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My research interests so far have ranged over the following areas:
- Cosmology, Galaxy formation, evolution and mergers,
- Properties of materials and the vacuum in ultrastrong magnetic fields,
- Magnetic and relativistic stellar structure,
- Physics of white dwarfs, black holes and neutron stars,
- Stellar evolution,
- General relativity.
Presently my research is mostly focussed on the last three bullets. The theme running though my work is how how is our understanding of astrophysical phenomena connected with our understanding of fundamental physics. Although my work so far has emphasized compact objects (white dwarfs, neutron stars and black holes), the physics of the early universe and cosmology in general also provides a window onto fundamental physics.
Taken to the extreme, the magnetic fields surrounding a neutron star can range from 108 to 1015 Gauss (the magnetic field at the surface of the Earth averages 0.6 Gauss). At the high end of this range lie the magnetars whose fields exceed the quantum-electrodynamic critical value of 4.4 x 1013 G. In such strong fields, the typical energy of an electron spiral around a magnetic field line exceeds its restmass energy.
The study of isolated cooling neutron stars continues to be a mainstay of my research program. I have developed both analytic and numerical models of the cooling of neutron stars including the effects of a strong magnetic field, and Don Lloyd, Lars Hernquist and I are currently developing the most comprehensive models of neutron star atmospheres. The radiation from the surface of a neutron star provides a excellent probe of the properties of atoms and light in strong magnetic fields.
Nir Shaviv and I have discovered a series of signatures of the quantum electrodynamic (QED) coupling of the radiation from the surface of a neutron star to the magnetic field surrounding it. The most important result is that QED-induced birefringence increases the observed polarization of the radiation from the surface of a neutron stars by about an order of magnitude. Optical polarimetry of nearby neutron stars on existing telescopes would be able to detect this increase, and X-ray polarimetry on the next generation of X-ray satellites could probe this effect on a wider variety of sources and in greater detail. The combination of optical and X-ray polarimetry would constrain the radius of the neutron star, and the strength and structure of the magnetic field surrounding it.
Millisecond Pulsars and LMXBs
Weakly magnetized neutron stars usually reveal themselves as millisecond pulsars. These quickly rotating stars are thought to form when an ordinary neutron star accretes material from a low mass companion in a low-mass x-ray binary (LMXB). The neutron star is spun up by the accreting material. It is still unclear how the neutron stars lose their magnetic fields as they are spun up.
Soon after the launch of RXTE, highly periodic oscillations were discovered in Type-I X-ray bursts, in which the fuel accumulated on the surface of a neutron star in an LMXB suddenly ignites and the flux of the star increases by several orders of magnitude for several seconds. The burning is thought to originate a particular point on the surface and quickly spread over the surface. Initially the oscillations were thought to be a signature of the hotspot as the star rotates. The frequency of the oscillation changes slightly during the burst which was thought to be due to the conservation of the angular momentum in the shearing atmosphere. I showed that if this is indeed the case, the evolution of the frequency of the oscillation coupled with a knowledge of the expansion and contraction of the atmosphere during the burst would provide a sensitive probe of general relativity. Relativistic corrections reduce the observed frequency shift by a factor or two to three relative to a Newtonian prediction.
Further study of the expansion of the atmosphere coupled with relativistic kinematics have indicated that conservation of angular momentum cannot account for the frequency shift observed in these sources. I have recently proposed that the observed oscillations are a hallmark of large-scale waves traveling through the cooling ocean during the decay of the burst. These waves have well understood analogues in oceans of the Earth, especially in the Pacific basin, and a straightforward scaling of their properties on the Earth to the condition on the surface of a neutron star accounts for the observed frequency shift and may provide deeper insight into nuclear burning of the surface of neutron stars.
In the same vein, Ramesh Narayan and I have been developing steady-state numerical and semianalytic models to understand the evolution of the accumulating atmosphere on the surface of a compact object up to the onset of the nuclear burning instability, i.e. the Type-I burst itself. This model predicts the regime of accretion rates and neutron star properties which allow stable nuclear burning on the surface of the star, i.e. no bursts. Furthermore, unlike fully time-dependent simulations of Type-I bursts, we can explore a wide range of neutron star parameters and physical assumptions to gain a broad understanding of X-ray novae in general.
My research into the physics of black holes like the kinematics of Type-I burst oscillations began not with the observations but with trying to understand the hallmarks of spacetime symmetries in general relativity. In the case of the black holes, I developed a model of a charged-starved magnetosphere of a spinning black hole. If a magnetic field with the strength typical of a radio pulsar threads a spinning stellar black hole, the result is a jet of highly energetic electrons and positrons with an initial luminosity exceeding 1051 ergs/s. The production of pairs results from the quantum electrodynamic treatment of the electromagnetic field surrounding the hole (i.e. the Wald field). Whether this could provide the engine for a gamma-ray burst depends on whether the magnetosphere remains starved of charge and if it does not on what are the properties of the magnetosphere. This second important aspect of the problem is a focus of my current research. Compact objects provide a laboratory to verify our understanding of nature at the extreme. The intense magnetic fields of neutron stars dwarf those produced on Earth a billion-fold, and the densities and pressures exceed the realm of Earth-bound experiment by a factor of a trillion or more. Neutron stars provide a unique opportunity to extrapolate and verify our theories of matter, energy and their interaction. Understanding black holes and their interaction with their surroundings also probes our understanding materials under extreme conditions; however, more crucially black holes test our understanding of spacetime itself. My research over the past few years has focussed of the theoretical and observational understanding of these objects.
A general theme threads my research interests: how is our understanding of astrophysical phenomena connected with our understanding of fundamental physics. The gross properties of neutron stars such as their mass and radius which we might learn definitively from observations of the thermal radiation from their surfaces, probes nuclear physics in an otherwise inaccessible realm. The details of their kinematics may probe the quark-gluon phase transition, and the understanding the dynamics of material and light near their surfaces can verify QED and general relativity. The dynamics of black holes, the matter and fields surrounding them most directly probes general relativity, but perhaps also QED in the context of the central engine of gamma-ray bursts. I intend to continue my research in the exciting area of the physics of compact objects. Starting in the distant past and proceeding to the present and from large scales to small, some of the questions that I would like to address in my future research:
- What is the nature of the initial singularity? Is it necessary?
- Can generic principles help constrain the properties of dark energy in the distant past and today?
- How does varying dark energy affect structure formation?
Now jumping down twenty orders of magnitude…
- Does QED play a role in producing or processing the radiation from soft-gamma repeaters? In classical gamma-ray bursts?
- What is the composition of neutron-star atmospheres (ionized, atomic or molecular)? What can we learn from their spectra?
- How can geophysical analogues help us understand neutron-star phenomena?
- What is the nature of nuclear matter in the core of a neutron star? Is the quark-hadron phase transition important in neutron-star cores? What can observations of neutron stars tell us about quantum chromodynamics?
- What is the nature of neutron-star collapse? Does it have any important observable consequences?
- How can black-hole electrodynamics help explain the observations and energetics of accreting black holes?