line and continuum intensity extraction in hifi data

Line and continuum intensity extraction in HIFI data

D. Teyssier ESAC

Outline 1.  Introduction: what this is about 2.  Line intensity extraction in HIFI spectra – Demo 3.  Continuum intensity extraction in HIFI spectra – Demo 4.  Best practice for cross-comparison with other instruments/observatories

– Demo

On-line Spectroscopy WS – 23-25 April 2014 - page 2

Introduction

•  This session focuses on line intensity and continuum level extraction in the framework of single pointing observations (point mode and spectral scan observations)

•  For mapping observations, similar concepts and rules will apply but additional tools are available in HIPE, that shall be presented at a later stage


Line intensity/area extraction •  For simple line profiles, the preferred approach should be to use the

Spectrum Fitter GUI (SFG)

–  Can be applied as interactive GUI –  Can be run as a command line code, written from scratch or based on

a GUI session export –  Detailed documentation at: http://herschel.esac.esa.int/hcss-doc-12.0/load/dag/html/Dag.Ch.SpectralFitting.SEII.html

•  When line profiles are too complex to be well represented by (multi-)line model fit, it is more accurate to simply sum up (integrate) all intensity channels within a given frequency range –  A task called integrator can be used for that matter –  It allows to take a simple baseline model into account based on

channels neighbouring the targeted line

DEMO: CO 6-5 line area extraction in NGC6302


Continuum level extraction (1) •  To first order, the continuum level is well represented by the mean

baseline intensity over the 4 GHz IF bandwidth •  However the following considerations need to be taken into account:

•  HIFI is Double-Sideband in nature so it measures the sum of the two sideband continua. Also it applies a SBR correction correct only for lines

•  Standing waves will not strictly cancel out if the IF bandwidth is not exactly a multiple of the number of cycles

•  There are known unruly IF ranges hitting at given LO tunings, that can affect the overall continuum level via baseline distortion

•  Strong/broad lines need to be properly ignored •  The error applying to this continuum will depend on efficiency of

rebinning on the radiometric noise •  Continuum levels in HEB data are strongly affected by Electrical

Standing Waves, so they must be corrected first •  Some observing modes are better suited for accurate continuum

estimate (DBS, preferably with continuum optimisation) – some others can actually NOT provide continuum info at all, e.g. FSW

•  Beware of continuum in OFFs !



Continuum level extraction (2)

Intermediate Frequency (MHz) ESW amplitude (K)

Inte

nsity

(K)

Con

tinuu

m le

vel

(K)

WBS-H WBS-V

Spurious continuum signal and ESW amplitude

Unruly IF ranges - examples

Band 6 resonances

Baseline distortion in band 3b

Continuum level extraction (3) •  Based on this, the following recommendations apply:

•  In HEB bands, ALWAYS correct from ESW first (see dedicated session in this workshop) – inspect the outcome in case of correction hiccups

•  In applicable bands (now 2a, 5a, 5b), undo the sideband ratio correction •  Wherever applies, remove fringes using FitHifiFringe, WITHOUT

baseline subtraction or division of any kind – one has to live with the residual baseline slopes…(they are part of the uncertainty)

•  Avoid ranges of the spectrum affected by either enhanced noise or very distorted baseline

•  In order to circumvent sideband ratio consideration, the continuum in the SSB spectrum at level 2 can be simply divided by 2, and considered to apply at the LO frequency

•  The noise in rebinned data may not decreased radiometrically for very large smoothing widths •  At large bin sizes, IF amplifiers will ingest noise that is expected to limit

the effective radiometric bandwidth •  Presently, we recommend to not smooth data with kernel width larger

than 100 MHz


Continuum level extraction (4) •  The mkRms task will perform some statistics computation on a given

spectrum, providing mean, median, RMS, etc, per sub-band: •  One of the strength of the task is an automatic line masking (a la FHF) •  It will also apply boxcar smoothing of a given width •  Alternatively, the pipeline will automatically compute and store spectra

statistics in obs > trendAnalysis > Statistics (no line masking though) •  H/V average is then up to the User – imbalance may always persist…

DEMO: Continuum around CH line in band 7a in W51

Before ESW correction Before ESW correction After ESW+FHF correction After ESW+FHF correction After ESW+FHF correction - V

After ESW+FHF correction - H


Comparison with PACS/SPIRE (1) •  Comparison with PACS and SPIRE data involves the conversion to flux

units •  This conversion can be performed using the task convertK2Jy, or

using analytical formulae •  An assumption on the source size must however be made •  For non point-like sources, analytical treatment exists for pure

Gaussian beams – for a more realistic beam the use of convertK2Jy is needed (note: this is not yet available in 12.0)

•  Further conversion to other unit can be done with the task convertUnits •  Analytical treatment:

S = (TA*×ηl/ηA) × (2kB/Ageom) × (ΩS/ΩΣ)

•  For point-like sources, K = ΩΣ/ΩS = 1 •  For non point-like sources, and a pure Gaussian beam of HPBW θb

•  K = (1-exp(-x2))/x2, where x=√ln2 (θS/θb), for a disk source of diam θS

•  K = 1/(1+x2), where x = (θS/θb) for a Gaussian source of HPBW θS



Comparison with PACS/SPIRE (2) •  PACS and SPIRE also have their tricks…

•  The source extend matters greatly for the accurate calibration of PACS or SPIRE data – see corresponding sessions in this workshop – the source morphology assumption must be consistent for all instruments

•  The very different spectral resolution compared to HIFI can imply line blending, which must be taken into account

•  PACS comparison in the red leak region is possible on line but not likely on continuum

•  Line flux extraction in SPIRE spectra is better done with sinc function fit on unapodized data. On top of that, multi-line, multi-species fitting is working better to account for the sinc side-lobe ringing and baselines

IRC+10216: CO-only fit

CO 6-5: 2.5e-15 W/m2

CO 7-6: 3.2e-15 W/m2

IRC+10216: multi-species fit

CO 6-5: 2.6e-15 W/m2

CO 7-6: 3.3e-15 W/m2

Comparison with other radio-tel. •  Comparison with other observatories using antenna temperature

scaling can in principle be done in temperature scale: •  Different dish size however implies a different beam dilution that must

be corrected – remember that Tmb is not Tb ! source morphology assumption kicks in again

•  Transiting to Jy may again be a better option since the antenna gain (Jy per K) will scale with the dish area


Questions ?


line and continuum intensity extraction in hifi data

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