Modeling GRB 091018A: Spectral Evolution and Evidence of A Superwind Progenitor (2012)

Undergraduates: Apurva Oza, Adam Trotter

Faculty Advisor: Daniel Reichart
Department: Physics & Astronomy

---

We model the GRB afterglow 091018A (redshift z= 0.971) using Galapagos, a versatile software employing genetic algorithms to output the best fit solution given our circumburst GRB parameters. Our data is mostly collected across 4 bands (BVRI) from PROMPT (Panchromatic Robotic Optical Monitoring and Polarimetry Telescopes) approximately 4.1 hours after the trigger, where we have succinct evidence of bumps either due to continuous energy injection or the addition of circumburst density discontinuties, in the jet. Additionally, we have added optical, ultraviolet, near-infrared and X-ray data with the help of ANIR, ANDICAM, UVOT, and Swift-XRT instruments. We have settled on a baseline fit that has constrained the circumburst density index k, to be -2.40 ; k values of -2 imply a wind interaction death of a massive star, similar to a Wolf-Rayet star’s. We present this particular GRB as a Superwind, suggesting a faster wind-driven circumburst medium. The constant interstellar medium (ISM), k=0 suggested by Wiersema et al. 2012, was ruled out with a 10.17 sigma confidence difference. We find strong evidence of absorption by neutral atomic hydrogen and based off of our extinction parameters indication of fragmented, gray dust implying a young star forming region. Additionally, we detect a rare early time spectral inversion due to the synchrotron peak crossing the cooling break in the UVOT bands. The crossover yields physical circumburst and jet information at early times. Our model extends to the host galaxy, observed by the Southern Astrophysical Research (SOAR) 4.1 m Telescope. With the unique linking capability in Galapagos, we will further our analysis to test variance in our k value across time slices, to discuss a non-constant density mediums in GRBs.