NIR Spectroscopy of Early Afterglow of Ultra-Long Gamma-Ray Burst 111209A

Abstract

In the recent years, few gamma-ray bursts have been observed with extremely long duration, over 1,000 s or 10,000 s, called ultra-long GRB. Among them, GRB 111209A at redshift z = 0:677 has the longest with an observer frame gamma-ray prompt emission duration 7 hr. We observed early NIR spectrum of this ULGRB using NASA's 3m IRTF. It is the only spectrum data that shows early afterglow in X-ray to Radio range. We use TAROT R-band data in addition to examining the

ux calibration. The NIR spectra shows synchrotron radiation with = 1:220:03 with electron power-law energy distribution index p 2:4. The thermal component was too weak so we cannot gure out any clue about it. To see whether ULGRBs can have a common origin as short/long GRB, we compare the spectrum of this GRB with another ULGRB, GRB 101225A so called "the Christmas burst". In contrast with GRB 111209A, GRB 101225A shows an early evolution of strong blackbody component. These mutually exclusive properties make the two ULGRB be considered with dierent progenitor candidates: core-collapse of a low metallicity blue supergiant for GRB 111209A and merger of helium star with a neutron star that underwent a common envelope phase for GRB 101225A. We also examine the validity of external shock model with two characteristic frequencies: c and m. Theoretically, it was possible to satisfy the observation using external shock model, however, it is not preferable for the real situation. Instead, we see the possibility for adopting magnetar model using synchrotron radiation theory.