MIT Astrophysics Brown Bag Lunch Series: Fall 2017

Mondays at 12:05 PM in 37-187 (unless otherwise noted)
MIT Kavli Institute for Astrophysics and Space Research
70 Vassar Street, Cambridge, MA


The Brown Bag lunch is a forum for visiting astronomers/astrophysicists to speak about their research. Talks begin at 12:05 and speakers should plan 40 minutes of material, to leave room for questions during and after the presentation. If you are visiting MIT and would like to give a lunch talk, you may either contact the organizers directly or have your local colleagues arrange a time. The present organizers of the series are Rana Ezzeddine and Paul Torrey.


Monday September 11


Discovery of the first eclipsing ULX

Ryan Urquhart 
ICRAR (Australia)
 

Abstract:
Ultraluminous X-ray sources (ULXs) are some of the most energetic stellar objects in the Universe. The nature of ULXs is still heavily debated; they have been interpreted as stellar-mass black holes/neutron stars accreting at super-Eddington rates, massive stellar-mass black holes accreting around Eddington and even intermediate-mass black holes accreting at sub-Eddington rates. In order to differentiate between these potential models, a constraint on the mass of the compact object is required. Understanding the accretion/ejection coupling and spectral properties of these extremely luminous sources are also important open questions in ULX research.

Our recent discovery of the first eclipsing ULX will allow us to make progress on these unsolved problems. The presence of eclipses allows us to place constraints on the physical parameters of the binary system, such as the mass function and orbital period. Eclipses tell us that the source is seen almost edge-on; therefore, we can test the relationship between geometry and spectral appearance of ULXs. The source also shows the presence of a radio jet and coincident optical nebula. Recent spectroscopic data have allowed us to determine the dynamics of the gas and the mechanism behind the nebular inflation. All of this enables us to explore the inflow/outflow coupling and power distribution in the most luminous compact stellar remnants.

Host: Jack Steiner


Monday September 18 -- Two Talks!

 

Characterizing the population of pulsars in the Galactic bulge with the Fermi Large Area Telescope

Mattia DiMauro
Stanford University

Abstract:
Several groups have demonstrated the existence of an excess in the gamma-ray emission around the Galactic Center (GC) with respect to the predictions from a variety of Galactic Interstellar Emission Models (GIEMs) and point source catalogs. The origin of this excess, peaked at a few GeV, is still under debate. A possible interpretation is that it comes from a population of unresolved Millisecond Pulsars (MSPs) in the Galactic bulge. We investigate the detection of point sources in the GC region using new tools which the Fermi-LAT Collaboration is developing in the context of searches for Dark Matter (DM) signals. These new tools perform very fast scans iteratively testing for additional point sources at each of the pixels of the region of interest. We show also how to discriminate between point sources and structural residuals from the GIEM. We apply these methods to the GC region considering different GIEMs and testing the DM and MSPs intepretations for the GC excess. Our analysis is capable of finding the characteristics of this putative population of MSPs in the Galactic bulge of our Galaxy. Additionally, we create a list of promising MSP candidates that could represent the brightest sources of a MSP bulge population.

 

The compact homes of tidal disruption events

Speaker: Or Graur
Harvard-Smithsonian Center for Astrophysics

Abstract:
When a star strays too close to the super-massive black hole (SMBH) at the heart of a galaxy, it will be tidally disrupted. Half of it will be thrown out of the system, while the rest will fall into the black hole. The circularization of the disrupted matter is predicted to produce a bright flare, referred to as a tidal disruption event (TDE). In the last few years, dozens of TDE candidates have been discovered and the field has taken off. The TDE rate is predicted to depend on the stellar conditions of the sub-parsec-scale environment around the SMBH, which are very hard to measure. I will show that global galaxy properties, on kpc scales, which are easy to measure, can be used to constrain the TDE rate. I will describe the types of galaxies that host these TDE candidates, and show that they significantly prefer galaxies with high stellar surface mass densities. Finally, I will present a statistical framework that ties between the TDE rate and global galaxy properties. When applying this model to a sample of TDE host galaxies with SDSS data, the results are consistent with the TDE rate being driven by dynamical relaxation of the stars around the SMBH.

 


Monday September 25 -- Two Talks!

Is black hole growth a universal process? Exploring selection effects in measurements of
supermassive black hole accretion rates and host galaxies
.
 

Mackenzie Jones

Dartmouth

Abstract:
At the center of essentially every massive galaxy is a monstrous black hole producing luminous radiation driven by the accretion of gas. These active galactic nuclei (AGN) can influence the evolution of their host galaxies, and by observing them we may trace the growth of black holes across cosmic time. However, our knowledge of the full underlying AGN population is hindered by complex observational biases that are difficult to untangle using conventional methods and theoretical models. My research attempts to untangle these biases by using a novel approach to simulate the impact of selection effects on multi-wavelength observations.

The most statistically powerful studies of AGN to date come from optical spectroscopic surveys, with some reporting a complex relationship between AGN accretion rates and host galaxy characteristics. However, the optical waveband can be strongly influenced by selection effects and dilution from host galaxy star formation. I have shown that when selection effects are accounted for in an optically selected AGN population, the Eddington ratio distribution is consistent with a broad power-law, as seen in the X-rays (Jones et al. 2016). This suggests that a simplistic mode of AGN accretion based on a universal Eddington ratio distribution may be enough to describe the full multi-wavelength AGN population.

 

Probing the Black Hole in the Center of Our Galaxy with Hydrogen Recombination Lines 

Lena Murchikova
CalTech

 

Abstract:
To more completely explore the impact of selection effects, I have expanded a semi-numerical galaxy formation simulation to include this straightforward prescription for AGN accretion and explicitly model selection effects. I have found that a simple model for AGN accretion can broadly reproduce the host galaxies and halos of the X-ray AGN population, and that different AGN selection techniques yield samples with very different host galaxy properties (Jones et al. 2017). Finally, I will discuss the capabilities of this simulation to build synthetic X-ray and multi wavelength SEDs in order to explore the synthesis of the X-ray background and the AGN populations that would be detected with the next generation of observatories.

 


Monday October 2

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Monday October 9 -- MIT Holiday


Monday October 16

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Speaker: Alex van Engelen

Affiliation: Canadian Institute for Theoretical Astrophysics

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Monday October 23 -- Two Talks!

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Cameron Liang

Affiliation: University of Chicago

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Ben Tal Margalit
Affiliation: Columbia University
 

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Monday October 30

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Speaker: Luis Nūnez

Leiden

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Monday November 6

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Monday November 13 -- Two Talks!

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Allison Kirkpatrick

Affiliation: Yale University

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Adam Jermyn

Affiliation: University of Cambridge

Adam Jermyn


Monday November 20

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Aleksey Generozov

Affiliation: Columbia University

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Monday November 27

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Monday December 4

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Yong Zheng

Affiliation: Columbia University

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