arXiv subjects

Pranab Ghosh

Publications and source records attributed to Pranab Ghosh.

At least 19 recordsLinked to original sources

arXiv2019

The X-ray Polarization Probe mission concept

The X-ray Polarization Probe (XPP) is a second generation X-ray polarimeter following up on the Imaging X-ray Polarimetry Explorer (IXPE). The XPP will offer true broadband polarimetery over the wide 0.2-60 keV bandpass in addition to imaging polarimetry from 2-8 keV. The extended energy bandpass and improvements in sensitivity will enable the simultaneous measurement of the polarization of several emission components. These measurements will give qualitatively new information about how compact objects work, and will probe fundamental physics, i.e. strong-field quantum electrodynamics and strong gravity.

astro-ph.IM↗

arXiv2019

Astro2020 Science White Paper: Using X-Ray Polarimetry to Probe the Physics of Black Holes and Neutron Stars

This white paper highlights compact object and fundamental physics science opportunities afforded by high-throughput broadband (0.1-60 keV) X-ray polarization observations. X-ray polarimetry gives new observables with geometric information about stellar remnants which are many orders of magnitude too small for direct imaging. The X-ray polarimetric data also reveal details about the emission mechanisms and the structure of the magnetic fields in and around the most extreme objects in the Universe. Whereas the Imaging X-ray Polarimetry Explorer (IXPE) to be launched in 2021 will obtain first results for bright objects, a follow-up mission could be one order of magnitude more sensitive and would be able to use a broader bandpass to perform physics type experiments for representative samples of sources.

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arXiv2016

Differentially Rotating White Dwarfs I: Regimes of Internal Rotation

Most viable models of Type Ia supernovae (SN~Ia) require the thermonuclear explosion of a carbon/oxygen white dwarf that has evolved in a binary system. Rotation could be an important aspect of any model for SN~Ia, whether single or double degenerate, with the white dwarf mass at, below, or above the Chandrasekhar limit. {\sl Differential rotation} is specifically invoked in attempts to account for the apparent excess mass in the super--Chandrasekhar events. Some earlier work has suggested that only uniform rotation is consistent with the expected mechanisms of angular momentum transport in white dwarfs, while others have found pronounced differential rotation. We show that if the baroclinic instability is active in degenerate matter and the effects of magnetic fields are neglected, both nearly-uniform and strongly-differential rotation are possible. We classify rotation regimes in terms of the Richardson number, Ri. At small values of Ri $\leq$ 0.1, we find both the low-viscosity Zahn regime with a non-monotonic angular velocity profile and a new differential rotation regime for which the viscosity is high and scales linearly with the shear, $σ$. Employment of Kelvin-Helmholtz viscosity alone yields differential rotation. Large values of Ri $\gg$ 1 produce a regime of nearly-uniform rotation for which the baroclinic viscosity is of intermediate value and scales as $σ^3$. We discuss the gap in understanding of the behavior at intermediate values of Ri and how observations may constrain the rotation regimes attained by nature.

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arXiv2013

White Paper for Blazar Observations with a GEMS-like X-ray Polarimetry Mission

In this document, we describe the scientific potential of blazar observations with a X-ray polarimetry mission like GEMS (Gravity and Extreme Magnetism SMEX). We describe five blazar science investigations that such a mission would enable: (i) the structure and the role of magnetic fields in AGN jets, (ii) analysis of the polarization of the synchrotron X-ray emission from AGN jets, (iii) discrimination between synchrotron self-Compton and external Compton models for blazars with inverse Compton emission in the X-ray band, (iv) a precision study of the polarization properties of the X-ray emission from Cen-A, (v) tests of Lorentz Invariance based on X-ray polarimetric observations of blazars. We conclude with a discussion of a straw man observation program and recommended accompanying multiwavelength observations.

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arXiv2013

White Paper on GEMS Study of Polarized X-rays from Neutron Stars

We examine the expected X-ray polarization properties of neutron-star X-ray sources of various types, e.g., accretion and rotation powered pulsars, magnetars, and low-mass X-ray binaries. We summarize the model calculations leading to these expected properties. We describe how a comparison of these with their observed properties, as inferred from GEMS data, will probe the essential dynamical, electromagnetic, plasma, and emission processes in neutron-star binaries, discriminate between models of these processes, and constrain model parameters. An exciting goal is the first observational demonstration in this context of the existence of vacuum resonance, a fundamental quantum electrodynamical phenomenon first described in the 1930s.

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arXiv2013

X-ray Polarization from Black Holes: GEMS Scientific White Paper

We present here a summary of the scientific goals behind the Gravity and Extreme Magnetism SMEX (GEMS) X-ray polarimetry mission's black hole (BH) observing program. The primary targets can be divided into two classes: stellar-mass galactic BHs in accreting binaries, and super-massive BHs in the centers of active galactic nuclei (AGN). The stellar-mass BHs can in turn be divided into various X-ray spectral states: thermal-dominant (disk), hard (radio jet), and steep power-law (hot corona). These different spectral states are thought to be generated by different accretion geometries and emission mechanisms. X-ray polarization is an ideal tool for probing the geometry around these BHs and revealing the specific properties of the accreting gas.

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arXiv2013

Collective Properties of X-ray Binary Populations of Galaxies III. The Low-mass X-ray Binary Luminosity Function

Continuing our exploration of the collective properties of low-mass X-ray binaries (LMXBs) in the stellar fields of normal galaxies, we compute in this paper the expected X-ray luminosity function (XLF) of LMXBs, starting from the results of the previous paper in the series (Paper II). We treat separately two classes of LMXB evolution, the first being close systems whose initial orbital periods are below the bifurcation period, wherein the companion is on the main sequence when Roche-lobe contact occurs, the subsequent evolution is driven by angular-momentum loss from the system, and the second being wider systems whose initial orbital periods are above the bifurcation period, wherein the companion is on the giant branch when Roche-lobe contact occurs, and the evolution of these systems is driven by the nuclear evolution of the companion. We obtain model luminosity profiles L(t) for individual LMXBs of both classes. We then compute the LMXB XLF by folding in the inputs for the pre-LMXB collective properties and formation rates with the above luminosity profiles. Because of the long timescale on which LMXBs evolve, one needs to keep track of the evolution of the star-formation rate (SFR) on the same timescale, and we use star-formation histories given by canonical models. We compare the observed LMXB XLF with our computed one, keeping in mind that we have included only neutron-star systems in this work, so that there would be an unaccounted-for population of black-hole binaries at the high-luminosity end. We show that a qualitative similarity already exists between the two, and we discuss the role of the giant fraction, i.e., that fraction of all LMXBs which harbors a low-mass giant companion, on the shape of the XLF at the high-luminosity end.

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arXiv2013

Collective Properties of X-ray Binary Populations of Galaxies II. Pre-Low-mass X-ray Binary Properties, Formation Rates, and Constraints

We continue exploring our understanding of the collective properties of X-ray binaries in the stellar fields (i.e., outside globular clusters) of normal galaxies, introduced in Paper I of this series, where we considered high-mass X-ray binaries (HMXBs). In this paper (Paper II of the series) and the companion paper (Paper III of the series), we consider low-mass X-ray binaries (LMXBs), whose evolutionary scenario is very different from that of HMXBs. In this paper, we consider the evolution of primordial binaries upto the stage where the neutron star just formed in the supernova explosion of the primary is in a binary with its low-mass unevolved companion, and this binary has circularized tidally, producing what we call a pre-low-mass X-ray binary (pre-LMXB). We study the constraints on the formation of such pre-LMXBs in detail (since these are low probability events), and calculate their collective properties and formation rate. To this end, we first consider the changes in the binary parameters in the various steps involved, viz., the common-envelope (CE) phase, the supernova, and the tidal evolution. This naturally leads to a clarification of the constraints. We then describe our calculation of the evolution of the distributions of primordial binary parameters into those of the pre-LMXB parameters, following the standard evolutionary scenario for individual binaries. We display the latter as both bivariate and monovariate distributions, discuss their essential properties, and indicate the influence of some essential factors on these. Finally, we calculate the formation rate of pre-LMXBs. The results of this paper are used in the next one (Paper III) to compute the expected X-ray luminosity function (XLF) of LMXBs, which is compared with observation.

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arXiv2011

Collective Properties of X-ray Binary Populations of Galaxies. I. Luminosity and Orbital Period Distributions of High-Mass X-ray Binaries

We introduce a method for obtaining the X-ray luminosity function (XLF) and the binary-period distribution of populations of high-mass X-ray binaries (HMXBs) in the stellar fields (i.e. outside globular clusters) of normal galaxies. We start from standard distributions of the parameters of those primordial binaries which are the progenitors of HMXBs, and follow the transformation of these ditributions with the aid of a Jacobian formalism as the former evolve into the latter through the processes of the first mass transfer and the supernova (SN) that follows. We discuss the distributions of the post-SN binaries and the HMXBs. We show that our calculated model XLF has a differential slope $\approx -1.6$ with a flattening at low luminosities, in excellent agreement with observations. The calculated binary-period distribution, which basically has a slightly sloping plateau-like character at intermediate periods, with a rise to this plateau at shorter periods and fall-off from it at longer periods, is in agreement with the observed distribution within observational uncertainties. We discuss the physical origin of these distributions. We demonstrate that, while the effects of both (a) the distribution of the properties of the massive companion in the HMXBs, and (b) the primordial orbital distribution and the SN dynamics are important, the former appear to be dominant in determining the XLF, and the latter in determining the HMXB binary-period distribution. We discuss the possible roles of stellar-mass black holes and ultra-luminous X-ray sources (ULX) in the observed "universal" XLF of HMXBs.

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arXiv2011

Understanding the X-ray luminosity function of high mass X-ray binaries

High mass X-ray binary luminosity function (XLF) is an important tool for studying binary evolution processes and also the mass loss and consequent evolution in massive stars. We calculate the XLF for neutron star binaries using the standard scenario for formation and evolution of these systems. A one to one relation between primordial binary parameters and the HMXB parameters is established. The probability density function is then transformed using the standard Jacobian formalism. It is shown that the model successfully explains some basic properties of the observed XLF.

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arXiv2009

Young pre-Low-Mass X-ray Binaries in propeller phase : Nature of the 6.7-hour periodic X-ray source 1E 161348-5055 in RCW 103

Discovery of the 6.7-hour periodicity in the X-ray source 1E 161348-5055 in RCW 103 has led to investigations of the nature of this periodicity. We explore a model for 1E 161348-5055, wherein a fast-spinning neutron star with a magnetic field $\sim 10^{12}$ G in a young pre-Low-Mass X-ray Binary (pre-LMXB) with an eccentric orbit of period 6.7 hr operates in the "propeller" phase. The 6.7-hr light curve of 1E 161348-5055 can be quantitatively accounted by a model of orbitally-modulated mass transfer through a viscous accretion disk and subsequent propeller emission (both Illarionov-Sunyaev type and Romanova-Lovelace et al type), and spectral and other properties are also in agreement. Formation and evolution of model systems are shown to be in accordance both with standard theories.

astro-ph.SR↗

arXiv2008

Evolution of Compact-Binary Populations in Globular Clusters: A Boltzmann Study II. Introducing Stochasticity

We continue exploration of the Boltzmann scheme started in Banerjee and Ghosh (2007, henceforth Paper I) for studying the evolution of compact-binary populations of globular clusters, introducing in this paper our method of handling the stochasticity inherent in dynamical processes of binary formation, destruction and hardening in globular clusters. We describe these stochastic processes as "Wiener processes", whereupon the Boltzmann equation becomes a stochastic partial differential equation, the solution of which requires the use of "Ito calculus" (this use being the first, to our knowledge, in this subject), in addition to ordinary calculus. We focus on the evolution of (a) the number of X-ray binaries $N_{XB}$ in globular clusters, and (b) the orbital-period distribution of these binaries. We show that, although the details of the fluctuations in the above quantities differ from one "realization" to another of the stochastic processes, the general trends follow those found in the continuous-limit study of Paper I, and the average result over many such realizations is close to the continuous-limit result. We investigate the dependence of $N_{XB}$ found by these calculations on two essential globular-cluster parameters, namely, the star-star and star-binary encounter-rate parameters $Γ$ and $γ$, for which we had coined the name Verbunt parameters in Paper I. We compare our computed results with those from CHANDRA observations of Galactic globular clusters, showing that the expected scalings of $N_{XB}$ with the Verbunt parameters are in good agreement with the observed ones. We indicate what additional features can be incorporated into the scheme in future, and how more elaborate problems can be tackled.

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arXiv2007

Evolution of Compact-Binary Populations in Globular Clusters: A Boltzmann Study. I. The Continuous Limit

We explore a Boltzmann scheme for studying the evolution of compact binary populations of globular clusters. We include processes of compact-binary formation by tidal capture and exchange encounters, binary destruction by dissociation and other mechanisms, and binary hardening by encounters, gravitational radiation and magnetic braking, as also the orbital evolution during mass transfer, following Roche lobe contact. For the encounter processes which are stochastic in nature, we study the probabilistic, continuous limit in this introductory work, deferring the specific handling of the stochastic terms to the next step. We focus on the evolution of (a) the number of X-ray sources N_{XB} in globular clusters, and (b) the orbital-period distribution of the X-ray binaries, as a result of the above processes. We investigate the dependence of N_{XB} on two essential cluster properties, namely, the star-star and star-binary encounter-rate parameters 'Gamma' and 'gamma', which we call Verbunt parameters. We compare our model results with observation, showing that the model values of N_{XB} and their expected scaling with the Verbunt parameters are in good agreement with results from recent X-ray observations of Galactic globular clusters, encouraging us to build more detailed models.

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arXiv2002

Cosmic Star Formation History and Deep X-ray Imaging in the XMM-NEWTON and CHANDRA Era

I summarize X-ray diagnostic studies of cosmic star formation in terms of evolutionary schemes for X-ray binary evolution in normal galaxies with evolving star formation. Deep X-ray imaging studies by CHANDRA and XMM-NEWTON are beginning to constrain both the X-ray luminosity evolution of galaxies and the log N - log S diagnostics of the X-ray background: I discuss these in the above context, summarizing current understanding and future prospects.

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arXiv2001

X-Ray Probes of Cosmic Star-Formation History

We discuss the imprints left by a cosmological evolution of the star formation rate (SFR) on the evolution of X-ray luminosities Lx of normal galaxies, using the scheme proposed by White and Ghosh (1998, WG98), wherein the evolution of Lx of a galaxy is driven by the evolution of its X-ray binary population. As indicated in WG98, the profile of Lx with redshift can both serve as a diagnostic probe of the SFR profile and constrain evolutionary models for X-ray binaries. We report here the first calculation of the expected evolution of X-ray luminosities of galaxies, updating the WG98 work by using a suite of more recently developed SFR profiles that span the currently plausible range. The first Chandra deep imaging results on Lx are beginning to probe the SFR profile of bright spirals: the early results are consistent with predictions based on current SFR models. Using these new SFR profiles, the resolution of the ``birthrate problem'' of low-mass X-ray binaries (LMXBs) and recycled, millisecond pulsars (WG98) in terms of an evolving global SFR is more complete. We discuss the possible impact of the variations in the SFR profile of individual galaxies and galaxy-types.

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arXiv2000

The structure of black hole magnetospheres. I. Schwarzschild black holes

We introduce a multipolar scheme for describing the structure of stationary, axisymmetric, force-free black-hole magnetospheres in the ``3+1'' formalism. We focus here on Schwarzschild spacetime, giving a complete classification of the separable solutions of the stream equation. We show a transparent term-by-term analogy of our solutions with the familiar multipoles of flat-space electrodynamics. We discuss electrodynamic processes around disk-fed black holes in which our solutions find natural applications: (a) ``interior'' solutions in studies of the Blandford-Znajek process of extracting the hole's rotational energy, and of the formation of relativistic jets in active galactic nuclei and ``microquasars'', and, (b) ``exterior'' solutions in studies of accretion disk dynamos, disk-driven winds and jets. On the strength of existing numerical studies, we argue that the poloidal field structures found here are also expected to hold with good accuracy for rotating black holes, except for maximum possible rotation rates. We show that the closed-loop exterior solutions found here are not in contradiction with the Macdonald-Thorne theorem, since these solutions, which diverge logarithmically on the hole's horizon $\cal H$, apply only to those regions which exclude $\cal H$.

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arXiv1999

Cosmic rays from remnants of quasars?

Considerations of the collision losses for protons traversing the 2.7 K black body microwave radiation field have led to the conclusion that the highest energy cosmic rays, those observed at $\geq 10^{20}$ eV, must come from sources within the present epoch. In light of this constraint, it is here suggested that these particles may be accelerated near the event horizons of spinning supermassive black holes associated with presently inactive quasar remnants. The required emf is generated by the black hole induced rotation of externally supplied magnetic field lines threading the horizon. Producing the observed flux of the highest energy cosmic rays would constitute a negligible drain on the black hole dynamo. Observations with upcoming air shower arrays and space missions may lead to the identification of candidate dormant galaxies which harbor such black holes. Although the highest energy events observed so far are accounted for within the context of this scenario, a spectral upper bound at $\sim 10^{21}$ eV is expected since the acceleration to higher energies appears to be precluded, on general grounds.

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arXiv1998

Viscous Stability of Relativistic Keplerian Accretion Disks

We investigate the viscous stability of thin, Keplerian accretion disks in regions where general relativistic (GR) effects are essential. For gas pressure dominated (GPD) disks, we show that the Newtonian conclusion that such disks are viscously stable is reversed by GR modifications in the behaviors of viscous stress and surface density over a significantly large annular region not far from the innermost stable orbit at $r=\rms$. For slowly-rotating central objects, this region spans a range of radii $14\lo r\lo 19$ in units of the central object's mass $M$. For radiation pressure dominated (RPD) disks, the Newtonian conclusion that they are viscously unstable remains valid after including the above GR modifications, except in a very small annulus around $r\approx 14M$, which has a negligible influence. Inclusion of the stabilizing effect of the mass-inflow through the disk's inner edge via a GR analogue of Roche-lobe overflow adds a small, stable region around \rms~for RPD disks, but leaves GPD disks unchanged. We mention possible astrophysical relevance of these results, particularly to the high-frequency X-ray variabilities observed by the $Rossi$ $X-ray$ $Timing$ $Explorer$.

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