arXiv subjects

Surajit Paul

Publications and source records attributed to Surajit Paul.

33 records · Page 2Linked to original sources

arXiv2020

Diffuse radio emission in the galaxy cluster SPT-CL J2031-4037: a steep spectrum intermediate radio halo?

The advent of sensitive low frequency radio observations has revealed a number of diffuse radio objects with peculiar properties that are challenging our understanding about the physics of the intracluster medium. Here, we report the discovery of a steep spectrum radio halo surrounding the central Brightest Cluster Galaxy (BCG) in the galaxy cluster SPT-CL J2031-4037. This cluster is morphologically disturbed yet has a weak cool core, an example of cool core/non-cool core transition system, which harbours a radio halo of $\sim 0.7$ Mpc in size. The halo emission detected at 1.7 GHz is less extended compared to that in the 325 MHz observation, and the spectral index of the part of the halo visible at 325 MHz to 1.7 GHz frequencies was found to be $-1.35 \pm 0.07$. Also, $P_{1.4\ \mathrm{GHz}}$ was found to be $0.77 \times 10^{24}$ W Hz$^{-1}$ which falls in the region where radio mini-halos, halo upper limits and ultra-steep spectrum (USS) halos are found in the $P_{1.4\ \mathrm{GHz}} - L_\mathrm{X}$ plane. Additionally, simulations presented in the paper provide support to the scenario of the steep spectrum. The diffuse radio emission found in this cluster may be a steep spectrum "intermediate" or "hybrid" radio halo which is transitioning into a mini-halo.

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arXiv2019

Probing the origin of diffuse radio emission in the cool-core of the Phoenix galaxy cluster

Cool core galaxy clusters are considered to be dynamically relaxed clusters with regular morphology and highly X-ray luminous central region. However, cool core clusters can also be sites for merging events that exhibit cold fronts in X-ray and mini-halos in radio. We present recent radio/X-ray observations of the Phoenix Cluster or SPT-CL J2344-4243 at the redshift of $z=0.596$. Using archival {\it Chandra} X-ray observations, we detect spiraling cool gas around the cluster core as well as discover two cold fronts near the core. It is perhaps the most distant galaxy cluster to date known to host cold fronts. Also, we present JVLA\footnote{Jansky Very Large Array\\ \url{https://science.nrao.edu/facilities/vla}} 1.52 GHz observations of the minihalo, previously discovered at 610 MHz with GMRT\footnote{Giant Metrewave Radio Telescope\\ \url{http://www.gmrt.ncra.tifr.res.in}} observations in the center of the Phoenix galaxy cluster. The minihalo flux density at 1.52 GHz is $9.65 \pm 0.97$ mJy with the spectral index between 610 MHz and 1.52 GHz being $-0.98 \pm 0.16$\footnote{$S_ν = ν^α$ where $S_ν$}. A possible origin of these radio sources is turbulence induced by sloshing of the gas in the cluster core.

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arXiv2019

Radio-relic and the diffuse emission trail discovered in a low mass galaxy cluster Abell 1697

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 $M_{500}^{X-ray}=2.9^{+0.8}_{-0.7}\times10^{14}~\rm{M_{\odot}}$, 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 $α(144,325,1400~\rm{MHz})=-0.98\pm0.01$ and magnetic field of $B_{eq}\sim0.6~μ$G. This relic, located in the northeast periphery of the cluster, is 300 kpc wide, exhibits a gradual spectral steepening across the width ($α_{144 \rm{MHz}}^{1.4\rm{GHz}}(inj)=-0.70\pm0.11$ to $α_{144 \rm{MHz}}^{1.4\rm{GHz}}(edge)=-1.19\pm0.15$), as well as indications of a co-spatial X-ray (ROSAT) shock and the radio relic emission. The radio power of the relic is $P_{1.4GHz}=8.5\pm1.1\times10^{23}~\rm{W\;Hz^{-1}}$, 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 ($790\times550$ kpc) diffuse radio emission towards the cluster center, that is likely an ultra-steep spectrum ($α_{144 \rm{MHz}}^{1.4\rm{GHz}}<-1.84$) radio source. This structure is also found to be older by at least 190 Myrs, has a very low surface brightness of $0.3~μ$Jy arcsec$^{-2}$ and magnetic field $B_{eq}\sim0.8~μ$G, 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.

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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.

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arXiv2019

Low-frequency radio study of MACS clusters at 610 and 235 MHz using the GMRT

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 ($α^{610}_{235}\sim -1.15$), giant ($\sim 1.6$~Mpc each) halo-relic structure and a close-by high-speed ($1769\pm^{148}_{359}$~km~s$^{-1}$) merger-shock ($\mathcal{M}=2.02\pm^{0.17}_{0.41}$) originated from a possible second merger in the cluster.

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arXiv2019

Manufacturing cosmic rays in the evolving dynamical states of galaxy clusters

Galaxy clusters are known to be reservoirs of Cosmic Rays (CRs), as inferred from theoretical calculations or detection of CR-derived observables. CR acceleration in clusters is mostly attributed to the dynamical activity that produces shocks. Shocks in clusters emerge out of merger or accretion, but which one is more effective in producing CRs? at which dynamical phase? and why? To this aim, we study the production or injection of CRs through shocks and its evolution in the galaxy clusters using cosmological simulations with the {\sc enzo} code. Particle acceleration model considered here is primarily the Diffusive Shock Acceleration (DSA) of thermal particles, but we also report a tentative study with pre-existing CRs. Defining appropriate dynamical states using the concept of virialization, we studied a sample of merging and non-merging clusters. We report that the merger shocks (with Mach number $\mathcal{M}\sim2-5$) are the most effective CR producers, while high-Mach peripheral shocks (i.e. $\mathcal{M}>5$) are mainly responsible for the brightest phase of CR injection in clusters. Clusters once merged, permanently deviate from CR and X-ray mass scaling of non-merging systems, enabling us to use it as a tool to determine the state of merger. Through a temporal and spatial evolution study, we found a strong correlation between cluster merger dynamics and CR injection. We observed that the brightest phase of X-ray and CR injection from clusters occur respectively at about 1.0 and 1.5 Gyr after every mergers, and CR injection peaks near to the cluster virial radius (i.e $r_{200}$). Delayed CR injection peaks found in this study deserve further investigation for possible impact on the evolution of CR-derived observables from galaxy clusters.

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arXiv2019

GMRT observations of extragalactic radio sources with steeply inverted spectra

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.

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arXiv2018

Towards building a first northern-sky sample of 'Extremely Inverted Spectrum Extragalactic Radio Sources (EISERS)'

We present here an extension of our search for EISERS (Extremely Inverted Spectrum Extragalactic Radio Sources) to the northern 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. A list of 15 EISERS candidates is presented here. It was assembled by applying a sequence of selection filters, starting with the two available large-sky radio surveys, namely the WENSS (325 MHz) and the ADR-TGSS (150 MHz). These surveys offer the twin advantages of being fairly deep (typical rms $<$ 10 mJy/beam) and having a sub-arcminute resolution. Their zone of overlap spreads over 1.3$π$ steradian in the northern hemisphere. Radio spectra are presented for the entire sample of 15 EISERS candidates, of which 8 spectra are of GPS type. Eleven members of the sample are associated with previously known quasars. Information on the parsec-scale radio structure, available for several of these sources, is also summarized.

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arXiv2018

`Rings' of diffuse radio emission surrounding the Bullet cluster

We present the discovery of ringlike diffuse radio emission structures in the peripheral regions of the Bullet cluster 1E 0657$-$55.8. Ring formations are spanning between 1--3 Mpc away from the center of the cluster, significantly further away from the two already reported relics. Integrated fluxes of four of the sub-regions in the inner `ring' from 4.5 to 10 GHz have also been reported. To understand the possible origin of these structures, here we present a maiden attempt of numerical modelling of a 3D and realistic `bullet' like event in a full cosmological ($Λ$CDM) environment with N-body plus hydrodynamics code. We report a simulated `bullet' found inside a (128 Mpc)$^3$ volume simulation with a speed of 2700 km s$^{-1}$, creating a high supersonic bow shock of Mach $M=3.5$ and a clear evidence of temporal separation of dark matter and baryons, assuring no challenge to $Λ$CDM cosmology from the bullet event as of now. We are also able to unveil the physics behind the formation of these observed multiple shock structures. Modelled radio emissions in our simulation support a complex combination of merger-associated processes that accelerates and re-accelerates fossil and cosmic-ray electrons. With a time evolution study and the computed radio emissions, we have shown that the ring like formation around the bullet is originated due to the interaction of the strong merger shocks with the accretion shocks at the periphery. The multiple shock structures observed are possibly originated from multiple mergers that have taken place at different times and much before the bullet event.

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arXiv2018

On the prospect of discovering `galaxy groups' through radio observations

Observed steep mass scaling of radio power from the available high mass clusters has ruled out the prospect of detection of 'galaxy groups'. But, the available simulations and observations of thermal emissions show that the groups are merger prone, thus non-virialised, indicating better visibility in the radio waves. Detection of radio emissions from them would help us to understand the scale-dependent particle acceleration mechanisms also groups can be a unique laboratory to test the models of cosmic magnetism and canbe the potential source of WHIMs. So, we have modelled radio emissions from the simulated structures using {\sc{ENZO}}. We present a model for computing magnetic field and for the first time, used the electron energy spectrum from both the Fermi I (DSA) and Fermi II (TRA) mechanisms to compute radio emissions. Computed radio power from more than 200 simulated objects, mass ranging $\geq 10^{13}$ to $2\times 10^{15} M_{\odot}$ show a new mass scaling of $M_{500} \propto P_{1.4\;GHz}^{2.17\pm 0.08}$ and a strong correlation scale of $L_X \propto P_{1.4\;GHz}^{1.08\pm 0.05}$. Both magnetic field and radio power are shown to have adequately replicated the available observations at high mass, allowing us to extend the results to further smaller masses. We report that groups below $10^{14}\;\rm{M_{\odot}}$ show the existence of 10s of nano to a sub-$μ$G magnetic field and about 10$^{19-23}$ W Hz$^{-1}$ of radio power, much higher than what existing mass scaling predicts. We found that the combined radio power from TRA and DSA electrons can only fit very well to all the observed `radio halos'. Finally, we have implemented this model on a real data set obtained from the Sloan Digital Sky Survey (SDSS). It predicts about 10s to 100s $μ$Jy/(10$\arcsec$ beam) of radio flux in groups indicating their detectability with existing and aplenty with the future radio telescopes.

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arXiv2016

Clusters of galaxies and the cosmic web with SKA

The intra-cluster and inter-galactic media (ICM, IGM) that pervade the large scale structure of the Universe are known to be magnetised at sub-micro Gauss to micro Gauss levels and to contain cosmic rays (CRs). The acceleration of CRs and their evolution along with that of magnetic fields in these media is still not well understood. Diffuse radio sources of synchrotron origin associated with the ICM such as radio halos, relics and mini-halos are direct probes of the underlying mechanisms of CR acceleration. Observations with radiotelescopes such as the GMRT, the VLA and the WSRT (0.15 - 2 GHz) have revealed scaling relations between the thermal and non-thermal properties of clusters and favour the role of shocks in the formation of radio relics and of turbulent re-acceleration in the formation of radio halos and mini-halos. Due to the limitations of current radio telescopes, wide-band studies and exploration of low mass and supercluster-scale systems is difficult. The Square Kilometer Array (SKA) is a next generation radio telescope that will operate in the frequency range of 0.05 - 20 GHz with unprecedented sensitivities and resolutions. The expected detection limits of SKA will reveal a few hundred to thousand new radio halos, relics and mini-halos providing the first large and comprehensive samples for their study. The wide frequency coverage along with sensitivity to extended structures will be able to constrain the CR acceleration mechanisms. The higher frequency (> 5 GHz) observations will be able to use the Sunyaev-Zel'dovich effect to probe the ICM pressure in addition to the tracers such as lobes of head-tail radio sources. The SKA also opens prospects to detect the "off-state" radio emission from the ICM predicted by the hadronic models and the turbulent re-acceleration models. [abridged]

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arXiv2012

Spectral and polarization study of the double relics in Abell 3376 using the GMRT and the VLA

Double radio relics in galaxy clusters are rare phenomena that trace shocks in the outskirts of merging galaxy clusters. We have carried out a spectral and polarization study of the spectacular double relics in the galaxy cluster A3376 using the Giant Metrewave Radio Telescope at 150 and 325 MHz and the Very Large Array at 1400 MHz. The polarization study at 1400 MHz reveals a high degree of polarization (~30%) and aligned magnetic field vectors (not corrected for Faraday rotation) in the eastern relic. A highly polarized (>60%) filamentary radio source of size ~300 kpc near the eastern relic and north of the bent-jet radio galaxy is detected for the first time. The western relic is less polarized and does not show aligned magnetic field vectors. The distribution of spectral indices between 325 and 1400 MHz over the radio relics show steepening from the outer to the inner edges of the relics. The spectral indices of the eastern and the western relics imply Mach numbers in the range 2.2 to 3.3. Remarkable features such as the inward filament extending from the eastern relic, the highly polarized filament, the complex polarization properties of the western relic and the separation of the BCG from the ICM by a distance >900 kpc are noticed in the cluster. A comparison with simulated cluster mergers is required to understand the complex properties of the double relics in the context of the merger in A3376. An upper limit (log(P(1.4GHz) W/Hz < 23.0) on the strength of a Mpc size radio halo in A3376 is estimated.

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arXiv2011

Discovery of the first giant double radio relic in a galaxy cluster found in the PLANCK Sunyaev-Zel'dovich cluster survey: PLCK G287.0+32.9

We report the discovery of large scale diffuse non-thermal radio emission in PLCK G287.0+32.9, an exceptionally hot (T ~ 13 keV), massive and luminous galaxy cluster, strongly detected by the PLANCK satellite in a recent, all-sky blind search for new clusters through Sunyaev-Zel'dovich effect. Giant Metrewave Radio telescope 150 MHz and Very Large Array 1.4 GHz radio data reveal a pair of giant (>1 Mpc) "arc" shaped peripheral radio-relics (signatures of shock waves) of unprecedented scale (linear separation ~4.4 Mpc at redshift 0.39), located at distances from the cluster center which are about 0.7 and 1.3 of the cluster's virial radius. Another possible giant relic and a radio-halo is detected closer to the cluster center. These relic sources are unique "signposts" of extremely energetic mergers and shocks (both ongoing and past), that are assembling and heating up this very massive galaxy cluster. They are also a probe of the filamentary cosmic-web structure beyond the cluster virial radius. Optical imaging with the IUCAA 2 meter telescope and XMM-Newton X-ray data confirm a very rich galaxy cluster with a morphologically disturbed core region, suggesting a dynamically perturbed merging system.

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arXiv2006

Giant Ringlike Radio Structures Around Galaxy Cluster Abell 3376

In the current paradigm of cold dark matter cosmology, large-scale structures are assembling through hierarchical clustering of matter. In this process, an important role is played by megaparsec (Mpc)-scale cosmic shock waves, arising in gravity-driven supersonic flows of intergalactic matter onto dark matter-dominated collapsing structures such as pancakes, filaments, and clusters of galaxies. Here, we report Very Large Array telescope observations of giant (~2 Mpc by 1.6 Mpc), ring-shaped nonthermal radio-emitting structures, found at the outskirts of the rich cluster of galaxies Abell 3376. These structures may trace the elusive shock waves of cosmological large-scale matter flows, which are energetic enough to power them. These radio sources may also be the acceleration sites where magnetic shocks are possibly boosting cosmic-ray particles with energies of up to 10^18 to 10^19 electron volts.

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arXiv2005

High and ultra-high energy cosmic ray acceleration in structure-formation shocks in Abell 3376 galaxy cluster

We describe multiwavelength optical, X-ray and radio evidences of energetic mergers in southern galaxy cluster Abell 3376 (A3376, redshift z=0.046, X-ray luminosity $ \rm L_{X}(0.1-2.4 keV)= 2.48 \times 10^{44} erg s^{-1}$). Here we report the rare discovery of gigantic, Mpc-scale non-thermal radio structures (`radio-arcs'), shaped like a pair of bow-shock fronts on the merger axis. We argue for the shock acceleration origin of the radio emission and discuss the possibilty of acceleration of very high energy cosmic ray particles in this and similar clusters. At redshift z=0.04, cluster A3376 is near enough that some of these primary or secondary cosmic ray components could actually be detected by new cosmic ray telescopes, particularly the energetic inverse Compton hard X-ray/gamma-ray photons upto extreme energies $E_γ \sim 100 {γ^{e}_{7}}^{2}$ GeV (where ${γ^{e}_{7}}$ is the electron Lorentz factor in units of $10^7$). Identification of radio or hard X-ray/gamma-ray emission from structure formation shocks holds a great promise for advancing current knowledge on shock formation in the IGM. It should provide the first direct evidence for such shocks, revealing the underlying large-scale cosmological flows. When combined with hard X-ray or gamma-ray detection, the radio signal will provide a direct measure of the unknown magnetic fields in the IGM, thus providing a better understanding of processes leading to IGM magnetization and also acceleration and propagation of ultra-high energy cosmic rays.

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