AR-2019-2020

not known observationally. The physical features of known stars are analyzed here, and found that compact star formation may be possible with repul- sive core. In the case of MCG in f ( T ) gravity com- pact stars may be obtained with anisotropic fluid ( p t > p r ), with maximum anisotropy at the center of the star, which however, is not found when MCG is absent. This has been studied in collaboration with Anirban Chanda, and Sagar Dey. NUSTAR and Swift observations of AMXP Swift J1756.92508 during its 2018 outburst We present here the timing and spectral analy- sis of the accreting millisecond pulsar (AMXP) SwiftJ1756.92508 during its recent outburst in 2018 using Swift and NuSTAR observations. The si- multaneous fitting of the Swift and NuSTAR spec- tra indicated that the source was in the hard state with a cut-off energy of about 74.58 keV. We also studied in detail the pulse profile of the AMXP and its dependence on energy. The colour- colour diagram of the source was different from those previously reported. We performed phase, and time-resolved spectral analysis using NuSTAR data. Pulse phase-resolved spectra were fitted with a power-law model, and significant changes in the spectral parameters with pulse phase were ob- served. The orbital phase and time-resolved spec- tra were fitted with a cut-off power-law model. The column density and photon index obtained from orbital phase spectral analysis were found to show some anti-correlation with the flux. Through time- resolved spectral analysis, we observed that the spectral parameters show positive correlation with each other and with the flux. We did not observe softening of the spectrum with time. No emission lines or Compton bump were observed in the spec- trum of the AMXP. This work has been done in collaboration with Binay Rai. Surajit Paul Radio relic and the diffuse emission trail discovered in low-mass galaxy cluster Abell 1697 We report the discovery of a putative radio relic, 830 kpc in length, and find towards the outskirts of galaxy cluster Abell 1697 ( z = 0 . 181), using the LOFAR Two Meter Sky Survey (LoTSS) at 144 MHz. An X-ray-inferred mass of M X − ray 500 = 2 . 9 +0 . 8 − 0 . 7 × 10 14 M places Abell 1697 among the least massive relic hosts. The relic is also detected at 325 MHz in the Westerbork Northern Sky Sur- vey (WENSS) and at 1.4 GHz in the NRAO VLA Sky Survey (NVSS), with an average spectral in- dex of α (144 , 325 , 1400 MHz) = − 0 . 98 ± 0 . 01 , and magnetic field of B eq ∼ 0 . 6 μ G. This relic, located in the north-east periphery of the cluster is 300 kpc wide, exhibits a gradual spectral steepening across the width ( α 1 . 4GHz 144MHz ( inj ) = − 0 . 70 ± 0 . 11 to α 1 . 4GHz 144MHz ( edge ) = − 1 . 19 ± 0 . 15), as well as indica- tions of a co-spatial X-ray (ROSAT) shock and the radio relic emission. The radio power of the relic is P 1 . 4 GHz = 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 ra- dio emission towards the cluster centre, which 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 sur- face brightness of 0 . 3 μ Jy arcsec − 2 and magnetic field B eq ∼ 0 . 8 μ G, similar to that of a radio phoenix. Finally, we discuss the possible mecha- nisms responsible for the relic and the trailing dif- fuse radio emission, invoking re-acceleration due to wake turbulence, as well as the revival of fossil elec- trons in the remnant radio lobes of active galactic nuclei (AGN) by the cluster merger shocks. This work has been done in collaboration with Sameer Salunkhe, Satish Sonkamble, Prateek Gupta, Tony Mroczkowski, and Somak Raychaudhury. Low-frequency radio study of MACS clusters using the GMRT at 610 and 235 MHz Studies have shown that mergers of massive galaxy clusters produce shocks and turbulence in the intra- cluster medium, the possible event that creates ra- dio 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 MAs- sive Cluster Survey (MACS) catalogue. We re- port the discovery of a very faint, diffuse, elon- gated 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 loca- tion, while no radio halos are detected. These high- redshift clusters ( z = 0 . 584 and 0 . 413) are among