Jillian Bornak Research/Teaching Assistant Entered: 2005 Office: 112 Astronomy Phone: (575)646-6399 Fax: (575)646-1602   E-mail: jbornak (append "@nmsu.edu")
B.A. Syracuse University, 2000

Research

I'm interested in stellar astronomy, particularly in compact objects such as white dwarfs, neutron stars, and black holes. The best place for me to study these objects is in binary systems, where the accretion of matter from a non-evolved star onto a compact object lead to a host of processes in the accretion disk itself and in jets launched by the disk. I am studying low-mass X-ray binaries (LMXBs) with Dr. Tom Harrison and Dr. Bernie McNamara in the Stellar research group. My work involves infrared photometry, but I am eager to add radio and X-ray data reduction to my skill set.

An infrared survey of LMXBs
My first project was an infrared survey of LMXBs with neutron star primaries. Similar programs have been conducted previously for white dwarf systems, but never for neutron star systems. We will create the first infrared catalog of neutron star binary counterpart stars, as no one seems to have many infrared magnitudes for neutron star systems. We don't know what the non-evolved stars of such systems look like: they could be normal main sequence or giants, or they could be stripped stars. By studying the secondaries we will estimate the masses of the neutron star and possibly constrain their radii. This will help us to constrain the neutron star equation of state. Additionally, we might observe behaviors of the systems such as disc and jet activity.

The peculiar source GX 17+2
My second project involves the peculiar low-mass X-ray binary GX 17+2. X-ray binaries were the first X-ray sources detected, for they are among the brightest objects in the X-ray sky, and consist of a compact object accreting matter from a donor star. The accretion disk is the brightest component of the system, with a temperature of millions of degrees, and dominates the X-ray emission. The secondary star may contribute to the optical-infrared emission. Evidence suggests these systems may also launch a compact, self-absorbed, collimated jet of relativistic material. Synchrotron radiation from this jet would be most visible in the radio, IR, and possibly >20 keV X-rays.

GX 17+2 is one of eight Z sources, so-called for the shape they trace out in an X-ray color-color diagram. It is a strong X-ray source, indicating continuous (and presume near-Eddington) accretion onto its neutron star primary. Identification of GX 17+2 in the infrared has been problematic. Callanan et al. (2002) had the the fortunate timing to observe GX 17+2 with Keck on two nights, one in which the source was IR bright and one IR faint, an impressive difference of about 4 magnitudes in the K band. This activity had never been seen in this source nor in any of the other Z sources. Intrigued by this unusual behavior, we studied this source over the course of three years, accumulating the largest dataset of time-resolved IR light curves of this source. We have detected 5 IR brightening events and determined them to be periodic. Futher, the light curves are not uniform in character. A possible explanation for this behavior is that the IR radiation is synchrotron in nature from a compact jet. The periodic activity could be explained as a precessing jet (either from a precessing disk or from the influence of a third body), which is the model for SS 433.

Dust production in classical novae
My thesis project involves the study dust formation in classical novae. A classical nova is thought to occur in a binary system consisting of a white dwarf accreting from a main sequence (or late type giant) via Roche Lobe overflow. The accreted material builds up as a layer on the surface of the white dwarf until the base temperature increases to the point when a thermonuclear runaway occurs, dominated by CNO reactions. This shell burning continues at Edddington luminosity for the white dwarf. Material is ejected at speeds from a few hundred to a thousand km/s. The shell starts as an optically thick fireball and expands and cools to be optically thin. Novae are classified by the speed at which they decrease in brightness. Classical novae are thought to recur every 10,000 years or so.

Some novae evidence a drop in optical/UV light with a corresponding increase in IR emission, indicating dust production. It seems that fast novae do not, on average, produce dust, while slow novae can evidence optically thick or thin dust emission or none at all. However, a unified model of dust production does not exist to explain these difference scenarios.

My thesis is based on OIR photometry and optical spectroscopy of the slow Nova Cen 1991. Initial results indicate Nova Cen produced dust but also has simultaneous contamination by optical light, indicating less than uniform ejecta. Evidence exists for clumpy ejecta in images of the GK Per and the light echo from V838 Mon, and I intend to model Nova Cen with clumpy ejecta using the DIRTY model. This presents a solution to the variable dust formation observed, with detection of dust depends on a line-of-sight through a clump.

I am pleased to acknowledge support from the New Mexico State University Higher Education Department (HED) graduate scholarship for women in the sciences, a graduate assistantship award, and the New Mexico Space Grant.

CV

Publications

"Additional Spitzer IRS Spectroscopy of Three Intermediate Polars: The Detection of a Mid-Infrared Synchrotron Flare from V1223 Sagittarii"
Harrison, T. E.; Bornak, J.; Rupen, M., Howell; S. B.; submitted to ApJ September, 2009