Research

Astrophysical Transients

I use radio observations to study a wide variety of astrophysical transients, including tidal disruption events (TDEs) and gamma ray bursts (GRBs). These events probe some of the most extreme environments in the Universe, from the regions surrounding supermassive black holes to the hearts of exploding stars. Recent highlights include:

  • Radio Observations of Outflows in Tidal Disruption Events (Alexander et al. 2016, ApJL 819, L25; Alexander et al. 2017a, ApJ 837, 153) 
    A stray star passing close to a supermassive black hole may be torn apart (tidally disrupted) by the black hole's gravity. Such tidal disruption events (TDEs) light up dormant systems and provide a unique view of the accretion and outflow of matter onto the black hole. Only a few such events have been discovered to date, but the existing data only shed light on accretion and not outflows. We used the Very Large Array radio telescope to obtain the first direct detection of an outflow from a normal tidal disruption event, ASASSN-14li (Alexander et al. 2016). The data allow us to quantify the outflow physical properties, to probe the circumnuclear environment of the black hole before the disruption occurred, and to conclude that such outflows may be commonly launched during TDEs. We are now building on this initial result by systematically observing new and archival TDEs in the radio (Alexander et al. 2017a), to better understand this population of events.

     

  • New Insights on Gamma-Ray Bursts with the VLA (Alexander et al. 2017b, ApJ, 848, 69) 
    Our research group has been awarded a large program to carry out radio observations of gamma-ray bursts (GRBs) using the Very Large Array in Socorro, NM. GRBs are short, intense bursts of gamma rays thought to be produced during two types of extreme events: the coalescence of two neutron stars orbiting each other and the explosions of certain types of massive stars. Our radio observations complement data at other wavelengths, revealing details of the burst physics and the pre-burst environment. Our unprecedented combination of rapid response to triggers, detailed time sampling, and broad frequency coverage has allowed us to explore both effects intrinsic to the burst and propagation effects that distort the radio emission from the GRB afterglow as it propagates through the Galactic interstellar medium. So far, we have reported the discovery of reverse shocks in GRB 160509A (Laskar et al. 2016) and GRB 160625B (Alexander et al. 2017b) and unusual late-time variability in GRB 160625B that is likely due to an extreme scattering event (Alexander et al. 2017b). Additional results are in prep.

     

  • Radio emission from the first binary neutron star merger detected in gravitational waves (Alexander et al. 2017c, ApJL, 848, 21) 
    On August 17, 2017 Advanced LIGO/Virgo detected GW170817, a gravitational wave (GW) event consistent with the merger of two neutron stars at a distance of ~40 Mpc. Binary neutron star mergers have been linked to a wide array of electromagnetic counterparts spanning the EM spectrum, most notably short gamma-ray bursts (SGRBs) and optical/NIR kilonova emission (e.g. Metzger & Berger 2012). SGRBs are beamed, making radio emission the best way to probe the most relativistic ejecta from off-axis mergers (Nakar & Piran 2011). After our identification of GW170817's kilonova with DECam (Soares-Santos et al. 2017), we triggered extensive multiwavelength follow-up with Chandra, HST, the VLA, ALMA, and ground-based optical and near-IR facilities. I led our group's effort to identify radio counterparts to Advanced LIGO/Virgo gravitational wave sources with the VLA and ALMA (Alexander et al. 2017c). We detected X-ray and radio emission consistent with either an off-axis relativistic jet observed at a viewing angle of ~20°-40° (Margutti et al 2017; Alexander et al. 2017c) or a cocoon of mildly relativistic material (as explored in Hallinan et al. 2017 and Mooley et al. 2017). We are continuing to monitor this source with the VLA (PI: Alexander) and our observations will distinguish between these models within months, providing the first direct constraints on the lateral structure of SGRB jets. We also predict a radio rebrightening of the source on timescales of years as the non-relativistic ejecta producing the kilonova emission slowly decelerates. This emission may remain detectable with next-generation radio facilities for decades, making GW170817 a prime target for long-term radio monitoring.


CMB Likelihood Analysis

For my first two and a half years of graduate school, I worked with John Kovac and the BICEP/Keck Array collaboration to study the polarization of the cosmic microwave background (CMB). For my Research Exam/Master's project, I implemented new multicomponent separation techniques for joint analysis of CMB datasets, with a focus on the BICEP/Keck Array series of experiments and Planck. These new techniques allowed us to model an observed polarization signal as a sum of contributions from galactic dust, galactic synchrotron, gravitational lensing by galaxy clusters, and (possibly) primordial gravitational waves. I helped develop new code to carry out a full multicomponent likelihood analysis and I created simulations to extensively validate this likelihood code. I also traveled to South Pole, Antarctica to install new 220 GHz focal planes on the Keck Array. For further details, please see:

  • The full list of BICEP/Keck publications and data products, especially the BICEP2/Keck Array/Planck joint analysis paper, which uses the results of the multicomponent likelihood analysis that I helped to develop and test.
  • My personal blog from my trip to Antarctica, for an overview of our science and a slice of daily life at the South Pole
  • A public talk on cosmology that I gave with two other graduate students, which places these results in the context of our broader quest to understand the origin and large-scale evolution of the Universe.

 

Undergraduate Projects
As an undergraduate, I pursued three separate astronomical research projects:

  • Quantifying the effects of noise peaks on two-point correlation functions in ground-based lensing data 
    I developed a new technique to measure the shear peak angular correlation function in existing deep imaging surveys, with applications for future survey data.

    Senior Thesis, Brown University, completed Spring 2012. Adviser: Ian Dell'Antonio.
    Won the Charles H. Smiley Prize for Excellent Contribution to the Astronomy Program.
     
  • A New Model for the Radio Emission from SN 1994I and an Associated Search for Radio Transients in M51 
    I conducted a search for radio transients in the nearby galaxy M51 using archival VLA observations of supernova 1994I and developed a new model for the supernova radio emission.
    Astronomy REU, Harvard-Smithsonian Center for Astrophysics, Summer 2011. Adviser: Alicia Soderberg.
    First-author publication in ApJ, AAS Poster (Winter 2012).
     
  • Stellar populations in fossil group galaxies
    I characterized the stellar populations of seven fossil galaxy groups using optical spectra obtained with the WIYN 3.5m telescope to learn more about their star formation histories and understand the origin of any observed AGN activity.

    Astronomy REU, University of Wisconsin-Madison, Summer 2010. Adviser: Eric Wilcots.
    AAS Poster (Winter 2011), Project website.