Amy Lien
NASA Goddard Space Flight Center
University of Maryland, Baltimore County
Code 661, Swift/BAT team
Office: Building 34, S225
Email: amy.y.lien at nasa.gov

Most of my recent work focuses on topics related to gamma-ray bursts. Here are some brief description about the gamma-ray burst study that I am involved in.



What are gamma-ray bursts?

Gamma-ray bursts are one of the most energetic explosions in the universe. When a gamma-ray burst occurs, it is often the brightest source in the entire gamma-ray sky. Due to the extreme brightness, gamma-ray bursts are detected in a wide range of distance, from our local universe to the early universe. In fact, gamma-ray bursts are one of the very few events that can be seen directly out to the era when the first star was expected to form. Gamma-ray bursts are thus powerful tools to study the environment in the early universe, and how the universe has evolved to its current stage.

Gamma-ray bursts were first discovered in the 1960s. Since then, our knowledge of these events has greatly advanced thanks to previous studies of both theoretical modelling and space and ground observations. Nowadays, gamma-ray bursts are usually classified into two groups, short and long, based on their burst durations, with the separation of about two seconds. Both the theoretical and observational evidences suggest that long gamma-ray bursts are originated from the collapse of massive stars, and thus are related to supernovae, while the short gamma-ray bursts are from the mergers of two neutron stars, or a neutron star and a black hole, and therefore also produce gravitational waves.

Gamma-ray bursts in the era of multi-messenger astronomy

Although gamma-ray bursts was originally detected in the gamma-ray wavelength, now we know that the emission actually spans a wide range of spectrum. While the gamma-ray emission usually only lasts for a few seconds to a few minutes, emission in the lower energy range (x-rays, UV, optical, and radio) can last for a much longer time (from days to years). In addition, gamma-ray bursts are known sources of gravitational waves, and potential sources of neutrinos and cosmic rays. Therefore, to gather a complete set of gamma-ray burst data requires covering not only photons from the entire electromagnetic spectrum, but also nutrinos, cosmic rays, and gravitational waves (the so-called "multi-messenger astronomy").

The Neil Gehrels Swift Observatory
To advance our knowledge of gamma-ray bursts through prompt observations in the x-rays, UV, and optical wavelengths, NASA launched the Neil Gehrels Swift Observatory in Nov. 20, 2004. The mission is often referred to as Swift, and has an international partnership from the United States, United Kindom, and Italy. Swift has three instruments onboard: the Burst Alert Telescope (BAT) that detects gamma-ray bursts in the gamma-ray/hard x-ray range (15-350 keV), the X-Ray Telescopes (XRT) that collects photons in the soft x-ray range (0.2-10 keV), and the UV/Optical Telescope (UVOT) that observes in the UV and optical wavelengths (170-650 nm).

Gamma-ray bursts from the Burst Alert Telescope (BAT) onboard Swift
The main instrument that I work with is The Burst Alert Telescope (BAT) onboard Swift. BAT can see a large portion (~1/6) of the sky of the sky at any time, and is responsible for finding gamma-ray bursts. For the past ~ 15 years, BAT has detected more than 1300 Gamma-ray Bursts (GRBs), of which about 1/3 of GRBs have distance measurements from the ground observatories, ranging from our local universe to the early universe when the first star was expected to form. We present the analyses of the BAT-detected GRBs in the third Swift/BAT GRB catalog. The result summaries and data products are available at the public website: The Swift/BAT GRB Catalog, which is continued to be updated with recent bursts.
Chasing gravitational wave counterparts in the Burst Alert Telescope onboard Swift
As the first gravitational waves signal was detected in 2015 by the the Laser Interferometer Gravitational-Wave Observatory (LIGO), a new window has been opened for physicists and astronomers to understand the universe. In addition, the first short gamma-ray bursts (GRB170817A) associated with gravitational waves detection (GW170817) was found in Aug. 17, 2017 by the Fermi gamma-ray telescope, confirming the long-lasting hypothesis that (at least some) short gamma-ray bursts are indeed originated from merging of two neutron stars.

Since 2015, Swift has been actively participated in the counterpart search and followup observations for gravitational waves detections. For the GRB170817A/GW170817 event, Swift was behind the Earth at the detection time and thus BAT could not see the event. However, XRT and UVOT participate in the followup observations and UVOT clearly detected the associated kilonova signal in the UV and optical wavelengths (Evans et al. 2017).

My work focuses on searching for counterparts in the Burst Alert Telescope (BAT) onboard Swift. For each gravitational waves detection, we search for potential astrophysical events in the BAT data around the LIGO/VIRGO detection time. Our search results are publicaly available on the the BAT gravitational waves summary page and also shared with the astronomy community through email notices via the Gamma-ray Coordinates Network (GCN).

Probing the early universe with long gamma-ray bursts
Due to the extreme luminosities of gamma-ray bursts, they are one of the very few astrophysical events that can be detected directly out to the early universe. Therefore, long gamma-ray bursts provide important insight of star-formation history in the early universe. We use the BAT-detected gamma-ray bursts and careful simulations of the BAT detection algorithm to estimate the gamma-ray burst rate out to the early universe. The result are presented in Lien et al. (2014).
Connectiong theoretical models to observations: the Swift/BAT trigger simulator
In order to maximize the number of gamma-ray burst detections, the Swift Burst Alert Telescope (BAT) adopts a complex algorithm for finding gamma-ray bursts, which includes over 600 search criteria. Aothough the method increased the chance of successfully finding gamma-ray bursts, it also introduce unknown and hard-to-quantified observational selection effect. To investigate this selection effect and explore the intrinsic gamma-ray burst properties, we develop a script that is capable of simulating the BAT trigger algorithm. We have used this "trigger simulator" to estimate the long gamma-ray burst rate at high redshift and explore their connection with the cosmic star-formation history. We also use the trigger simulator to examine the detectability of other transients, such as short gamma-ray bursts, theoretical gamma-ray bursts from the early universe, and the gravitational wave event GW170817.
Finding gamma-ray bursts in future space missions
In addition to the current space telescopes, scientists continue to proposed new ideas for future missions. I am involved in several potential future space telescopes that includes gamma-ray burst science and searching for gravitational wave counterparts. These missions includes the Transient Astrophysics Observatory (TAO), the Transient Astrophysics Probe (TAP), the All-sky Medium Energy Gamma-ray Observatory (AMEGO), BurstCube, the Gamow explorer (exploring the early universe), and High Resolution Energetic X-ray Imager SmallSat Pathfinder (HSP). Most of my participation focuses on simulations for gamma-ray burst detections for these future missions.