In the recent years, few gamma-ray bursts have been observed with extremelylong duration, over 1,000 s or 10,000 s, called ultra-long GRB. Among them, GRB111209A at redshift z = 0:677 has the longest with an observer frame gamma-rayprompt emission duration 7 hr. We observed early NIR spectrum of this ULGRBusing NASA;;s 3m IRTF. It is the only spectrum data that shows early afterglow inX-ray to Radio range. We use TAROT R-band data in addition to examining theux calibration. The NIR spectra shows synchrotron radiation with= 1:220:03with electron power-law energy distribution index p 2:4. The thermal componentwas too weak so we cannot gure out any clue about it. To see whether ULGRBscan have a common origin as short/long GRB, we compare the spectrum of thisGRB with another ULGRB, GRB 101225A so called ;;the Christmas burst;;. Incontrast with GRB 111209A, GRB 101225A shows an early evolution of strongblackbody component. These mutually exclusive properties make the two ULGRBbe considered with dierent progenitor candidates: core-collapse of a low metallicityblue supergiant for GRB 111209A and merger of helium star with a neutron starthat underwent a common envelope phase for GRB 101225A. We also examine thevalidity 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 foradopting magnetar model using synchrotron radiation theory.
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NIR Spectroscopy of Early Afterglow of Ultra-Long Gamma-Ray Burst 111209A
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