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Benbow, Wystan: Very High Energy Gamma-Ray Astronomy with VERITAS

Project Title: Very High Energy Gamma-Ray Astronomy with VERITAS 

Project Advisor: Dr. Wystan Benbow, 617-496-7597, Observatory P-323, wbenbow@cfa.harvard.edu

Background: VERITAS (Very Energetic Radiation Imaging Telescope Array System) is a stereoscopic array of four atmospheric-Cherenkov telescopes that are sensitive to very high energy (VHE; E> 100 GeV) gamma rays. Located at the F.L.Whipple Observatory in southern Arizona, USA, the array began operation in 2007, and is currently the most sensitive VHE observatory in the world. The VERITAS Collaboration, which consists of ~80 scientists from institutions in the U.S.A., Canada, Germany, and Ireland, is carrying out observations that cover a broad range of science topics. VERITAS seeks to both identify new sources of VHE gamma rays, and to perform in-depth studies (e.g. spectral, temporal and morphological measurements) of the known VHE sources to better understand their underlying fundamental processes. VERITAS continues to the emergent field of VHE gamma-ray astrophysics, where in the past fifteen years the VHE source catalog has grown from ~10 to ~200 objects. VERITAS is a also keystone facility for the high-growth field of multi-messenger astrophysics.

Scientific Questions: What are the sources of high-energy neutrinos and gravitational waves? What is the population of extragalactic very high energy gamma-ray emitters? What are the underlying non-thermal mechanisms behind these powerful particle accelerators? How do supermassive black holes accrete matter and produce powerful jets? How do AGN jets accelerate particles and are they sources of ultra-high energy cosmic rays? What is the origin of, and the timescales of, the extreme variability observed in VHE gamma-ray emitting blazars? 

Scientific Methodology: The SAO VERITAS group focuses on VHE observations of extragalactic objects including: active galactic nuclei (primarily blazars), radio galaxies, starburst galaxies, gamma-ray bursts and dark-matter dominated structures (e.g. galaxy clusters and dwarf galaxies). Since VHE gamma-ray sources emit radiation over ~20 orders of magnitude in energy, these studies often involve collaboration with experiments at lower energies (e.g., the Fermi Gamma-ray Space Telescope, several X-ray satellites (Chandra, Swift, XMM, Suzaku), and numerous optical and radio facilities). The multi-wavelength data are used to search for temporal flux correlations and variability time scales, and to generate spectral energy distributions enabling the non-thermal processes behind the observed emission to be modeled. Nearly every VERITAS observation also has multi-messenger astrophysics implications, and these science efforts also often involve correlation analyses of high-energy signals across all known astronomical messengers: photons, neutrinos, cosmic rays, and gravitational waves. A major goal of the SAO group is to publish the VERITAS AGN catalog, the first long-term, intensive multi-wavelength study of the entire (Northern) VHE AGN catalog, and an interpretation of these data. Members of the SAO group are expected to spend time at the VERITAS site observing and assisting with upgrades to various subsystems of the array, as well as in developing the next-generation of VHE gamma-ray instrumentation.

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Deluca, Ed: Solar Magnetic Field Modeling, Active Region Structure and Stability, Developing the Scientific Basis for Space Weather

Project Title: Solar Magnetic Field Modeling, Active Region Structure and Stability, Developing the Scientific Basis for Space Weather.

Project Advisor: Edward DeLuca, 617-496-7725, Observatory P136, edeluca@cfa.harvard.edu

Background: Sigmoidal active regions in the solar corona are a main source of coronal mass ejections and flares. Such regions are commonly observed by coronal imagers. This study will use observations from the Hinode X-ray Telescope (XRT) and the Solar Dynamics Observatory (SDO).

Scientific Questions: What is the magnetic structure and topology of sigmoids? Are flux rope topologies prevalent and what what are their parameters? Is flux cancellation the main mechanism for creating sigmoids? What magnetic instabilities are responsible for the eruption of sigmoids?

Scientific Methodology: This project considers both data of sigmoidal active regions observed with XRT and SDO, and magnetic field modeling. The Coronal Modeling System (CMS) will be used to model the regions. Development of new software may be necessary to accommodate the data analysis and modeling effort.

Doeleman, Sheperd: Imaging Supermassive Black Holes

Project Title: Imaging Supermassive Black Holes

Project Advisor: Dr. Sheperd Doeleman, 617-496-7762, Observatory M215, sdoeleman@cfa.harvard.edu

Background: The Event Horizon Telescope (EHT) is a Very Long Baseline Interferometry (VLBI) array operating at the shortest possible wavelengths, which can resolve the event horizons of the nearest supermassive black holes. Initial observations with the EHT have revealed Schwarzschild radius scale structure in SgrA*, the 4 million solar mass black hole at the Galactic Center, and in the much more luminous and massive black hole at the center of the giant elliptical galaxy M87. Over the next 2 years, this international project will add new sites and increase observing bandwidth to focus on astrophysics at the black hole boundary. The EHT will have an unprecedented combination of sensitivity and resolution with excellent prospects for imaging strong GR signatures near the horizon, detecting magnetic field structures through full polarization observations, time-resolving black hole orbits, testing GR, and modeling black hole accretion, outflow and jet production. In April 2017, the EHT team completed its first observing campaign with the potential for horizon imaging. 

Scientific Questions: Our group is focusing on some of the most fundamental questions in astronomy that can only be answered with observations that resolve the event horizon of a black hole. How do black holes accrete matter? Simulations show that an interplay between magnetic fields and hot gas surrounding a black hole results in instabillities that drive matter to the event horizon, and the EHT will look for signatures of these physical processes. How do black holes launch jets that pierce entire galaxies? Some supermassive black holes power directed outflows that redistribute matter and energy on galactic scales, but the process is not well understood. By imaging the magnetic fields thought to accelerate charged particles at the black hole boundary, the EHT will test models for how jets are launched. Does General Relativity hold at the event horizon - was Einstein right? Close to the black hole, the strong gravity distorts light emitted by the infalling gas into a 'silhouette' or 'shadow'. The EHT is aiming to image this shadow whose shape and size is predicted by Einstein's Field Equations. Detection of this silhouette feature would confirm that millions of solar masses can be contained within a few Schwarzschild radii - all but cementing the existence of black holes. How does matter orbit black holes? Separate confirmation and testing of GR can be accomplished by time-resolving the orbits of material close to the black hole. The EHT can use non-imaging techniques to search for orbital signatures near the Innermost Stable Circular Orbit. 

Scientific Methodology: Our group uses numerical simulations to refine imaging algorithms and tests of strong field GR near a black hole. We also develop cutting edge instrumentation that we bring to remote mountain tops and install at mm and submm wavelength observatories. Each site has an atomic clock that enables us to synchronize and compare recordings made at sites around the Globe, each observing the same black hole at the same time. This technique, known as VLBI, synthesizes a virtual telescope as big as the Earth with unparalleled magnifying power. Students interested in instrumentation, signal processing algorithms and black hole astrophysics will find a lot to do in this project.

Other links related to this project:

Drake, Jeremy: High Energy Stellar Physics

Project Title: High Energy Stellar Physics

Project Advisor: Dr. Jeremy J. Drake, 617-496-7850, Observatory B-441, jdrake@cfa.harvard.edu

Background: Stars exhibit a range of energetic phenomena: hot coronae found on young protostars and stars like the Sun accretion thermal radiation from hot white dwarfs, novae and neutron stars. These phenomena are all characterised by plasmas that radiate copiously in the X-ray range and can be studied with satellite observatories above the Earths atmosphere.

Scientific Questions: What heats the coronae of stars? How do stellar outer atmospheric phenomena affect stellar and planetary evolution - star formation itself, protoplanetary disks, angular momentum loss through stellar winds and mass ejections, and the evolution of binary systems? What is the nature of the outer layers of neutron stars? What is happening in violent novae explosions?

Scientific Methodology: Our studies have recently concentrated on X-ray observations of stars using the Chandra and XMM-Newton observatories, and multi-dimensional photoionisation and radiative transfer models of protoplanetarty disks. High resolution X-ray diffraction grating spectra provide detailed information on individual objects, whereas CCD imaging spectroscopy provides lower resolution information on larger samples of objects, such as young pre-main sequence star clusters. Other observations compliment these studies for example, optical high resolution spectroscopy has been used to obtain information on elemental abundances that are of interest for probing outer atmospheric abundance anomalies in stars. Protoplanetary disk models are employed to investigate disk structure and ionisation under the influence of energetic phenomena.

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Elvis, Martin: Astronomical Prospecting: Steps to Asteroid Mining

Project Title: Astronomical Prospecting: Steps to Asteroid Mining

Project Advisor: Dr. Martin Elvis, 617-495-7442, Observatory B-424, melvis@cfa.harvard.edu

Background: Asteroids number in the millions and the total mass of industrially useful raw materials they contain is far vaster than the accessible materials in the Earth's crust. There are many potentially ore-bearing asteroids, but as a fraction of the total they are quite rare. As a result asteroid mining is likely to proceed in a multi-step process, like terrestrial mining, from initial surveys to final extraction. Astronomical techniques must be the first step in prospecting the asteroids.

Scientific Questions: How can we identify potential ore-bodes among the many asteroids given that most are just "dumb rock"? We are investigating two approaches: (1) Remote prospecting via large astronomical telescopes are preferred as they are cheap and can prospect large numbers of asteroids rapidly. However the information returned is limited. (2) Proximity prospecting, using instruments on spacecraft within a kilometer or so of the asteroid, provides far more detailed information, if the right instruments are used. But this approach is expensive to apply to many asteroids.

Scientific Methodology: (1) for telescopic prospecting we are beginning a campaign with the PISCO instrument on a 6.5m Magellan telescope in Chile; PISCO takes 4-color images simultaneously, and gets high signal-to-noise in 2 minutes, allowing both spectral types and accurate orbits to be obtained from the same data. (2) CfA scientists have developed miniature X-ray optics and radiation hard X-ray sensors that will make great proximity prospecting tools as well as enabling X-ray navigation for deep space missions; we are developing these into a system and will propose it at every opportunity.

Papers related to this project:

(1) Elvis, M., 2016, "Astronomical Prospecting: A Necessary Precursor to Asteroid Mining", 66th International Astronautical Congress, IAC-15-D4.3.10.

(2) Galache J.L., Beeson, C.L., McLeod, K.K., and Elvis, M., 2015, "The need for speed in Near-Earth Asteroid characterization", Planetary and Space Science, Volume 111, p. 155-166.

Golub, Leon: Dynamics of the Solar Corona 

Project Title: Dynamics of the Solar Corona 

Project Advisor: Dr. L. Golub, 617-495-7177, Observatory P-132, lgolub@cfa.harvard.edu

Background: Hot, X-ray emitting plasmas are ubiquitous throughout astrophysics, and the mechanism(s) responsible for their heating is poorly understood. 

Scientific Questions: What causes the heating and dynamics of the hot, magnetized solar outer atmosphere? What combinations of observations and modeling can be carried out to determine the mechanisms involved? 

Scientific Methodology: A combination of theory, modelling and experiment: calculate the plasma properties resulting from proposed instability mechanisms, model the observable effects, and compare to observations of the solar corona. 

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Golub, Leon: Heating of Hot Magnetized Plasmas

Project Title: Heating of Hot Magnetized Plasmas 

Project Advisor: Dr. L. Golub 

Background: Hot, X-ray emitting plasmas are ubiquitous throughout astrophysics, and the mechanism(s) responsible for their heating is poorly understood. 

Scientific Questions: What are the observable consequences of the different mechanisms proposed for heating of the solar coronal plasma? Can we distinguish among them via direct observation? 

Scientific Methodology: A combination of theory, modelling and experiment: calculate the plasma properties resulting from proposed dissipation mechanisms, model the observable effects, and compare to observations of the solar corona. 

Other links related to this project:

Green, Paul: Spectroscopic Variability of Quasars

Project Title: Spectroscopic Variability of Quasars

Project Advisor: Paul J. Green

Background: The physics of supermassive black hole accretion is rather poorly understood, but I use both multi-wavelength properties and variability to study the near-nuclear environment.

Scientific Questions:  What does photometric variability tell us about the size of the quasar accretion disk, and how disturbances propagate? How do the broad emission lines change in response? Is the rare, strongestvariability seen in "Changing Look Quasars" a different phenomenon, or just the tail of the quasar variability distribution? What is the quasar duty cycle, and can they turn completely off and on again?

Scientific Methodology:  Our study of the spectroscopic variability of quasars probes both long and short-term optical variability in quasars, tracing changes in the power-law continuum and corresponding changes in the broad emission lines. The primary scientific goal is to understand the surprisingly rapid and significant variability of Changing Look Quasars, using optical spectroscopy, optical photometry (from either existing surveys or dedicated follow-up observing programs), and X-ray observations. The primary dataset is the Time Domain Spectroscopic Survey (TDSS) of SDSS-IV, which will be expanded in SDSS-V under the Black Hole Mapper (BHM) program.

Other links related to this project:

Hora, Joseph: Investigating Massive Star Formation in the Cep OB4 Association

Project Title: Investigating Massive Star Formation in the Cep OB4 Association

Project Advisor: Dr. Joseph L. Hora, 617-496-7548, Observatory M-232, jhora@cfa.harvard.edu

Background: The presence and formation of massive stars have major impacts on galactic structure and evolution, but the important mechanisms that control this process are still not well understood (e.g., see reviews by Zinnecker & Yorke 2007, Motte et al. 2017). Winds, ionizing flux, and outflows from massive stars and supernovae will affect the structure and inject turbulence in molecular clouds. Massive O and B stars typically form in loosely organized groups called OB associations, which also contain many low mass stars. It is likely that most stars in the Galaxy originated from OB associations, so determining the processes at work in these regions is critical to the understanding of star formation. 

Scientific Questions: We are investigating several questions about how massive stars form and influence their natal molecular clouds. For example, one outstanding question is how high mass stars can trigger or otherwise enable subsequent generations of star formation in a molecular cloud. Winds from massive stars and supernovae can sweep up material into shock fronts or compress the nearby cloud, which would allow parts of the cloud to reach sufficient densities for gravitational collapse, such as in the "collect and collapse" (CC) process proposed by Elmegreen & Lada (1977). However, other mechanisms have been proposed, such as the radiation-driven implosion (RDI) model proposed by Reipurth (1983). This process involves the ionization front from an H II region passing over a molecular condensation, which can trigger shocks in the condensation and compress it, which can lead to forming clumps that eventually collapse to form stars. This mechanism can lead to YSOs and clusters forming in the heads of pillars of molecular material that extend into the H II region. Another possibility is that the stars form as a result of collisions between molecular clouds. Evidence for this process that we could observe would include finding YSOs embedded in the junction between the colliding molecular clouds. The low-mass stars provide a "fossil record" of the star formation in OB associations, and are key to understanding some of the most fundamental questions in star formation.

Scientific Methodology: We have recently completed the first Spitzer/IRAC deep study of one of the closest OB associations, Cepheus OB4 (Cep OB4), in order to have the best sensitivity and spatial resolution on the environments of massive stars and to identify its population of lower mass young stars. Cep OB4 is a ~2 Myr old association at a distance of ~800 pc, and is an ideal region for comparison to star formation models for several reasons. It is offset from the densest part of the Galactic plane and relatively nearby, enabling us to resolve individual objects at a scale of ~1400 AU and detect young stellar objects (YSOs) down to a few tenths of a solar mass. Also, the region has a simple geometry, with the OB stars and H II region surrounded by a roughly circular molecular and dusty cloud into which the bubble is expanding, providing a testbed for sequential star formation models. The project will involve reducing and analyzing the IRAC data, along with existing near-IR and other available datasets from the WISE and Herschel infrared space missions, to identify YSOs and young clusters and test models of star formation.

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McCarthy, Michael: Laboratory Spectroscopy of Highly-Reactive Molecules of Astrophysical Interest

Project Title: Laboratory Spectroscopy of Highly-Reactive Molecules of Astrophysical Interest

Project Advisor: Michael C. McCarthy, 617-495-7262 or 617-495-9848, P-256, mmccarthy@cfa.harvard.edu

Background: Understanding the chemical composition in the interstellar medium can provide much insight into a variety of astrophysical processes, allowing one to derive important physical properties such as mass loss, temperature, density, fractional ionization, etc. Many of the key chemical intermediates found in space are highly reactive or unstable species, generally unknown or unfamiliar on Earth, such as radicals, carbenes, and positively and negatively-charged ions. Unambiguous astronomical detection of these reactive intermediates frequently requires highly accurate measurements of their rotational spectra throughout the radio band. Using highly sensitive laboratory instrumentation and production techniques developed at SAO, such measurements are undertaken, yielding precisely rest frequencies to guide dedicated radio astronomical searches for new molecules.

Scientific Questions: What are the key chemical intermediates in astronomical sources? What methods and techniques can be used to detect this species in the laboratory? How can these intermediates be used to provide new insight into astrophysical process?

Scientific Methodology: Chemically unstable molecules of astronomical interest are produced and detected in the radio band using custom instrumentation. Laboratory searches are often undertaken in collaboration with leading theoretical groups here and abroad because computational predictions serve as a useful guide to experiment. Target reactive species are synthesized by applying an electrical discharge to a mixture of precursor gases, as the gas mixture rapidly expands to form an ultra-cold molecular beam. Fourier transform microwave spectroscopy is used in the centimeter-wave band to conduct spectral surveys at frequencies predicted by theory. Follow-up investigations to confirm the carrier of the rotational lines or to extend the frequency range of the laboratory measurements are often undertaken as part of this effort.

 

 

McCollough, Michael: A Multi-Wavelength Study of the Relativistic Jet Source Cygnus X-3

Project Title: A Multi-Wavelength Study of the Relativistic Jet Source Cygnus X-3

Project Advisor: Dr. Michael L. McCollough, 617-496-2119, Observatory B-240, mmccollough@cfa.harvard.edu

Background: Cygnus X-3 is one of the most enigmatic X-ray binaries to have been studied. Its X-ray flux shows a 4.79 hr modulation associated with its orbital period. While the period is typical of a low mass system IR observations have shown that the mass donating companion is a massive Wolf-Rayet star. Cygnus X-3 has two major X-ray states (low/hard and high/soft), shows correlative activity between the radio and hard X-ray, and relativistic jets have been observed in the system (~0.9c).

Scientific Questions: Among the issues we are seeking to address in this study are:

Hard X-Ray/Gamma-Ray Continuum: We seek to understand the nature of the hard X-ray/gamma-Ray continuum associated with major radio flares. Is it due to synchrotron or inverse Compton? Are the processes producing this emission nonthermal or thermal in nature?

Annihilation Lines: The major radio flares in Cygnus X-3 have been linked to relativistic jets. The composition of these jets is a major point of interest. Are they a pair plasma (electrons and positrons) or do baryons play major role in their makeup? The detection of annihilation lines make help answer this question.

Timing Properties: Do the major flares have a distinctive timing signature? The RXTE observations probe times very close to the creation of these major flares.

Properties of Cygnus X-3's Wind: The Chandra observations (supported by the RXTE) observations will allow a detailed (phased resolved) measurement of the parameters and nature of the wind associated with Cygnus X-3.

Multi-Wavelength Correlations: We will look for and study the correlations between the different wavelenghts (radio, X-ray, Gamma-Ray, IR). These will be examined relative to previously discovered correlations.

Scientific Methodology: Since early 2002 Cygnus X-3 had been in an unusually long quiescent state (~ 1300 days). At the start of 2006 Cygnus X-3 transitioned from a radio quiescent (low/hard) state to a flaring (high/soft) state. Among the activities that have been observed are an extended quenched state (high X-ray, very low radio, and very low hard X-ray emission), rapid (< 1 day) bright flares (~ 3 Jy), and three major radio flares (~ 14 Jy). 

During this active state, a major international multi-wavelength observing campaign has been undertaken. This campaign includes observations in the radio (Ryle, RATAN-600), IR (PAIRITEL), UV/Optical (Swift), X-ray (Chandra, RXTE, INTEGRAL, Swift), hard X-ray (RXTE, INTEGRAL, Swift), and Gamma-ray (INTEGRAL, Whipple). 

This project involves the analysis of these various data sets with particular emphasis on the spacecraft data (Chandra, INTEGRAL, RXTE, and Swift). We will be using XSPEC, FTOOLS, CIAO, and OSA (INTEGRAL Data Analysis) software to analyze the various data sets.

Other links related to this project:

Myers, Philip: Forming the Stars: Is it a Magnetically Driven Process?

Project Title: Forming the Stars: Is it a Magnetically Driven Process?

Project Advisor: Philip Myers, 617-495-7295, M-318, pmyers@cfa.harvard.edu, and Ian Stephens (Worcester State University)

Background: To understand the role of magnetic fields in star formation, we need to understand their structure. As of now, how magnetic fields help or hinder star formation is poorly constrained. Fields can potentially regulate or even control the flow and accretion of mass from the interstellar medium to regions of dense, star-forming gas. This uncertainty may be changing since we now have a treasure trove of new data showing magnetic field patterns in star-forming regions, from the SOFIA telescope (an observatory on a Boeing 747 airplane) on the scale of protostellar groups and clusters, and from ALMA on the scale of young circumstellar disks. The student will primarily analyze the magnetic field structure with data from SOFIA to investigate the importance of fields in forming both large-scale structure (e.g., filamentary streamers) and smallscale structure (e.g., circumstellar disks). From these data, the student will determine the relative importance of magnetic fields, turbulence, outflows, and gravity for forming stars. In a related program, ALMA observations will probe the role of magnetic fields in guiding gas flow in accreting and rotating circumstellar disks.

Scientific Questions: How do magnetic field patterns and strengths near dense, star-forming gas relate to the gas structure? The field lines near dense gas often have one of three patterns: (1) they are nearly perpendicular to the surfaces of the streamers and their clumps, as if the fields are guiding the inward motions of accreting gas; (2) they are nearly parallel to the streamers, as if they are channeling flow along the streamers; and (3) they have more complex structure suggesting flows dominated by turbulence rather than gravity. We believe we now have enough information to learn how these magnetic structures relate to the star formation in their associated dense gas.

Scientific Methodology: We will catalog the incidence of these magnetic patterns in the numerous star-forming regions observed in our SOFIA program to understand how often each pattern is associated with regions of high and low gas density and star formation. We will then test these properties against simple models based on flux freezing of the gas to the field, on gas motions observed in molecular spectral lines, and on gravitational forces in filamentary and spherical geometry.

Links/Information Related to this Project:

Recent Observing Programs:

"Fieldmaps" - https://www.sofia/usra.edu/science/data/legacy-programs

"BOPS" -  https://almascience.eso.org/observing/highest-priority-projects

Recent Analysis Papers: Myers et al. 2018, ApJ, 868, 51; Myers et al. 2020, ApJ, 896, 163

Predoc Duration:

1-2 years depending on availability of funds with a nominal starting date of June 1, 2021.

Randall, Scott: The Structure and Physics of Galaxy Clusters

Project Title: The Structure and Physics of Galaxy Clusters

Project Advisor: Dr. Scott Randall, 617-496-7738, Observatory B-418, srandall@cfa.harvard.edu

Background: Clusters of galaxies are the largest gravitationally bound structures in the Universe. As such, they are excellent signposts for the study of cosmology and the growth of large scale structure. Galaxy clusters contain a hot, X-ray emitting atmosphere of plasma called the intracluster medium (ICM). There is an apparent negative feedback loop between outbursts of a cluster's central supermassive black hole (SMBH) and the ICM, although the details of this interaction are not fully understood. Radio observation reveal that clusters sometimes contain diffuse radio structures, such as radio halos, relics, and phoenixes. It is believed that cluster mergers power the formation of these radio structures, but here too the detailed physics is unclear. Multiwavelength observations of merging clusters can also, in some cases, allow constraints to be placed on the nature of dark matter. On larger scales, clusters connect with the cosmic web in their outskirts, where observations are challenging due to the low density and surface brightness of the ICM. Understanding the physics of the ICM is important for the use of clusters in cosmological studies, and has implications for galaxy evolution, plasma physics, accretion physics, and the growth of supermassive black holes.

Scientific Questions: What physical processes are at work in the low-density ICM at the outskirts of galaxy clusters, where the ICM interfaces with the cosmic web? How do connecting large scale structure filaments affect the ICM in cluster outskirts? How, in detail, is the negative feedback loop established between the central SMBH and the ICM? Is the amount and distribution of cold molecular gas consistent with a model where gas condenses out of the ICM and feeds the central SMBH? What can merging clusters tell us about the self-interaction cross section of dark matter particles? What is the nature of the diffuse radio structures seen in some merging clusters?

Scientific Methodology: This work will focus on using multiwavelength observations (with a focus on X-ray observations) and other techniques to study multiple aspects of galaxy cluster physics. Submillimeter observations (e.g., with ALMA) map the cool molecular gas in the cores of clusters, which is thought to feed their central supermassive black holes and establish a negative feedback loop by heating the surrounding hot, X-ray emitting ICM. X-ray and radio observations reveal the connection between physical processes in the ICM and diffuse radio sources. Constraints can be placed on the self-interaction cross section of dark matter, by comparing the offsets between the diffuse gas (X-ray), galaxy number density (optical), and total mass (optical lensing) peaks with results from numerical simulations. Finally, X-ray observations of the outskirts of clusters allow us to study the physics of the virialization region, where the ICM connects to the large scale cosmic web. A comparison with mock observations from state of the art hydrodynamical cosmological simulations will test our predictions in this region.

Reeves, Kathy: Plasma Heating and Energy Partition in Solar Flares and Coronal Mass Ejections

Project Title: Plasma Heating and Energy Partition in Solar Flares and Coronal Mass Ejections

Project Advisor: Kathy Reeves, 617-496-7563, Observatory P141, kreeves@cfa.harvard.edu

Background: Solar eruptions, in the form of solar flares and coronal mass ejections (CMEs), are the largest energy release events in the solar system and the main driver of space-weather disturbances at Earth. It is widely accepted that magnetic reconnection is the main process that enables the release of magnetic energy in solar eruptions. However, the details of the associated energy transfer into thermal and kinetic energy, and of the associated heating and distribution of plasma are still poorly understood.

Scientific Questions: What are the physical mechanisms that heat plasma during the impulsive phase of solar flares? How is the released energy partitioned? What are the physical mechanisms responsible for heating plasma in the region of the current sheet in the late phase of solar flares? How are supra-arcade plasma sheets formed? How are the recently discovered hot plasma channels formed and heated to temperatures of more than 10 MK in the early stages of an eruption? How is plasma in coronal mass ejections heated during an eruption, and how does it evolve?

Scientific Methodology: We will, for the first time, analyze energy transfer and plasma heating and evolution using state-of-the-art, fully thermodynamic, magnetohydrodynamic simulations of solar flares and coronal mass ejections. The simulations we will analyze are produced with the Magnetohydrodynamic Algorithm outside a Sphere (MAS) code, developed and maintained by Predictive Science Inc. (PSI). We will complement our numerical investigations with detailed analysis of high-cadence and high-resolution observations from current spacecraft, such as the Atmospheric Imaging Assembly instrument on the Solar Dynamics Observatory mission.

Sobolewska, Malgosia: X-Raying the Youngest Extragalactic Radio Jets

Project Title: X-Raying the Youngest Extragalactic Radio Jets

Project Advisor: Dr. Malgosia Sobolewska, 617-496-7929, Observatory B217, msobolewska@cfa.harvard.edu

Background: The onset of supermassive black hole (SMBH) accretion is still poorly understood. Observations suggest that gas falling onto a SMBH forms an accretion disk/corona system often enshrouded in a dense obscuring dust. Some of the infalling material may be redirected away from the black hole in the form of relativistic jets which are able to alter a galaxy's rate of star formation by redistributing matter that would otherwise collapse to form new stars. Thus, jets play an important role in the evolution of galaxies.

Scientific Questions: Our study targets young newly-triggered jets identified with radio observations. We aim at answering the following scientific questions: What are the conditions required to launch a jet from an active galactic nucleus (AGN)? How do the jets expand within the host galaxy? What are the X-ray signatures of such ejecta and expansion? Can the dense obscuring dust confine the jets and affect their expansion? What is the impact of AGN jets on the host galaxy?

Scientific Methodology: Primarily, the data for this project were acquired by our group and are from the Chandra, XMM-Newton and NuSTAR X-ray observatories. The student will have the opportunity to learn about these observatories and apply/modify advanced models to extract the X-ray information about the physics of the young jets, the environments in which they expand, and jet interactions with the interstellar medium. Broadband radio-to-gamma ray study will require collecting the multiband AGN data from the literature.

Links to papers related to this project:

"The Impact of the Environment on the Early State of Radio Source Evolution," Sobolewska et al. (2019)

"First Hard X-ray Observation of a Compact Symmetric Object: A Broadband X-ray Study of a Radio Galaxy OQ+208 with NuSTAR and Chandra," Sobolewska et al. (2019)

X-ray Properties of the Youngest Radio Sources and Their Environments," Siemiginowska, Sobolewska et al. (2016)

The Chandra X-ray Observatory

The X-ray Multi-Mirror Mission (XMM-Newton)

The Nuclear Spectroscopic Telescope Array (NuSTAR)

Stark, Antony: The South Pole Telescope (SPT)

Project Title: The South Pole Telescope (SPT)

Project Advisor: Antony Stark, 617-495-7256, Observatory M-205, astark@cfa.harvard.edu

Background: The SPT is a 10m diameter millimeter wave telescope located at Amundsen-Scott South Pole Station in Antarctica. It has been in continual operation since 2007, resulting in over 40 major publications on a variety of topics that are fundamental to cosmology and high-redshift astrophysics. SPT is operated by an informal consortium of 70 scientists from 20 institutions including the Harvard-Smithsonian Center for Astrophysics. CfA astronomer Antony A. Stark, as one of the founders of the project, can provide unrestricted access to all SPT consortium data. The project is manpower limited, with a great many interesting projects available to pre-doctoral students. Consortium policy is to encourage independent work by students and to reward those efforts with first-authorships.

Scientific Questions: SPT science falls into three broad categories: cosmology by direct observation of features in the Cosmic Microwave Background including E- and B-mode polarization and lensing; cosmology and astrophysics of galaxy clusters discovered via the Sunyev-Zeldovich effect; and the astrophysics of highly-redshifted galaxies that happen to be unusually bright because they are behind a strong gravitational lens. The SPT is among the few instruments in the world that is currently constraining cosmological models and the properties of neutrinos. Future observations will determine the tensor-scalar ratio, running, kinetic S-Z effect, the structure of matter between z = 0 and z = 1000, the timescale of reionization, the number and masses of neutrino species, and the history of Dark Energy. Galaxy cluster projects will study the ensemble of clusters in the context of cosmology as well as the physics of intergalactic gas, star formation and populations of stars in cluster galaxies. Our sample of highly-magnified high-z galaxies allow study of star and galaxy formation in the very early Universe. That data can be used, for example, to study the possible existence of a large-scale gradient in the fine structure constant.

Scientific Methodology: The SPT is engaged in several long-term survey projects to produce deep (~ 3 microK rms) images of 10% of the sky near the south galactic pole at 90, 150, and 230 GHz. Six years of data, comprising the first Sunyaev-Zeldovich effect survey and the first two deep polarization surveys are complete. Survey work is expected to continue for at least the next 5 years. The sensitivity of the SPT has recently been greatly improved with the successful commissioning of the SPT3G detector system. SPT is by far the most sensitive CMB instrument, currently operating at brightness levels 30X deeper than Planck at 3X higher resolution. Detections in the survey are followed up with a wide variety of observations in the radio, infrared, visual, UV and X-ray. Harvard-Smithsonian participants in this project routinely observe with the Hubble, Spitzer, and Chandra Space Telescopes, the Magellan, Gemini, and VLT telescopes, and the ALMA and ATCA radio telescopes.

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Steiner, Jack: X-ray Studies of Dynamic Accretion onto Stellar-Mass Black Holes

Project Title: X-ray Studies of Dynamic Accretion onto Stellar-Mass Black Holes

Project Advisor: Dr. James ("Jack") Steiner, 617-496-7988, Observatory B-243, james.steiner@cfa.harvard.edu

Background: Stellar-mass black holes in X-ray binary systems are discovered when they erupt into year-long X-ray outbursts during which they are among the brightest X-ray objects on the sky, and after which they fade more than a millionfold into years or decades of quiescence. The emission from these outbursts are peaked in the X-rays and characterized by a rich pattern of behavior, the characteristics of which reveal properties of the accretion flow (in the form of a 10-million-degree accretion disk and a 100-times hotter corona of tenuous electrons) and of the black hole itself (its mass and spin). Over the course of an outburst cycle, the black hole transitions through a sequence of spectral-timing states associated with changes in the geometry of the disk-corona system, and with the appearance and evolution of quasi-periodic oscillations (QPOs) in which up to ten percent of the system emission is flickering at a characteristic frequency.

Scientific Questions: Data with high cadence and high sensitivity are in hand from a handful of black-hole systems covering a range of spectral-timing states. The student will have the opportunity to engage with any of an array of related projects including (1) measuring the spin of black hole systems; (2) modeling quasi-periodic oscillations (QPOs) in black hole systems and relating QPO changes to spectral evolution of the black-hole system; (3) modeling the geometrical and spectral characteristics during a black hole outburst; (4) using the premier Magellan telescopes to measure orbital characteristics and determine the masses of quiescent black hole systems. Any student interested in applied machine learning with the requisite background would have the opportunity to explore a number of spectral-timing data sets. 

Scientific Methodology: Principally, the data in question are from X-ray observatories including Chandra, NICER and NuSTAR, which will be analyzed in spectral and Fourier domains to study accretion flows onto stellar-mass black holes. Any student interested in applied machine learning with the requisite background would have the opportunity to apply these techniques to a rich set of spectral-timing data sets.

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Wargelin, Bradford: Stellar Coronae and Cycles in the Optical, UV, and X-rays

Project Title: Stellar Coronae and Cycles in the Optical, UV, and X-rays

Project Advisor: Bradford J. Wargelin, 617-496-7702, Observatory P-445, bwargelin@cfa.harvard.edu

Background: Stellar cycles are known in many late-type stars (11 years in the Sun), but have only recently been reported for a few fully convective stars (~M3.5 and later), including Proxima Cen (M5.5Ve). This surprising discovery, along with the advent of massive optical surveys and monitoring programs, is driving major progress in our understanding of low-mass stellar evolution and magnetic field generation. The Swift, XMM-Newton, and Chandra X-ray observatories have collected X-ray and UV data on Proxima on a semi-regular basis since 2009, with optical monitoring data (ASAS, ASAS-SN, and MEarth) since 2000, revealing a 7-yr cycle and 83-day rotation period.Roughly half a dozen other stars have data on X-ray cycles, as well as sometimes extensive optical monitoring and/or spectral (Ca II HK) data. We have recently begun an X-ray monitoring program on three additional stars.

Scientific Questions: How do properties of magnetic activity cycles correlate in the X-ray, UV, and optical? How do cycle amplitudes vary as a function of Rossby number, stellar type, age, etc.? What can we infer about differential rotation and starspot size and longevity from rotational modulation observed in different bands? How do coronal temperatures, element abundances, and X-ray/UV/optical flaring vary over a cycle? What can we learn about coronae from large flare evolution? How do we develop robust methods to measure quiescent X-ray/UV emission levels in active stars?

Scientific Methodology: Work can be tailored to individual interests but may include periodicity studies (Lomb-Scargle, auto-correlation, etc.) and cross calibration of optical monitoring data sets such as ASAS, ASAS-SN, MEarth, and TESS. X-ray data will be extracted and analyzed with the HEAsoft, SAS/Hera, or CIAO software packages (for Swift XRT/UVOT, XMM EPIC/RGS, or Chandra HRC/ACIS/HETG/LETG, respectively). X-ray/UV evidence for coronal mass ejections will be assessed, and CME properties modeled.

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