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High Energy Astrophysics

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Questions

  • How did the first supermassive black holes in the Universe feed and grow? What are the effects of cosmic environment, from voids to superclusters, on the growth of supermassive black holes?
  • How can Chandra X-ray observations be used to classify supernova explosions (and hence their progenitors), measure ejecta elemental composition, and study the details of the explosion process?
  • What are the mechanisms controlling the prompt MeV to multi-GeV emission of Gamma-Ray Bursts (GRBs) observed with the Fermi and Swift spacecrafts?
  • What controls the GRB afterglow behavior, and what do they tell us about their progenitors and their environment?
  • What are the beaming angles of GRB afterglows? What accounts for the dramatic flaring seen in X-ray afterglows of GRBs?
  • What are the properties of the Population III GRBs expected at redshifts 10-20, at the dawn of the reionization era? How can GRBs be used to probe the high-redshift universe?
  • What physical processes control the spectra and variability of active galaxies, both at high energies and across the electromagnetic spectrum?
  • How have the X-ray properties of starburst and normal galaxies evolved over cosmic time, and what does this imply about the evolution of their accreting X-ray binary populations?

Discoveries and Milestones

  • Niel Brandt has performed the most sensitive X-ray surveys of the extragalactic universe, which have significantly constrained the cosmic growth of supermassive black holes, determined the importance of black-hole accretion within the overall cosmic energy budget, measured the active galactic nucleus content of forming massive galaxies, and detected X-ray emission from cosmologically distant starburst and normal galaxies. These surveys have increased the number of X-ray detected active galaxies at z > 4 by more than an order of magnitude, showing that X-ray emission is a universal and stable property of accreting supermassive black holes out to the reionization epoch.
  • Peter Meszaros proposed and contributed in large part to the development of the standard model of GRB afterglows, which successfully predicted the properties of the subsequently detected afterglows, and which is widely used to interpret the observations.  He developed the model of internals shocks, and subsequently a model of photospheric jet emission which accounts with many of the properties of the prompt GRB emission.
  • The Mission Operations Center for NASA's Swift satellite, launched in 2004 and dedicated to the study of gamma-ray bursts, is located at Penn State; more than 20 Penn State scientists and students are involved in the study of these distant fireballs. Swift received the top science ranking of all small and medium NASA astrophysics missions in 2008.
  • Members of the Swift XRT group (Burrows, Nousek, Fox, Meszaros, Falcone, et al.) are pioneering the use of X-ray afterglows of GRBs to understand their underlying physics. Major results include: discovery of flaring in GRB afterglows that indicates prolonged central engine activity, X-ray afterglows of short GRBs allowing them to be studied in detail for the first time; and discovery of the most distant GRB detected to date (z=8.1), which proves that star formation and metal enrichment occurs back to less than one billion years after the Big Bang.
  • David Burrows has precisely localized the shock front in some supernova remnants (e.g. G292+1.8) and has mapped relativistic jets in the Crab Nebula.
  • Niel Brandt has used spectroscopy from the Chandra X-ray Observatory to study winds from active galaxies, ranging from local Seyfert galaxies to distant quasars. The hundreds of detected atomic features have provided qualitatively new information about wind physical conditions, kinematics, and geometry, and X-ray spectroscopy of distant quasars has clarified their wind geometries, mass-ejection rates, and role in feedback.

Current Projects

  • Peter Meszaros is working to identify a mechanism that will explain the delay of the GeV emission relative to the MeV emission in GRBs observed by Fermi, based on upscattering of photospheric photons by internal shock electrons.
  • David Burrows is using the Chandra X-ray Observatory to track the birth of a supernova for the first time (SN1987A) and to reveal how the ejecta interact with the circumstellar material in unprecedented detail.
  • Swift was recently recommended for continued operations through 2014. Members of the Swift XRT group are continuing to observe GRB afterglows.
  • Abe Falcone is leading an effort to use Swift to identify and study the X-ray counterparts to >200 unidentified gamma ray sources detected by the Fermi Gamma Ray Telescope. Falcone is also using Swift to study particle acceleration gamma ray emission processes of known GeV-TeV gamma ray sites, including blazars and X-ray binaries with compact objects.
  • John Nousek is US Co-Chair of the International X-ray Observatory (IXO) Instrument Working Group. IXO was recently recommended to receive $170M for technology development of instruments and telescope optics to prepare for a new start in the 2020 timeframe.
  • Peter Meszaros is developing a model of the transition between the prompt and the afterglow emission of GRBs, based on a leptonic synchrotron and inverse Compton interpretation.
  • The JANUS mission was recently proposed to NASA by Derek Fox (Project Scientist), John Nousek (Mission Operations Lead), David Burrows (XCAT Lead), Abe Falcone (XCAT Deputy Lead), and Jamie Kennea (Science Data Center Lead).
  • Peter Meszaros is calculating the "additional" (second) high energy spectral component of GRBs, based on radiation of hadronic cascade secondaries.

Student Highlights

  • Jianfeng Wu has led the best X-ray studies of mini-BAL quasars to date, and he is currently leading research on a potential class of intrinsically X-ray weak quasars.
  • Judy Racusin carried out the definitive study of X-ray light curves for GRBs. Her work covered nearly 100 objects and uniformly treated the appearance or non-appearance of jet breaks, allowing physically relevant constraints on the energy of the internal explosions powering GRBs.
  • Bin Luo led X-ray and multiwavelength studies of active galaxies in deep X-ray surveys, particularly the Chandra Deep Field-South. He won the Department's 2009 Downsborough Graduate Fellowship.
  • Haoning He found that the GeV emission at the earliest times from a short GRB cannot be explained by an external foward shock synchrotron model, contrary to the results of previous studies.
  • Peter Brown performed the definitive study of UV light curves of Supernovae using the Swift satellite. His work expanded the number of well studied light curves by more than an order of magnitude and will be critical to studies of the extra-galactic distance scale using Type Ia supernovae.
  • Brendan Miller used X-ray and multiwavelength data to provide new insights into jets and outflows in radio-loud quasars.
  • Adam Morgan determined the optimal co-addition for obtaining the maximum sensitivity for Swift UVOT images.
  • Koji Mori discovered relativistic motion in the Crab Nebula.
  • Bret Lehmer characterized the X-ray properties and evolution of cosmologically distant galaxies using the deepest X-ray surveys.  He won the Department's 2007 Downsborough Graduate Fellowship and an Einstein Fellowship.
  • Ann Hornschemeier obtained the the first measurements of the X-ray properties of cosmologically distant starburst and normal galaxies. She won a Chandra Fellowship and the Annie J. Cannon Award of the American Astronomical Society.
  • Sarah Gallagher conducted pioneering X-ray studies of the winds in broad absorption line quasars using sensitive hard X-ray data. She won a Spitzer Fellowship.
  • Dennis Just led the best X-ray survey to date of the properties of the most-luminous quasars in the Universe.
  • Lee Bassett led a Chandra survey of some of the first radio-loud quasars to form in the Universe.

Participants

  • Faculty
  • Postdocs and Research Associates
Pat Broos
Margaret Chester
Jonathan Gelbord
Abe Falcone
Konstantin Getman
Dirk Grupe
Jamie Kennea
Bin Luo
Bettina Posselt
Cristian Saez
Michael Siegel
Kenji Toma
Yongquan Xue
Monica Young
  • Students
Nurten Filiz Ak
Michael Kuhn
Jianfeng Wu


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