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Astronomy Seminar Abstracts

Bradford Snios (Center for Astrophysics)
Extragalactic Jets

Extragalactic jets emitted from active galactic nuclei are known to propagate with bulk relativistic motions over kiloparsec-scale distances. These large-scale outflows generate luminous emission across multiple energy bands, including X-rays. In my talk, I will discuss recent research on proper motions and variability in X-ray-bright jets. Using multi-epoch observations spanning several years, proper motions from X-ray emission are detected for the first time in extragalactic systems, with superluminal motions measured up to speeds of 6.3c. Brightness variations up to 73% are found for the X-ray knots in the jets, and spectroscopic modeling is used to determine cooling emission mechanisms from the sources. These results provide invaluable insight into the kinematic evolution of these exotic extragalactic systems.

Amruta Jaodand (Caltech)
Enigmatic transitional millisecond pulsars

A newly discovered class of pulsars called transitional millisecond pulsars switch between low mass X-ray binary (LMXB) to radio millisecond pulsar (RMSP) states. These transitions are swift (~days) and accompanied by changes in electromagnetic emission. They also confirm an evolutionary link between LMXBs and RMSPs as proposed by the pulsar recycling mechanism. Hence, tMSPs provide a great avenue to answer questions such as: what triggers the state transition, when does the recycling process truly end, what accretion mechanism is at play in X-ray state and what will the radio pulsar's final spin rate be? Systematic, multi-wavelength observational campaigns in this state have resulted in surprising finds such as: i) persistent, multi-year-long, low-level (L< 1034 ergs/s) accretion state with coherent pulsations; ii) extremely stable, bi-modal X-ray light curves; iii) radio outflows; and iv) uninterrupted pulsar spin down in the X-ray state. In my talk I will review currently known observational facts about the tMSPs while highlighting new key findings which reveal how these systems have altered our understanding of low level accretion onto neutron stars.

Michael Pajkos (MSU)
Determining Core Collapse Supernova Features using Gravitational Waves

Core collapse supernovae (CCSNe) mark the brilliant end to the lives of massive stars. From creating compact objects to catalyzing the chemical enrichment of galaxies, CCNSe influence many areas of astronomy. To better understand these explosions, gravitational waves (GWs) can be used as probes of the supernova center. In this talk, I will review results of my research by explaining the physics responsible for creating GWs during each stage of a CCSN. I will then describe the types of GW signals unique to each stage. Lastly, I will propose a new method GW astronomers can use that connects observable GWs to the internal physics of the CCSN, reveals information about its progenitor, and potentially predicts explosion properties even before shock breakout.

Vivienne Baldassare (Washington State University)
AGN and low-mass galaxies

The present-day population of supermassive black holes in low-mass galaxies offers a window into massive black hole formation in the early universe. While we cannot yet observe the formation of "black hole seeds" at high redshift, the fraction of small galaxies that host a supermassive black hole—and the properties of those black holes—are thought to depend on the mechanism by which these they form. However, black holes in the smallest galaxies can be difficult to find, requiring creative new approaches. I will discuss recent work showing that long-term optical photometric variability in low-mass galaxies can identify active galactic nuclei that are missed by other selection techniques. I will present an analysis of the nuclear variability of more than 70,000 nearby galaxies and discuss our sample of low-mass, variability-selected supermassive black holes. Using this sample, we also begin to place meaningful constraints on the present-day black hole occupation fraction at low galaxy stellar masses.

Emily Rauscher (University of Michigan)
The Peril and Promise of Three-Dimensional Planets

Much to the consternation of spherical cows everywhere, planets are three-dimensional objects. As we try to measure properties of their atmospheres, it may be inappropriate to pretend that all regions of the planet have the same temperature and composition. In fact, we may trick ourselves and retrieve biased values when we use 1-D models to interpret atmospheric characterization measurements. This is particularly true for the high signal-to-noise type of exoplanet known as “hot Jupiters” because of the intense stellar heating they receive on their permanent day sides. In this talk, I will discuss how we can turn this challenge into an opportunity, using 3-D models of exoplanet atmospheres to uncover the influence of complex physics in different types of observations and thereby empirically constrain the inherently 3-D structure of these planets. The necessity of a 3-D approach to exoplanet atmospheric characterization will only increase as we move into the era of JWST and Extremely Large Telescopes, promising a future of exquisitely detailed measurements.

Jessica Lu (University of California, Berkeley)
Searching for Black Holes in the Milky Way

The population of stellar mass black holes in the Milky Way is almost entirely unexplored. Only a dozen black holes are confidently known in our Galaxy—all in binaries. As a result, many basic properties of black holes remain uncertain at the order of magnitude level, including the total number of black holes in the Milky Way, the mass function, the binary fraction, and whether black holes receive kicks at birth. To constrain these properties, we need to find and study a larger population of black holes, both in isolation and in binary systems. High-precision astrometry is opening a new window onto black holes. I will present progress on our search for black holes using both gravitational lensing for isolated black holes and astrometric wobble for black holes in binaries.

Aarran Shaw (University of Nevada-Reno)
New Results on Magnetic Cataclysmic Variables

Cataclysmic variables (CVs) are interacting binaries in which a white dwarf is accreting matter from a main sequence companion. A common subclass of CVs are the so-called magnetic CVs, in which the white dwarf is highly magnetic, such that the accretion disk is disrupted and matter instead flows along the field lines on to the surface of the white dwarf. Though CVs, both magnetic and non-magnetic, have been well-studied for decades, they are still proving to be important for answering open questions about binary evolution, accretion physics and even cosmology. Here I will discuss some recent results regarding magnetic CVs, in particular focusing on a recent large hard X-ray survey of magnetic CVs with NuSTAR. In addition I will introduce the recent discovery of some systems transitioning to unusual low-flux states, which could shine a light on the changing accretion mechanisms at work in these sources.

Manisha Shrestha
Novel polarimetric technique to constrain the magnetic field structure and strength of Gamma-ray burst jets

Optical polarimetry can play a key role in diagnostic of magnetic field strength/order/geometry in diverse time varying astrophysical sources such as active galactic nuclei (AGN), X-ray binaries(XRBs), and gamma-ray bursts(GRBs). For distant objects such as GRBs, polarization can probe the physical conditions at spatial scale that is not possible via other observational techniques. Generally polarization is measured via ratio of fluxes by taking consecutive exposures, however for rapidly varying objects such as GRBs, it is not an effective way to observe polarization. Liverpool Telescope (LT) has utilized rapidly rotating polaroid to overcome this problem and created a series of polarimeters RINGO, RINGO2, and RINGO3 which have successfully detected early-time optical polarimetry of various GRBs. MOPTOP is a next-generation dual beam polarimeter that replaced RINGO3 in LT. I will present photometric and polarimetric results of various GRBs observed by RINGO3. In addition, I will present the results of MOPTOP characterization, which shows an improvement in the sensitivity of MOPTOP compared to RINGO3.

Alex Pizzuto
High energy astrophysical neutrinos

Abstract unavailable