We study neutron star astrophysics and gravitational waves.

Neutron stars are the collapsed remnants of massive stars, the most
compact astrophysical objects outside of black holes, and the place where
matter makes it's last stand against the overwhelming force of gravity. Inside
a neutron star, matter is compressed into strange new forms, pushing the limits
of our understanding of condensed matter and particle interactions.

Like many stars, neutron stars can be found in binary systems, where
pairs of neutron stars orbit. As they do so, they lose energy to gravitational
waves, eventually spiraling towards each other and colliding. This will
release huge amounts of energy in gravitational-wave signals that LIGO, the
Laser Interferometer Gravitational-wave Observatory, hopes to observe. These
collisions may also explain some of the Gamma Ray Bursts seen by space telescopes like NASA's
Fermi.

My group and I work to understand and model how neutron stars
interact, collide, and radiate energy, so that observations can tell us new
things about their structure and composition.

My research relies on the hard work of many students here at CSUF. They
include:

Areas of Research

Dense matter and gravitational waves

We search for the gravitational-wave signals from binary systems in LIGO data
using templates calculated from the expected motions of the two objects. For
systems involving neutron stars, most of this motion is approximate. Each
neutron star or black hole is described by a point mass, and we add relativistic
corrections to the Newtonian dynamics of the system.

The best indirect evidence
for the existence of gravitational waves described by General Relativity comes
from observing the binary dynamics of nearby neutron-star systems like the double
pulsar system PSR
J0737-3039 and comparing it to these approximate models.

The systems we
observe in our neighborhood aren't detectable by LIGO; they orbit each other
every few hours and radiate weakly. LIGO could only see them only after they
spiral into tight enough orbits that they zip around more than ten times per
second. For the systems we see in our galaxy, this won't happen for many millions of
years. However, Advanced LIGO will be able to listen to many
galaxies at a time; enough that we expect to catch about forty of these binary
neutron star mergers
every year.

The point-particle and post-Newtonian description will inevitably break
down as the stars make their final approach; tidal forces will begin to distort
the stars, and then their gravitational interaction will no longer have only
small corrections to the Newtonian dynamics. Eventually, when the two stars
crash into each other, their behavior won't be well-described by an orbit at all.
Numerical simulations are required to understand what happens then.

Student projects

Incorporate tidal terms with realistic coefficients into the waveforms used by
the LIGO Scientific Collaboration (the LIGO
Algorithm Library,) so that they can be used in parameter estimation studies
and to analyze future gravitational-wave detections.

Where will the templates used to search for gravitational waves break down?
Use results of post-Newtonian calculations and numerical simulations to
determine; when matter effects might lead to mistaken flags for poor waveform
matches, or where they might contaminate parameter estimation.

Crust shattering and gamma-ray flares

A recent project looks at what might cause small flares before the main
Gamma Ray Burst in a merging neutron star scenario. You can read a synopsis
in Physics from a recent Physical Review Letters publication which was an
Editor's Suggestion. My collaborator Dave Tsang has a webpage
explaining the resonant shattering scenario.

Student projects

The crust response to the tidal field of the
other star has not been fully explored. Even though this probably won't crack
the crust directly, perhaps it would trigger other effects. How do different
crusts get strained by tidal forces?

The frequency of the resonance depends on properties of the neutron-star
matter. We have only looked at a few examples. Can we find systematic
relationships between features of the equation of state and the frequency of
the shattering resonance?

Research projects for students

Research in astrophysics will let you learn more about the Universe and
help push the frontiers of human knowledge. Each research
student can personally contribute something new to the scientific community.

The skills you build doing research will give you a head start for a future
academic career. They are also used outside of physics. Each
student will produce a publically-viewable portfolio of work: open-source code
repositories, online demos and writeups, documentation, and/or published
papers. Specific things you can work on:

Using python, a scripting language: a workhorse in astronomy as
well as other sciences, used in today's software industry.

Use of modules like numpy (numeric python) and scipy (scientific python)

Use of Mathematica, matlab/pylab

Code development in c/c++

Unix-style command-line interface

Submitting jobs to computer clusters for parallel processing. Managing clusters. Writing parallelizable code.

Data analysis: dealing with big data. Pattern matching. Signal processing.

Technical writing. The LaTeX typesetting system.

Making clear figures and graphical communication.

Writing web demos for outreach, teaching, and illustrating ideas.

Applied mathematics:

working with general relativity, understanding
coordinates, framing differential equations, using perturbation theory.