arXiv subjects

S. Mandal

Publications and source records attributed to S. Mandal.

At least 73 records · Page 4Linked to original sources

arXiv2020

The life cycle of radio galaxies in the LOFAR Lockman Hole field

Radio galaxies are known to go through cycles of activity, where phases of apparent quiescence can be followed by repeated activity of the central supermassive black hole. A better understanding of this cycle is crucial for ascertaining the energetic impact that the jets have on the host galaxy, but little is known about it. We used deep LOFAR images at 150 MHz of the Lockman Hole extragalactic field to select a sample of 158 radio sources with sizes $> 60^{\prime\prime}$ in different phases of their jet life cycle. Using a variety of criteria (e.g. core prominence combined with low-surface brightness of the extended emission and steep spectrum of the central region) we selected a subsample of candidate restarted radio galaxies representing between 13% and 15% of the 158 sources of the main sample. We compare their properties to the rest of the sample, which consists of remnant candidates and active radio galaxies. Optical identifications and characterisations of the host galaxies indicate similar properties for candidate restarted, remnant, and active radio galaxies, suggesting that they all come from the same parent population. The fraction of restarted radio galaxies is slightly higher with respect to remnants, suggesting that the restarted phase can often follow after a relatively short remnant phase (the duration of the remnant phase being a few times 10$^{7}$ years). This confirms that the remnant and restarted phases are integral parts of the life cycle of massive elliptical galaxies. A preliminary investigation does not suggest a strong dependence of this cycle on the environment surrounding any given galaxy.

astro-ph.GA↗

arXiv2020

Penning Spectroscopy and Structure of Acetylene Oligomers in He Nanodroplets

Embedded atoms or molecules in a photoexcited He nanodroplet are well-known to be ionized through inter-atomic relaxation in a Penning process. In this work, we investigate the Penning ionization of acetylene oligomers occurring from the photoexcitation bands of He nanodroplets. In close analogy to conventional Penning electron spectroscopy by thermal atomic collisions, the n = 2 photoexcitation band plays the role of the metastable atomic $1s2s$ $^{3,1}S$ He$^\ast$. This facilitates electron spectroscopy of acetylene aggregates in the sub-kelvin He environment, providing the following insight into their structure: The molecules in the dopant cluster are loosely bound van der Waals complexes rather than forming covalent compounds. In addition, this work reveals a Penning process stemming from the n = 4 band where charge-transfer from autoionized He in the droplets is known to be the dominant relaxation channel. This allows for excited states of the remnant dopant oligomer Penning-ions to be studied. Hence, we demonstrate Penning ionization electron spectroscopy of doped droplets as an effective technique for investigating dopant oligomers which are easily formed by attachment to the host cluster.

physics.atm-clus↗

arXiv2020

Coupling between Exciton-Polariton Corner Modes through Edge States

Recently realized higher order topological insulators have taken a surge of interest among the theoretical and experimental condensed matter community. The two-dimensional second order topological insulators give rise to zero-dimensional localized corner modes that reside within the band gap of the system along with edge modes that inhabit a band edge next to bulk modes. Thanks to the topological nature, information can be trapped at the corners of these systems, which will be unhampered even in the presence of disorder. Being localized at the corners, the exchange of information among the corner states is an issue. Here we show that the nonlinearity in an exciton polariton system can allow the coupling between the different corners through the edge states based on optical parametric scattering, realizing a system of multiple connectible topological modes.

cond-mat.mes-hall↗

arXiv2020

One-way reflection-free exciton-polariton spin filtering channel

We consider theoretically exciton-polaritons in a strip of honeycomb lattice with zigzag edges and it is shown that the interplay among the spin-orbit coupling, Zeeman splitting, and an onsite detuning between sublattices can give rise to a band structure where all the edge states of the system split in energy. Within an energy interval, one of the spin polarized edge states resides with the gap-less bulk having opposite spin. Being surrounded by opposite spin and the absence of the backward propagating edge state ensures both reflection free and feedback suppressed one-way flow of polaritons with one particular spin in the system. The edge states in this system are more localized than those in the standard topological polariton systems and are fully spin polarized. This paves the way for feedback free spin-selective polariton channels for transferring information in polariton networks.

cond-mat.mes-hall↗

arXiv2019

New insights in giant molecular cloud hosting S147/S153 complex: signatures of interacting clouds

In order to understand the formation of massive OB stars, we report a multi-wavelength observational study of a giant molecular cloud hosting the S147/S153 complex (size ~90 pc X 50 pc). The selected complex is located in the Perseus arm, and contains at least five HII regions (S147, S148, S149, S152, and S153) powered by massive OB stars having dynamical ages of ~0.2 - 0.6 Myr. The Canadian Galactic Plane Survey 12CO line data (beam size ~100".4) trace the complex in a velocity range of [-59, -43] km/s, and also reveal the presence of two molecular cloud components around -54 and -49 km/s in the direction of the complex. Signatures of the interaction/collision between these extended cloud components are investigated through their spatial and velocity connections. These outcomes suggest the collision of these molecular cloud components about 1.6 Myr ago. Based on the observed overlapping zones of the two clouds, the collision axis appears to be parallel to the line-of-sight. Deep near-infrared photometric analysis of point-like sources shows the distribution of infrared-excess sources in the direction of the overlapping zones of the molecular cloud components, where all the HII regions are also spatially located. All elements put together, the birth of massive OB stars and embedded infrared-excess sources seems to be triggered by two colliding molecular clouds in the selected site. High resolution observations of dense gas tracer will be required to further confirm the proposed scenario.

astro-ph.GA↗

arXiv2019

On the possibility of a terahertz light emitting diode based on a dressed quantum well

We consider theoretically the realization of a tunable terahertz light emitting diode from a quantum well with dressed electrons placed in a highly doped p-n junction. In the considered system the strong resonant dressing field forms dynamic Stark gaps in the valence and conduction bands and the electric field inside the p-n junction makes the QW asymmetric. It is shown that the electrons transiting through the light induced Stark gaps in the conduction band emit photons with energy directly proportional to the dressing field. This scheme is tunable, compact, and shows a fair efficiency.

cond-mat.mes-hall↗

arXiv2019

Revived Fossil Plasma Sources in Galaxy Clusters

It is well established that particle acceleration by shocks and turbulence in the intra-cluster medium can produce cluster-scale synchrotron emitting sources. However, the detailed physics of these particle acceleration processes is still not well understood. One of the main open questions is the role of fossil relativistic electrons that have been deposited in the intra-cluster medium by radio galaxies. These synchrotron-emitting electrons are very difficult to study, as their radiative life time is only tens of Myrs at GHz frequencies, and are therefore a relatively unexplored population. Despite the typical steep radio spectrum due to synchrotron losses, these fossil electrons are barely visible even at radio frequencies well below a GHz. However, when a pocket of fossil radio plasma is compressed, it boosts the visibility at sub-GHz frequencies, creating so-called radio phoenices. This compression can be the result of bulk motion and shocks in the ICM due to merger activity. In this paper, we demonstrate the discovery potential of low frequency radio sky surveys to find and study revived fossil plasma sources in galaxy clusters. We used the 150~MHz TGSS and 1.4 GHz NVSS sky surveys to identify candidate radio phoenices. A subset of three candidates were studied in detail using deep multi-band radio observations (LOFAR and GMRT), X-ray (\textit{Chandra} or \textit{XMM-Newton}) and archival optical observations. Two of the three sources are new discoveries. Using these observations, we identified common observational properties (radio morphology, ultra-steep spectrum, X-ray luminosity, dynamical state) that will enable us to identify this class of sources more easily, and help to understand the physical origin of these sources.

astro-ph.CO↗

arXiv2019

Charge-exchange dominates long-range interatomic Coulombic decay of excited metal-doped He nanodroplets

Atoms and molecules attached to rare gas clusters are ionized by an interatomic autoionization process traditionally termed 'Penning ionization' when the host cluster is resonantly excited. Here we analyze this process in the light of the interatomic Coulombic decay (ICD) mechanism, which usually contains a contribution from charge exchange at short interatomic distance, and one from virtual photon transfer at large interatomic distance. For helium (He) nanodroplets doped with alkali metal atoms (Li, Rb), we show that long-range and short-range contributions to the interatomic autoionization can be clearly distinguished by detecting electrons and ions in coincidence. Surprisingly, ab initio calculations show that even for alkali metal atoms floating in dimples at large distance from the nanodroplet surface, autoionization is largely dominated by charge exchange ICD. Furthermore, the measured electron spectra manifest ultrafast internal relaxation of the droplet into mainly the 1s2s 1^S state and partially into the metastable 1s2s 3^S state.

physics.atm-clus↗

arXiv2019

Study of the long-term evolution of the accretion dynamics of GX 339-4

We study the dynamical behaviour of the galactic black hole source GX 339-4 during 2002-2011 outbursts using RXTE, Swift(XRT), XMM-Newton(PN) archival data. We present the spectral evolution of the source using four outbursts data and discuss their similarities/differences between outbursts. We infer that the second peak in 2002/03 and 2004/05 outbursts can be due to a second instant of triggered instability in the accretion disc due to irradiation from the central X-ray source after peak-I. This propagates in viscous time scale and takes ~80-90 days after peak-I to produce peak-II. This unifies all four outbursts having a long rising time of ~90 days. The dynamical evolution of accretion parameters have been studied by modeling the individual observed spectrum with two-component accretion disc model where a Keplerian accretion disc produces the soft photons and the hard part of the spectrum originates from a hot sub-Keplerian central corona. A generic mathematical model has been proposed to understand the evolution of accretion parameters for sources like GX 339-4 which have longer rising time. Also, the possible differences of physical scenario for outbursts with shorter rising time are also discussed.

astro-ph.HE↗

arXiv2019

Anti-chiral edge states in an exciton polariton strip

We present a scheme to obtain anti-chiral edge states in an exciton-polariton honeycomb lattice with strip geometry, where the modes corresponding to both edges propagate in the same direction. Under resonant pumping the effect of a polariton condensate with nonzero velocity in one linear polarization is predicted to tilt the dispersion of polaritons in the other, which results in an energy shift between two Dirac cones and the otherwise flat edge states become tilted. Our simulations show that due to the spatial separation from the bulk modes the edge modes are robust against disorder.

cond-mat.mes-hall↗

arXiv2018

Ultra-steep spectrum emission in the merging galaxy cluster Abell 1914

A number of radio observations have revealed the presence of large synchrotron-emitting sources associated with the intra-cluster medium. There is strong observational evidence that the emitting particles have been (re-)accelerated by shocks and turbulence generated during merger events. The particles that are accelerated are thought to have higher initial energies than those in the thermal pool but the origin of such mildly relativistic particles remains uncertain and needs to be further investigated. The galaxy cluster Abell 1914 is a massive galaxy cluster in which X-ray observations show clear evidence of merging activity. We carried out radio observations of this cluster with the LOw Frequency ARay (LOFAR) at 150 MHz and the Giant Metrewave Radio Telescope (GMRT) at 610 MHz. We also analysed Very Large Array (VLA) 1.4 GHz data, archival GMRT 325 MHz data, CFHT weak lensing data and Chandra observations. Our analysis shows that the ultra-steep spectrum source (4C38.39; $α\lesssim -2$), previously thought to be part of a radio halo, is a distinct source with properties that are consistent with revived fossil plasma sources. Finally, we detect some diffuse emission to the west of the source 4C38.39 that could belong to a radio halo.

astro-ph.CO↗

arXiv2018

The LOFAR Two-metre Sky Survey - II. First data release

The LOFAR Two-metre Sky Survey (LoTSS) is an ongoing sensitive, high-resolution 120-168MHz survey of the entire northern sky for which observations are now 20% complete. We present our first full-quality public data release. For this data release 424 square degrees, or 2% of the eventual coverage, in the region of the HETDEX Spring Field (right ascension 10h45m00s to 15h30m00s and declination 45$^\circ$00$'$00$''$ to 57$^\circ$00$'$00$''$) were mapped using a fully automated direction-dependent calibration and imaging pipeline that we developed. A total of 325,694 sources are detected with a signal of at least five times the noise, and the source density is a factor of $\sim 10$ higher than the most sensitive existing very wide-area radio-continuum surveys. The median sensitivity is S$_{\rm 144 MHz} = 71\,μ$Jy beam$^{-1}$ and the point-source completeness is 90% at an integrated flux density of 0.45mJy. The resolution of the images is 6$''$ and the positional accuracy is within 0.2$''$. This data release consists of a catalogue containing location, flux, and shape estimates together with 58 mosaic images that cover the catalogued area. In this paper we provide an overview of the data release with a focus on the processing of the LOFAR data and the characteristics of the resulting images. In two accompanying papers we provide the radio source associations and deblending and, where possible, the optical identifications of the radio sources together with the photometric redshifts and properties of the host galaxies. These data release papers are published together with a further $\sim$20 articles that highlight the scientific potential of LoTSS.

astro-ph.GA↗

arXiv2018

The spectacular cluster chain Abell 781 as observed with LOFAR, GMRT, and XMM-Newton

A number of merging galaxy clusters show the presence of large-scale radio emission associated with the intra-cluster medium (ICM). These synchrotron sources are generally classified as radio haloes and radio relics. Whilst it is commonly accepted that mergers play a crucial role in the formation of radio haloes and relics, not all the merging clusters show the presence of giant diffuse radio sources and this provides important information concerning current models. The Abell 781 complex is a spectacular system composed of an apparent chain of clusters on the sky. Its main component is undergoing a merger and hosts peripheral emission that is classified as a candidate radio relic and a disputed radio halo. We used new LOw Frequency ARay (LOFAR) observations at 143 MHz and archival Giant Metrewave Radio Telescope (GMRT) observations at 325 and 610 MHz to study radio emission from non-thermal components in the ICM of Abell 781. Complementary information came from XMM-Newton data, which allowed us to investigate the connection with the thermal emission and its complex morphology. The origin of the peripheral emission is still uncertain. We speculate that it is related to the interaction between a head tail radio galaxy and shock. However, the current data allow us only to set an upper limit of $\mathcal{M} < 1.4$ on the Mach number of this putative shock. Instead, we successfully characterise the surface brightness and temperature jumps of a shock and two cold fronts in the main cluster component of Abell 781. Their positions suggest that the merger is involving three substructures. We do not find any evidence for a radio halo either at the centre of this system or in the other clusters of the chain. We place an upper limit to the diffuse radio emission in the main cluster of Abell 781 that is a factor of 2 below the current radio power-mass relation for giant radio haloes.

astro-ph.CO↗

arXiv2018

Semiconductor quantum well irradiated by a two-mode electromagnetic field as a terahertz emitter

We study theoretically the nonlinear optical properties of a semiconductor quantum well (QW) irradiated by a two-mode electromagnetic wave consisting of a strong resonant dressing field and a weak off-resonant driving field. In the considered strongly coupled electron-field system, the dressing field opens dynamic Stark gaps in the electron energy spectrum of the QW, whereas the driving field induces electron oscillations in the QW plane. Since the gapped electron spectrum restricts the amplitude of the oscillations, the emission of a frequency comb from the QW appears. Therefore, the doubly-driven QW operates as a nonlinear optical element which can be used, particularly, for optically controlled generation of terahertz radiation.

cond-mat.mes-hall↗

arXiv2018

LOFAR discovery of a double radio halo system in Abell 1758 and radio/X-ray study of the cluster pair

Radio halos and radio relics are diffuse synchrotron sources that extend over Mpc-scales and are found in a number of merger galaxy clusters. They are believed to form as a consequence of the energy that is dissipated by turbulence and shocks in the intra-cluster medium (ICM). However, the precise physical processes that generate these steep synchrotron spectrum sources are still poorly constrained. We present a new LOFAR observation of the double galaxy cluster Abell 1758. This system is composed of A1758N, a massive cluster hosting a known giant radio halo, and A1758S, which is a less massive cluster whose diffuse radio emission is confirmed here for the first time. Our observations have revealed a radio halo and a candidate radio relic in A1758S, and a suggestion of emission along the bridge connecting the two systems which deserves confirmation. We combined the LOFAR data with archival VLA and GMRT observations to constrain the spectral properties of the diffuse emission. We also analyzed a deep archival Chandra observation and used this to provide evidence that A1758N and A1758S are in a pre-merger phase. The ICM temperature across the bridge that connects the two systems shows a jump which might indicate the presence of a transversal shock generated in the initial stage of the merger.

astro-ph.CO↗

arXiv2018

Accretion Flow Dynamics During 1999 Outburst of XTE J1859+226 - Modeling of Broadband Spectra and Constraining the Source Mass

We examine the dynamical behavior of accretion flow around XTE J1859+226 during the 1999 outburst by analyzing the entire outburst data ($\sim$ 166 days) from RXTE Satellite. Towards this, we study the hysteresis behavior in the hardness intensity diagram (HID) based on the broadband ($3 - 150$ keV) spectral modeling, spectral signature of jet ejection and the evolution of Quasi-periodic Oscillation (QPO) frequencies using the two-component advective flow model around a black hole. We compute the flow parameters, namely Keplerian accretion rate (${\dot m}_d$), sub-Keplerian accretion rate (${\dot m}_h$), shock location ($r_s$) and black hole mass ($M_{bh}$) from the spectral modeling and study their evolution along the q-diagram. Subsequently, the kinetic jet power is computed as $L^{\rm obs}_{\rm jet}\sim 3 - 6 \times 10^{37}$ erg~s$^{-1}$ during one of the observed radio flares which indicates that jet power corresponds to $8-16\%$ mass outflow rate from the disc. This estimate of mass outflow rate is in close agreement with the change in total accretion rate ($\sim 14\%$) required for spectral modeling before and during the flare. Finally, we provide a mass estimate of the source XTE J1859+226 based on the spectral modeling that lies in the range of $5.2 - 7.9 M_{\odot}$ with 90\% confidence.

astro-ph.HE↗

arXiv2018

Observation of a superconducting glass state in granular superconducting diamond

The magnetic field dependence of the superconductivity in nanocrystalline boron doped diamond thin films is reported. Evidence of a glass state in the phase diagram is presented, as demonstrated by electrical resistance and magnetic relaxation measurements. The position of the phase boundary in the H-T plane is determined from resistance data by detailed fitting to zero-dimensional fluctuation conductivity theory. This allows determination of the boundary between resistive and non-resistive behavior to be made with greater precision than the standard ad hoc onset/midpoint/offset criterion.

cond-mat.supr-con↗

arXiv2018

Can ultraprecision polishing techniques improve the coherence times of nitrogen-vacancy centers in diamond?

We investigate the correlation between surface roughness and corresponding $T_2$ times of nearsurface nitrogen-vacancy centers (~7 nm/ 5 keV implantation energy) in diamond. For this purpose we compare five different polishing techniques, including both purely mechanical as well as chemical mechanical approaches, two different substrate sources (Diam2tec and Element Six) and two different surface terminations (O- and H-termination) during nitrogen-vacancy forming. All coherence times are measured and compared before and after an oxygen surface treatment at 520 °C. We find that the coherence times of shallow nitrogen-vacancy centers are surprisingly independent of surface roughness.

cond-mat.mes-hall↗