Gregory R. Sivakoff
	Associate Professor
	Department of Physics

Compact Object Research Pages

Why Study Compact Objects?

Studies of white dwarfs, neutron stars, and black holes (compact objects) can reveal a wide range of important astrophysical phenomena. Some key phenomena include the following:

• relationships of compact objects with star formation and galaxy formation
• the universal processes of accretion and jet formation
• fundamental gravitational physics

Because of this rich reservoir of open scientific questions, my primary research focuses on multi-wavelength observational studies of compact objects, ranging from Galactic sources as close as a thousand light years away to accreting compact objects (X-ray binaries) in nearby galaxies millions of light years away, to Active Galactic Nuclei billions of light years away. In the text below, I introduce some topics that will help explain what compact objects are, and why they reveal such a wide range of important astrophysical phenomena.

A Compact Object Primer

First, I will introduce the different types of compact objects.

Compact Objects Are The End Points of Stellar Evolution

Due to gravity, all matter in the Universe tends to want to collapse unless other forces can balance gravity. In stars, the collapse of material leads to such high temperatures and densities, that the atoms that make up a star can fuse together, which is how stars generate energy. During most of the life of a star, the radiation pressure from these nuclear processes balances the gravitational processes exquisitely. However, eventually the fuel runs out and gravity begins to win. Compact objects are formed in the death throes of a star. In all compact objects, the stellar remnant is much smaller than the star (or stars) it came from.

There are four types of compact objects we know exist and two we believe may exist:

White Dwarves

White dwarfs are the stellar remnants of stars like our Sun (stars that are generally less than 8 times the mass of the Sun). When the fuel of low-mass stars begins to run out, a quantum mechanical effect from packing electrons close together produces a balancing force (electron degeneracy pressure). This can occur as long as the white dwarf is less than 1.4 times as massive as the Sun (during the lifetime of a star, large amounts of the star can be ejected). White dwarfs as massive as the Sun are only as large as the Earth.

Neutron Stars

Neutron stars are the stellar remnants of stars a little more massive than our Sun (stars that are generally 8 - 25 times the mass of the Sun). If a star is more massive than 1.4 times the mass of the Sun when it runs out of fuel, the electron degeneracy pressure is not strong enough to halt gravitational collapse. Further collapse occurs until the protons and electrons combine into neutrons. Eventually, a second quantum mechanical effect from packing neutrons close together produces another balancing force (neutron degeneracy pressure). For these more massive stars, Neutron stars are observed to have masses between 1.4 and 2.5 times the mass of the Sun, but are only as large as cities.

Stellar-Mass Black Holes

For the most massive stars, there is no force capable of balancing gravity and the star collapses into singularity, called a black hole. The gravitational field of a black hole is so strong that not even nearby light can escape it. Although the singularity has no physical size, astronomers often talk about the distance light has to be from a black hole for it to be able to escape. For a non-spinning black hole that distance is about 3 kilometers.for every solar mass of the black hole.

Super-Massive Black Holes

Somehow (astronomers are still trying to figure out how), black holes at the middle of galaxies grow tremendously, becoming hundreds of thousands to billions of times as massive as the Sun. These objects are known as super-massive black holes.

Intermediate-Mass Black Holes (?)

There are ongoing searches to find black holes with masses between those of stellar-mass black holes and super-massive black holes. Astronomers call these elusive objects intermediate-mass black holes. There are some pieces of evidence that point to intermediate black holes in the cores of some massive globular clusters; however, this is still hotly debated.

Quark Stars (??)

Some scientists believe that some neutron stars may become so dense that the neutrons eventually turn into strange quarks, which might provide an additional source of pressure such that there is a type of star in between a neutron star and a black hole. No "quark star" has ever been definitively detected and few astronomers believe in their existence.

Ongoing and Recent Projects

Demographic Studies of Low-Mass X-Ray Binaries in Early-Type Galaxies

The exceptional imaging quality of the Chandra X-ray Observatory has helped reveal that the X-ray emission in early-type (elliptical and lenticular galaxies) comes from both diffuse interstellar gas at millions of degrees and point sources. These point sources are dominated by compact objects (neutron stars and black holes) accreting material from a low-mass binary companion star. Recent Chandra observations of these low-mass X-ray binaries were the first to image large populations of neutron stars and stellar-mass black holes in a normal elliptical galaxy. Combining X-ray and optical data, we have found that low-mass X-ray binaries are often associated with globular clusters, important tracers of galaxy evolution. Thus, low-mass X-ray binaries may help trace the evolution history of entire galaxies, as well as the population of stars that eventually form low-mass X-ray binaries. For my thesis, I showed that the low-mass X-ray binaries in globular clusters were formed from dynamical interactions, as opposed to the evolution of primordial binaries, and that metallicity plays a key role. This result raises two important questions: are the low-mass X-ray binaries in the fields of galaxies formed primordially or dynamically in globular clusters that no longer exist in the galaxies, and why do metal richer globular clusters host more X-ray binaries? I am leading several projects that will address these issues.

• Studying the X-ray binary population in depth with deep, multi-epoch Chandra observations of the nearest early-type galaxies. I am currently leading the Chandra Centaurus A Very Large Project studies of point sources. This large project will provide the most in-depth study of X-ray binaries in an early-type galaxy to date.
• Proving that some portion of the field low-mass X-ray binaries must be different from globular cluster low-mass X-ray binaries by comparing the luminosity functions of low-mass X-ray binaries in globular clusters to those in the field. Recent targets include Centaurus A, NGC 3379, NGC 4278, & NGC 4697.
• Deriving the fraction of field low-mass X-ray binaries that formed primordially by comparing the spatial distributions of stars, low-mass X-ray binaries, and globular clusters: Current targets include elliptical galaxies Centaurus A, NGC 3379, NGC 4278, NGC 4365, & NGC 4697. Study of lenticular galaxy NGC 1023 was interrupted by the failure of Advanced Camera for Surveys on the Hubble Space Telescope.
• Determining what role secondary parameters such as metallicity play in the formation of X-ray binaries in dense stellar systems by performing multi-wavelength censuses of compact object binaries in Galactic globular clusters. Our current target is NGC 3201, a sparse Galactic globular cluster.

Variability Studies of X-Ray Binaries

• Discovering rare source behavior and a more general overview of X-ray binary behavior to better understand accretion through multi-epoch variability studies of low-mass X-ray binaries in early-type galaxies. Current targets include elliptical galaxies Centaurus A, NGC 1023, NGC 4365, & NGC 4697.
• Probing how jets are launched and collimated through monitoring entire outbursts of Galactic black hole X-ray binaries, neutron star X-ray binaries, and white dwarf cataclysmic variables with simultaneous X-ray, optical, infrared, and radio monitoring. I am the lead Co-Investigator of the Jet Acceleration and Collimation Probe Of Transient X-Ray Binaries (JACPOT XRB) project , which is combining the unparalleled angular resolution of the VLBA with multi-wavelength monitoring to create unprecedented data that will determine the similarities and differences between jet formation and evolution in accreting stellar mass compact objects.

Demographic Studies of Active Galactic Nuclei

• Probing the role of environment by studying X-ray identified Active Galactic Nuclei in clusters of galaxies. Recent projects include studying the spatial distribution of Active Galactic Nuclei in local galaxy clusters and how the fraction of Active Galactic Nuclei in galaxy clusters evolves with redshift.
• Discovering new properties of the class of Broad Absorption Line Quasars through correlative studies of large catalogs. Recent projects include the discovery that the fraction of Broad Absorption Line Quasars was about twice as large as originally believed and that the fraction of Broad Absorption Line Quasars dropped to nearly zero for the most radio luminous AGN.
• Tracing the relative accretion history of optical quasars that are and are not strong radio sources using correlative studies of large catalogs. Recently, we have found that radio quasars grow much more slowly than optical quasars.

Testing Fundamental Physics With Compact Objects

• Probing the neutron star equation of state with observations of middle-aged neutron stars. Current projects include determining the temperature and distance of only the third known middle-aged neutron stars within one kiloparsec, PSR J1741-2054