CRAFTS Narrow-band data-cube pre-release


    The HIFAST project, as a part of the CRAFTS survey has been oberving the Galactic 21cm emission of atomic Hydrogen (HI). The project is aiming to publish its first public data release during the first half of 2023. The attached cube is a pre-release sample of a portion of the southern sky including the Orion Molecular Cloud Complex. The HIFAST data will be released in stages, and this map represents the region we intend to publish in the first stage.
    The map covers a region from 4 hours to 8 hours in Right Ascension, and from -13 to -3 degrees in Declination. The map covers LSR velocities from -100 to +100 km/sec which includes all the Galactic HI emission in this region. Currently, the best published map of this region was done with the HI4PI survey. That survey had a beam size of ~16.5 arcminutes, while FAST achieves a beam size ~3 arcminutes at a better sensitivity and spectral resolution. The improved resolution offers new details not otherwise visible, such as fine filamentary structures, shocks, HI shells, magnetic striations, HI Narrow Self-Absorption within cold molecular clouds, and large scale structures. Further it shows interactions between the Inter Galactic Medium and our own Galactic Halo with segments of the Magellanic Stream. The GALFA map which was made with the Arecibo telescope has similar sensitivity, beam size, and spectral resolution as our map, but is confined to a much smaller region of the sky due to Arecibo's restricted sky visibility. FAST is able to view more than twice as much sky. This offers us the first high-resolution single-dish HI imaging of the Orion Nebula, and the Orion Molecular Cloud Complex visible in this image.
    The first HIFAST public data release will include this map with additional improvements and improved sensitivity.
Technical Details:
Right Ascension Range: 4 hours to 8 hours (J2000)
Declination Range: -13 to -3 degrees (J2000)
Velocity Range: -100 to +100 km/sec (LSR)
Projection: Cartesian
Original beam size: ~3 arcmin
Effective beam size: 4 arcmin
Polarization: Stokes I
Sensitivity: Non-uniform, ~0.17K RMS per 0.2 km/sec channel or better
    The data reduction for this map was completed with our HIFAST project pipeline, and represents only ~0.3% of the data acquired and processed thus far. The electronics gain was corrected using the new High Cadence CAL technique which allows commensal observations as part of the CRAFTS survey. RFI rejection was performed in two stages. One stage flagged and removed Narrow-band RFI, however these frequency ranges are protected and thus the RFI from exterior sources was minimal. A portion of the data taken early during the survey is impacted by strong RFI from a faulty compressor, this often impacted the HI line itself, and required careful consideration including manual cleaning of the affected data in order to avoid harming the real HI emission. This process is still ongoing. The ~1MHz ripple is present in all FAST data. The ripple has been removed to the ~0.02K level in most cases so that it is lower than an individual channel's noise. Baseline removal was performed through a three stage process that included median filtering to determine the bandpass shape over longer timescales and polynomial fitting over short timescales, and a final post-processing step to correct any faulty fits. The map as a whole, as well as invidual drifts, and indiviual beams has been bootstrapped to the HI4PI survey in flux. Self-calibration in flux will be achieved through a new technique that takes advantage of FAST's sensitivity and is intended for the second data release.
    The aforementioned RFI from a Helium compressor impacted much of the older data in this image. Presently there are still several artifacts visible in the map, most of them are caused by the baseline removal algorithm failing due to the presence of this compressor RFI. We are in the process of manually removing these artifacts in order for the map to reach publication ready status.
    There are several strong continuum sources in the image which make the receiver's response non-linear when the telescope drifts through them. Most notable is the source in Orion. At present these data have been removed as the results there are not reliable. We are working on a method to recover an estimate of the true flux of that individual source and the HI line therein prior to publication.

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