Giant Metrewave Radio Telescope (uGMRT)

• Giant Metrewave Radio Telescope (uGMRT) is an upgraded low-frequency radio interferometric array near Pune, India, equipped with wideband receivers and state-of-the-art correlators.
• Its enhanced capabilities provide sub-arcsecond resolution and μJy sensitivity across 120–1500 MHz, enabling deep continuum, spectral, and time-domain investigations.
• The uGMRT supports diverse scientific programs from extragalactic surveys to cosmic dawn 21-cm cosmology and pulsar timing, complementing SKA precursors like MeerKAT and LOFAR.

The Giant Metrewave Radio Telescope (uGMRT) is an upgraded low-frequency radio interferometric array situated near Pune, India, designed for sensitive continuum, spectral line, and time-domain studies across a range of astrophysical environments. Building on the legacy Giant Metrewave Radio Telescope, the uGMRT incorporates wideband receivers, improved sensitivity, and new backend systems to address key problems in galaxy evolution, cosmic magnetism, large-scale structure, the interstellar and intergalactic medium, pulsars, transients, and cosmic dawn science. Its operational frequency range covers roughly 120–1500 MHz, providing unique leverage for spectral and morphological studies of both Galactic and extragalactic radio sources. The following sections detail the uGMRT’s technical capabilities, notable scientific programs, data processing infrastructure, and its impact across diverse areas of astrophysics.

1. Instrument Architecture and Sensitivity

The uGMRT comprises 30 parabolic antennas (each 45 m in diameter) distributed over a Y-shaped array with maximum baselines up to 25 km, enabling sub-arcsecond angular resolution at higher bands. The major leap in capability with the uGMRT stems from its new wide-band receiver systems, which extend instantaneous bandwidths up to 200–400 MHz per band (e.g., Band 3: 250–500 MHz, Band 4: 550–900 MHz, Band 5: 1000–1460 MHz), and their significantly improved system temperatures and digital correlators.

Improvements include:

• Continuum sensitivity: Achieving μJy/beam rms levels over wide fields; e.g., 4–9 μJy/beam at 650 MHz in deep extragalactic fields (Lal et al., 11 Sep 2025).
• Spectral dynamic range: Enabling detection of optical depths as low as 10⁻³ in HI 21 cm absorption against compact background sources (Kanekar, 2014).
• Polarimetry: Full-Stokes imaging is feasible within certain limits, subject to depolarization and RFI characteristics.
• Instantaneous frequency coverage: Spans 120–1500 MHz (split across four bands), enabling spectral index and turnover studies for sources ranging from pulsars and AGN to diffuse cluster emission.

These improvements allow highly competitive surveys—often equaling or complementing SKA precursor facilities (e.g., MeerKAT, LOFAR) in sensitivity at low frequencies, spatial resolution, and spectral coverage (Roy, 2018, Lal et al., 11 Sep 2025).

2. Extragalactic Surveys and Source Populations

The uGMRT plays a fundamental role in deep extragalactic radio surveys and population studies, particularly in synergy with contemporary wide-area projects:

• superMIGHTEE: Combines uGMRT (400, 650 MHz) and MeerKAT data (1.28 GHz) to produce sub-arcsecond, μJy-sensitivity images over 9.9 deg² (650 MHz) and 6.9 deg² (400 MHz) in the XMM-LSS, COSMOS, and E-CDFS fields (Lal et al., 11 Sep 2025). The resulting catalogs cover ~40,000 radio sources, with a redshift range up to z ≈ 4 (median z = 1).
• Spectral Evolution: Broadband spectra show a marked population transition near S ≈ 10 mJy: above this, the AGN component dominates, exhibiting steep optically thin synchrotron indices between α ≈ –0.57 and –0.75 (Sν ∝ ν^α); below this threshold, star-forming galaxies contribute significantly, exhibiting flatter indices (α ≈ –0.51 between 650–1280 MHz), and the fractional occurrence of peaked (GPS/CSS) spectra increases with decreasing flux density. The frequency coverage enables quantitative modeling of compact versus extended, and thermal versus nonthermal spectral characteristics.
• Orientation and Population Studies: Targeted surveys at 322 MHz (plus archival MHz–GHz data) of radio-loud BAL and non-BAL quasars show that BAL quasars are significantly more likely to exhibit steep/peaked spectra (indicative of jet axes further from the line of sight), supporting an orientation scenario but also suggesting a need for more complex models given the presence of flat/inverted spectrum BAL quasars (Hayashi et al., 11 Apr 2024).

3. Cluster, Galaxy, and Large-Scale Structures

The uGMRT’s spectral sensitivity, wide bandwidth, and high dynamic range are leveraged for studies of cluster physics, diffuse radio halos/relics, and mapping cosmic magnetic fields:

• Radio Megahalos: The first detection of a radio megahalo in PLCKG287.0+32.9 at 300–850 MHz, with emission extending ~3.2 Mpc (approaching R₅₀₀), reveals an outer region with a steep α ≤ –1.5 and a radio emissivity ~20× lower than the inner halo. The azimuthally averaged surface brightness shows a distinct flattening beyond ~0.5 R₅₀₀, matching characteristics predicted in cluster merger simulations involving late-stage turbulent reacceleration (Salunkhe et al., 15 Mar 2025).
• Diffuse Emission in Low-Mass Clusters: Systematic searches detect radio halos and relics even in clusters with M₅₀₀ < 5×10¹⁴M_⊙, extending the nonthermal parameter space and supporting the occurrence of powerful merger-driven particle acceleration at lower cluster masses (Paul* et al., 2021).
• Spectral Index Mapping of Distant Halos: uGMRT Bands 3/4 used in conjunction with LOFAR (144 MHz) differentiate between ‘classical’ halos (α ≈ –1.0 to –1.4) and ultra–steep spectrum (USS, α < –1.5) systems, supporting turbulent re-acceleration models and demonstrating that a significant fraction of high-z, massive clusters host very steep spectrum halos (Gennaro et al., 2021).
• Galaxy-Scale Magnetism and Halo Studies: Deep mapping of local galaxies (e.g., NGC3344, NGC3627, NGC3623, NGC4096, NGC4594, NGC4631) at 0.3–0.5 GHz reveals radio halos and extended disks previously undetected at higher frequencies, with scale lengths (from exponential fits, f(x) = a exp(–x/b)) up to 75% larger at 0.4 GHz compared to ~1.5 GHz. Spectral index maps show progressive steepening (α ~ –1.5) in the outer halo regions, providing constraints on cosmic ray propagation and magnetic field structure (Manna et al., 4 Nov 2024).
• Coma Cluster Imaging: High-resolution imaging at 250–500 MHz and 550–850 MHz provides spatially resolved spectral index and equipartition pressure mapping for both normal and ram pressure–stripped galaxies. Observations of head–tail galaxies, radio relics (e.g., B1253+275), ‘slingshot’ tails, and transverse spectral gradients reveal complex dynamical and radiative histories of cluster galaxies (Lal et al., 2022).

4. Cosmology, Neutral Hydrogen, and Cosmic Dawn

• 21-cm Cosmology: The uGMRT Band 1 (50–80 MHz) opens a direct observational window into the Cosmic Dawn (z ≈ 15–20). Simulation-based studies show that a 10σ detection of a strongly enhanced HI 21-cm power spectrum—potentially arising if baryon–dark matter (b–DM) interactions amplify the spatial fluctuations—can be achieved in 70 hours for optimistic foreground scenarios (Chatterjee et al., 2018). Signal and noise estimates use visibility-based power spectrum estimators:

$$P(k) = \frac{r'}{\tilde{Q}} \int \langle V(U,\nu)V^*(U,\nu+\Delta \nu) \rangle d(\Delta\nu), \ P_n(k_g) = \frac{T_\mathrm{sys}^2 r' r^2}{\Delta t N_p \tilde{\eta} \tau(k_g)}$$

where $P(k)$ is the HI signal, $P_n(k_g)$ is the noise power, and all terms are defined in (Chatterjee et al., 2018). The uGMRT remains noise-limited at long integration; in contrast, SKA-Low becomes cosmic variance limited on many k-modes.

• Damped Lyman-α Absorbers and HI Absorption: The detection of HI 21 cm absorbers at z ≈ 1.2 (Chowdhury et al., 2020) and z ≈ 2 (Kanekar, 2014) demonstrates the role of uGMRT's wide-band receivers in extending high-N$_\mathrm{HI}$ DLA samples. The canonical equation employed is:

$$N_\mathrm{HI} = 1.823 \times 10^{18} \left( \frac{T_s}{f} \right) \int \tau_{21\;\rm cm} dV \quad \rm{[cm}^{-2}\rm{]}$$

where $T_s$ is the spin temperature and $f$ is the covering factor. High opacity systems ($\gtrsim 10^{22}$ cm$^{-2}$) trace regions of high dust and molecular content, while spin temperature–metallicity anti-correlation measurements constrain star formation and gas cooling in high-redshift galaxies.

• Foreground Characterization: Measurements of the angular power spectrum of foregrounds (DGSE, point sources) using the Tapered Gridded Estimator (TGE) at 325 MHz yield power laws $\mathcal{C}_\ell = A \ell^{-\beta}$ with $\beta$ ≃ 2.28–2.55, underpinning foreground removal strategies vital for 21-cm tomography (Chakraborty et al., 2019).

5. Pulsars, Transients, and Time-Domain Applications

• Wideband Pulsar Timing and PTA Science: The uGMRT supports simultaneous observations in widely separated bands (e.g., 300–500 & 1260–1460 MHz), enabling precision measurements of pulse times of arrival (ToA) and dispersion measures (DM) via both narrowband and principal-component-derived wideband template techniques (Nobleson et al., 2021, Tarafdar et al., 2022). The wideband method jointly fits for ToA and DM with sub-μs/sensitivity, mitigating interstellar dispersion and providing data suitable for nanohertz gravitational wave searches as part of the Indian Pulsar Timing Array.
• Pulsar Spectral Studies: Broadband coverage enables robust identification and modeling (via free–free thermal absorption models) of gigahertz-peaked spectrum (GPS) pulsars, refining turnover frequencies and constraining local absorption environments (Rożko et al., 2021).
• Discovery and Localization of Fast Radio Bursts (FRBs): The combination of incoherent array mode and simultaneous interferometric imaging enables arcsecond localization of bursts from repeating FRBs, e.g., FRB 20201124A, and reveals persistent emission from host galaxies. Distribution properties (fluence, width, waiting times) and upper limits on scattering are measurable, with implications for progenitor models and host environments (Marthi et al., 2021).

6. Pipeline Infrastructure and Data Processing

Automated imaging and data reduction for uGMRT are exemplified by the CAPTURE pipeline (Kale et al., 2020), which, built atop CASA, supports:

• Comprehensive flagging (short and long baselines, narrow-band RFI),
• Stepwise, user-configurable flag-calibrate-image pipelines,
• Iterative self-calibration and conservative flagging to preserve extended emission,
• Science-ready products augmented with precise primary beam corrections (performed externally, e.g., wbpbgmrt).

This approach is modular and adaptable to various arrays, facilitating efficient and reproducible processing of wideband, high-data-rate observations.

7. Special Programs: Surveys of the Milky Way and Ultra-High Energy Sources

• Galactic Plane Surveys: The Metrewave Galactic Plane with the uGMRT Survey (MeGaPluG) demonstrates mosaiced continuum maps with 25″ resolution and sub-5 mJy/beam point source sensitivity at 400 and 650 MHz, enabling joint studies of extended and compact Galactic sources. Snapshot imaging, self-calibration, and auto-masking with advanced RFI excision strategies are integral (Dokara et al., 2023).
• PeVatron Candidate Identification: Deep uGMRT mapping near LHAASO ultra-high-energy gamma-ray sources revealed disk-jet morphologies in extended radio counterparts—a configuration consistent with microquasar engines capable of PeV particle acceleration. Analysis employs synchrotron and hadronic emission models to interpret multiwavelength associations (Mahanta et al., 18 Dec 2024).

Summary Table: uGMRT Capabilities and Science Domains

Science Area	Frequency Bands (MHz)	Techniques & Products
Extragalactic deep	250–900, 1200–1500	Continuum imaging, spectral indices, population stats
Cosmic dawn/21-cm	50–80, 250–500	Power spectrum, APS, DLA absorption
Pulsar/Timing array	300–500, 1260–1460	Wideband timing, dual-band, precision DM/ToA
Clusters/diffuse	300–900	Halo, relic, megahalo spectral/structural mapping
Milky Way/Plane	300–750	Mosaiced surveys, galactic structure, SNR/HII imaging
PeVatron searches	650	Disk-jet morphology, particle acceleration diagnostics

These capabilities position the uGMRT as a premier facility for probing nonthermal phenomena, time-variable and cosmological signals, structure formation, cosmic magnetism, and high-energy astrophysics across the low-frequency radio spectrum. Its modular, automated data frameworks, and broad spectral leverage complement facilities such as MeerKAT, LOFAR, and eventually, the SKA.