Mysterious Gamma-Ray Burst
(Long) Gamma-Ray Burst (GRB) is the most energetic explosion in the Universe. While the Sun releases vast energy during its long lifetime, 10 billion years, GRBs release the equivalent energy in only several seconds. The origin is a mystery in astronomy.
Imaginary picture of GRB
(NASA / SkyWorks Digital)。
I identified a GRB host galaxy, i.e., a galaxy in which GRB occurred, with Japanese Subaru Telescope (Right figure). After my detailed follow-up observations, we found that the metallicity, amount of elements heavier than Helium, is much higher than a metallicity predicted by standard theoretical models of GRBs. The standard models predict that a massive star explodes as a supernova and relativistic jets are generated in the central region of the supernova. When the jet is beamed toward us, it is observed as a GRB. Theoretically, low-metallicity environment is necessary for massive stars to launch the relativistic jets. Why does GRB happen in high-metallicity environment? Our new finding provided us a new mystery of GRB.
The GRB host galaxy and the afterglow detected with Subaru Telescope. (a) About 9 hours after the burst. (b) About 34 hours after the burst. (c) the afterglow, i.e., (a)-(c).
Iron abundances of GRB host galaxies
To reveal the mystery we opened a new frontier of GRB hosts, i.e., composition of Oxygen and Iron.
In previous studies Oxygen has been used as an indicator of metallicity. No ones payed attention to Iron abundance very much. This is because the emission line from Iron is very weak compared with that of Oxygen and the detection is not easy observationaly. The largest optical telescopes in the world including Subaru and Very Large Telescope (VLT) allowed us to address this project (Figure below). I, for the first time, found that iron abundances of GRB host galaxies are quite low regardless of Oxygen abundances. Stellar evolution models predict that a massive star can not maintain an enough rotational speed to launch relativistic jets when the Iiron abundance is high. We, for the first time, revealed that the traditionally utilized Oxygen abundance is not a good indicator but only supernovae that has low iron abundances can explode as GRBs.
The spectrum of a neaby GRB host galaxy obtained by the Vey Large Telescope of European Southern Observatory. The Iron emission line is marked by a blue arrow.
This project is based on the following publications.
Tetsuya Hashimoto, Ravi Chaudhary, Kouji Ohta, Tomotsugu Goto, Francois Hammer, Albert K. H. Kong, Ken'ichi Nomoto, and Jirong Mao, `Why Are Some Gamma-Ray Bursts Hosted by Oxygen-rich Galaxies?’, The Astrophysical Journal, Volume 863:95 (14pp), (2018) August 10.
Hashimoto, Tetsuya; Perley, Daniel A.; Ohta, Kouji; Aoki, Kentaro; Tanaka, Ichi; Niino, Yuu; Yabe, Kiyoto; Kawai, Nobuyuki , `THE STAR FORMATION RATE AND METALLICITY OF THE HOST GALAXY OF THE DARK GRB 080325 AT z = 1.78’, The Astrophysical Journal, 806:250 (9pp), (2015) June 20.
T. Hashimoto, K. Ohta, K. Aoki, I. Tanaka, K. Yabe, N. Kawai, W. Aoki, H. Furusawa, T. Hattori, M. Iye, K. S. Kawabata, N. Kobayashi, Y. Komiyama, G. Kosugi, Y. Minowa, Y. Mizumoto, Y. Niino, K. Nomoto, J. Noumaru, R. Ogasawara, T.-S. Pyo, T. Sakamoto, K. Sekiguchi, Y. Shirasaki, M. Suzuki, A. Tajitsu, T. Takata, T. Tamagawa, H. Terada, T. Totani, J. Watanabe, T. Yamada and A. Yoshida, ```Dark’’ GRB 080325 in a Dusty Massive Galaxy at z ~ 2’, The Astrophysical Journal, 719:378–384, (2010) August 10.