China’s first X-ray satellite, Insight-HXMT, has made an important contribution to gravitational wave astronomy by setting a strict upper limit on electromagnetic radiation produced during a recently discovered gravitational wave event. 

On Oct 16 2017, the Laser Interferometer Gravitational Wave Observatory (LIGO) and the European Gravitational Observatory Virgo announced the discovery of gravitational wave event GW170817. 

It contrasts with four previous gravitational wave events, which were caused by the merging of black holes; this event is thought to have been created by the merging of two neutron stars. It is widely believed that this kind of gravitational wave event also produces strong electromagnetic (EM) emissions at the time of merger.

An artist’s impression of two stars orbiting each other (left). The system emits gravitational waves as the orbits shrink (middle). When the stars merge (right), there is a resulting powerful emission of gravitational waves. (Image by NASA)

Dozens of observatories joined in a campaign to search for the EM counterpart of this event, searching at frequencies from radio to very high energy gamma-rays. Insight-HXMT, which was launched on June 15 2017, made an important contribution by setting a stringent upper limit on the event’s prompt emission flux in the 0.2-5 MeV range. 

Artist's impression of the Insight-HXMT satellite in space (Image by IHEP)

Only four X-ray and gamma-ray telescopes (Fermi, INTEGRAL, Insight-HXMT, and Konus-Wind) were monitoring the sky region of GW170817 when the event took place. Of these, Insight-HXMT has the largest detection area and best time resolution in the 0.2-5 MeV range. 

About two seconds after the gravitational wave event, a weak short gamma-ray burst (GRB 170817A) independently triggered the Fermi Gamma Burst Monitor (GMB). The GBM localization was broadly consistent with the gravitational wave location given by the LIGO detector. Soon after, this gamma-ray burst was confirmed by offline analysis of data from the INTEGRAL satellite’s SPI-ACS system. 

However, differing from theoretical predictions, the emission of GW170817 is weak and soft, and so the flux in the MeV energy band is extremely low. Neither Insight-HXMT nor any of the other telescopes has detected any significant emission in this energy range. 

Insight-HXMT then set the most stringent MeV flux upper limit, which is important for constraining the overall properties of GW170817. The results of Insight-HXMT’s observations will be published in a paper on the multi-messenger astrophysics of GW170817 to appear in The Astrophysical Journal Letters, and in a separate paper to be published in SCIENCE CHINA Physics, Mechanics & Astronomy. 

According to Einstein’s theory of general relativity, the merging (coalescing) of two compact stars creates ripples in space-time known as gravitational waves. On Sept 14, 2015, LIGO detected the first gravitational event. In honor of this achievement, the leaders of the LIGO experiment, Rainer Weiss, Barry Barish, and Kip Thorne were awarded the 2017 Nobel Prize in Physics.

Source:Institute of Physics,CAS