Research

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, L25Alexander 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) and mm (ALMA, PI: Alexander), to better understand this population of events.
  • New Insights on Gamma-Ray Bursts with the VLA (Alexander et al. 2017b, ApJ, 848, 69; Alexander et al. 2019, ApJ, 870, 67)
    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). We also characterize strong scattering behavior in the afterglow of GRB 161219B in unprecedented detail (Alexander et al. 2019). 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; Alexander et al. 2018, ApJL, 863, 18)
    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; Alexander et al 2018). We detected X-ray and radio emission consistent with an off-axis relativistic jet observed at a viewing angle of ~25°, 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(Alexander et al. 2017c). This emission may remain detectable with next-generation radio facilities for decades, making GW170817 a prime target for long-term radio monitoring.