THCA AST3-2 telescope observes GW170817's optical counterpart
On August 17, 2017, the now famous LIGO gravitational wave detector and VIRGO, the italo-french detector simultaneously observed a gravitational wave signal (GW170817). The joint detection allowed a more precise positioning of the event, located at about 130 million light years in a 31 square degrees area. This precision, which was not possible with LIGO-only observations allowed more than 70 observatories and telescopes in the world to point to the position in a most exciting and beautifully orchestrated series of multi-band follow-up observations (see Figure 1) of the electromagnetic signals associated to a gravitational event. This is the first ever observed neutron star collision, and it is in both gravitational waves and its electromagnetic counterparts (from high energy gamma-rays, X-rays, optical, and radio wavelengths) and will be certainly remembered as the start of gravitational waves astronomy.
Previous gravitational waves observed by LIGO produced “chirps” lasting a fraction of a second, but the Aug. 17 chirp lasted approximately 100 seconds. The chirp source was immediately considered to be much less massive than the black holes seen to date, and it was likely to be from neutron stars.
The gravitational wave event and associated electromagnetic observations provide a rich spectrum of results with tests of fundamental physics, models of neutron star collisions, and nuclear astrophysics.
When neutron stars collide, they give off gravitational waves and gamma rays, along with powerful jets that emit light across the electromagnetic spectrum. The gamma-ray burst detected by Fermi and INTEGRAL, is a short gamma-ray burst, confirming that some short gamma-ray bursts are generated by the merging of neutron. Furthermore, the observed flash of gamma rays indicates that the collision is not due to black holes (black hole collisions are not expected to give light). Although the gamma ray bust is one of the closest to Earth that has been observed so far, the signal in gamma was rather weak. It is possible that the gamma rays flash is off axis, but more observations will be necessary to confirm this explanation and new insights are already being developed. The material that is left over from the neutron star collision/merger is blown out far out into space, and its glow is detected in optical telescopes. This post-merger phenomenon is called a “kilonova”. The many optical telescopes follow-up provide light curves that have been compared and are are confirming some kilonovae models. In addition, spectroscopic observations by eg, the U.S. Gemini Observatory, the European Very Large Telescope, and the Hubble Space Telescope reveal signatures of synthesized material, including lead, gold and platinum, solving a long lasting mystery of where about half of all elements heavier than iron are produced.
The LIGO Scientific Collaboration research group at Tsinghua University is led by Prof. Junwei Cao. The group is mainly focused on gravitational wave data analysis using advanced computing technologies. Tsinghua Center for Astrophysics contributed to the construction of the Antarctica AST3-2 telescopes with a grant from Tsinghua University to Prof. Charling Tao. Wang Xiaofeng, Charling Tao and their teams were lucky to take part in this beautiful symphony of observations and analysis:
- AST3-2 provided several i-band measurements one day after the GW170817 detection in the NGC4993 Galaxy, where the two neutron stars collided (cf Figure 1 upper right corner).
- Prof. Wang Xiaofeng and his team also participate to the MASTER consortium which provided the first non-american observation with optical band, 11.3 hours after (cf lower left corner of Figure 1).
Telescopes around the world continue and will continue to observe the afterglow of the neutron star merger and gather further evidence about various stages of the merger, its interaction with its surroundings, and the processes that produce the heaviest elements in the universe.
We are also prepared in THCA to follow future events and understand the associated physics in this new era of gravitational wave - multi-wavelength astronomy.
Some references :
1. http://iopscience.iop.org/article/10.3847/2041-8213/aa91c9/pdf
Multi-messenger Observations of a Binary Neutron Star Merger (2017, ApJ, 848, L12)
2. http://doi.org/10.1016/j.scib.2017.10.006
Optical Observations of LIGO Source GW 170817 by the Antarctic Survey Telescopes at Dome A, Antarctica (2017, Science Bulletin, accepted)
3. http://lanl.arxiv.org/abs/1710.05846
Follow up of GW170817 and its electromagnetic counterpart by Australian-led observing programs (2017, PASA, accepted)
4. http://lanl.arxiv.org/abs/1710.05461
MASTER optical detection of the first LIGO/Virgo neutron stars merging GW170817 (2017, ApJ Letter, accepted)