Studies have shown that mergers of massive galaxy clusters produce shocks and turbulence in the intra-cluster medium, the possible event that creates radio relics, as well as the radio halos. Here we present GMRT dual-band (235 and 610 MHz) radio observations of four such clusters from the MAssive Cluster Survey (MACS) catalogue. We report the discovery of a very faint, diffuse, elongated radio source with a projected size of about 0.5 Mpc in cluster MACSJ0152.5-2852. We also confirm the presence of a radio relic-like source (about 0.4 Mpc, previously reported at 325 MHz) in MACSJ0025.4-1222 cluster. Proposed relics in both these clusters are found apparently inside the virial radius instead of their usual peripheral location, while no radio halos are detected. These high-redshift clusters (z = 0.584 and 0.413) are among the earliest merging systems detected with cluster radio emissions. In MACSJ1931-2635 cluster, we found a radio mini-halo and an interesting highly bent pair of radio jets. Further, we present here a maiden study of low frequency (GMRT 235 & 610 MHz) spectral and morphological signatures of a previously known radio cluster MACSJ0014.3-3022 (Abell 2744). This cluster hosts a relatively flat spectrum (α 610 235 ∼ −1.15), giant (∼ 1.6 Mpc each) halo-relic structure and a close-by high-speed (1769± 148 359 km s −1 ) mergershock (M = 2.02± 0.17 0.41 ) originated from a possible second merger in the cluster.

We report quasi-simultaneous GMRT observations of seven extragalactic radio sources at 150, 325, 610 and 1400 MHz, in an attempt to accurately define their radio continuum spectra, particularly at frequencies below the observed spectral turnover. We had previously identified these sources as candidates for a sharply inverted integrated radio spectrum whose slope is close to, or even exceeds α c = +2.5, the theoretical limit due to synchrotron self-absorption (SSA) in a source of incoherent synchrotron radiation arising from relativistic particles with the canonical (i.e., power-law) energy distribution. We find that four out of the seven candidates have an inverted radio spectrum with a slope close to or exceeding +2.0, while the critical spectral slope α c is exceeded in at least one case. These sources, together with another one or two reported in very recent literature, may well be the archetypes of an extremely rare class, from the standpoint of violation of the SSA limit in compact extragalactic radio sources. However, the alternative possibility that free-free absorption is responsible for their ultra-sharp spectral turnover cannot yet be discounted.

We report the discovery of a putative radio relic, 830 kpc in length and found toward the outskirts of galaxy cluster Abell 1697 (z = 0.181), using the LOFAR Two Meter Sky Survey (LoTSS) at 144 MHz. With an X-ray-inferred mass of M X−ray 500 = 2.9 +0.8 −0.7 × 10 14 M , this places Abell 1697 among the least massive relic hosts. The relic is also detected at 325 MHz in the Westerbork Northern Sky Survey (WENSS) and at 1.4 GHz in the NRAO VLA Sky Survey (NVSS) with an average spectral index of α(144, 325, 1400 MHz) = −0.98 ± 0.01 and magnetic field of B eq ∼ 0.6 µG. This relic, located in the northeast periphery of the cluster, is 300 kpc wide, exhibits a gradual spectral steepening across the width (α 1.4GHz 144MHz (in j) = −0.70 ± 0.11 to α 1.4GHz 144MHz (edge) = −1.19 ± 0.15), as well as indications of a co-spatial X-ray (ROSAT) shock and the radio relic emission. The radio power of the relic is P 1.4GHz = 8.5 ± 1.1 × 10 23 W Hz −1 , which is found to be in good agreement with the expected empirical correlation between the radio power and Largest Linear Size (LLS) of relics. The relic is trailed by extended (790 × 550 kpc) diffuse radio emission towards the cluster center, that is likely an ultra-steep spectrum (α 1.4GHz 144MHz < −1.84) radio source. This structure is also found to be older by at least 190 Myrs, has a very low surface brightness of 0.3 µJy arcsec −2 and magnetic field B eq ∼ 0.8 µG, similar to that of a radio phoenix. Finally, we discuss the possible mechanisms responsible for the relic and the trailing diffuse radio emission, invoking re-acceleration due to wake turbulence, as well as the revival of fossil electrons from an old AGN activity by the cluster merger shocks.

We present an extension of our search for 'Extremely Inverted Spectrum Extragalactic Radio Sources' (EISERS) to the northern celestial hemisphere. With an inverted radio spectrum of slope α > +2.5, these rare sources would either require a non-standard particle acceleration mechanism (in the framework of synchrotron self-absorption hypothesis), or a severe free-free absorption which attenuates practically all of their synchrotron radiation at metre wavelengths. By applying a sequence of selection filters, a list of 15 EISERS candidates is extracted out by comparing two large-sky radio surveys, WENSS (325 MHz) and TGSS-ADR1 (150 MHz), which overlap across 1.03π steradian of the sky. Here we report quasi-simultaneous GMRT observations of these 15 EISERS candidates at 150 MHz and 325 MHz, in an attempt to accurately define their spectra below the turnover frequency. Out of the 15 candidates observed, two are confirmed as EISERS, since the slope of the inverted spectrum between these two frequencies is found to be significantly larger than the critical value α c = +2.5: the theoretical limit for the standard case of synchrotron self-absorption (SSA). For another 3 sources, the spectral slope is close to, or just above the critical value α c . Nine of the sources have GPS type radio spectra. The parsec-scale radio structural information available for the sample is also summarised.A few years ago, we combined the TIFR GMRT SKY SURVEY (TGSS/DR5) at 150 MHz with the 352 MHz WISH survey, to search for extragalactic radio sources whose

Low-mass (M500 &lt; 5 × 1014 M⊙) galaxy clusters have been largely unexplored in radio observations, because of the inadequate sensitivity of existing telescopes. However, the upgraded Giant Metrewave Radio Telescope (uGMRT) and the Low Frequency ARray (LoFAR), with unprecedented sensitivity at low frequencies, have paved the way to study less massive clusters more closely than before. We have started the first large-scale programme to systematically search for diffuse radio emission from low-mass galaxy clusters, chosen from the Planck Sunyaev–Zel’dovich cluster catalogue. We report here the detection of diffuse radio emission from four of the 12 objects in our sample, shortlisted from the inspection of the LoFAR Two-Meter Sky Survey data release 1 (LoTSS-I), followed up by uGMRT Band 3 deep observations. The clusters PSZ2 G089 (Abell 1904) and PSZ2 G111 (Abell 1697) are detected with relic-like emission, while PSZ2 G106 is found to have an intermediate radio halo and PSZ2 G080 (Abell 2018) seems to be a halo-relic system. PSZ2 G089 and PSZ2 G080 are among the lowest-mass clusters discovered with a radio-relic and a halo-relic system, respectively. A high ($\sim \! 30{{\ \rm per\ cent}}$) detection rate, with powerful radio emission (P1.4 GHz ∼ 1023 W Hz−1) found in most of these objects, opens up prospects of studying radio emission in galaxy clusters over a wider mass range, to much lower-mass systems.