Wednesday Colloquium


"DRAGON million-body globular cluster simulations: The possible origin of detected Gravitational Wave of black hole binary merging"

Long Wang (KIAA Kavli Institute for Astronomy and Astrophysics, Peking University)

Introducing the DRAGON simulation project, we present direct N-body simulations of four massive globular clusters (GCs) with 10^6 stars and 5% primordial binaries at a high level of accuracy and realism. The GC evolution is computed with NBODY6++GPU and follows the dynamical and stellar evolution of individual stars and binaries, kicks of neutron stars and black holes, and the effect of a tidal field. Our model show a large fraction of black holes (BHs) are retained in the GCs. This BHs form a dense system can significantly change the GC evolution. Since recently the gravitational waves (GW) emitted by a BH-BH binary is discovered by advanced-LIGO. We also investigate the BH properties in our models and check whether the BH-BH binaries can reach the merging cases via GW radiations. The detailed comparison with Monte-Carlo methods (MOCCA and CMC) for BH properties are also provided.


"The coming of age of X-ray polarimetry: the X-ray Imaging Polarimetry Explorer (XIPE)"

Giorgio Matt (University of Rome)

X-ray astronomy is now a mature and consolidated field, as far as imaging, spectroscopy and timing are concerned. On the other hand, X-ray polarimetry is very much lagging behind, the only positive measurement dating back to the 70s. This is due to the combination of instrinsic difficulties (polarimetry is a photon hungry technique) and to technical limitations. The advent of a new type of detectors, based on the photoelectric effect, make it now possible to meaningfully search for polarization on hundreds of X-ray sources. I will review the main open astrophysical questions that will benefit from X-ray polarimetry (including the use of cosmic sources as laboratory for fundamental physics), and present XIPE, a mission selected by ESA for a phase A study (with the final selection expected in summer 2017).


"X-ray reverberation in accreting black holes"

Barbara De Marco (Max-Planck-Institut für extraterrestrische Physik)

Accreting black hole (BH) systems are characterized by strong, aperiodic variability on a wide range of time scales. The variable primary X-ray radiation interacts with any surrounding matter, including the accretion disc. Short light travel time delays are expected between the primary and the reprocessed emission in the disc. These, so-called “X-ray reverberation lags” can be used to map the geometry of the very inner regions of the accretion flow. I will review recent studies of X-ray reverberation lags in accreting BH systems, discussing the analogies between X-ray reverberation lags in active galactic nuclei (AGN) and in BH X-ray binaries (BHXRB), and showing how the latter give indirect evidence of evolving disc geometry during the outburst.


"New results of the accretion-jet model in X-ray binaries and AGNs "

Fu-Guo Xie (谢富国) (Shanghai Astronomical Observatory)

The central engine in X-ray binaries (XRBs) and AGNs is under active debate in recent years. One leading theoretical scenario is the accretion-jet model. This model has achieved great success, as it can explain a large body of observed phenomena in these systems, including the broadband spectrum, the timing properties, and the radio/X-ray luminosity correlation (the so-called fundamental plane in AGNs, where black hole mass is taken into account). In this talk, I will discuss several new progresses, e.g. the numerical simulation realization of the two-phase accretion stage during the outburst evolution in XRBs, the three branch relationship between the index and X-ray flux in XRBs and AGNs, and the unified paradigm on the two radio/X-ray correlations observed in XRBs