O: 301-286-4678

F: 301-286-0677

Goddard Space Flight Center

Mail Code 661

Greenbelt, MD 20771

The Swift Gamma-Ray Burst Explorer

For their short durations, gamma-ray bursts (GRBs) are the most luminous explosions since the Big Bang. They are able to accelerate ejecta to velocities extremely close to the speed of light, making them useful probes of physical regimes that are impossible to replicate in our laboratories. And because of their powerful, long-lived afterglow emission, we can use GRBs as probes of distant galaxies, back when the universe was only in its infant stages.

As Deputy Project Scientist for the Swift mission, my work is focused on:

- Constraining the energy release from these explosions. Because of a large degree of collimation (narrow jets typically ~ 10 degrees wide), this geometric correction is vital to understanding the progenitor system(s) and explosion mechanism.

- Using multi-wavelength photometry and spectroscopy to study the environments of GRBs. Afterglow observations sample the immediate (< pc scale) circumstellar environment and hence the progenitor systems. Optical (and NIR) spectroscopy illuminate material along the line of sight from the interstellar and intergalactic medium.

- Optically dark GRBs are useful tools to study dust in distant galaxies, and may ultimately serve as probes of the epoch of reionization (when the neutral H in the universe was photo-ionized by the first stars and/or galaxies). New and upcoming NIR facilities such as RATIR and RIMAS will greatly facilitate such work.

- Identifying the progenitors of short-duration GRBs, and using them to prepare for the first detection of gravitational wave emission from the Advanced LIGO/VIRGO detectors.

The Palomar Transient Factory (PTF)

The Palomar Transient Factory (PTF) is a wide-field (7.2 deg2), high-cadence survey of the optical sky designed with two primary goals: 1) To systematically study known classes of transient and/or variable sources (e.g., Type Ia supernovae, RR Lyra stars), and 2) To uncover new classes of transient sources by probing unexplored regimes of phase space (sensitivity and time scale). Since operations began in earnest in 2009, PTF has discovered over 2000 spectroscopically confirmed supernovae, and observed over 20,000 deg2. My areas of interest within the project include:

Tidal Disruption Flares (TDFs): A tidal disruption flare results when a main sequence star wanders too close to the super-massive black hole present in the center of all bulge galaxies; tidal forces overcome the stars self-gravity, and the stellar debris form a relatively long-lived accretion disk feeding the black hole. We hope that in the long term TDFs may provide a unique probe of the central black hole mass in distant, otherwise quiescent galaxies, thereby helping us to better understand the process of black growth and evolution. As a wide-field, blind survey, PTF offers a unique opportunity to discover such events in the local universe, where detailed studies can help us better understand the physical processes involved in these systems. We have already uncovered one particularly intriguing candidate, a luminous short-lived flare from the nucleus of a star-forming galaxy (more info here).

More recently, I have helped to identify a new class of high-energy outbursts that appear also to result from the tidal disruption process. Most interestingly, we have compelling evidence that the long-lived, luminous X-ray and radio emission from these sources is associated with the birth of a relativistic jet. Understanding the connection between these relativistic tidal disruption flares (are they the same phenomenon viewed from different angles, or is there some particular ingredient required to generate a relativistic jet?), and what they might tell us about the central supermassive black holes in these otherwise quiescent galaxies, has now become an area of intense research. You can read more about these fascinating sources here, here, and here.

Orphan GRB Afterglows: A GRB-like outflow may lack associated high-energy emission for several reasons, including geometry (i.e., the explosion is viewed outside the narrow cone of the jet) and lack of sufficient expansion velocity (i.e., if the ejecta were loaded with a sufficient baryon fraction). Such orphan afterglows have long been posited to exist, but we now may have evidence for the first such detection from PTF (PTF11agg). Unless we have been fortunate, PTF11agg may represent a new class of relativistic outbursts lacking in high energy emission entirely (a dirty fireball). We are currently conducting a high-cadence experiment with PTF to search for an additional examples, as understanding the nature of these sources will be vital to prepare for the era of the Large Synoptic Survey Telescope.

Supernovae: As a member of the Transients in the Local Universe / Core-Collapse Supernova project within PTF, I have been actively exploring the end states of massive (M > 8 Msun) stars through their violent deaths. My primary interests include:

- Understanding the nature of the mysterious super-luminous supernovae (particularly those lacking clear evidence for interaction with a dense circumstellar environment).

- Using prompt observations of the shock breakout phase to constrain fundamental properties of the progenitor star (i.e., radius).

- High-resolution spectroscopic studies of type Ia supernovae, which probe the immediate (sub pc-scale) environment of these outbursts, and ultimately may yield valuable diagnostics of their progenitor system(s).


Photo credit: NASA