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Einstein-Maxwell dilaton-axion (EMDA) gravity provides a simple framework to investigate the signatures of string theory. The axion and the dilaton fields arising in EMDA gravity have important implications in inflationary cosmology and in addressing the late time acceleration of the universe. It is therefore instructive to explore the implications of such a model in explaining the astrophysical observations. In this work we explore the role of EMDA gravity in explaining the observed shadows of black holes (M87* and Sgr A*) released by the Event Horizon Telescope (EHT) collaboration. The Kerr-Sen metric represents the exact, stationary and axisymmetric black hole solution of EMDA gravity. Such a black hole is characterized by the angular momentum $a$ acquired from the axionic field and the dilatonic charge $r_2$ arising from string compactifications. We study the role of spin and the dilaton charge in modifying the shape and size of the black hole shadow. We note that black holes with larger dilaton charge cast a smaller shadow. We investigate the consequences of such a result in addressing the EHT observations of M87* and Sgr A*. Our analysis reveals that the shadow of M87* exhibits a preference towards the Kerr scenario. However, when 10% offset in the shadow diameter is considered, $0.1\lesssim r_2\lesssim 0.3$ is observationally favored within 1-$\sigma$. The shadow of Sgr A* on the other hand shows a preference towards the Kerr-Sen scenario since the central value of its shadow can be better explained by a non-zero dilaton charge $0.1 \lesssim r_2 \lesssim 0.4$. However, when the 1-$\sigma$ interval is considered the Kerr scenario is included. We discuss the implications of our results., Comment: 28 pages, 7 figures

We study the method of extended gravitational decoupling in obtaining static black hole solutions satisfying Einstein's equations with a tensor vacuum. The source has quite generic characteristics and satisfies the strong energy condition. The stationary, axisymmetric counterpart of the static metric is obtained by applying the Newman-Janis and Azreg-A\"{i}nou algorithms. The thermodynamics of the rotating solution is studied and the expressions of various thermodynamic quantities are derived. The dependence of the temperature, free energy and specific heat on the horizon radius is studied for various values of the hairy parameter and the black hole spin. Such a study reveals that small hairy black holes are thermodynamically more stable compared to large hairy black holes, and that the horizon radius and temperature range for which the rotating hairy black holes can be in thermodynamic equilibrium with the surroundings depends non-trivially on the hairy parameters. We further discuss the first law of black hole thermodynamics for the hairy case and discuss its implications., Comment: 30 Pages, 17 Figures, references added, discussion extended, Published version

We show that the observed angular diameter of the shadow of the ultra compact object Sgr A*, favours the existence of an extra spatial dimension. This holds irrespective of the nature of the ultra compact object, i.e., whether it is a wormhole or, a black hole mimicker, but with the common feature that both of them have an extra dimensional origin. This result holds true for the mass and the distance measurements of Sgr A* using both Keck and the Gravity collaborations and whether we use the observed image or, the observed shadow diameter. In particular, the central value of the observed shadow or, the observed image diameter predicts non-zero hairs inherited from the extra dimensions., Comment: v2, published version, 12 pages, 4 figures

We study the prospect of Bardeen black holes in explaining the observed shadow of Sgr A* and M87*. Bardeen black holes are regular black holes endowed with a magnetic monopole charge that arise in Einstein gravity coupled to non-linear electrodynamics. These black holes are interesting as they can evade the r = 0 curvature singularity arising in general relativity. It is therefore worthwhile to look for signatures of Bardeen black holes in astrophysical observations. With two successive release of black hole images by the Event Horizon Telescope (EHT) collaboration, the scope to test the nature of strong gravity has substantially increased. We compare the theoretically computed shadow observables with the observed image of Sgr A* and M87*. Our analysis reveals that while the observed angular diameter of M87* favors the Kerr scenario, the shadow of Sgr A* can be better explained by the Bardeen background. This indicates that although rare, certain black holes exhibit a preference towards regular black holes like the Bardeen spacetime., Comment: 24 pages, 8 figures

With the recent release of the black hole image of Sgr A* alongside the earlier image of M87*, one can now really hope to acquire a better understanding of the gravitational physics at the horizon scale. In this paper, we investigate the prospect of the regular black hole scenario with a Minkowski core in explaining the observed shadow of M87* and Sgr A*. Regular black holes generally appear in Einstein gravity coupled to non-linear electrodynamics and are interesting as they can evade the r = 0 curvature singularity arising in general relativity. Using the previously determined mass and distance we compute the observables associated with the black hole shadow. These when compared with the observed angular diameter reveal that the shadow of M87* and Sgr A* favor the regular black hole scenario with a small but non-zero charge. The implications are discussed., Comment: 23 pages, 9 figures, matches published version (JCAP accepted)

Regular black holes arising in Einstein gravity coupled to non-linear electrodynamics are worth studying as they can circumvent the r = 0 curvature singularity arising in general relativity. In this work we explore the signatures of regular black holes with a Minkowski core from the quasar continuum spectrum. We use thin-disk approximation to derive the theoretical luminosity from the accretion disk and compare it with the optical data of eighty Palomar Green quasars. Our analysis based on error estimators like the chi-square, the Nash-Sutcliffe efficiency, the index of agreement etc. reveal that optical observations of quasars favor the Kerr scenario compared to black holes in non-linear electrodynamics. The implications are discussed., Comment: 22 pages, 6 figures

Scenario of a bouncing universe is one of the most active area of research to arrive at singularity free cosmological models. Different proposals have been suggested to avoid the so called 'big bang' singularity through the quantum aspect of gravity which is yet to have a proper understanding. In this work, on the contrary, we consider three different approaches, each of which goes beyond General Relativity but remain within the domain of classical cosmological scenario, to address this problem. The hallmark of all these approaches is that the origin of the bouncing mechanism is somewhat natural within the geometrical framework of the model without any need of incorporating external source by hand. In the context of these scenarios ,we also discuss various constraints that these viable cosmological models need to satisfy., Comment: Accepted by GRG in the memorial volume for Prof. Thanu Padmanabhan. arXiv admin note: text overlap with arXiv:2109.00345, arXiv:2202.13186

Quasi-periodic oscillations (QPOs), in particular, the ones with high frequencies, often observed in the power spectrum of black holes, are useful in understanding the nature of strong gravity since they are associated with the motion of matter in the vicinity of the black hole horizon. Interestingly, these high frequency QPOs (HFQPOs) are observed in commensurable pairs, the most common ratio being 3:2. Several theoretical models are proposed in the literature which explain the HFQPOs in terms of the orbital and epicyclic frequencies of matter rotating around the central object. Since these frequencies are sensitive to the background spacetime, the observed HFQPOs can potentially extract useful information regarding the nature of the same. In this work, we investigate the role of regular black holes with a Minkowski core, which arise in gravity coupled to non-linear electrodynamics, in explaining the HFQPOs. Regular black holes are particularly interesting as they provide a possible resolution to the singularity problem in general relativity. We compare the model dependent QPO frequencies with the available observations of the quasi-periodic oscillations from black hole sources and perform a \c{hi} 2 analysis. Our study reveals that most QPO models favor small but non-trivial values of the non-linear electrodynamics charge parameter. In particular, black holes with large values of non-linear electrodynamics charge parameter are generically disfavored by present observations related to QPOs., Comment: 24 pages, 2 figures, 4 tables. arXiv admin note: text overlap with arXiv:2201.00679

Quasi-periodic oscillations (QPOs) observed in the power spectrum of black holes are unique observational probes to the background spacetime since they can be directly related to the timescales associated with the motion of matter orbiting in the vicinity of the black hole horizon. In this regard, the high frequency QPOs (HFQPOs) are particularly interesting as they occur in commensurable pairs, the most common ratio being the 3:2 twin peak QPOs. The theoretical models which aim to explain these QPOs express the observed frequencies in terms of the epicyclic motion of test particles in a given background spacetime. In this work we study the signatures of Bardeen spacetime from the observed QPOs in the black hole power spectrum. Bardeen black holes are rotating, regular black holes with a magnetic monopole charge. Such regular backgrounds are theoretically interesting as they can potentially evade the curvature singularity, otherwise unavoidable in general relativistic black holes. We perform a chi-square analysis by comparing the available observations of the quasi-periodic oscillations from black hole sources with the relevant theoretical models and note that the Kerr black holes in general relativity are observationally more favored compared to black holes with a monopole charge. Our analysis reveals that black holes with very high monopole charge are disfavored from QPO related observations., Comment: 23 pages, 2 figures

Black holes carrying a magnetic monopole charge are a subject of interest for a long time. In this work we explore the possibility of an observational evidence of such black holes carrying a magnetic monopole, namely the Bardeen rotating black holes. We derive the theoretical spectrum from the accretion disk surrounding a Bardeen black hole using the thin-disk approximation. We compare the theoretically derived spectrum in comparison to the optical data of eighty Palomar Green quasars to constrain the monopole charge parameter $g$ and the spin parameter $a$ of the quasars. From our analysis we note that the Kerr-scenario in \gr\ is observationally more favored than black holes with a monopole charge. We arrive at such a conclusion using error estimators like $\chi^2$, the Nash-Sutcliffe efficiency, the index of agreement and their modified forms. In particular, black holes with $g \geq 0.03$ are outside $99\%$ confidence interval. The implications are discussed.

We propose a cosmological scenario in which the universe undergoes through a non-singular bounce, and after the bounce, it decelerates having a matter-like dominated evolution during some regime of the deceleration era, and finally at the present epoch it evolves through an accelerating stage. Our aim is to study such evolution in the context of Chern-Simons corrected F(R) gravity theory and confront the model with various observational data. Using the reconstruction technique, and in addition by employing suitable boundary conditions, we determine the form of F(R) for the entire possible range of the cosmic time. The form of F(R) seems to unify a non-singular bounce with a dark energy epoch, in particular, from a non-singular bounce to a deceleration epoch and from a deceleration epoch to a late time acceleration era. It is important to mention that the bouncing scenario in the present context is an asymmetric bounce, in particular, the Hubble radius monotonically increases and asymptotically diverges at the late contracting era, while it seems to decrease with time at the present epoch. Such evolution of the Hubble radius leads to the primordial perturbation modes generate at the deep contracting era when all the perturbation modes lie within the horizon. We calculate the scalar and tensor power spectra, and as a result, the primordial observables are found to be in agreement with the latest Planck 2018 constraints. In this regard, the Chern-Simons term seems to have considerable effects on the tensor perturbation evolution, however keeping intact the scalar part of the perturbation with that of in the case of a vacuum F(R) model, and as a result, the Chern-Simons term proves to play an important role in making the observable quantities consistent with the Planck results. Furthermore the theoretical expectation of the dark energy observables are confronted with the Planck+SNe+BAO data., Comment: "Physics of the Dark Universe" accepted. Abstract is reduced due to arxiv limitation

We present and discuss a new family of topological hairy dyonic black hole solutions in asymptotically anti-de Sitter (AdS) space. The coupled Einstein-Maxwell-Scalar gravity system, that carries both the electric and magnetic charges is solved, and exact hairy dyonic black hole solutions are obtained analytically. The scalar field profiles that give rise to such black hole solutions are regular everywhere. The hairy solutions are obtained for planar, spherical, and hyperbolic horizon topologies. In addition, analytic expressions of regularized action, stress tensor, conserved charges, and free energies are obtained. We further comment on different prescriptions for computing the black hole mass with hairy backgrounds. We analyze the thermodynamics of these hairy dyonic black holes in canonical and grand canonical ensembles, and we find that both electric and magnetic charges have a constructive effect on the stability of the hairy solution. For the case of planar and hyperbolic horizons, we find thermodynamically stable hairy black holes which are favoured at low temperatures compared to the non-hairy counterparts. We further find that, for a spherical hairy dyonic black hole, the thermodynamic phase diagram resembles to that of a Van der Waals fluid not only in canonical but also in the grand canonical ensemble., Comment: 41 pages, many figures, clarification and references added, published version

An exact solution for the bulk 5-dimensional geometry is derived for F(R) gravity with non-flat de-Sitter 3-branes located at the $M_4 \times Z_2$ orbifold boundaries. The corresponding form of F(R) that leads to such an exact solution of the bulk metric is derived which turns out to have all positive integer powers of R.In such a scenario the stability issue of the modulus (radion field) is analyzed critically for different curvature epochs in both Einstein and Jordan frames. The radion in the effective 4-d theory exhibits a phantom epoch making this model viable for a non-singular bounce. Simultaneous resolution of the gauge hierarchy problem is exhibited through the resulting stable vaue of the radion field in the effective $3+1$ dimensional theory., Comment: PRD Accepted

Quasi-periodic oscillations, often present in the power density spectrum of accretion disk around black holes, are useful probes for the understanding of gravitational interaction in the near-horizon regime of black holes. Since the presence of an extra spatial dimension modifies the near horizon geometry of black holes, it is expected that the study of these quasi-periodic oscillations may shed some light on the possible existence of these extra dimensions. Intriguingly, most of the extra dimensional models, which are of significant interest to the scientific community, predicts the existence of a tidal charge parameter in black hole spacetimes. This tidal charge parameter can have an overall negative sign and is a distinctive signature of the extra dimensions. Motivated by this, we have studied the quasi-periodic oscillations for a rotating braneworld black hole using the available theoretical models. Subsequently, we have used the observations of the quasi-periodic oscillations from available black hole sources, e.g., GRO J1655 -- 40, XTE J1550 -- 564, GRS 1915 + 105, H 1743 + 322 and Sgr A* and have compared them with the predictions from the relevant theoretical models, in order to estimate the tidal charge parameter. It turns out that among the 11 theoretical models considered here, 8 of them predict a negative value for the tidal charge parameter, while for the others negative values of the tidal charge parameter are also well within the 1-$\sigma$ confidence interval., Comment: Published version, 37 pages, 9 figures

We explore the possibility of a non-singular bounce in our universe from a warped braneworld scenario with dynamical branes and a non-zero brane cosmological constant. Such models naturally incorporate a scalar sector known as the radion originating from the modulus of the theory. The presence of brane cosmological constant renders the branes to be non-flat and gives rise to a potential and a non-canonical kinetic term for the radion field in the four dimensional effective action. The kinetic term exhibits a phantom-like behavior within the domain of evolution of the modulus which leads to a violation of the null-energy condition often observed in a bouncing universe. The interplay of the radion potential and kinetic term enables the evolution of the radion field from a normal to a phantom regime where the universe transits from a contracting era to an expanding epoch through a non-singular bounce. Analysis of the scalar and tensor perturbations over such background evolution reveal that the primordial observables e.g., the amplitude of scalar perturbations $\mathcal{A}_s$, tensor to scalar ratio $r$ and the scalar spectral index $n_s$ are in agreement with the current constraints reported by the Planck satellite. The implications are discussed., Comment: JCAP accepted

String inspired models can serve as potential candidates to replace GR in the high energy regime where quantum gravity is expected to play a vital role. Such models not only subsume the ultraviolet nature of gravity but also exhibit promising prospects in resolving issues like dark matter and dark energy, which cannot be adequately addressed within the framework of GR. The Einstein-Maxwell dilaton-axion (EMDA) theory, which is central to this work is one such string inspired model arising in the low energy effective action of the heterotic string theory with interesting implications in inflationary cosmology and in the late time acceleration of the universe. It is therefore important to survey the role of such a theory in explaining astrophysical observations, e.g. the continuum spectrum of black holes which is expected to hold a wealth of information regarding the background metric. The Kerr-Sen spacetime corresponds to the exact, stationary, and axisymmetric black hole solution in EMDA gravity, possessing dilatonic charge and angular momentum originating from the axionic field. In this work, we compute the theoretical spectrum from the accretion disk around quasars in the Kerr-Sen background assuming the thin accretion disk model due to Novikov \& Thorne. This is then used to evaluate the theoretical estimates of optical luminosity for a sample of eighty Palomar-Green quasars which are subsequently compared with the available observations. Our results based on $\chi^2$ analysis indicate that the dilaton parameter $r_2\sim 0.2$ is favored by optical observations of quasars which is further corroborated by other error estimators e.g., the Nash-Sutcliffe efficiency, the index of agreement, and their modified versions. We further report that strong dilaton charges ($r_2>1.6$) are disfavored by quasar optical data and the spins associated with the quasars are also estimated., Comment: 14 pages, 6 figures, 1 Table (accepted for publication in MNRAS)

The Einstein-Maxwell dilaton-axion (EMDA) gravity arises in the low energy effective action of the heterotic string theory and provides a simple framework to explore the signatures of the same. The dilaton and the axion fields inherited in the action from string compactifications have interesting consequences in inflationary cosmology and in explaining the present accelerated expansion of the universe. It is therefore worthwhile to search for the footprints of these fields in the available astrophysical observations. Since Einstein gravity is expected to receive quantum corrections in the high curvature domain, the near horizon regime of black holes seems to be the ideal astrophysical laboratory to test these deviations from general relativity. Exact, stationary and axisymmetric black hole solution in EMDA gravity corresponds to the Kerr-Sen spacetime which carries dilaton charge, while the angular momentum is sourced by the axion field. The ballistic jets and the peak emission of the continuum spectrum from the accretion disk are believed to be launched very close to the event horizon and hence should bear the imprints of the background spacetime. We compute the jet power and the radiative efficiency derived from the continuum spectrum in the Kerr-Sen background and compare them with the corresponding observations of microquasars. Our analysis reveals that Kerr black holes are more favored compared to Kerr-Sen black holes with dilaton charges., Comment: Matches published version

Low surface brightness (LSBs) superthins constitute classic examples of very late-type galaxies, with their disc dynamics strongly regulated by their dark matter halos. In this work we consider a gravitational origin of dark matter in the brane world scenario, where the higher dimensional Weyl stress term projected onto the 3-brane acts as the source of dark matter. In the context of the braneworld model, this dark matter is referred to as the \emph{`dark mass'}.This model has been successful in reproducing the rotation curves of several low surface brightness and high surface brightness galaxies. Therefore it is interesting to study the prospect of this model in explaining the vertical structure of galaxies which has not been explored in the literature so far. Using our 2-component model of gravitationally-coupled stars and gas in the external force field of this \emph{dark mass}, we fit the observed scale heights of stellar and atomic hydrogen (HI) gas of superthin galaxy `UGC7321' using the Markov Chain Monte Carlo approach. We find that the observed scaleheights of `UGC7321' can be successfully modelled in the context of the braneworld scenario. In addition, the model predicted rotation curve also matches the observed one. The implications on the model parameters are discussed., Comment: 20 pages, 2 figures (Accepted for publication in MNRAS)

The final fate of gravitational collapse of massive stars has been a subject of interest for a long time since such a collapse may lead to black holes and naked singularities alike. Since, the formation of naked singularities is forbidden by the cosmic censorship conjecture, exploring their observational differences from black holes may be a possible avenue to search for these exotic objects. The simplest possible naked singularity spacetime emerges from the Einstein massless scalar field theory with the advantage that it smoothly translates to the Schwarzschild solution by the variation of the scalar charge. This background, known as the Janis-Newman-Winicour spacetime is the subject of interest in this work. %A further advantage of working with this spacetime is that it smoothly translates to the Schwarzschild solution by the variation of the scalar charge. We explore electromagnetic observations around this metric which involves investigating the characteristics of black hole accretion and shadow. We compute the shadow radius in this spacetime and compare it with the image of M87*, recently released by the Event Horizon Telescope Collaboration. Similarly, we derive the expression for the luminosity from the accretion disk and compare it with the observed optical luminosity of eleven Palomar Green quasars. Our analysis indicates that the shadow of M87* and the quasar optical data consistently favor the Schwarzschild background over the Janis-Newman-Winicour spacetime. The implications of this result are discussed., Comment: 32 pages, 4 new figures added; accepted for publication in Physical Review D

Detection of axion field can unfold intriguing facets of our universe in several astrophysical and cosmological scenarios. In four dimensions, such a field owes its origin to the completely anti-symmetric Kalb-Ramond field strength tensor. Its invisibility in the solar system based tests compels one to look for its signatures in the strong field regime. The recent observation of the shadow of the supermassive black hole in the galaxy M87 ushers in a new opportunity to test for the footprints of axion in the near horizon region of black holes, where the gravity is expected to be strong. In this paper, we explore the impact of axion on the black hole shadow and compare the result with the available image of M87*. Our analysis indicates that axion which violates the energy condition seems to be favored by observations. The implications are discussed., Comment: 20 pages, 6 figures

The recent observation of the shadow of the supermassive black hole M87*, located at the centre of the M87 galaxy, by the Event Horizon Telescope collaboration has opened up a new window to probe the strong gravity regime. In this letter, we explicitly demonstrate the consequences of this observation on brane world black holes, characterized by existence of a negative tidal charge. Our results based on three observables associated with the shadow, namely, angular diameter, deviation from circularity and axis ratio reveal that the existence of a negative tidal charge is more favoured, possibly marking a deviation from general relativity., Comment: v3, Published as Rapid Communication in PRD, 16 pages, 6 figures

Extra dimensions, which led to the foundation and inception of string theory, provide an elegant approach to force-unification. With bulk curvature as high as the Planck scale, higher curvature terms, namely f(R) gravity seems to be a natural addendum in the bulk action. These can not only pass the classic tests of general relativity but also serve as potential alternatives to dark matter and dark energy. With interesting implications in inflationary cosmology, gravitational waves and particle phenomenology it is worth exploring the impact of extra dimensions and f(R) gravity in black hole accretion. Various classes of black hole solutions have been derived which bear non-trivial imprints of these ultraviolet corrections to general relativity. This in turn gets engraved in the continuum spectrum emitted by the accretion disk around black holes. Since the near horizon regime of supermassive black holes manifest maximum curvature effects, we compare the theoretical estimates of disk luminosity with quasar optical data to discern the effect of the modified background on the spectrum. In particular, we explore a certain class of black hole solution bearing a striking resemblance with the well-known Reissner Nordstrom de Sitter/anti-de Sitter/flat spacetime which unlike general relativity can also accommodate a negative charge parameter. By computing error estimators like chi-square, Nash-Sutcliffe efficiency, index of agreement, etc. we infer that optical observations of quasars favor a negative charge parameter which can be a possible indicator of extra dimensions. The analysis also supports an asymptotically de Sitter spacetime with an estimate of the magnitude of the cosmological constant whose origin is solely attributed to f(R) gravity in higher dimensions., Comment: 32 pages, 5 figures, 2 tables

Continuum spectrum emitted by the accretion disk around quasars hold a wealth of information regarding the strong gravitational field produced by the massive central object. Such strong gravity regime is often expected to exhibit deviations from general relativity (GR) which may manifest through the presence of extra dimensions. Higher dimensions, which serve as the corner stone for string theory and M-theory can act as promising alternatives to dark matter and dark energy with interesting implications in inflationary cosmology, gravitational waves and collider physics. Therefore it is instructive to investigate the effect of more than four spacetime dimensions on the black hole continuum spectrum which provide an effective astrophysical probe to the strong gravity regime. To explore such a scenario, we compute the optical luminosity emitted by a thin accretion disk around a rotating supermassive black hole albeit in the presence of extra dimensions. The background metric resembles the Kerr-Newman spacetime in GR where the tidal charge parameter inherited from extra dimensions can also assume negative signature. The theoretical luminosity computed in such a background is contrasted with optical observations of eighty quasars. The difference between the theoretical and observed luminosity for these quasars is used to infer the most favoured choice of the rotation parameter for each quasar and the tidal charge parameter. This has been achieved by minimizing/maximizing several error estimators, e.g., chi-squared, Nash-Sutcliffe efficiency, index of agreement etc. Intriguingly, all of them favour a negative value for the tidal charge parameter, a characteristic signature of extra dimensions. Thus accretion disk does provide a significant possibility of exploring the existence of extra dimensions through its close correspondence with the strong gravity regime., Comment: References Updated, Published Version, 32 pages, 4 figures, 1 Table

The galactic black hole candidate Cygnus X-1, one of the brightest sources in the sky, is the first ever black hole candidate to be discovered. Despite being a very well-studied object due to its persistent brightness in X-rays, there has been much difficulty in determining its mass since its discovery. In spite of continuous efforts since the early nineteen seventies, there is yet no concensus on its mass for nearly four decades. The uncertainties in the mass measurements are due to the high degree of error involved in the measurement of its distance. In 2011, Orosz et al. constrained the mass of this object to be M = 14.8 +/- 1.0 M_Sun using dynamical methods. In this paper, we obtained the mass of Cygnus X-1, using a completely independent method, namely, carrying out the spectral analysis using Two Component Advective Flow (TCAF) solution based fits file and the archival data of RXTE PCA instrument. Our result does not require the distance of the source or the information about the companion. Each spectral fit with the TCAF gives one best fitted mass. Averaging fitted masses of Cygnus X-1 over a span of five months of observation during its persistent hard phase, mass of the source comes out to be M_avg = 14.20 +/- 0.36 M_Sun, which is consistent with the dynamically estimated mass., Comment: 4 pages, 2 figures, 1 table

The black hole transient H1743-322 exhibited several outbursts with temporal and spectral variability since its discovery in 1977. These outbursts occur at a quasi-regular recurrence period of around $0.5-2$ years, since its rediscovery in March 2003. We investigate accretion flow dynamics around the Low Mass X-ray Binary H1743-322 during its 2004 outburst using the RXTE/PCA archival data. We use Two Component Advective Flow (TCAF) solution to analyse the spectral data. From the fits with TCAF solution, we obtain day to day variation of physical accretion rates of Keplerian and sub-Keplerian components, size of the Compton cloud and its other properties. Analysis of the spectral properties of the 2004 outburst by keeping fitted normalization to be in a narrow range and its timing properties in terms of the presence and absence of QPOs, enable us to constrain the mass of the black hole in a range of $10.31 M_{\odot} - 14.07 M_{\odot}$ which is consistent with other estimates reported in the literature., Comment: 24 pages, 7 figures, 2 tables (Published in MNRAS)

We analyze several outbursts of a few transient sources using Proportional Counter Array (PCA) data (2.5-25 keV) as well as All Sky Monitor (ASM) data (1.5-12 keV) of Rossi X-ray Timing Explorer (RXTE) satellite. We find a time delay between the arrival times of the Keplerian disc component and the halo of the Two-Component Advective Flow (TCAF) when the spectral data is fitted with TCAF solution. We compare this time delay from the spectral fits with the TCAF solution of the transient low mass X-ray binaries (LMXBs) e.g., GX 339-4, H 1743-322 and MAXI J1836-194 with that of the high mass X-ray Binary (HMXB), Cyg X-1. We find that several days of time delays are observed in LMXBs while for Cyg X-1 the delay is negligible. We interpret the large delay to be due to the viscous delay of a large Keplerian component to reach the inner region as compared to nearly free-fall time taken by the low angular momentum halo component. The delay is of the order of a few days for the low mass X-ray binaries (LMXBs) where the feeding is primarily through the Roche-lobe. However, it is negligible in a wind-fed system like Cyg X-1 since a very small Keplerian disc is created here by slowly redistributing the low angular momentum of the wind. As a consequence, sporadic soft or intermediate spectral states are observed.

Warped braneworld models has the potential to provide a plausible resolution to the gauge hierarchy problem but at the cost of introduction of an additional field, namely the inter-brane separation or, the radion field. Providing an appropriate stabilization scheme for the radion field has long been sought for. In this work, we discuss dynamical stabilization of the radion field in the context of early universe cosmology. We show that by allowing the branes to be \emph{non-flat} and \emph{dynamical} a radion potential is naturally generated which can trigger the inflationary phase of the early universe. Interestingly, the kinetic term of the radion during the inflationary epoch exhibits a phantom-like behavior and the dynamical evolution of the radion field to its stable value marks the exit from the phantom phase as well as the inflationary era simultaneously. We show that the amalgamation and synergy of the radion potential along with its kinetic term can not only trigger the inflationary scenario but also provide a successful exit from the same while maintaining concordance with current observation., Comment: v2, published version, 26 pages, 4 figures

The presence of axionic field can provide plausible explanation to several long standing problems in physics such as dark matter and dark energy. The pseudo-scalar axion whose derivative corresponds to the Hodge dual of the Kalb-Ramond field strength in four dimensions plays crucial roles in explaining several astrophysical and cosmological observations. Therefore, the detection of axionic hairs/Kalb-Ramond field which appears as closed string excitations in the heterotic string spectrum may provide a profound insight to our understanding of the current universe. The current level of precision achieved in solar-system based tests employed to test general relativity, is not sufficient to detect the presence of axion. However, the near horizon regime of quasars where the curvature effects are maximum seems to be a natural laboratory to probe such additions to the matter sector. The continuum spectrum emitted from the accretion disk around quasars encapsulates the imprints of the background spacetime and hence acts as a storehouse of information regarding the nature of gravitational interaction in extreme situations. The surfeit of data available in the electromagnetic domain provides a further motivation to explore such systems. Using the optical data for eighty Palomar Green quasars we demonstrate that the theoretical estimates of optical luminosity explain the observations best when the axionic field is assumed to be absent. However, axion which violates the energy condition seems to be favored by observations which has several interesting consequences. Error estimators, including reduced $\chi^{2}$, Nash-Sutcliffe efficiency, index of agreement and modified versions of the last two are used to solidify our conclusion and the implications of our result are discussed., Comment: v2, Revised, accepted for publication in JCAP, 19 pages, 3 figures

Continuum spectrum from black hole accretion disc holds enormous information regarding the strong gravity regime around the black hole and hence about the nature of gravitational interaction in extreme situations. Since in such strong gravity regime the dynamics of gravity should be modified from the \EH one, its effect should be imprinted on the continuum spectrum originating from the black hole accretion. To explore the effects of these alternative theories on the black hole continuum spectrum in an explicit manner, we have discussed three alternative gravitational models having their origin in three distinct paradigms --- (a) higher dimensions, (b) higher curvature gravity and (c) Horndeski theories. All of them can have signatures sculptured on the black hole continuum spectrum, distinct from the standard general relativistic scenario. Interestingly all these models exhibit black hole solutions with tidal charge parameter which in these alternative gravity scenarios can become negative, in sharp contrast with the Reissner-Nordstr\"om black hole. Using the observational data of optical luminosity for eighty Palomer Green quasars we have illustrated that the difference between the theoretical estimates and the observational results gets minimized for negative values of the tidal charge parameter. As a quantitative estimate of this result we concentrate on several error estimators, including reduced $\chi^{2}$, Nash-Sutcliffe efficiency, index of agreement etc. Remarkably, all of them indicates a negative value of the tidal charge parameter, signaling the possibility of higher dimensions as well as scalar charge at play in those high gravity regimes., Comment: v2, Revised, Published in PRD; 30 pages, 9 figures

The stability of the modular field in a warped brane world scenario has been a subject of interest for a long time. Goldberger \& Wise ( GW ) proposed a mechanism to achieve this by invoking a massive scalar field in the bulk space-time neglecting the back-reaction. In this work, we examine the possibility of stabilizing the modulus without bringing in any external scalar field. We show that instead of flat 3-branes as considered in Randall-Sundrum ( RS ) warped braneworld model, if one considers a more generalized version of warped geometry with de-Sitter 3-brane, then the brane vacuum energy automatically leads to a modulus potential with a metastable minimum. Our result further reveals that in this scenario the gauge hierarchy problem can also be resolved for appropriate choice of brane cosmological constant., Comment: 8 pages, 4 figures

We investigate nucleosynthesis inside the gamma-ray burst (GRB) accretion disks and in the outflows launched from these disks mainly in the context of Type II collapsars. We report the synthesis of several unusual nuclei like 31P, 39K, 43Sc, 35Cl and various isotopes of titanium, vanadium, chromium, manganese and copper in the disk. We also confirm the presence of iron-group and alpha-elements in the disk, as shown by previous authors. Much of these heavy elements thus synthesized are ejected from the disk and survive in the outflows. While emission lines of several of these elements have been observed in the X-ray afterglows of GRBs by BeppoSAX, Chandra, XMM-Newton etc., Swift seems to have not found these lines yet., Comment: Accepted for publication in the Conference series the Astronomical Society of India published by the Bulletin of the Astronomical Society of India (4 pages, 1 figure)

We investigate nucleosynthesis inside the outflows from gamma-ray burst (GRB) accretion disks formed by the Type II collapsars. In these collapsars, massive stars undergo core collapse to form a proto-neutron star initially and a mild supernova explosion is driven. The supernova ejecta lack momentum and subsequently this newly formed neutron star gets transformed to a stellar mass black hole via massive fallback. The hydrodynamics and the nucleosynthesis in these accretion disks has been studied extensively in the past. Several heavy elements are synthesized in the disk and much of these heavy elements are ejected from the disk via winds and outflows. We study nucleosynthesis in the outflows launched from these disks by using an adiabatic, spherically expanding outflow model, to understand which of these elements thus synthesized in the disk survive in the outflow. While studying this we find that many new elements like isotopes of titanium, copper, zinc etc. are present in the outflows. 56Ni is abundantly synthesized in most of the cases in the outflow which implies that the outflows from these disks in a majority of cases will lead to an observable supernova explosion. It is mainly present when outflow is considered from the He-rich, 56Ni/54Fe rich zones of the disks. However, outflow from the Si-rich zone of the disk remains rich in silicon. Although, emission lines of many of these heavy elements have been observed in the X-ray afterglows of several GRBs by Chandra, BeppoSAX, XMM-Newton etc., Swift seems to have not detected these lines yet., Comment: 15 pages including 8 figures; accepted for publication in The Astrophysical Journal

Stellar mass black holes (SMBHs), forming by the core collapse of very massive, rapidly rotating stars, are expected to exhibit a high density accretion disk around them developed from the spinning mantle of the collapsing star. A wide class of such disks, due to their high density and temperature, are effective emitters of neutrinos and hence called neutrino cooled disks. Tracking the physics relating the observed (neutrino) luminosity to the mass, spin of black holes (BHs) and the accretion rate (M_dot) of such disks, here we establish a correlation between the spin and mass of SMBHs at their formation stage. Our work shows that spinning BHs are more massive than non-spinning BHs for a given M_dot. However, slowly spinning BHs can turn out to be more massive than spinning BHs if M_dot at their formation stage was higher compared to faster spinning BHs., Comment: 7 pages including 3 figures and supplementary information; accepted for publication in Physical Review Letters

We investigate nucleosynthesis inside the gamma-ray burst (GRB) accretion disks formed by the Type II collapsars. In these collapsars, the core collapse of massive stars first leads to the formation of a proto-neutron star and a mild supernova explosion is driven. However, this supernova ejecta lack momentum and falls back onto the neutron star which gets transformed to a stellar mass black hole. In order to study the hydrodynamics and nucleosynthesis of such an accretion disk formed from the fallback material of the supernova ejecta, we use the well established hydrodynamic models. In such a disk neutrino cooling becomes important in the inner disk where the temperature and density are higher. Higher the accretion rate (dot{M}), higher is the density and temperature in the disks. In this work we deal with accretion disks with relatively low accretion rates: 0.001 M_sun s^{-1} \lesssim dot{M} \lesssim 0.01 M_sun s^{-1} and hence these disks are predominantly advection dominated. We use He-rich and Si-rich abundances as the initial condition of nucleosynthesis at the outer disk, and being equipped with the disk hydrodynamics and the nuclear network code, we study the abundance evolution as matter inflows and falls into the central object. We investigate the variation in the nucleosynthesis products in the disk with the change in the initial abundance at the outer disk and also with the change in the mass accretion rate. We report the synthesis of several unusual nuclei like {31}P, {39}K, {43}Sc, {35}Cl, and various isotopes of titanium, vanadium, chromium, manganese and copper. We also confirm that isotopes of iron, cobalt, nickel, argon, calcium, sulphur and silicon get synthesized in the disk, as shown by previous authors. Much of these heavy elements thus synthesized are ejected from the disk via outflows and hence they should leave their signature in observed data., Comment: 13 pages including 4 figures; to appear in Research in Astronomy and Astrophysics

We investigate nucleosynthesis inside the gamma-ray burst (GRB) accretion disks formed by the Type II collapsars and outflows launched from these disks. We deal with accretion disks having relatively low accretion rates: 0.001 M_sun s^{-1} <~ Mdot <~ 0.01 M_sun s^{-1} and hence they are predominantly advection dominated. We report the synthesis of several unusual nuclei like 31P, 39K, 43Sc, 35Cl and various isotopes of titanium, vanadium, chromium, manganese and copper in the disk. We also confirm that isotopes of iron, cobalt, nickel, argon, calcium, sulphur and silicon get synthesized in the disk, as shown by previous authors. Much of these heavy elements thus synthesized are ejected from the disk and survive in the outflows. Indeed, emission lines of many of these heavy elements have been observed in the X-ray afterglows of several GRBs., Comment: 3 pages including 1 figure; to appear in the Proceedings of 13th Marcel Grossman Meeting (MG13), Stockholm, Sweden, 1-7 July 2012; based on the talk given in the meeting

We show that the electric field driven surface instability of visco-elastic films has two distinct regimes: (1) The visco-elastic films behaving like a liquid display long wavelengths governed by applied voltage and surface tension, independent of its elastic storage and viscous loss moduli, and (2) the films behaving like a solid require a threshold voltage for the instability whose wavelength always scales as ~ 4 x film thickness, independent of its surface tension, applied voltage, loss and storage moduli. Wavelength in a narrow transition zone between these regimes depends on the storage modulus., Comment: Accepted for Publication in Physical Review Letters