X-ray binaries and microquasars
X-ray binaries are among the brightest X-ray objects in the sky. They are powered by the release of gravitational energy during accretion of mass by the compact object (either a neutron star or a stellar-mass black hole) from its companion star. Mass accretion may take place via Roche-lobe overflow or capture of stellar wind, but the details of the process are not yet understood. In some systems, highly collimated jets of relativistic particles are also seen; the systems are referred to as microquasars. There is growing observational evidence that suggests the central engines of microquasars, gamma-ray bursts and active galactic nuclei are qualitatively similar, despite vastly different physical scales. We actively pursue X-ray and gamma-ray observations of X-ray binaries, often in the context of multiwavelength campaigns, to characterize their temporal and spectral properties and thus to gain insights into the origin of X-ray/gamma-ray radiation, the coupling between accretion flows and jets, the composition of jets, as well as to probe relativistic effects in the strong-gravity regime.
Contact: Wei Cui, Hua Feng
Radio-loud active galactic nuclei
Active galactic nuclei (AGN) are also gravitationally powered systems. They are thought to harbor a supermassive black hole (of millions to billions of solar masses) accreting matter from its surrounding media. They radiate across nearly the entire electromagnetic spectrum, from radio to gamma-ray wavelengths. Based on their radio properties, AGN are divided into radio-loud and radio-quiet varieties, generally indicating the presence and absence of relativistic jets, respectively. One of the most exciting advances in high energy astrophysics over the past decade has been the detection of a significant population of radio-loud AGN at GeV-TeV gamma ray energies. Among them, we are particularly interested in blazars, which are AGN with jets directed nearly at us. Their spectral energy distributions (SEDs) invariably show two characteristic peaks, with one at optical-X-ray energies and the other at GeV-TeV energies. The lower SED peak is generally attributed to synchrotron radiation from relativistic electrons (and positrons) in the jets, but the origin of the higher peak is still being debated, depending on the composition of the jets. We observe blazars in gamma rays with the VERITAS telescopes and the LAT detector on the Fermi satellite, often with supporting observations at radio, optical, and X-ray wavelengths, and use the data to study their SEDs and variability.
Contact: Wei Cui
Ultraluminous X-ray sources
Ultraluminous X-ray sources (ULXs) are accreting compact objects found in external galaxies. They are not coincident with the galactic nuclei and are not supermassive black holes. Their apparent luminosities exceed the Eddington limit of a stellar mass black hole, implying that they may contain a missing population of black holes with masses intermediate between stellar mass and supermassive black holes, or reflect physics with supercritical accretion. We actively pursue X-ray observations of these objects, often in the context of multiwavelength campaigns, to characterize their spectral and temporal properties and thus gain insights into the physical processes.
Contact: Hua Feng