Seminars 2015
Norbert Werner
Kavli Institute for Particle Astrophysics and Cosmology, Stanford University
How supermassive black holes and star-formation sculpt the visible Universe
Abstract
Norbert Werner
In the course of structure formation, only a small fraction of the baryons turned into stars - most remain in a diffuse intergalactic medium. The growth of galaxies is regulated by feedback processes, such as energy and momentum input from supernovae, and jets and winds of accreting supermassive black holes. These processes, collectively called galactic feedback, can limit or even inhibit star formation, and thus a detailed knowledge of how they work is essential for our understanding of galaxy formation and evolution. I will start my talk by presenting recent observational results on the role of supermassive black holes in keeping the most massive galaxies 'red and dead'. Then, I will 'zoom out' to the outskirts of galaxy clusters where we also find hints that supermassive black holes played an important role in the distant past. X-ray observations with the Suzaku satellite reveal a remarkably homogeneous distribution of iron out to the virial radius of the nearby Perseus Cluster, requiring that most of the metal enrichment of the intergalactic medium occurred before the cluster formed, probably more than ten billion years ago, during the period of maximal star formation and black hole activity. Finally, I will talk about the upcoming ASTRO-H satellite which will revolutionize X-ray spectroscopy and our understanding of how feedback processes couple to the intergalactic medium.
Mordehai Milgrom
Weizmann Institute, Rehovot, Israel
Scale invariance at low accelerations: An alternative to dark matter
Abstract
Mordehai Milgrom
Galactic systems exhibit large mass discrepancies: The observed matter in
them falls very short of providing enough gravity to hold them together.
The mainstream solution of this conundrum is the evocation of large
quantities of “dark matter”, which purportedly supplies the extra gravity.
Its nature is not known, but it is clear that it cannot be made of any
presently known form of matter. The MOND paradigm offers a different
solution: a breakdown of standard dynamics (gravity and/or inertia)
in the limit of low accelerations (below some acceleration constant a0),
such as are found in galactic systems. In this limit, dynamics become
space-time scale invariant, and is controlled by a scale-invariant
gravitational constant that replaces G. With the new dynamics, the various
detailed manifestations of the galactic mass discrepancies disappear with
no need for exotic dark matter. I will briefly describe the achievements
and the remaining shortcomings of the paradigm.
Francesco Ursini
Universita Roma Tre, Rome, Italy
The importance of taking the broad view: the NuSTAR AGN Physics Program
Abstract
Francesco Ursini
After a brief description of NuSTAR - a NASA hard X-ray satellite in orbit since June 13, 2012 - and of its scientific objectives and main results, I will present and discuss the activities and results of the AGN Physics Working Group. In particular, I will discuss the NuSTAR impact on our understanding of: the hot corona responsibile for the main X-ray emission; the strong gravity effects close to the BH; the absorption/reflection components from distant matter; the soft X-ray emission.
Nick Kylafis
University of Crete, Greece
Accretion and ejection in black-hole X-ray transients
Abstract
Nick Kylafis
Black-hole X-ray transients (BHTs) often exhibit during their outbursts a characteristic q-shaped curve in a hardness-luminosity diagram. A rich phenomenology has been accumulated over the years regarding this diagram. It is desirable to have a physical picture of BHTs over the entire q-shaped curve, which hopefully will have predictive power. Such a physical picture is proposed here and it relies on two assumptions, easily justifiable. The first is that the mass-accretion rate to the black hole in a BHT outburst has a generic “bell-shaped” form. This is guaranteed by the observational fact that all BHTs start their outburst and end it at the quiescent state, i.e., at very low accretion rates. The mass-accretion rate increases, reaches values close to the Eddington rate, and decreases again. The second assumption is that at low accretion rates the accretion disk is geometrically thick, ADAF-like, while at high accretion rates it is thin. This assumption is generally accepted. Unlike phenomenological pictures typically invoked for BHTs, our physical picture explains a) the difference between type C and type B QPOs, b) the formation and the destruction of jets, and c) why BHTs traverse the q-shaped curve always in the counterclockwise direction and that no BHT is expected to ever traverse the entire q-curve in the clockwise direction. Our physical picture explains the q-shaped curve and its associated phenomenology with only one parameter, the accretion rate.
Pavel Kroupa
Helmholtz-Institut für Strahlen- und Kernphysik, Bonn
Is star formation a stochastic or a regulated process?
Abstract
Pavel Kroupa
The stellar initial mass function (IMF) is usually assumed to be a
probability density distribution function. Recent data appear to question
this interpretation though, and I will discuss alternative applications and
results concerning the possibly true nature of the IMF. Empirical evidence
has emerged that the IMF becomes top-heavy in intense star bursts and at
low metallicity. Related to the IMF are binary star distribution functions, and
these evolve through dynamical processes in embedded star clusters. The
insights gained from these considerations lead to a mathematically
computable method for calculating stellar populations in galaxies, with
possibly important implications for the matter cycle in galaxies. It turns
out that the galaxy-wide IMF, the IGIMF, becomes increasingly top-heavy
with increasing galaxy-wide star formation rate, while at the same time the
binary fraction in the galactic field decreases.
Q. Daniel Wang
Astronomy Department, University of Massachusetts, Amherst
A close-up of the nearest supermasive black hole – Sgr A*
Abstract
Q. Daniel Wang
Supermassive black holes (SMBHs) are typically low in radiation luminosity. But the collective mechanical feedback from such radiatively inefficient SMBHs could rival or even exceed that from active galactic nuclei and could hence strongly affect galaxy evolution. Our own Galaxy's SMBH, Sgr A*, provides an excellent laboratory to study this radiatively inefficient accretion state. A close-up view of its x-ray emission based on recent 3 megaseconds of Chandra observations is providing new insights into the interplay of the SMBH with its environment. The X-ray spectrum of Sgr A* shows multiple emission lines of highly ionized ions. This, together with the lack of the neutral iron K-alpha line, suggests that the accretion of the SMBH is truly in a hot mode. A spectral modeling further suggests that the accretion must be accompanied by an outflow, which likely removes more than 99% of the material initially captured by the SMBH. This conclusion is confirmed in a fitting of the spatially-resolved X-ray emission structure with accretion/outflow simulations, which also enables us to determine its angular momentum. Both the fitted position and inclination angles of the flow are consistent with those of the surrounding disk of massive stars, confirming that their stellar winds are feeding the SMBH. If time allows, I will discuss what we are learning from a systematic time variability analysis and ongoing multi-wavelength monitoring of Sgr A*, as well as a comparison with other nearby low-luminosity SMBHs.
Pavel Kroupa
Helmholtz-Institut für Strahlen- und Kernphysik, Bonn
Testing the standard cold dark matter cosmological model
Abstract
Pavel Kroupa
The dual-dwarf-galaxy theorem, according to which two types of galaxies must exist and which must be true in the standard model of cosmology, appears to be ruled out by astronomical data: both types of dwarf galaxy, those with putative exotic dark matter and those known to not contain dark matter even if it were to exist, cannot be distinguished by observation. Furthermore, the arrangement of satellite galaxies in rotating disk-like vast near-polar structures around the Milky Way and Andromeda galaxies and the frequent occurrence of anisotropic flattened satellite populations around major galaxies, seem to very strongly support the conclusion that only one type of satellite dwarf galaxy exists, namely the type without dark matter. Also, the orbital decay implied by dynamical friction on the putative dark matter halos is not evident in interacting galaxies. Dynamically relevant cold or warm dark matter therefore seems not to be present. Instead and as suggested by Milgrom, scale-invariant dynamics is showing a new direction for understanding the astrophysics of galaxies. Galaxies are observed to be simple systems following laws that result from scale-invariant dynamics which do not emanate from the haphazard merging history of halos of exotic dark matter. As a result, the present-day cosmological description of galaxy formation and evolution appears to need major revision.
Prashanth Mohan
ARIES (Aryabhatta Research Institute of Observational Sciences), Nainital, India
Radiatively driven relativistic jets
Abstract
Prashanth Mohan
We present a general relativistic (GR) model of jet variability in active galactic nuclei due to orbiting blobs in helical motion along a funnel or cone shaped magnetic surface anchored to the accretion disk near the black hole. Considering a radiation pressure driven flow in the inner region, we find that it stabilizes the flow yielding Lorentz factors ranging between 1.1-7 at small radii for reasonable initial conditions. Assuming these as inputs, simulated light curves (LCs) for the funnel model include Doppler and gravitational shifts, aberration, light bending and time delay. These LCs are studied for quasi-periodic oscillations (QPOs) and the power spectral density (PSD) shape and yield an increased amplitude (~12%); a beamed portion and a systematic phase shift with respect to that from a previous special relativistic model. The results strongly justify implementing a realistic magnetic surface geometry in a GR framework to describe effects on emission from orbital features in the jet close to the horizon radius. Accepted for publication in The Astrophysical Journal (arXiv:1503.06551).
Martin C. Weisskopf, Stephen L. O'Dell
NASA Marshall Space Flight Center
The Imaging X-ray Polarimetry Explorer (IXPE): An international collaboration
Abstract
Martin C. Weisskopf, Stephen L. O'Dell
The Imaging X-ray Polarimetry Explorer (IXPE) is a mission recently proposed in response to a NASA solicitation announcement for an Astrophysics Small Explorer (SMEX). IXPE will use x-ray polarimetry to expand observation space dramatically, providing fresh insights into x-ray emission mechanisms and geometry of cosmic sources-including pulsar-wind nebulae, isolated and accreting neutron stars, and stellar-mass and supermassive black holes. The two-year mission is very low-risk, utilizing mature flight elements and combining expertise for x-ray optics at NASA Marshall Space Flight Center and for polarization-sensitive x-ray detectors at the Istituto Nazionale di Fisica Nucleare (INFN) Pisa and Istituto Nazionale di Astrofisica (INAF) Istituto di Astrofisica e Planetologia Spaziali (IAPS). The science team is comprised of many interested researchers throughout the world. The expected performance and scientific capability of IXPE will be described.
On behalf of the IXPE collaboration.
Michal Zajaček
I. Institute of Physics, University of Cologne, Germany
Monitoring the Dusty S-cluster object (DSO/G2) around the Galactic center black hole: core-less cloud vs. enshrouded star scenario
Abstract
Michal Zajaček
We have monitored the Dusty S-cluster object (DSO/G2) during its closest approach to the Galactic Center supermassive black hole in 2014 with integral spectroscopy imager ESO VLT/SINFONI (Valencia-S. et al., 2015). We report on our findings, i.e. ionized-hydrogen emission from the DSO that remains spatially compact before and after the peribothron passage. Before 2014 May, we detect red-shifted Br-gamma and Pa-alpha emission lines at about 40 mas east of Sgr A*, indicating a (pre-peribothron) LOS velocity of +2700+/-60 km/s. Then, no blueshifted emission above the noise level is detected at the position of Sgr A* or upstream the presumed highly-elliptical orbit. The analysis of data after May 2014 shows a spatially compact blue-shifted Br-gamma emission line with (post-peribothron) LOS velocity of -3320+/-60 km/s and no significant red-shifted component.
The detection of DSO/G2 object as a compact single-peak emission line source is in contradiction with the original hypothesis of a core-less cloud that is necessarily tidally stretched, hence producing double-peak emission line profile around the pericentre passage. Therefore we suggest that the DSO/G2 source is a dust-enshrouded star, presumably young, rather than a core-less gas cloud.
I will discuss details of the model, specifically how the accretion of material onto the stellar surface from the circumstellar disk can contribute significantly to the line emission and the observed large line width of the order of 10 angstrom. Given the high eccentricity of the DSO/G2 orbit, the Roche lobe radius shrinks to approx. 1 AU at the pericentre, which can cause an extended circumstellar disk/envelope to be tidally perturbed and finally detached from the putative star. We will discuss whether and when this material can reach the immediate accretion zone of Sgr A*, potentially causing the increase of its activity in the future.
I will comment on the recent ESO press release, http://www.eso.org/public/news/eso1512/ .
Petr Slany
Institute of Physics, Silesian University, Opava
Electrically charged tori in spherical gravitational and dipolar magnetic fields
Abstract
Petr Slany
A Newtonian model of non-conductive, charged, perfect fluid tori orbiting in combined spherical gravitational and dipolar magnetic fields is presented and stationary, axisymmetric toroidal structures are analyzed. Matter in such tori exhibits a purely circulatory motion and the resulting convection carries charges into permanent rotation around the symmetry axis. As a main result, we demonstrate the possible existence of off-equatorial charged tori and equatorial tori with cusps that also enable outflows of matter from the torus in the Newtonian regime. These phenomena qualitatively represent a new consequence of the interplay between gravity and electromagnetism. From an astrophysical point of view, our investigation can provide insight into processes that determine the vertical structure of dusty tori surrounding accretion disks (ApJSS, 205, id. 3, 2013).
Frederic Marin
Astronomical Institute, Academy of Sciences, Prague
Exploring active galactic nuclei by polarisation of radiation in infrared/optical/ultraviolet/X-ray spectral domains
Abstract
Frederic Marin
The center of the Milky Way harbours a super-massive compact dark body that influences motion of stars and gas in its neighborhood. Evidence points to a 106 solar-mass black hole, or one of even more exotic alternatives. Parameters of the black hole and the nuclear star cluster have been measured by methods of photometry and spectroscopy in near-infrared, X-ray and millimeter radio regimes. Modern techniques have been employed: interferometry and high-resolution imaging with adaptive optics (on the contrary, visible light is not suitable to study Galactic center because of high extinction along our line of sight). Nevertheless, the mentioned methods provide very important but limited information about three-dimensional distribution of gas around the Galactic center. Our current knowledge is restricted to the projected images and radial velocities of matter. Working towards future technique of X-ray polarimetry, the authors of the present paper propose a new approach that will allow to gather missing pieces information. The main observational results will become available with one of the proposed X-ray Imaging Polarimetry missions where the Czech team participates in preparations for the science consortium. In the recently published paper, for the first time, technical details are calculated to show how a feasible mission can provide us with detailed knowledge about the past history and present structure of the Galactic center. One of exciting topics that the authors address is whether the currently inactive Milky Way behaved like an active galaxy in the recent past.