Unveiling the Progenitors of Short-duration Gamma-ray Bursts

Gamma-ray bursts (GRBs) are relativistic explosions which originate at cosmological distances, and are among the most luminous transients in the universe. Following the prompt gamma-ray emission, a fading synchrotron "afterglow" is detectable at lower energies. While long-duration GRBs (duration > 2 sec) are linked to the deaths of massive stars, the progenitors of short-duration GRBs (duration < 2 sec) have remained elusive. Theoretical predictions formulated over the past two decades have suggested that they are the mergers of two compact objects, involving either two neutron stars (NS-NS) or a neutron star and a black hole (NS-BH). Such merging systems are also important to understand because they are premier candidates for gravitational wave detections with upcoming facilities and are considered likely sites of heavy element nucleosynthesis. The launch of the Swift satellite in 2004, with its rapid multi-wavelength monitoring and localization capabilities, led to the first discoveries of short GRB afterglows and therefore robust associations to host galaxies. At a Swift detection rate of ~8 events per year, the growing number of well-localized short GRBs enables comprehensive population studies of their afterglows and environments for the first time. In this thesis, I undertake a multi-wavelength observational campaign to address testable predictions for the progenitors of short GRBs. From their local environments, I show that short GRBs explode in diffuse regions of their host galaxies and are weakly correlated with the distribution of stellar mass and star formation in their host galaxies. I study the host galaxy demographics for the entire population and find that ~20-40% of short GRBs originate from elliptical galaxies, implying an older stellar progenitor. From their afterglows, I present evidence that some short GRBs are collimated in narrow jets of ~5-10 degrees, directly affecting the true energy scale and event rate. Finally, taking advantage of a decade of broad-band afterglow observations at radio through X-ray wavelengths, I find that short GRBs have median isotropic-equivalent energies of ~10^51 erg and that their local environments have low densities, ~10^-3-10^-2 cm^-3. Taken together, this thesis comprises several lines of independent evidence to demonstrate that short GRBs originate from the mergers of two compact objects, and also provides the first constraints on the explosion properties for a large sample of events. With the direct detection of gravitational waves from compact object mergers on the horizon, these studies provide necessary inputs to inform the next decade of joint electromagnetic-gravitational wave search strategies.Astronom

Extrasolar planet transit photometry at Wallace Astrophysical Observatory

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2008.Includes bibliographical references (p. 42-43).Extrasolar planet transit photometry is a relatively new astronomical technique developed over the past decade. Transit photometry is the measurement of a star's brightness as an orbiting planet passes in front of the star as seen from the Earth. Recently, members of MIT's Planetary Astronomy Lab (PAL) have launched an observing program for extrasolar planet transits at Wallace Astrophysical Observatory (WAO), which houses the 24-inch telescope used in this work. The purpose of this thesis is to enable students and faculty to easily perform transit photometry at WAO and assess the feasibility of transit photometry there. The PAL extrasolar planetary database currently has 36 planetary candidates, 23 of which are observable at WAO due to their positive declinations 6 (in the Northern celestial hemisphere). The maintenance of this database is described. Prediction methods used in Mathematica to determine when transits will occur at WAO for a given period of time are discussed. The transits at WAO are prioritized based on frequency of transit, transit depth and celestial location of parent stars, using the prediction period of 01-20-2008 to 05-30-2008. This prediction period is compared to four others spanning 2007-2009. These results suggest that the best planetary candidates at WAO for the fall are XO-3b, WASP-lb and HAT-P-6b and for the spring are HAT-P-3b, TrES-3 and XO-3b. A typical observing plan is produced based on the planetary candidate TrES-3, including finder charts for the highest frequency transiting planets in Spring 2008. Data reduction and analysis using either the standard IDL routine phot or the "Make_Lightcurve.nb" Mathematica notebook are described. A partial transit of XO2b taken at WAO is presented. Given WAO's recent upgrade by PAL along with the data presented here, the feasibility for successful extrasolar planet transit photometry projects at WAO is high.by Wen-fai Fong.S.B

A VLA Study of High-redshift GRBs I - Multi-wavelength Observations and Modeling of GRB 140311A

We present the first results from a recently concluded study of GRBs at $z\gtrsim5$ with the Karl G. Jansky Very Large Array (VLA). Spanning $1$ to $85.5$ GHz and 7 epochs from 1.5 to 82.3 d, our observations of GRB 140311A are the most detailed joint radio and millimeter observations of a GRB afterglow at $z\gtrsim5$ to date. In conjunction with optical/near-IR and X-ray data, the observations can be understood in the framework of radiation from a single blast wave shock with energy $E_{\rm K,iso}\approx8.5\times10^{53}$ erg expanding into a constant density environment with density, $n_0\approx8\,{\rm cm}^{-3}$. The X-ray and radio observations require a jet break at $t_{\rm jet}\approx0.6$ d, yielding an opening angle of $\theta_{\rm jet}\approx4^{\circ}$ and a beaming-corrected blast wave kinetic energy of $E_{\rm K}\approx2.2\times10^{50}$ erg. The results from our radio follow-up and multi-wavelength modeling lend credence to the hypothesis that detected high-redshift GRBs may be more tightly beamed than events at lower redshift. We do not find compelling evidence for reverse shock emission, which may be related to fast cooling driven by the moderately high circumburst density.Comment: 16 pages, 13 figures, submitted to Ap

Forward Modeling of Double Neutron Stars: Insights from Highly-Offset Short Gamma-Ray Bursts

We present a detailed analysis of two well-localized, highly offset short gamma-ray bursts---GRB~070809 and GRB~090515---investigating the kinematic evolution of their progenitors from compact object formation until merger. Calibrating to observations of their most probable host galaxies, we construct semi-analytic galactic models that account for star formation history and galaxy growth over time. We pair detailed kinematic evolution with compact binary population modeling to infer viable post-supernova velocities and inspiral times. By populating binary tracers according to the star formation history of the host and kinematically evolving their post-supernova trajectories through the time-dependent galactic potential, we find that systems matching the observed offsets of the bursts require post-supernova systemic velocities of hundreds of kilometers per second. Marginalizing over uncertainties in the stellar mass--halo mass relation, we find that the second-born neutron star in the GRB~070809 and GRB~090515 progenitor systems received a natal kick of $\gtrsim 200~\mathrm{km\,s}^{-1}$ at the 78\% and 91\% credible levels, respectively. Applying our analysis to the full catalog of localized short gamma-ray bursts will provide unique constraints on their progenitors and help unravel the selection effects inherent to observing transients that are highly offset with respect to their hosts.Comment: 18 pages, 7 figures, 1 table. ApJ, in pres

Collective Spin Modes in Superconducting Double Layers

We investigate a double layer system with tight-binding hopping, intra-layer and inter-layer interactions, as well as a Josephson like coupling. We find that an antiferromagnetic spin polarization induces additional spin-triplet pairing (with $S_z =0$) to the singlet order parameter. This causes an undamped collective mode in the superconducting state below the particle-hole threshold, which is interpreted as a Goldstone excitation.Comment: 7 pages, latex, 2 postscript figure