Talk detail

Participant	Mukhopadhyay, Banibrata
Institution	Indian Institute of Science - Department of Physics - Bangalore - Karnataka - India

Session	AP2	Accepted		Order	Time
Talk	Oral abstract	Title	Does black hole spin play a key role in the FSRQ/BL Lac dichotomy?
Co-authors
Abstract	The majority of the identified extragalactic sources in gamma-ray catalogs of EGRET and Fermi are blazars. It is believed that jets emerging from blazars are almost aligned to the line-of-sight. Observationally, blazars can be divided into two classes: flat spectrum radio quasars (FSRQs) and BL Lacs. The later class usually exhibits lower luminosity and harder power law spectra at gamma-ray energies than the former class. Here we attempt to explain the high energy properties of FSRQs and BL Lacs from Fermi gamma-ray space telescope observations. It was argued previously that the difference in accretion rates is mainly responsible for the large observed luminosity mismatch between the classes. However, when intrinsic luminosities are derived by correcting for beaming effects, this difference in luminosity between the two classes is significantly reduced. In order to explain this difference in intrinsic luminosities, we propose that spin plays an important role to reveal the dichotomy of luminosity distributions between BL Lacs and FSRQs. As the outflow power of a blazar increases with increasing spin of central black hole, we suggest that the spin plays a crucial role in making BL Lac sources low luminous and slow rotators compared to FSRQ sources.

Session	BH2	Accepted		Order	Time
Talk	Oral abstract	Title	Stochastically driven instability and turbulence in accretion disks
Co-authors
Abstract	Origin of turbulence in accretion disks is an important issue. While such flows are Rayleigh stable, they exhibit magnetorotational instability. However, the issue appears extremely non-trivial when any magnetohydrodynamic instability in disks becomes weaker or ruled out. The present talk explores the effect of stochastic noise in accretion flows. We essentially concentrate on a small section of such a flow which is nothing but a plane shear flow supplemented by the Coriolis effect. In order to solve the problem, we first establish the magnetized version of the celebrated Orr-Sommerfeld and Squire equations, which have not been established in the literature yet. It is found that such stochastically driven flows exhibit large temporal and spatial correlations, and hence large energy dissipations indicating instability, conditionally. Interestingly, such stochastically driven flows reveal a unique universality class, independent of angular velocity profile (e.g. whether it is a Keplerian disk or a galactic flat rotation curve). This work, to the best of our knowledge, is the first attempt to understand origin of instability and turbulence in the three-dimensional accretion disks based on a modern statistical physics tool.

Session	SF1	Accepted		Order	Time
Talk	Oral abstract	Title	Constraining the central magnetic field of magnetars
Co-authors
Abstract	The magnetars are believed to be highly magnetized neutron stars having surface magnetic field 10^14 - 10^15 G. It is believed that at the center, the magnetic field may be higher than that at the surface. We study the effect of the magnetic field on the neutron star matter and hence on the mass-radius relation of neutron stars. We model the nuclear matter with the relativistic mean field approach considering the possibility of appearance of hyperons at higher density. We find that the effect of magnetic field on the matter of neutron stars and hence on the mass-radius relation is important, when the central magnetic field is atleast of the order of 10^17 G. Very importantly, the effect of strong magnetic field reveals anisotropy to the system. Moreover, if the central field is of the order of 10^19 G, then the matter becomes unstable which limits the maximum magnetic field at the center of magnetars.

Session	SO1	Accepted		Order	Time
Talk	Oral abstract	Title	Model of a disk-outflow coupled system: Disk-outflow symbiosis
Co-authors
Abstract	Several observational evidences and deeper theoretical insights reveal that accretion and outflow/jet are strongly correlated. We model an advective disk-outflow coupled dynamics, incorporating explicitly the vertical flux. Inter-connecting dynamics of outflow and accretion essentially upholds the conservation laws. We investigate the properties of the disk-outflow surface and how is it dependent on the Kerr parameter of the black hole. The energetics of such a symbiotic system strongly depend on the mass, accretion rate and spin of the black holes. The model clearly shows that the outflow power extracted from the disk increases strongly with the spin of the black hole, inferring that the power of the observed astrophysical jets increases with the increasing spin of the central object. In case of blazars (BL Lacs and Flat Spectrum Radio Quasars), most of their emission are believed to be originated from their jets. It is observed that BL Lacs are relatively low luminous than Flat Spectrum Radio Quasars (FSRQs). Their luminosity is presumably linked to the power of the jet, which in turn reflects that the nuclear regions of the BL Lac objects have a relatively low spinning black hole compared to that in the case of FSRQ. If the extreme gravity is responsible for powering strong outflows and jets, then the spin of the black hole, perhaps, is the fundamental parameter to account for the observed astrophysical processes in an accretion powered system.

Session	SO1	Accepted		Order	Time
Talk	Poster abstract	Title	Nonlinear signatures in observed data of black holes and neutron stars
Co-authors
Abstract	We study X-ray data from RXTE/PCA satellite for black hole and neutron star sources: Cyg X-1, Cyg X-2, Cyg X-3, Sco X-1, to search for nonlinear temporal variabilities. Along with some preliminary studies like the analyses of autocorrelation, distribution and Higuchi's dimension (D), we adopt a nonlinear time series analysis technique, which basically computes the correlation dimension (D_2), as a diagnostic tool for this. We find that these sources show strong temporal variability. In some observations, we find D_2 saturating to a low value and D<2, presumably indicating fractal nature of the source. In other cases, D_2 does not saturate to a low value with D ~ 2, arguing the source for stochastic or random. The data exhibiting lower saturated D_2 have lower ratio of expected Poisson noise to the variability, whereas the ratio is higher in absence of lower saturated D_2. Hence, overall the sources reveal the variation in temporal classes. Note that the black hole source GRS 1915+105 was also shown to be transiting between 12 different temporal classes. Our analyses indicate that other compact sources (including neutron stars) also reveal transitions between temporal classes.