Dissecting accretion and outflows in accreting white dwarf binaries

1.4 MB·English

by  D. de Martino

Dissecting accretion and outflows in
accreting white dwarf binaries
5
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0
2

n
a
White Paper in Support of the Mission Concept of the
J

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Large Observatory for X-ray Timing
1

]
E
H
.
h Authors
p
-
o D.deMartino1,G.Sala2,S.Balman3,F.Bernardini1,4,A.Bianchini5,M.Bode6,
tr J.-M.Bonnet-Bidaud7,M.Falanga8,J.Greiner9,P.Groot10,M.Hernanz11,G.Israel12,
s
a J.Jose2,C.Motch13,M.Mouchet14,A.J.Norton15,A.Nucita16,M.Orio17,J.Osborne18,
[ G.Ramsay19,P. Rodriguez-Gil20,S.Scaringi9,21,A.Schwope22,I.Traulsen22,F.Tamburini5

1
v
7
6 1 INAF–OsservatorioAstronomicodiCapodimonte, Barcelona,Spain
7 Naples,Italy 12INAF–OsservatoriodiMonteporzio,Rome,Italy
2 2 Univ.PolitecnicadeCatalunyaandIEEC,Barcelona, 13ObservatoireAstronomiquedeStrasbourg,Stras-
0
Spain bourg,France
.
1 3 MiddleEastTechnicalUniversity,Ankara,Turkey 14LUTH,ObservatoiredeMeudonandUniversitéParis
0 4 NewYorkUniversityAbuDhabi,AbuDhabi,United Diderot,Paris,France
5 ArabEmirates 15TheOpenUniversity,WaltonHall,UnitedKingdom
1
5 UniversitádiPadova,Padova,Italy 16UniversitádelSalento,Lecce,Italy
:
v 6 LiverpoolUniversity,Liverpool,UnitedKingdom 17INAF–OsservatorioAstronomicodiPadova,Padova,
Xi 7 CEASaclay,DSM/Irfu/Serviced’Astrophysique, Italy
Gif-Sur-Yvette,France 18UniversityofLeicester,Leicester,UnitedKingdom
r
a 8 InternationalSpaceScienceInstitute(ISSI),Bern, 19ArmaghObservatory,Armagh,UnitedKingdom
Switzerland 20InstitutodeAstrofisicadeCanarias,LaLaguna,Spain
9 MaxPlanckInstitutfürExtraterrestrischePhysik, 21InstituteofAstronomy,Leuven,Belgium
Garching,Germany 22Leibniz-InstitutfürAstrophysikPotsdam(AIP),Pots-
10RadboudUniversity,Nijmegen,TheNetherlands dam,Germany
11InstituteofSpaceSciences,ICE(CSIC-IEEC),
Dissectingaccretionandoutflowsinaccretingwhitedwarfbinaries
Preamble
TheLargeObservatoryforX-rayTiming,LOFT,isdesignedtoperformfastX-raytimingandspectroscopy
with uniquely large throughput (Feroci et al. 2014). LOFT focuses on two fundamental questions of
ESA’s Cosmic Vision Theme “Matter under extreme conditions”: what is the equation of state of ultra-
dense matter in neutron stars? Does matter orbiting close to the event horizon follow the predictions of
generalrelativity? ThesegoalsareelaboratedinthemissionYellowBook(http://sci.esa.int/loft/
53447-loft-yellow-book/)describingtheLOFT missionasproposedinM3,whichcloselyresembles
theLOFT missionnowbeingproposedforM4.
TheextensiveassessmentstudyofLOFT asESA’sM3missioncandidatedemonstratesthehighlevel
ofmaturityandthetechnicalfeasibilityofthemission,aswellasthescientificimportanceofitsunique
core science goals. For this reason, the LOFT development has been continued, aiming at the new M4
launch opportunity, for which the M3 science goals have been confirmed. The unprecedentedly large
effectivearea,largegrasp,andspectroscopiccapabilitiesofLOFT’sinstrumentsmakethemissioncapable
of state-of-the-art science not only for its core science case, but also for many other open questions in
astrophysics.
LOFT’s primary instrument is the Large Area Detector (LAD), a 8.5m2 instrument operating in the
2–30keVenergyrange,whichwillrevolutionisestudiesofGalacticandextragalacticX-raysourcesdown
totheirfundamentaltimescales. ThemissionalsofeaturesaWideFieldMonitor(WFM),whichinthe
2–50keVrangesimultaneouslyobservesmorethanathirdoftheskyatanytime,detectingobjectsdownto
mCrabfluxesandprovidingdatawithexcellenttimingandspectralresolution. Additionally,themissionis
equippedwithanon-boardalertsystemforthedetectionandrapidbroadcastingtothegroundofcelestial
brightandfastoutburstsofX-rays(particularly,Gamma-rayBursts).
This paper is one of twelve White Papers that illustrate the unique potential of LOFT as an X-ray
observatoryinavarietyofastrophysicalfieldsinadditiontothecorescience.
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Dissectingaccretionandoutflowsinaccretingwhitedwarfbinaries
1 Summary
Accreting white dwarf binaries, cataclysmic variables, symbiotics, double-degenerates and novae, represent
idealastrophysicalenvironmentstostudyaccretionandoutflowprocessesinawidevarietyofplasmaconditions.
AmongthewhitedwarfaccretorsonefindsthepossibleprogenitorsofSupernovaetypeIa. Theyaresources
ofX-rayemissionwhosepropertiesstronglydependonfundamentalparameterssuchastheaccretionrate,the
massandthemagneticfieldoftheaccretingwhitedwarf. Thus,understandingtheprocessesofX-rayemission
andtheevolutionofcloseaccretingwhitedwarfbinariesisofutmostimportanceinawidercontext. TheX-rays
powerfullydiagnosetheconditionsshapingtheaccretionflowclosetothewhitedwarfaswellasthemechanisms
ofmassejectionandenergetics. Inrecentyearsaccretingwhitedwarfbinarieshavesurprisinglybeenfoundtobe
hardX-rayemitterswithahugevarietyofspectralandvariabilitycharacteristics. Understandingtheunderlying
physicalprocessesrequireshighsensitivityandtimingcapabilitiesasofferedbytheLOFT mission. Thethree
mainquestionstobeaddressedare:
Howdoesmatteraccreteontowhitedwarfs? LOFT willaddressthisbystudyingselectedsamplesoften
•
magneticandnon-magneticsystemseachtodetectforthefirsttimelow-amplitudefastaperiodicand
periodicvariabilitiesinthehardX-raysandperformingphase-resolvedspectroscopy.
Howdoesmassejectionworkinnovaexplosions? LOFT willinvestigatethisbymonitoring threenovae
• ∼
peryeartodetecttheonsetofhardX-rayemissionanditsspectralvariabilityalongoutburstaswellasto
characteriseforthefirsttimefastvariabilityinthehardX-rayregime.
Whatcausesdwarfnovaeoutburstdiversityandwhataretheconditionsfordisc-jetlaunching? LOFT
•
willaddressthisbyfollowing threedwarfnovaeperyearthroughtime-resolvedspectroscopyandfast
∼
timinginthehardX-rayswithunprecedenteddetails.
Thus LOFT will be crucial to measure the spectral and temporal properties of the poorly known hard X-ray
tailsinthesesystems. WhenLOFT isexpectedtobeoperational,widefieldareasurveyswillhaveprovided
statisticallysignificantsamplestoallowdetailedinvestigationincoordinationwiththelargefacilitiesforeseenin
thepost-2020timeframe.
2 Introduction
Accretingwhitedwarf(WD)binariesrepresentthemostcommonendproductsofclosebinaryevolution. They
encompass various classes of systems: the cataclysmic variables (CVs) in which the donor (secondary) star
is a late type main sequence star in a close orbit (P 1.4–14hr), the AM CVn stars where the donor is
orb
∼
a (semi)degenerate He-rich star in an ultracompact binary (P 10–65min) and the symbiotics where the
orb
∼
donor star is a late type giant or Mira star in wide (P years) orbit. Mass transfer occurs either through
orb
∼
Roche-Lobe overflow in the CVs and AMCVn stars or via stellar wind in the symbiotics. These binaries
display a wide range of timescales of variabilities (from sec–mins up to months–years) that make them an
ideal laboratory to study accretion/ejection processes in non-relativistic regimes. Disc instabilities, disc-jet
connection and magnetic accretion, that are common processes occurring in a wide variety of astrophysical
objectsfrompre-mainsequencestarstoBHbinaries,makeaccretingWDsachallenginglinktounveiluniversal
connections. Furthermore, thermonuclear runaways occurring during nova explosions have the potential to
unveil the accretion/ignition mechanisms, the chemical enrichment of the interstellar medium as well as the
effectsofaccretionontotheWDthatmaydriveitsmasstogrowovertheChandrasekharlimitandexplodeas
SNIa,akeyaspectforourunderstandingoftheprogenitors. Inaddition,populationstudiesofgalacticX-ray
sourceshaverecentlyrecognizedacrucialroleofaccretingWDbinaries. Ofparticularimportanceistheorigin
oftheGalacticRidgeX-rayEmission(GRXE),oneofthegreatmysteriesinX-rays,thoughttobecomposedof
Page3of12
Dissectingaccretionandoutflowsinaccretingwhitedwarfbinaries
20 50 10 3
CD IndBY Cam V834A CMen Her −1V
e 10 2
d fraction [%] 1105 V709 CaNsY LuApO PVs1c2−−2 3 00 S..15g rkkss 234000 Magnetic field stren −1alized counts s k 110
e g m
Puls 5 −− 15..00 kkss th [MG Nor10 −1
− 10.0 ks 10 ]
4
o
ati 2
0 0 R 0
10 1 10 2 10 3 10 4 2.0 5.0 10.0 20.0
LAD count rate [cts s−1] Energy [keV]
Figure1: Left: Feasibilityofdetectingfast 1HzQPOsinLADlightcurvesegmentsofdifferentlength(fromtopto
∼
bottom: 0.1–10ks). ThegreylinesshowtheminimumamplitudesvsLADcountratesforwhichperiodicitiescanbe
detectedata4σconfidencelevel,basedonZ2 statistics(Buccherietal. 1983). VerticallinesshowtheexpectedLAD
1
countratesforasampleofX-rayhardmCVsduringbrightstates,derivedfrompower-lawfitstoSwift/BATspectrafrom
Baumgartneretal.(2013). Thesamplecoversawiderangeofmagneticfieldstrengthsfrom11MG(CDInd)to40MG
(BYCam)forthePolarsandB.10MGfortheIntermediatePolars. Right: Simulated10ksLADspectraofafaintmCV
(blue;RXJ2133+51,aftermodelB2ofAnzolinetal. 2009)andabrightpolar(AMHer)(red)withF =3.5 10 11
and2.1 10 10ergcm 2s 1,respectively. Bothcompositespectracompriseabsorption,amulti-tem3−p2e0rkaetVurepost×-sho−ck
− − −
×
region,aComptonreflectioncontinuumandaGaussianFeKαfluorescentline. Theresidualscomparedtomodelsnot
accountingforreflection(lowerpanel)elucidatetheopportunitiesopenedbyLOFT evenforfaintobjectsandtheprospects
forphase-resolvedspectroscopy.
anon-negligiblepopulationofCVs(Revnivtsevetal. 2009;Revnivtsevetal. 2011;Yuasaetal. 2012;Hong
2012;Warwick2014).
TheX-raydomainiscrucialtounderstandthephysicalconditionofmatterclosetothecompactstar. Despite
importantprogressachievedinrecentyears,therearemanyopenquestionsthatstilldeservefullexploitation,
buthamperedbythelowquiescentluminosities(L 1029 33ergs 1)andtheunpredictabilityoflarge-scale
X − −
∼
variations, that can reach luminosities of 1032 1034ergs 1 in dwarf novae (DNe) outbursts and up to
−
∼ −
1038ergs 1 inNovaexplosions. Theuniquecombinationofhightemporal,moderatespectralcapabilitiesand
−
∼
sensitivityofthe
ladinstrument, aswellasthewidefieldcoverageoftheWFMinstrumentonboardLOFT,willbecrucialto
uncoverthewiderangeofvariabilitiesinthemostlyunexploredhardX-raydomain.
The major scientific goals are here summarized for accreting WD binaries for which LOFT will provide
breakthroughresults. InSect.3wewilladdresshowLOFT willallowtostudyX-rayoscillationsandaperiodic
variability to probe details of the accretion flow and its instabilities in CVs. In Sect. 4 we will discuss the
importance of the evolution and onset of hard X-ray emission in novae. In Sect. 5 we will address LOFT’s
capability to investigate the large diversity in the X-ray behaviours in DN outbursts and whether in these
scaled-downsystemsofNS/BHbinariesadisc-jetconnectionisalsoatwork. Finally,inSect.6theimportance
ofsuchX-raymissionwillbediscussedinthecontextofforeseenfacilitiesinotherwavelengthdomainsinthe
post-2020timeframe.
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Dissectingaccretionandoutflowsinaccretingwhitedwarfbinaries
3 How does matter accrete onto WDs? Probing accretion through oscillations
3.1 Accretion in WDs
AccretionofmatterontoWDcantakeplacethroughadisc,aring-likestructureoranaccretionstreamdepending
ontheWDmagneticfieldstrength,thebinaryorbitalperiodandmassaccretionrate. Thephysicalconditions
ofthematerialimpingingontotheWDandemittingX-raysarestilltobeinvestigated. Innon-magneticCVs
theWDmagneticfieldstrengthislowenough,B . 105G,toallowtheformationofanaccretiondiscand
WD
X-raysareemittedatthediscBoundaryLayer(BL).InhighlymagnetisedCVs(mCVs),B & 107G,material
WD
isdirectlyaccretedontotheWDpolesflowingalongthemagneticfieldlines. Apartialaccretiondisccanbe
formedatintermediatefieldstrengths. AttheWDpolesastand-offshockisformed. Thepost-shockaccretion
columnisthoughttocoolviabremstrahlung(hardX-rays),recombinationprocessesandcyclotronradiation.
TherelativeproportionofthesecoolingmechanismsstronglydependsontheWDmagneticfield(Fischer&
Beuermann2001).
BoththeBLandtheshockradiateintheX-raysasubstantialfractionofgravitationalenergyandthustheir
structureandstabilitycanbeefficientlyprobedthroughspectralandtemporalX-rayproperties. Ofparticular
importanceistheWDmassthatcanbedeterminedeveninfaintsources,thankstotheextendedcoverageand
uniquesensitivityofLOFT inthehardX-rays. ThisiscrucialtounderstandtheroleofWDbinariesasSNIa
progenitors. Mostly unexplored are non-periodic variations that uncover a wide range of timescales (from a
fewsectohundreds-thousandsofsec). Theseareobserveduptonowmainlyatopticalwavelengths,inboth
non-magneticandmagneticsystems. SearchesofX-raycounterpartswerecarriedoutinafewbrightsystems
leadingtopartial,sometimescontroversialresults.
3.2 QPOs in magnetic cataclysmic variables
Oneofthemostchallengingaspectsistheoriginof1–10squasi-periodic-oscillations(QPOs)inmCVs. Optical
narrowQPOswithafewpercentamplitudeweredetectedinsomemCVsofthepolartype(Middleditchetal.
1997), but not in all of them. If these represent oscillations arising in the post-shock region they should be
detectedintheX-raysaswell. AttemptstosearchtheX-raycounterpartshashoweverfailedsofar,providing
onlyupperlimits (. 10–20%)(Beardmore& Osborne1997;Imamuraetal. 2000;Pandel&Cordova2002;
Ramsay et al. 2007; Bonnet-Bidaud et al. 2014), mainly due to the lack of adequate sensitivity in the hard
X-rays. TheoriginofQPOsinmCVsisdebated,buthasgreatpotentialtoallowanunderstandingofradiative
shocks. ExtendedshocksarepredictedtobethermallyunstablegivingrisetoQPOs,inbothopticalandX-ray
domains. QPO characteristics (frequency, amplitude and phase) are related to the cooling timescales of the
emittingplasma(Wu2000). Pureradiativeshockswerehoweverstronglyquestionedonthebasisofstability
studies. Inparticular,inthepresenceofstrongmagneticfields(& 107G),cyclotroncoolinghasasignificant
roleindampingoscillations. Alternativeinhomogeneousaccretionornoise-drivenshockoscillationmodels
werealsoinvoked(Steiman-Cameronetal. 1994). Animportantconstrainonthemodelscanbeprovidedby
variationsofamplitudeandphaseoftheoscillationsandtheirenergydependence. Theloweramplitudesofthe
0.3-1HzQPOstogetherwiththeirstrongdependenceontherotationalphasemade,uptonow,thedetection
a challenge because it requires high sensitivity in the hard X-rays, where bremsstrahlung dominated shocks
emitmost. ThehighsensitivityandenergycoverageoftheLADwillbethencrucialtodetectfastX-ray
QPOs in mCVs down to amplitudes of 1% (Fig. 1, left panel). To fully test the shock structure stability, a
welldefinedsampleof 10hardX-rayemittingmCVscoveringawiderangeofWDmagneticfieldstrengthsis
∼
needed. ThisshouldincludemoderatefieldPolarsandthelow-fieldIntermediatePolarstobeobservedalong
their rotational cycles (few tens of minutes to a few hours, depending on their magnetic field strength). The
sampleshouldalsoincludethefewknowneclipsingsystemstoallowdirectmeasuresofthesizeandbrightness
distributions over the accretion spot thus uncovering the complex interplay between the magnetic field and
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Dissectingaccretionandoutflowsinaccretingwhitedwarfbinaries
matterintheaccretionregions. Inthiswayitwillbepossibletoprobeshockstabilityagainstawiderangeof
physicalconditionsofthepost-shockregiontobetracedwithphase-resolvedspectroscopyalongthespincycle
(Fig1,rightpanel). ParticularlyimportantwillbetheabilitytoconstraintheComptonreflectioncomponents,
ubiquitousinthesesystems,throughthefluorescenceFe Kαlineandcontinuumthatpeaksinthe10–30keV
range.
3.3 Rapid Oscillations in dwarf novae
Rapidfluxoscillations(DNOs)of 3–40sareobservedinhighmassaccretionrateCVs–mostlyduringDN
∼
outbursts(seealsoSect.5)andmainlyatopticalwavelengthswithafewdetectionsintheEUVorsoftX-rays
(seereviewinWarner2004). TheopticalDNOsarefoundtohavehighcoherenceandlowamplitude( 1 2%).
∼ −
Theyrelatetotheoutburstevolution,appearingduringtherise,persistingatmaximumanddisappearingduring
thedeclineofoutbursts. Theirperioddecreasesduringtherisetothemaximum,whileitincreasesduringthe
outburstdecay. ThefewsystemsmonitoredintheX-raysshowthatDNOsduringoutburstsoccurinthesoft
X-rays. Ontheotherhand,low-coherenceandlongperiod(mins)QPOswithamplitudes 10 40%aremainly
∼ −
observedinthehardX-raysandgenerallyappearduringthehardX-rayturn-onattheendofopticaloutbursts
(Wheatleyetal. 1996). Theircharacteristicperiodsandamplitudesvaryalongtheoutburst,butthesechangesdo
notseemtoberelatedtospectralvariations(Wheatleyetal. 2003).
The optical QPOs and DNOs (period ratio 15) correlate as the low and high-frequency QPOs of LMXBs
strengtheningtheevidenceofageneralrelationinallinteractingbinaries,irrespectiveofwhethertheprimary
is a BH, NS or WD (Warner et al. 2003). They are thought to be manifestations of disc accretion onto a
verylow-magneticfieldcompactobject. TheopticalDNOsarebelievedtobeduetomagneticcouplinginan
equatorialbeltwhiletheQPOstoreprocessingintheaccretiondiscoroscillationinthediscitself(Warner2004).
ThehardX-rayQPOscannothavethesameoriginandmayreflectvariationsinthemassaccretionrateatthe
innerBL.TheirappearanceisrelatedtothetransitionfromopticallythicktoopticallythinoftheBLandthus
haveagreatpotentialtoinferdetailsofBLregimes. ThestudyoffastoscillationsintheX-raysisthuscrucial
buthamperedsofarduetothelackofX-raycoverageoverabroadenergyrangeandadequatemonitoringofthe
differentevolutionofoutbursts(seeSect.5foroutburstdiversity). LOFT willbeabletodetecttheoscillations
andmonitorthefrequencychangealongoutburstevenwhensourceswillreachquiescentfluxes.
3.4 Aperiodic variability
Broad-bandvariability(flickering)isknowntobeanubiquitousfeatureamongstallaccretingcompactbinaries.
Inparticular,bothX-rayandopticalaperiodicvariabilitiesinCVshavebeenrecognisedforoveradecade. A
“fluctuatingaccretiondiscmodel”,wherevariationsinthemasstransferrategeneratedthroughoutthedisccouple,
multiplicatively,togiverisetotheobservedflickering,wasoriginallydevelopedforX-raybinaries(XRBs)and
ActiveGalacticNuclei(AGN)(Arevaloetal. 2006;Ingram&vanderKlis2013). Thismodelwassuccessfully
appliedtohighS/Nopticallightcurvesofthenova-likeMVLyr(Scaringietal. 2012;Scaringi2014). Itwas
alsoappliedtotheX-raylightcurvesofafewnon-magneticCVsinquiescence(Balman&Revnivtsev2012)
allowingtoinferdisctruncationfromtheobservedbreakintheX-raypowerspectra. Apossibleinfluenceofthe
weakmagneticfieldsinthesesystemswasalsoclaimed(Anzolinetal. 2010). TheX-rays,especiallythehard
range,diagnosetheinnerdiscregionsandhencearecrucialtoestablishthedrivingmechanismandwhether
discsaretrulytruncatedinquiescence. ThisrequiresmuchhigherS/Nachievedsofar,becausethesesystems
areratherfaintinquiescence. HighqualityandhightemporalresolutionlightcurveswillbeobtainedbyLOFT
allowingthefirstfaircomparisonofX-rayflickeringinCVswiththoseofXRBs. LOFT thusholdsthepotential
tounifyaperiodicvariabilityacrossallaccretingcompactsources,irrespectiveoftheaccretortype.
TostudybothX-rayQPOsandflickeringinnonmagneticCVs,asampleof 10systemsneedstobeexplored
∼
both in quiescence and outburst (see also Sect. 5). The sample should encompass a range of orbital periods,
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Dissectingaccretionandoutflowsinaccretingwhitedwarfbinaries
henceofmasstransferrates,toovercomeselectioneffectsandtodefinethestatisticalpropertiesofnon-periodic
variabilities. Fig.3,rightpanel,showsthegreatcapabilityofLOFT todetectQPOsandthefrequencybreak
expectedforasourceatcomparablefluxlevelsofSSCygduringtheturn-onofhardX-raysattheendofthe
opticaloutburst.
4 How does mass ejection work in nova explosions?
NovaoutburstsoccurwhenthepressureatthebaseoftheaccretedenvelopeontheWDinaclosebinarysystem
ishighenoughtoinitiateahydrogenthermonuclearrunawaythatleadstotheejectionofapartoftheaccreted
envelope; theexplosiondoesnotdisrupttheWD,sothattheclassicalnova(CNe)phenomenonrecursevery
103–105years. Recurrentnovae(RNe)showoutburstsrecurringoverahumanlifetimescale. Abouthalfofthem
havelongorbitalperiodswitharedgiantcompanion(symbioticRNe,SyRN).RNearethoughttohostamassive
WD,closetotheChandrasekharlimitandtheirhighmasstransferrateenablefrequentoutbursts. Theyarethus
consideredcandidateSNIaprogenitors, withthefinalfateoftheWDdependingonthebalancebetweenthe
massaccretionandejectionprocesses.
NovaeareluminousX-raysources(seestatisticsandreviewsinOrioetal. 2001;Orioetal. 2012;Schwarz
etal. 2011),withemissionspanningfromtheverysofttothehardX-rayenergyrange. WhilethesoftX-rays
probethecontinuousnuclearburningontheH-richenvelopeontheWDsurface,thehardX-raysarebelieved
totracemassoutflowsandtheonsetofresumedaccretion(Mukai&Ishida2001;Bodeetal. 2006;Sokoloski
etal. 2006;Nelsonetal. 2012;Orioetal. 2014). Thedetailsofthemassoutflow,theshapingandevolution
oftheejectedshellarestillverypoorlyknown. Recenteffortsinthestudyoftheejectamorphologyinclude
modellingoftheopticalandX-rayemissionlineprofilesandradioimaging(O’Brienetal. 2006;Orlandoetal.
2009;Ribeiroetal. 2013;Chomiuketal. 2014),butbetterqualityX-raydataatanearlypost-outburststagewill
beessentialtoprogress.
Ourlackofadetailedknowledgeoftheprocessesoccurringintheexpandingejectahasbeenrecentlyevident
withtheunprecedenteddetectionbyFermi/LATofveryhighenergy(VHE)gammaraysatenergieslargerthan
100MeV in four novae, during the first 2–3 weeks after outburst (Ackermann et al. 2014). In fact, particle
accelerationearlyaftertheexplosionwaspredictedfortheSyRNRSOph2006outburst(Tatischeffetal. 2007).
Such a mechanism produces gamma rays, which could not be detecetd before the advent of Fermi in 2008.
RSOphshowedcopiousX-rayfluxinthe10–50keVrangefromthebeginningoftheoutburstandforabout
3weeks,withRXTE andSwift/BAT(Sokoloskietal. 2006;Bodeetal. 2006). Thefactthatthefirstgamma-ray
novadetectedbyFermi/LATwastheSyRNV407Cyg2010(Abdoetal. 2010)wasinitiallyinterpretedasbeing
aprocessrelatedtothestrongshocksoftheexpandingejectacollidingwiththedenseredgiantwind. However,
theadditionalFermi-LATdetectionsofthreemorenovaewithoutgiantcompanionsshowthattheacceleration
of particles to high energies also occurs in the CNe ejecta, without invoking the dense red giant wind of the
SyRN.ThefurtherdetectionofevolvingradioemissionfromnovaV959Monhasrecentlybeenshowntobea
newtooltopinpointthelocationoftheparticleaccelerationresponsibleforthegamma-raysdetectedbyFermi
(Chomiuketal. 2014). Thus,themonitoringoftheearlyhardX-rayemissionofnovaeincoordinationwith
radiocoveragehastheuniquepotentialtounderstandwhetherhardX-raysaretrulyrelatedtotheprocessesof
particleacceleration.
ThecentralsupersoftX-raysourceshowsvariabilityandperiodicitiesonmanydifferenttimescales,ranging
from days to less than a minute. Periodicities of several hours can be attributed to orbital modulation, as in
V5116Sgr(Salaetal. 2008)whilethosewithtimescalesoffractionsofhourscanberelatedtotheWDspin.
Variationsat 19–37minwerefoundinsomenovaealsoinM31(Leibowitzetal. 2006;Dobrotka&Ness2010;
∼
Nessetal. 2011;Pietschetal. 2011)). Incontrast,atransient35smodulationfoundinthesoftX-rayemission
ofRSOphwassuggestedtoberelatedtoH-burninginstabilities(Nessetal. 2007;Osborneetal. 2011). Such
shortperiodoscillationshavenowbeenfoundinseveralothernovae. However,thelackofenoughsensitivity
Page7of12
Dissectingaccretionandoutflowsinaccretingwhitedwarfbinaries
Figure2: Simulations of the spectral evolu-
(cid:13) (cid:14) (cid:14)
tion of the SyRN RS Oph over its outburst, (cid:12)(cid:12) (cid:12)(cid:13) (cid:12)(cid:13) (cid:12)(cid:13) L()*1(cid:0)(cid:21)+
bothasseenbyLAD(upperpanel)andbythe F(cid:11) F(cid:11)(cid:12) F(cid:11)(cid:12) F(cid:11)(cid:12)
D(cid:3)(cid:4)(cid:5)
WFM (lower panel). The spectra include a
thermalplasmamodel(mekal)withsolarabun- 0 D(cid:3)(cid:4)(cid:6)
0
dances, evolving from kT = 8.2keV and a 40
flux of 3.2 10−9ergcm−2s−1 (0.5–20keV) -(cid:24)
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haspreventedhigh-resolutiontimingstudiesinthehardX-raysoriginatingintheejectaand,possibly,inthe
processofresumingaccretion.
LOFT willbeuniquelyabletobothmonitorwiththeWFMtheskyforbrightnovaoutbursts,andfollowthem
withLADtocharacterisespectralvariabilityaswellasfastvariabilitiesinthehardX-rayemissionthatwould
yieldinformationontheprocessesoccurringduringmassejection. ThebrightestandfastestRN,withfluxofthe
sameorderofmagnitudeasthatofRSOph(locatedat1.6kpc,Hjellmingetal. 1986),wouldtriggertheWFM
evenatadistanceof5kpcimmediatelyaftertheoutburst(Fig.2,lowerpanel). ThustheWFMwithitslarge
fieldofviewisanidealinstrumenttodiscoverthem. FornovaenotbrightenoughtotriggertheWFM,LAD
observationswouldbetriggeredfewdaysafteropticaldiscoveryandanalysiswithsomepre-establishedtrigger
criterion (based on distance, and/or fast evolution of the nova). Simulations show that the early hard X-ray
emission(asforexampledetectedinRSOphwithRXTEandSwift/BAT(Sokoloskietal. 2006;Bodeetal.
2006))willprovideahighrateintheLAD(Fig.2,upperpanel)toallowfasttimingstudiesforthecontinuumon
timescalesshorterthanasecondorafewminsforfluxesusuallydetectedinhardX-raysfornovaeinoutburst,
aswellastoinferspectralvariations(e.g.,theFelinecomplex)downtotensofminutes. Inthiswayitwillbe
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Dissectingaccretionandoutflowsinaccretingwhitedwarfbinaries
possibletoprobeindetailstheejectionprocessandtheshocksintheexpandingshellsduringtheoutburst.
ThankstotheSwiftobservationsofnovaeinthelastfewyears,weknowthataroundhalfofthediscovered
GalacticnovaearehardX-raysources(intheSwift/XRThardbandupto10keV).Thisrepresentsatotalof2–3
novaeperyear. TheLAD’sexceptionaleffectiveareawouldprovidespectrawithhighstatisticsforallofthem,
makingpossiblethefirstaccuratestudyofasampleof 10novaeduringthelifetimeofLOFT.Furthermore,
∼
whethersomenovaeremainhardX-rayemittersseveralyearsaftereruption,possiblyduetoamagneticnature
oftheWD,isanexcitingissue(Hernanz&Sala2002)thatLOFT willbeabletoaddress.
5 What causes DN outburst diversity and what are the conditions for disc-jet launching?
DNoutbursts(L 1032–1034ergs 1)arebelievedtobeduetodiscinstabilities. Theknowledgeofviscosityis
X −
∼
crucialtounderstandangularmomentumtransportinaccretiondiscs. Progresscanbemadebystudyingthose
phenomenathatchangeonviscoustimescales. ThegreatadvantageofDNearetheiroutbursttimescales(hence
viscoustimescales)ofonlyafewdaysorweeks,whichare,thus,easytoaccess. DNoutburstsarecharacterized
byopticalbrighteningby2–5maglastingtypicallyforaweekandrecurrencetimeofseveraltensofdays. Much
longeroutburstrecurrencetimesoccurinlowmassaccretionratesystems. Theoutburstcyclesareunderstoodas
duetodiscinstabilities(Lasota2001). Howeververylittleisknownabouttheinnerdiscregionsandhowthey
changefromoutbursttoquiescence. ThesearebesttracedintheX-rayregime(Baskilletal. 2005). TheX-ray
outburstspectrumhasshownhintsofthepresenceofmorethanonethermalcomponent: asoft(5–30eV)and
ahardone(10keV).Theformeroriginatesfromtheboundarylayeralthoughthefluxistoolowcomparedto
boundarylayermodels. Thismaybeattributedtoadensitygradientintheopticallythickboundarylayerso
thatahotopticallythinlayerisstillpresentandemitshardX-rays(Done&Osborne1997),thoughcolderand
weak(Popham1999). ThehardX-rayfluxinsteadrecoversthequiescentlevelattheendoftheopticaloutburst.
Onlyafewsystemshadintensemulti-wavelengthmonitoringcoveringmultipleoutbursts(Collins&Wheatley
2010). SomeDNeshowanoptical/X-rayanti-correlatedbehaviour,wherethehardX-rayfluxissuppressed
duringopticaloutburst. Others,likeSSCyg(Wheatleyetal. 2003;McGowanetal. 2004)ortheoldnovaand
magneticCVGKPer(Evansetal. 2009),havealsoshownincreasesofhardX-raysduringthetransitionphase
fromopticalquiescencetooutburstaswellasduringthefadingoftheoutbursts. HoweverthereareDNe,likeU
Gem(Güveretal. 2006)orGWLib(Bycklingetal. 2009),thatdonotshowhardX-raysuppression,butX-ray
enhancementduringopticaloutburst. SSCygalsoshowedsimilarbehaviourduringanomalousoutbursts. The
diversityofbehavioursdependsonseveralfundamentalparameters(Narayan&Popham1993),butstillfarfrom
beingunderstood. Arecentproposalthatdiversitymightberelatedtoacriticalvalueoftheaccretionratethat
dividesopticallythinandopticallythickBLregimes(Fertigetal. 2011)needstobeassessedwithobservations
ofalargesampleofDNe.
BesidestheonsetofDNOsandQPOsduringoutburst,DNehaveshownanotherstrikingsimilaritytotransient
LMXBs,furthersuggestinguniversalrelationsamongaccretingWDs,NSandBHs. Thesurprisingdetectionof
atransientradioemissioninSSCygitselfduringoutburst(Koerdingetal. 2008)wasinterpretedastheonsetof
aradiojet. DNewerebelievedtobeunabletopowerjets. Theinferredjetpowerandtherelationtotheoutburst
cycle were found analogous to those seen in XRBs, thus suggesting that the disc-jet coupling mechanism is
ubiquitous. TheadditionaldetectionofradioemissionfromV3885Sgr,aCVaccretingatahighratecomparable
toSSCyg(Koerdingetal. 2011),seemstosupportthatradiojetsalsooccurinbothsteadyandtransientCVs. It
hasbeenrecentlyshownthatnova-likeCVscanhaveopticallythininefficientaccretiondiscboundarylayer
(ADAF-like)(Balmanetal. 2014),allowingthepossibilitytoretainenergytopowerjets. Ifthiswerethecase,a
radio/X-raycorrelation,similartothatobservedinNSandBHbinaries(Coriatetal. 2011),shouldbepresentin
thesesystems.
Sofar,ahandfulofDNehavebeenobservedthroughmultipleoutburstsanditiscrucialtoobservesystems
spanning a range of orbital periods with adequate multi-outburst coverage. Bright outbursts such as those
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Dissectingaccretionandoutflowsinaccretingwhitedwarfbinaries
50
0 Anomalous Outburst 1
00 SS Cyg SS Cyg
1 Outburst 40
1
00 Quiescence
−1V 1 00
e 0
−1ounts s k 10 Power 1001
c
ed  1
aliz 10
m
or 1
n 0.
1
1
0
0.2 5 10 20 10−3 0.01 0.1
Energy [keV] Frequency [Hz]
Figure3: Left: Fromtoptobottom: Simulated10ksLADspectraofSSCygduringanomalousandnormaloutburstsand
inquiescence(F = 7,2,0.3 10 10ergcm 2s 1,respectively). SpectraarebasedonMcGowanetal. 2004that
3−20keV × − − −
includeathermalcomponent(mekal)andagaussianat6.4keV(EW = 82eV).Thetemperaturevariesfrom26keVto
20keVinoutburstsdecreasingto8keVinquiescence,recoveredat.4%level. Right: Simulated6kspowerspectraof
SSCygduringtwophasesofX-rayturn-onduringanormaloutburst(F =2,1.5 10 10ergcm 2s 1,redandblack
linesrespectively). Abrokenpowerlawwithbreakfrequency2 1032−H20zkeiVsassumed×. Th−eQPOsha−ve−periodsof155s
−
×
(blackline)and245s(redline)andrmsamplitudesof40%and30%,respectively(afterWheatleyetal. 2003).
displayedbySSCygorGKPerwillbeeasilydetectedbytheWFMandthenfollowed-upbytheLAD(Fig.3,
left panel), allowing spectral parameters to be determined at accuracy better than 4%. Fainter sources down
tothefluxlevelsasthatofGWLib( 2 10 11ergcm 2s 1)canbemonitoredbytheLADfromtheirearly
− − −
∼ ×
phasesonwardsthroughToOalertsfromopticalground-basedtelescopes. Itwillbepossibletorecoverthehard
X-raysuppressionandre-appearancetounderstandthediversityofbehaviours. Acrucialdeterminationwillbe
theWDmassthroughthestudyofthehardX-raytails. ShortorbitalperiodDNe(.2hr)havelong(several
yrs)dutycyclesandlong(months)outburstdurations,whilelong(&3hr)orbitalperiodDNedisplayshorter
durations ( weeks) and duty cycles (months) of outbursts. The monitoring of a sample of 10 DNe during
∼ ∼
LOFT lifetime,withmultipleoutburstcoverageforthosewithshortdutycycles,willallowtocharacterisenot
onlythespectralevolutionoftheoutbursts,butalsothenon-periodicvariations(seeSect.3)withunprecedented
detailsintheharddomain. Furthermore,properToOalertswithradiofacilitieswillbecrucialtoexplorewhether
theX-rayandradioluminositiesarerelated,thusprovingthesuspecteddisc-jetconnectionuniversalamong
LMXBsandCVs.
6 Context in post-2020 time frame
MostofthesciencegoalsthatcanbeachievedbyLOFT willgreatlybenefitfromforeseensynergiesamong
facilitiesinthepost-2020timeframeindifferentenergydomains.
PopulationofgalactichardX-raysources: RecentstudiesofGRXE(Revnivtsevetal. 2009;Revnivtsev
•
et al. 2011; Yuasa et al. 2012; Warwick 2014) and galactic bulge (Hong 2012) have revealed the
contributionofdiscretefaintsourcesthatabove2–3keVaremostidentifiedasmCVs. Thisimpliesthat
the galactic X-ray source composition should take into account a large population of faint hard CVs.
TheSwift/BATandINTEGRAL/IBIShardX-raysurveyshaveindeedrevealedthat 7–8%ofdetected
∼
sources( 20%ofgalacticsources)areaccretingWDbinaries,mostofthemagnetictype. Surprisingly,a
∼
numberofhardX-rayemittingnon-magneticCVs(mostsymbiotics)werealsoidentified. Thesecouldbe
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