arXiv subjects

Shubham Bhagat

Publications and source records attributed to Shubham Bhagat.

4 recordsLinked to original sources

arXiv2022

Deciphering the ultra-steep spectrum diffuse radio sources discovered in the cool-core cluster Abell~980

Clusters of galaxies are excellent laboratories for studying recurring nuclear activity in galactic nuclei since their hot gaseous medium can vastly prolong the detectability of their radio lobes via better confinement. We report here a multi-band study of the sparsely studied galaxy cluster Abell 980, based on our analysis of {\it{Chandra}} X-ray and the GMRT (150 and 325 MHz) and EVLA (1.5 GHz) radio archival data, revealing an unusually rich phenomenology. It is shown to be a quasi-relaxed cluster with a cool core ($T\sim4.2$ keV) surrounded by a hot and extensive intracluster medium (ICM) at $T\sim6.8$ keV. The radio emission shows a rich diversity, having (i) two large diffuse sources of ultra-steep spectrum (USS), extending to opposite extremities of the ICM, each associated with an X-ray brightness discontinuity (cold front), (ii) a bright radio-double of size $\sim55$ kpc coinciding with the central BCG, and (iii) a diffuse radio source, likely a mini-halo of size $\sim110$~kpc around the BCG which possesses a huge ellipsoidal stellar halo of extent $\sim 80$~kpc. The association of cold fronts with two highly aged (~ 260 Myr) USS sources in a cool-core cluster, makes it a very rare system. These USS sources are probably radio lobes from a previous episode of jet activity in the BCG, driven buoyantly towards the outskirts of the X-ray halo, thereby creating the cold fronts. A deeper radio image of this cluster may provide a rare opportunity to verify the recently proposed alternative model which explains radio mini-haloes as the aggregate radio emission from Type I supernovae occurring in the giant stellar halo extended across the cluster core.

astro-ph.GA↗

arXiv2021

uGMRT detection of cluster radio emission in low-mass Planck~SZ clusters

Low-mass ($M_{\rm{500}}<5\times10^{14}{\rm{M_\odot}}$) galaxy clusters have been largely unexplored in radio observations, due to the inadequate sensitivity of existing telescopes. However, the upgraded GMRT (uGMRT) and the Low Frequency ARray (LoFAR), with unprecedented sensitivity at low frequencies, have paved the way to closely study less massive clusters 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 (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 ($\sim30\%$) detection rate, with powerful radio emission ($P_{1.4\ {\rm GHz}}\sim10^{23}~{\rm{W~Hz^{-1}}}$) 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.

astro-ph.GA↗

arXiv2020

Strain-induced tuning of optical properties of layered Mo$S_2$

The sensitive correlation between optical parameters and strain in Mo$S_2$ results in a totally different approach to tune the optical properties. Usually, an external source of strain is employed to monitor the optical and vibrational properties of a material. It is always challenging to have a precise control over the strain and its consequences on material properties. Here, we report the presence of a compressive strain in Mo$S_2$ crystalline powder and nanosheets obtained via the process of ball-milling and probe sonication. The diffraction peaks in the X-ray diffraction pattern shift to higher 2$θ$ value implying a compressive strain that increases with the processing time. The absorption spectra, photoluminescence and Raman modes are blue-shifted w.r.t the bulk unprocessed sample. The observed blue-shift is attributed to the presence of compressive strain in the samples. Whereas in thin nano-sheets of Mo$S_2$, it is very likely that both quantum confinement as well as strain result in the observed blue-shift. These results indicate that by optimizing the processing conditions and/or time, a strain of desired amount and hence tunable shift in optical properties of material can be achieved.

cond-mat.mes-hall↗

arXiv2019

GMRT observations of a first sample of Extremely Inverted Spectrum Extragalactic Radio Sources (EISERS) candidates in the Northern sky

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 1.03$π$ 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.

astro-ph.GA↗