HiFAST:An HIdata calibration and imaging pipeline for FAST

The Five-hundred-meter Aperture Spherical radio Telescope(FAST)has the largest aperture and a 19-beam L-band receiver,making it powerful for investigating the neutral hydrogen atomic gas(HI)in the universe.We present HiFAST(https://hifast.readthedocs.io),a dedicated,modular,and self-contained calibration and imaging pipeline for processing the H?data of FAST.The pipeline consists of frequency-dependent noise diode calibration,baseline fitting,standing wave removal using an FFT-based method,flux density calibration,stray radiation correction,and gridding to produce data cubes.These modules can be combined as needed to process the data from most FAST observation modes:tracking,drift scanning,On-The-Fly mapping,and most of their variants.With HiFAST,the root-mean-square(RMS)noises of the calibrated spectra from all 19 beams were only slightly(~5%)higher than the theoretical expectation.The results for the extended source M33 and the point sources are consistent with the results from Arecibo.The moment maps(0,1 and 2)of M33 agree well with the results from the Arecibo Galaxy Environment Survey(AGES)with a fractional difference of less than 10%.For a common sample of 221 sources with signal-to-noise ratio S/N>10 from the Arecibo Legacy Fast ALFA(ALFALFA)survey,the mean value of fractional difference in the integrated flux density,Sint,between the two datasets is approximately 0.005%,with a dispersion of 15.4%.Further checks on the integrated flux density of 23 sources with seven observations indicate that the variance in the flux density of the source with luminous objects(Sint>2.5 Jy km s-1)is less than 5%.Our tests suggest that the FAST telescope,with the efficient,precise,and user-friendly pipeline HiFAST,will yield numerous significant scientific findings in the investigation of the H?in the universe.

Peering into the Milky Way by FAST:Ⅳ.Identification of two new Galactic supernova remnants G203.1+6.6and G206.7+5.9

A 5°×7°sky area containing two large radio structures of G203.1+6.6 and G206.7+5.9 with a size of about 2.°5 and 3.°5 respectively is scanned by using the L-band 19-beam receiver of the Five-hundred-meter Aperture Spherical radio Telescope(FAST).The FAST L-band receiver covers a frequency range of 1.0-1.5 GHz.Commissioning of the receiving system,including the measurements of the half-power beam width,gain,and main-beam efficiency is made by observing the calibrators.The multi-channel spectroscopy backend mounted to the receiver allows an in-band spectral-index determination.The brightness-temperature spectral indices of both objects are measured to beβ~-2.6 to-2.7.Polarized emission is detected from the archival Effelsbergλ11 cm data for all the shell structures of G203.1+6.6 and G206.7+5.9.These results clearly indicate a non-thermal synchrotron emitting nature,confirming that G203.1+6.6 and G206.7+5.9 are large shell-type supernova remnants(SNRs).Based on morphological correlation between the radio continuum emission of G206.7+5.9 and the H i structures,the kinematic distance to this new SNR is estimated to be about 440 pc,placing it in the Local Arm.

Peering into the Milky Way by FAST:Ⅱ.Ionized gas in the inner Galactic disk revealed by the piggyback line observations of the FAST GPPS survey

As one of the major components of the interstellar medium,the ionized gas in our Milky Way,especially the low-density diffuse component,has not been extensively observed in the radio band.The Galactic Plane Pulsar Snapshot(GPPS)survey covers the sky area within the Galactic latitude of±10°around the Galactic plane visible by the Five-hundred-meter Aperture Spherical radio Telescope(FAST),and the spectral line data are simultaneously recorded during the pulsar survey observations.With an integration time of 5 min for each beam,the GPPS survey project provides the most sensitive piggyback spectra for tens of radio recombination lines(RRLs)in the band of 1000-1500 MHz for Hnα,Henα,Cnα,as well as Hnβand Hnγ.We processed the spectral data of RRLs,and obtained a sensitive averaged HnαRRL map of a sky area of 88 square degrees in the inner Galaxy of 33°≤l≤55°and∣b∣≤2°0.The final spectral data of the HnαRRLs have a spatial resolution of∼3′,a spectral resolution of 2.2 km s^(−1),and a typical spectral RMS noise of 0.25 mJy beam−1 or 6.3 mK in main-beam brightness temperature.The FAST GPPS HnαRRL observations are sensitive down to an emission measure of about 200 cm^(−6) pc if a 3σdetection limit is required.The new HnαRRL map shows complex structural features dominated by a number of HII regions and large extended diffuse ionized gas regions.We detect about 94%of the known HII regions and confirm 43 WISE HII regions in the observed sky area.Several large HII regions or star-forming complexes in the distant outer Galaxy are resolved in the map of HnαRRLs.Extended RRL features of the diffuse ionized gas are detected.In addition,the GPPS piggyback spectral-line data also provide sensitive detection for other kinds of RRLs,such as the Henα,Cnα,Hnβ,and HnγRRLs.The RRL data products of the GPPS survey will be published and updated at http://zmtt.bao.ac.cn/MilkyWayFAST/.

Peering into the Milky Way by FAST:Ⅰ.Exquisite HI structures in the inner Galactic disk from the piggyback line observations of the FAST GPPS survey

Neutral hydrogen(Hi)is the fundamental component of the interstellar medium.The Galactic Plane Pulsar Snapshot(GPPS)survey is designed for hunting pulsars by using the Five-hundred-meter Aperture Spherical radio Telescope(FAST)from the visible Galactic plane within|b|≤10°.The survey observations are conducted with the L-band 19-beam receivers in the frequency range of 1.0-1.5 GHz,and each pointing has an integration time of 5 min.The piggyback spectral data simultaneously recorded during the FAST GPPS survey are great resources for studies on the Galactic Hi distribution and ionized gas.We process the piggyback Hi data of the FAST GPPS survey in the region of 33°≤l≤55°and|b|≤2°.The rms of the data cube is found to be approximately 40 m K at a velocity resolution of 0.1 km s^(-1),placing it the most sensitive observations of the Galactic Hi by far.The high velocity resolution and high sensitivity of the FAST GPPS Hi data enable us to detect weak exquisite Hi structures in the interstellar medium.Hi absorption line with great details can be obtained against bright continuum sources.The FAST GPPS survey piggyback Hi data cube will be released and updated on the web:http://zmtt.bao.ac.cn/Milky Way FAST/.