Background-Source separation in astronomical images with Bayesian Probability Theory

In this work a new method for the detection of faint, both point-like and extended, astronomical objects based on the integrated treatment of source and background signals is described. This technique is applied to public data obtained by imaging methods of high-energy observational astronomy in the X-ray spectral regime. These data are usually employed to address current astrophysical problems, e.g. in the fields of stellar and galaxy evolution and the large-scale structure of the universe. The typical problems encountered during the analysis of these data are: spatially varying cosmic background, large variety of source morphologies and intensities, data incompleteness, steep gradients in the data, and few photon counts per pixel. These problems are addressed with the developed technique. Previous methods extensively employed for the analysis of these data are, e.g., the sliding window and the wavelet based techniques. Both methods are known to suffer from: describing large variations in the background, detection of faint and extended sources and sources with complex morphologies. Large systematic errors in object photometry and loss of faint sources may occur with these techniques. The developed algorithm is based on Bayesian probability theory, which is a consistent probabilistic tool to solve an inverse problem for a given state of information. The information is given by a parameterized model for the background and prior information about source intensity distributions quantified by probability distributions. For the background estimation, the image data are not censored. The background rate is described by a two-dimensional thin-plate spline function. The background model is given by the product of the background rate and the exposure time which accounts for the variations of the integration time. Therefore, the background as well as effects like vignetting, variations of detector quantum efficiency and strong gradients in the exposure time are being handled properly which results in improved detections with respect to previous methods. Source probabilities are provided for individual pixels as well as for correlations of neighboring pixels in a multi-resolution analysis. Consequently, the technique is able of detecting point-like and extended sources and their complex morphologies. Furthermore, images of different spectral bands can be combined probabilistically to further increase the resolution in crowded regions. The developed method characterizes all detected sources in terms of position, number of source counts, and shape including uncertainties. The comparison with previous techniques shows that the developed method allows for an improved determination of background and source parameters. The method is applied to data obtained by the ROSAT and Chandra X-ray observatories whereas particularly the detection of faint and extended sources is improved with respect to previous analyses. This lead to the discovery of new galaxy clusters and quasars in the X-ray band which are confirmed in the optical regime using additional observational data. The new technique developed in this work is particularly suited to the identification of objects featuring extended emission like clusters of galaxies

3D Detection and Characterisation of ALMA Sources through Deep Learning

We present a Deep-Learning (DL) pipeline developed for the detection and characterization of astronomical sources within simulated Atacama Large Millimeter/submillimeter Array (ALMA) data cubes. The pipeline is composed of six DL models: a Convolutional Autoencoder for source detection within the spatial domain of the integrated data cubes, a Recurrent Neural Network (RNN) for denoising and peak detection within the frequency domain, and four Residual Neural Networks (ResNets) for source characterization. The combination of spatial and frequency information improves completeness while decreasing spurious signal detection. To train and test the pipeline, we developed a simulation algorithm able to generate realistic ALMA observations, i.e. both sky model and dirty cubes. The algorithm simulates always a central source surrounded by fainter ones scattered within the cube. Some sources were spatially superimposed in order to test the pipeline deblending capabilities. The detection performances of the pipeline were compared to those of other methods and significant improvements in performances were achieved. Source morphologies are detected with subpixel accuracies obtaining mean residual errors of $10^{-3}$ pixel ($0.1$ mas) and $10^{-1}$ mJy/beam on positions and flux estimations, respectively. Projection angles and flux densities are also recovered within $10\%$ of the true values for $80\%$ and $73\%$ of all sources in the test set, respectively. While our pipeline is fine-tuned for ALMA data, the technique is applicable to other interferometric observatories, as SKA, LOFAR, VLBI, and VLTI

Bayesian and Machine Learning Methods in the Big Data era for astronomical imaging

The Atacama Large Millimeter/submillimeter Array with the planned electronic upgrades will deliver an unprecedented amount of deep and high resolution observations. Wider fields of view are possible with the consequential cost of image reconstruction. Alternatives to commonly used applications in image processing have to be sought and tested. Advanced image reconstruction methods are critical to meet the data requirements needed for operational purposes. Astrostatistics and astroinformatics techniques are employed. Evidence is given that these interdisciplinary fields of study applied to synthesis imaging meet the Big Data challenges and have the potentials to enable new scientific discoveries in radio astronomy and astrophysics.Comment: 8 pages, 5 figures, proceedings International Workshop on Bayesian Inference and Maximum Entropy Methods in Science and Engineering, IHP, Paris, July 18-22, 202

A BRAIN study to tackle image analysis with artificial intelligence in the ALMA 2030 era

An ESO internal ALMA development study, BRAIN, is addressing the ill-posed inverse problem of synthesis image analysis employing astrostatistics and astroinformatics. These emerging fields of research offer interdisciplinary approaches at the intersection of observational astronomy, statistics, algorithm development, and data science. In this study, we provide evidence of the benefits of employing these approaches to ALMA imaging for operational and scientific purposes. We show the potential of two techniques, RESOLVE and DeepFocus, applied to ALMA calibrated science data. Significant advantages are provided with the prospect to improve the quality and completeness of the data products stored in the science archive and overall processing time for operations. Both approaches evidence the logical pathway to address the incoming revolution in data rates dictated by the planned electronic upgrades. Moreover, we bring to the community additional products through a new package, ALMASim, to promote advancements in these fields, providing a refined ALMA simulator usable by a large community for training and/or testing new algorithms.Comment: 9 pages, 5 figures, MaxEnt2023 conferenc

The Second-Generation Guide Star Catalog: Description and Properties

The GSC-II is an all-sky database of objects derived from the uncompressed DSS that the STScI has created from the Palomar and UK Schmidt survey plates and made available to the community. Like its predecessor (GSC-I), the GSC-II was primarily created to provide guide star information and observation planning support for HST. This version, however, is already employed at some of the ground-based new-technology telescopes such as GEMINI, VLT, and TNG, and will also be used to provide support for the JWST and Gaia space missions as well as LAMOST, one of the major ongoing scientific projects in China. Two catalogs have already been extracted from the GSC-II database and released to the astronomical community. A magnitude-limited (R=18.0) version, GSC2.2, was distributed soon after its production in 2001, while the GSC2.3 release has been available for general access since 2007. The GSC2.3 catalog described in this paper contains astrometry, photometry, and classification for 945,592,683 objects down to the magnitude limit of the plates. Positions are tied to the ICRS; for stellar sources, the all-sky average absolute error per coordinate ranges from 0.2" to 0.28" depending on magnitude. When dealing with extended objects, astrometric errors are 20% worse in the case of galaxies and approximately a factor of 2 worse for blended images. Stellar photometry is determined to 0.13-0.22 mag as a function of magnitude and photographic passbands (B,R,I). Outside of the galactic plane, stellar classification is reliable to at least 90% confidence for magnitudes brighter than R=19.5, and the catalog is complete to R=20.Comment: 52 pages, 33 figures, to be published in AJ August 200

Gamma-ray spectroscopy of positron annihilation in the Milky Way

T. Siegert, et al., “Gamma-ray spectroscopy of positron annihilation in the Milky Way”, Astronomy & Astrophysics, Vol. 586, January 2016, https://doi.org/10.1051/0004-6361/201527510. Reproduced with permission from Astronomy & Astrophysics, © ESO”.Context. The annihilation of positrons in the Galaxy's interstellar medium produces characteristic gamma-rays with a line at 511 keV. This gamma-ray emission has been observed with the spectrometer SPI on ESA's INTEGRAL observatory, confirming a puzzling morphology with bright emission from an extended bulge-like region, while emission from the disk is faint. Most known or plausible sources of positrons are, however, believed to be distributed throughout the disk of the Milky Way. Aims: We aim to constrain characteristic spectral shapes for different spatial components in the disk and bulge using data with an exposure that has doubled since earlier reports. Methods: We exploit high-resolution gamma-ray spectroscopy with SPI on INTEGRAL based on a new instrumental background method and detailed multi-component sky model fitting. Results: We confirm the detection of the main extended components of characteristic annihilation gamma-ray signatures, altogether at 58σ significance in the 511 keV line. The total Galactic 511 keV line intensity amounts to (2.74 ± 0.25) × 10-3 ph cm-2 s-1 for our assumed model of the spatial distribution. We derive spectra for the bulge and disk, and a central source modelled as point-like and at the position of Sgr A*, and discuss spectral differences. The bulge (56σ) shows a 511 keV line intensity of (0.96 ± 0.07) × 10-3 ph cm-2 s-1 together with ortho-positronium continuum equivalent to a positronium fraction of (1.080 ± 0.029). The two-dimensional Gaussian that represents the disk emission (12σ) has an extent of 60+10-5 degrees in longitude and a rather large latitudinal extent of 10.5+2.5-1.5 degrees; the line intensity is (1.66 ± 0.35) × 10-3 ph cm-2 s-1 with a marginal detection of the annihilation continuum and an overall diffuse Galactic continuum of (5.85 ± 1.05) × 10-5 ph cm-2 s-1 keV-1 at 511 keV. The disk shows no flux asymmetry between positive and negative longitudes, although spectral details differ. The flux ratio between bulge and disk is (0.58 ± 0.13). The central source (5σ) has an intensity of (0.80 ± 0.19) × 10-4 ph cm-2 s-1.Peer reviewe

Positron annihilation signatures associated with the outburst of the microquasar V404 Cygni

This document is the Accepted Manuscript version of the following article: Thomas Siegert, et al, ‘Positron annihilation signatures associated with the outburst of the microquasar V404 Cygni’, Nature: International Journal of Science, Vol. 531: 341-343, March 2016, DOI: https://doi.org/10.1038/nature16978. Content in the UH Research Archive is made available for personal research, educational, and non-commercial purposes only. Unless otherwise stated, all content is protected by copyright, and in the absence of an open license, permissions for further re-use should be sought from the publisher, the author, or other copyright holder.Microquasars1, 2, 3, 4 are stellar-mass black holes accreting matter from a companion star5 and ejecting plasma jets at almost the speed of light. They are analogues of quasars that contain supermassive black holes of 106 to 1010 solar masses. Accretion in microquasars varies on much shorter timescales than in quasars and occasionally produces exceptionally bright X-ray flares6. How the flares are produced is unclear, as is the mechanism for launching the relativistic jets and their composition. An emission line near 511 kiloelectronvolts has long been sought in the emission spectrum of microquasars as evidence for the expected electron–positron plasma. Transient high-energy spectral features have been reported in two objects7, 8, but their positron interpretation9 remains contentious. Here we report observations of γ-ray emission from the microquasar V404 Cygni during a recent period of strong flaring activity10. The emission spectrum around 511 kiloelectronvolts shows clear signatures of variable positron annihilation, which implies a high rate of positron production. This supports the earlier conjecture that microquasars may be the main sources of the electron–positron plasma responsible for the bright diffuse emission of annihilation γ-rays in the bulge region of our Galaxy11. Additionally, microquasars could be the origin of the observed megaelectronvolt continuum excess in the inner Galaxy.Peer reviewe

Search for 511 keV Emission in Satellite Galaxies of the Milky Way with INTEGRAL/SPI

Reproduced with permission from Astronomy & Astrophysics. © 2018 ESO.The positron annihilation gamma-ray signal in the Milky Way (MW) shows a puzzling morphology: a very bright bulge and a very low surface-brightness disk. A coherent explanation of the positron origin, propagation through the Galaxy and subsequent annihilation in the interstellar medium has not yet been found. Tentative explanations involve positrons from radioactivity, X-ray binaries, and dark matter (DM). Dwarf satellite galaxies (DSGs) are believed to be DM-dominated and hence promising candidates in the search for 511 keV emission as a result of DM annihilation into electron-positron pairs. The goal of this study is to constrain possible 511 keV gamma-ray signals from 39 DSGs of the MW and to test the annihilating DM scenario. We use the spectrometer SPI on INTEGRAL to extract individual spectra for the studied objects. As the diffuse galactic emission dominates the signal, the large scale morphology of the MW has been modelled accordingly and was included in a maximum likelihood analysis. Alternatively, a distance-weighted stacked spectrum has been determined. Only Reticulum II (Ret II) shows a 3.1 sigma signal. Five other sources show tentative 2 sigma signals. The mass-to-511-keV-luminosity-ratio shows a marginal trend towards higher values for intrinsically brighter objects, opposite to the V band mass-to-light-ratio, which is generally used to uncover DM in DSGs. All derived flux values are above the level implied by a DM interpretation of the MW bulge signal. The signal from Ret II is unlikely to be related to a DM origin alone, otherwise, the MW bulge would be about 100 times brighter than what is seen. Ret II is exceptional considering the DSG sample, and rather points to enhanced recent star formation activity, if its origins are similar to processes in the MW. Understanding this emission may provide further clues regarding the origin of the annihilation emission in the MW.Peer reviewe