List of Past CTC Theory Lunches : 01-Jan-2017 to 01-Jun-2017

Date:   Monday 20-Feb-2017
Speaker:   Stephen Walker, NASA Goddard
Title:  Probing magnetic fields in galaxy clusters

Unraveling the properties of magnetic fields permeating galaxy clusters is one of the most significant challenges in understanding the physics of the intracluster medium, and how it responds to phenomena such as AGN feedback. Here I discuss two of our recent works, each using different techniques to probe the cluster magnetic field. Firstly, deep Chandra observations of the ICM in nearby clusters such as Perseus and Centaurus show evidence of Kelvin-Helmholtz instabilities, brought about by the shear created as their cores slosh around in response to minor mergers, which are highly sensitive to the magnetic field strength. Comparing these structures to state-of-the-art hydrodynamic simulations provides unique constraints on the average, cluster wide, magnetic field. Secondly , HST observations of extremely narrow and long H-alpha filaments in the cores of these clusters, which have likely been dragged out through AGN feedback, provide further insights into the magnetic field in the most dynamically active parts of clusters.

Date:   Monday 13-Mar-2017
Speaker:   Dr. Iair Arcavi (UCSB)
Title:  "New Ways of Doing Time-Domain Astronomy: Supernovae, TDEs and Beyond"

Observational time-domain astronomy is undergoing a transformation, with novel techniques allowing us to do science that was not accessible through conventional means. I will show how recent advances are leading to new and surprising discoveries about supernovae, are allowing us to study some of the more mysterious aspects of tidal disruption events, and are enabling efficient and cheap gravitational wave followup programs. These new observing modes are also preparing us for an even more exciting and challenging future with the Large Synoptic Survey Telescope generating millions of alerts per night.

Date:   Monday 10-Apr-2017
Speaker:   Thomas Dauser (Dr. Karl Remeis Observatory, Bamberg)
Title:  "Modeling Relativistic Reflection: A More Self-Consistent Approach"

Reflection of X-ray photons at the innermost regions of the accretion disk around black holes carries is strongly affected by strong gravity effects. Analyzing X-ray spectra of such sources therefore allows us to constrain characteristic parameters of the system such as the spin of the black hole. I will show how a more self-consistent modeling of these relativistic reflection features generally yields better constraints. These improvements are supported by a systematic analysis of combined XMM-Newton and NuSTAR observations of a sample of AGN.

Date:   Monday 17-Apr-2017
Speaker:   Ori Fox (STScI)
Title:  "Supernovae and their Progenitor Systems (or lack thereof)"

Despite the robust empirical supernova (SN) classification scheme in place, the underlying progenitor systems remain ambiguous for many subclasses. The most straightforward constraint relies on a detection of the progenitor star in high-resolution pre-explosion images. Such a direct identification is typically not feasible, however, even with modern telescopes such as Hubble. Instead, astronomers are forced to rely on supernova "forensics," where the surrounding circumstellar medium can yield direct clues about the mass loss from the star in the years leading up to the SN explosion. Although progenitor discussions have historically considered mostly single star systems, I will focus a significant portion of the discussion on the impact binary stars may have on our understanding of these results.

Date:   Monday 24-Apr-2017
Speaker:   Joel Coley (NASA/GSFC)
Title:  "A Multi-wavelength Analysis on the Gamma-ray Binaries 1FGL J1018.6-5856 and LMC P3"

Gamma-ray emission dominates the spectral energy distribution of gamma-ray binaries, which are often thought to be the early stage in the evolutionary history of the X-ray binaries. Consisting of a compact object and a high mass optical companion, gamma-ray binaries are characterized by GeV and/or TeV emission modulated on their orbital period. Often, this attributed to inverse Compton scattering of UV photons in the relativistic jet of an accreting compact object or particle acceleration in the shock from winds of the donor star and pulsar powered by the rotation of a rapidly spinning neutron star. Here, I use multi-wavelength observations with Fermi, Swift, ATCA, SOAR and SALT to provide detailed temporal and spectral information on the two most luminous gamma-ray binaries, 1FGL J1018.6-5856 and LMC P3.

The gamma-ray binary 1FGL J1018.6-5856, discovered by the Fermi Large Area Telescope, consists of an O6 V(f) star and suspected rapidly spinning pulsar. I exploit the ~8 yr gamma-ray data to search for long-term changes in the properties of the 16.531+/-0.006 day orbital modulation. The best-fit X-ray spectral model consists of a featureless absorbed power law, evidence that 1FGL J1018.6-5856 is a non-accreting system. I find the radio amplitude modulation to decline with increasing frequency, suggesting the presence of free-free absorption.

The newly discovered LMC P3/ CXOU J053600.0–673507 is currently the only gamma-ray binary found in the Large Magellanic Cloud. Consisting of an O5 III(f) star and a compact object orbiting their center of mass, LMC P3 is four times more luminous than similar binary systems in the gamma-ray and an order of magnitude more luminous in the X-ray and radio bandpasses. This extreme behavior could be driven by a large spin-down power from the suspected pulsar as well as a higher UV photon seed density of the O5 III star, but the details of the high energy emission region remain perplexing. The optical radial velocity semi-amplitude is consistent with a neutron star at various orbital inclinations, but a black hole nearly face on to our line of sight cannot be ruled out. This implies that the progenitor of the compact object, presumably more massive than the O5 III star was below the critical mass to form a black hole.

Date:   Monday 8-May-2017
Speaker:   Yajie Yuan (Princeton)
Title:  "Magnetoluminescence"

Abstract: Many powerful and variable gamma-ray sources, including pulsar wind nebulae (particularly the Crab Nebula), active galactic nuclei and gamma-ray bursts, seem capable of accelerating particles to gamma-ray-emitting energies efficiently over very short time scales. These are likely due to rapid dissipation of electromagnetic energy in a highly magnetized, relativistic plasma. We term such a process as “magnetoluminescence”. One possible scenario is that in the highly magnetized outflow of the prime mover, there may exist tangled plasma configurations with free magnetic energy; these may go through an ideal instability and relax to a lower energy state over light crossing time scales. During such processes extended E>B or E||B regions can be formed and sustained as the particles are accelerated up to the radiation reaction limit, removing the electromagnetic energy in the form of gamma-ray emission. In order to test this conjecture, we devise simple models of magnetized, relativistic plasma configurations, which allow us to study in detail the macroscopic instability that leads to dramatic dissipation of electromagnetic energy. In this talk, I will present our findings so far, from theoretical analysis, MHD simulations and fully kinetic simulations. I will also discuss the astrophysical applications of these results, and future prospects in understanding the aforementioned extreme gamma-ray sources. Some new ideas about the possibility of similar processes underlying the heating of the lamppost corona above accretion disks will also be briefly outlined.

Date:   Tuesday 30-May-2017
Speaker:   Maan Hani (Univ. of Victoria)
Title:  Galaxy Mergers Moulding The CGM

Galaxy evolution is primarily driven by the life cycle of gas. The circumgalactic medium is a key part in this cycle, viz. the CGM is a major gas reservoir where inflowing cold gas mixes with outflowing metal enhanced gas and ultimately settles onto the ISM. Galaxy-galaxy mergers have been hypothesized to play a critical role in shaping the CGM's chemical and physical structure. In addition to being a fundamental evolutionary path for galaxies to grow their stellar mass and trigger star formation and AGN activity, galaxy mergers are known to vigorously reshuffle the gas reservoir due to the strong tidal torques and feedback processes which are induced by the interaction. However, detailed predictions for the effect of galaxy mergers on the CGM are currently lacking. In this talk, I will present a new analysis of the CGM of major mergers using cosmological zoom-in hydrodynamical simulations. This analysis provides the first constraints on the interplay between mergers and the CGM in a self-consistent cosmological framework. I will demonstrate the long lasting effects of mergers on the metal content of the CGM and its ionization, specifically highlighting the importance of mergers in moulding the CGM of galaxies: increasing its metal content by factors of 2-3, preferentially increasing the covering fractions of the high ionization species, and the large radial extent of such enhancements. Finally, I will use these simulations to make predictions of absorption line column densities and associated impact parameters in the CGM of post-merger galaxies, that can be directly tested with observations (i.e. with HST-COS).

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