Sarah Yost
Assistant Professor
CSB/SJU Physics and Astronomy

Contact info:

Office: PENGL 113
Phone: 363-3187
email: syost ("at")

Publication list


Fall 2011 Course Pages

Past Course Pages

Research Interests

Gamma-Ray Bursts (GRBs)

Concept: GRBs are the fraction-of-a-second to minutes-long gamma-ray flashes accompanying the deaths of (certain) massive stars. They are the most luminous (energy per time) events in the electromagnetic universe. They occur in other galaxies, at a detectable rate (BATSE project) of approximately 1 per universe per day. GRB emission is very irregular in brightness during the event, but the events are followed by a longer-lived, fainter, smoother ''afterglow'' light from radio to X-ray wavelengths. This ''afterglow'' has the signature (redshift) indicating that they occur in distant galaxies. Some are definitively associated with core-collapse supernovae. Other events have failed to yield any supernova signature despite being near enough that we should have detected one. Therefore, the origin of some of the events is unclear. The Wikipedia article provides more details.

My past work on GRBs has involved fitting comprehensive broadband afterglow datasets to an implementation of the relativistic blastwave ''fireball'' model, and participating in the rapid followup efforts of the ROTSE project.

At present, I work with public, archival data. I am particularly interested in how the properties of the gamma-ray event itself may (or may not) be connected to the early afterglow. There are different patterns of behaviour in the transition from GRB to smoothly-decaying afterglow light.

A past paper (2007) on the subject (ApJ 669, 1107, link to free preprint)

I have been working with a student researcher to determine the optical onset properties (delay time, width of the onset bump, etc) of many events and compare them to the variability of the gamma-ray outburst itself. The optical onset delay is connected to the Lorentz factor or kinematically-scaled thickness of the initial relativistic ejecta from the central engine of the GRB. The variability of the prompt gamma-rays is related to the irregularity of that outflow. An initial check of a few events appeared to show evidence for a correlation between these properties, and we are investigating this with a more statistically significant sample. (This work is being prepared for American Astronomical Society meeting 219, January 2012.)

Further information, Useful links. (under construction)

Variable Stars

Summer research projects

Concept: To verify whether targets in the archival Northern Sky Variability Survey are short-duration periodic variables. The targets must (1) NOT be classified as a variable star in the databases and (2) have a high ''variability index'', as given by the NSVS classification [WS technique, as modified by Stetson (1996)].

Targets are observed for blocks of time, preferably at least 2 hrs per target, repeated on 2 or 3 nights. Lightcurves are analyzed for periodic variability with a period-finder.

This works best for continuously-variable objects. We cannot rule out widely detatched eclipsing binaries (which have a constant brightness for a large fraction of the time). However, we can determine whether stars are continuously variable with periods of less than or on order of a few hours, with amplitudes of 0.1 mag or more. (Variable types which can potentially be identified include contact binaries, RR Lyraes, and AM CvN stars.)

Further information, useful links, etc