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Correcting the extended-source calibration for the Herschel-SPIRE Fourier-transform spectrometer

Valtchanov, I., Hopwood, R., Bendo, G., Benson, C., Conversi, L., Fulton, T., Griffin, Matthew J., Joubaud, T., Lim, T., Lu, N., Marchili, N., Makiwa, G., Meyer, R.A., Naylor, D. A., North, Christopher, Papageorgiou, Andreas, Pearson, C., Polehampton, E.T., Scott, J., Schulz, B., Spencer, L.D., Van der Wiel, M.H.D. and Wu, R. 2018. Correcting the extended-source calibration for the Herschel-SPIRE Fourier-transform spectrometer. Monthly Notices of the Royal Astronomical Society 475 (1) , pp. 321-330. 10.1093/mnras/stx3178

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We describe an update to the Herschel-SPIRE Fourier-Transform Spectrometer (FTS) calibration for extended sources, which incorporates a correction for the frequency-dependent far-field feedhorn efficiency, ηFF. This significant correction affects all FTS extended-source calibrated spectra in sparse or mapping mode, regardless of the spectral resolution. Line fluxes and continuum levels are underestimated by factors of 1.3-2 in the Spectrometer Long-Wavelength band (SLW, 447-1018 GHz; 671-294 μm) and 1.4-1.5 in the Spectrometer Short-Wavelength band (SSW, 944-1568 GHz; 318-191 μm). The correction was implemented in the FTS pipeline version 14.1 and has also been described in the SPIRE Handbook since Feb 2017. Studies based on extended-source calibrated spectra produced prior to this pipeline version should be critically reconsidered using the current products available in the Herschel Science Archive. Once the extended-source calibrated spectra are corrected for ηFF, the synthetic photometry and the broadband intensities from SPIRE photometer maps agree within 2-4% -- similar levels to the comparison of point-source calibrated spectra and photometry from point-source calibrated maps. The two calibration schemes for the FTS are now self-consistent: the conversion between the corrected extended-source and point-source calibrated spectra can be achieved with the beam solid angle and a gain correction that accounts for the diffraction loss.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Physics and Astronomy
Subjects: Q Science > QB Astronomy
Q Science > QC Physics
Publisher: Oxford University Press
ISSN: 0035-8711
Date of First Compliant Deposit: 6 December 2017
Date of Acceptance: 5 December 2017
Last Modified: 19 Sep 2019 14:45

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