List of Past CTC Theory Lunches : 01-Jan-2016 to 01-Jun-2016

Date:   Monday 11-Jan-2016
Speaker:   Or Graur (NYU)
Title:  "Revealing the Progenitors of Explosive Transients with Spectroscopic Surveys"

We still do not know what types of stellar systems end up exploding as most types of supernovae (SNe). In my talk, I will show how we can use observed correlations between the SN rates and various host-galaxy properties to constrain the progenitor scenarios of different types of SNe. Most of the results I will present were achieved via a spectroscopic SN survey conducted among galaxy spectra from the Sloan Digital Sky Survey. I will also show how this survey paves the way to transform any massive spectroscopic galaxy survey into a transient survey at no extra cost. This has particular applications to upcoming projects such as WFIRST and the Dark Energy Survey Instrument (DESI).

Date:   Monday 25-Jan-2016
Speaker:   Jonathan Zrake (Stanford University, KIPAC)
Title:  "Relativistic turbulence and the Crab Nebula gamma-ray flares"


Date:   Monday 01-Feb-2016
Speaker:   Foteini Oikonomou (Penn State University)
Title:  "A multi-messenger quest for the sources of ultra-high energy cosmic rays"

Ultra-high energy, > 10^19 eV, cosmic rays (UHECRs) are now routinely detected by gigantic extensive air shower detectors. Although the origin of these particles remains a mystery, their existence implies the existence of extreme, extragalactic, astrophysical sources that challenge models of high energy astrophysical acceleration. The recently discovered PeV neutrinos by IceCube, and very high energy gamma-rays detected with the Fermi-LAT, and ground based Imaging Atmospheric Cherenkov Telescopes may offer hints about the origin of UHECRs. I will summarise the current state of experimental efforts, and the open questions that the multi-messenger searches for transient emission, that have recently commenced, may help addressing.

Date:   Monday 08-Feb-2016
Speaker:   Danilo Morales Teixeira (UMD)
Title:  "Tilted thin accretion disks: theory vs numerical simulations"

In this talk I am going to talk about the standard theory of tilted accretion disks since Papaloizou & Pringle (1983) and compare with the recent results from numerical simulations done using SPH and GRMHD and show that the physics proposed by Papaloizou & Pringle is not right and also the problems with the SPH simulations that have been investigated recently by Morales Teixeira el et (2014) and Zhuravlev et al (2014).

Date:   Monday 22-Feb-2016
Speaker:   Sarah Antier (CEA Saclay)
Title:  "Gamma-rays burst science with the future SVOM mission"

SVOM (Space-based multiband astronomical Variable Objects Monitor) is a Sino-French space mission to be launched in 2021. SVOM, is a highly versatile astronomy satellite, with built-in multi-wavelength capabilities, autonomous repointing and dedicated ground follow-up, in order to detect and localize the prompt emission of Gamma-Ray Bursts and measure the properties of the GRB afterglow. In the first part of my presentation, I will review the performances of the different instruments on board SVOM and on ground. During the second part of my presentation, I will focus on the number and the characteristics of the bursts expected to be detected by ECLAIRs (the hard X-ray (4-250 keV) coded mask imaging telescope) and GRM (the Gamma-ray Monitor; 20 keV - 5 MeV). In order to evaluate this, we performed a series of simulations for which we created mock bursts in the ECLAIRs energy range, using existing catalogs from CGRO/BATSE, Fermi/GBM and Swift/BAT on the one hand, and using an intrinsic population done by the Swift Team on the other.

Date:   Monday 07-Mar-2016
Speaker:   Mia Bovill (STScI)
Title:  "Lost and Found: The Dwarfs of Centaurus A"

I will present work using the interface of simulations and observations to undertake the first detailed study of the complete dwarf satellite population of a galactic system beyond the Local Group. In this talk, I will present initial results from a set of high resolution cosmological N-body simulations of Centaurus A, a 10^13 solar mass halo approximately 3 Mpc from the Local Group. When coupled with our Survey of Centaurus A's Baryonic Structure (SCABS), this work will be a powerful addition to near field cosmology. In addition, I will discuss a possible formation scenario for the observed high M/L Centaurus A globular clusters (Taylor et al, 2015) within a cosmological context.

Date:   Monday 04-Apr-2016
Speaker:   James Guillochon (CfA)
Title:  "Tidal disruptions of stars by supermassive black holes: dynamics, light, and relics"

Most supermassive black holes in the local universe lie dormant, with only one in a hundred accreting at their Eddington limits. Aside from this active minority, and the black holes in nearby galaxies that we can observe to influence the dynamics of stars and gas, most remain difficult to study. Tidal disruptions of stars by supermassive black holes give these dormant black holes a chance to be seen once every ~10,000 years, and each tidal disruption brings along with it a host of observable signatures that can be studied from gigaparsecs away, from the moment of the disruption to millennia after a disruption has occurred. In my talk I will present work I have done on tidal disruptions of stars, and describe their dynamics, observational signatures from real-time monitoring, and relics of disruption that may exist in plain sight.

Date:   Monday 11-Apr-2016
Speaker:   Ana Mosquera (Naval Academy)
Title:  "Using galaxies as natural telescopes: Disentangling the quasars’ X-ray emission regions with microlensing"

As accreting supermassive black holes, AGNs are among the most physically interesting objects in the Universe. They also help to regulate star formation and the evolution of galaxies. One of their defining characteristics is their X-ray emission, but the origin and spatial extent of this emission is still unknown in large part because we have lacked the means to resolve the central engine. However, microlensing in multiply imaged gravitationally lensed quasars allows us to zoom in on in the structure of AGN and explore their physics in more detail. Quantitative microlensing X-ray size constraints exist for 6 systems, setting the first upper limits for these emitting regions. Obtaining these pioneering results was possible due to the high resolution and sensitivity of the CXO. The next frontier is to explore possible correlations of the X-rays sizes with black hole mass and spectral index, as well as to set constraints on the scaling of the size with X-ray energy. In my talk I will briefly describe our methodology and results, and discuss key aspects of future surveys that would help us to obtain unprecedented results in the field.

Date:   Monday 18-Apr-2016
Speaker:   Francesco Miniati (ETH Zürich)
Title:  "Particle acceleration by turbulence and giant source of diffuse radio emission in the intracluster medium"

Galaxy Clusters (GC) are the largest virialized structures in the cosmic web, serving at the same time as unique cosmological probes and laboratory of plasma astrophysics. While most of a GC’s mass is made of non baryonic dark matter, the majority of the baryonic matter is in the form of a hot, thin, magnetised plasma, namely the intra-cluster-medium (ICM). GC form through gravitational instability which on the one hand drives supersonic accretion flows, large scale shocks, and giant turbulent eddies and on the other cause GCs to merge in a hierarchical fashion, i.e., small GC form first and then merge to form larger ones.

A significant fraction of the largest GCs in the universe, produces giant regions of diffuse radio emission extending over distances of millions of light years. The nature of this phenomenon is unknown but, upon inspection, it reveals that phenomenon appears to be triggered by major merger events, mergers of GC of similar mass (mass ratio < 1/3). The prevailing, and arguably most convincing, working model for the origin of radio halos is the idea that relativistic particles are accelerated by compressional motions following the excitation of considerable turbulence during a major merger event. The acceleration rate depend both on the amount of turbulent energy released during a merger event but also the subtle workings of plasma processes at microscopic scales.

Thanks to recent advances in computational techniques and supercomputer power we are now capable of modelling the statistical properties of the ICM turbulence during the formation and evolution of a GC in a fully cosmological context. This allows us to quantify the acceleration rate associated to the ICM compressional turbulence, its evolution during times of slow mass accretion as well as its rise and fall during major merger events. I will discuss the general implications for the origin of diffuse radio emission in GC and, having constrained the macroscopic physics that enter the expressions of the acceleration rate, the remaining dependence on the unknown plasma microphysics which turns out surprisingly significant.

Date:   Monday 25-Apr-2016
Speaker:   Nicholas Stone (Columbia)
Title:  Rates of Stellar Tidal Disruption Events: Too High and Too Low!

Recent advances in optical time domain astronomy have rapidly expanded our sample of tidal disruption events (TDEs), the luminous multiwavelength flares produced when an unlucky star is tidally shredded by a supermassive black hole (SMBH). The growing sample of TDE candidates can be analyzed to infer per-galaxy event rates, and we are now confronted with two puzzles. Theoretical calculations predict a typical SMBH to tidally disrupt at least one star every ~10^4 years, but observationally inferred rates are at least an order of magnitude lower than this. Moreover, the majority of existing optically-selected TDEs appear to reside in an extremely rare subtype of galaxies (post-starburst galaxies, which make up ~0.1-1% of all z=0 galaxies). This both worsens the rate discrepancy for normal galaxies and indicates that TDE rates are enhanced above simple theoretical expectations in rare post-starburst galaxies. In this talk, I will summarize my past work to produce robust TDE rate calculations calibrated with observations of real galaxies, and discuss possible solutions to these two rate discrepancies. I will also discuss recent observational work to test hypotheses about the overproduction of TDEs in post-starburst galaxies. Our findings suggest that this phenomenon may be tied to an overdense central stellar population produced in the starburst.

Date:   Monday 02-May-2016
Speaker:   Sylvain Veilleux (UMD)
Title:  Mrk 231: The Nearest Laboratory to Study Quasar Feedback in Action, Slim Accretion Disks, but not Binary Black Holes

At a distance of only 178 Mpc, Mrk 231 is the nearest quasar known. In recent years, Mrk 231 has become the archetype of galactic-scale quasar-driven winds. These outflow events are purported to self-regulate the growth of the black hole (BH) and spheroidal component of the galaxy and explain the relatively tight BH-spheroid mass relation. In this presentation, I will first summarize the latest evidence for quasar feedback in action in this system. Next, I will present new ultraviolet HST spectra suggesting that Mrk 231 is also the nearest example of weak-lined "wind-dominated" quasars with high Eddington ratios and geometrically thick ("slim") accretion disks. I will argue that these data are inconsistent with the recently proposed binary black hole model.

Date:   Monday 09-May-2016
Speaker:   Shoucun Hu (Purple Mountain Observatory)
Title:  "New observations of (4179) Toutatis from the Chang'e-2 flyby mission and future Chinese mission to asteroids"

Chang’e-2 was the first successful spacecraft that had visited a near-Earth asteroid 4179 Toutatis. New geological features for Toutatis are exposed in the flyby mission. The rotational dynamics, boulders' origination and the crater retention age of Toutatis are studied based on the obtained optical images. Besides, the future Chinese asteroid mission plan is also introduced. The major scientific objectives, as well as the payloads and potential targets are briefly summarized.

Date:   Monday 06-Jun-2016
Speaker:   Catia de Jesus Silva (University of Amsterdam)
Title:  "Spectral-timing analysis of warm absorbers in AGN - The case study of NGC 4051"

AGN feedback may play an important role in galaxy and cluster evolution. In the case of Seyfert I galaxies, outflows are highly ionized and remarkably complex in their structure. While the observed spectra provides information on the column density and outflowing velocities of the gas, determining its radial location, and thus its impact on the surrounding environment, is not trivial. Monitoring the response of the gas to the variability of the ionizing continuum, results in an estimation of a response time, from which the density of the gas, and hence its distance, can be determined. We have performed a detailed analysis through a time-dependent photoionization code in combination with spectral and Fourier spectral-timing techniques, to study in detail the response of the gas due to variations in the ionizing luminosity. Our results suggest that the observed X-ray time lags may carry a signature of the warm absorber response time to changes in the ionizing continuum (Silva, Uttley & Costantini 2016, submitted to A&A). X-ray time lags of AGN often show a characteristic behaviour, displaying a hard lag at low Fourier frequencies and a soft lag at high Fourier frequencies. The soft lag has been explained through reverberation from reflected emission, and it has been suggested that propagation of fluctuations in the accretion flow may explain the hard lag. We have found that the warm absorber alone, in NGC 4041, is capable of producing a soft lag (~100s), on timescales of ~ hours. This will have implications for the modelling of the hard lags, observed on the same timescales. In this talk, I will show that using spectral-timing analysis together with a time-dependent photoionization model is an extremely powerful method, not only to assess the contribution of the warm absorber to the X-ray time lags of AGN, but also having the potential to provide important diagnostics on the warm absorber location and gas density. In particular, our method is extremely promising for future missions, when higher signal to noise grating or calorimeter data become available.

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