arXiv subjects

Alexander Roman

Publications and source records attributed to Alexander Roman.

25 records · Page 2Linked to original sources

arXiv2022

Transverse Vector Decomposition Method for Analytical Inversion of Exoplanet Transit Spectra

We develop a new method for analytical inversion of binned exoplanet transit spectra and for retrieval of planet parameters. The method has a geometrical interpretation and treats each observed spectrum as a single vector $\vec r$ in the multidimensional spectral space of observed bin values. We decompose the observed $\vec{r}$ into a wavelength-independent component $\vec{r}_\parallel$ corresponding to the spectral mean across all observed bins, and a transverse component $\vec{r}_\perp$ which is wavelength-dependent and contains the relevant information about the atmospheric chemistry. The method allows us to extract, without any prior assumptions or additional information, the relative mass (or volume) mixing ratios of the absorbers in the atmosphere, the scale height to stellar radius ratio, $H/R_S$, and the atmospheric temperature. The method is illustrated and validated with several examples of increasing complexity.

astro-ph.EP↗

arXiv2022

Uncertainties associated with GAN-generated datasets in high energy physics

Recently, Generative Adversarial Networks (GANs) trained on samples of traditionally simulated collider events have been proposed as a way of generating larger simulated datasets at a reduced computational cost. In this paper we point out that data generated by a GAN cannot statistically be better than the data it was trained on, and critically examine the applicability of GANs in various situations, including a) for replacing the entire Monte Carlo pipeline or parts of it, and b) to produce datasets for usage in highly sensitive analyses or sub-optimal ones. We present our arguments using information theoretic demonstrations, a toy example, as well as in the form of a formal statement, and identify some potential valid uses of GANs in collider simulations.

hep-ph↗

arXiv2022

Unsupervised Machine Learning for Exploratory Data Analysis of Exoplanet Transmission Spectra

Transit spectroscopy is a powerful tool to decode the chemical composition of the atmospheres of extrasolar planets. In this paper we focus on unsupervised techniques for analyzing spectral data from transiting exoplanets. We demonstrate methods for i) cleaning and validating the data, ii) initial exploratory data analysis based on summary statistics (estimates of location and variability), iii) exploring and quantifying the existing correlations in the data, iv) pre-processing and linearly transforming the data to its principal components, v) dimensionality reduction and manifold learning, vi) clustering and anomaly detection, vii) visualization and interpretation of the data. To illustrate the proposed unsupervised methodology, we use a well-known public benchmark data set of synthetic transit spectra. We show that there is a high degree of correlation in the spectral data, which calls for appropriate low-dimensional representations. We explore a number of different techniques for such dimensionality reduction and identify several suitable options in terms of summary statistics, principal components, etc. We uncover interesting structures in the principal component basis, namely, well-defined branches corresponding to different chemical regimes of the underlying atmospheres. We demonstrate that those branches can be successfully recovered with a K-means clustering algorithm in fully unsupervised fashion. We advocate for a three-dimensional representation of the spectroscopic data in terms of the first three principal components, in order to reveal the existing structure in the data and quickly characterize the chemical class of a planet.

astro-ph.EP↗

arXiv2021

Analytical Modelling of Exoplanet Transit Specroscopy with Dimensional Analysis and Symbolic Regression

The physical characteristics and atmospheric chemical composition of newly discovered exoplanets are often inferred from their transit spectra which are obtained from complex numerical models of radiative transfer. Alternatively, simple analytical expressions provide insightful physical intuition into the relevant atmospheric processes. The deep learning revolution has opened the door for deriving such analytical results directly with a computer algorithm fitting to the data. As a proof of concept, we successfully demonstrate the use of symbolic regression on synthetic data for the transit radii of generic hot Jupiter exoplanets to derive a corresponding analytical formula. As a preprocessing step, we use dimensional analysis to identify the relevant dimensionless combinations of variables and reduce the number of independent inputs, which improves the performance of the symbolic regression. The dimensional analysis also allowed us to mathematically derive and properly parametrize the most general family of degeneracies among the input atmospheric parameters which affect the characterization of an exoplanet atmosphere through transit spectroscopy.

astro-ph.EP↗

arXiv2021

Finding Wombling Boundaries in LHC Data with Voronoi and Delaunay Tessellations

We address the problem of finding a wombling boundary in point data generated by a general Poisson point process, a specific example of which is an LHC event sample distributed in the phase space of a final state signature, with the wombling boundary created by some new physics. We discuss the use of Voronoi and Delaunay tessellations of the point data for estimating the local gradients and investigate methods for sharpening the boundaries by reducing the statistical noise. The outcome from traditional wombling algorithms is a set of boundary cell candidates with relatively large gradients, whose spatial properties must then be scrutinized in order to construct the boundary and evaluate its significance. Here we propose an alternative approach where we simultaneously form and evaluate the significance of all possible boundaries in terms of the total gradient flux. We illustrate our method with several toy examples of both straight and curved boundaries with varying amounts of signal present in the data.

hep-ph↗

arXiv2016

Constraints on the FRB rate at 700-900 MHz

Estimating the all-sky rate of fast radio bursts (FRBs) has been difficult due to small-number statistics and the fact that they are seen by disparate surveys in different regions of the sky. In this paper we provide limits for the FRB rate at 800 MHz based on the only burst detected at frequencies below 1.4 GHz, FRB 110523. We discuss the difficulties in rate estimation, particularly in providing an all-sky rate above a single fluence threshold. We find an implied rate between 700-900 MHz that is consistent with the rate at 1.4 GHz, scaling to $6.4^{+29.5}_{-5.0} \times 10^3$\,sky$^{-1}$\,day$^{-1}$ for an HTRU-like survey. This is promising for upcoming experiments below a GHz like CHIME and UTMOST, for which we forecast detection rates. Given 110523's discovery at 32$σ$ with nothing weaker detected, down to the threshold of 8$σ$, we find consistency with a Euclidean flux distribution but disfavour steep distributions, ruling out $γ> 2.2$.

astro-ph.HE↗

arXiv2015

Dense magnetized plasma associated with a fast radio burst

Fast Radio Bursts are bright, unresolved, non-repeating, broadband, millisecond flashes, found primarily at high Galactic latitudes, with dispersion measures much larger than expected for a Galactic source. The inferred all-sky burst rate is comparable to the core-collapse supernova rate out to redshift 0.5. If the observed dispersion measures are assumed to be dominated by the intergalactic medium, the sources are at cosmological distances with redshifts of 0.2 to 1. These parameters are consistent with a wide range of source models. One fast radio burst showed circular polarization [21(7)%] of the radio emission, but no linear polarization was detected, and hence no Faraday rotation measure could be determined. Here we report the examination of archival data revealing Faraday rotation in a newly detected burst - FRB 110523. It has radio flux at least 0.6 Jy and dispersion measure 623.30(5) pc cm$^{-3}$. Using Galactic contribution 45 pc cm$^{-3}$ and a model of intergalactic electron density, we place the source at a maximum redshift of 0.5. The burst has rotation measure -186.1(1.4) rad m$^{-2}$, much higher than expected for this line of sight through the Milky Way and the intergalactic medium, indicating magnetization in the vicinity of the source itself or within a host galaxy. The pulse was scattered by two distinct plasma screens during propagation, which requires either a dense nebula associated with the source or a location within the central region of its host galaxy. Keeping in mind that there may be more than one type of fast radio burst source, the detection in this instance of source-local magnetization and scattering favours models involving young stellar populations such as magnetars over models involving the mergers of older neutron stars, which are more likely to be located in low density regions of the host galaxy.

astro-ph.HE↗