The Five-hundred-meter Aperture Spherical radio Telescope (FAST) has passed national acceptance and finished one pilot cycle of ‘Shared-Risk’ observations. It will start formal operation soon. In this context, this paper describes testing results of key fundamental parameters for FAST, aiming to provide basic support for observation and data reduction of FAST for scientific researchers. The 19-beam receiver covering 1.05–1.45 GHz was utilized for most of these observations. The fluctuation in electronic gain of the system is better than 1% over 3.5 hours, enabling enough stability for observations. Pointing accuracy, aperture efficiency and system temperature are three key parameters for FAST. The measured standard deviation of pointing accuracy is 7.9″, which satisfies the initial design of FAST. When zenith angle is less than 26.4°, the aperture efficiency and system temperature around 1.4 GHz are ∼0.63 and less than 24 K for central beam, respectively. The sensitivity and stability of the 19-beam backend are confirmed to satisfy expectation by spectral Hi observations toward NGC 672 and polarization observations toward 3C 286. The performance allows FAST to take sensitive observations for various scientific goals, from studies of pulsars to galaxy evolution.

The Five-hundred-meter Aperture Spherical radio Telescope (FAST) was completed with its main structure installed on September 25, 2016, after which it entered the commissioning phase. This paper aims to introduce the commissioning progress of the FAST over the past two years. To improve its operational reliability and ensure effective observation time, FAST has been equipped with a real-time information system for the active reflector system and hierarchical commissioning scheme for the feed support system, which ultimately achieves safe operation of the two systems. For meeting the high-performance indices, a high-precision measurement system was set up based on the effective control methods that were implemented for the active reflector system and feed support system. Since the commissioning of the FAST, a low-frequency ultra-wideband receiver and 19-beam 1.05-1.45 GHz receiver have been mainly used. Telescope efficiency, pointing accuracy, and system noise temperature were completely tested and ultimately achieved the acceptance indices of the telescope. The FAST has been in the process of national acceptance preparations and has begun to search for pulsars. In the future, it will still strive to improve its capabilities and expand its application prospects. Keywords: Radio telescopes and instrumentation, astronomical observations, radio wave receivers, algorithms and implementation, control systems 95.55.Jz; 95.85.-e; 07.57.Kp; 07.05.Kf; 07.05.Dz Till date, several observation modes, such as tracking, drift scanning, andbasketweave scanning, have been realized, which means that the functional commissioning tasks have been completed. Now, the sensitivity of the FAST has reached 2,000 m 2 /K, the system noise temperature has been controlled below 20 K (19-beam 1.05-1.45 GHz receiver), and the pointing accuracy of the feed receivers has reached about 16″. The FAST has already begun to search for pulsars in batches.Several researches based on the FAST telescope have been reported. Qian et al. [3] reported their observation and basic parameters of the first pulsar discovered by the FAST, PSR J1900−0134. Zhang et al. [4] used FAST parameters obtained from the commissioning data to estimate the sensitivity of the CRAFTS extragalactic HI survey and predict its survey capacity in the future. Yu et al. [5] observed the abnormal emission shift event of PSR B0919+06 using the FAST with the ultra-wideband receive system. They found the potential existence of a slow-drifting mode between two major abnormal events. A sequence of dimmed pulses was observed during one of those events at all frequency bands. Lu, Peng et al. [6] reported the analysis of three rotating radio transients (RRATs), namely J1538+2345, J1854+0306, and J1913+1330, observed using the FAST. The derived burst rates of the three RRATs are higher than previous results owing to the high sensitivity of the FAST. Lu et al. [7] showed both the mean and single pulses of PSR B2016+28, observed in detail using the FAST. Wang, Zhu, Guo, et al. [8] developed a pulsa...

Fast radio bursts (FRBs) are highly dispersed radio bursts prevailing in the universe [1][2][3] . The recent detection of FRB 200428 from a Galactic magnetar [4][5][6][7][8] suggested that at least some FRBs originate from magnetars, but it is unclear whether the majority of cosmological FRBs, especially the actively repeating ones, are produced from the magnetar channel. Here we report the detection of 1863 polarised bursts from the repeating source FRB 20201124A 9 during a dedicated radio observational campaign of Five-hundred-meter Aperture Spherical radio Telescope (FAST). The large sample of radio bursts detected in 88 hr over 54 days indicate a significant, irregular, short-time variation of the Faraday rotation measure (RM) of the source during the first 36 days, followed by a constant RM during the later 18 days. Significant circular polarisation up to 75% was observed in a good fraction of bursts. Evidence suggests that some low-level circular polarisation originates from the conversion from linear polarisation during the propagation of the radio waves, but an intrinsic radiation mechanism is required to produce the higher degree of circular polarisation. All of these features provide evidence for a more complicated, dynamically evolving, magnetised immediate environment around this FRB source. Its host galaxy was previously known 10-12 . Our optical observations reveal that it is a Milky-Way-sized, metal-rich, barred-spiral galaxy at redshift z = 0.09795 ± 0.00003, with the FRB source residing in a low stellar density, interarm region