14.01.2008
Lecture Day: Annual reports from the Center for Theoretical Astrophysics 2007
Seminars in 2008
13.02.2008
An X-ray investigation of supermassive black holes and their environments
This talk focuses on two projects that were motivated by recent X-ray observations of NGC 2992, a Seyfert 1.9 galaxy, from RXTE and Suzaku. First, I will discuss how high-resolution, time-resolved X-ray spectroscopy can be used to resolve the space-time metric around a black hole. Future instrumentation, such as X-ray calorimeters, are expected to be able to significantly improve the constraints on key physical parameters such as the black-hole angular momentum (spin). With improved X-ray instrumentation and accurate spectral modeling, we will be able to measure the peak energies of Fe lines corresponding to the extremes of the orbits of localized flares (hot spots) close to the black hole. A year-long monitoring campaign of NGC 2992 by RXTE indicates the detection of a possible hot spot from the inner accretion disk of the system. Theoretically, high-resolution measurements of the Fe line resulting from a hot spot on the accretion disk should yield constraints on both the location of the spot and the black hole spin, with fewer assumptions about the system than other spin-measurement methods require. We investigate the possibility of finding such constraints, making use of transfer function calculations for the Kerr metric given by Dovciak et al. (2004), for given peak energies and disk inclination angles as a function of uncertainties in the other pertinent observable parameters of the system. The Suzaku data of NGC 2992 reveals X-ray spectral components from both the inner regions of the AGN (ie, accretion disk) and more distant matter (molecular torus). Deficiencies in current models make it difficult to model the distant matter properly. Therefore, we are developing a spectral fitting routine to model the toroidal reprocessor in AGNs that may be used with an arbitrary input spectrum. The model will be applicable to both Compton-thick and Compton-thin type II AGNs, as well as type I AGNs since it will yield direct constraints on the geometry and column density of distant matter out of the line of sight. The routine will make use of pre-calculated grids of Green's functions to model spectral data in the energy range of 0.2 keV to 1 MeV, and will include up-to-date atomic data to accurately model the Fe K-alpha, Fe K-beta, Ni K-alpha and O K-alpha emission lines. This fitting tool will be made publicly available for use with the X-ray spectral fitting package XSPEC.
25.02.2008
Weak coupling among epicyclic modes in slender tori
In past thirty years, the oscillations of fluid tori orbiting around massive compact objects were studied systematically, namely in the context of a stability problem of thick accretion disks. We further examine importance of a nonlinear coupling among epicyclic oscillations of the slender torus. Since the geodesic equations are separable, earlier studies, based on epicyclic motion of test particles, used additional `ad-hoc' force to wake up the resonance. We show that in the case of epicyclic modes of the fluid torus the situation is different and that both epicyclic oscillations are naturally coupled by pressure and gravity. We estimate strength of this coupling.
03.03.2008
Warped young stellar disc in the Galactic Centre
Within the central parsec of the Galaxy, several tens of young stars orbiting a central supermassive black hole are observed. Subset of these stars form a coherently rotating disc. Yet another observations reveal a massive molecular torus which lies at radius ~ 2 pc from the centre. We consider gravitational influence of the molecular torus upon the stars of the stellar disc. We estimate rate of precession of individual stellar orbits and show that it is highly sensitive upon the orbital semi-major axis and its inclination with respect to the plane of the torus. Under an assumption that both structures are stable on the time-scale > 6 Myr, we determine limits on the mass of the torus and its orientation based on parameters of the stellar disc. We further suggest that all young stars observed in the Galactic centre may have a common origin in a single coherently rotating structure with an opening angle < 5 deg, which was partially destroyed (warped) during its lifetime by the gravitational influence of the molecular torus.
17.03.2008
Introduction to Smoothed Particle Hydrodynamics – I
Lecture outline: Brief review of two different methods of representing a fluid: – Eulerian – construct a grid, fill grid cells with fluid and evolve system by moving fluid between grid cells – Lagrangian – define discrete packets of fluid and evolve the boundaries between them, or evolve the positions of the packets using the forces between them SPH is a Lagrangian method – Review of fluid equations in Lagrangian form ∗ Mass conservation ∗ Momentum cnservation ∗ Energy conservation Outline of how SPH works – Originally a Monte Carlo technique – Fluid represented by discrete particles (usually equal mass) – Particle mass smoothed in space by kernel function – Definition of particle properties as averages over particle and neighbours – leads to errors O(h2) – Adjust h to keep number of neighbours approximately constant The details – Spline kernel (compact support) – Derivation of fluid equations in SPH ∗ Mass conservation ∗ Momentum cnservation ∗ Energy conservation – Equation of state (required to close equations) ∗ Isothermal ∗ Adiabatic – Evolving the smoothing length ∗ Benz method using ∇.v ∗ More explicit techniques, eg GADGET method
31.03.2008
Introduction to Smoothed Particle Hydrodynamics – II
* Brief review of Lecture 1 * Advantages of SPH for studying astrophysical problems – Evolution of smoothing length automatically gives best resolution where gas density is highest (usually, but not always, desirable) – Do not need to define or evolve a grid – no restrictions on geometry of systems that can be studied – System under study does not need to have a boundary – useful for studying objects such as molecular clouds or processes such as stellar collisions in which boundaries cannot be meaningfully defined * Inclusion of extra physics in SPH for astrophysical applications – Self gravity (i.e. gravitational field generated by the gas itself) * Another avantage to SPH over grid codes: implementation of self–gravity relatively easy * Gravity between neighbours kernel smoothed * Otherwise, essentially an N–body problem * N–body problem has N2 complexity, Binary or octal tree to cut down on computation time – Radiative transfer * Brief review of problems in astrophysics where radiative transfer important HII regions Accretion discs Protostars Quasar feedback * Stromgren volume method for HII regions * Flux–limited diffusion method (e.g. Whitehouse and Bate) – Magnetic fields * Brief review of problems in astrophyscs where magnetic fields important Turbulent molecular clouds Accretion discs Collapsing molecular cores * Dan Price’s methods
09.04.2008
Measurements prospects of X-ray photoelectric polarimetry
The study of Astronomical Sources in the X-ray band has been till now largely performed without studying the degree and the angle of polarization, despite the great interest, proved by a large literature. The development of instruments based on the photoelectric effect allows today to overcome the lack of sensitivity of instruments built to date and based on Bragg diffraction or Thomson scatterings. In this presentation the performances and the development of one of the most advanced project, the Gas Pixel Detector built by the INFN of Pisa and by the IASF of Rome, will be discussed. Moreover the prospects of measurements which involve this instrument will be also presented, like the EXP2 mission on board the Chinese mission HXMT and the Italian mission POLARIX.
16.04.2008
Jet formation in the brightest Low Mass X-ray Binaries
Low Mass X-ray Binaries can be classified on the basis of luminosity into the Atoll sources (L = 10^36 - 10^38 erg/s) and the Z-track sources (L > 10^38 erg/s). Both types display characteristic shapes in hardness-intensity diagrams which in the Z-track sources have a Z shape indicating three states of the sources with major physical differences between the states. Although we understand accretion physics in some detail, and the phenomena taking place in LMXB, the basic nature of the Z-track and Atoll sources and the changes between states has not been understood. The importance of the Z-track sources is the presence of jets in one state of the source only, and this allows us to determine by spectral analysis the conditions under which jets are, and are not, formed. We will present results for a number of sources based on which we propose that strong radiation pressure of the neutron star is the necessary condition for jet formation.
28.04.2008
Introduction to Smoothed Particle Hydrodynamics – III
Lecture outline. • Star formation – Brief review of star formation in turbulent clouds Observations of turbulence Dissipation of turbulent energy Onset of graviational instability, Jeans mass – Simulating turbulent clouds in SPH – Simulating gravitational collapse and star formation in SPH Sink particles Formation of sink particles Accretion of gas onto sink particles • Feedback from young stars – Review of problem of molecular cloud lifetimes/cloud dispersal – Review of star formation efficiency problem – Review of triggering/self–propagating star formation – Review of main sources of stellar feedback: HII regions · Simple ionisation physics · Spitzer solution for HII region expansion Stellar winds · Momentum–driven solution · Pressure–driven solution Supernovae · Blast wave solution
12.05.2008
Coordinated multi-wavelength observations of Sgr A*
The compact source Sgr A* can be associated with the massive black hole at the center of the Milky Way. It shows strong variability from the radio to the X-ray wavelength domain. Here we report on the latest (May 2007, and the preceding years) simultaneous NIR/sub-millimeter/X-ray observations using the NACO adaptive optics (AO) instrument at the European Southern Observatory's Very Large Telescope, the Australian Telescope Compact Array, the US mm-array CARMA, and other telescopes. We interpret the results using a model in which spots are on relativistic orbits around Sgr A* and discuss the possibility of a jet or outflow from such a disk. The general relativistic effects in strong gravitational regime influence the light-curve variations and also the observed polarization angle and degree, which allow us to constrain physical parameters close to the event horizon. We include shearing and expansion for modeling the data. The results could also be used to constrain the magnetic field structure inside the plasma close to the marginally stable orbit.
19.05.2008
Introduction to Smoothed Particle Hydrodynamics – IV
Lecture outline. • Brief review of feedback from Lecture 3 • All feedback mechanisms drive shocks into the surrounding gas • Basic shock physics – Rankine–Hugoniot equations – conservation of mass, momentum and (if appropriate) energy across shock front – Importance of cooling of shocked gas on evolution, i.e. isothermal versus adiabatic shocks • Modelling shocks in SPH – Artificial viscosity – numerical trick to model processes occurring at length scales many orders of magnitude smaller than resolution of simulation – Methods of implementing artificial viscosity in SPH Von Neumann Richtmyer Balsara etc... • Some examples of shock simulations – Spherical HII region – Spherical wind – HII region in non–uniform sloud (from Dale et al., 2005) – more complicated probem! Discuss difficulties of expelling gas/disrupting clusters with stellar feedback • Self–gravitating shells and triggered star formation – Review gravitational fragmentation in 3–D – Gravitational fragmentation of thin shells Onset of fragmentation Mass spectum of fragments expected Discuss relation to observations
10.06.2008
The X-ray spectra of LINERs: starburst or active galactic nuclei emission?
The aim of our work is to understand LINER nature and to assess whether they can be classified as Active Galactic Nuclei or Starburst galaxies. Active Galactic Nuclei are some of the most luminous sources in the Universe. The primary source of their luminosity is conversion of gravitational energy in radiation via accretion on a Super Massive Black Hole. Starbursts are galaxies with strong star formation mechanisms and a strong contribute in their spectra from HII regions. LINERs were identified the first time in 1980 (by Heckman); they are characterized by the strong luminosity of narrow emission line of low ionization elements. According to some interpretations, their central source is a Low Luminosity AGN, but there are also interpretations of LINERs as Starburst galaxies. Both hypotheses are able to explain the spectral energy distributions of these sources. In this project we studied a sample of LINERs. The work was divided in three steps: 1) modelling the X-ray spectra of LINERs, 2) study of their multi- wavelength properties, 3) summary and interpretation of the results. In the first step, we studied the spectra with two models: the first one was a LLAGN model, the second one was a Starburst model. For those sources with good enough spectra, we measured the equivalent width of the Fe K emission line, or alternatively put upper limits to it. In this step, we came to the conclusion that almost all the sources were better modelled by a LLAGN model, than by a Starburst model. In the second step, we compared the X-ray, far infrared, radio, [OIII]5007 emission line, optical luminosities. We compared hard X-rays and FIR luminosities, in order to assess whether or not these sources could be classified as Starburst galaxies. In fact, tight linear relation holds between the X-ray and far infrared luminosities for the star forming galaxies (Ranalli et al. 2003): the aim of this diagnostic test was to check if this correlation could be still valid for our sample. We compared hard X-rays and [OIII]5007 emission line luminosities, in order to assess whether or not these sources could be classified as AGN. In fact, a strong linear correlation between X-rays and [OIII]5007 emission line luminosities was found for samples of AGN (Panessa et al. 2006): the aim of this diagnostic test was to check if this correlation could be still valid for our sample. In the third step of this work, we discussed the results of the X- ray study and multi- wavelength properties of the sample. We found that many objects of our sample are classified as AGN, other LINERs have both an AGN and a Starburst source, and only one LINER is classified as a Starburst.
16.06.2008
Point processes as a mathematical tool to describe black-hole accretion disc stochastical variability
Radiation from accreting black holes varies on different time-scales. In X-rays, the observed light-curve, is a complicated noisy curve that can be represented by a broad-band power spectrum. The `hot spots' on the accretion disc have been identified a possible contributor to this variability. These spots are supposed to occur on the surface of an accretion disc following its irradiation by coronal flares. Various schemes have been proposed in which spots are mutually interconnected in some way. We discuss the statistical properties of model light-curves, constructed as a sum of contributions from many point-like sources that are orbiting above an underlying accretion disc. Particularly, we examine the power spectral profiles generated by different kinds of avalanching mechanisms, using the framework of the theory of point processes.
24.06.2008
Recent progress on the Fe Kalpha fluorescent line in X-ray spectra of Active Galaxies
Recent work on the Fe line progresses along two lines of research: one tries to explain the lack of variability of the line with extreme relativistic effects; in some other papers modeling the data using advanced models of absorption reveals that the line (in, e.g., MCG-6-30-15) does not need to be extremely broad as previously reported. I am going to review these recent developments.
07.07.2008
1D and 2D reverberation mapping – application to NGC 4151
1D and 2D reverberation mapping (RM) is a proven technique that is used to measure the size of the broad emission-line region and central black hole mass in active galactic nuclei. We applied this technique on results of a long-term spectral monitoring of the galactic nucleus of NGC 4151 (from 1996 to 2006). We considered three characteristic periods where both Hβ and Hα profiles were similar: 1996–1999, 2000–2001 and 2002–2006. 1D RM gives a more realistic estimation of the dimension of the BLR than during other periods. Further, the time lags obtained by binning intervals of three years within the whole monitoring period indicate the permanent presence of a small component of the BLR (0.3–0.7 light days; Shapovalova et al. 2008). Also, using the 2D RM of the Hα line of NGC 4151, we analyzed the BLR structure in this active galaxy. We found that the line wings have different response to the continuum variation than the line core. This type of variation indicates an outflow model for the BLR of NGC 4151.
01.09.2008
The formation of brown dwarfs
I will explain why understanding how brown dwarfs form is a critical problem in astronomy. I will review the possible mechanisms that have been proposed, and suggest that the available evidence points to brown dwarfs forming by fragmentation of discs around more massive stars.
04.09.2008
SPH simulations of expanding HII regions
We will begin with a short description of the stages found during the evolution of an HII region, and how this leads to the formation of stars. A short introduction to Smoothed Particle Hydrodynamics will be presented and how we numericaly treat the expansion of an HII region. Finally, we will show examples of ionization of different kinds of interstellar clouds.
15.10.2008
Extreme magnetic fields in the Universe
The numerical simulations of the last years show that the plasma phenomena should have a dominant role in the star formation from the creation of the protostellar cloud, making possible the creation of the primary globules without having to accomplish the Jean’s criterion regarding the minimum size of the nebula and even without a "starter" shock wave from a neighbor supernova. In the same manner the numerical simulations show that the arms of the galactic spirals can be the result of the electromagnetic interaction of global magnetic fields and need not to be only gravitational manifestations. Nowadays it also seems that the most energetic particles, which are observed in the cosmic rays, were accelerated in the plasma spatial filaments inside an electric double layer. Therefore the possible picture that we can make of our Universe changes, since the Universe is not only gravitational interaction, as we have thought until recently. In the formation of the Universe has contributed in the same manner the electromagnetic interaction and its diverse manifestations. With the introduction of the X-ray studies we have come literally with an attack from plasma physics to the study of our Universe.
13.11.2008
Advanced ray-tracing in Kerr spacetime
With increasingly improving resolution of astronomical detectors the ray-tracing technique is and must often be used to accurately calculate the emission properties of sources. This talk will describe the most advanced method of ray-tracing in Kerr space-time by evaluation of elliptic integrals, which has been recently improved to include necessary routines for investigating asymmetric time-dependent configurations and also to include the polarization characteristics.