arXiv subjects

Somasri Sen

Publications and source records attributed to Somasri Sen.

At least 19 recordsLinked to original sources

arXiv2026

Spectral evolution of NS binary system GX 349+2 using AstroSat observations

We present a broadband spectral study of the Sco-like Z source GX 349+2 using AstroSat LAXPC and SXT observations obtained during 2019 and 2024. The X-ray light curves exhibit large variability, and the hardness-intensity diagram traces the characteristic Z-shaped track. Using hardness ratio and intensity, the Z-track is segmented into six regions comprising normal branch, soft apex, and an extended flaring branch. We perform simultaneous broadband spectral fitting in the energy range 0.7-25.0 keV using two model configurations, tbabs*(thcomp*bbodyrad+diskbb) and tbabs*(bbodyrad+thcomp*diskbb), to study the evolution of emission from the neutron-star boundary layer, accretion disk, and Comptonizing corona along the Z-track. The comparative analysis reveals that, irrespective of the adopted Comptonization geometry, the inferred inner disk radius remains large (~100 km) and exhibits no systematic inward motion as the source luminosity increases. This suggests that the accretion disk remains truncated and the disk luminosity tends to saturate, consistent with a radiation pressure influenced inner disk. The relatively small variation in the inferred mass accretion rate across the branches indicates that changes in $\dot{M}$ alone cannot account for the observed Z-track evolution. Instead, the boundary-layer component becomes progressively dominant toward the flaring branch, with both its temperature and flux increasing with total luminosity, while the relative contribution from the disk decreases. These results indicate a redistribution of accretion power toward the boundary layer and the associated Comptonizing region, suggesting that the primary energy dissipation in GX 349+2 occurs within the boundary layer region of the neutron star.

astro-ph.HE↗

arXiv2026

Dissipative Cosmology and the Nature of Dark Energy: Insights from Bulk Viscosity with DESI DR2 observations

We explore a cosmological model in which dark energy is described by a bulk viscous fluid, providing a dissipative mechanism for late-time cosmic acceleration. Considering both minimally and non-minimally coupled scenarios, we constrain the model using SNe Ia, DESI DR2 BAO, and Planck 2018 CMB data. We find that viscous effects can successfully mimic dynamical dark energy and yield improved fits over $\Lambda$CDM, particularly in the interacting non-minimal case. Our results demonstrate that dissipative processes offer a viable and physically motivated alternative to the cosmological constant in explaining the current accelerated expansion of the universe.

astro-ph.CO↗

arXiv2025

Bouncing Cosmologies in modified gravity with space time torsion

We explore the possibility of realizing a non-singular bounce in the early universe within the framework of modified gravity with spacetime torsion. In Einstein Cartan theory, torsion is embedded in the spacetime by adding an antisymmetric part in affine connection . We consider generalized version of the framework as $f(\bar{R})$, $\bar{R}$ being the scalar of the modified curvature tensor. $f(\bar{R})$ gravity is recast in Einstein frame as non-minimally coupled scalar tensor theory where the scalar field gets coupled with a rank 2 antisymmetric torsion field through derivative couplings. We investigate whether the introduction of three additional torsion-dependent terms in Einstein frame help to realize a bounce. We first explore this cosmological system in the background of a homogeneous and isotropic FRW spacetime but inclusion of the torsion terms are insufficient to produce a bounce in this symmetric setting. Motivated by this limitation, we relax the symmetry and generalize the background to include inhomogeneity and anisotropy. In this setup, the dynamics is modified in such a way that a bouncing solution is possible without invoking phantom fields or energy condition violations. We have found the exact solutions of all the fields and reconstructed the modified gravity form. We have addressed the behaviour of the fields under perturbation and investigated the stability of the solutions. Constraints on the model parameters have also been derived based on cosmological observations.

gr-qc↗

arXiv2025

Quasi-Periodic Oscillations due to radiative feedback mechanism between the disc and corona

Compact object systems exhibit Quasi-Periodic Oscillations (QPOs) as revealed by peaked features in their power density spectra. It has been known that stochastic variations in the accretion disc will propagate to the corona after a time delay and that the hard X-rays from the corona impinge back on the disc, giving reflection spectral features. Here, we show that the combination of these two effects makes a simple radiative feedback system between the corona and the disc, which naturally produces the observed QPOs whose primary frequency corresponds to the inverse of the time delay. The analytical form of the expected power spectra can be statistically compared with the observed ones. Hence, for the first time, a physical model is used to describe and fit the AstroSat observed power spectra of the black hole systems MAXI J1535-571 and GRS 1915+105, including the QPO, its harmonics, as well as the broadband components.

astro-ph.HE↗

arXiv2024

Modelling the Energy-dependent broadband variability in the black-hole transient GX 339-4 using Astrosat and NICER

We present a spectro-timing analysis of the black hole X-ray transient GX 339-4 using simultaneous observations from Astrosat and NICER during the 2021 outburst period. The combined spectrum obtained from NICER, LAXPC, and SXT data is effectively described by a model comprising a thermal disk component, hard Comptonization component, and reflection component with an edge. Our analysis of the Astrosat and NICER spectra indicates the source to be in a low/hard state, with a photon index of ~1.64. The Power Density Spectra (PDS) obtained from both Astrosat and NICER observations exhibit two prominent broad features at 0.22 Hz and 2.94 Hz. We generated energy-dependent time lag and fractional root mean square (frms) at both frequencies in a broad energy range of 0.5-30 keV and found the presence of hard lags along with a decrease in variability at higher energy levels. Additionally, we discovered that the correlated variations in accretion rate, inner disc radius, coronal heating rate, and the scattering fraction, along with a delay between them, can explain the observed frms and lag spectra for both features.

astro-ph.HE↗

arXiv2024

Absence of antisymmetric tensor fields : Clue from f(R) model of gravity

One of the surprising aspects of the present Universe, is the absence of any noticeable observable effects of higher-rank antisymmetric tensor fields in any natural phenomena. Here, we address the possible explanation of the absence of the higher rank antisymmetric tensor fields within the framework of a general class of $f(R)$ gravity represented by $f (R) = R +\alpha_n R^n$. We explore the setup in Einstein frame, where the higher curvature is manifested in terms of a scalar field with a potential through a conformal transformation. The evolution of different cosmological parameters is studied in the background of FRW universe. We show that while different cosmological parameters mimic their standard behaviour at different epochs for different forms of higher curvature gravity (i.e. different values of n ), the positive values of the scalar field in the models provide an additional suppression for the massless modes of higher rank antisymmetric field, The Starobinsky model identified with $n=2$, provides heavier suppression compared to the others $n\neq2$. The result does not change even with the inclusion of the Cosmological Constant. Thus, our result reveals that a general class of modified gravity models can successfully explain the suppression of the massless modes of higher rank antisymmetric tensor fields leading to their invisibility in the present universe.

astro-ph.CO↗

arXiv2023

Investigating the Energy-Dependent Temporal Nature of Black Hole Binary System H 1743-322

Black hole X-ray binaries routinely exhibit Quasi Periodic Oscillations (QPOs) in their Power density spectrum. Studies of QPOs have demonstrated immense ability to understand these dynamical systems although their unambiguous origin still remains a challenge. We investigate the energy-dependent properties of the Type-C QPOs detected for H 1743-322 as observed with AstroSat in its two X-ray outbursts of 2016 and 2017. The combined broadband LAXPC and SXT spectrum is well modelled with a soft thermal and a hard Comptonization component. The QPO exhibits soft/negative lags i.e. variation in soft band lags the variation in hard band, although the upper harmonic shows opposite behaviour i.e. hard/positive lags. Here, we model energy-dependent properties (fractional root mean square and time-lag variation with energy) of the QPO and its upper harmonic individually with a general scheme that fits these properties by utilizing the spectral information and consequently allows to identify the radiative component responsible for producing the variability. Considering the truncated disk picture of accretion flow, a simple model with variation in inner disk temperature, heating rate and fractional scattering with time delays is able to describe the fractional RMS and time-lag spectra. In this work, we show that this technique can successfully describe the energy-dependent features and identify the spectral parameters generating the variability.

astro-ph.HE↗

arXiv2023

Probing the soft state evolution of 4U 1543-47 during its 2021 outburst using AstroSat

4U 1543-47 underwent its brightest outburst in 2021 after two decades of inactivity. During its decay phase, AstroSat conducted nine observations of the source spanning from July $1^{st}$ to September $26^{th}$, 2021. The first three observations were performed with an offset of 40 arcmin with AstroSat/LAXPC, while the remaining six were on-axis observations. In this report, we present a comprehensive spectral analysis of the source as it was in the High/Soft state during the entire observation period. The source exhibited a disk-dominated spectra with a weak high-energy tail (power-law index $\geq2.5$) and a high inner disk temperature ($\sim$0.84 keV). Modelling the disk continuum with non-relativistic and relativistic models, we find inner radius to be significantly truncated at $>$$10~R_g$. Alternatively, to model the spectral evolution with the assumption that the inner disk is at the ISCO, it is necessary to introduce variation in the spectral hardening in the range $\sim$1.5-1.9.

astro-ph.HE↗

arXiv2022

On the energy dependence of the QPO phenomenon in the black hole system MAXI J1535-571

Previous analysis of AstroSat observations of the black hole system MAXI J1535-571, have revealed the presence of a strong Quasi-Periodic Oscillation (QPO) whose frequency is correlated with the high energy spectral index. Here, we fit the spectra as emitted from a truncated disc with an inner hot corona, study the QPO frequency dependence on other spectral parameters and model the energy dependent r.m.s and time-lag of the QPO to identify the physical spectral parameters whose variation are responsible for the QPO. The QPO frequency is found to also correlate with the scattering fraction (i.e. the fraction of the soft photons Comptonized) and its dependence on the accretion rate and inner disc radii is consistent with it being the dynamical frequency. The time-lag between the hard and soft photons is negative for QPO frequency > 2.2 Hz and is positive for lesser values, making this the second black hole system to show this behaviour after GRS 1915+105. Modelling the energy dependent time-lag and r.m.s requires correlated variation of the accretion rate, inner disc radii and the coronal heating rate, with the latter having a time-lag compared to the other two for QPO frequencies less than < 2.2 Hz and which changes sign (i.e. the coronal heating variation precedes the accretion rate one) for higher values. The implications of the results are discussed.

astro-ph.HE↗

arXiv2021

Do cosmological observations allow a negative $\Lambda$?

In view of the recent measurement of $H_{0}$ from HST and SH0ES team, we explore the possibility of existence of a negative cosmological constant (AdS vacua in the dark energy sector) in the Universe. In this regard, we consider quintessence fields on top of a negative cosmological constant and compare such construction with $\Lambda$CDM model using a different combination of CMB, SnIa, BAO and $H_{0}$ data. Various model comparison estimators show that quintessence models with a negative $\Lambda$ is either preferred over $\Lambda$CDM or performs equally as $\Lambda$CDM model. This suggests that the presence of a negative $\Lambda$ (AdS ground state) in our Universe, which can naturally arise in string theory, is consistent with cosmological observations.

astro-ph.CO↗

arXiv2021

Detection of low-frequency breaks in power density spectrum of GX 339-4 in faint low/hard state observations using AstroSat data

We present the spectro-timing analysis of the black hole binary system GX 339-4 using AstroSat data extracted at the beginning of outbursts in 2017 and 2019. The joint spectral fitting of LAXPC and SXT data revealed that the source was in a faint low/hard state for both 2017 and 2019 observations with a nearly equal photon index of $\sim$1.57 and $\sim$1.58 and with Eddington ratio, $L/L_{Edd}$, of 0.0011 and 0.0046 respectively. The addition of a reflection component into the spectral modelling improved the fit ($\Delta\chi^2$ $\approx~6$ for 2017 and $\Delta\chi^2$ $\approx~7$ for 2019), pointing towards the presence of weak reflection features arising due to irradiation of the accretion disk. The power density spectrum (PDS) consisted of strong band-limited noise with a break at low frequencies, described with a combination of few zero-centered Lorentzian. The fitting revealed a low-frequency break at $\sim$6~mHz for 2017 and at $\sim$11~mHz for 2019 observation, whose detection is validated by results from independent detectors (LAXPCs and SXT). The break frequency is roughly consistent with results obtained from earlier observations that showed an evolution of the frequency with flux, which is in accordance with the truncated disk model. Associating the break frequency with viscous time scale of the accretion disk, we estimated a truncation radius of $\sim$93 and $\sim$61 gravitational radius for 2017 and 2019 observation, respectively.

astro-ph.HE↗

arXiv2020

Identifying the radiative components responsible for Quasi-Periodic Oscillations of black hole systems

While the dynamical origin of the variability observed in Galactic Black hole systems, such as quasi-periodic oscillations (QPOs), are still a matter of debate, insight into the radiative components responsible for such behaviour can be obtained by studying their energy dependent temporal behaviour. In particular, one needs to ascertain which variations of the parameters of the best fit time-averaged spectral components reproduce the observed energy dependent fractional r.m.s and time-lags. However, to obtain meaningful interpretation, the standard spectral component parameters have to be recast to physically relevant ones. Then, the energy dependent temporal variations that their fluctuations will cause, needs to be predicted and compared with observations. In this work, we describe a generic method to do this and apply the technique to the ~ 3-4 Hz QPOs observed in the black hole system GRS 1915+105 as observed by AstroSat where the time-averaged spectra can be represented by emission from a truncated disk and a hot thermal Comptonizing coronae in the inner regions. We find that the QPOs and their harmonic can be explained in terms of correlated local accretion rate variations in the disk, the truncated disk radius, the optical depth and the heating rate of the coronae with time-delays between them. We highlight the potential of such techniques to unravel the radiative process responsible for variability using high quality spectral and temporal data from AstroSat and NICER.

astro-ph.HE↗

arXiv2013

Towards a characterization of fields leading to black hole hair

In the present work, it is shown that an asymptotically flat spherical black hole can have a nontrivial signature of any field for an exterior observer if the energy momentum tensor of the corresponding field is either tracefree or if the trace falls off at least as rapidly as inverse cube of the radial distance. In the absence of a general No Hair Theorem, this result can provide a characterization of the fields leading to black hole hair.

gr-qc↗

arXiv2011

GCG Parametrization for Growth Function and Current Constraints

We study the linear growth function $f$ for large scale structures in a cosmological scenario where Generalised Chaplygin Gas (GCG) serves as dark energy candidate. We parametrize the growth index parameter as a function of redshift and do a comparative study between the theoretical growth rate and the proposed parametrization. Moreover, we demonstrate that growth rates for a wide range of dark energy models can be modeled accurately by our proposed parametrization. Finally, we compile a data set consisting of 28 data points within redshift range (0.15,3.8) to constrain the growth rate. It includes direct growth data from various projects/surveys including the latest data from the Wiggle-Z measurements. It also includes data constraining growth indirectly through the rms mass fluctuation $\sigma_8(z)$ inferred from Ly-$\alpha$ measurements at various redshifts. By fitting our proposed parametrization for $f$ to these data, we show that growth history of large scale structures of the universe although allows a transient acceleration, one cannot distinguish it at present with an eternally accelerating universe.

astro-ph.CO↗

arXiv2011

Matter-gravity interaction in a multiply warped braneworld,

The role of a bulk graviton in predicting the signature of extra dimensions through collider-based experiments is explored in the context of a multiply warped spacetime. In particular it is shown that in a doubly warped braneworld model, the presence of the sixth dimension, results in enhanced concentration of graviton Kaluza Klein (KK) modes compared to that obtained in the usual 5-dimensional Randall-Sundrum model. Also, the couplings of these massive graviton KK modes with the matter fields on the visible brane turn out to be appreciably larger than that in the corresponding 5- dimensional model. The significance of these results are discussed in the context of KK graviton search at the Large Hadron Collider (LHC).

hep-ph↗

arXiv2009

A Randall-Sundrum scenario with bulk dilaton and torsion

We consider a string-inspired torsion-dilaton-gravity action in a Randall-Sundrum brane world scenario and show that, in an effective four-dimensional theory on the visible brane, the rank-2 antisymmetric Kalb-Ramond field (source of torsion) is exponentially suppressed. The result is similar to our earlier result in [12], where no dilaton was present in the bulk. This offers an explanation of the apparent invisibility of torsion in our space-time. However, in this case the trilinear couplings ~ Tev^{-1} between dilaton and torsion may lead to new signals in Tev scale experiments, bearing the stamp of extra warped dimensions.

hep-th↗

arXiv2007

Bulk antisymmetric tensor fields in a Randall-Sundrum model

We consider bulk antisymmetric tensor fields of various ranks in a Randall-Sundrum scenario. We show that, rank-2 onwards, the zero-modes of the projections of these fields on the (3+1) dimensional visible brane become increasingly weaker as the rank of the tensor increases. All such tensor fields of rank 4 or more are absent from the dynamics in four dimensions. This leaves only the zero-mode graviton to have coupling $\sim 1/M_P$ with matter, thus explaining why the large-scale behaviour of the universe is governed by gravity only. We have also computed the masses of the heavier modes upto rank-3, and shown that they are relatively less likely to have detectable accelerator signals.

hep-th↗

arXiv2004

Parity violation in four and higher dimensional spacetime with torsion

The possibility of parity violation in a gravitational theory with torsion is extensively explored in four and higher dimensions. In the former case,we have listed our conclusions on when and whether parity ceases to be conserved, with both two-and three-index antisymmetry of the torsion field. In the latter, the bulk spacetime is assumed to have torsion, and the survival of parity-violating terms in the four dimensional effective action is studied, using the compactification schemes proposed by Arkani-Hamed-Dimopoulos-Dvali and Randall-Sundrum. An interesting conclusion is that the torsion-axion duality arising in a stringy scenario via the second rank antisymmetric Kalb-Ramond field leads to conservation of parity in the gravity sector in any dimension. However, parity-violating interactions do appear for spin 1/2 fermions in such theories, which can have crucial phenomenological implications.

hep-th↗