Peking University, Jan. 17, 2016: Dong Subo, astronomer and a Thousand Youth Talent Plan research professor at The Kavli Institute for Astronomy and Astrophysics at Peking University, led an international research team which searched for supernovae in Chile back in the summer of 2015. What they observed  was stunning – a supernova explosion which is two times more powerful than the previous record holder. The result was published in Science on Jan. 15, and Dong Subo was the corresponding author.

The supernova was discovered in Chile by the All Sky Automated Survey for SuperNovae (ASASSN), of which Dong Subo is a member. The star, named ASASSN-15lh, is a rare breed – it is classified as a "superluminous supernova”. It is 3.8 billion light years away from the Earth – among the closest supernovae ever observed. During the supernova explosion, luminosity of the star reached 570 billion times the luminosity of the sun, and is approximately 20 times brighter than the Milky Way combined. According to Dong, “ASASSN-15lh is the most powerful supernova discovered in human history. The explosion’s mechanism and power source remain shrouded in mystery because all known theories meet serious challenges in explaining the immense amount of energy ASASSN-15lh has radiated.”
 

The discovery of the ASASSN-15lh and its explosion triggered widespread interest among astronomers around the world. Telescopes including NASA’s “Swift” started to follow this rare event. To this day, researchers are still observing the supernova with optic, X-ray and radio wave bands.

The first spectral line of the ASASSN-15lh was observed by B. J. Shappee of Carnegie Observatories in Chile. Astronomers generally use spectral lines to analyze projectiles from supernova explosions to determine their chemical components and physical conditions. This analysis in turn helps astronomers classify supernovae and understand the physical process of their explosions. However, the spectral lines of ASASSN-15lh was significantly different from all the other supernovae discovered by ASASSN. This initially perplexed many astronomers.

While discussing the issue with Universidad Diego Portales Professor Jose Prieto and The Ohio State University Professor K. Z. Stanek, Dong Subo suddenly realized that ASASSN-15lh could be a superluminous supernova. According to his estimations, if ASASSN-15lh was indeed 3.8 billion light years away from us, then its most significant spectral characteristics should be very similar to the spectrum of a superluminous supernova discovered in 2010. They immediately contacted various other grander telescope, but due to bad weather and instrumental malfunctions, the required observations were delayed. In the end, SaurabhJha of Rutgers University verified that ASASSN-15lh is indeed a superluminous supernova using the 10 meter Southern African Large Telescope (SALT). Dong was overjoyed when he realized that they have discovered the most powerful supernova explosion in history when he heard the news from his colleague at SALT.

Multiple hypotheses have been suggested to explain the mechanism of an explosion of such a high level of energy. Dong’s research team at the School of Physics at Peking University plans to utilize more advanced equipment, such as the Hubble Space Telescope, to reveal the secrets behind ASASSN-15lh.

"ASASSN-15lh may lead to new thinking and new observations of the whole class of superluminous supernova, and we look forward to plenty more of both in the years ahead,” said Dong.

Written by: Xu Liangdi
Edited by: Zhang Jiang
Source: http://pkunews.pku.edu.cn/xxfz/2016-01/13/content_292629.htm