arXiv subjects

Oleg M. Smirnov

Publications and source records attributed to Oleg M. Smirnov.

At least 19 recordsLinked to original sources

arXiv2026

A new look at old devils. II: New insights on classical radio galaxies from MeerKAT and uGMRT

This paper presents a detailed morphological and spectral analysis of a sample of ten radio galaxies observed with the MeerKAT in L-band ($856\, \text{-}\,1712\ \mathrm{MHz}$) and the upgraded Giant Metrewave Radio Telescope in Band-4 ($550\,\text{-}\,850\ \mathrm{MHz}$). Our main goals are to revisit the current classification scheme for classical radio galaxies and study the properties of new radio features in jets, lobes and hot spots, which are becoming increasingly numerous due to the sensitivity and imaging capabilities of current radio interferometers, and suggest previous unexplored interaction mechanisms between the radio plasma and the external medium. The sample from the 4C catalogue includes FR I, FR II, wide-angle and narrow-angle tailed sources as well as FR 0 radio galaxies from FR0CAT, with redshifts ranging from $0.04\,\text{-}\,0.20$. The high angular resolution, $\sim4^{\prime\prime}\text{-}10^{\prime\prime}$ total intensity images are presented, revealing complex structures in the jets, lobes and hotspots of these sources. The integrated spectra of the sources, constructed using flux density measurements from our observations and archival data, reveal spectral breaks and slopes indicative of radiative ageing, with ages spanning $\sim40\,\text{-}\,242\ \mathrm{Myr}$. While the sample broadly confirms the classical FR classification as a useful first-order scheme, the substructures within the radio emission of sources highlight the need for models that incorporate environmental complexity and episodic jet activity to fully describe radio-galaxy evolution.

astro-ph.GA↗

arXiv2026

The burgeoning rise of the 21-cm forest I: constraints on the optical depth of the intergalactic medium at $5.38 < z < 5.84$

The redshifted 21-cm line is a promising probe of the Epoch of Reionization, during which the first generation of stars ionized the InterGalactic Medium (IGM). We aimed to constrain the IGM neutral Hydrogen fraction and spin temperature via redshifted 21-cm absorption against high-redshift radio sources. We analysed an 18-hour observation of the radio-loud quasar PSO J352.4034-15.3373 ($z = 5.84$) in the 203-222.5 MHz band. We obtained a continuum image with a $0.66$ mJy/beam rms noise and a spectrum with $3.6$ mJy/beam rms noise per 390 kHz-wide channel. We fit the quasar spectrum with a power-law continuum modified by an intervening 21-cm absorption, testing two scenarios: an island model, a residual cold neutral Hydrogen patch surviving at the end of reionization, and a global model, approximating the overall decline of the IGM HI fraction and spin temperature with redshift. Combining archival data spanning 150 MHz to 3.0 GHz with our measurements, we determined a quasar flux density of $86.8 \pm 1.1$ mJy at 200 MHz and a spectral index of $-0.88 \pm 0.02$ across the full frequency range. We found no evidence for the 21-cm absorption from intervening neutral Hydrogen at $5.38<z<5.84$. Assuming the island model, we set a 95% confidence lower limit on the IGM spin temperature of $1.73$ K in residual HI regions. Assuming the global model, we constrained the 21-cm optical depth to $τ_{21}< 0.02$ (95% C.L.). These results provide constraints on the 21-cm optical depth near the end of reionization, over the $5.38 < z < 5.84$ range, and confirm that the IGM was heated above the adiabatic cooling limit ($\sim 0.8$ K at $z = 5.68$), consistent with theoretical predictions and with 21-cm power spectrum measurements at higher redshifts. Our results also disfavour the presence of extremely cold HI regions at $z < 5.84$ and open the way to future 21-cm absorption from high-redshift sources.

astro-ph.CO↗

arXiv2026

SMGPS: A study of Galactic HII regions with extended morphology

We present a study of ionised hydrogen ($\textrm{H}\scriptstyle\mathrm{II}$) regions in the Galactic Plane using data from the SARAO MeerKAT Galactic Plane Survey (SMGPS). The SMPGS is a wide-field, wide-band 1.3 GHz radio continuum survey ($251^\circ \leq l \leq 358^\circ$ and $2^\circ \leq l \leq 61^\circ$ at $\quad |b| \leq 1^\circ.5$) that has enabled us to trace the diffuse emission enveloping recently formed massive stars. Our multifrequency synthesis images reveal faint and extended emission that was previously overlooked by $\textrm{H}\scriptstyle\mathrm{II}$ region surveys. We report the distances and Lyman-photon flux ($N_{\mathrm{Ly}}$) measurements for 1,327 Galactic $\textrm{H}\scriptstyle\mathrm{II}$ regions from which we characterise the spectral types for candidate ionising stars. The spectral types range from B2 to O4. The typical stellar spectral type responsible for ionisation is the B0, which constitutes about $\text{16%}$ of our catalogue, corresponding to a mean $\log(N_{\mathrm{Ly}}) = 47.5\ \mathrm{s}^{-1}$. Moreover, as a result of the lack of radio recombination line (RRL) velocity measurements for faint $\textrm{H}\scriptstyle\mathrm{II}$ regions, we identify the effective completeness limit at $\log(N_{\mathrm{Ly}}) \approx 47.6\ \mathrm{s}^{-1}$. The multiwavelength approach reveals that the physical radius at 1.3 GHz and in the mid-infrared are well correlated with a slope of $1.15 \pm 0.02$. We find clear power-law relations between $N_{\mathrm{Ly}}$ and physical radius, and an inverse correlation between electron density and radius ($n_{\rm e} \propto R^{-0.73}$). However, no significant correlation is observed between the $N_{\mathrm{Ly}}$ and Galactocentric distance, suggesting that the observed trends are governed primarily by local star-forming environments rather than large-scale Galactic gradients.

astro-ph.GA↗

arXiv2026

Commensal image plane transient search methods with the SKAO

This chapter outlines the key, state of the art, techniques required to conduct commensal image plane transient searches. Using significant experience and expertise from conducting transient searches with the SKA pathfinders, we have developed efficient fast imaging strategies, automated transient detection pipelines, determined careful filtering techniques to avoid artifacts, and identified the most useful triggered reprocessing tools. Using these strategies, we will be ideally placed to optimally and reliably detect transient sources from commensal image plane transient surveys. The tools and methods presented in this chapter can be used for all SKA-Mid and SKA-Low array deployments from AA* inclusive.

astro-ph.IM↗

arXiv2026

Fast Simultaneous Surveys with On-the-Fly Mapping

The SKAO is a next-generation radio telescope that will transform our understanding of the formation and evolution of radio galaxies, quasars, transients, and other cosmic sources. Surveys conducted on precursor telescopes can inform us about the capabilities and challenges to be overcome in preparation for SKAO science. The MeerKAT Large Area Synoptic Survey (MeerKLASS), is a pathfinder large area survey to probe cosmology using the single-dish ${\rm H\hspace{0.5mm}}{\scriptsize {\rm I}}$~intensity mapping (IM). MeerKLASS provides an additional wide, high angular-resolution commensal survey by utilizing the ``On-the-Fly'' (OTF) imaging technique. The survey target is to cover 10,000 sq. degrees in the Southern sky avoiding the Galactic plane, using the UHF-band (544-1088 MHz). The survey aims to achieve an r.m.s. of 25 $μ$Jy/beam and $14''$ resolution. In this chapter we discuss the survey strategy, observational status, the development of the MeerKLASS OTF imaging pipeline and the science prospects of the data products. OTF imaging, which relies upon constant elevation fast scanning, comes with its own set of challenges, such as smearing. Here we detail how we have mitigated them and the current scientific impact. Significantly fast survey speed ($\sim 3.5$ sq. deg per min), commensality with ${\rm H\hspace{0.5mm}}{\scriptsize {\rm I}}$~IM survey, deep and large area continuum images make MeerKLASS OTF an economic survey technique. Lessons from this technique will be valuable for the upcoming SKA-Mid, where we expect a better resolution $(< 2'')$ and sensitivity $(\sim 7μ{\rm Jy/beam})$ of the OTF images with the AA4 configuration. Furthermore, improvements in the correlator will result in negligible smearing effects in the imaging.

astro-ph.IM↗

arXiv2026

Identification of a Radio Counterpart to SN 2025ulz in the S250818k Localization Area

On 2025 August 18, the LIGO-Virgo-KAGRA collaboration reported S250818k, a sub-threshold gravitational-wave (GW) candidate consistent with a binary neutron star (NS) merger potentially involving a sub-solar-mass NS. Optical follow-up by the Zwicky Transient Facility identified AT2025ulz, a transient temporally coincident with the GW trigger that initially resembled a kilonova but was later classified as a young stripped-envelope Type IIb supernova (SN), dubbed SN 2025ulz. A key question is whether SN 2025ulz harbors fast, possibly collimated, non-thermal ejecta indicative of a central engine, as invoked in "superkilonova" scenarios linking sub-solar-mass NSs to accretion-disk fragmentation or core fission. We present early-to-late-time multi-band radio observations of SN 2025ulz obtained with the Karl G. Jansky Very Large Array as part of the JAGWAR program, complemented by observations with the upgraded Giant Metrewave Radio Telescope and MeerKAT. We detect a faint but significant radio counterpart to SN 2025ulz at 6-10 GHz. The data are consistent with non-thermal emission from SN ejecta interacting with circumstellar material, favoring a compact progenitor and relatively fast ejecta akin to those of Type cIIb SNe. Our data are also consistent with emission from an off-axis jet peaking at about 50-100 days after the GW trigger. Overall, our radio detection is compatible with a superkilonova scenario and would motivate future systematic multi-wavelength follow-up of core-collapse events coincident with sub-solar NS GW candidates, should the association between S250818k and SN 2025ulz be supported by offline GW analyses.

astro-ph.HE↗

arXiv2025

The MeerKLASS L-band On-the-Fly Continuum Survey: Data Release 1

The MeerKAT Large Area Synoptic Survey (MeerKLASS) collaboration has acquired multiple passes of L-band (856-1712 MHz) scanning observations over a 268 deg$^2$ sky region. This scanning enables efficient, large-area sky surveys by continuously scanning the MeerKAT array back and forth at fixed elevation while recording data at 2 sec intervals, progressively covering the survey region as the Earth rotates. We employ a novel on-the-fly (OTF) interferometric imaging technique to construct continuum images and catalogs from 16 hours of scan data. These data products, constituting the first MeerKLASS L-band data release (DR1), consist of high-fidelity radio continuum images and a catalogue of 34,874 radio sources detected with a SNR$>$9. The resulting Stokes I images achieve a median noise level of 33 $μ$Jy\,beam$^{-1}$ and a median angular resolution of approximately $25.5''\times 7.8''$. Cross-comparisons with previous surveys confirm the consistency of our flux density scale within 5\% and astrometric precision within $1.5''$. Additionally, flux densities measured across the seven sub-bands enable in-band spectral-index estimates for the detected sources, providing insights into their physical properties and the broader source population. We compute the differential source counts, finding good agreement with existing measurements and validating our end-to-end processing. This data release demonstrates the effectiveness of scanning surveys when combined with OTF interferometric imaging. Commensal intensity mapping and interferometric imaging offers a dramatic enhancement of survey science per invested hour of observations and could therefore be an appealing option for next generation facilities like SKA-Mid.

astro-ph.IM↗

arXiv2025

The MeerKLASS On-the-Fly continuum survey: pipeline design and validation

The MeerKAT Large Area Synoptic Survey (MeerKLASS) is designed to map large areas of the Southern sky for cosmology using the single-dish HI intensity mapping (IM) technique, while simultaneously delivering a wide, high angular-resolution interferometric survey. We present the design and first results of the MeerKLASS On-the-Fly (OTF) continuum data, which exploits the visibilities recorded during fast, constant-elevation scans. This observing mode enables fast commensal imaging over several hundred of square degrees on a nightly basis. We describe the OTF survey strategy and pipeline, focusing on handling challenges introduced by the current MeerKAT fixed-delay correlation observing mode, which causes decorrelation (smearing). We implement a correction scheme based on time-dependent phase rotation, direction-dependent PSF modeling, and wide-band faceted deconvolution with \texttt{DDFacet}. Using UHF-band and pilot L-band data, we demonstrate the recovery of high-quality 2-second snapshot images and deep mosaics over hundreds of square degrees. After smearing correction we are able to achieve a resolution of $23.3$arcsec and $14$ arcsec with an rms sensitivity of $35 μ{\rm Jy\,beam}^{-1}$ and $ 33 μ{\rm Jy\,beam}^{-1}$ in the UHF and L-band respectively. The full survey will cover $10,000 \, {\rm deg}^{2}$ at 544-1088 MHz, and after the delay tracking fix implemented we expect to reach $\sim 25 μ{\rm Jy\,beam}^{-1}$ at $14$ arcsec resolution. The continuum OTF data products will support diverse science goals, including galaxy and AGN evolution, diffuse cluster emission, large-scale structure and cosmology, rotation-measure synthesis, and transient searches. MeerKLASS-OTF thus establishes an efficient path to wide-area commensal surveys with MeerKAT and provides a key technical precursor for SKA-Mid.

astro-ph.IM↗

arXiv2025

Mining the time axis with TRON. I. Millisecond pulsars in Omega Centauri, Terzan 5 and 47 Tucanae detected through MeerKAT interferometric imaging

Medium-timescale (minutes to hours) radio transients are a relatively unexplored population. The wide field-of-view and high instantaneous sensitivity of instruments such as MeerKAT provides an opportunity to probe this class of sources, using image-plane detection techniques. We aim to systematically mine archival synthesis imaging data in order to search for medium-timescale transients and variables that are not detected by conventional long-track image synthesis techniques. We deploy a prototype blind transient and variable search pipeline named TRON. This processes calibrated visibility data, constructs high-time cadence images, performs a search for variability on multiple timescales, and extracts lightcurves for detected sources. As proof of concept, we apply it to three MeerKAT observations of globular clusters, known to host transient or variable sources. We detect a previously known eclipsing MSP suspected to be a `black widow' system, in the globular cluster Omega Centauri, with a light curve confirming the eclipsing nature of the emission. We detect a previously known `red back' eclipsing MSP in the globular cluster Terzan 5. Using observations of the globular cluster 47 Tucanae, we detect two known millisecond pulsars (MSPs), and one previously reported MSP candidate, with hints of eclipsing behaviour.

astro-ph.IM↗

arXiv2025

Mining the time axis with TRON. II. MeerKAT detects a stellar radio flare from a distant RS CVn candidate

astro-ph.IM↗

arXiv2024

Africanus II. QuartiCal: calibrating radio interferometer data at scale using Numba and Dask

Calibration of radio interferometer data ought to be a solved problem; it has been an integral part of data reduction for some time. However, as larger, more sensitive radio interferometers are conceived and built, the calibration problem grows in both size and difficulty. The increasing size can be attributed to the fact that the data volume scales quadratically with the number of antennas in an array. Additionally, new instruments may have up to two orders of magnitude more channels than their predecessors. Simultaneously, increasing sensitivity is making calibration more challenging: low-level RFI and calibration artefacts (in the resulting images) which would previously have been subsumed by the noise may now limit dynamic range and, ultimately, the derived science. It is against this backdrop that we introduce QuartiCal: a new Python package implementing radio interferometric calibration routines. QuartiCal improves upon its predecessor, CubiCal, in terms of both flexibility and performance. Whilst the same mathematical framework - complex optimization using Wirtinger derivatives - is in use, the approach has been refined to support arbitrary length chains of parameterized gain terms. QuartiCal utilizes Dask, a library for parallel computing in Python, to express calibration as an embarrassingly parallel task graph. These task graphs can (with some constraints) be mapped onto a number of different hardware configurations, allowing QuartiCal to scale from running locally on consumer hardware to a distributed, cloud-based cluster. QuartiCal's qualitative behaviour is demonstrated using MeerKAT observations of PSR J2009-2026. These qualitative results are followed by an analysis of QuartiCal's performance in terms of wall time and memory footprint for a number of calibration scenarios and hardware configurations.

astro-ph.IM↗

arXiv2024

Africanus III. pfb-imaging -- a flexible radio interferometric imaging suite

The popularity of the CLEAN algorithm in radio interferometric imaging stems from its maturity, speed, and robustness. While many alternatives have been proposed in the literature, none have achieved mainstream adoption by astronomers working with data from interferometric arrays operating in the big data regime. This lack of adoption is largely due to increased computational complexity, absence of mature implementations, and the need for astronomers to tune obscure algorithmic parameters. This work introduces pfb-imaging: a flexible library that implements the scaffolding required to develop and accelerate general radio interferometric imaging algorithms. We demonstrate how the framework can be used to implement a sparsity-based image reconstruction technique known as (unconstrained) SARA in a way that scales with image size rather than data volume and features interpretable algorithmic parameters. The implementation is validated on terabyte-sized data from the MeerKAT telescope, using both a single compute node and Amazon Web Services computing instances.

astro-ph.IM↗

arXiv2024

Africanus IV. The Stimela2 framework: scalable and reproducible workflows, from local to cloud compute

Stimela2 is a new-generation framework for developing data reduction workflows. It is designed for radio astronomy data but can be adapted for other data processing applications. Stimela2 aims at the middle ground between ease of development, human readability, and enabling robust, scalable and reproducible workflows. It represents workflows by linear, concise and intuitive YAML-format "recipes". Atomic data reduction tasks (binary executables, Python functions and code, and CASA tasks) are described by YAML-format "cab definitions" detailing each task's "schema" (inputs and outputs). Stimela2 provides a rich syntax for chaining tasks together, and encourages a high degree of modularity: recipes may be nested into other recipes, and configuration is cleanly separated from recipe logic. Tasks can be executed natively or in isolated environments using containerization technologies such as Apptainer. The container images are open-source and maintained through a companion package called cult-cargo. This enables the development of system-agnostic and fully reproducible workflows. Stimela2 facilitates the deployment of scalable, distributed workflows by interfacing with the Slurm scheduler and the Kubernetes API. The latter allows workflows to be readily deployed in the cloud. Previous papers in this series used Stimela2 as the underlying technology to run workflows on the AWS cloud. This paper presents an overview of Stimela2's design, architecture and use in the radio astronomy context.

astro-ph.IM↗

arXiv2024

Africanus I. Scalable, distributed and efficient radio data processing with Dask-MS and Codex Africanus

New radio interferometers such as MeerKAT, SKA, ngVLA, and DSA-2000 drive advancements in software for two key reasons. First, handling the vast data from these instruments requires subdivision and multi-node processing. Second, their improved sensitivity, achieved through better engineering and larger data volumes, demands new techniques to fully exploit it. This creates a critical challenge in radio astronomy software: pipelines must be optimized to process data efficiently, but unforeseen artefacts from increased sensitivity require ongoing development of new techniques. This leads to a trade-off among (1) performance, (2) flexibility, and (3) ease-of-development. Rigid designs often miss the full scope of the problem, while temporary research code is unsuitable for production. This work introduces a framework for developing radio astronomy techniques while balancing the above trade-offs. It prioritizes flexibility and ease-of-development alongside acceptable performance by leveraging Open Source data formats and software. To manage growing data volumes, data is distributed across multiple processors and nodes for parallel processing, utilizing HPC and cloud infrastructure. We present two Python libraries, Dask-MS and Codex Africanus, which enable distributed, high-performance radio astronomy software with Dask. Dask is a lightweight parallelization and distribution framework that integrates with the PyData ecosystem, addressing the "Big Data" challenges of radio astronomy.

astro-ph.IM↗

arXiv2024

uGMRT sub-GHz view of the Sausage cluster diffuse radio sources

CIZA J2242.8+5301, or the Sausage cluster, is well studied over a range of frequencies. Since its first discovery, a lot of interesting features and unique characteristics have been uncovered. In this work, we report some more new morphological features using the uGMRT band-3 and band-4 data. In the north relic, we observe variation in spectral index profiles across the relic width from the east to west, which may indicate a decrease in downstream cooling rate in that direction. We re-confirm the presence of an additional ~ 930 kpc relic in the north. We classify the filamentary source in the downstream region to be a narrow angle tail (NAT) radio galaxy. The bright arc in the east relic shows sub-structure in the spectral index profile, which may indicate the presence of finer filaments. We further report the presence of a double-strand structure in the east relic similar to the 'Toothbrush' relic. We categorize the bright 'L' shaped structure in the southern relic to be a NAT radio galaxy, as well as trace the actual ~ 1.1 Mpc relic component. We re-confirm the existence of the faint southern extent, measuring the relic length to be ~ 1.8 Mpc. Furthermore, we suggest the southern relic to be a union of individual component relics rather than a single giant filamentary relic. Lastly, based on the morphological symmetry between northern and southern relics, we suggest a schematic shock structure associated with the merger event in an attempt to explain their formation scenario.

astro-ph.CO↗

arXiv2024

A Multi-Frequency View of the Radio Phoenix in the Abell 85 Cluster

Radio phoenices are complex and filamentary diffuse radio sources found in both merging and relaxed clusters. The formation of these sources was proposed to be due to adiabatic compression of old Active Galactic Nucleus (AGN) plasma in shock waves. Most of the previous spectral studies of these sources were limited to integrated spectral indices, which were found to be very steep as well as showing a curved spectrum. Here, we have performed a multi-frequency investigation of the radio phoenix in the Abell 85 cluster. Owing to the sensitive high-resolution observations, we found some of the finer filamentary structures that were previously undetected. We produced resolved spectral index maps of the radio phoenix between 323, 700, and 1280 MHz. The orientation of the filaments, as well as the gradient across the spectral index maps, suggest the possible direction of the shock motion from northeast to southwest. The integrated spectrum of the radio phoenix was found to be very steep and curved toward high-frequencies. Furthermore, the spectral index of the filaments was found to be less steep compared to the non-filamentary regions, implying greater energy injection in the filaments. The observed features in the radio phoenix in the Abell 85 cluster seem to be in support of the adiabatic shock compression mechanism.

astro-ph.HE↗

arXiv2024

fast-resolve: Fast Bayesian Radio Interferometric Imaging

Context: Interferometric imaging is algorithmically and computationally challenging as there is no unique inversion from the measurement data back to the sky maps, and the datasets can be very large. Many imaging methods already exist, but most of them focus either on the accuracy or the computational aspect. Aims: This paper aims to reduce the computational complexity of the Bayesian imaging algorithm resolve, enabling the application of Bayesian imaging for larger datasets. Methods: By combining computational shortcuts of the CLEAN algorithm with the Bayesian imaging algorithm resolve we developed an accurate and fast imaging algorithm which we name fast-resolve. Results: We validate the accuracy of the presented fast-resolve algorithm by comparing it with results from resolve on VLA Cygnus A data. Furthermore, we demonstrate the computational advantages of fast-resolve on a large MeerKAT ESO 137-006 dataset which is computationally out of reach for resolve. Conclusions: The presented algorithm is significantly faster than previous Bayesian imaging algorithms, broadening the applicability of Bayesian interferometric imaging. Specifically for the single channel VLA Cygnus A datasets fast-resolve is about $144$ times faster than resolve. For the MeerKAT dataset with multiple channels the computational speedup of fast-resolve is even larger.

astro-ph.IM↗

arXiv2024

A Spectropolarimetric Study of Pictor A Radio Galaxy with MeerKAT

We present the results of a polarimetric study from our new high-sensitivity L-band (0.8--1.7 GHz) observation of Pictor A with the MeerKAT radio telescope. We confirm the presence of the radio jet extending from the nucleus to the western hotspot of this source. Additionally, we show the radio emission expected to be coincident with previously observed X-ray emission in the radio lobes, confirming that the emission mechanism is of inverse Compton origin, as suggested by a previous study. Our spectropolarimetric analysis using the RM-Synthesis technique reveals a relatively uniform mean RM distribution across the lobes of Pictor A, with most lines-of-sight exhibiting single-peaked Faraday spectra. However, a number of the lines-of-sight exhibit single peaked spectra with a wide base or multiple peaks, suggesting the presence of multiple Faraday components or a Faraday thick structure along Pictor A's lines-of-sight. We also confirm the asymmetry in RM variability and depolarization between the two lobes of this source which were reported in a previous study.

astro-ph.GA↗