A staff worker of the Purple Mountain Observatory displays an image at a press conference at the observatory in Nanjing, east China's Jiangsu Province, Oct. 16, 2017. Chinese scientists on Monday announced observation of the "optical counterpart" of gravitational waves coming from the merger of two binary neutron stars using a survey telescope in Antarctica. The gravitational waves were first discovered by the U.S.-based Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors on Aug. 17. The Chinese telescope independently observed optical signals resulting from the merger the next day, according to the Chinese Center for Antarctic Astronomy. It was the first time humans have detected gravitational waves and the corresponding electromagnetic phenomena resulting from a binary neutron star merger. (Xinhua/Li Xiang)
by Xinhua writer Guo Shuang
LOS ANGELES, Oct. 17 (Xinhua) -- Chinese researchers have played a significant role in opening a new era of "Multimessenger astronomy," marked by the first detection of both electromagnetic radiation and gravitational waves and light in the same cosmic collision.
The discovery is also the first verification of a "kilonova" explosion, confirming binary neutron star collisions as one source for the universe's heaviest elements, such as gold and uranium.
The story goes back 60 years to the groundbreaking stellar nucleosynthesis paper published in 1957, which offered a successful model of element formation. For decades, however, no one could identify the site of the process and the source of the neutrons.
In the end of the 20th century, astrophysicists were chatting about mergers of neutron stars, which eject a small fraction of matter with a subrelativistic velocity.
Until 20 years ago, a young Chinese called Xin Li and the late pioneering astrophysicist Bohdan Paczynski first presented the model of mergers of neutron stars, and analytical formulae for the associated electromagnetic radiation in the highly cited paper Transient events from neutron star mergers, which was published in 1998.
"Chinese researchers did a remarkable job in the wonderful discovery...Chinese researcher Li has made the first theory prediction," said Suijian Xue, professor and deputy director of the National Astronomical Observatories of China (NAOC) under Chinese Academy of Sciences.
"I am delighted. It is the critical next step to explore in detail the physics involved," Li said after being announced to have for the first time detected the ripples in space and time known as gravitational waves and light from a spectacular collision of two neutron stars.
"However, there is now much more work to be done. We still need more detection like this to prove the model," Li added.
In 2015, detectors from the U.S.-based Laser Interferometer Gravitational-Wave Observatory (LIGO) confirmed the existence of gravitational waves produced during the merger of two black holes, predicted by Albert Einstein's theory of general relativity 100 years ago.
The detection marked the beginning of a new era in observational astronomy, yet no neutron stars had ever been known to have merged until this August.
GLITTERING PROSPECTS AT CHINA'S TELESCOPE
On Aug. 17, the gravitational waves signal, GW170817, was detected by LIGO's twin detectors, respectively located in Livingston, Louisiana, and Hanford, Washington.
The fifth detected source as it was, the source of gravitational waves was historical since a glowing aftermath of the collision of two neutron stars was visible by traditional telescopes.
The Chinese telescope independently observed optical signals resulting from the merger the next day, among some 70 telescopes on the ground or from space across the world.
"While many observatories contributed to the broad set of companion observations, the Chinese observatory, by having half a year of night per year is ideally suited to this challenging task," Gary Sanders, deputy director and project manager of LIGO in 1994-2004, told Xinhua.
Chinese team then conducted immediate follow-up on the gravitational waves source GW170817 in the nearby galaxy NGC4993 all the way from Dome A in Antarctica, using a fully-robotic telescope called AST3.
"Thanks to the optical detection of GW170817, reported from the AST3-2 telescope and at other southern telescopes, a day after its detection by LIGO, scientists are now closer to answering that question with a fascinating story of binary stellar evolution," Australian astronomer Jeremy Mould said in an article Glittering prospects at Dome A.
The electromagnetic afterglow decays very quickly. "Most observatories have to wait for the next night to observe the decaying light signal. Only the global Las Cumbres network and the Chinese Antarctic observatories are in a favorable position to quickly seek the afterglow signals," said Sanders, project manager of Thirty Meter Telescope Project (TMT).
"This bold Chinese observatory was able to catch the signal quickly and even measure the amount of highly excited matter ejected quickly from the coalescence," Sanders told Xinhua, adding that the exciting accomplishment shows that "the gamble to locate in Antarctica has paid off handsomely."
MORE PRIZES AHEAD
"If I were to name the new era, I would call it the era of the dark universe. That is why, when China announces (hopefully sometime later this year) the Kunlun Dark Universe Survey Telescope, it will be such a pioneering step," Mould said.
China built its first Antarctic expedition station -- Kunlun Station -- in 2009, about 7.3 km from Dome A, and has sent astronomers to the region every year since 2007 to build an astronomical observatory on Dome A, the highest location in Antarctica, about 4,093 meters above sea level.
The Antarctic Survey Telescopes (AST3), located at Dome A, Antarctica, is uniquely situated for rapid response time-domain astronomy with its continuous night-time coverage during the austral winter.
Installed at Kunlun Station in 2015 and upgraded this May, the second Chinese Antarctic Survey Telescope, AST3-2, is the largest visible telescope in Antarctic, operating in a fully automatic control mode to accomplish observation toward different scientific targets.
There are only two observatories on the Antarctic plateau, Dome A (China) and Dome C (Europe). "When KDUST and DATE5 are built by China, Kunlun Station will be the world's most powerful remote-controlled observatory, other than those in Space," said Mould.
The cosmic collision of the two neutron stars set up ripples in spacetime that raced outward at the speed of light. The glowing aftermath of the event, rich in heavy radioactive elements, is similar to a supernova but on a smaller scale, thus is called a kilonova.
The brightness and time evolution of the optical transient associated with GW170817 are broadly consistent with the predictions of models involving merging binary neutron stars, according to a paper published Monday in Science Bulletin on AST3 observations of GW170817.
"I agree with Li, more of these, presumably in the year ahead, will help develop the model," said Mould, "Beyond stellar scales, there are more glittering prizes for gravity wave astronomy ahead."