Further Evidence for the Bimodal Distribution of Neutron-star Masses

The Astrophysical Journal, Volume 719, Issue 1, pp. 722-727 (2010)

Josiah Schwab, Philipp Podsiadlowski and Saul Rappaport

We use a collection of 14 well-measured neutron-star masses to strengthen the case that a substantial fraction of these neutron stars were formed via electron-capture (e-capture) supernovae (SNe) as opposed to Fe core-collapse SNe. The e-capture SNe are characterized by lower resultant gravitational masses and smaller natal kicks, leading to lower orbital eccentricities when the e-capture SN has led to the formation of the second neutron star in a binary system. Based on the measured masses and eccentricities, we identify four neutron stars, which have a mean post-collapse gravitational mass of ~1.25 M _sun, as the product of e-capture SNe. We associate the remaining 10 neutron stars, which have a mean mass of ~1.35 M _sun, with Fe core-collapse SNe. If the e-capture SN occurs during the formation of the first neutron star, then this should substantially increase the formation probability for double neutron stars, given that more systems will remain bound with the smaller kicks. However, this does not appear to be the case for any of the observed systems and we discuss possible reasons for this.

Compass Summer Program

I'm teaching for the Berkeley Compass Project summer program.  This year's topic is "How does a Wind Turbine Work?".

Outstanding GSI Award

I was selected as one of the recipients of the 2009-2010 Outstanding Graduate Student Instructor Award in recognition of excellence in teaching in the Physics Department.

Physics 8B

This semester I'm one of the GSIs for Physics 8B: Introductory Physics.  This is an undergraduate course for non-majors being taught by Profs. Mike DeWeese and Catherine Bordel.

Here's the course description:

Introduction to electricity, magnetism, electromagnetic waves, optics, and modern physics. The course presents concepts and methodologies for understanding physical phenomena, and is particularly useful preparation for upper division study in biology and architecture.

Galaxy-Scale Strong-Lensing Tests of Gravity and Geometric Cosmology: Constraints and Systematic Limitations

The Astrophysical Journal, Volume 708, Issue 1, pp. 750-757 (2010)

Josiah Schwab, Adam Bolton and Saul Rappaport

Galaxy-scale strong gravitational lenses with measured stellar velocity dispersions allow a test of the weak-field metric on kiloparsec scales and a geometric measurement of the cosmological distance-redshift relation, provided that the mass-dynamical structure of the lensing galaxies can be independently constrained to a sufficient degree. We combine data on 53 galaxy-scale strong lenses from the Sloan Lens ACS Survey with a well-motivated fiducial set of lens-galaxy parameters to find (1) a constraint on the post-Newtonian parameter γ = 1.01 ± 0.05, and (2) a determination of Ω_Λ = 0.75 ± 0.17 under the assumption of a flat universe. These constraints assume that the underlying observations and priors are free of systematic error. We evaluate the sensitivity of these results to systematic uncertainties in (1) total mass-profile shape, (2) velocity anisotropy, (3) light-profile shape, and (4) stellar velocity dispersion. Based on these sensitivities, we conclude that while such strong-lens samples can, in principle, provide an important tool for testing general relativity and cosmology, they are unlikely to yield precision measurements of γ and Ω_Λ unless the properties of the lensing galaxies are independently constrained with substantially greater accuracy than at present.