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 main project involves the peculiar LMXB GX 17+2.
X-ray binaries were among the first X-ray sources detected, for they are
among the brightest objects in the X-ray sky. X-ray binaries consist of a compact object accreting matter from a donor star, whether that companion is a main sequence star, an evolved giant, or another compact object. In an LMXB the donor object fills its Roche Lobe, and the gas flows toward the compact object in a thin stream across the L1 point to form a disk around the compact object. Viscous processes serve to help the gas shed angular momentum, moving the gas from the outer edge of the disk to the inner edge, where it then is accreted by the compact object. A jet of relativistic material can also be launched, but the exact mechanism is unclear. The process is similar in a high mass X-ray binary, except the compact object accretes material via the stellar wind of the donor, usually an early-type star or a giant.
GX 17+2 has a neutron star primary, deduced from observed type I X-ray bursts, but the identity of the companion star is unknown. The X-ray activity of this source has been well-studied, for it is one of eight known Z sources, so called for the Z-shaped track they trace out in an X-ray color-color diagram (hard color versus soft color). These sources are extremely bright and interpreted as neutron stars accreting at the Eddington limit, the limit beyond which the accretion flow onto the compact object is disrupted by the radation produced by the accreting material itself.
Identification of GX 17+2 in the infrared has been problematic. Historically, it has been confused with NP Ser, a field star which lies almost directly in front of it; however, recent work of Callanan et al. (2002) has shown NP Ser to be a normal G star and the X-ray source to be 0.9" to the North. Callanan et al. (2002) also 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 behavior is unusual and we studied this source further using NICFPS
on the ARC 3.5m at APO and SQIID on the 2.1m at KPNO. We now have the
largest dataset of time-resolved IR observations of this source, including
simultaneous light curves in J, H, and K. We have
detected 5 IR brightening events. Based on our data and that of
Callanan
et al. (2002), we have determined the IR brightening events to be periodic. Futher, the light curves are not uniform in character, some having a sharp peak and others displaying a long plateau, or in peak magnitude.
A possible explanation for this behavior is that the IR radiation is
synchrotron in nature from a compact jet, as has been seen in some black
hole XRBs. 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, or as the periastron activity
in an eccentric
binary, which is the model for Cir X-1. Further observations are planned,
and a paper on this topic is in preparation.
I am pleased to acknowledge support from the Department of Higher
Education (HED) graduate scholarship for women in the sciences and the New
Mexico Space Grant.
Meetings
211th AAS, 2008 January 7-11
The Infrared Variability of GX17+2 and Low-Mass X-ray Binary Jets
Bernie McNamara, Jillian Bornak, Tom Harrison, & M. Rupen
Are There Any Other Old Novae Like V1500 Cyg?
Joni Johnson, Tom Harrison, Joe Wellhouse, Jillian Bornak, Ryan Campbell, J. E. Lyke, S. B. Howell, P. Szkody, & F. Cordova
A Population Explosion: The Nature and Evolution of X-ray Binaries in Diverse Environments, 2007 October 28 - November 2
Multiwavelength Observations of the Highely-Reddened Z source GX 17+2
Jillian Bornak, Tom Harrison, , Bernie McNamara, & M. Rupen
23rd New Mexico Symposium, 2007 October 19
Multiwavelength Observations of the Highely-Reddened Z source GX 17+2
Jillian Bornak, Tom Harrison, , Bernie McNamara, & M. Rupen
Webpages
