Radio Telescope Data Research Articles

A Multiwavelength Approach to Constraining the Merger Properties of ACT-CL J0034.4+0225

ACT-CL J0034.4+0225 is a previously unrecognized merging galaxy cluster at z = 0.38588 ± 0.00068. Our primary evidence is provided by a 21 ks Chandra image that shows two surface brightness peaks separated by ∼49″ (259 kpc) surrounded by an extended cluster gas distribution. Each gas peak contains a brightest cluster galaxy, offset from the gas peak. We collect new South African Large Telescope optical spectra that, when augmented by archival data, yield redshifts for the two BGCs and 58 other cluster members. Archival Giant Metrewave Radio Telescope and MeerKAT data reveal a radio halo that encompasses the X-ray peaks. We provide and compare three X-ray-based mass estimates (5.0 × 1014 M ⊙, 6.4 × 1014 M ⊙, and 8.6 × 1014 M ⊙). The Planck and ACT Sunyaev–Zel’dovich masses are ≈5.8 × 1014 M ⊙. We constrain the merger state and properties by comparing them to an existing suite of N-body/hydrodynamical models using the measured gas peak separation (259 kpc, projected) and radial velocity difference (0–1000 km s−1). This constrains the epoch of the merger to be within ∼100 Myr of first pericenter passage. A strong lensing analysis constrains the mass ratio to be in the range 1:1–1:20, while the cluster morphology prefers values near the equal-mass range.

Reliability of In-band and Broadband Spectral Index Measurement: Systematic Study of the Effect of Signal-to-noise Ratio for uGMRT Data

Low-radio-frequency spectral index measurements are a powerful tool for distinguishing between different emission mechanisms and, in turn, understanding the nature of the sources. Besides the standard method of estimating the “broadband” spectral index of sources from observations in two different frequency “bands,” if the observations were made with large instantaneous bandwidth, the “in-band” spectral index can be determined, either using images of emission at multiple frequency ranges within a band or using the novel Multi Term-Multi Frequency Synthesis (MT-MFS) imaging algorithm. Here, using simulated upgraded Giant Metrewave Radio Telescope (uGMRT) data, we have systematically studied the reliability of various methods of spectral index estimation for sources with a wide range of signal-to-noise ratios (S/Ns). It is found that for synthetic uGMRT point-source data, the MT-MFS imaging algorithm produces in-band spectral indices for S/N ≲ 100 that have errors ≳0.2, making them unreliable. However, at a similar S/N, the sub-band splitting method produces errors ≲0.2, which are more accurate and unbiased than the in-band spectral indices. The broadband spectral indices produce errors ≲0.2 even for S/N ≳ 15, and hence they are most reliable if there are no higher-order variations in the spectral index. These results may be used to improve the uGMRT observation and data analysis strategies, depending on the brightness of the target source.

SDN-Based Multicast Traffic Engineering for Radio Telescope Data Networks

SDN-Based Multicast Traffic Engineering for Radio Telescope Data Networks

Identification of Fast Radio Bursts using Transfer Learning Approach with Data Augmentation

The universe has many mysteries, such as pulsars, dying stars, supernovae, and fast radio bursts (FRBs), FRBs are millisecond long radio signals, detected as a spike in radio-telescope data. Identification of Fast Radio Bursts from available data involves manual inspection of exhaustive data/plots. Radio Frequency Interference in pose a major challenge in identification of Fast Radio Bursts due to their abundance in the observatory data. We present a machine-learning-aided system, which screens telescope-generated data and identifies potential Fast Radio Burst candidates in it. Proposed system employs Convolutional Neural Networks and Transfer Learning to classify potential Fast Radio Bursts from Radio Frequency Interference from data recorded by the uGMRT. We have used data simulation tools to synthesize additional samples in order to make up for the paucity of data. The VGG16-based model displayed the best receiver operating characteristics curve with the area under curve being 0.90 along with an accuracy of 90.67%.

Enhancing Neutral Hydrogen (HI) Detection in Galaxies through Optimized Matched Filter Analysis

Neutral Hydrogen (HI) is a fundamental component in the composition of galaxies, providing crucial insights into their structure and dynamics. In this research, we propose a novel approach to improve the detection of H1 spectral lines in radio telescope data by employing a matched filter algorithm. The algorithm capitalizes on the convolution of a template function, optimized using the "Busy" function that defines the line profile properties, with a randomly generated H1 peak. This H1 peak is subsequently embedded within noisy data, accounting for the effects of telescope measurements, such as thermal broadening and radio-frequency interference. The incorporation of the Voigt profile in the noise generation ensures a realistic representation of these effects. Through comprehensive analysis, we demonstrate that a matched filter with a multi-layered convolution scheme yields the highest efficiency in recovering H1 peaks. The effectiveness of peak recovery is shown to be influenced by the injected noise profiles, allowing for a direct comparison of the matched filter’s performance concerning the Integrated and Peak Signal to Noise ratios. This research contributes to the advancement of observational techniques in astrophysics and deepens our understanding of the distribution and properties of Neutral Hydrogen in galaxies. It represents the first such exploration of this technique in the context of the Canadian Hydrogen Observatory and Radio Transient Detector (CHORD), which is under construction at the Dominion Radio Astronomy Observatory in Penticton, BC and which will survey the HI sky more broadly and deeply than has been previously possible.

MFPIM: A Deep Learning Model Based on Multimodal Fusion Technology for Pulsar Identification

With the development of radio telescope technology, the quantity and types of acquired pulsar candidate data have increased dramatically. However, it is difficult to accurately identify pulsar candidates. Therefore, we propose to use multimodal fusion technology, called the multimodal fusion-based pulsar identification model (MFPIM), to build a deep learning model to improve the efficiency and accuracy of pulsar candidate identification. MFPIM treats each diagnostic plot of pulsar candidates as a modality and uses multiple convolutional neural networks to extract effective features from the diagnostic plots. After fusing the features, the commonality of different modalities in high-dimensional space is obtained to ensure that the model can take full advantage of the complementarity between diagnostic plots and thus identify pulsar candidates, achieved better classification performance than other current supervised learning algorithms. In addition, a channel attention mechanism is used in the model to enable it to learn the importance of different channel features so that the model focuses more on the channel information in the input data that is more meaningful for classification, reducing the model size while extracting pulsar diagnostic map features more accurately. We conducted experiments on the Five-hundred-meter Aperture Spherical radio Telescope (FAST) data set, and the results show that MFPIM can effectively identify the pulsars in the FAST data set with an identification accuracy of over 98%. To further verify the robustness of the model, we applied the MFPIM to the High Time Resolution Universe data set using transfer learning, with the test accuracy and F1 score reaching over 99%.

Star formation history of ≤ z ≤ mass-selected galaxies in the ELAIS-N1 Field

ABSTRACT We measure the specific star formation rates (sSFRs) of K-band selected galaxies from the European Large Area ISO Survey North 1 by stacking Giant Metrewave Radio Telescope data at 610 MHz. We identify a sample of star-forming galaxies (SFGs), spanning ${0.1\le \, {z}\, \le \, 1.5}$ and $\rm {10^{8.5}\lt \, {{\mathit{ M}}_{\star }}/{{M}_{\odot }}\lt 10^{12.4}}$, using a combination of multiwavelength diagnostics obtained from the deep LOw Frequency ARray Two-metre Sky Survey multiwavelength catalogue. We measure the flux densities in the radio map and estimate the radio SFR in order to probe the nature of the galaxies below the noise and confusion limits. The massive galaxies in our sample have the lowest sSFRs, which is in agreement with previous studies. For the different populations, we show that the sSFR–mass relation steepens with redshift, with an average slope of $\rm {\langle \beta _{All} \rangle \, =\, -0.49\pm 0.01}$ for the whole sample, and $\rm {\langle \beta _{SFG} \rangle \, =\, -0.42\pm 0.02}$ for the SFGs. Our results indicate that galaxy populations undergo ’downsizing’, whereby most massive galaxies form their stars earlier and more rapidly than low-mass galaxies. Both populations show a strong decrease in their sSFR towards the present epoch. The sSFR evolution with redshift is best described by a power law ${(1\, +\, {z})^{n}}$, where $\rm {\langle {\mathit{ n}}_{ALL}\rangle \sim 4.94\pm 0.53}$ for all galaxies and $\rm {\langle {\mathit{ n}}_{SFG}\rangle \sim 3.51\pm 0.52}$ for SFGs. Comparing our measured sSFRs to results from literature, we find a general agreement in the sSFR–M⋆ plane.

Systematic performance of the ASKAP fast radio burst search algorithm

ABSTRACT Detecting fast radio bursts (FRBs) requires software pipelines to search for dispersed single pulses of emission in radio telescope data. In order to enable an unbiased estimation of the underlying FRB population, it is important to understand the algorithm efficiency with respect to the search parameter space and thus the survey completeness. The Fast Real-time Engine for Dedispersing Amplitudes (fredda) search pipeline is a single pulse detection pipeline designed to identify radio pulses over a large range of dispersion measures (DM) with low latency. It is used on the Australian Square Kilometre Array Pathfinder (ASKAP) for the Commensal Real-time ASKAP Fast Transients (CRAFT) project. We utilize simulated single pulses in the low- and high-frequency observation bands of ASKAP to analyse the performance of the pipeline and infer the underlying FRB population. The simulation explores the signal-to-noise ratio (S/N) recovery as a function of DM and the temporal duration of FRB pulses in comparison to injected values. The effects of intrachannel broadening caused by dispersion are also carefully studied in this work using control data sets. Our results show that for Gaussian-like single pulses, >85 per cent of the injected signal is recovered by pipelines such as fredda at DM <3000 pc cm−3 using standard boxcar filters compared to an ideal incoherent dedispersion match filter. Further calculations with sensitivity implies at least ∼10 per cent of FRBs in a Euclidean universe at target sensitivity will be missed by fredda and heimdall, another common pipeline, in ideal radio environments at 1.1 GHz.

SDSS-FIRST-selected interacting galaxies

Context. In the hierarchical model of evolution of the Universe, galaxy mergers play an important role, especially at high redshifts. Interactions among galaxies appear to be associated with incidences of radio-loudness in quasars and it is of interest to study the galaxies that are in the process of interacting with each other, where there is at least one nucleus that is active in the radio regime. Aims. In order to understand the various processes taking place within colliding galaxies, it is important to study the radio and optical properties of these sources, as well as any possible correlations that might exist. Methods. To this end, we present optical long-slit spectroscopy data for ten pairs of interacting galaxies selected from SDSS-FIRST at redshifts of ∼0.05, observed using the multi-object double spectrographs at the Large Binocular Telescope. Results. We used line fluxes extracted from the spectra of the nuclear regions of galaxies to plot optical diagnostic diagrams and estimate the masses of the central supermassive black holes, as well as their Eddington ratios. Additionally, we used previously published Effelsberg radio telescope data at 4.85 GHz and FIRST survey data at 1.4 GHz to estimate radio spectral slopes and the radio-loudness parameters for all of the radio-detected sources. We also used WISE data to plot a mid-infrared colour-colour diagram. Conclusions. We see that while the sample of galaxies covers all of the classes on the optical diagnostic diagrams, the sources that are radio-detected fall in the composite or transition region of the diagram. Additionally, we notice a trend of the highest radio-loudness parameter in a pair of interacting galaxies being associated with the galaxy that hosts the more massive central supermassive black hole. We do not see any obvious trends with respect to the radio spectral slope, radio-loudness parameter, and Eddington ratio. With respect to the mid-infrared data of the galaxies detected by WISE, we see that most of them have some type of contribution from star formation, however, two of them seem to have a significant contribution from an AGN as well.

High-performance computing for SKA transient search: Use of FPGA-based accelerators

This paper presents the High-Performance computing efforts with FPGA for the accelerated pulsar/transient search for the SKA. Case studies are presented from within SKA and pathfinder telescopes highlighting future opportunities. It reviews the scenario that has shifted from offline processing of the radio telescope data to digitizing several hundreds/thousands of antenna outputs over huge bandwidths, forming several 100s of beams, and processing the data in the SKA real-time pulsar search pipelines. A brief account of the different architectures of the accelerators, primarily the new generation Field Programmable Gate Array-based accelerators, showing their critical roles to achieve high-performance computing and in handling the enormous data volume problems of the SKA is presented here. It also presents the power-performance efficiency of this emerging technology and presents potential future scenarios.

Long-term scintillation studies of EPTA pulsars

Context. Interstellar scintillation analysis of pulsars allows us to probe the small-scale distribution and inhomogeneities of the ionized interstellar medium. From the frequency scale of scintillation, one can estimate the geometric time delays from multipath propagation, a source of (typically) unmodeled, correlated noise in pulsar timing. Interstellar scintillation analysis of well-timed pulsars is useful to quantify the effects of time delays and may lead to improved timing precision, enhancing the probability of detecting gravitational waves. Aims. Our priority is to present the data set and the basic measurements of scintillation parameters of pulsars, employing long-term scintillation observations carried out from 2011 January to 2020 August by the European Pulsar Timing Array radio telescopes in the 21-cm and 11-cm bands. Additionally, we aim to identify future possible lines of study using this long-term scintillation data set. Methods. The autocorrelation function of dynamic spectra has been used to estimate the scintillation bandwidth vd and scintillation timescale τd. Results. We present the long-term time series of vd and τd for 13 pulsars. Sanity checks and comparisons indicate that the scintillation parameters of our work and previously published works are mostly consistent. For two pulsars, PSRs J1857+0943 and J1939+2134, we were able to obtain measurements of the vd at both bands, which allowed us to derive the time series of frequency scaling indices with a mean and a standard deviation of 2.82 ± 1.95 and 3.18 ± 0.60, respectively. We found some interesting features which will be studied in more detail in subsequent papers in this series: (i) in the time series of PSR J1939+2134, where vd and τd sharply decrease associated with a sharp increase in the dispersion measure; (ii) PSR J0613-0200 and PSR J0636+5126 show a strong annual variation in the time series of the τd; and (iii) PSR J1939+2134 shows a weak anticorrelation between the scintillation timescale and the dispersion in Westerbork Synthesis Radio Telescope data.

The Unusually Weak and Exceptionally Steep Radio Relic in A2108

Abstract Mergers between galaxy clusters often drive shocks into the intracluster medium, the effects of which are sometimes visible via temperature and density jumps in the X-ray and via radio emission from relativistic particles energized by the shock’s passage. A2108 was selected as a likely merger system through comparing the X-ray luminosity to the Planck Sunyaev–Zeldovich signal, where this cluster appeared highly X-ray underluminous. Follow-up observations confirmed it to be a merging low-mass cluster featuring two distinct subclusters, both with a highly disturbed X-ray morphology. Giant Metrewave Radio Telescope (GMRT) data in bands 2, 3, & 4 (covering 120–750 MHz) show an extended radio feature resembling a radio relic near the location of a temperature discontinuity in the X-rays. We measure a Mach number from the X-ray temperature jump (  X = 1.6 ± 0.2 ). Several characteristics of radio relics (location and morphology of extended radio emission) are found in A2108, making this cluster one of the few low-mass mergers (M L−M = 1.8 ± 0.4 × 1014 M ⊙) likely hosting a radio relic. The radio spectrum is exceptionally steep (α = −2 at the lowest frequencies), and the radio power is very weak (P 1.4 GHz = 1 × 1022 W Hz−1). To account for the shock/relic offset, we propose a scenario in which the shock created the relic by re-accelerating a cloud of pre-existing relativistic electrons and then moved away, leaving behind a fading relic. The electron-aging timescale derived from the high-frequency steepening in the relic spectrum is consistent with the shock travel time to the observed X-ray discontinuity. However, the lower flux in GMRT band 4 data causing the steepening could be due to instrumental limitations and deeper radio data are needed to constrain the spectral slope of the relic at high frequencies. A background cluster, 4′ from the merger, may have contributed to the ROSAT and Planck signals, but SZ modeling shows that the merger system is still X-ray underluminous, supporting the use of this approach to identifying merger-disrupted clusters.

Imaging results from the legacy Giant Metrewave Radio Telescope Galaxy Cluster Key Project

ABSTRACT We have used archival Giant Metrewave Radio Telescope (GMRT) data to image and study 39 galaxy clusters. These observations were made as part of the GMRT Key Project on galaxy clusters between 2001 and 2004. The observations presented in this sample include 14 observations at 610 MHz, 29 at 325 MHz, and three at 244 MHz covering a redshift range of 0.02–0.62. Multifrequency observations were made for eight clusters. We analysed the clusters using the spam processing software and detected the presence of radio halo emission for the first time in the clusters RXC J0510−0801 and RXC J2211.7−0349. We also confirmed the presence of extended emission in 11 clusters which were known from the literature. In clusters where haloes were not detected, upper limits were placed using our own semi-automated program. We plot our detections and non-detections on the empirical LX−P1.4 and M500−P1.4 relation in radio halo clusters and discuss the results. The best fits follow a power law of the form $L_{500} \propto P_{1.4}^{1.82}$ and $M_{500} \propto P_{1.4}^{3.001}$ which is in accordance with the best estimates in the literature.

Resolving the Emission Regions of the Crab Pulsar’s Giant Pulses

Abstract The Crab pulsar has striking radio emission properties, with the two dominant pulse components—the main pulse and the interpulse—consisting entirely of giant pulses. The emission is scattered in both the Crab Nebula and the interstellar medium, causing multipath propagation and thus scintillation. We study the scintillation of the Crab’s giant pulses using phased Westerbork Synthesis Radio Telescope data at 1668 MHz. We find that giant pulse spectra correlate at only ∼2%, much lower than the one-third correlation expected from a randomized signal imparted with the same impulse response function. In addition, we find that the main pulse and the interpulse appear to scintillate differently; the 2D cross-correlation of scintillation between the interpulse and main pulse has a lower amplitude and is wider in time and frequency delay than the 2D autocorrelation of the main pulses. These lines of evidence suggest that the giant pulse emission regions are extended, and that the main pulse and interpulse arise in physically distinct regions that are resolved by the scattering screen. Assuming the scattering takes place in the nebular filaments, the emission regions are of order a light-cylinder radius, as projected on the sky. With further very long baseline interferometry and multifrequency data, it may be possible to measure the distance to the scattering screens, the size of giant pulse emission regions, and the physical separation between the pulse components.

CAPTURE: a continuum imaging pipeline for the uGMRT

We present the first fully automated pipeline for making images from the interferometric data obtained from the upgraded Giant Metrewave Radio Telescope (uGMRT) called CAsa Pipeline-cum-Toolkit for Upgraded Giant Metrewave Radio Telescope data REduction - CAPTURE. It is a python program that uses tasks from the NRAO Common Astronomy Software Applications (CASA) to perform the steps of flagging of bad data, calibration, imaging and self-calibration. The salient features of the pipeline are: i) a fully automatic mode to go from the raw data to a self-calibrated continuum image, ii) specialized flagging strategies for short and long baselines that ensure minimal loss of extended structure, iii) flagging of persistent narrow band radio frequency interference (RFI), iv) flexibility for the user to configure the pipeline for step-by-step analysis or special cases and v) analysis of data from the legacy GMRT. CAPTURE is available publicly on github (https://github.com/ruta-k/uGMRT-pipeline, release v1.0.0). The primary beam correction for the uGMRT images produced with CAPTURE is made separately available at https://github.com/ruta-k/uGMRTprimarybeam. We show examples of using CAPTURE on uGMRT and legacy GMRT data. In principle, CAPTURE can be tailored for use with radio interferometric data from other telescopes.

Dark matter annihilation in ω Centauri: Astrophysical implications derived from the MWA radio data

We present an analysis of Murchison Widefield Array radio telescope data from $\omega$ Cen, possibly a stripped dwarf spheroidal galaxy core captured by our Galaxy. Recent interpretations of Fermi-LAT $\gamma$-ray data by Brown {\it et al.} (2019) and Reynoso-Cordova {\it et al.} (2019) suggest that $\omega$ Cen may contain significant Dark Matter. We utilise their best-fit Dark Matter annihilation models, and an estimate of the magnetic field strength in $\omega$ Cen, to calculate the expected radio synchrotron signal from annihilation, and show that one can usefully rule out significant parts of the magnetic field - diffusion coefficient plane using our current observational limits on the radio emission. Improvement by a factor of 10-100 on these limits could constrain the models even more tightly.

An Automated Approach to Modeling Jupiter’s Synchrotron Radiation from Radio Telescope Observations

In this study, we used 2.295 GHz radio telescope data to study synchrotron radiation from Jupiter’s magnetosphere. We processed scans of Jupiter and calibrators taken by the Goldstone Apple Valley Radio Telescope on various dates, developing and automating algorithms for outlier removal, baseline subtraction, and Gaussian fitting in order to determine the peak intensity of each scan. Comparing the peak intensities of Jupiter to those of the calibrators and the known fluxes of the calibrators, we computed the flux of Jupiter on each scan. Plotting Jupiter’s flux against the longitude facing Earth at the time of each scan revealed a periodic relationship between the variables and thus a model for expected synchrotron flux from Jupiter observed at a given longitude. This estimate can corroborate other flux measurements of Jupiter at similar frequencies, such as those taken by the microwave radiometer on the Junoprobe.

Investigating the Physical Conditions in Extended System Hosting Mid-infrared Bubble N14

Abstract To observationally explore physical processes, we present a multiwavelength study of a wide-scale environment toward l = 13.°7–14.°9 containing a mid-infrared bubble N14. The analysis of 12CO, 13CO, and C18O gas at [31.6, 46] km s−1 reveals an extended physical system (extension ∼59 pc × 29 pc), which hosts at least five groups of the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) 870 μm dust clumps at d ∼3.1 kpc. These spatially distinct groups/subregions contain unstable molecular clumps and are associated with several Class I young stellar objects (mean age ∼0.44 Myr). At least three groups of ATLASGAL clumps associated with the expanding H ii regions (including the bubble N14) and embedded infrared dark clouds, devoid of the ionized gas, are found in the system. The observed spectral indices derived using the Giant Metrewave Radio Telescope (GMRT) and THOR radio continuum data suggest the presence of nonthermal emission with the H ii regions. The high-resolution GMRT radio continuum map at 1280 MHz traces several ionized clumps powered by massive B-type stars toward N14, which are considerably young (age ∼103–104 yr). Locally, the early stage of star formation is evident toward all the groups of clumps. The position–velocity maps of 12CO, 13CO, and C18O exhibit an oscillatory-like velocity pattern toward the selected longitude range. Considering the presence of different groups/subregions in the system, the oscillatory pattern in velocity is indicative of the fragmentation process. All these observed findings favor the applicability of the global collapse scenario in the extended physical system, which also seems to explain the observed hierarchy.

Low(er) frequency follow-up of 28 candidate, large-scale synchrotron sources

AbstractWe follow up on a report by Vacca et al. (2018) of 28 candidate large-scale diffuse synchrotron sources in an 8°× 8°area of the sky (centred at RA 5h0m0s; Dec 5°48ʹ00ʹʹ). These sources were originally observed at 1.4 GHz using a combination of the single-dish Sardinia Radio Telescope and archival NRAO VLA Sky Survey data. They are in an area with nine massive galaxy clusters at$z \approx 0.1$and are candidates for the first detection of filaments of the synchrotron cosmic web. We attempt to verify these candidate sources with lower frequency observations at 154 MHz with the Murchison Widefield Array and at 887 MHz with the Australian Square Kilometre Array Pathfinder (ASKAP). We use a novel technique to calculate the surface brightness sensitivity of these instruments to show that our lower frequency observations, and in particular those by ASKAP, are ideally suited to detect large-scale, extended synchrotron emission. Nonetheless, we are forced to conclude that none of these sources are likely to be synchrotron in origin or associated with the cosmic web.

Type Ib Supernova Master OT J120451.50+265946.6: Radio-emitting Shock with Inhomogeneities Crossing through a Dense Shell

Abstract We present radio observations of Type Ib supernova (SN) Master OT J120451.50+265946.6. Our low-frequency Giant Metrewave Radio Telescope (GMRT) data, taken when the SN was in the optically thick phase for observed frequencies, reveal inhomogeneities in the structure of the radio-emitting region. The high-frequency Karl G. Jansky Very Large Array data indicate that the shock is crossing through a dense shell between ∼47 and ∼87 days. The data ≥100 days onward are reasonably well fit with the inhomogeneous synchrotron self-absorption model. Our model predicts that the inhomogeneities should smooth out at late times. Low-frequency GMRT observations at late epochs will test this prediction. Our findings suggest the importance of obtaining well-sampled wide-band radio data in order to understand the intricate nature of the radio emission from young supernovae.