I find all sorts of mysteries in physics and astronomy interesting. Thus, I am prepared to expand my research fields at any time.
Currently my research interests include:
Recently I became very interested in gamma-ray bursts (GRBs) and supernovae. This was mainly caused by two facts: (1) Since the launch of Swift, the research on GRBs and their relation to supernovae has entered a completely new era. The fraction of GRBs (long and short) with measured redshifts and observed afterglows has increased rapidly. The association of (at least some) long GRBs with type Ibc supernovae has been laid on a solid foundation with the discovery of GRB 060218 and the SN 2006aj associated with it. (2) Despite the exciting progresses in observations, the nature of GRBs is still a great mystery and we do not have a standard model for GRBs yet. This gives us a large space for imagination and intellectual games.
Specific topics that I am interested in include:
The GRB-Supernova Connection Relation between the GRB and supernova parameters, the physical origin, and the nature of the GRB-supernova connection.
Supernova Shock Breakout The shock breakout event from Type Ibc supernovae (some of them are associated with GRBs) whose progenitors are believed to be Wolf-Rayet stars with strong stellar winds. Interaction of the supernova shock wave with the winds and the interstellar medium.
Precursors of GRBs Precursor activities (in X-ray or gamma-ray band) have been observed in some GRBs. Study on the physical process which produces the precursor is important for revealing the nature of GRBs.
Variability of GRBs It has been discovered that the variability of GRB lightcurves is correlated to the peak luminosity of the GRB. More luminous GRBs tend to have a more spiky lightcurve. Physical origin underlying the relation is still unclear.
Short GRBs Intrinsic parameters distinguishing short GRBs from long GRBs, and the progenitor of short GRBs. For a long time people have believed that short GRBs are produced by the merger of two neutron stars (or a black hole and a neutron star). However, this scenario is challenged by the fact that no supernova signature has been found in the afterglows of short GRBs.
GRBs and Cosmology Various problems in GRBs as a tool (e.g., as a standard candle) to probe cosmology.
My Ph.D. thesis was on the magnetic interaction between a black hole and an accretion disk, and extraction of energy from the black hole. It was found that a magnetic field bridging a black hole and an accretion disk plays an important role in the transportation of energy and angular momentum, and has an important effect on the spectrum of the energy radiated by the disk. When the black hole rotates faster than the disk, the spin energy of the black hole provides an energy source for the radiation of the disk in addition to disk accretion. Such a disk has a radiation efficiency higher than that predicted by the standard theory.
In collaboration with R. Narayan et al., I have investigated the blackbody radiation spectrum from a thin Keplerian disk around a Kerr black hole and written a number of important codes for computing it. The codes have been compiled into a single subroutine: KERRBB, and installed into Xspec 12 &mdash a standard X-ray data analysis package. Since then, KERRBB has been a standard tool for analyzing the spectra of X-ray binaries and estimating the spin of black holes. Applying KERRBB to GRS 1915+105, a very stringent lower limit on the spin of the black hole was obtained: a > 0.98, close to the maximum rotation.
In collaboration with R. Sunyaev, I am working on the following two projects aiming to understand the growth of supermassive black holes at high redshift:
Super-Eddington Accretion Process Super-Eddington accretion, which involves a mass accretion rate exceeding the Eddington rate but a luminosity limited by the Eddington luminosity, is expected to take place at high redshifts when the Universe is young and merger of galaxies is frequent. Despite several decade efforts, physical details of super-Eddington accretion are still not well understood and many important questions remain to be answered.
Evolution of the Black Hole Spin during Merger of Galaxies Merger of galaxies affects not only the mass accretion rate onto a black hole, but also the orientation of the black hole spin relative to the axis of the disk as the radiation efficiency of the disk.
The nature of the quasi-periodic oscillations (QPOs) in X-ray binaries is still not answered. Since QPOs have been observed in both neutron star and black hole systems, people generally believe that QPOs originate from the accretion flow surrounding the central mass rather than from the surface of the accreting object. However, Jeremy Goodman, Ramesh Narayan, and I show that nonaxisymmetric g-mode and p-mode waves in an unmagnetized accretion disk either are strongly absorbed at corotation, or are amplified but with a very weak gain. Our results have eliminated a promising explanation for QPOs.
Considering the magnetohydrodynamic (MHD) instability in a magnetized disk, R. Narayan and I have found that the gas in the disk is highly unstable to the Rayleigh-Taylor and Kelvin-Helmholtz instabilities. Our model has provided some interesting links between the MHD instabilities and kilohertz QPOs, the frequency ratio 2:3 observed in some twin QPOs can be explained.
It has been known that the magneto-rotational instability rapidly develops in a magnetized disk with differential rotation, which leads to the formation of self-sustained turbulence and transportation of angular momentum. However, I have found that large-scale magnetic fields can also transport angular momentum in a disk, without dissipation of energy. It is an interesting problem whether a large-scale magnetic field is stable, and under what conditions a large-scale magnetic field is destroyed by MHD instabilities to form small-scale and chaotic magnetic fields.
Gravitational lensing directly probes the mass distribution in the Universe, so an investigation on gravitational lensing can tell us important informations about galaxies, dark matter, large-scale structures in the Universe, and constrain the cosmological models.
J. P. Ostriker and I have developed a semi-analytical approach to study the gravitational lensing of distant quasars by foreground mass clumps (galaxies, clusters of galaxies, etc) in various cosmological models. We have found that the probability of lensing is extremely sensitive to the mass density profile of lenses (galaxies and dark matter halos) and somewhat less so to the mean mass density in the Universe and the amplitude of primordial fluctuations. Lenses with a Navarro-Frenk-White (NFW) density profile are very much less effective in producing multiple images than that with an singular isothermal sphere (SIS) profile. For example, when all other parameters are fixed (the cosmological parameters, the number density of lenses, and the amplitude of primordial fluctuations), in a flat universe for a quasar at redshift 1.5 the integral probability of lensing by NFW halos is smaller than that by SIS halos by more than two orders of magnitude. This means that lensing also sensitively probes small-scale structures in the Universe &mdash while the fact is known, its importance has not been appreciated sufficiently.
When we applied our model to explain the observed numbers of lensing events discovered in the JVAS/CLASS survey, we found that the mass clumps in the Universe cannot be described by a universal mass density profile. This result is in agreement with the observations on the mass distribution in galaxies and clusters of galaxies, which show that spiral and elliptical galaxies are well described by SIS profiles but clusters of galaxies and dwarf galaxies usually have shallower inner density slopes.
Einstein's theory of general relativity predicts the existence of closed timelike curves (CTCs), but some previous calculations have shown that the renormalized stress-energy tensor of the quantum vacuum polarization diverges in a spacetime with CTCs. Based on those calculations, in 1992 S. W. Hawking proposed the chronology protection conjecture which claimed that the laws of physics do not allow the appearance of CTCs. Since then, many people have challenged this conjecture. In 1998, J. R. Gott and I found a self-consistent vacuum for quantum fields in Misner space (a flat space with CTCs), for which the renormalized stress-energy tensor is zero everywhere. This implies that CTCs can exist at least at the level of semi-classical quantum gravity.
Adopting that the laws of physics permit the existence of CTCs, J. R. Gott and I have proposed that the Universe may be created from itself: tracing backward in time we may eventually encounter a region of CTCs. Our model predicts that a time arrow exists in our Universe, which is consistent with our experiences.
I have calculated the vacuum polarization of a scalar field in an anti-de~Sitter space, aiming to interpret the cosmological constant in our Universe in the scenario of a brane universe. I have found that the vacuum polarization naturally gives rise to a small but nonzero cosmological constant in a brane world living in the anti-de Sitter space. The value of the predicted cosmological constant is in agreement with that indicated by current cosmological observations, if the anti-de Sitter space has a curvature radius ~ 0.001 cm and the scalar field has a mass ~ 0.01 eV. Probing gravity down to a scale ~ 0.001 cm, which is attainable in the near future, will provide a test of the model.
A legal copy of my PhD thesis can be obtained from ProQuest LLC. If you are looking for KERRBB, please go to this site.
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