Updated: 19 March 2015

MIT Kavli Institute for Astrophysics and Space Research

2015 Postdoc Symposium

April 29-30, May 1 11:45am−1:30pm in 37-252 (Marlar Lounge)

This annual event features the MKI-affiliated postdocs presenting 15 minute talks (with 5 minutes of questions) on their past, present, and/or future research to the MKI community. Lunch will be provided, and the talks will promptly begin at noon. There will be three speakers each day; the full schedule with abstracts is below.

Speaker Schedule

April 29, 2015

Moderator: Niels Warburton

Speaker: Salvatore Vitale
Title: Use of gravitational waves to measure alignment of spins in compact binaries
Abstract: Coalescences of compact objects, neutron star and black holes, in binary systems are very promising sources of gravitational waves for the ground based detectors Advanced LIGO and Virgo. Much about compact binaries is still uncertain, including how often they are formed in the Universe, and some key details about their formation channels. One of the key open questions about compact binary coalescences is whether or not common envelope evolution is highly efficient in aligning spins with the orbital angular momentum. In this talk we show how gravitational waves detected by Advanced LIGO and Virgo can be used to verify if spins are preferentially aligned with the orbital angular momentum in compact binaries made of two black holes or a neutron star and a black hole.

Speaker: Lia Corrales
Title: X-ray Studies of Astrophysical Dust
Abstract: Dust in the foreground of bright point sources will scatter X-rays through arcminute scale angles, producing a diffuse `halo' image. The scattering halo intensity is strongly sensitive to the dust grain size distribution, spatial distribution, and composition. The X-ray scattering cross-section is also highly sensitive to the large end of the dust grain distribution, which is difficult to measure with other wavelengths. I will demonstrate how a Bayesian analysis of the scattering halo around Cyg X-3 yields a grain size distribution. Examination of the energy resolved halos can also be used to constrain the optical properties of astrophysical dust.

Speaker: Slawomir Gras
Title: Observation of Parametric Instability in Advanced LIGO
Abstract: Gravitational wave interferometric detectors are the most sensitive position meters in existence, aiming to measure strain sensitivities of the order of 10^{-23}. In order to achieve such incredible sensitivities, the Advanced LIGO detectors employ 4 km long Fabry-Perot cavities in the interferometer arms, storing nearly 1 MWatt of laser light when operating at full power. In this extreme environment, the optomechanical interaction between the radiation pressure and the arm cavities' 40 kg optics causes parametric instabilities: energy transfer from the arm cavity field to the mechanical mode of the interferometer optics, in a runway positive feedback process. Since they were recognized in 2001, parametric instabilities have been extensively studied as a limit to high power operation of gravitational wave detectors. First observed at the Louisiana Observatory, parametric instabilities have now been seen in both Advanced LIGO detectors. Thanks to previous research, a number of mitigation techniques have been developed, one of which was successfully applied at both observatories. While parametric instabilities remain a concern for high power operation, thanks to many years of theoretical and experimental work, now informed by these direct observations, the challenge faced by gravitational wave detectors is now clear and well understood. In this talk I will present the observations, and briefly discuss implications for Advanced LIGO.

April 30, 2015

Moderator: Salvatore Vitale

Speaker: Aleksander Sadowski
Title: Thin accretion disks - theory vs observations vs simulations
Abstract: I will discuss our understanding of thin accretion disks and challenges the confrontation with observation brings. I will then present state-of-art techniques of simulating them in general relativity and argue that they are no less challenging. Finally, I will present first simulations of sub-Eddington disks and discuss the implications they bring.

Speaker: Kevin Schlaufman
Title: The Best and Brightest Metal-poor Stars
Abstract: Since halos form from the inside out, the oldest stars at a given metallicity are found near the center of a halo on the most tightly-bound orbits. The oldest stars in the Milky Way are therefore the most metal-poor stars in -- but not necessarily of -- the bulge. We have developed a new selection that uses only public infrared photometry to identify metal-poor star candidates through their lack of molecular absorption near 4.6 microns. Our selection is as efficient as previous techniques, yet is capable of finding bright metal-poor stars in areas of high reddening. A pilot survey based on our technique has already identified three of the most metal-poor stars known in the bulge. We find that with the exception of scandium, all three stars follow the abundance trends identified previously for metal-poor halo stars. These three stars have the lowest [Sc/Fe] abundances yet seen in alpha-enhanced giant stars in the Galaxy. The only place that a similar abundance pattern has been observed is the ultra-faint dwarf spheroidal galaxy Coma Berenices, which is thought to have an age of 13.9 ± 0.3 Gyr. Theoretical models predict that there is a 30% chance that at least one of these stars formed at z > 15, while there is a 70% chance that at least one formed at 10 < z < 15. These observations imply that by z ~ 10, the progenitor galaxies of the Milky Way had both reached [Fe/H] ~ -3.0 and established the abundance pattern observed in extremely metal-poor stars.

Speaker: Victoria Grinberg
Title: Understanding the origin of hard X-rays in black hole binaries using polarization
Abstract: X-ray binaries are among the brightest X-ray objects in the sky, yet the origin of the hard X-ray emission remains highly disputed. New studies of X-ray polarization, however, offer the unique opportunity of breaking the degeneracy among proposed models, but they are hampered by low sensitivity. These studies thus require bright sources and long exposures. The first X-ray binary for which X-ray polarization successfully has been observed is the black hole binary Cyg X-1 \96 a target of a multi-year-long INTEGRAL key program. The fact that its emission above 400 keV is highly polarized suggests synchrotron jet emission. Here, we go a step further and investigate the links between the behavior of the polarized high energy tail and emission at other wavelengths. To do so, we combine data from all-sky monitors such as MAXI, (MIT-built!) RXTE-ASM, Swift-BAT, and Fermi-GBM with 7 Msec INTEGRAL exposure and conduct the first state-resolved polarization analysis of an X-ray binary to-date.

May 1, 2015

Moderator: Aleksander Sadowski

Speaker: Robert Penna
Title: Black hole jets as circuits
Abstract: The Blandford-Znajek (BZ) model is the Standard Model of black hole-spin powered jets. Unfortunately, it depends on an arbitrary function, \Omega_F, which fixes the angular velocity of magnetic field lines. This function is typically fixed using large-scale simulations. Amazingly, the answer is always \Omega_F ~ 0.5 \Omega_H, where \Omega_H is the angular velocity of the black hole event horizon. We give a new explanation for this universality. We use the black hole membrane paradigm. In this formulation, the black hole magnetosphere forms a closed circuit and Omega_F/\Omega_H is the circuit efficiency.

Speaker: Meng Su
Title: PANGU: a high resolution sub-GeV gamma-ray telescope
Abstract: In this talk, I will briefly describe a proposed space mission for the low energy gamma-ray observations. The highly successful Fermi Gamma-ray Space Telescope has proved the great potential of studying astrophysics, cosmology and fundamental physics in high energy gamma rays. One area of improvement is in the 10 MeV to 1 GeV region, where the PSF of Fermi is limited by the presence of the Tungsten converters. Another area is the polarization measurement which has not yet been achieved at this energy range. It is also crucial to have a gamma-ray all sky survey mission running in parallel with missions covering higher energy range from tens of GeV to PeV, including HERD, CTA, and LHAASO. PANGU (PAir-productioN Gamma-ray Unit) is a mission candidate proposed to the ESA-CAS joint mission. PANGU will use a fully active tracker with thin silicon strip detectors to achieve a PSF of ~1° at 100 MeV, which is a factor of five better than Fermi. With the much improved PSF and the capability of polarization measurement for the first time in this crucial energy range, PANGU will enhance our capability to search for dark matter annihilation signal from the Galactic center, besides many other science goals it could achieve.

Speaker: Niels Warburton
Title: Inspiral of compact objects into massive black holes
Abstract: Binary systems with small mass ratios are potential sources for gravitational-wave detectors. Intermediate-mass-ratio systems (typical mass ratios around 10^{-2} to 10^{-3}) are potential sources for ground based detectors (LIGO,VIRGO,KAGRA) and extreme-mass-ratio systems (typical mass ratios around 10^{-5}) are potential sources for future space-based missions such as eLISA. In this talk I will discuss progress modeling these systems with black hole perturbation theory.