Transient effects in Herschel/PACS spectroscopy

2.8 MB·

by  Dario Fadda

Astronomy&Astrophysicsmanuscriptno.DarioFadda (cid:13)cESO2016
August19,2016
Transient effects in Herschel/PACS spectroscopy
DarioFadda1,(cid:63),JefferyD.Jacobson2,andPhilipN.Appleton2
1 InstitutodeAstrofisicadeCanarias,E-38205,LaLaguna,Tenerife,Spain
UniversidaddeLaLaguna,Dpto.deAstrofísica,E-38206LaLaguna,Tenerife,Spain
2 NASAHerschelScienceCenter—CaliforniaInstituteofTechnology,MC100-22,Pasadena,CA91125,USA
e-mail:[email protected],[email protected]
August19,2016
6
1 ABSTRACT
0
2 Context.TheGe:GadetectorsusedinthePACSspectrographonboardtheHerschelspacetelescopereacttochangesoftheincident
fluxwithacertaindelay.Thisgeneratestransienteffectsontheresultingsignalwhichcanbeimportantandlastforuptoanhour.
g Aims.Thepaperpresentsastudyoftheeffectsoftransientsonthedetectedsignalandproposesmethodstomitigatethemespecially
u
inthecaseofthe“unchopped”mode.
A
Methods.Sincetransientscanarisefromavarietyofcauses,weclassifiedtheminthreemaincategories:transientscausedbysudden
variationsofthecontinuumduetotheobservationalmodeused;transientscausedbycosmicrayimpactsonthedetectors;transients
8 causedbyacontinuoussmoothvariationofthecontinuumduringawavelengthscan.Weproposeamethodtodisentangletheseeffects
1
andtreatthemseparately.Inparticular,weshowthatalinearcombinationofthreeexponentialfunctionsisneededtofittheresponse
variationofthedetectorsduringatransient.Analgorithmtodetect,fit,andcorrecttransienteffectsispresented.
M] Results.Thesolutionproposedtocorrectthesignalfortheeffectsoftransientssubstantiallyimprovesthequalityofthefinalreduction
withrespecttothestandardmethodsusedforarchivalreductioninthecaseswheretransienteffectsaremostpronounced.
I Conclusions.Theprogramsdevelopedtoimplementthecorrectionsareofferedthroughtwonewinteractivedatareductionpipelines
h. inthelatestreleasesoftheHerschelInteractiveProcessingEnvironment.
p
Keywords. methods:dataanalysis—techniques:spectroscopic—infrared:general
-
o
r
t
s1. Introduction In this paper we briefly review the observational modes of
a
the PACS spectrograph and the data reduction techniques. We
[TheHerschelspaceobservatory(Pilbrattetal.2010)completed
describehowthe“unchopped”modeisparticularlysensitiveto
itsmissiononApril29,2013afterperformingatotalof23,400
3 suddenchangesinincidentflux,andcosmicrayglitches,which
vhours of scientific observations during almost 4 years of activ- cause memory effects in the detector responses. Finally, we in-
9ity ( 1446 operational days). One of the most used instruments troducethetechniqueusedtocorrectmostoftheseeffects,and
2aboard Herschel was the PACS spectrometer (Poglitsch et al. show a few selected examples. The methods and software de-
72010).Approximatelyonequarterofthetotalscientifictimewas
scribedinthepaperarenowimplementedintheHerschelInter-
7devoted to PACS spectroscopy. Although most of the observa-
activeProcessingEnvironment(HIPE1)(Ott2010).Thecompar-
0tions were performed in the standard “chop-nod” mode, a sub-
isonspresentedinthepaperarebetweenthearchivedSPG(stan-
.stantialfraction(30%oftheobservations,correspondingtothe
1 dard product generation) products, generated with HIPE 14.0
025%ofthetotalspectroscopytime)usedtwoalternativemodes. andcalibrationdataversion72,andourpipelineinHIPE15.
6The“wavelengthswitching”modewasreleasedtousersafterthe
1startofthemissiontoallowPACSspectrometerobservationsto
v:bemadeincrowdedfieldswherechoppingwasnotpossible.A 2. ThePACSspectrometer
yearlater,thismodewasreplacedbytheso-called“unchopped”
i
Xmode.BytheendoftheHerscheloperationmission,thismode WerecommendreadingPoglitschetal.(2010)andthePACSob-
rwasusedforapproximatelyone-thirdofallPACSspectroscopic servationalmanualforadetaileddescriptionofthePACSspec-
aobservations. The primary focus of this paper is to describe an trometer. As a way to introduce some terminology used in the
optimalwaytoreducedatatakeninthe“unchopped”mode. paper,wegivehereaconcisedescriptionoftheinstrumentand
The PACS spectrometer was able to observe spectroscopi- howitworks.
callybetween60µmand210µmusingGe:Gaphoto-conductor ThePACSspectrometerwasanIFU(integralfieldunit)com-
arrays. This type of detectors suffers from systematic memory posedbyamatrixof5×5spatialpixels(spaxelsorspacemod-
effectsoftheresponsewhichcanbiasthephotometryofsources ules)coveringafieldof47(cid:48)×47(cid:48) squarearcminutes.The5×5
andincreasethenoiseinthesignal.Sucheffectshavebeendoc- pixel image passed into an image slicer, which rearranged the
umentedandstudiedforsimilardetectorsonpreviousspaceob- 5×5two-dimensionalimageintoone-dimension(1×25),which
servatories such as ISOCAM (Coulais & Abergel 2000; Lari thenfedaLittrow-mounteddiffractiongratingwhereitoperated
etal.2001)andMIPS(Faddaetal.2006,Fig.17). at1st,2nd,and3rdorder.Thefirstorder(red)andsecondorthird
(cid:63) MovingtotheSOFIAScienceCenter(USRA) 1 www.cosmos.esa.int/web/herschel/hipe-download
Articlenumber,page1of14
A&Aproofs:manuscriptno.DarioFadda
order(blue)wereseparatedwithadichroicbeam-splitter,where
the spectra were re-imaged onto separate detectors. The wave-
lengthrangecoveredis51-105µmfortheblue(choiceof51-73
and 71-105 µm for the 3rd and 2nd order) and 102-220 µm for
thered,respectively.
The dispersed light was detected by two (low and high-
stressed) Ge:Ga photo-conductors arrays with 25 × 16 pixels.
In the following we call spectral pixels the individual pixels of
thetwoarrays.Wecall“module”or“spaxel”(fromspatialpixel)
thesetof16spectralpixelscorrespondingtothedispersedlight
fromasinglepatchof9.4×9.4squarearcsecondsonthesky.
Since the instantaneous wavelength range covered by 16
spectral pixels is small (∼1500 km/s for many observations),
the grating was typically stepped through a range of grating
angles during an observation. This operation is referred as a
“wavelength scan”. During a standard observation, the desired Nod B Chop 2
−10.58
wavelength range is covered by moving the grating back and Off
forth. These movements are called up- and down-scans, since
s] Slew
thewavelengthseenbyasinglespectralpixelfirstincreasesand g
e−10.62
d Chop2
then decreases as the grating executes a scan first in one direc- c [ On
tion,andthenback. e Chop 1
D
Theothermobilepartofthesystemisthechopperwhichlies −10.66
at the entrance to the whole PACS instrument. This is a mirror
which allows the IFU to point at different parts of the sky. In Chop1 Nod A Off
standardchop-nodmode,thechoppingmirrorisusedtoalterna- −10.70
tivelypointrapidlytothesourceandthenabackgroundposition,
310 320 330 340 350
whilemaintainingthesametelescopepointing.Thelargestchop- Time [s]
perthrowavailableis6arcminutes,whichisalimitationofthe
chop-nodmode.Ifthechoppermirrorismovedtolargerangles,
Fig.1.Exampleofchop-nodobservationfootprints(top),andobserved
itcanaccesstwointernalcalibrators(BB1andBB2)whichare
position projected on the sky (bottom). In the first nod, the detector
usedforcalibrationduringtheslewtothetargetsource.
alternatesbetweentwoopticalpathstoobservetheobjectandtheback-
Sincethemirrorispassivelycooled,theemissionofthetele- ground. These paths have slightly different telescope background sig-
scopeissignificant,andtypicallydominatesthetotalsignal.For nals.Inthesecondnod,theopticalpathsareinvertedafterslewingthe
instance,inthered,theemissionseenbyagivenpixelat120µm telescope.Byaddingthetwoobject-backgrounddifferences,itispossi-
isaround300Jy.Thishightelescopebackgroundlevel,although bletoremovetheeffectsofthedifferenttelescopebackgroundlevels.
not generally desirable in IR astronomy, has the advantage that
ithelpstomitigatetransienteffectsinthedetectorsbyhelpingto
effects. This paper is directed at ways of solving some of these
keepthesignallevelconstantatthedetectors.Asecondproperty
problemswithintheHIPEsoftware.
ofthedominanttelescopesignalisthatitisalwayspresent,and
canbeusedasarelativelyconstantreferenceflux.Thisproperty
canbepositivelyexploitedintheanalysisofthesePACSdata. 3.1. Chop-nodmode
The standard way to observe with the PACS spectrometer was
the “chop-nod” mode. During a wavelength scan, the chopper
3. Observationalmodes
modulatesbetweenan“on-source”and“off-source”position.In
Spectroscopic observations consist of a calibration block fol- thisway,anyvariationoftheresponseslowerthanthechopping
lowedbyaseriesofscienceblocks.Thecalibrationblockisper- timecanbesubtractedusingthe“off-source”constantreference.
formedwhenthetelescopeisslewingtoreachthetargetandin- However,sincetheon-andoff-sourcechopsignalsfollowdiffer-
volvesrapidchoppingbetweenthetwointernalreferenceblack- ent optical paths through the telescope, the observed telescope
bodies.Thesciencetargetisthenobservedinoneormorebands background emission is slightly different in the two chop posi-
using either the “chop-nod” or the “unchopped” observational tions. Toremovethis effect,the telescopeisnodded. Inthe left
mode. The “chop-nod” mode is used for isolated sources and panelofFigure1weshowthefirstnodposition(nodB)whenthe
involves the continuous chopping between target and an “off” sourceisobservedinthechop2position.Theright-handpanel
position. The “unchopped mode” is used to observe crowded showsthesituationafterasmallslewwhichplacesthesourcein
fields or extended sources. It consists of a staring observation chopposition1wheretheobservationisrepeated(nodA).The
of the science target followed by an observation of the refer- optical paths used to observe source and off-field are inverted
ence“off”positionbymovingthetelescope.Thismodereplaced so that the average of the “source minus off” signals is largely
the“wavelength-switching”modeusedforearlierobservations. unaffectedbythedifferenttelescopebackgrounds.
Instead of chopping, the “wavelength switching” used a wave- Two types of submodes are defined in the chop-nod mode.
length modulation to move the line on the detector array by a Inthecasetheobservationinvolvesanunresolvedline,the“line
wavelength equivalent to the FWHM of the line. This allowed chop-nod”modedefinesanoptimalwaytoobservethelineand
thedifferentialspectrumtobemeasured,butitwasfoundtobe the surrounding continuum. If a larger wavelength range is re-
inefficient during the verification phase and deprecated. Obser- quired,the“rangechop-nod”modecanbeusedtomanuallyen-
vations in “unchopped mode” suffered from detector transient tertherangeofwavelengthstobescannedbythegrating.
Articlenumber,page2of14
DarioFadda etal.:TransienteffectsinHerschel/PACSspectroscopy
Thedatareductionisdoneoneachindividualpixel.Thesim- This methodfrees theresult fromthe effects ofthe variablere-
plestreductiontechniqueusesthecalibrationblocktocompute sponse although it requires the knowledge of the dark. Since
theaverageresponseduringtheobservation.Theresponseisde- PACS has no shutter, the dark was measured in the laboratory
fined for each pixel as the ratio between the measured and ex- beforethelaunchofHerschel.Itisnotknownwithgreataccu-
pectedsignalfromtheinternalcalibratorswhichwasmeasured racyalthoughitsvalueisnegligiblewithrespecttothetelescope
inthelaboratory.Themeasuredsignaliscomposedofthetele- background.Thetelescopebackgroundwasderivedinflightby
scope background (T and T in the two chopping positions), calibrating the emission of the mirror with observed spectra of
A B
sourceflux(s),anddark(d)signal.Thetelescopebackgroundis brightasteroids.
dominated by the emission of the primary mirror, and depends
on the temperature of the spot which is seen by the detector.
Becauseofdifferentialvariationsinthemirrortemperature,and 3.2. Unchoppedmode
its emissivity with position on the mirror, the different chopper At the end of the verification phase, the “wavelength switch-
positionstendtoseedifferingdegreesoftelescopebackground. ing” mode (see Poglitsch et al. 2010) was released3 and used
Although, during a given observation, the overall temperature in a few key programs. Several disadvantages became appar-
of the mirror changed slowly, it was measured to vary due to ent with this mode during its early use, leading to its replace-
changes in illumination of the spacecraft by the Sun on longer mentwiththe“unchopped”mode.Forexample,byconstruction,
timescalesthanasingleobservation. the continuum of the source could not be measured. Moreover,
ForthemeasuredsignalC,inV/s,inachop-nodobservation only observations of unresolved lines could be measured cor-
wehave: rectly. Asymmetric lines or lines broader than the wavelength
switchingintervalwerefoundtobedifficulttorecover.Forlarge
C =(T +d+s)·R(t) C = (T +d)·R(t)
1 A 2 B wavelength ranges, SED shape variations can lead to peculiar
(1)
C(cid:48) = (T +d)·R(t) C(cid:48) =(T +d+s)·R(t) baselines in the final differential spectrum. This made observa-
1 A 2 B tionsoverlargewavelengthrangeschallenging.Finally,therapid
for the two chop positions in the first (C and C ) and second switching of the grating between two distant wavelength posi-
1 2
nods(C(cid:48) andC(cid:48)),respectively.HereR(t) = ρ ·r(t)istheprod- tionscreatedmechanicaloscillationswhichrequiredalongtime
uctofth1erelativ2espectralresponsefunction(λRSRF)ρ andthe todamp.Thiseffectwasmoresevereinspacethanduringground
λ
response function r(t) (V s−1 Jy−1) which can vary with time. testing,leadingtolongertimeintervalsbetweenusefulobserva-
T andT correspondtothesignaldetectedfromthetelescope tionalsamplesandthuspreventinganefficientremovalofrapid
A B
backgroundatthechopperpositionsAandB,respectively. responsevariations.
The simplest technique consists in estimating the signal It appeared clear that performing a slow grating scan while
sourcebycomputingthedifferentialsignal,sˆ,betweenthechop- staring at an object, followed by a similar observation pointed
pingpositionsandcombiningthenods: to an “off” position, yielded superior results compared with
“wavelength-switching”. This “unchopped” mode had the ad-
sˆ=[(C −C )+(C(cid:48) −C(cid:48))]/(2·ρ ·r ), [Jy] (2) vantage of allowing large wavelength range scans, such as far-
1 2 1 2 λ CB
IR SEDs of confused or extended regions. Unlike the “chop-
where rCB = (cid:104)r(t)(cid:105) is the average response estimated from the nod” mode, where the two chopper positions sample different
calibrationblock.Aswecansee,themethodworksifr(t)isap- opticalpathsandthereforedifferentmirrortemperatures,inthe
proximatelyconstantduringachoppingperiod(1sec)sothatthe “unchopped”modetheopticalpathsusedforon-andoff-source
subtraction cancels the effect of a transient. This approach was observationsarethesame.Therefore,themirrortemperaturein
usedbythesocalledSPG(standardproductgeneration)pipeline the on- and off-source are exactly the same. Moreover, in the
to populate the Herschel Science Archive (HSA) for chop-nod caseofchop-nod,thefootprintofthedetectorontheskyrotates
observations2untilrecently. slightlybetweenthetwochoppositions(seethePACSobserver
SinceSPG13,analternativeapproachexploitingtheknowl- manual, Figure 4.7). The effect, though small, is worse for the
edge of the telescope background is used. This technique does largerchopperthrows.Thismeansthatinthetwonodpositions,
notrequiretheapplicationoftheRSRFandresponsecorrections theonlyspaxelseeingpreciselythesamepartofthesourceisthe
sinceitusestheratiosbetweentheobservationsinthetwochop- centralone.Spaxelsfurtherfromthecenterbecomesuccessively
pingdirections.Nevertheless,theaccuracyoftheresultsdepends mismatchedinthetwonods.So,thereductionworksbestforthe
on the knowledge of the mean telescope background. Also, ra- centralspaxel.Thisproblemdoesnotexistforthe“unchopped”
tios introduce more noise in the final signal with respect to the mode because the observation of the on- and off-positions are
standardreduction. madewithafixedcentralchopperposition.
Informulae,ifwedefinenormalizationas: The “unchopped” line and range modes were released on
September2010,supersedingtheprevious“wavelengthswitch-
C −C C(cid:48) −C(cid:48) 2s
N = 1 2 + 1 2 = , (3) ing” mode4. A specific “unchopped” mode for bright lines was
C1+C2 C1(cid:48) +C2(cid:48) TA+TB+2d+s releasedonApril20115.
In “unchopped” mode, a complete wavelength scan is done
the signal normalized to the average telescope background can
first on the “on-source” (hereafter ON) position and then later
beexpressedas:
3 herschel.esac.esa.int/Docs/AOTsReleaseStatus/
s = s = N . (4) PACS_WaveSwitching_ReleaseNote_20Jan2010.pdf
(cid:104)T(cid:105)+d (T +T )/2+d 1−N/2 4 herschel.esac.esa.int/Docs/AOTsReleaseStatus/
A B
PACS_Unchopped_ReleaseNote_20Sep2010.pdf
The source flux can be therefore derived by multiplying the 5 herschel.esac.esa.int/twiki/pub/Public/PacsAotReleaseNotes/
normalizedsignalbythetelescopebackground(darkincluded). PACS_UnchoppedReleaseNote_BrightLines_15Apr2011.pdf
Note that all the AOT release notes will be provided on the Herschel
2 http://www.cosmos.esa.int/web/herschel/science-archive LegacyLibrarywebsitestartingfrom2017.
Articlenumber,page3of14
A&Aproofs:manuscriptno.DarioFadda
10
BB1
9 2.5
BB1
8
BB2 2.0
] 7
s
V/ 6
[ Calibration block 1.5
x 5 BB2
u
Fl 4 Previous Transient 1.0 Calibration block
3 values
Observation Previous
2 0.5 values Obs
1 Red array Blue array
0.0
0 10 20 30 40 50 60 70 80 90 100 110 120 10 20 30 40 50 60 70 80 90 100 110 120
1.10 1.10
1.05 1.05
>
x
u
Fl1.00 1.00
<
ux / 0.95 BB1 0.95 BB1
Fl BB2 BB2
0.90 0.90
Red array Blue array
0.85 0.85
20 30 40 50 60 70 80 90 100 110 20 30 40 50 60 70 80 90 100 110
Time [s] Time [s]
Fig.2.Transientsaffectingthecalibrationblockforthered(left)andblue(right)arrays.Duringthecalibrationblock,thedetectorsseealternatively
thetwoblackbodies(toppanels).Sincetheinitialfluxissignificantlydifferentfromthefluxesoftheinternalblackbodies,astrongtransientoccurs
duringthefirstminuteoftheobservation.Thesignalsnormalizedtothemedianvaluesofeachblackbody(bottompanels)showhowthesame
transientaffectsthefluxesfromthetwoblackbodies(blueandredlines).Thetransienteffectismorepronouncedfortheredarray.
ona“off-source”(hereafterOFF)positionclearofsourceemis- evident after sudden variations of flux on detector arrays (see,
sion. Unfortunately, variations of the response during the ON e.g., Coulais&Abergel2000).InFigure2weshowthesignal
scan cannot be corrected using the OFF scan because the two detected during the calibration block when the chopper points
scansareperformedatdifferenttimes.So,althoughunchopped alternativelybetweenthetwointernalblackbodies(whichhave
observationsoffersomeadvantagesoverthechop-nodmode,the differenttemperatures).Passingfromthepreviousobservationto
effects of transients require mitigation. In this paper, we show theinternalblackbodiescausesacleartransienteffectwhichis
thetypicaltransientsfoundinthesignal,andsometechniquesto more important in the case of the red array (left panels). When
modelandsubtractthem. thesignalisnormalizedtotheasymptoticflux(bottompanels),
Threedifferentsub-modesexistfortheunchoppedmode: theresponsevariationbecomesevident.Itisinterestingtonote
(leftbottompanel)thattheshorttransientsduetocosmichitson
– unchoppedline,forsinglelineobservations.
thearrayhaveatimescalelongerthanthechoppingtime.Thisal-
– unchoppedbrightline,forsinglebrightlines.Itis30%more
time efficient than the standard line mode since bright lines lowsthecorrectionoftheseeffectsusingthe“chop-node”mode.
Wecanclassifythreeseparatetypesoftransients:
requirelesscontinuumtocomputethelineintensity.
– unchopped range, for observations of the continuum or a – continuum–jump transients: transients due to a sudden
complexoflines. change of the incident flux from one almost constant level
A further difference between these sub-modes is that the refer- toanotheralmostconstantlevel;
– cosmicraytransients:transientsinducedbycosmicrayhits
enceOFFisobservedduringthesameAOR(AstronomicalOb-
whichproduceglitches,followedbyaresponsevariation;
servation Request) in thecase of the “unchopped line”, but has
– scan dependent transients: transients along a wavelength
tobeprovidedasaseparateAORinthe“unchoppedrange”.In
scan produced by rapid variations of the (dominant) tele-
ordertoensuretheobservationsareperformedsequentially,the
scope background and continuum (in the case of a bright
OFFobservationintheunchoppedrangescanisconcatenatedto
target).
themainobservationwithintheobservationsequence.
ItisworthmentioningthattheStandardProductGeneration
(SPG)pipeline,whichpopulatestheHSA,doesnotuseanytype
4. Transients
of transient correction. So the products available in the archive
The term “transient” refers to a delayed response of a detector for the “frequency switched” and “unchopped-mode” observa-
to the variation of the incident flux. Transients are particularly tions contain many transient effects that could adversely affect
Articlenumber,page4of14
DarioFadda etal.:TransienteffectsinHerschel/PACSspectroscopy
science goals. However, from HIPE 14 onward, users have the
5
option of processing their data with special scripts designed 1
1.
specifically to correct transients. The current paper describes
0
1
how these corrections are made. In the following we describe 1.
indetailthedifferenttypesoftransients. 5
0
1.
0
0
4.1. Continuum-jumptransients 1.
5
9
Asuddenchangeintheilluminationofadetectorpixelpassing 0.
from one flux level to another, can induce a major transient in 0
2
thesignal.Thisoccurstypicallyatthebeginningofeachobser- 1. 0.00 0.05 0.10 0.15
vationjustafterthecompletionofthecalibrationblock.Because
5
of the difference between the flux from internal calibrators and 1.1 g
k
the telescope background, a transient is usually visible during B
0
theentireobservation. 1.1 Tel
cahsaonAugrnecoeot.hfFebroarncdoex,moammrcophnlaenf,ogtrhemeoufoswfearthvmieslaekynignhtdahvooefcrcteruqarunseswisethenidtlsetwiosbowsedhriveffnienrga- 1.05 gnal /
entlinesoccurringatdifferentwavelengths.Whenthegratingis 00 Si
commandedtoaccessadifferentpartofthespectrum,orchange 1.
to a completely different region of the spectrum, a jump in the 95
0. Time [hour]
signalusuallyoccursbecauseofthechangingemissionspectrum
of the telescope background at different wavelengths (see Sec-
tion 5.2). If the source is very bright, differences in continuum 0.0 0.3 0.6 0.9 1.2 1.5 1.8
levelfromthesourcecanalsoinduceatransient.
Ifthesourcecontinuumisnegligiblewithrespecttothetele-
Fig. 3. When the signals from a spaxel are normalized to the ex-
scope background, it is possible to normalize the signal during pectedtelescopebackgroundalong-termtransientafterthecalibration
the whole observation to the telescope background which has block becomes clear. In this example (an observation of M81, ObsID
beenpreviouslycalibrated.Thetransientduetothejumpincon- 1342269535)anOFFpositionisobservedatthebeginningandatthe
tinuumbetweenthecalibrationblockandthetargetobservation endoftheobservation(bluedots),whilea2x2rasterobservationisper-
appearsclearly,seeFigure3.Itispossible,inthiscase,tofitthe formedinthemiddle(greendots).Sincethecontinuumemissionofthe
behaviourwithamodelandsubtractitfromtheobservation.Itis objectisnegligiblewithrespecttothetelescopebackground,itispossi-
bletofitthegenerallong-termtransient(redline)andcorrectthesignal.
interestingtonotethat,withoutthiscorrection,anOFFobserva-
Notethatmanyhugetransientsappearinindividualspectralpixelsbe-
tioncanhaveameasuredfluxgreaterthananONobservationif
causeofcosmicrayhitsonthedetectors.Ifthiscorrectionisnotmade,
ithappenstobeperformedduringaperiodofresponsestabiliza-
andanaveragebackgroundissubtractedfromthesignal,thefinalim-
tion. This effect is not taken into account by the SPG pipeline,
age will present an artificial gradient and the flux will be negative in
so that many observations in the archive have an artificial and someregions.Topinsetpanel:aclose-upoftheinitialpartofthesignal
confusingnegativebackground. justafterthecalibrationblock,showingthestrongtransient.Theperi-
Someobservationsthatusedtheunchoppedmodecontained odicvariationinthesignalisaleft-overfromtheimperfecttelescope
requestsformorethanonebandinasingleAOR.Forextended backgroundestimateusedfornormalization.Theemptyspacesbetween
sources, a typical raster observation allows one to cover an ex- pointsoccurduringtelescopeslewing.
tended region by moving the telescope to cover a grid of posi-
tions. However, for efficiency reasons, the AOR was designed
tocyclethroughalltherequestedbandsateachrasterposition, bandchangesappeartolimitthequalityoftheobservation,even
beforemovingthetelescopetothenextpositionintherasterse- withthemostadvanceddatareduction.Indeed,weareabletosee
quence.Anunexpectedsideeffectofthisstrategywasthegen- jump-transients even in the blue channel, which has the fastest
eration of transients at each band change. It is not possible to responsestabilizationcomparedwiththered(seeFigure4).
remove these jump–transients in a clean way in case of such
multiple-bandobservations.
There is an important lesson to be learned from this expe- 4.2. Cosmicraytransients
rience with PACS. In retrospect, it would have been better to
observe the complete raster in a single band, before cycling to ItiswellknownthatforGe:Gadetectors,energeticcosmicrays
the next band. This would have minimized the effects of band- usuallyproduceglitchesinthesignal,followedbyresponsevari-
inducedjump-transient.Weadvisethatanyfuturemissionwhere ations. Depending on the energy of the cosmic ray, the glitch
transient could be an issue, should avoid as much as possible can be followed by a tail or the variation can be more compli-
suddenchangesinfluxduringtheobservation.Thisisprobably cated (lowering temporarily the response). A similar behavior
thecaseofFIFI-LSonboardSOFIA,whichisessentiallyaclone wasnotedinthepastforpixelsintheISOCAMarrayontheIn-
of the PACS spectrograph. Unfortunately, when the observa- frared Space Observatory (see, for example, Lari et al. 2001).
tionalmodewasintroducedforHerschel,thereductionpipeline Wewillshowthattheresponsevariationcanbedescribedwitha
wasnotfullydeveloped,anditwasverydifficulttoevaluateall combinationofexponentialfunctions(seeSection5).Themain
theeffects.Sincetheresponsewasknowntostabilizerelatively challenge to correcting and masking the damage in the signal
fast, it was assumed that the effect of a band change was mini- fromcosmicrayhitsistoselectthemostsignificantevents,and
mal,comparedwiththeadvantageofbeingmoretimeefficient. thenfindthestartingtimewhichmarksthebeginningofthedis-
Experiencelatershowedthatjump-transienttransientcausedby continuityinthesignal.
Articlenumber,page5of14
A&Aproofs:manuscriptno.DarioFadda
300
CR transients ] Transients
y
J 250
[
l
Bkg gna 200
el si
T d
nal / lize 150 Band 3 Band 3
Sig Drift ma 100 Band 1 Band 2 Band 1 Band 2
r
o
N
50 On−source Off−source
0
−0.05 0.00 0.05 0.10 0.15 0.20 0.25 0.30
Time [hours] Time [hours]
Fig.4. Long-termtransientforthepixelsofmodule16inthebluechan- Fig. 5. The central spaxel of an observation of NGC6543 (ObsID
neloftheobsid1342246963.Oneofthepixelsshowsalsoatransient 1342215704) with three lines observed. For each observation block,
duetoacosmicrayhit.Thereisa30%variationbetweenthetwoends thesignalineachwavelengthscanisdividedbythelastscanandnor-
oftheobservation.Blueandgreendotsrefertooff-targetandon-target malized to its asymptotic median flux. The red line shows the tran-
partsoftheobservation,respectively. sientmodelfitted.Thefirstblockcomesafterthecalibrationblock(not
shown)whosefluxishigherthanthecontinuumduringthispartofthe
observation.Thethreelinesareobservedfirstonthetarget(greendots)
4.3. Scandependenttransients andthenontheOFFposition(bluedots).Thegapsinthesesciencedata
correspond to the time spent moving the grating, and slewing to the
Finally,inthecaseofobservationsspanninganextendedwave- OFFposition.Alsointhiscasesomepixelsappeartobehavedifferently
lengthrange,rapidvariationofthecontinuumduringthewave- becauseoftheimpactofcosmicrays.
length scan produced transients in the signal (see Section 5.4
foranexample).Insomeobservations,thislasteffectisrespon-
revisittheindividualpixelresponsesbynormalizingtheirsignal
sible for different apparent fluxes in the source spectrum for
to this “first guess” spectrum. In this way, each individual de-
upward and downward scans over the same wavelength range. tectorpixelresponsetocanbestudiedfortheeffectsofcosmic
Luckily, many PACS observations using the unchopped mode
raytransients,andtheseCRtransientscanbeeithercorrectedor
targeted only one line with a short range scan. For these cases,
masked.Inthefollowing,wedescribethemodelusedtofitthe
scan-dependent transients had negligible effects. It was found
responseasafunctionoftimeandthealgorithmsappliedtothe
to be important only for observations which scanned over ex- signaltomitigatetheeffectofthetransients.
tended wavelength ranges. As a result, two different interac-
tive pipeline scripts (accessible within HIPE as so–called ipipe
scripts)havebeenwrittentotreatthe“unchoppedline”and“un- 5.1. Model
choppedrange”casesseparately.
Theresponsevariationscanbedescribedingeneralusingacom-
binationofexponentialfunctions.Wefoundthatacombination
ofthreeexponentialfunctionscloselydescribetheseveraldiffer-
5. Detectingandmodelingtransients
entcasesoftransientsobserved.
To detect, model, and correct transients in the signal, one has
to decouple the signal and the effect of the transients. The best
waytoproceedistoobtainanestimateoftheresponsebynor- f(t)=a+ashorte−t/τshort +amediume−t/τmedium +alonge−t/τlong (5)
malizingthesignaltotheexpectedspectrum.Aswewillseein Theparameteracorrespondstothemedianlevelofthenor-
thefollowingsections,ifthesourceemissionisnegligiblewith malizedsignal.Inthecasethattheresponseisobtainedbynor-
respecttothetelescopebackground,theidealwaytoproceedis malizing the signal to the last scans, a is fixed to unity by con-
tonormalizethesignaltotheexpectedtelescopebackground.If struction. The three exponential functions have very different
the source emission is not negligible, one has to have a better timescales.Afirstfunctionaccountsforthefastvariationofthe
guessofthesignal.Onepossibilityistoassumethatthedetector signal just after the flux change or a cosmic hit. The timescale
eventually stabilizes after a jump in the continuum, so that one is between 1 and 10 seconds. The second term has an interme-
canusethelastscansneartheendoftheobservingsequenceto diate timescale (between 10 and 80 seconds). Finally, the third
estimate the “true” continuum level. This asymptotic level can term accounts for the long-term variation of the signal and has
then be used to normalize the entire signal to reveal the jump– the longest timescale (between 200 and 1200 seconds). In the
transient.Oncethetransientisrevealedinthismanner,itcanbe caseofcosmicrayinducedtransients,thea coefficientisal-
short
fittedandremovedfromthesignal. wayspositive,whereasforthecontinuum-jumptransientsa
short
Once the transients due to expected continuum-jumps are canbeeitherpositiveornegativedependingonthedirectionof
corrected, it is possible to obtain a first estimate of the final the jump. The parameter space considered for the fitting in the
spectrum,combiningalldatafromallindependentupanddown twocasesissimilar.
scans,andmeasurementsmadewithall16pixelscombinedto- In the case of cosmic rays, the time constant τ is kept
short
gether. Oncethisaverage spectrumis created,itcan beused to lower than one second. For the continuum-jump transient, the
Articlenumber,page6of14
DarioFadda etal.:TransienteffectsinHerschel/PACSspectroscopy
parameter space for τ considered is between 1 and 10 sec-
short
onds.Fortheothertwotimeconstants,theparameterboundaries
are10<τ <80secondsandτ >100seconds.
medium long
5.2. Correctingcontinuum-jumptransients
If the continuum of the source is negligible with respect to the
telescope background, it is possible to see the long-term tran-
sient caused by the calibration block all along the observation
bynormalizingthesignaltotheexpectedtelescopebackground
(seeFigure3).Ifthiseffectisnotcorrectedandanaverageback-
ground is subtracted from the signal, there will be parts of the
image with a signal lower than the average background which
will have negative fluxesat the end of the datareduction. Even
worse, in mapping observations, each raster position taken at a
different phase of the transient will have a different continuum
levelresultinginanartificialgradientinthefinalimage(seelater
inFigure15).
Unfortunately, this correction depends on the knowledge of
thetelescopebackgroundemission.Thecurrentmodelavailable
throughHIPEisfairlyaccuratefortheredarray,whilethesitu-
ationisslightlyworseforthebluearray.
There are several situations where the simple correction
above will not work. In the case of a very bright continuum,
thenormalizationofthesignalbythetelescopebackgroundwill
not be appropriate, and this will make any correction difficult.
Anothercaseisthatofamulti-bandobservationtakenconsecu-
tively.InFigure5weshowanexampleofanobservationtakenin
3differentbands.Atthebeginningofeachobservationalblock
(observationofoneband)thereisacleartransientintroducedby
thesuddenvariationofthecontinuumlevel. Fig.6.Anexampleofhowweestimatethe“Guess”spectrumforthe
Tocorrectthetransientsinbothofthesecases,wecannor- C+ line of Arp 220 (obsID 1342202119). We compute the histogram
malizethesignaltothefluxdetectedduringthelastwavelength of the normalized χ2 of each individual spectral scan with respect to
themedianspectrum(toppanel).Thebluelineshowsthewellbehaved
scanofeachobservationalblock.Wemaketheassumptionthat
scansandtheredlineshowsthosetoberejected.TheGuessspectrum
the detector response has stabilized by this time. This assump-
(bottompanel;greenline)istheresultofco-addingthewellbehaved
tionworkswellinthecaseoflongobservations(∼1hr).Inthe
scans(bluelines).Thescanswithχ2 deviatingmorethan3σfromthe
caseofshorterobservations,alittlebiaswillremaininthedata
medianvaluearediscarded(redlines).
sincetheasymptoticlevelhasnotyetbeenreached.
weobtainanapproximationtothefinalspectrumcladdingallthe
5.3. Correctingtransientscausedbycosmicrays
individual pixels that contribute to the scan (the “Guess” spec-
Themostdisruptivetransientsarecausedbytheimpactofcos- trum). Next, the signal for each individual pixel is normalized
mic rays on the detectors. Since they happen at random times by this spectrum to obtain an estimate of the response. At this
withdifferentenergies,theycauseunpredictablechangesinthe point,thediscontinuitiesintheresponseareidentified,andonly
baseline of the data which can last from several minutes up to the most significant are selected. We fit the model (equation 5)
onehour.Themaindifficultyinfittingthesetransientswithour totheresponseintheintervalbetweentwoconsecutivediscon-
model is to disentangle the real signal from the variation in re- tinuities. A correction is then applied to the original signal to
sponsecausedbythecosmicrayimpact,andtodetectthetime attempttoremovetheeffectofthecosmicrays.Inthefollowing,
whentheimpactoccurs.Cosmicrayeventsaresofrequentthat, wedescribeindetaileachoneofthesesteps.
inpractice,wemustsetathresholdabovewhichweattemptto
correctthesignal.
5.3.1. Estimatingthe“Guess”spectrum
Lebouteiller et al. (2012) developed a procedure to iden-
tify features associated with transients using a non-parametric We assume that the continuum jump transients have been cor-
method(multi-resolutiontransformofthesignalandidentifica- rected to this point. We need to identify the scans affected by
tionofpatternsonatypicalscale).Thefeaturesfoundwerethen severe transients, and discard them to obtain a good first guess
fitted with an exponential, and subtracted from the signal. This of the spectrum. This step is particularly important in the case
algorithm is identical to the one developed for the treatment of oflowredundancy,sinceevenafewbadscanscancompromise
ISOCAMdata(Starcketal.1999). thecoaddedspectrum.Theproceduretoobtainaguessspectrum
Theapproachwedescribedinourpaperisdifferentinboth isiterative.Thefirststepconsistsofcomputingamedianspec-
the way we identify the cosmic ray events, and how remove trumbycoaddingallthewavelengthscansforthespectralpixels
them.OurmethodsaresimilartothosedevelopedbyLarietal. whichcontributetoagivenspaxel.Thespectrumisobtainedby
(2001) for the treatment of ISOCAM data, although the algo- computing the median of all the valid fluxes for each bin of a
rithms presented here differs in several significant details. First wavelength grid: the median spectrum. Scans with very devi-
Articlenumber,page7of14
A&Aproofs:manuscriptno.DarioFadda
240
235
0.12
230
225 se0.08
n
o
220 p
es0.04
215 r
d
e
210 er 0.00
205 Filt
−0.04
200
195
−0.08
190
1.3
1.20
1.15
1.2
1.10
e
nse1.05 ons
o p1.1
esp1.00 Res
R
0.95
1.0
0.90
0.85 0.9
4200 4400 4600 4800 5000 5200 4500 4600 4700 4800 4900 5000
Frame No. Frame No.
Fig.7.Toppanel:Acomparisonoftheobservedsignalofasinglespec-
Fig.8.Onthetoppanel,partoftheresponseconvolvedwiththefirst
tralpixel(cyan)withthe“Guess”signal(blue;seetext).Thesignalis
derivative of the Gaussian with dispersion equal to two frame units.
plottedagainsttheFrameNumber,whichisafunctionoftimeduring
Thiswidthgivesthebestsensitivitytosuddenchangesoftheresponse
the scan. We show three scans in the figure (two up and one down)
caused by cosmic ray hits on the detectors. The green lines mark the
whichcapturestheC+lineshowninthepreviousfigure.Justafterthe
region of ±5 times the local dispersion and the red circles marks the
framenumber4720astrongcosmicrayhitsthedetectorproducinga
startingpointsoftransients.Inthebottompanel,theresponsivityforthe
suddenchangeoftheresponseofthedetector.Bottompanel:Theratio
sameblockofframeswitheachintervalbetweentwoconsecutivedis-
ofthetwocurves,i.e.theresponseofthepixelasafunctionofframe
continuitiesintheresponsefittedwithourtransientmodel(darkgreen).
number.Manytransientsareclearlyvisible,inparticularthehugetran-
Theredpointsmarktheframemaskedbecausetheintervalbetweentwo
sientafterframenumber4720.
consecutivediscontinuitiesistooshort(lessthan10frames).
ating values compared with the median spectrum are rejected.
Thescanswithχ2 valueswhichdeviatemorethan3σfromthe naliscomparedwiththesignalfromasinglepixel,allowingthe
median of the χ2 distribution are discarded and a new median discontinuitiestobeeasilyseen.
spectrum is computed (see top panel of Figure 6). To obtain a Tocorrectlyfitthetransients,onehastoidentifythestarting
robust estimate of the dispersion of the distribution, we make point,i.e.themomentatwhichthediscontinuityinfluxappears
useofthemedianabsolutedeviation(orMAD).Theprocedure inthesignal.Thisisaclassicalproblemofsignaltheoryandit
isiteratedthreetimestoobtainacleanmedianspectrumwhich hasbeenshown(Canny1986)thattheoptimalfiltertofinddis-
serves as “Guess” spectrum. Figure 6 shows visually how the continuities in a mono-dimensional signal is the first derivative
process works in the case of an observation of the C+ line of ofaGaussian.ThedispersionoftheGaussianwasfoundempiri-
Arp220.Theprocedureworksbecauseofthehighredundancy callytobetwoframeunits(equivalentto0.25s).Ifweconvolve
of the PACS data, which has a minimum of four up-and-down theresponsewiththisfilter,spikesappearjustaftercosmicrays
scansforeachofthe16spectralpixels. impactthedetector.AsshowninFigure8,wecanselectthemost
importantonesbycomparingtheirintensitytothelocalnoisein
ourresponseestimate.Inouralgorithmwemakeacutof5σto
5.3.2. Findingsignificantdiscontinuities selectthesignificantevents.
Bynormalizingthesignalofeachpixelbytheexpectedfluxof
the Guess spectrum, we obtain an estimate of the response as
5.3.3. Maskingandcomputingtheresponse
a function of time for each pixel. This function can be used to
study the effect of cosmic rays on the signal by selecting the Atthispointwe breakthesignalinintervalsbetweentwo con-
mostimportantones,fittingthetransientsafterthem,andeven- secutive discontinuities, and fit the model (equation 5) in each
tuallymaskingpartofthesignalexcessivelydamagedbytheef- oneoftheseintervals.Ifthecosmicraysoccurtooclosetogether
fectofcosmicrayimpactsonthedetector.InFigure7weshow (less than 10 frame units) we simply mask that part of the sig-
an example using the M82 C+ spectrum. Here the Guess sig- naltimeline.Theresultingmodelresponseandmaskedregions
Articlenumber,page8of14
DarioFadda etal.:TransienteffectsinHerschel/PACSspectroscopy
200
190
180
170
160
x
u
l 150
F
140
130
120
SPG
110
190
180
170
160
x
u 150
l
F 140
130
120
TC Fig.10. Signalvariationsasafunctionofthewavelengthscan.Tosee
110 the effect of the flux variations on the measured signal, we can com-
parethesignalmeasuredduringtheup-scans(blueparts)withtheone
100
1800 2000 2200 2400 2600 2800 measuredduringthedown-scans(redparts).
Frame No.
lengthscanthereisalmostnovariationintheincidentfluxwhich
Fig. 9. Comparison between masking in the SPG pipeline (top) and
couldcausetransienteffectsonthesignal.Thisisnotthecasein
transient-correction pipeline (bottom). Signal is blue and masked
observations of extended wavelength ranges, where the contin-
frames are represented as red circles. The SPG pipeline masks a few
deviant frames as GLITCHES, while the transient-correction pipeline uum can vary substantially during the scan. This leads to tran-
masksframeseitherasGLITCHESorasUNCORRECTED.Asaresult, sient behaviour which becomes apparent when comparing the
thesignalfromtheSPGpipelinewillcontainunmaskedresidualsofbig scansintwooppositedirections.InFigure10,weshowthatdur-
transients,whilethetransient-correctionpipelinewilleithercorrectthe ing up and down scans, the incident flux can change by, in this
transientsormaskedthemcompletely.Thisleadstoabigreductionof example,afactorof7.Thisleadstoalargetransient.
noise in the final reduction with respect to the product from the SPG
In Figure 11 (top panel) the ratio of the two fluxes shows
pipeline.
clearly the effect. Here we consider only one scan for a given
spaxel.Fortheup-scan(bluepoints),weseethefluxdecreases
arethenappliedtotheoriginalsignaltoremovetheeffectofthe aswescantolongerwavelengths.However,whenwescanback
with a down-scan, (red points) there is a systematically lower
transients.Thisautomaticallycorrectsthesignalforvariationsin
signal compared with the up scan. This is due to transient be-
theresponseaftercosmicrayimpacts,aswellascorrectingfor
largeoffsetsinindividualscansrelativetothemediansignal. haviorcausedbytherapidlychangingfluxduringthescans.The
effect is clearly shown in the ratio of the up and down scans
Onemaindifferencewiththereductionpipelineforchopped
(bottom panel of Figure 11). When the flux is changing very
spectroscopyisthat,atthispoint,thereisnoneedfortheappli-
rapidly (between 145–190 µm), the up-scan flux is 10% higher
cationofaspectralflatfieldcorrection.Inthestandardpipeline,
thanthedown-scanflux.However,from190–210µm,wherethe
this is a separate task that takes into account small variations fluxchangeismuchsmaller(onlyafactoroftwo),theeffectof
oftheresponseinthedifferentspectralpixels.Ourmodelingis
thetransientisalsosmaller(lessthan2%).
able to automatically rescale all the pixels relative to the same
Before we describe in detail how we apply a correction for
spatial module in each observational block without any further
thiskindoftransient,wefirstneedtodemonstratethatthetran-
operation.
sient caused by a sudden increase in continuum has a similar
The net effect of all our corrections is to significantly de-
formtothatofasuddendecreaseincontinuum.Wewillusethis
crease the noise in the final coadded spectrum compared with
symmetricalbehaviouroftransientstoprovideasolutiontothe
the SPG pipeline. Although the SPG pipeline attempts to mask
problem. To prove this symmetry, we consider an observation
largertransients,importantresidualsremain(seeFigure9).
whichwasespeciallydesignedtoinducetransientbehaviourof
thedetectorsduringobservationalday27(obsID1342178054).
In this observation, the grating was moved to several different
5.4. Correctingscandependenttransients
positions to have different levels of flux on the detectors. After
For many of the PACS spectra, only a small wavelength range each move, the grating was halted for 4 minutes to allow sta-
is observed. In this case we can consider that the continuum is bilization. We show in Figure 12 a part of this observation to
essentially constant during the observation. So, during a wave- demonstratetheresponsetosuddenjumpsinrecordedflux.The
Articlenumber,page9of14
A&Aproofs:manuscriptno.DarioFadda
1.1
malized Flux01..90
Nor
0.8
1.2
0 400 800
Frame No.
0.9
x
u
Fl
0.6
0.3
2500 3500 4500
Frame No.
Fig. 12. Transient effects on the measured signal when changing be-
tweentwogratingpositionsperformedduringanengineeringobserva-
tion during OD 27 (see text). In the inset, comparison of the upward
(blue)anddownward(red)transients.Thedownwardtransienthasbeen
flipped to compare the two behaviours. For a regime of flux changes
similar to the one found in the range observations, the two transients
followessentiallythesamebehaviour.Thisjustifiesthechoiceofget-
Fig.11. Toppanel:fluxintheup-scan(blue)comparedtothefluxin tingthemediansignalbetweenup-anddown-scansignals.
thedown-scan(red).Whentheincidentfluxisdecreasing(bluelinebe-
tween140and195µm)themeasuredfluxishigherthanthatmeasured
when the flux is increasing (red line in the same wavelength region). of the symmetry, we can assume that the differences between
Thesituationisinvertedbetween195and205µm,whilethetwofluxes thefluxesinFigure11isentirelyduetotransientbehavior.This
areverysimilarinthewavelengthregionbeyond205µmwherethereis
means that a reasonable solution is to scale the up- and down-
nobigvariationinflux.Bottompanel:ratiobetweenup-anddown-scan
scan values of the flux to the average of the two values at each
measuredfluxesforallthe16pixelinaspacemodule.Thebehaviouris
wavelengthinterval.Withthissimplesolution,accordingtothe
verysimilar.
observationinFigure12,thefluxrecoverediswithinafewper-
centoftheasymptoticincidentflux.Wenotethatachangeofa
factor of three in the continuum in one minute (as in the above
figureincludesthesignalreadoutjustbefore(blackline)asud-
example)isanextremecase.Inmostobservations,thechanges
denchangeinthepositionofthegrating,whichtheninducesan
influxthroughoutascanarelikelytobemuchless.
upward transient and new stabilization (blue line). After a suit-
ableinterval,thegratingischangedbacktotheoriginalposition
leading to a downward jump and transition (red line), followed
6. Examples
againbyaperiodofstabilization.Toinvestigatetheshapeofthe
upwardanddownwardtransientsinducedbythischangeincon- Inthefollowingsectionweprovideafewexampleswhichshow
tinuum, we show in the inset figure, the normalized signal for the improvements introduced by our transient-correction algo-
the upward– and downward–going signal, but with the down- rithms, as compared with the standard unchopped-mode HIPE
ward signal flipped in sign to allow a close comparison. If we analysis(SPGpipeline,vers.14).Weremindthatitisimportant
ignorethecosmicraytransientswhicharepresent,thebehavior toavoidexecutingpipelinescriptsblindlywithoutexaminingthe
oftheupward–going(blue)anddownward–going(red)transient effects of the corrections in the different steps of the reduction
responsesarealmostidentical. process.
In the above example, the largest part of the transient re- In this section we also show a few comparisons of obser-
sponse occurs within a time interval of approximately one vations made with unchopped and chop-nod mode of the same
minute,andtheexamplewechosecorrespondedtoachangein targets,toshowhowthereductiondescribedinthispapergives
flux of about a factor of three. We recall that in Figure 11, the consistentfluxcalibrationforthesameobservedlines.
up-and-down scan induced changes in continuum which were
largerthanthisoverthewholescan(whichlastedtwominutes),
6.1. Line
but are about the same order (roughly a factor of three) in one
minute. Therefore, the test of the hypothesis that the symmetry Asanexampleoftheimprovementinthereductionofalineun-
intheshapeoftheupwardanddownwardtransientisvalid. chopped observation, we show here an observation of Arp 220
If we make the assumption that indeed the upward– and (obsID1342202119).WeshowinFigure13(topleftpanel)the
downward–going transients have similar behaviors (except for cloud of flux–points for the central spaxel of the spectrometer
the sign), this suggests a workable way to correct for the tran- arrayafterpassingtherawdatathroughtheSPGpipeline.Inthe
sientsinalongscanoverarapidlychangingcontinuum.Because toprightpanelofthesamefigure,thesameobservationwaspro-
Articlenumber,page10of14