The puzzling case of the accreting millisecond X-ray pulsar IGR J00291+5934: flaring optical emission during quiescence

0.23 MB·

by  M. C. Baglio

Astronomy&Astrophysicsmanuscriptno.IGRJ00291_paper_arXive (cid:13)cESO2017
January11,2017
The puzzling case of the accreting millisecond X–ray pulsar IGR
J00291+5934: flaring optical emission during quiescence(cid:63)
M.C.Baglio1,2, S.Campana2, P.D’Avanzo2, A.Papitto3, L.Burderi4, T.DiSalvo5, T.MuñozDarias6,7, N.
Rea8,9, D.F.Torres8,10
1 Universitàdell’Insubria,DipartimentodiScienzaeAltaTecnologia,ViaValleggio11,I-22100Como,Italy
e-mail:[email protected]
2 INAF,OsservatorioAstronomicodiBrera,ViaE.Bianchi46,I-23807Merate,Italy
3 INAF,OsservatorioAstronomicodiRoma,ViadiFrascati33,I-00044MontePorzioCatone,Italy
4 DipartimentodiFisica,UniversitàdegliStudidiCagliari,SPMonserrato-Sestukm0.7,09042Monserrato,Italy
7 5 UniversitàdegliStudidiPalermo,DipartimentodiFisicaeChimica,viaArchirafi36,I-90123Palermo,Italy
1 6 InstitutodeAstrofísicadeCanarias,E-38205LaLaguna,Tenerife,Spain
0 7 DepartamentodeAstrofísica,Univ.deLaLaguna,E-38206LaLaguna,Tenerife,Spain
2 8 InstituteofSpaceSciences(CSIC-IEEC),CampusUAB,CarrerCanMagranss/n,E-08193Barcelona,Spain
9 AntonPannekoekInstituteforAstronomy,UniversityofAmsterdam,Postbus94249,NL-1090GEAmsterdam,TheNetherlands
n 10 InstitucióCatalanadeRecercaiEstudisAvançats(ICREA)Barcelona,Spain
a
J

9

ABSTRACT
]
E
We present an optical (gri) study during quiescence of the accreting millisecond X-ray pulsar IGR J00291+5934 performed with
H
the10.4mGranTelescopioCanarias(GTC)inAugust2014.Althoughthesourcewasinquiescenceatthetimeofourobservations,
. itshowedastrongopticalflaringactivity,morepronouncedathigherfrequencies(i.e.thegband).Aftersubtractingtheflares,we
h
tentativelyrecoveredasinusoidalmodulationatthesystemorbitalperiodinallbands,evenwhenasignificantphaseshiftwithrespect
p
to an irradiated star, typical of accreting millisecond X-ray pulsars, was detected. We conclude that the observed flaring could be
-
o a manifestation of the presence of an accretion disc in the system. The observed light curve variability could be explained by the
r presenceofasuperhump,whichmightbeanotherproofoftheformationofanaccretiondisc.Inparticular,thediscatthetimeofour
t observationswasprobablypreparingthenewoutburstofthesource,whichoccurredafewmonthslater,in2015.
s
a
Keywords.
[

1
v
1. Introduction on,systemsalternatingwithinashorttimefromtheAMXPstate
1
to the millisecond radio pulsar state were discovered (named
2AccretingmillisecondX-raypulsars(AMXPs)areasubclassof
transitionalpulsars),whichfullyvalidatedthepicture(Archibald
3
transient low-mass X-ray binaries (LMXBs) hosting a weakly
2 etal.2009;Papittoetal.2013;deMartinoetal.2013;Bassaetal.
magnetised fast-spinning (a few ms) neutron star (NS) that ac-
0 2014;Royetal.2015).
cretes matter from a low-mass companion (M (cid:46) 1M ). Typi-
. (cid:12)
1cally, their spin periods span between 1.7 and 5.4 ms, whereas Optical observations of AMXPs during quiescence are the
0theorbitalperiodsrangebetween40minand19hr(seePatruno onlywaytostudythecompanionstar,sinceduringoutburstsits
7 emissionisotherwiseoverwhelmedbythatoftheaccretiondisc.
&Watts2012forareview).
1 Theopticallightcurveresultingfromtheseobservationsisusu-
Formanyyears,AMXPswerethoughttobetheprogenitors
: allymodulatedattheorbitalperiodofthesource.Whenthelight
vof millisecond radio pulsars. According to this recycling sce-
inario (Alpar et al. 1982), millisecond radio pulsars arise from curve is dominated by the elongated, Roche-lobe deformed ge-
X
ometry of the companion star, a double-humped light curve is
thespin-upoftheneutronstarshostedinLMXBsasaresultof
r observedwithtwoequalmaximaatthedescendingandascend-
the transfer of angular momentum (and mass) from their com-
a
ing nodes of the companion star (orbital phases 0.25 and 0.75,
panion stars. After a mass accretion phase, during which the
respectively) and two unequal minima at superior and inferior
systemappearsasabright(possiblytransient)X-raysource,the
conjunction (orbital phases 0 ad 0.5, respectively). In systems
mass transfer rate declines and allows for the re-activation of a
withsmallorbitalseparations,however,thelightcurvecanpos-
rotation-poweredpulsarthatemitsfromtheradiotothegamma-
sess one single maximum at phase 0.5 when irradiation from
rayband.ThefirstAMXPthatconfirmedthisscenariowasSAX
the compact object plays a major role and dominates geomet-
J1808.4–3658,inwhichacoherentpulsedsignalat2.5msinthe
ric effects (see e.g. D’Avanzo et al. 2009). A single-maximum
X-rays was discovered (Wijnands & van der Klis 1998). Later
lightcurveisoftenobservedintransientLMXBsinquiescence,
(cid:63) Based on observations made with the Gran Telescopio Canarias whichtestifiestothestrengthofirradiationeffects.Sincetheob-
(GTC),installedintheSpanishObservatoriodelRoquedelosMucha- servedquiescentX-rayluminosityisnotsufficienttoaccountfor
chos of the Instituto de Astrofísica de Canarias, in the island of La theobservedopticalmodulation,theenergyreleasedbyarotat-
Palma. ingmagneticdipolehasbeensuggestedasthemainresponsible
Articlenumber,page1of6
A&Aproofs:manuscriptno.IGRJ00291_paper_arXive
factorforirradiationinquiescentLMXBsandAMXPs(Burderi 3. Results
etal.2003;Campanaetal.2004).
The system IGR J00291+5934 is an AMXP that contains As can be observed in Fig. 1, the light curves of IGR
J00291+5934areplaguedbystrongflaringactivitythatispromi-
the fastest X–ray pulsar (1.67 ms) as compact object and has
nentinalltheobservedbandswithasmallpreferenceforshorter
anorbitalperiodof2.46hr(Gallowayetal.2005).Thesystem
wavelengths,andwithupto1magvariability.Wetriedtofilter
wasfirstdetectedduringanoutburston2004December2inan
the flares by determining a magnitude threshold by visual in-
INTEGRAL observation (Eckert et al. 2004; Shaw et al. 2005).
spectionforeachband.Onthiswedefinetheflaringactivity(i.e.
Thecompanionstaristhoughttobeahotbrowndwarf,derived
g = 23.25,r = 22.25,andi = 21.40).Unfortunately,adetailed
for an isotropic distribution of inclinations and a NS mass of
analysis such as reported in Jonker et al. (2008) is not possible
2M (Galloway et al. 2005). The source distance has not been
(cid:12) with our dataset because of the poor temporal resolution. Af-
firmly determined to date (a lower limit of ∼ 4 kpc has been
ter this rough filtering, we tried to recover the possible orbital
given by Galloway et al. 2005 by equating the long-term mass
modulation, hidden by the flares (see Fig. 1). We fit the three
accretionratededucedfromthefluenceobservedinoutburstto
folded light curves together, imposing them to have the same
theexpectedgravitational-wave-drivenmasstransferrate).
orbital phase; the reduced χ2 of the best fit is not compelling
IGR J00291+5934 was first observed during quiescence by
( χ2/dof ∼ 187.2/18), however, probably due to a combined
Chandra and ROSAT in 2005 at a 0.5-10 keV luminosity level effect of the very small errors and residual flaring activity. The
of∼1032ergs−1(Jonkeretal.2005)andwaslaterconfirmedby
resultsofthefitsarereportedinTable1.
XMM-Newtonobservations(Campanaetal.2008).
The source IGR J00291+5934 appears brighter than in
TheopticalcounterpartwasidentifiedwithanR∼17.4mag
D’Avanzo et al. 2007 and Jonker et al. 2005 (0.6-0.8 mag and
star during outburst with weak He II and Hα emission (Fox 0.2-0.3 mag, respectively). This effect is confirmed even when
& Kulkarni 2004; Roelofs et al. 2004; Torres et al. 2008; see takingthedifferentfiltersusedbytheseauthorsintoaccount(see
also Lewis et al. 2010). The quiescent optical counterpart was
Sec.4).Inaddition,thephaseofthemaximumisnotat0.5(su-
discovered by D’Avanzo et al. (2007) and Torres et al. (2008)
periorconjunctionofthecompanionstar)butat0.21±0.02.A
as a faint R = 23.2 ± 0.1 object. Sinusoidal variability at the
smallshiftwasalsofoundbyJonkeretal.(2005),whofoundthe
knownorbitalperiodwithanRand I semi-amplitudeof∼0.2-
maximumatphase0.34±0.03.
0.3 mag suggested strong irradiation (D’Avanzo et al. 2007).
The semi-amplitude of the modulations (Table 1) is in line
Based on VRIJHK photometry, the required irradiating lumi-
with previous results by D’Avanzo et al. (2007), who found a
nosityis∼ 5×1033 ergs−1,whichismuchhigherthantheob-
modulation of 0.22±0.09 by combining R and I data, but not
served X-ray luminosity in quiescence (D’Avanzo et al. 2007).
with Jonker et al. (2005), who found a very small amplitude in
Jonker et al. (2008) observed IGR J00291+5934 in quiescence
theI bandof0.06±0.01.However,weemphasisethatallthese
for several orbital periods in the I band. They found evidence
resultsarestronglydependentontheflarefiltering.
of strong flaring (up to 1 mag). After removal of the strongest
Inordertoinvestigatethenatureoftheflaringactivity,weex-
opticalflares,aweakorbitalmodulationisstillpresent.
tractedthespectralenergydistribution(SED,gri)aroundphase
In this paper we investigate the quiescent optical emission
0.2 (where a prominent flare is present) and around phase 0.9
of IGR J00291+5934 in more detail through observations per-
(when the system is likely quiescent). Data in the r band were
formed with the 10.4m Gran Telescopio Canarias (GTC). We
taken as reference, and gi data (mag vs MJD) were linearly in-
obtainedg,r,idataovermorethanoneorbitalcycle(seeSect.2).
terpolated to estimate the flux at exactly the same phase (Fig.
Strongflaringvariabilityisalsoapparentinourdata.Wederive
2).Datawerethencorrectedforintrinsicabsorptionevaluatedin
anorbitalmodulationandspectralenergydistributionsclosetoa
D’Avanzo et al. (2007) and quoted in the last column of Table
flarepeakandduringanintervaloflowflaringactivity(Sect.3). 1. The two SEDs are markedly different. The flare SED can be
The discussion is presented and conclusions are drawn in Sect. fitted with a power law with index α = 0.31±0.35 (1σ c.l.),
4.
indicatingthattheemissionpeakislikelyathigherfrequencies
than the g band. The index is fully consistent with what is pre-
dicted for a multi-temperature accretion disc with T > 30,000
2. Observationsanddataanalysis
K.The“quiescent”SEDinsteaddecreaseswithfrequency,with
ThesourceIGRJ00291+5934wasobservedinquiescencedur- a power-law best fit of α = −1.06 ± 0.35. This is difficult to
interpret and could arise at the downturn of an accretion disc
ing the night of 2014 August 31, using the Optical System
spectrum, immediately after the peak (in this case, a disc tem-
for Imaging and low-Intermediate-Resolution Integrated Spec-
perature of T ∼ 10,000 K is needed), or it could be the tail of
troscopy (OSIRIS; Cepa et al. 2000) camera mounted on the
the emission coming from the companion star (a brown dwarf;
GTCwiththeg,r,ifilters(PI:A.Papitto).Theseeingwasgood
and remained < 1(cid:48)(cid:48). Fifteen images in each g,r,i filters were Gallowayetal.2005).
taken,withanexposuretimeof240s,180s,and150s,respec-
tively (and cycling among filters), with the aim of covering the
4. Discussion
2.48hrorbitalperiod.
Image reduction was carried out using the standard proce- Wereportastudyoftheoptical(gri)lightcurvesoftheLMXB
dures, which consist of subtracting an average bias frame and IGRJ00291+5934performedduringquiescence,followingtwo
dividing by a normalised flat frame. Aperture photometry was studies in the NIR–optical of the quiescent light curves of the
performedonthefieldusingthedaophottask(Stetson1987)for samesystem(D’Avanzoetal.2007;Jonkeretal.2008)thatre-
allthesourcesinthefield.Thephotometriccalibrationwasmade porteddifferentresults.
against the Stetson standard field star PG1528 (Stetson 2000). D’Avanzoetal.(2007)providedopticalandNIRphotometry
Weperformeddifferentialphotometrywithrespecttoaselection ofthesourceduringquiescence,withdataacquiredin2005,less
ofisolatedandnon-saturatedfieldstars,inordertominimiseany thanoneyearaftertheendofanoutburstin2004(Eckertetal.
possiblesystematiceffect. 2004;Shawetal.2005),andfoundsinusoidalvariabilitymodu-
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M.C.Baglioetal.:OpticalphotometryofIGRJ00291+5934duringquiescence
Fig.1.Leftpanel:Fromtoptobottom,g,r,ilightcurves(magvs.MJD)ofthesystemIGRJ00291+5934(dots)andofacalibrationstar(squares).
Circleddotsrepresentthepossibleflaringactivity.Rightpanel:Fromtoptobottom,g,r,ilightcurves(magvs.orbitalphase)ofthesystemIGR
J00291+5934.Circleddotsrepresentthepointscorrespondingtotheflaringactivity.Superimposedonthelightcurveswesketchtheconstantplus
sinusoidalfitofthedata,carriedoutafterfilteringtheflaresandimposingthesamephaseonallthelightcurvesandwiththeperiodfixedto1.In
therband,thepointcorrespondingtophase0.73isconsideredasaflareforcontinuitywiththeotherbands.Phase0correspondstotheinferior
conjunctionofthecompanionstar.WeevaluatedthephasesbasedontheX-rayephemeridesofGallowayetal.(2005).Twoperiodsofthesystem
aredrawnforclarity.
latedatthesource2.46horbitalperiod,withasemi–amplitude tributionofIGRJ00291+5934,consistentwithanirradiatedstar
of 0.2–0.3 mag, in both the R and the I band. The light curves only,withnoneedforaresidualaccretiondisc.
peakedatphase∼0.5,suggestingastronglyirradiatedcompan-
ionstar,asexpectedforAMXPs.Thisconclusionwasalsosup- In light of these results, Jonker et al. (2008) reported puz-
portedbythemodellingoftheNIR/opticalspectralenergydis- zlingfindings.In2006thesource,stillinquiescence,showeda
strongflickeringactivityintheIband,withflaresupto∼1mag
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Table1.ResultsofthephotometryofIGRJ00291+5934.Themeanmagnitudesarenotcorrectedforreddening.Thereddeningisobtainedstarting
fromtheN valuereportedinD’Avanzoetal.(2007)(lastcolumn).Errorsareindicatedatthe90%c.l.
H
Filter λ TotalExposure Mean Amplitude A
c λ
(Å) (s) Magnitude
g 4770 3600 23.45±0.02 0.15±0.04 2.73±0.07
r 6231 2700 22.47±0.01 0.10±0.02 1.88±0.07
i 7625 2250 21.65±0.02 0.15±0.03 1.38±0.07
over,theorbitalphasecorrespondingtothemaximumbrightness
is 0.21±0.02 in all bands, significantly shifted with respect to
what is expected for an irradiated object (phase 0.5; D’Avanzo
et al. 2007). A similar phase shift was also detected by Jonker
etal.(2008).
From its discovery in 2004 up to now, IGR J00291+5934
underwentthreedifferentoutbursts,in2004(Eckertetal.2004;
Shaw et al. 2005), 2008 (Lewis et al. 2010), and 2015 (Cum-
mings et al. 2015; Sanna et al. 2015; Russell & Lewis 2015;
Sanna et al. 2016; Ferrigno et al. 2016; Patruno 2016). After
eachoutburst,theaccretiondiscthatsurroundsthecompactob-
ject in an LMXB is expected to empty out, leaving the com-
panion star alone as the only (or main) responsible source for
theNIR–opticalemission.Thiscaneasilyexplainwhyafterqui-
escence begins, an observer usually sees a sinusoidal (or ellip-
soidal)modulatedlightcurveintheNIR/optical.However,dur-
Fig. 2. Spectral energy distributions (SED) of the system IGR ingquiescencethereplenishmentoftheaccretiondisccanstart
J00291+5934intwodifferentepochs:phase0.2(redsqares)andphase again,thusexplainingwhyinagreatnumberofX–raybinaries
0.9(bluedots).Phase0.2correspondstoastrongflareactivityforthe
during quiescence a contribution in the optical that is due to a
system.ThefitsofthetwoSEDswithapowerlawareshown(dashed
residual accretion disc is indeed observed (see e.g. D’Avanzo
anddottedlines,phase0.2and0.9,respectively).Oppositebehaviours
canbeobserved(α=0.31±0.35andα=−1.06±0.35,respectively). etal.2009).InthecaseofIGRJ00291+5934,itispossiblethat
afterthe2004outburstthecontributionoftheaccretiondisc,at
Allthepointsarecorrectedforreddening,theparametersofwhichare
reportedinTable1. thattimealmostempty,totheopticalemissionwasminimal,thus
leavingthecompanionstartodominatetheoverallemissionand
explainingthesinusoidallightcurvesreportedinD’Avanzoetal.
(2007).AsshowninD’Avanzoetal.(2007),thecompanionstar
anddurationof∼ 10min,thathidasinusoidalmodulationthat wasnotabletoaccountfortheobservedopticalmodulation,thus
is mosst likely due to the irradiated companion star. When the rulingoutastrongcontributionfromtheaccretiondisc.Oursug-
flaresweresubtracted,Jonkeretal.(2008)wasfinallyabletoob- gestionwasthenthatthecompanionstarwasstronglyirradiated
servethesinusoidalmodulationoftheI bandlightcurve,which by an additional energy source, which we tentatively identified
waspeakedatphase0.34.Moreover,anoverall(non-significant) intherelativisticwindofaspinningneutronstar.
brighteningofthesourceof0.5±0.2magwasobservedwithre- Afterthat,theaccretiondisccontinuedreplenishing,getting
specttowhatwasfoundbyD’Avanzoetal.(2007),consistently readyforthenewoutburstin2008.Exactly699daysbeforethe
with a somewhat enhanced activity of the source in 2006 with outburstbegan,Jonkeretal.(2008)observedthesystemagainin
respecttonormalquiescence(2005). theopticalin2006,andthecontributionoftheaccretiondiscwas
Our observations were acquired in 2014, eight years after indeednolongernegligible:thesystemwasbrighterandshowed
thosereportedinJonkeretal.(2008).Duringthistime,thesys- flares.Thesamemighthaveoccurredevensomeyearslater:our
temunderwentalong-lastingoutburst(∼ 100days;Lewisetal. observationsin2014weremadeonly327daysbeforethe2015
2010) in 2008 and then returned to quiescence, where it re- outbursttookplace,thuspossiblyexplainingwhyastrongflar-
mained until the epoch of our observations. Similarly to what ing activity was detected, together with the brightening of the
isreportedinJonkeretal.(2008),ourlightcurvesdonotshow sourcewithrespecttothelevelofnormalquiescence(D’Avanzo
aclearsinusoidaltrend(Fig.1).Inparticular,astrongflickering etal.2007).AsimilarbehaviourwasalsostatedinWangetal.
activityisdetectedinallbands,abovealling,withflaresupto (2013)fortheAMXPSAXJ1808.4-3658,forwhichabrighten-
∼1maganddurationoftheorderoftensofminutes(i.e.compa- ingoftheaccretiondiscemission1.5monthsbeforetheonsetof
rabletothoseobservedbyJonkeretal.2008).Afterexcludingall abrightoutburstwasobserved.
flares(Fig.1),thelightcurvesshowahintofasinusoidalmod- The observations carried out by Jonker et al. (2008) and
ulation at the known orbital period, particularly in the i band, in this work instead show a source with a higher luminosity
where the irradiated companion star is expected to contribute and strong flaring. An orbital modulation in the optical band is
the most. The fit of the light curves with a constant plus sinu- barely visible in the data, even if it is detected at a level simi-
soidmodelresultedinai–bandmeanmagnitudeof21.65±0.02, lartothemodulationobservedbyD’Avanzoetal.(2007).This
whichcorrespondstoI =21.18±0.02,accordingtothetransfor- suggests that the companion star contribution remained simi-
mationsreportedinJordietal.(2006).Thisshowsthatthesys- lar, but that the disc contribution grew considerably. With the
tembrightenedfurtherby0.72±0.09magin2014withrespect growth of the accretion disc, an increase in X-ray luminosity
to2006(Jonkeretal.2008),withasignificanceof7.8σ.More- might also be expected. This was not observed, however, as
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M.C.Baglioetal.:OpticalphotometryofIGRJ00291+5934duringquiescence
confirmed by pointed Swift observations on 2014, August 16 mustthenberelatedtothetimescaleofshearingintheaccretion
(ID:00031258002). In particular, no source is detected in a 20- disc,andshouldbemoreefficientintheouterregionsofthedisc,
pixel radius around the position of the target, with a 3σ upper wheretheamountofshearingishigher(seeZuritaetal.2003).
limit on the count rate of 9×10−3 countss−1, which translates Ifweassumethatthedurationofflaresislinkedtotheirlocation
intoanupperlimitof6×10−13ergcm−2s−1ontheunabsorbedX- intheaccretiondisc,flareswiththelongestdurationshouldthen
rayflux(forN =6×1021cm−2andapowerlawwithindex2.4; bethemostsignificant.Sincetotestthisscenariowearestrongly
H
Torresetal.2008).Thetwostatements(largerdiscemissionin limitedbythedurationofourobservationsandstatistics,wecan
theopticalandlackofX–rayemission)canbereconciledassum- leavethispossibilityopen.
ingsomeinefficientaccretionmechanism,suchasthepropeller
mechanism(seee.g.Campanaetal.2016).
The observation of a strong flaring activity (Fig. 1) shows 5. Conclusions
thatthecompanionstarofthesystemisnottheprincipalplayer
Wepresentedtheresultsofopticalgriphotometryoftheaccret-
intheopticalemissionofthesystem.Theobservationofacon- ing millisecond X–ray pulsar IGR J00291+5934 during quies-
sistent phase shift of the optical light curves (after the subtrac-
cence. Observations were carried out with the GTC equipped
tion of the flares) with respect to what is expected for an irra-
withOSIRISon2014,August31.
diated AMXP (phase 0.5) is further evidence. In particular, the
The system displays a strong flaring activity in all bands,
phase shift suggests that at least part of the modulation that is
above all in the g band. This flaring activity is comparable to
sketched in Fig. 1 may arise in a somewhat different location,
the activitiy previously observed by Jonker et al. (2008) in in-
likeanasymmetryinanaccretiondisc(hotspotorsuperhump),
tensity(∼1mag)andduration.Whentheflaresweresubtracted,
with a periodicity that is different from the orbital period. Our
weobservedanindicationofasinusoidalmodulationatthesys-
lightcurvesseemstosuggestanon–negligiblecontributionofa
temorbitalperiod.Asinthecaseoftheobservationsreportedin
newlyformedaccretiondisc.Thesameisalsosupportedbythe
Jonkeretal.(2008),aphaseshiftofthelightcurveswithrespect
spectralenergydistributionbuiltduringaflaringevent(Fig.2),
to phase 0.5 is detected. All our optical light curves are con-
whichisfullyconsistentwithamulti-temperatureaccretiondisc
sistentwithanenhancedactivityofthesource,withasignificant
withT > 30,000K(seeSect.3).Asimilarconclusionwasalso ∆Iwithrespecttotheobservationsduringquiescencereportedin
reached by Jonker et al. (2008), who proposed a superhump as
D’Avanzoetal.(2007)of0.7±0.1mag.Finally,thespectralen-
thepossiblecauseforboththeobservedvariabilityandthephase
ergydistributionbuiltduringaflare(atphase∼0.2)wasfittedby
shift of their light curve. This is doubtful, however: although apowerlawwithindexα=0.31±0.32,whichisconsistentwith
someconditionsfortheformationofasuperhumparefullfilled
what is predicted for a multi-colour black body of an accretion
in the case of IGR J00291+5934 (e.g. the mass ratio between
discwithT >30,000K.Alltheseresultscanbeexplainedwhen
thecompanionstarandthecompactobjectis(cid:46)0.3;Whitehurst
we consider that the principal player in the quiescent emission
&King1991),othersarenot.Inparticular,itisknownthatsu- ofIGRJ00291+5934ofourdatasetisnotthecompanionstar,as
perhumpstendtodevelopmainlyduringoutbursts,thatis,when
expectedforaquiescentLMXB,buttheaccretiondisc.Thedisc,
theaccretiondiscextensionisthelargest,andthisisnotthecase
afteritwasemptiedoutduringthe2008outburst,startedreplen-
for the observations reported in Jonker et al. (2008) and in this
ishinginpreparationtothenextoutburst(whichoccurredinJuly
work. However, in some rare cases superhumps have been de-
2015). In this way, both the observed brightening of the source
tectedevenduringquiescence(Neilsenetal.2008),whichleaves
and the phase shift of the light curves can be explained, since
thishypothesisstillopen.
the main optical emitter in the system might be an asymmetry
Mass transfer instabilities, magnetic reconnection events,
inthedisc,likeahotspotorasuperhump.Thecontroversialre-
andX-rayreprocessingintheaccretiondisc(Hynesetal.2002;
sultsreportedinD’Avanzoetal.(2007)andJonkeretal.(2008)
Zurita et al. 2003) might in principle also explain a strong flar-
couldalsobeaccountedforinthisscenario:inthefirstcase,the
ing activity in LMXBs. No enhanced X-ray activity has been
systemwasobservedaftertheendofanoutburst,whichproba-
detected in IGR J00291+5934 near the epoch of our observa-
blylefttheaccretiondiscalmostempty,thusexplainingwhythe
tionsin2014.Forthisreason,X-rayreprocessingcanbesafely
typical modulation due to the irradiated companion star alone
ruled out. It has to be noted that X–ray flares are observed in
wasobserved.Inthelatter,instead,thesystemwaspreparingit-
some transitional millisecond pulsars (XSS J12270-4859 and
selfforthe2008outburst;thustheaccretiondiscwasnolonger
PSR J1023+0038; de Martino et al. 2013; Patruno et al. 2014;
emptyandprobablystronglycontributedtothequiescentoptical
Bogdanov et al. 2015). However, in these cases, the flaring ac-
emissionofthesystem(asinthecaseofthe2014dataset).
tivityisonlydetectedintheX-raysandhasdifferenttimescales
Accordingtothispicture,wethusconcludethattheobserved
than in IGR J00291+5934, suggesting that a different mecha-
2014 flaring activity might indicate an accretion disc during
nismisatplay.
quiescence. Magnetic reconnection events in the disc might be
Instabilities in the mass transfer rate from the donor star
a likely possibility. Further multi–wavelength optical observa-
mightinducevariabilityatthehotspot,forexample.Thisisthe
tions during quiescence, possibly over longer timescales, could
point where accreting matter impacts the accretion disc. If this
shed light on the true origin of the quiescent flares of IGR
isthecase,theflaringactivityshouldbehigherwhenthevisibil- J00291+5934.
ityofthehotspotisatitsmaximum,thatis,atphase0.8(Zurita
etal.2003).Sinceatphase0.8oursystemseemstobequiet(Fig. Acknowledgements. PDAandSCacknowledgetheItalianSpaceAgency(ASI)
forfinancialsupportthroughtheASI-INAFcontractI/004/11/.APacknowledges
1),however,wecanexcludethispossibilityaswell.
supportviaanEUMarieSklodowska-CurieIndividualFellowshipundercon-
Magnetic reconnection events in the disc could also trigger
tractNo.660657-239TMSP-H2020-MSCA-IF-2014,aswellasfruitfuldiscus-
theflares(Hynesetal.2002;Zuritaetal.2003).Inparticular,dif- sionwiththeinternationalteamon“Thedisk-magnetosphereinteractionaround
ferentialmotionsintheaccretiondiscplasmamayproducemag- transitionalmillisecondpulsars”atISSI(InternationalSpaceScienceInstitute),
Bern.TMDacknowledgessupportviaaRamónyCajalFellowship(RYC-2015-
netic fields, and when the reconnection between vertical field
18148)andbytheSpanishMinisteriodeEconomiaycompetitividadundergrant
linesofoppositesignsoccurs,someenergyisdissipated,leading
AYA2013-42627.NRandDFTacknowledgesupportbytheSpanishMinisterio
to the emission of flares. In this scenario, the duration of flares deEconomiaycompetitividad(MINECO)undergrantAYA2015-71042-Pand
Articlenumber,page5of6
A&Aproofs:manuscriptno.IGRJ00291_paper_arXive
theGeneralitatdeCatalunyaviagrantSGR2014-1458aswellastheProgramme
CERCA.NRissupportedbyanNWOVidiAward.
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