The 500-meter aperture spherical radio telescope (FAST) sits in the middle of mountains Photo: Xinhua

 
The 500-meter aperture spherical radio telescope (FAST), also known as China's Tianyan or "Sky Eye," has discovered 201 pulsars, including some faintest pulsars, 40 millisecond pulsars, and 16 pulsars in binaries.

The discoveries were published on Thursday in Research in Astronomy and Astrophysics. 

Located in Pingtang county, Southwest China's Guizhou Province, FAST was completed in 2016 and is the world's largest single-aperture radio telescope.

Pulsars are a compact remnant left behind after bright massive stars die. They have the strongest magnetic fields, highest density, and fastest rotation in the universe, and show significant relativistic effect in a system of binary compact stars, the Global Times learned from China's National Astronomical Observatories (NAOC).

A total of about 3,000 pulsars have been uncovered since the first discovery in 1968, among which about 400 have a period of less than 30 milliseconds and are stable in rotation, said Han Jinlin, the leader of the research team from the NAOC of Chinese Academy of Sciences for the discoveries. 

Han's team designed a snapshot survey strategy so that all visible sky near the Milky Way can be observed for five minutes by FAST and a small patch of sky can be fully hunted in 21 minutes. They named this project the Galactic Plane Pulsar Snapshot survey (GPPS).

This is the first sensitive search for weak pulsars down to the microJy level, which means it has been selected as one of the five key science projects for FAST. Such a survey can detect pulsars with a flux density down to five microJy, about a magnitude weaker than the previous survey, said the NAOC. 

About 5 percent of the planed sky has been searched, and 201 pulsars have been uncovered. 

"At this early stage of the project, this is an impressive total," commented Professor R.N. Manchester, from CSIRO Astronomy and Space Science, Australia.

Among the newly discovered pulsars, some have strange properties on the pulse dispersion.

Dispersion is a good indicator for pulsar distance that measures the total electron density along the path from a pulsar. The higher the dispersion measure, the farther the pulsar distance is, said NAOC.

The GPPS survey uncovered pulsars with very high dispersion measure, which challenge the current best models of electron density distribution in the Milky Way, it said. 

According to the best knowledge of electron distribution in the Milky Way, the pulsars are expected to be located outside the Milky Way. However, it is more likely that these pulsars are located in the Milky Way, it said. 

About 40 pulsars found in the GPPS survey have a period of less than 30 milliseconds, which are newly discovered millisecond pulsars. 

"The GPPS survey has already increased the number of known MSPs by nearly 10 percent, a remarkable achievement," Manchester was quoted as saying in a NAOC report. Among them, 14 have a companion around, so do the two long period pulsars. "No doubt some of these will turn out to be excellent probes of gravitational theories".

The GPPS survey discovered many pulsars with special features, some have emission switched on and off, or even just a few pulses over many minutes. 

For many previously known pulsars, the FAST survey received data with extremely high signal to noise ratio, which improved the parameters for 64 pulsars.