. But we show the usual method for assigning absorbed photon momentum to gas in numerical radiation-hydrodynamics simulations (integrating over cell volumes or evaluating at cell centres) can severely underestimate the RP force in the immediate vicinity around unresolved (point/discrete) sources (and subsequently underestimate its effects on bulk gas properties), unless photon mean free paths are highly resolved in the fluid grid. The existence of this error is independent of the numerical radiation transfer (RT) method (even in exact ray-tracing/Monte Carlo methods), because it depends on how the RT solution is interpolated back onto fluid elements. Brute-force convergence (resolving mean free paths) is impossible in many cases (especially where UV/ionizing photons are involved). Instead, we show a 'face-integrated' method – integrating and applying the momentum fluxes at interfaces between fluid elements – better approximates the correct solution at all resolution levels. The 'fix' is simple and we provide example implementations for ray-tracing, Monte Carlo, and moments RT methods in both grid and mesh-free fluid schemes. We consider an example of star formation in a molecular cloud with UV/ionizing RP. At state-of-the-art resolution, cell-integrated methods underestimate the net effects of RP by an order of magnitude, leading (incorrectly) to the conclusion that RP is unimportant, while face-integrated methods predict strong self-regulation of star formation and cloud destruction via RP.
关键词： galaxies: active,galaxies: formation,hydrodynamics,methods: numerical,stars: formation
更新于2025-09-23 15:22:29
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The optical morphologies of galaxies in the IllustrisTNG simulation: a comparison to Pan-STARRS observations
摘要： We have generated synthetic images of ～27 000 galaxies from the IllustrisTNG and the original Illustris hydrodynamic cosmological simulations, designed to match Pan-STARRS observations of log10(M?/M(cid:4)) ≈ 9.8–11.3 galaxies at z ≈ 0.05. Most of our synthetic images were created with the SKIRT radiative transfer code, including the effects of dust attenuation and scattering, and performing the radiative transfer directly on the Voronoi mesh used by the simulations themselves. We have analysed both our synthetic and real Pan-STARRS images with the newly developed statmorph code, which calculates non-parametric morphological diagnostics – including the Gini–M20 and concentration–asymmetry–smoothness statistics – and performs 2D S′ersic ?ts. Overall, we ?nd that the optical morphologies of IllustrisTNG galaxies are in good agreement with observations, and represent a substantial improvement compared to the original Illustris simulation. In particular, the locus of the Gini–M20 diagram is consistent with that inferred from observations, while the median trends with stellar mass of all the morphological, size and shape parameters considered in this work lie within the ～1σ scatter of the observational trends. However, the IllustrisTNG model has some dif?culty with more stringent tests, such as producing a strong morphology–colour relation. This results in a somewhat higher fraction of red discs and blue spheroids compared to observations. Similarly, the morphology–size relation is problematic: while observations show that discs tend to be larger than spheroids at a ?xed stellar mass, such a trend is not present in IllustrisTNG.
关键词： galaxies: statistics,galaxies: formation,galaxies: structure,methods: numerical,techniques: image processing
更新于2025-09-23 15:22:29