Ayan Bhattacharjee

We present the first contemporaneous X-ray and optical polarimetric measurement of the extremely high-synchrotron-peaked (HSP) blazar H 1426+428. The X-ray polarimetric observations were undertaken using the Imaging X-ray Polarimetry Explorer (IXPE) on 2024 May 27 and 2024 July 5. The IXPE pointings were accompanied by contemporaneous optical observations of the Observatorio de Sierra Nevada, Calar Alto Observatory, and the Perkins Telescope Observatory. While we observed the X-ray degree of polarization to be >20%, the polarization in the optical band was found to be only 1%–3%. This trend has been observed in several HSP blazars with available optical and X-ray polarimetric data and is typically explained in terms of energy stratification downstream of a shock. However, we observed a significant difference between the optical and X-ray polarization angles, a feature that has been observed in certain HSP blazars, such as Mrk 421, but remains a relatively rare or underreported phenomenon. We discuss possible scenarios for these findings within the framework of a partially turbulent jet model.

Radio galaxies are classified into two primary categories based on their morphology: center-brightened FR-I and edge-brightened FR-II. It is believed that the jet power and interactions with the ambient medium govern the deceleration and decollimation of the jet-spine flows, which, in turn, influence this dichotomy. Using high-resolution, three-dimensional relativistic hydrodynamic simulations, we follow the development of flow structures on sub-kiloparsec to kiloparsec scales in kinetically dominant low-power relativistic jets. We find that the bulk Lorentz factor of the jet spine and the advance speed of the jet head, which depend on the energy injection flux and the jet-to-background density contrast, primarily determine the dynamics and structures of the jet-induced flows. The entrainment of ambient gas and the background density and pressure gradient may also play significant roles. To emulate radio morphology, we produce the synthetic maps of the synchrotron surface brightness for the simulated jets, by employing simple models for magnetic field distribution and nonthermal electron population and considering relativistic beaming effects at different inclination angles. Both the flow structures and radio maps capture the longitudinal and transverse structures of the jet-spine and shear layer, consistent with observations. We also compare different background effects and argue that the loss of pressure confinement beyond the galactic core may be a key factor in the flaring and disruption of FR-I jets. Our results confirm that mildly relativistic jets could explain the one-sidedness or asymmetries with the boosted main jet and deboosted counterjet pairs.

Radio galaxies are classified into two primary categories based on their morphology: center-brightened FR-I and edge-brightened FR-II. It is believed that the jet power and interactions with the ambient medium govern the deceleration and decollimation of the jet-spine flows, which, in turn, influence this dichotomy. Using high-resolution, three-dimensional relativistic hydrodynamic simulations, we follow the development of flow structures on sub-kpc to kpc scales in kinetically dominant low-power relativistic jets. We find that the bulk Lorentz factor of the jet spine and the advance speed of the jet head, which depend on the energy injection flux and the jet-to-background density contrast, primarily determine the dynamics and structures of the jet-induced flows. The entrainment of ambient gas and the background density and pressure gradient may also play significant roles. To emulate radio morphology, we produce the synthetic maps of the synchrotron surface brightness for the simulated jets, by employing simple models for magnetic field distribution and nonthermal electron population and considering relativistic beaming effects at different inclination angles. Both the flow structures and radio maps capture the longitudinal and transverse structures of the jet-spine and shear layer, consistent with observations. We also compare different background effects and argue that the loss of pressure confinement beyond the galactic core may be a key factor in the flaring and disruption of FR-I jets. Our results confirm that mildly relativistic jets could explain the one-sidedness or asymmetries with the boosted main jet and deboosted counterjet pairs.

The Galactic microquasar GRS 1915+105 shows rich variability that is categorized into different classes. In this paper, we report the temporal and spectral analysis of GRS 1915+105 to study the properties of the accretion flow when the light curve shows θ class variability. For this purpose, we use the Large Area X-ray Proportional Counter data from the Target of Opportunity observations of India’s first multiwavelength astronomy satellite AstroSat. The θ class is marked by the recurrent appearance of U-shaped regions in the light curve, where the photon count rate first decreases rapidly and then increases slowly. For our analysis, we use U-shaped regions of the first two orbits (02345 and 02346) on 2016 March 4. In both cases, the dynamic power-density spectra (PDS) showed significant power at around 4–5 Hz, suggesting the presence of a low-frequency quasi-periodic oscillation (QPO) around that frequency interval. The QPO frequency is found to increase with time when the energy flux is also enhanced. From the evolution of the spectra, we determine the evolution of the accretion flow parameters in these two observations. Fitting the spectra with the transonic flow solution-based two-component advective flow (TCAF) model in the 4–25 keV energy band shows that the Keplerian disk accretion rate increases with the increase in radiation intensity, while the location of the centrifugal pressure-driven shock front decreases. In both these data, a gradual increment of power-law photon index with intensity is observed, suggesting the progressive softening of the source.

We use Smoothed Particle Hydrodynamics to study viscous accretion flows around a weakly magnetic neutron star. We show the formation of multiple ``boundary" layers in presence of both cooling and viscosity. We find that with the introduction of a small viscosity in a sub-Keplerian flow, much like the wind accretion in HMXBs such as Cir X-1, only a single Normal Boundary Layer (NBOL) forms to adjust the rotational velocity component. With the increase of viscosity, the region extends radially and beyond some critical value, a RAdiative KEplerian Disk/layer (RAKED) forms between the sub-Keplerian flow and the NBOL. When viscosity is increased further only NBOL and RAKED remain. In all such cases, the CENtrifugal pressure dominated BOundary Layer (CENBOL) is formed, away from the star, as in the case of black holes. This is the first self-consistent study where such a transition from sub-Keplerian flows has been reported for neutron stars. We also identify the connection between accretion and ejection of matter, following the Two-Component Advective Flow for black holes, for neutron stars. The results are crucial in the understanding of the formation of disks, boundary layers and outflows in wind dominated neutron star systems.

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.

We carry out the first robust numerical simulation of accretion flows on a weakly magnetized neutron star using smoothed particle hydrodynamics (SPH). We follow the two-component advective flow (TCAF) paradigm for black holes and focus only on the advective component for the case of a neutron star. This low-viscosity sub-Keplerian flow will create a Normal Boundary Layer (NBOL) right on the star surface in addition to the CENtrifugal pressure supported BOundary Layer (CENBOL) present in a black hole accretion. These density jumps could give rise to standing or oscillating shock fronts. During a hard spectral state, the incoming flow has a negligible viscosity, causing a larger sub-Keplerian component as compared to the Keplerian disk component. We show that our simulation of flows with cooling and a negligible viscosity produces precisely two shocks and strong supersonic winds from these boundary layers. We find that the specific angular momentum of matter dictates the locations and the nature of oscillations of these shocks. For flows with low angular momentum, the radial oscillation appears to be preferred. For flows with higher angular momentum, the vertical oscillation appears to become dominant. In all cases, asymmetries with respect to the Z = 0 plane are seen, and instabilities set in due to the interaction of the inflow and outgoing strong winds. Our results capture both the low- and high-frequency quasi-periodic oscillations without invoking magnetic fields or any precession mechanism. Most importantly, these solutions directly corroborate observed features of wind-dominated high-mass X-ray binaries, such as Cir X-1.