NASA Technical Reports Server (NTRS) 20150002685: Variability of a Stellar Corona on a Time Scale of Days: Evidence for Abundance Fractionation in an Emerging Coronal Active Region

0.32 MB·English

by  NASA Technical Reports Server (NTRS)

A&A550,A22(2013) Astronomy
DOI:10.1051/0004-6361/201220491 &
(cid:2)c ESO2013 Astrophysics
Variability of a stellar corona on a time scale of days
Evidence for abundance fractionation in an emerging coronal active region
R.Nordon1,2,E.Behar3,andS.A.Drake4
1 Max-Planck-InstitutfürExtraterrestrischePhysik(MPE),Postfach131285741Garching,Germany
2 SchoolofPhysicsandAstronomy,TheRaymondandBeverlySacklerFacultyofExactSciences,Tel-AvivUniversity,
69978Tel-Aviv,Israel
e-mail:[email protected]
3 PhysicsDepartment,TechnionIsraelInstituteofTechnology,32000Haifa,Israel
e-mail:[email protected]
4 NASA/GSFC,Greenbelt,20771Maryland,USA
e-mail:[email protected]
Received3October2012/Accepted1December2012
ABSTRACT
Elementalabundanceeffectsinactivecoronaehaveeludedourunderstandingforalmostthreedecades,sincethediscoveryofthefirst
ionizationpotential(FIP)effectonthesun.Thegoalofthispaperistomonitorthesamecoronalstructuresoveratimeintervalof
sixdaysandresolveactiveregionsonastellarcoronathroughrotationalmodulation.Wereportonfouriso-phaseX-rayspectroscopic
observationsoftheRSCVnbinaryEIEriwithXMM-Newton,carriedoutapproximatelyeverytwodays,tomatchtherotationperiod
ofEIEri.WepresentananalysisofthethermalandchemicalstructureoftheEIEricoronaasitevolvesoverthesixdays.Although
thecoronaisrathersteadyinitstemperaturedistribution,theemissionmeasureandFIPbiasbothvaryandseemtobecorrelated.An
activeregion,predatingthebeginningofthecampaign,repeatedlyentersintoourviewatthesamephaseasitrotatesfrombeyond
thestellarlimb.Asaresult,theabundancestendslightly,butconsistently,toincreaseforhighFIPelements(aninverseFIPeffect)
withphase.WeestimatetheabundanceincreaseofhighFIPelementsintheactiveregiontobeofabout75%overthecoronalmean.
Thisobservedfractionationofelementsinanactiveregionontimescalesofdaysprovidescircumstantialcluesregardingtheelement
enrichmentmechanismofnon-flaringstellarcoronae.
Keywords.stars:atmospheres–stars:coronae–stars:abundances–X-rays:stars–stars:individual:EIEridani
1. Introduction micro-flaring, which is observed on the sun, but perhaps with
differentstatistics(e.g.,Caramazzaetal.2007).
Thebeststudiedcool-starcoronaisthatofourownsun,which Solar coronal chemical abundances are different from the
also providesthe only spatially resolvedstellar coronal source solarphotosphericcomposition.Thesolarcoronaitselfis non-
of extreme ultraviolet (EUV) and X-rays. From X-ray images uniforminthatrespect.Whileincoronalholes,theabundances
of the sun we know that the solar corona in its active state is are close to photospheric,abundances in active regions and in
non-uniform and that a significant fraction of the emission in the solar wind are significantly different. The abundance pat-
thesebandscomesfrommagneticloopstructuresconcentrated ternsdependonthefirstionizationpotential(FIP;Meyer1985;
inactiveregionsaroundphotosphericsun-spots.Evidently,these Feldman 1992; Sheeley 1995). Elements with FIP < 10 eV
structuresevolveinbrightnessontimescalesofhours.Themor- (low FIP) are enriched in the corona relative to elements with
phology as well as the metal abundances vary on longer time FIP > 12 eV (high FIP). Averagingover the entire solar disk,
scales of days. Coronal structures on the Sun may survive for theenrichmentfactorisabout4(Feldman&Widing2002,and
a few weeks, although tracking them continuously from the referencestherein).SuchaFIP bias,in varyingintensities, has
groundisdifficultduetothe∼24.5daysolarrotationperiod. beenreportedformanycoronalstructures,fromclosedmagnetic
Active coronaeof other cool stars can be up to five orders loopstostreamers(seereviewsby Kohletal.2006;Doschek&
of magnitude more luminous and an order of magnitude hot- Feldman2010).Currentlythereisnogenerallyacceptedmodel
terthanthesolarcorona.Althoughunresolved,theyshowmany thatexplainstheFIPbias.
characteristicssimilartothoseofthesolarcorona,andarethere- TheFIPeffectbecameevenmorepuzzlingwhenhighreso-
forebelievedtobestructuredandtoevolveinasimilarway.For lutionX-rayspectrafromXMM-NewtonandChandrarevealed
the most part, late-type stars have envelopes in which convec- thatactivestellarcoronaedonotfollowthesolarFIPpattern.In
tionanddifferentialrotationinteractto drivea dynamo,which somecases,thehigh-FIPelementsareevenenrichedcompared
producesmagneticflux tubes.Thefluxtubes float intothe up- tothelow-FIPones,aneffectlabeledtheinverse-FIP(IFIP)ef-
per atmosphere and eventually the corona. It is generally un- fect(Brinkmanetal.2001).LaterstudiesrevealedthatFIPand
clear,however,whetherstellarcoronaeactuallyhavelargercov- IFIPbiasesarecorrelatedwithcoronalactivity(andage):Highly
eringoftheirstellarphotosphere,orevenqualitativelydifferent activestarsshowanIFIPeffect,whilelessactivecoronaehavea
morphologies,such as inter-binarymagnetic structures. Stellar solarFIPbias(Audardetal.2003;Telleschietal.2005),imply-
coronae show high variability, often attributed to continuous ingatransitiononstellar-evolutiontimescalesofGyr.
ArticlepublishedbyEDPSciences A22,page1of9
A&A550,A22(2013)
Table1.Observinglog.
Obs.ID Starttime Endtime Exposure[ks]
060317201 2009-08-01T00:02:50 2009-08-01T09:13:05 32.84
060317301 2009-08-02T23:06:16 2009-08-03T09:24:46 36.92
060317401 2009-08-04T22:20:49 2009-08-05T07:42:41 33.54
060317501 2009-08-06T20:59:39 2009-08-07T07:31:30 37.48
Onthefaceofit,thecorrelationwithactivitysupportedpre- M4-5 secondary,with a possible 19-year orbit of a third body
vioussuggestionsthatabundancesmaybelinkedtoflares(e.g., (Washuettletal.2009a).
Güdeletal.1999,2004;Ostenetal.2003;Nordonetal.2006). LongtermphotometricmonitoringofEIEriseemedtoindi-
However, direct systematic measurements of abundance varia- catea∼11yrbrightnesscyclethatwasconjecturedtobeanana-
tionsduringflaresshowthatregardlessofcoronalFIP/IFIPbias, logofthesolaractivitycycle(Oláh&Strassmeier2002),butun-
duringlarge flares the abundancestend to photosphericvalues likethesun,longtermDopplerimagingofEIEribyWashuettl
(Nordon&Behar2007,2008).ThemechanismforFIP/IFIPis, et al. (2009b) shows little variability of its cool starspots
therefore, related either to the long-term evolution of coronal over11yr.As forthehotcorona,Ostenetal.(2002)observed
structures,or tothe continuousmicro-flaringthat producesthe EI Eri with ASCA (and EUVE) and fitted its spectrum with a
quiescenceemission,butnottolargeflares. 2T model of kT = 0.72 keV and 2.2 keV, but the abundances
Inrecentyears,amodelhasbeenproposedinwhichabun- couldnotbetightlyconstrainedwithASCA.TheXMM-Newton
dancefractionationisdrivenbymagnetohydrodynamics(MHD) CCDspectraofashortflareonEIErion2009-08-06duringthe
waves(Laming2004,2009,2012).This“pondermotive”forceis presentcampaignwasanalyzedbyPandey&Singh(2012),who
controlledbytheenergydensityofMHDwavesatthebasesof suggestedtheflareoriginatedfromaloopthesizeof∼R∗ (pri-
thecoronalloops.Woodetal.(2012)studiedasampleofX-ray mary),whichisaboutatenthofthebinaryseparationdistance.
faint(L < 1029 ergs−1)dwarfsandfoundnocorrelationwith
X
X-ray luminosity or surface flux, but a good correlation with
spectral type instead. According to this relation M dwarfs are 2. Observations,data,andanalysis
IFIP biased while early G dwarfs are FIP biased. Wood et al.
InTable1welistthelogofthefourXMM-Newtonobservations
(2012)claim that this shift from FIP to IFIP can be explained
of EI Eri. Note the start times for each observation,which are
bytheinversionofthepondermotiveforceatcertainconditions.
separatedbyapproximatelytheEIEriorbitalperiodof168.5ks.
ThelatterstudyislimitedtoX-rayfaintdwarfsandmaynotbe
applicabletothetypicalhighlyactivebinariesfromotherstud-
iesthataremorethantwoorderofmagnitudemoreluminousin 2.1.Datareduction
X-rays,emittedmostlybyagiantorsub-giantstar(RSCVntype
XMM-Newton data were reduced using the Science Analysis
systems).
Software(SAS–ver.10.0.0)inthestandardprocessingchains
Despite the assumed similarity between solar and stellar
asdescribedintheABCGuidetoXMM-NewtonDataAnalysis1,
coronae, we have lacked a study of the evolution of a stellar
andusingthemost updatedcalibrationfiles (date22.06.2010).
corona on time scales of days, which is the relevant time for
PileupwascheckedusingtheepatplotroutineoftheSASsoft-
coronalloopevolutiononthesun.Thegoaloftheobservations
warepackage.Nosignificantpileupwasdetectedineitherofthe
presentedinthispaperistoseekandmeasurethevariabilityof
EPICinstrumentsandthuseventsfromsingletoquadruplewere
temperatures and abundances in an active corona over several
retained.Backgroundwaslowduringalltheobservationsandno
days.Inordertoavoidconfusionbetweenrotationalvariability,
significantamountoftimewaslost.
whichisinevitableduetotherapidrotationtypicalofactivestel-
lartargetsandthecoronabeingcomposedofdiscreteactivere-
gionsasonthesun,andgenuinecoronal-activityvariability,the 2.2.Spectralanalysis
present observing campaign is designed to return to the target
We measure the temperature and abundance properties of the
eachtimeatexactlythesameorbitalphase.
EIEricoronabyfittingthermalplasmamodelstotheobserved
spectra. XMM-Newton comprises several different instruments.
1.1.EIEri Thelongerwavelengths(6−38Å)arecoveredbythetwohigh
resolution reflection grating spectrometers (RGS) that resolve
To carry out a few iso-phase observations, we chose the tar-
get of EI Eridani (EI Eri, HD 26337), an X-ray bright (L = theemissionlines.Thehighenergyendupto15keViscovered
1031.1 erg/s)RS CVnbinary,whoseorbitalperiodis 1.95dxays bythetwoEPIC-MOSandsingleEPIC-pnCCDs,thatprovide
lowerspectralresolution.
(168.5ks),whichisverysimilartotheexactly2.0dayorbitalpe-
Tostudythethermalandchemicalcompositionofthesource
riodofXMM-NewtonaroundEarth.ThisenabledXMM-Newton
plasma,itisimperativetofirstproperlyaccountforitsemission
to observe the exact same orbital phase of EI Eri during four
consecutiveorbits,andforsufficientlylongexposureseachtime measuredistribution(EMD)thatisdefinedas
(∼35ks).EIEriisanearby(50.8pc,vanLeeuwen2007),rapidly EMD(T)=n n dV/dT. (1)
rotating(vsini=51kms−1),non-eclipsing,rotationallylocked, e H
close RS CVn binary with an inclination angle to the rotation wheren andn aretheelectronandprotonnumberdensities,
axis of ∼56◦ ±4.5◦ (Washuettl et al. 2009a). These properties respectiveely, anHd dV/dT is the temperature distribution of the
makeEIEria populartargetforDopplerimagingandstarspot plasma volume. The EMD is usually characterized by several
monitoring that indicate the primary star to be highly active.
EIEriconsistsofasolar-massG5IVprimaryandamuchfainter 1 Seehttp://heasarc.gsfc.nasa.gov/docs/xmm/abc
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R.Nordonetal.:Variabilityofastellarcoronaonatimescaleofdays
discretetemperaturebins.Here,weuseseventemperaturebins
with identical abundances. To allow for a uniform analysis of
thefourobservations,thetemperaturesofthemiddlefivecom-
ponents kT = 0.7, 1.0, 1.5, 2.0, and 2.5 keV are held fixed,
while the lowest and highest temperatures are allowed to vary
and are determined by fitting the model to the observed spec-
tra.WefittheobservedspectrausingtheXSPEC(Arnaud1996)
softwarepackageandaVAPEC(Smithetal.2001)plasmaspec-
tralmodel.Forhighstatistics,wefitthetwoRGS,EPIC-pn,and
EPIC-MOS1spectraofeachobservation(orbit)simultaneously.
EPIC-MOS2 was used in timingmode and its spectra are sys-
tematicallyoffsetbya small amountfromEPIC-pnandEPIC-
MOS1thatagreewitheachotherverywell.Forthisreason,we
excludeitfromthespectralfitting.Weignoretheverylowen-
ergy(E<0.35keV)portionsoftheCCDspectra.Visualinspec-
tion of each RGS spectrum indicates the models do an overall
goodjoboffittingthelines.Seetheappendixforthespectraand
Fig.1. EPIC-pn light curves during the four consecutive EI Eri rota-
fitted model. The absorptioncolumn density towards EI Eri is
tionorbits,indicatedbysequentialIDnumbers.Thetimeaxishasbeen
negligible(NH = 1.2 × 1020 cm−2,Pandey&Singh2012)and folded to represent the rotation phase within the 1.95 days period of
wasnotincludedinthemodel. EIEri.
Thesevenplasmacomponentsprovideonlyageneralideaof
thetemperaturedistributionintheplasma.Itisknown(seemost
recently, Testaetal.2012)thattheEMDreconstructionshould Thisbrighteningwithphaseismostdramaticduringthethird
be treated with caution. More specifically, the EMD is highly observation(401),∼4daysintothemonitoring,butshouldstill
degenerate between adjacent components. Therefore, we keep notbeconfusedwithacoronalflarethatwouldtendtosharply
trackofthecovariancematrices(betweenparameters),whichare risewithinafewksandquicklydecayover∼10ks.Indeed,dur-
fullytakenintoaccountwhenpropagatingtheerrorsofall de- ing the last exposure(Obs. 501),a large flare is superimposed
rivedquantities.Duetothisdegeneracy,themoreimportantand onthegraduallyincreasinglightcurve,risingsharply(∼1.5ks)
reliable quantity is(cid:2)(cid:3)the cumula(cid:4)tive, or progressively integrated abouthalfwaythroughtheexposureanddecayingexponentially
onatimescaleof∼4.8ks.Foranalysispurposes,wetherefore
emissionmeasure EMDdT ,whichwedenotebyIEM.The
furthersplit observation501into its quiescenceandflare peri-
calculated errors for the IEM are more meaningful than those
ods,asindicatedinFig.1,andrefertothemasObs.501Qand
of the EMD, as they are essentially independentof the choice
501F,respectively.
oftemperaturebins,as opposedto errorson theEMD that are
affectedbythearbitrarychoiceoftemperaturesandbytheprox- The mean count rate between visits (∼130 ks) also varies,
increasing within the first 4 days, between Obs. 201 and 401
imityoftheothertemperaturecomponents.MonteCarlo(MC)
by ∼20%, and then decreasing by ∼30% by the beginning of
simulationsareusedforerrorestimates.TheMCsimulationran-
Obs.501.Interestingly,justbeforetheflareand∼5daysintothe
domlyrollsanewsetofEMDvaluesaccordingtothecorrelation
monitoring,thesteadyquiescentlevelisobservedtobebackto
matrixandindividual-componenterrorsobtainedfromthebest-
itsinitiallevelwhenthemonitoringstarted(Obs.201,301).
fitsolution.Thecentral68%ofthedistributionforeachpointin
theIEMis takenastheequivalentofthe1-σconfidenceinter-
val.Foramoredetaileddescriptionoftheerrorpropagationsee 3.2.Temperature
AppendixB.
InFig.2weplottheEMD(T)foreachobservationbasedonour
7-temperaturesbest-fittedmodels,whereeachEMDbinisdueto
asingletemperaturecomponent.Thewidthofeachplottedbin
3. Results
isarbitraryanddesignedtogiveanimpressionofacontinuous
3.1.Lightcurves distribution,althoughinrealitytheseareiso-T components.As
with the light curves, the EMD of observations 201, 301, and
BysynchronizingtheXMM-Newtonexposureswiththerotation 501Qare fairly similar, primarilyshowingemission of plasma
phaseofEIEri,weareabletoobservethesamestellarsurface uptokT (cid:6) 2keV,whiletheEMDofobservation401features
during each visit, and to thus remove most of the rotation ef- some excess high-T emission up to kT (cid:6) 4 keV. During the
fects.TheEPIC-pnlightcurvesofthefourexposuresareplotted flare (Obs. 501F), significant emission measure is observed at
inFig.1versustherotaionalphase.Onlyabout20%oftheorbit, kT ≥4keV.
i.e.,35ksor∼0.4days,iscoveredduringeachvisit,andconsec- TheIEMsforeachobservationarethenplottedinFig.3.The
utivevisitsareseparatedbytheremainderoftheEIEriperiod IEMinobservations201,301and501Qareconsistentwithno
(∼2days). variationinthetotalemissionmeasure(seethehightemperature
Betweenphases0–0.1,all4exposuresshowasteadycount endofFig.3),andshowverysmallvariationsbelow1keV.On
rate with some variability on time scales of minutes, but at theotherhand,theEIEricoronaduringobservation401hasless
a low amplitude that is not much above the Poisson noise. emissionmeasurebelow1keV,butahighertotal,implyingasig-
This variability is typical of active coronae, and has been as- nificantexcessofplasmaathighertemperatures,mostlybetween
cribedtomicro-flaring.Thesecondhalfofeachobservation,i.e., 2.5–3.5keV.Duringtheshortflare(Obs.501F),onecanseean
phases0.1−0.2,featuresagradualriseinfluxthatisinterpreted excessofemissionmeasurebetween1–2keVandanothersmall
astherecurrentappearanceofanactiveregiononthatpartofthe componentabove3keVthatbringsthetotalemissionmeasureto
star. alevel∼40%higherthanthequiescentstatesofObs.201,301,
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A&A550,A22(2013)
201 301
401 501Q
501F
Fig.4.VariabilityoftheEMD-weightedmeantemperature.Horizontal
errorbarsindicatetheexposuretimeofeachobservation.
kT[keV]
Fig.2. Emission measure distribution as obtained by the best-fitted
7Tmodelsofeachobservation.
Fig.5.CoronalabundancesrelativetoFeandexpressedrelativetothe
solarX/Feabundanceratio(Asplundetal.2009).Pointsfromdifferent
observationsareprogressivelyshiftedtotherightalongthehorizontal
axisintheorderinwhichtheyweretaken(∼2daysapart)toenhance
visibilityandtoemphasizethetimevariability.
Fig.3.CumulativelyintegratedemissionmeasuredistributionIEMcal-
culatedfromtheEMDsofFig.2.Errorstakethedegeneracybetween
thedifferenttemperaturecomponentsofthemodelsintoaccount. dramaticallybeforeandduringtheflare.Temperaturevariability
duringquiescenceis moresubtle,withameankT of1.60keV
andastandarddeviationof0.13keV.
andthepre-flare501Q,whichisalsosomewhatlowerthandur-
ing Obs. 401. In short, the emission measure of the quiescent
3.3.Abundances
coronaofEIEricanvarybetweenrotationorbits,andontime
scale of days, but mostly in the 1−2 keV temperature regime. InFig.5weplottherelativeabundancesofthemajorelements
Excessemissionmeasureofhotterplasmaupto4−5keVisob- with respect to Fe, in solar-abundance units (Asplund et al.
servedbothinactiveregions(Obs.401),andmoredramatically 2009),andasafunctionoftheirfirstionizationpotential(FIP).
duringtheflare(Obs.501F). In the plot, the data points are progressively shifted along the
Aconcisewaytoquantifytheplasmaconditionsisthrough x-axisaccordingto the orderin time in whichtheywere mea-
itsmeantemperature(cid:8)kT(cid:9),weightedbytheEMDofeachcom- sured,thuscreatingatime-likesequenceforeachelement.The
ponent.TheerrorsareagaincomputedusinganMCsimulation, abundancesare expressedrelative to the Fe abundanceinstead
while taking the correlations between the fitted EMDs of the ofthemorecommonabsolutevaluesrelativetoH.Theabsolute
variousmodelcomponentsintoaccount.Theresulting(cid:8)kT(cid:9)val- FeabundancewithrespecttoHis0.32,0.32,0.38and0.31so-
ues are plottedin Fig. 4 for each observationand against time lar in observations 201, 301, 401 and 501Q respectively, with
elapsed from the beginning of the first observation. A similar a typical error of ±0.03. In the APEC model and the global
pattern as before emerges. When the source is brighter (e.g., spectral fit approach in the XSPEC software that we use, the
Obs.401)itisalsohotteronaverage.Obs.201and301donot absolute abundance is constrained by the flux in the Fe lines
strictly follow this pattern, but the temperature decrease from and the overall fit to the continuum. While lines are emitted
Obs.301toObs.201isinsignificant.Intermsof(cid:8)kT(cid:9),weob- fromalimitedrangeoftemperautes,continuumfluxatallRGS
serve temperature variations on time scales of days, and most wavelengths is contributed from all temperature components.
A22,page4of9
R.Nordonetal.:Variabilityofastellarcoronaonatimescaleofdays
Absoluteabundanceisthereforesensitivetotheunderlyingtem-
peraturedistribution,whichisnotsowellconstrainedatthehigh
temperatures.Conversely,estimatesofabundancesrelativetoFe
relyonwell-measuredlineratiosthattendtoemergefromalim-
ited and overlappingrange of temperatures.The large number
of observedFe ions and lines, whose peak emissivities span a
wide temperature range from kT ∼ 0.3−1.5 keV makes these
measurementssignificantlylesssensitivetothetemperaturedis-
tributionin the plasma. The selection of the referenceelement
for describing abundances does not affect the general pattern
ofabundancevariations.We normalizetheabundanceratiosto
those of the solar photospheric composition of Asplund et al.
(2009)inordertoplotthemonthesamescaleandtodiscussthe
FIPeffect.
As seen in Fig. 5, EI Eri shows the typical inverse FIP ef-
fectobservedinthemostactivecoronae(Brinkmanetal.2001;
Audardetal.2003).Inparticular,theNeabundanceisenriched
Fig.6.FIPbias(FBfromEq.(2))foreachobservationandasafunc-
by a factor∼3 comparedto Fe, and a lesser enrichmentis ob- tionoftime.Horizontalerrorbarsindicatetheexposuretimeofeach
served for C, N, and O (all high FIP) over Fe (low FIP), The observation.
abundanceratioofthelow-FIPelementsMgandSitoFeisap-
proximatelysolar,althoughwithrespecttoH,theyareallsub-
solar(∼0.3). On the other hand, the present flare on EI Eri does not follow
Littlevariationoftheabundanceswiththelevelofactivityis thistrend,buttheweakabundancevariationobservedisconsis-
observed.TheO(highFIP)abundancedoesseemtobelowdur- tentwiththeabundancevariationsobservedinflaresoncoronae
ingthehigh-statesofObs.401and501F(flare),andhighduring with similar quiescent FB values of about −0.2 (see Fig. 5 in
the low-activitystate of Obs. 501Q.This is reminiscentof the Nordon&Behar2008).
chromosphericevaporationeffectofsolar-compositionmaterial
during flares in an otherwise (quiescent) inverse-FIP coronae,
4. Rotationeffects
whichwasobservedbyNordon&Behar(2007,2008)inasam-
ple of flares. ThehighlyenrichedNe abundanceis slightly re- The four exposures that all cover the same rotation phase al-
ducedinthehighstateofObs.401.However,despitethegood lowustostudythedependenceoftemperaturesandabundances
signal,nosignificantvariationintheNeabundancecanbede- onphase,byfurtherbreakingupeachobservationintosmaller
tectedduringanyotherperiod,notevenduringtheflare,which segmentsand co-addingthe similar phase segmentsfromeach
would be expected if chromospheric evaporation takes place. observation. If active coronal regions are compact, and sparse
The Mg/Fe abundance ratio comes down by ∼20% during the inlongitude,thentherotationwouldcreatevariationsintheob-
flare, but this can notbe a FIP relatedeffect, as bothelements servedX-rayemissionproperties,asactiveregionsrotateinand
arelow-FIP. out ofview,overthe limb ofthe star. If the activeregionslast
Tobetterquantifytheabundancevariations,asinglenumber longerthanthemonitoringtimeof6days(assolaractiveregions
thatquantifiestheFIPbiascanbedefined(followingNordon& do),thenstackingdatafromthesamephase,butfromdifferent
Behar2008) orbits,canincreasethesignalofthisrotationalmodulation.
FB=(cid:5)(cid:5)log(AZ/AZ(cid:10))F(cid:6)IP<10eV(cid:6)−(cid:5)log(AZ/AZ(cid:10))FIP>10eV(cid:6) (2) tatesDbuyrin7g2edaecghrefuesll,oi.bes.,er2v0a%tioonfotfh∼e3o6rbkist.(TThabelein1c)l,inEaItiEonriarno--
where log(A /A ) istheerror-weightedmeanabundance(in gle of the rotationaxis to the line of sight of 56◦ implies only
Z Z(cid:10)
log)ofthelow/highFIPelements,relativetoasetofreference about10%ofitssurfacegraduallyappearsduringtheobserva-
abundances,inthiscasethesolarabundances.Weusethetradi- tionandadifferent10%graduallymovesoutofsight.Wedivide
tionalsolardistinctionbetweenlowFIP(<10eV)andhighFIP eachobservationintofourquartersof ∼9ks each,duringeach
(>10 eV). A positive FB value indicates a solar-like FIP bias, ofwhichtheEIErisystemrotatesby18degrees.Thedataare
whileanegativeFBvalueindicatesanIFIPbias. thenreducedinquartersandthespectraofeachinstrumentare
TheFIPbiasesinthefiveobservations(201,301,401,501Q, combined per phase bin. The two late phase bins of Obs. 501
501F)areshowninFig.6.TheFBvariationsoverthecourseof are excluded from the analysis, due to the flare (see Fig. 1).
thepresentcampaignarenotdramatic,butstillsometrendscan Subsequently,we performthe same EM andabundanceanaly-
be identified. The FB in Fig. 6 generally varies with activity, sisasbefore,butforeachphase-stackedspectra.
namelytheFBincreases,i.e.,tendsto(solar)photosphericval- Figure 7 shows the IEM for each phase bin, Q1–Q4. The
ues,astheactivityincreases,risingfromObs.201toObs.301, first three phase bins show the same temperature structure. In
andfurtherinObs.401,andthendecreasinginObs.501Qwhen thelastphasebin(Q4),thereisanexcessofemissionattemper-
the count rate returns to its lower level. For comparison, see atures below ∼1.5 keV, which slightly increases the total IEM
the light curves in Fig. 1. This trend breaks down during the of Q4, althoughit is just consistent with the previous quarters
flarewhentheactivitystronglyincreases,buttheFBdecreases. towithintheerrors.Thisexcessemissionmeasureisaresultof
At face value, this means that as the (quiescent) coronal ac- the rise in flux observedin the light curves(Fig. 1) duringthe
tivity increases,perhapsmoresmall active regionsappear,and last quarter of all of the observations, but most conspicuously
more high-FIP elements are preferentially brought up into the in Obs. 401. A similar rise in flux can be seen also in the last
corona.Thisprocessis reminiscentoftheevaporationofchro- quarter of Obs. 501, which is somewhat masked by the flare,
mospheric material observed in the largest flares on the most and therefore excluded from the analysis. Figure 7 can be ex-
active(strongestIFIP)coronae(Nordon&Behar 2007,2008). plained by an active region that is not visible at all duringthe
A22,page5of9
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Fig.9.CoronalabundancesrelativetoFeandexpressedrelativetothe
Fig.7.Cumulativelyintegratedemissionmeasuredistribution(IEM)as solarX/Feabundanceratio(Asplundetal.2009).Pointsfromdifferent
afunctionofthefourphasebinsQ1−Q4. phasebinsQ1−Q4areprogressivelyshiftedtotherightalongthehor-
izontalaxistoenhancevisibilityandtoemphasizethevariabilitywith
phase.
Fig.8. Mean temperature for the four rotation phase bins. Very little
evolutionisobservedcomparedtothedays-scaletemperaturevariations
Fig.10.FIPbias(FB)asafunctionofrotationphaseaveragedoverall
(Fig.4).
availableobservationsinfouriso-phaseintervals.Horizontalerrorbars
reflectthe∼9ksdurationineachbin.
twoearlierquarters,thatcomesintoviewfrombeyondthelimb
during Q3 or Q4 (i.e., after ∼40 degrees of rotation), and that between consecutive bins. However, Mg, O and Ne, the three
graduallybrightensduringthefourdaysafterthebeginningof elements with the best signal do appear to vary monotonously
theXMM-Newtonmonitoring,i.e.,itbecomesparticularlybright withphase.TheabundanceratioofMg/Fedecreaseswhilethat
duringObs.401andObs.501. ofO/FeandNe/Feincreases.Theotherelementstendtoshow
InFig.8weplotthemeantemperatureasafunctionofphase. asimilarmonotonicvariation,althoughwithmuchlargeruncer-
ThevalueofkTeff is verystablebetweentheobservedrotation tainties.Whilethetrendisoflowsignalineachelement,there-
phases, despite the added emission measure. In fact, it shows peatedpatterninseveralelementsmakesitmorebelievable,and
much less scatter than the scatter from one observation to the lesslikelyduetonoise.Moreover,sincethetrendofthelow-FIP
other over several days (Fig. 4). The lack of temeprature vari- Mgisoppositetothatofthehigh-FIPNeandO,wesuspectit
ationissomewhatexpected,sinceeachphaserepresentatime- is due to a FIP related effect and not due to only Fe changing
averageof4−6days,andvariationsonshortertimescalestendto withphase.
averageout.Notethatheretoo,thetwolastquartersofObs.501 AsinSect.3.3,wewishtoenhancetheindividualabundance
areexcludedfromtheanalysisduetotheflare.Theemergingac- signals as to reflect potential FIP related effects by computing
tiveregion(presumablyamagneticloop)reachesthemeantem- the FIP bias from Eq. (2) for each phase-stacked spectra. The
peratureof(kT ∼ 1.5keV),whichis verysimilar tothemean FIP bias as a function of phase is plotted in Fig. 10. The FB
temperatureofthecoronalregionsontherestoftheEIEriob- shows a mild, yet monotonous decrease with phase. This can
servedarea. beinterpretedastheemergingactiveregioncontributinganin-
Themeanabundancesforeachrotationphasebinareplotted verse FIP bias. In other words, the active region(Q3, Q4) has
inFig.9.Again,thedatapointsareprogressivelyshiftedalong slightly more high-FIP elements than the background corona
thehorizontalaxisfromlefttoright,inordertogiveanideaof (Q1,Q2).Notethatthistime-averagedrotationeffectisdifferent
theevolutionwithphase.Thevariationinabundancewithphase fromthetime-evolutionobservedoverdays(Fig.6),inwhichthe
for each element is very small, and statistically insignificant FBincreaseswithincreasedactivity.Here,theincreasedactivity
A22,page6of9
R.Nordonetal.:Variabilityofastellarcoronaonatimescaleofdays
comingfromaspecificregiononthestaristhelikelycauseof Indeed, on the sun, it has been reported that solar coronal
thedecreaseinFB. loops evolve on time scales of days (e.g., Widing & Feldman
The weakness of the observedsignal is not unexpected,as 2001). Newly emerging structures tend to have photospheric
thefractionoftheEIErisurfacerotatinginandoutofourview composition,butdevelopthe solarFIPbias withinafewdays.
isnotverylarge,∼5%betweenquarters.Saywemeasureduring Moreover,onthesun,thetypicalenhancementoflowFIPabun-
rotationphaseQ4,inwhichtheactiveregionisvisible,amean dancesrelativetophotosphericvaluesbyafactorof4isreached
abundance A(4) which deviates from the mean abundance dur- within2−3daysandcontinuestoevolvefurther,reachingafac-
Z
ingphaseQ1 A(1) (inwhichtheactiveregionisnotvisible)by torof8ormore.Theanalogousinterpretationandoneinagree-
Z mentwiththepresentobservationsisthatnewcoronalloopson
α≈0.1,i.e.A(4) =(1+α)A(1).Thetotalcountrateisincreasing
Z Z EI Eri emerge with photospheric composition and develop an
withphase,indicatingthatthiseffectislikelyduetoadifferent
IFIPbiasovertimescalesofdays.ThespatialmeanIFIPbiasis
abundancein the active regionwhich is rotatinginto view,for
determinedbymeanageoftheloops.Thus,periodsofgradual
whichAregion = (1+β)A(1).Sincethecontributionstothespec- brighteningofthecoronaarecharacterizedbytheemergenceof
Z Z
trumofboththe backgroundandtheactive-regionabundances youngloops that reduce the global IFIP bias, while periodsof
areweightedbytheirrespectiveemissionmeasures,andweat- gradual dimming are characterized by aging of existing loops
tributealltheexcessemissionmeasureduringQ4overQ1tothe thatincreasetheglobalIFIPbias.Notethatthenegativecorrela-
activeregion,thenonecaneasilyshowthat: tionbetweenemissionmeasureandIFIPbiasasdescribedabove
α relatestothedaysscaleevolutionofagivenstellarcoronaandis
β= 1−(EM(1)/EM(4)) (3) notatoddswiththegeneralpositivecorrelationfoundbetween
activitylevelandtendencytoIFIPbiasinstellarpopulations.
where EM(4) and EM(1) are the total emission measures in
Rotational modulation in X-rays and EUV has been re-
Q4 and Q1 respectively. According to Fig. 7, the EM ratio is
portedforHR1099(Agrawal&Vaidya1988;Drakeetal.1994;
EM(4)/EM1 ≈ 1.15.Therefore,theactualabundancevariation
Audardetal.2001),fromlightcurves,butphasedependentspec-
intheactiveregionbycomparisonwiththespatial-meanback-
troscopy was not possible, due to the constant flaring of that
groundcoronaisattheβ≈0.75level,ora75%strongerIFIPef- source,whichmakesitdifficulttodisentanglebetweenrotational
fectthanthemeancorona.Admittedly,thisconclusionforEIEri
modulationofactiveregionsandtheirintrinsicactivityvariabil-
is based on a marginal signal and is, thus, associated with ap- ity. A different, yet notable, phase dependent measurement is
preciableuncertainty,butitdoespointtosignificantabundance
thatoflineprofilesbyBrickhouseetal.(2001),whichenabled
variationsin a compactactive regionwith respect to the mean
latitudedeterminationofactivecoronalregionsonthestellarbi-
corona.
nary44iBoo.Thepresentmeasurement,however,isthefirstto
measurephasedependentabundances.
5. Discussionandconclusions The active regionthat appears towards the end of each ex-
posure persists throughout the six days of the observing cam-
ThelightcurvesandabundancepatternsinthecoronaofEIEri
paign. It therefore likely contains loops that are at least a few
show an active region appearing over the limb as the corona
daysold.Inthescenariowhichweproposedabove,theseloops
rotates with the star. Although on time scales of days, the in-
willhavealreadyevolved(overdays,possiblystartingevenbe-
creaseoftheactivitylevelseemstoleadtoanincreaseoftheFB
forethebeginningofthecampaign)andtheirabundancesdevel-
(i.e.,lessIFIPbias,abundancesmoreakintothephotosphere),
opedastrongIFIPbias,strongerthanthecoronalmeanbiasthat
the appearance of the active region with phase features an in-
includesyoungerloops.Thus,asthisregioncomesintoview,the
crease inhigh-FIPelementsand a loweredFB (i.e. a tendency
spatiallyaveragedabundancestendtowardsastrongerIFIPbias
toastrongerIFIPbias).Twodifferenteffectsareatplay:oneis
(Fig.10).
theevolutionoftheemission measureandcoronalabundances
After showing that large flares tend to counter the coronal
onatimescaleofdaysbetweenexposures,spatiallyintegrated
FIP effect (Nordon & Behar 2007, 2008) – the present work
andaveragedovertheentirevisiblesurfaceofthestar.Theother
promotes the notion that the abundances in individual coro-
isanalready-existing,compactandbrightactive-regionthatper-
nalactiveregions,differfromthemeancoronalabundancesas
sistsoverthesixdaysdurationofthemonitoringcampaignand
well. It is not clear whether large flares show coronal abun-
rotates into view towards the end of each exposure. Thus, the
dances that are closer to photospheric composition than the
activeregionsismanifestedasvariationswithphaseinsteadof
quiescent corona because of added material from the photo-
time.
sphere/chromosphere (chromospheric evaporation) or because
One cannot tell whether the days-scale variations in count
theseflareshappenpreferentiallyinyoungactiveregionswhere
rates, emission measure, and abundances are due to the evo-
theabundancesareclosertophotosphericthantheglobalmean.
lution of existing coronal loops, or due to the appearance of
Theabundancesmeasuredduringtheflareobservedinthiscam-
new coronal loops and disappearance of old loops. However,
paignshowanincreaseIFIPbias(morenegativeFB,Fig.6)in-
the correlation in which an increase in emission measure and
steadofthetypicaltendencytobringabundancestowardspho-
countratescorrespondstoadecreaseintheIFIPbias(increase
inFB,closertophotosphericcomposition)offersaninteresting tospheric composition.If the flare is related to the limb active
regionthat is stronglyIFIP biased,it is possible that the mean
interpretation.Ifthebrighteningisduetotheemergenceofnew
abundances,weightedbetweentheflaringactiveregion,evapo-
loops,andthoseappearinitiallywithnearlyphotosphericcom-
ratedchromosphericmaterialandsurroundingquiescentcorona
position, then the mean coronal abundances will tend towards
endupbeingdominatedbytheabundancesoftheactiveregion.
photosphericcompositionduringthese times. Conversely,dur-
ing times in which no new loops emerge and existing loops While the proposedscenarioinwhichyoungloopsemerge
age, evolve in composition and perhaps some fade, the mean withphotospheric-likecompositionandthendevelopovertime-
abundanceswilltendtoresemblethecompositionoftheolder, scaleofdaystoanIFIPbiascanexplainourobservations,itdoes
evolvedloops,i.e.differentthanphotospheric. notexplainwhytheabundancesevolveanIFIPeffectinsteadof
A22,page7of9
A&A550,A22(2013)
Fig.A.1.RGSspectrum(RGS1andRGS2combined)ofObs.ID060317201. Plottedmodelisasimultaneousfittoallinstruments(seeEPIC
spectrainFig.A.2).
Fig.A.2.EPIC-pnandEPIC-MOS1spectraofObs.ID060317201.Plottedmodelisasimultaneousfittoallinstruments(seeRGSspectrumin
Fig.A.1).
asolar-likeFIPeffect.Foratheorythatattemptstoresolvethis the uncertainties in the normalizations of the various compo-
issueseeLaming(2004,2009,2012). nentsarehighlycorrelated,especiallywhenmanycomponents
are used.Thecorrelationtendsto be negativeandincreasesas
Acknowledgements. ThisworkwassupportedbyTheIsraelScienceFoundation thecomponentsarecloserintemperature,i.e.,adjacenttemper-
(grant#1163/10)andbytheMinistryofScienceandTechnology.
aturescancompensateforeachother.Thisresultsinlargeerrors
on each individualtemperaturecomponent,whichcan be seen
AppendixA: Spectra inFig.2.Ontheotherhand,theexchangeofemissionmeasure
betweenclose-temperaturecomponentshas onlya small effect
Samplespectraandbest-fittedmodelarepresentedin this sec- onthecumulative,orintegratedemissionmeasure(IEM).Still,
tion. The X-ray spectra obtained from our first observation thespectrallineemissivitiesaretemperaturedependentandthus
ID 060317201, as measured by all XMM-Newton instruments theIEMisnotperfectlyconserved.Inaddition,therearetheun-
used in the analysis are shown in Fig. A.1. This observation correlateduncertaintiesassociatedwiththe limitedstatistics of
hasthelowestcountrates(seeFig.1).Consequently,thespec- thedata.Toestimatethehighlynonlinearcontributionsofthese
traofthethreeotherobservationshaveslightlybettersignal-to- effectsontheuncertaintiesintheIEM,wealludeaMonte-Carlo
noiseratios,butareoverallsimilartothoseshowninFig.A.1. simulation.
InSect. 4, eachobservationis dividedintofoursegments,and
The fitting algorithm in XSPEC gives, along with the best
thefouriso-phasesegmentsarethenstacked,resultinginspec-
fitmodel,thecovariancematrixofsecondderivativescalculated
trathatare,again,withnumbersofphotonsandsignal-to-noise
at the best-fit solution. Here, we use this covariancematrix to
ratiosthatarecomparabletothoseoftheindividualobservations.
redraw a random,new set of model EMD values on the given
temperaturegrid(seeFig.3)thatiscloseinitsstatisticalprop-
AppendixB: IEMand(cid:2)kT(cid:3)errorestimates ertiestotheoriginalbest-fitmodel,inthesensethatitproduces
spectral fits to the data that are almost as good as the best-fit
TheEMDsolutionstocoronalemissiontendtobehighlydegen- model.To generatea randomset of numberscorrespondingto
erateasemissionmeasurecanbetradedbetweendifferent,but a correlation matrix R, one can decompose R into R = U(cid:12)U.
close temperatures with little effect on the resulting spectrum. If X is a vector of uncorrelatedrandomvalues, X = XU is a
c
Thus,whenrepresentingtheEMDwithisothermalcomponents, setofcorrelatedrandomvaluescorrespondingtoR.Theprocess
A22,page8of9
R.Nordonetal.:Variabilityofastellarcoronaonatimescaleofdays
of generating random EMDs is repeated numerous times, and Fludra,A.,&Schmelz,J.T.1999,A&A,348,294
foreachEMDsolutionthecorrespondingIEM(kT)iscalculated Güdel,M.,Linsky,J.L.,Brown,A.,&Nagase,F.1999,ApJ,511,405
ateachtemperaturepointonthegrid.Thus,wegeneratealarge Güdel,M.,Audard,M.,Reale,F.,Skinner,S.L.,&Linsky,J.L.2004,A&A,
416,713
sampleofIEM(kT)valuesateachtemperaturepoint,fromwhich
Kohl,J.L.,Noci,G.,Cranmer,S.R.,&Raymond,J.C.2006,TheA&AR,13,
weadoptthecentral68%astheequivalentofthe1-σuncertainty 31
intervalthatis,then,plottedinFigs.3and7.Foreachgenerated Laming,J.M.2004,ApJ,614,1063
randomEMDwealsocalculatetheemission-measureweighted Laming,J.M.2009,ApJ,695,954
mean temperature(cid:8)kT(cid:9). The central 68% of the (cid:8)kT(cid:9) distribu- Laming,J.M.2012,ApJ,744,115
Meyer,J.P.1985,ApJS,57,173
tionis adoptedas the1-σuncertaintyintervalon (cid:8)kT(cid:9), plotted
Nordon,R.,&Behar,E.2007,A&A,464,309
inFigs.4and8. Nordon,R.,&Behar,E.2008,A&A,482,639
Nordon,R.,Behar,E.,&Güdel,M.2006,A&A,446,621
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