Peering into the Milky Way by FAST

The Five-hundred-meter Aperture Spherical radio Telescope (FAST) has the largest collecting area for radio waves, with an illumination area of 300m in diatemter and the aperture efficiency of about 60%. It has an excellent performance on pointing, with an uncertainty of less than 8 arcseconds in general (see Jiang et al 2020), and can track a source for a few hours with its full gain in a zenith angle of less than about 26.5 degree. The tracking observations can further improve the sensitivity with integration time. Together with the mounted 19-beam L-band (1000 - 1500MHz) receivers which have a system temperature of about 20 K at the primary focus, FAST is currently the most sensitive radio telescope at the L-band over the world.

As one of the five FAST key science projects approved by the FAST science committee at the end of 2019, the Galactic Plane Pulsar Snapshot (GPPS) survey formally started observations in 2020 February. The goal of the GPPS survey is to make the most sensitive pulsar survey on the Galactic plane in |b| < 10 degree of the Galactic latitudes, with the highest priority given to the area within |b| < 5 degree. The snapshot observation mode was specially designed for the FAST GPPS survey, in which a cover can be fully observed with 4 nearby pointings with 3 quick beam-switchings. The GPPS survey is very successful, discovering so many pulsars, as seen in this webpage.

During the GPPS survey, a set of independent spectrum digital backends are connected to the 19-beam L-band receivers, and the four polarization products of spectral line data of 1024k channels covering the frequency band of 1000 - 1500 MHz are simultaneously recorded. These piggyback spectral data have a great sensitivity due to the huge collecting area of FAST and also the 5-min integration time each beam, hence are valuable resources to study the Galactic interstellar medium with an excellent spectral resolution (~0.477 kHz).

Peering into the Milky Way by using FAST

By using the GPPS survey data, properties of the Milky Way can be investigated in many aspects:

• Using the GPPS piggyback spectral data, the Galactic HI gas can be detected in great details with an unprecedented sensitivity;
• Using the GPPS piggyback spectral data, the Galactic ionized gas can be detected via the radio recombination lines (RRLs) with a great sensitivity and resolution over a very wide sky region;
• Using the polarization data of weak pulsars recorded during the GPPS survey, the Galactic magnetic fields can be revealed in much wide regions in the Galactic halo and disk. Faraday rotation measures of weak pulsars, most of which are newly discovered by the FAST, would be hard to be measured without FAST;
• Using the scanning mode of the FAST observations and also the tracking of the GPPS observations, the radio continuum emission of our Galaxy can be detected with a great sensitivity and resolution.