Vivek Gupta

We present Very Large Telescope/X-Shooter spectroscopy for the host galaxies of 12 fast radio bursts (FRBs) detected by the Australian SKA Pathfinder observed through the “Fast and Unbiased FRB Host Galaxy (FURBY)” Large Programme at the European Southern Observatory, which imposes strict selection criteria on the included FRBs and their host galaxies to produce a homogeneous and well-defined sample. We describe the data reduction and analysis of these spectra and report their redshifts, line-emission fluxes, and derived host properties. From the present sample, this paper focuses on the faint host of FRB 20230708A ( m _R = 22.53 ± 0.02) identified at low redshift ( z = 0.1050). This indicates an intrinsically very low-luminosity galaxy ( L ≈ 10 ⁸ L _⊙ ), making it the lowest-luminosity nonrepeating FRB host to date by a factor of ∼3 and slightly dimmer than the lowest-luminosity host for repeating FRBs. Our spectral energy distribution fitting analysis reveals a low stellar mass ( M _* ≈ 10 ^8.0 M _⊙ ), low star formation rate (SFR ≈ 0.04 M _⊙ yr ⁻¹ ), and very low metallicity( 12 + log ( O / H ) ∼ ( 8.0 − 8.3 ) ), distinct from the more massive galaxies (log( M / M _⊙ ) ∼ 10) that are commonly identified for nonrepeating FRBs. Its discovery demonstrates that FRBs can arise in the faintest, metal-poor galaxies of the Universe. In turn, this suggests that at least one FRB progenitor channel must include stars (or their remnants) created in very low metallicity environments. This indicates better prospects for detecting FRBs from the high- z Universe where young, low-mass galaxies proliferate.

We present deep optical and near-infrared observations of the host galaxies of 34 fast radio bursts (FRBs) detected by the Commensal Real-time ASKAP Fast Transient (or CRAFT) survey on the Australian SKA Pathfinder (ASKAP) to compare the locations of FRBs relative to their host light distributions. Incorporating three additional FRBs from the literature, for a total of four repeating and 33 apparently nonrepeating FRBs, we determine their projected galactocentric offsets and find a median of 4 . 2 − 2.5 + 5.7 kpc ( 1 . 0 − 0.6 + 1.5 r e ). We model their host surface-brightness profiles and develop synthetic spatial distributions of their globular clusters (GCs) based on host properties. We calculate the likelihood the observed location of each FRB is consistent with the smooth light of its host galaxy, residual (primarily spiral) substructure, or GC distributions. The majority of FRBs favor locations within the disks of their galaxies, while only 11% ± 5% favor a GC origin, primarily those with galactocentric offsets ≳3 r _e . At z < 0.15, where spiral structure is apparent in 86% of our sample of FRB hosts, we find ≈20%–46% of FRBs favor an association with spiral arms. Assuming FRBs derive from magnetars, our results support multiple formation channels, with the majority of progenitors associated with massive stars and a minority formed through dynamical channels. However, the moderate fraction of FRBs associated with spiral structure indicates that high star formation efficiency of the youngest and most massive stars is not a predominant driver in the production of FRB progenitors.

We report the detection of a burst of emission over a 695.5–1031.5 MHz bandwidth by the Australian Square Kilometre Array Pathfinder. The burst was localized through analysis of near-field time delays to the long-decommissioned Relay 2 satellite and exhibited a dispersion measure of 2.26 × 10⁻⁵ pc cm⁻³—69.7 total electron content units, consistent with expectations for a single pass through the ionosphere. After coherent dedispersion, the burst was determined to be less than 30 ns in width, with an average flux density of at least 300 kJy. We consider an electrostatic discharge (ESD) or plasma discharge following a micrometeoroid impact to be plausible explanations for the burst. ESDs have previously been observed with the Arecibo radio telescope, but on 1000 times longer timescales. Our observation opens new possibilities for the remote sensing of ESD, which poses a serious threat to spacecraft, and reveals a new source of false events for observations of astrophysical transients.

Searches for impulsive, astrophysical transients are often highly computationally demanding. A notable example is the dedispersion process required for performing blind searches for fast radio bursts (FRBs) in radio telescope data. We introduce a novel approach – efficient summation of arbitrary masks (ESAM) – which efficiently computes 1D convolution of many arbitrary 2D masks and can be used to carry out dedispersion over thousands of dispersion trials efficiently. Our method matches the accuracy of the traditional brute force technique in recovering the desired signal-to-noise ratio while reducing computational cost by around a factor of 10. We compare its performance with existing dedispersion algorithms, such as the fast dispersion measure transform algorithm, and demonstrate how ESAM provides freedom to choose arbitrary masks and further optimise computational cost versus accuracy. We explore the potential applications of ESAM beyond FRB searches.

The emerging population of long-period radio transients (LPTs) shows both similarities and differences with normal pulsars. A key difference is that their radio emission is too bright to be powered solely by rotational energy. Various models have been proposed (including both white dwarf or neutron star origins), and their nature remains uncertain. Known LPTs have minutes-to-hours-long spin periods, while normal pulsars have periods ranging from milliseconds to seconds. Here, we report the discovery of PSR J0311+1402, an object with an intermediate spin period of 41 s, bridging the gap between LPTs and normal pulsars. PSR J0311+1402 exhibits low linear (∼25%) and circular polarization (∼5%) and a relatively steep spectral index (∼ −2.3), features similar to normal pulsars. However, its observed spin-down properties place it below the pulsar death line, where pair production and thus radio emission are expected to cease. The discovery of PSR J0311+1402 suggests the existence of a previously undetected population within this intermediate period range, presumably missed due to selection biases in traditional pulsar search methods. Finding more such objects is important to fill the current gap in neutron star spin periods, improving our understanding of the relationships among rotation-powered pulsars and LPTs.

The Molonglo Cross was first commissioned in 1965, as a transit radio (408 MHz) interferometer with the largest collecting area in the Southern hemisphere. In 1981, the telescope was redeveloped as an Earth-rotation synthesis interferometer using only the East-West arm (843 MHz), known as the Molonglo Observatory Synthesis Telescope. While the East-West arm was revitalised in the 2010s, the (slightly larger) North-South arm, which consists of two co-linear paraboloid cylindrical reflectors spanning 2× 778 ×12.7 m, had not been used for over 40 years. Re-fitting this 19800 m² collecting area with modern electronics is a cost-effective way of producing a significant survey instrument. The upgrades made to the entire signal chain of the NS arm from the antenna through the transport, digitisation and digital signal processing are described, along with the subsequent performance of the system. The instrument was designed to undertake pulsar timing and searching for dispersed single pulses (e.g., from Fast Radio Bursts [FRBs]). The upgraded system operated across the 806.25-856.25 MHz frequency range, and had a primary beam that spans 12.7×2.5 deg. It had dual linear polarisation capability and a lower system temperature and wider bandwidth compared to the East-West system. The digital signal processing was performed on servers with Graphics Processing Units, which enabled low-latency, high-speed data processing, and made use of pipelines built from existing and custom codes. It timed around 70 pulsars per day while running concurrent FRB searches at nearly 100% duty cycle during its operation.

Fast radio bursts (FRBs) are a promising tool for studying the low-density universe as their dispersion measures (DM) are extremely sensitive probes of electron column density. Active galactic nuclei (AGN) inject energy into the intergalactic medium, affecting the DM and their scatter. To determine the effectiveness of FRBs as a probe of AGN feedback, we analysed three different AGN models from the EAGLE simulation series. We measured the mean DM–redshift relation, and the scatter around it, using 2.56 × 10⁸ sightlines at 131 redshift (z) bins between 0 ≤ z ≤ 3. While the DM–redshift relation itself is highly robust against different AGN feedback models, significant differences are detected in the scatter around the mean: weaker feedback leads to more scatter. We find that ∼10⁴ localized FRBs are needed to discriminate between the scatter in standard feedback and stronger, more intermittent feedback models. The number of FRBs required is dependent on the redshift distribution of the detected population. A lognormal redshift distribution at z = 0.5 requires approximately 50 per cent fewer localized FRBs than a distribution centred at z = 1. With the Square Kilometre Array expected to detect >10³ FRBs per day, in the future, FRBs will be able to provide constraints on AGN feedback.