Talks

The discovery of persistent radio sources (PRSs) associated with three repeating fast radio bursts (FRBs) has provided crucial insight into the local environments of these enigmatic sources. Here, we present deep radio observations of the fields surrounding four active repeating FRBs—FRB 20220912A, FRB 20240114A, FRB 20240619D and FRB 20230607A —using the upgraded Giant Metrewave Radio Telescope (uGMRT) at low radio frequencies and Karl G. Jansky Very Large Array  (VLA) at high frequencies. Towards FRB 20240114A, we report the detection of a compact radio source at 650 MHz with a flux density of 65.6 ± 8.1 μJy/beam. Our spectral index measurements, host galaxy star formation rate, and constraints on the physical size of the source strongly support the interpretation that this is a persistent radio source (PRS) associated with the FRB. We also present preliminary results on the variability of the PRS over a month-long baseline. For FRB 20220912A, while our uGMRT data indicate that the radio emission is likely associated with star formation in the host galaxy, we will also present high-resolution follow-up observations from the VLA across the C, X, Ku, and K bands (5–21 GHz). These reveal a compact radio source at the FRB location nature of which is still unclear. In addition, we will present very recent results from our imaging study of FRB 20230607A, a repeater with an extremely high rotation measure (RM ≈ 12,000 rad/m²), Using uGMRT Band 4 and Band 5. For FRB 20240619D, we place upper limits on radio emission from an associated PRS or from star formation in the host galaxy. The detection of the PRS associated with FRB 20240114A is a valuable addition to the small but growing sample of PRSs associated with FRBs, reinforcing the idea that such sources arise from a magnetoionic medium surrounding the FRB sources.

Fast radio bursts (FRBs) are one of the most intriguing discoveries in the last two decades. Their cosmological distances imply that these short-lived events emit enormous amount of energy, and the exact mechanism and origin of such energy release is still an active topic of research. With huge magnetic energy reservoirs, magnetars are leading candidate sources of FRBs. The first detection of a FRB-like burst from the Galactic magnetar SGR 1935+2154 in April 2020 by CHIME and STARE2, the Galactic FRB 20200428D, provided an irrefutable observational link between FRBs and magnetars. Since then, a few more bright bursts have been detected from SGR 1935+2154 with fluences in excess of 1 kJy ms, including the burst in October 2022 that was detected at two widely separated frequencies with CHIME and GBT. While detection of these bursts strengthen the hypothesis that some of the FRBs originate from magnetars, further observational links between magnetars and FRBs have remained scarce. In this talk, I will present a detailed account of some of the magnetar observations including bursts observed from SGR 1935+2154 in this context, and discuss future approaches that could help in understanding the physical connections between magnetars and FRBs.

Radio interferometric observations, specially at low frequencies, are often hindered by radio frequency interferences (RFIs), making data processing a time-intensive challenge. With next-generation radio telescopes producing increasingly large datasets, the demand for automated data processing solutions has grown critical. We present GARUDA (Generic AI-based GMRT-tUned Radio Data Analysis pipeline), a novel automated pipeline designed for uGMRT data reduction. Written in Python and utilising modular CASA for calibration, GARUDA includes GNET, our custom Deep Learning based RFI detection model. With only two tunable parameters, GNET ensures flexibility and ease of use across diverse observations and frequency bands. The pipeline handles system issues and performs RFI excision, producing high-quality calibrated data ready for imaging. In this talk, I will present the capabilities of GARUDA, integrated with the LOFAR Transients Pipeline (TraP), for searching Long-Period Transients (LPTs) in GMRT archival data. This dataset spans nearly 20 years and offers adequate time resolution and sensitivity at low radio frequencies -- where LPTs are typically most luminous. We have processed a large set of Galactic plane observations from the GMRT archive and identified several intriguing transient candidates, which will also be highlighted.

Fast Radio Bursts (FRBs) are brief yet highly energetic pulses of radio emission, the origin of which remains largely uncertain. FRBs are classified into two categories based on their repetition behavior: repeaters and non-repeaters. Distinct progenitor models have been proposed to account for these differences, with magnetars often associated with repeaters and cataclysmic events with non-repeaters. Therefore, elucidating the differences between these two populations is essential for constraining their underlying origins. However, the difficulty in accurately localizing FRBs has posed a significant challenge in confirming whether their progenitors are indeed distinct. In this study, we estimate the galaxy number density around FRBs listed in the CHIME catalog 1 by utilizing the WISE × PS1 galaxy catalog. Our methodology emphasizes the large-scale galactic environments surrounding FRBs, thereby it is independent of precise localization. This approach enables the inclusion of a significantly larger sample—26 repeaters and 238 non-repeaters—approximately twice the number of currently localized FRBs. If repeaters and non-repeaters originate in distinct galactic environments, it could imply different host and progenitor types. Conversely, similar environments might suggest a common progenitor. Here, we present our findings by comparing the density increments of both repeaters and non-repeater sources. The Kolmogorov-Smirnov (KS) test for the distributions of galaxy number densities around the FRB sources indicates no significant difference between repeaters and non-repeaters with a p-value of 0.405. Our finding suggests that repeaters and non-repeaters could share similar galactic environments and, hence, similar host and progenitor types. In addition to this, we find that the majority of FRBs occur in underdense galactic environments compared to randomly selected regions, exhibiting a mild preference for young stellar populations.

Unimodular Henneaux-Teitelboim (HT) Gravity provides a fully diffeomorphism-invariant route to unimodular gravity by promoting the cosmological constant to a conjugate variable with an associated “unimodular time”. I will present two complementary applications. In 4D minisuperspace, starting from the connection representation, unimodular Hartle–Hawking wave packets yield a unitary inner product and normalizable states whose peaks track classical FRW dynamics. This work reframes “creation from nothing” as the emergence of a semiclassical Universe from interference of incident/reflected packets near the bounce, without invoking Vilenkin’s contour. In parallel, I will introduce a 2D cousin obtained via a centrally extended JT/KSY construction. In de Sitter quantum cosmology, superposing Lambda-eigenstates produces unitary evolution in unimodular time, controlled interference near T=0, and sharply separated WKB branches at late times. The same mechanism naturally accommodates transient quantum deformations of global dS and suggests a topology change interpretation in which contracting/expanding branches play the role of Universe/anti-Universe pairs.

There are thousands of FRB sources detected so far, out of which a few hyper-active sources can be studied in great detail due to their repeating nature. I will present a detailed broadband polarimetric analysis of the newest hyper-active repeater FRB 20240114A, providing insights into its central engine and surrounding environment. This FRB source was first detected by CHIME/FRB. As part of the FRB monitoring campaign called ÉCLAT (Extragalactic Coherent Light from Astrophysical Transients) with the Nançay Radio Telescope, we recorded roughly 800 bursts from FRB 20240114A to date at 1 - 2 GHz. In addition, using the Ultra-broadband receiver (UBB) on the Effelsberg 100-m telescope, spanning from 1.3 - 6 GHz, we also recorded around 900 bursts from multiple epochs until mid-2025. Through long-term monitoring of this source and studying its polarimetric properties, we have seen huge polarization position angle (PPA) variation from burst-to-burst within an observation, and apparently low rotation measure (RM) variations over ~10 months. I will present the statistics of variability between linear and circular polarization, as well as PPA variations both within and between the bursts of this FRB and across a wide bandwidth. I will then compare FRB 20240114A to other known repeaters and to predictions made by progenitor and emission models.

Fast Radio Bursts (FRBs) are millisecond-duration, high-energy radio transients of extragalactic origin. FRB 20201124A is a repeating source that has shown significant activity across a broad frequency range. In this presentation, I will present a spectro-temporal analysis of FRB 20201124A using observations from the uGMRT conducted between 8 May and 15 June 2021. Bursts were detected in Band 4 (550–850 MHz) and Band 5 (1060–1460 MHz), with high-frequency activity ceasing after 24 May. On 28 May, multiple bursts were detected in Band 4 but none in Band 5, suggesting possible spectral evolution. Two burst pairs separated by less than 300 ms were identified, consistent with short repetition timescales or potential sub-second periodicity. The waiting time and energy distributions show a bimodal structure, indicating the presence of distinct emission modes. The fluence distribution follows a broken power-law, similar to other known repeating FRBs. These results provide constraints on the spectral and temporal behaviour of FRB 20201124A.