The detection of variable radio emission from the fast rotating magnetic hot B-star HR7355 and evidence for its X-ray aurorae

0.94 MB·

by  P. Leto

MNRAS000,1–??() Preprint13February2017 CompiledusingMNRASLATEXstylefilev3.0
The detection of variable radio emission from the fast rotating
magnetic hot B-star HR7355 and evidence for its X-ray aurorae
P. Leto1 ⋆, C. Trigilio1, L. Oskinova2, R. Ignace3, C. S. Buemi1, G. Umana1,
A. Ingallinera1, H. Todt2, F. Leone4
1INAF-OsservatorioAstrofisicodiCatania,ViaS.Sofia78,95123Catania,Italy
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2InstituteforPhysicsandAstronomy,UniversityPotsdam,14476Potsdam,Germany
1
3DepartmentofPhysics&Astronomy,EastTennesseeStateUniversity,JohnsonCity,TN37614,USA
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4Universita´deglistudidiCatania,ViaS.Sofia78,I-95123Catania,Italy
2

b
e
F

0
ABSTRACT
1
InthispaperweinvestigatethemultiwavelengthspropertiesofthemagneticearlyB-typestar

HR7355.Wepresentitsradiolightcurvesatseveralfrequencies,takenwiththeJanskyVery
]
R LargeArray,andX-rayspectra, takenwith the XMM-Newton X-raytelescope.Modelingof
theradiolightcurvesfortheStokesIandVprovidesaquantitativeanalysisoftheHR7355
S
magnetosphere.AcomparisonbetweenHR7355andasimilaranalysisfortheApstarCUVir,
.
h allowsustostudyhowthedifferentphysicalparametersofthetwostarsaffectthestructureof
p
therespectivemagnetosphereswherethenon-thermalelectronsoriginate.Ourmodelincludes
-
o acoldthermalplasmacomponentthataccumulatesathighmagneticlatitudesthatinfluences
r theradioregime,butdoesnotgiverisetoX-rayemission.Instead,thethermalX-rayemission
st arisesfromshocksgeneratedbywindstreamcollisionsclosetothemagneticequatorialplane.
a TheanalysisoftheX-rayspectrumofHR7355alsosuggeststhepresenceofanon-thermal
[ radiation. Comparison between the spectral index of the power-law X-ray energy distribu-

tion with the non-thermalelectron energy distribution indicates that the non-thermalX-ray
2
v componentcouldbetheauroralsignatureofthenon-thermalelectronsthatimpactthestellar
9 surface,thesamenon-thermalelectronsthatareresponsiblefortheobservedradioemission.
7 Onthebasisofouranalysis,wesuggestanovelmodelthatsimultaneouslyexplainstheX-ray
6 andtheradiofeaturesofHR7355andislikelyrelevantformagnetospheresofothermagnetic
7 earlytypestars.
0
. Keywords: stars:early-type–stars:chemicallypeculiar–stars:individual:HR7355–stars:
1
magneticfield–radiocontinuum:stars–X-rays:stars.
0
7
1
:
v
i 1 INTRODUCTION magnetosphere), whereas it can freely propagate along directions
X
near the magnetic poles (Shore 1987; Leone 1993). Observable
Stellar magnetism at the top of the main sequence is not typical,
r signaturesofplasmatrappedinsidestellarmagnetospherescanbe
a butneitherisitanextremelyrarephenomenon.Infactabout10%
recognized in the UV spectra (Shoreetal. 1987; Shore&Brown
of the OB-type stars display strong and stable magnetic fields
1990) and in the Hα line (Walborn 1974). The interaction of
(Grunhutetal.2012b;Fossatietal.2015).Thehotmagneticstars
a radiatively driven wind with the stellar magnetosphere has
are mainly characterized as oblique magnetic rotators (OMR): a
been well studied. As examples, see Poe,Friend&Cassinelli
dipolar magnetic field topology with field axis misaligned with
(1989) for an application of the Weber&Davis (1967) model to
respect to the rotation axis (Babcock 1949; Stibbs 1950). The
radiativelydrivenwinds,windcompressionmodelswithmagnetic
existenceofsuchwell-orderedmagneticfieldsareacauseofinho-
fields (WCFields) (Ignace,Cassinelli&Bjorkman 1998), and the
mogeneous photospheres, giving rise to observable photometric,
magnetically torqued disk (MTD) model (Cassinellietal. 2002).
spectroscopicandmagneticvariabilitythatcanbeexplainedinthe
Such interaction causes an accumulation of hot material close to
frameworkoftheOMR.Earlytypemagneticstarsaresufficiently
the magnetic equatorial plane, as described by the magnetically
hottoproducearadiativelydrivenstellarwind,thatinthepresence
confinedwindshock(MCWS)model(Babel&Montmerle1997).
of their large-scale magnetic fields may be strongly aspherical.
Thewindplasma arisingfromthetwoopposite hemispheres col-
The wind plasma accumulates at low magnetic latitudes (inner
lidesclosetothemagneticequatorialplane,shockingtheplasmato
radiateX-rays (ud-Doula&Naze´ 2016 has made a recent review
oftheX-rayemissionfrommagnetichotstars).Inthepresenceofa
⋆ E-mail:[email protected]
(cid:13)c TheAuthors
2 P.Leto et al.
strongmagneticfield,thestellarwindplasmaisconfinedwithinthe
Table1.SummaryofstellarparametersforHR7355
stellarmagnetosphereandforcedtorigidlyco-rotatewiththestar
(Townsend&Owocky 2005; ud-Doula,Townsend&Owocky
Parameter Symbol ref.
2006; ud-Doula,Owocky&Townsend 2008). The proto-
type of a rigidly rotating magnetosphere (RRM) is σOriE Distance[pc] D 236 1
(Groote&Hunger1997;Townsend,Owocki&Groote2005).Ev- Reddening[mag] E(B−V) 0.065 1
idenceofcircumstellarmatterboundtothestrongstellarmagnetic Mass[M⊙] M∗ 6 1
field has been reported in a few other cases (Leoneetal. 2010; EquatorialRadius[R⊙] R∗ 3.69 1
Bohlender&Monin 2011; Grunhutetal. 2012a; Riviniusetal. PhotosphericTemperature[K] Tphot 17500 1
2013;Hubrigetal.2015;Sikoraetal.2015). RotationalPeriod[days] Prot 0.5214404 2
PolarMagneticField[Gauss] Bp 11600 1
Thestellarrotationplaysanimportantroleinestablishingthe
RotationAxisInclination[degree] i 60 1
sizeoftheRRM,andthedensityoftheplasmatrappedinside.The MagneticAxisObliquity[degree] β 75 1
rotationworksinoppositiontothegravitationalinfallofthemag-
netospheric plasma, leading to a large centrifugal magnetosphere References: (1)Riviniusetal.2013;(2)Oksalaetal.2010;
(CM)(Maheswaran&Cassinelli2009;Petitetal.2013).
TheexistenceofaCMfilledbystellarwindmaterialisasuit- larlypolarizedfluxdensity.Themodelsimulationoftheradiolight
able condition to give rise to non-thermal radio continuum emis- curves,alongwithasimulationoftheX-rayspectrumofHR7355,
sion that was first measured for peculiar magnetic B and A stars are used to significantly constrain the physical parameters of its
(Drakeetal. 1987; Linskyetal. 1992; Leone,Trigilio&Umana stellarmagnetosphere.Onthisbasis,wesuggestascenariothatsi-
1994). In accord with the OMR model, their radio emission is multaneouslyexplainsthebehavior ofHR7355atbothradioand
cyclic owing to stellar rotation (Leone 1991; Leone&Umana X-raywavelengths.
1993),suggestiveofopticallythickemissionarisingfromastable In Section2 we briefly introduce the object of this study,
RRM. The radio emission features are characterized by a simple HR7355. The observations used in our analyses are presented in
dipolarmagneticfieldtopologyandhavebeensuccessfullyrepro- Section3.TheradiopropertiesofHR7355arediscussedindetail
ducedusinga3Dmodelthatcomputesthegyrosynchrotronemis- in Section4. Section5 describes the model, while stellar magne-
sion(Trigilioetal.2004;Letoetal.2006). tosphere ispresented inSection6. Analysisof X-rayemission of
In this model, the non-thermal electrons responsible for the HR7355 is provided in Section6.2. The considerations on auro-
radio emission originate in magnetospheric regions far from the ralradioemissioninHR7355aregiveninsectionSection7,while
stellarsurface,wherethekineticenergydensityofthegasishigh Section8summarizestheresultsofourwork.
enough to brake the magnetic field lines forming current sheets.
These regions arethe siteswhere themildly relativisticelectrons
originate. In the magnetic equator, at around the Alfve´n radius,
2 MAGNETICEARLYB-TYPESTARHR7355
there is a transitional magnetospheric “layer” between the inner
confined plasma and the escaping wind. Energetic electrons that Theearly-typemainsequencestar(B2V)HR7355(HD182180)is
recirculatethroughthislayerbacktotheinnermagnetosphere ra- characterizedbyasurfaceoverabundanceofhelium(Riviniusetal.
diateradiobythegyrosynchrotronemissionmechanism. 2008).Thisstarevidencesalsoaverystrongandvariablemagnetic
Theanalysisofradioemissionsfrommagneticearly-typestars field(Oksalaetal.2010;Riviniusetal.2010).Themagneticcurve
isapowerfuldiagnostictoolforthestudyofthetopologyoftheir of HR7355 changes polarity twice per period, and was modeled
magnetospheres.Theradioradiationatdifferentfrequenciesprobes intheframeworkof theOMR byamainlydipolar field,withthe
the physical conditions of the stellar magnetosphere at different magneticaxissignificantlymisalignedwithrespecttotherotation
depths,eventopologiesarecomplex(Letoetal.2012).Hence,the axis.
radioemissionofthehotmagneticstarsprovidesafavoredwindow Amongtheclassofthemagneticearly-typestars,HR7355is
tostudytheglobalmagneticfieldtopology,thespatialstratification anextraordinarily fastrotator.OnlytheB2.5V typestar HR5907
of the thermal electron density, the non-thermal electron number (Grunhutetal. 2012a) has a shorter rotation period. The rotation
density, andinteractionsbetweenstellarrotation,wind, andmag- periodofHR7355(≈0.52days)setsitclosetothepointbreak-up,
netic field. In fact, the above physical parameters can be derived givingrisetoastrongdeformation fromspherical (Riviniusetal.
bycomparingthemulti-wavelengthradiolightcurves,forthetotal 2013).ThemainstellarparametersofHR7355arelistedinTable1.
andcircularlypolarizedfluxdensity,withsyntheticlightcurvesus- HR7355hostsastrongandsteadymagneticfield,indicating
ingour3Dtheoreticalmodel.Itisthenpossible,tostudyhowsuch theexistenceofaco-rotatingmagnetosphere(Riviniusetal.2013)
stellarpropertiesasrotation,wind,magneticfieldgeometryaffect suitableforgiving risetonon-thermal radiocontinuum emission.
theefficiencyoftheelectronaccelerationmechanism. HR7355hasafluxdensityat1.4GHzof7.9±0.6[mJy]aslisted
Inparticular,itisimportanttoapplytheradiodiagnostictech- bytheNVSS(Condonetal.1998).Atthetabulatedstellardistance,
niquesonasampleofmagneticearly-typestarsthatdifferintheir weestimatearadioluminosityof≈5×1017[ergs−1Hz−1],mak-
stellarrotationperiods,magneticfieldstrengths,andfieldgeome- ingHR7355oneofthemostluminousmagnetichotstarsatradio
tries.Tothisend,weconductedaradiosurveyofarepresentative wavelengths.Thus,HR7355isanidealtargettostudytheeffects
sampleofhotmagneticstarsusingtheKarlG.JanskyVeryLarge offastrotationandthehighmagneticfieldstrengthformagneto-
Array(VLA).Thesestarsprobe different combinations ofsource sphericradioemissions.
parametersowingtotheirdifferentphysicalproperties.Thispaper To obtain information on the stellar wind parameters of
presentsthefirstresultsofthisextensivestudy. HR7355weretrieveditsarchivalUVspectra(sp39549,sp39596)
Here wepresent theanalysis of the radioemission fromthe obtained by the International Ultraviolet Explorer (IUE). These
fast rotating, hot magnetic star HR7355. We were able to repro- UV spectra were analyzed by means of non-LTE iron-blanketed
ducemulti-wavelengthradiolightcurvesforthetotalandthecircu- modelatmospherePoWR,whichtreatsthephotosphereaswellas
MNRAS000,1–??()
ThemagnetosphereofHR7355 3
2
Table2.VLAobservinglog,Code:15A-041
S
3
P - ν ∆ν Epoch conf. Fluxcal Phasecal
3
x V [GHz] [GHz]
u I
fl Si
d 6/10/15 2 15-Apr-10 B 3C286 1924−2914
e
z1 22/33/44 8 15-Apr-10 B 3C286 1924−2914
i
al 6/10/15 2 15-Jun-01 BnA 3C286 1924−2914
m 22/33/44 8 15-Jun-01 BnA 3C286 1924−2914
r
o
N
observations weredone using thefull array configuration at each
observingbands,withoutsplittingtheinterferometerinsub-arrays.
0 Toobservealltheselectedskyfrequencies,theobservationswere
1380 1390 1400 1410 1420
carriedoutcyclicallyvaryingtheobservingbands.
λ/Ao The data were calibrated using the standard calibration
pipeline, working on theCommon Astronomy SoftwareApplica-
tions (CASA), and imaged using the CASA task CLEAN. Flux
Figure1.DetailoftheIUEobservation(bluesolidline)vs.aPoWRmodel
densitiesfortheStokesIandVparameterswereobtainedbyfitting
(redsolidline)withTeff=15.7kK, log(M˙)=−11andv∞=500kms−1. a two-dimensional gaussian at the source position in the cleaned
The synthetic spectrum was calculated for a rotating atmosphere with
vsini=320kms−1asdescribedinShenaretal.(2014). maps.Thesizeofthegaussianprofileiscomparablewiththearray
beam, indicating that HR7355 is unresolved for all the analyzed
radio frequencies. The minimum array beam size is 0.18×0.12
the the wind, and also accounts for X-rays (Gra¨feneretal. 2002; [arcsec2], obtained with the BnA array configuration at 44 GHz.
Hamann&Gra¨fener 2003; Sanderetal. 2015). Already the first Theerrorswerecomputedasthequadraticsumofthefluxdensity
inspection of the spectra reveals a lack of asymmetric line pro- error, derived from the bidimensional gaussian fitting procedure,
fileswhichwouldbeexpectedforspectrallinesformedinastellar andthemaprmsmeasuredinafieldarealackinginradiosources.
wind.Infact,Riviniusetal.(2013)wereabletofittheIUEspectra
ofHR7355withastaticmodelthatdoesnotincludestellarwind,
3.2 X-ray
showingthatitscontributionmustbesmall,andatbestonlyupper
limitcouldbeobtainedbyUVspectrallinemodeling. We retrieved and analyzed archival X-ray observations of
We attempted to estimate the upper limit for the mass-loss HR 7355 obtained with the XMM-Newton on 2012-09-25 (Ob-
rate that would be still consistent with the IUE observations. sID0690210401,Naze´etal.2014),andlasting≈2.5hr.Allthree
We found that in our models for the stellar parameters given by (MOS1, MOS2, and PN) European Photon Imaging Cameras
Riviniusetal. (2013) and the assumed mass-loss rate of about (EPICs)wereoperatedinthestandard,full-framemodeandathick
M˙ =10−10M⊙yr−1 only the SiIV resonance line is sensitive to UV filter (Turneretal. 2001; Stru¨deretal. 2001). The data were
themass-lossrate(seeFig.1).Assphericalsymmetryisassumed reduced using the most recent calibration. The spectra and light-
forthePoWRmodelswhileRiviniusetal.(2013)founddifferent curveswereextractedusingstandardproceduresfromaregionwith
temperaturesforthepoleandequatorregions,wecangiveonlya diameter≈15′′.Thebackgroundareawaschosentobenearbythe
roughestimateforthemass-lossrate.Forthelowertemperatureof starandfreeofX-raysources.Toanalyzethespectraweusedthe
15.7kK atthepoleregiontheSiIVresonancelineisweakerthan standardspectralfittingsoftwareXSPEC(Arnaud1996).Theabun-
forthehighertemperaturesoftheequatorregion.Thereforewein- dancesweresettosolarvaluesaccordingtoAsplundetal.(2009).
fertheupperlimitofthemass-lossrateforthelowertemperature TheadopteddistancetothestarandinterstellarreddeningE(B−V)
andfindavalueofaboutM˙ <10−11M⊙yr−1forasphericalsym- arelistedinTable1.
metricsmoothwindtobeconsistentwiththeIUEobservation.We
alsochecked for theeffect of X-raysvia super-ionization but did
notfindamajorimpactontheSiIVresonanceline.
4 THERADIOPROPERTIESOFHR7355
4.1 Radiolightcurves
3 OBSERVATIONSANDDATAREDUCTION The magnetosphere of HR7355 shows evidence of strong and
variableradioemission.TheVLAradiomeasurementswerephase
3.1 Radio
foldedusingtheephemerisgivenbyOksalaetal.(2010):
Broadbandmulti-frequencyobservationsofHR7355werecarried
outusingtheKarlG.JanskyVeryLargeArray(VLA),operatedby JD=2454672.80+0.5214404E [days]
theNationalRadioAstronomyObservatory1(NRAO),indifferent
epochs. Table2reportstheinstrumental andobservational details andaredisplayedinFig.2.Theleftpanelsshowthenewradiodata
foreachobservingepoch.TomaximizetheVLAperformancesthe fortheStokesI(RCP+LCP,respectivelyRightandLeftCircular
Polarizationstate2),witheachobservingradiofrequencyshownin-
dividually.InthetoppanelofFig.2isalsoshownthevariabilityof
1 TheNationalRadioAstronomyObservatoryisafacilityoftheNational
Science Foundation operated undercooperative agreement byAssociated
Universities,Inc. 2 VLAmeasurements ofthecircularpolarization stateareinaccordance
MNRAS000,1–??()
4 P.Leto et al.
Figure2. LeftandrighttoppanelsshowthemagneticcurveofHR7355anddatatakenbyOksalaetal.(2010)andRiviniusetal.(2010).Theotherleftpanels
showtheradiolightcurvesforStokesIobtainedatalltheobservingfrequencies.Rightpanelsdisplaytherotationalmodulationofthefractionalcircularly
polarizedradioemission.
thelogitudinalcomponentofthemagneticfield(Be)(Oksalaetal. At the higher frequencies (ν>22 GHz), the shapes of the
2010;Riviniusetal.2010). lightcurvearemorecomplex,andanyrelationwithvariabilityin
TheradiolightcurvesforStokesIarevariableatallobserved Beisnolongersimple.Furthermore,itappearsthattheaveragera-
frequencies.Relativetothemedian,theamplitudesofthevariation, diospectrumofHR7355isrelativelyflatfrom6to44GHz(c.f.,
with frequency are respectively: ≈60% at 6 GHz, ≈65% at 10 toppanelinFig.3).Theerrorbarofeachpointshowninthefigure
GHz,≈62%at15GHz,≈77%at22GHz,≈60%at33GHzand isthestandarddeviationofthemeasurementsperformedatagiven
≈39%at44GHz. frequency. Thespectral index of the mean radio spectrum of this
TheJD0oftheHR7355ephemerisreferstotheminimumof hotmagneticstariscloseto−0.1,likefree-freeemissionfroman
thephotometriclightcurve(Oksalaetal.2010),correspondingtoa opticallythinthermalplasma.Hence,withoutanyinformationre-
nullintheeffectivemagneticfieldcurve,andtoaminimumemis- gardingthefractionofthecircularlypolarizedradioemissionand
sionfortheHα(Riviniusetal.2013).Interestingly,theradiolight itsvariability,thetotalradiointensityalonecaneasilybemistak-
curvesatν615GHzshowanindicationofaminimumemission enly attributed to a Bremsstrahlung radiation. Interestingly, a flat
closetoφ=0(seetheleftpanelsofFig.2). radiospectrumhasbeenalreadydetectedinsomeothersmagnetic
Despitenothavingfullcoverageofrotationalphase,theradio chemicallypeculiarstars(Leoneetal.2004).
lightcurveatν615GHzevidencesamaximumatφ≈0.2,close
tothemaximumeffectivemagneticfieldstrength,followedbyan-
otherminimumthatbecomesprogressivelylessdeepwithincreas- 4.2 Circularpolarization
ingfrequency.Therotationalphasescoveringthenegativeextrema
Circularly polarized radio emission is detected from HR7355
of themagnetic curve are not observed at radio wavelengths, but
above the 3σ detection level, revealing a non-thermal origin for
therisingfluxessuggestthattheradiolightcurvesatν615could
becharacterizedbyanothermaximum.Theradiodatafortotalin- theradioemission.TherightpanelsofFig.2showthefractionπc
ofthecircularlypolarizedfluxdensity(StokesV/StokesI,where
tensityseemstoshow2peakspercycle,thatarerelatedtothetwo
StokesV=RCP−LCP)asafunctionoftherotationalphase,and
extremaof themagneticfieldcurve. Comparison between thera-
foralltheobservedfrequencies.InthetoprightpanelofFig.2,the
dioandthemagneticcurvesalsoindicatesaphaselagbetweenthe
magneticfieldcurveisagainshownforreference.
radiolightcurvesandthemagneticone.
πc is variable as HR7355 rotates, and the amplitude of the
variationrisesastheradiofrequencyincreases.Itappearsthatthe
with the IAU and IEEE orientation/sign convention, unlike the classical amplitudeoftheintensityvariationislargerwhenthecircularpo-
physicsusage. larization is smaller. In particular πc ranges between: ≈−5% to
MNRAS000,1–??()
ThemagnetosphereofHR7355 5
Figure3.Toppanel:radiospectrumofHR7355obtainedaveragingallthe
VLAmeasurementsperformedatthesameobservingband.Theerrorbars
areforthestandarddeviationoftheaverageddata.Thedottedlinerepre-
sentsthepower-lawthatbestfittheaveragespectrum(spectralindex≈0.1). Figure4.Meridionalcross-sectionofthemagnetosphericmodelforahot
Bottompanel:standarddeviationsoftheradiomeasurements,respectively magneticstar,characterized byasimpledipolar magneticfield,andwith
fortheStokesIandV. thedipolaraxismisalignedwithrespecttotherotationaxis(themisalign-
mentamplitude isarbitrary). Thegreyarea indicates the thermal plasma
trappedwithintheinnermagnetosphere.Theequatorialregionofthemag-
netosphereoutsidetheAlfve´nsurface(dashedline)isasiteofmagneticre-
connectionevents,capableofacceleratingtheinsituplasma.Thelength(l)
ofthenon-thermalaccelerationregion,justoutsidetheequatorialAlfve´nra-
dius(RA),isalsoshown.Themagneticshell,middle-magnetosphere,where
5%at6GHz,≈−9%to8%at10GHz,≈−10%to8%at15GHz, thenon-thermalelectrons(indicatedbythesmallvectors)propagatetoward
thestellarsurface,radiatebythegyro-synchrotronemissionmechanism,is
≈−5%to14%at22GHz,≈−13%to20%at33GHz,≈−16%
delimitedbythetwopicturedmagneticfieldlines.
to26%at44GHz.
Toparameterizetheamplitudeoftheradiolightcurvesforthe
totalandpolarizedintensity,thebottompanelofFig.3showsthe
5 THEMODEL
variationofthestandarddeviation(σ)ofallthemeasurementsoc-
curingatthesamefrequency,asafunctionoffrequency.Thestan- In previous papers (Trigilioetal. 2004; Letoetal. 2006), we de-
dard deviation of the Stokes I measurements is largest atν622 veloped a3D model tosimulate the gyrosynchrotron radio emis-
GHz,whereasat33and44GHz,thestandarddeviationsdramati- sion arising from astellar magnetosphere defined by a dipole. In
callydecrease,confirmingthedecreasingofthelightcurveampli- thecaseofthehotmagneticstars,thescenarioattributestheorigin
tudesdiscussed inSec.4.1.Bycontrast thestandarddeviation of of their radio emission as the interaction between the large-scale
themeasurementsofthecircularlypolarizedfluxdensityincreases dipolarmagneticfieldandtheradiativelydrivenstellarwind.
as the frequency increases. Considering Fig. 3 (bottom panel),σ Following this model the plasma wind progressively accu-
valuesfortheStokesIandVmeasurementsareevidentlyinversely mulates in the magnetospheric region where the magnetic field
related. lines are closed (inner magnetosphere). The plasma tempera-
Comparing the curves of variation of πc with the magnetic ture linearly increases outward, whereas its density linearly de-
field curve, a positive degree of circular polarization is detected creases(Babel&Montmerle1997;ud-Doulaetal.2014).Outside
when the north magnetic pole isclose to the line-of-sight, and is the Alfve´n surface, the magnetic tension is not able to force co-
negativewhenthesouthpoleismostnearlyalignedwiththeview- rotation of the plasma. Similar to the case of Jupiter’s magneto-
ing sightline. When the magnetic poles are close to the direction sphere (Nichols 2011), the co-rotation breakdown powers a cur-
oftheline-of-sight,weobservemostoftheradiallyorientedfield rentsheetsystemwheremagneticreconnectionacceleratesthelo-
lines.Inthiscasethegyrosynchrotronmechanismgivesrisetora- cal plasma up to relativistic energies (Usov&Melrose 1992). A
dio emission that is partially polarized, respectively right-handed fractionofthenon-thermalelectrons,assumedtohaveapower-law
forthenorthpoleandleft-handedforthesouthpole.Thisbehavior energyspectrumandanisotropicpitchangledistribution(i.e.,the
of the gyrosynchrotron polarized emission has already been rec- anglebetweenthedirectionsoftheelectronvelocityandthelocal
ognized,atν615GHz,inthecasesofCUVir(Letoetal.2006) magneticfieldvector), candiffuseback tothestarwithinamag-
andσOriE (Letoetal. 2012), that have magnetospheres defined neticshellthatweheredesignateasthe”middlemagnetosphere”.
mainlybyamagneticdipole,similartothecaseofHR7355.Our This non-thermal electron population has a homogeneous spatial
newradiomeasurementsshow,forthefirsttime,thisbehaviorper- density distribution within the middle magnetosphere, owing to
sisting up to ν=44 GHz for HR7355. Furthermore, the light magnetic mirroring. A cross-section of the stellar magnetosphere
curvesofπc show thatthemagneticfieldcomponent closetothe modelispicturedinFig.4.
stellarsurfacecanbetracedwiththecircularlypolarizedemission The non-thermal electrons moving within the middle mag-
athighfrequency. netosphere radiate at radio wavelengths by the gyro-synchrotron
MNRAS000,1–??()
6 P.Leto et al.
Table3.
Freeparameters
Symbol Range Simulationstep
Alfve´nradius[R∗] RA 8–50 ∆logRA≈0.1
Thicknessofthemiddlemagnetosphere[R∗] l 1–40 ∆logl≈0.2
Nonthermalelectrondensity[cm−3] nr 102–105 ∆lognr≈0.1
Relativisticelectronenergypower-lawindex δ 2–3 ∆δ=0.5
Thermalelectrondensityatthestellarsurface[cm−3] n0 108–1010 ∆lognp0≈0.25
Modelsolutionswithδ=2.5
Thermalelectrondensityatthestellarsurface n0=3×109[cm−3]
Alfve´nradius RA=12.5–40[R∗]
Columndensityofthenonthermalelectrons nr×l=10(12.95±0.09)×R(A2.68±0.07)[cm−2]
Modelsolutionswithδ=2
Thermalelectrondensityatthestellarsurface n0=2–5×109[cm−3]
Alfve´nradius RA=10–40[R∗]
Columndensityofthenonthermalelectrons nr×l=10(12.48±0.04)×R(A2.44±0.03)[cm−2]
emissionmechanism.Tosimulatetheradioemissionarisingfrom 5.1 ModelingtheHR7355radioemission
thesenon-thermalelectrons,themagnetosphereofthestarissam-
Onthebasisofthemodeldescribedinprevioussection,weseekto
pled in a three dimensional grid, and the physical parameters
reproducethemulti-wavelengthradiolightcurvesofHR7355for
needed to compute the gyrosynchrotron emission and absorption
StokesIandV.ThealreadyknownstellarparametersofHR7355,
coefficientsarecalculatedateachgridpoint.
neededforthesimulationsarelistedinTable1.TheAlfve´nradius
As a first step, we set the stellar geometry: rotation axis in-
(R )andthelengthofthecurrentsheet(l)havebeenassumedas
A
clination(i),and tiltof the dipolemagnetic axis(β), and thepo-
free parameters. For the sampling step, we adopt a variable grid
larfieldstrength(Bp).Inthestellarreferenceframe,assumedwith with a narrow spacing (0.1 R ) for distances lower then 8 R , a
∗ ∗
thez-axiscoincidingwiththemagneticdipoleaxis,thespacesur-
middlespacing(0.3R )between8and12R ,andaroughspacing
∗ ∗
roundingthestarissampledina3Dcartesiangrid,andthedipolar
(1R )fordistancesbeyond12R .
∗ ∗
magneticfieldvectorcomponentsarecalculatedateachgridpoint.
Following results obtained from simulations of radio emis-
Giventhestellarrotationalphase(φ),thefieldtopologyisthenro-
sions of other hot magnetic stars (Trigilioetal. 2004; Letoetal.
tatedin theobserver reference frame (seeprocedure described in
2006),thelow-energycutoffofthepower-lawelectronenergydis-
App.AofTrigilioetal.2004).
tribution has been fixed at 100 keV, corresponding to a Lorentz
Inthesecondstep,welocatethemagnetosphericsubvolume factorγ=1.2.Thetemperatureofthethermalplasmaatthestellar
wheretheunstableelectronpopulationpropagates.Thisspatialre- surfacehasbeensetequaltothephotosphericone(giveninTab.1),
gionisdelimitedbytwomagneticfieldlines.Theinnerlineinter- whereasitsdensity(n )hasbeenassumedasafreeparameter.The
0
ceptsthemagneticequatorialplaneatadistanceequaltotheAlfve´n assumedvaluesofthemodel,freeparameters,andthecorrespond-
radius(RA).Theouterlineinterceptstheequatorialplaneatadis- ingsimulationstepsarelistedinTable3.Adoptingthesestellarpa-
tance RA+l, with l being the width of the current sheet where rameters,wewereabletosimulateradiolightcurvesfortheStokes
magneticreconnectionacceleratesthelocalplasmauptorelativis- I and V that closely resemble the measurement of HR7355. The
ticenergies.Withineachgridpointofthemiddlemagnetosphere, correspondingrangesofthemodelparametersarereportedinTa-
thenon-thermalelectronshaveaconstantnumberdensity(nr).By ble3.
contrasttheinnermagnetosphereisfilledbyathermalplasmawith TheFig.5displaystheenvelopeofthesimulatedlightcurves,
densityandtemperaturethatarefunctionsofthestellardistanceas fortheStokesIandVrespectively,thatcloselymatchtheobserved
previouslydescribed. ones.Thisenvelopewasobtainedfromthesimultaneousvisualiza-
In the third step, given the observing radio frequencyν, we tion of the whole set of simulations performed using the combi-
calculate the emission and absorption coefficients for the gyro- nations of themodel free parameters listedas model solutions in
synchrotron emission (Ramaty 1969) at the grid points that fall Table3. The simulations indicate that gyro-synchrotron emission
withinthemiddlemagnetosphere.Foreachgridpointoftheinner fromadipole-shapedmagnetospherecancloselyreproducetheob-
magnetosphere, the free-free absorption coefficient (Dulk 1985), servationsofHR7355.Thelow-frequencyStokesIradioemission
the refractive index, and the polarization coefficient for the two shows a clear phase modulation, that becomes progressively less
magneto-ionicmodes(Klein&Trotter1984)arecomputed.Weare evident asthefrequency increases. Conversely thesimulationsof
abletosolvenumericallytheradiativetransferequationalongthe thelightcurvesfortheStokesVindicatesthatthecircularlypolar-
directions parallel to the line-of-sight for the Stokes I and V (as izedemissionisstronglyrotationallymodulated,withanamplitude
describedintheApp.AofLetoetal.2006).Scalingtheresultfor thatincreaseswithfrequency.Suchbehaviorofthesimulatedradio
thestellardistance,andrepeatingtheseoperationsasafunctionof lightcurvesisconsistentwiththemeasurements.
therotationalphase,φ,syntheticstellarradiolightcurvesarecal- Tohighlighttheclosematchbetweensimulationsandobser-
culated,andthensimulationsarecomparedwithobservations. vations,wealsocomparedthesimulatedradiospectrawiththeob-
MNRAS000,1–??()
ThemagnetosphereofHR7355 7
Figure5.ThefilledcirclesaretheHR7355observedradiolightcurves,respectivelyofthetotalintensity(StokesI),andofthecircularlypolarizedfluxdensity
(StokesV).Thegreyareasaretheenvelopeofthemodeledlightcurvesobtainedbyusingthecombinationsofthefreeparametersabletogeneratesynthetic
lightcurvesthatclosematchtheobservedones.ThecorrespondingmodelsolutionsarelistedinTable3.Toppanelsshownthemodelsolutionswithδ=2.5,
bottompanelsthosewithδ=2.
servedspectrum.Thesyntheticspectrawererealizedaveragingthe originate.Theeffectoftheplasmainertiatothemagneticfieldcon-
simulatedlightcurvesateachfrequency.Inthetopandmiddlepan- figurationisanissueoutsidethelimitofourmodel.Inanycase,this
elsofFig.6,theobservedspectrumofHR7355isshownagain,and mismatchbetweenthedipolarandthetruestellarmagnetictopol-
thesuperimposedshadedarearepresentstheenvelopeofthesimu- ogycouldexplainthedifferencesbetweenobservationsandsimula-
latedspectrafortheStokesI.InthebottompanelofFig.6thestan- tions.Furthermore,hasbeenproventhatwithintheHR7355mag-
darddeviation(σ)oftheobservedandsimulatedmulti-frequency netospheretherearecloudthedenseplasma(withlinearsize≈2
lightcurves(StokesIandV)havebeencompared. Inthecaseof R )co-rotatingwiththestar(Riviniusetal.2013),thatcouldaffect
∗
themodelsimulations,morethanonespectrumwasproduced.The therotationalmodulationofthestellarradioemission.Themodel-
σvaluespicturedinthebottompanelofFig.6aretheaveragesof ingapproachfollowedtosimulatetheradiolightcurvesofHR7355
thestandarddeviationscorrespondingtothewholesetofsimulated doesnottakeintoaccountforthepresenceofsuchmaterial.Onthe
lightcurves.ThetoppanelofFig.6referstothemodelsimulations basisofthisconsiderationswecannotexcludethatthespectralin-
withparametersδ=2.5,whereasthemiddlepanelisfortheδ=2 dexofthenonthermalelectronscouldbeclosetoδ=2.5.Inany
case.TheσofthesimulatedHR7355radioemissionseemstobe case, the higher dispersion of the simulations with respect to the
larger than the observed ones. Such behavior is confirmed when observationscanbeexplainedasaconsequenceofthecoveragefor
looking at the bottom panel of Fig. 6. In particular the σ of the theobservedradiolightcurvesnotbeingcomplete.Infact,weare
lightcurveswithδ=2.5arehighest.Thisbehaviorsuggeststhat missingsome portions of thelight curves that areexpected to be
thevalueofδ=2,forthespectralindexofthenon-thermalelec- highly variable. On the other hand, the frequency dependence of
tron,could beclosetothetruevalue. But wemust alsotakeinto thestandarddeviationsofthesimulatedStokesIandVradiolight
accountthat,themagnetosphereofthisrapidlyrotatingstarcould curvesaresimilartotheobservedones(seebottompanelofFig.6).
beoblate,whereasourmodelassumesasimpledipole.Thestretch- Thisisfurtherevidenceofthegoodfitofourmodelfordescribing
ingofthemagnetospherecouldaffectthemagneticfieldtopology theradiomagnetosphereofHR7355.
oftheregionswheretheradioemissionattheobservedfrequencies
MNRAS000,1–??()
8 P.Leto et al.
Figure7.Graphicalviewoftheanalyticequationthatdescribesthecolumn
density of the non-thermal electrons (given in Table 3), calculated close
totheacceleration site,asafunctionofthevalues ofRA thatareableto
givesimulatedradiolightcurvesmatchingtheobservedones.Thedotted
linecorresponds tothemodelsolutions withδ=2;thedashedlinewith
δ=2.5.NoteacceptablemodelsolutionsarenotfoundforRA<12.5R∗
withδ=2.5.Thegreyareashighlightthesolutionuncertainty.
Figure6.Topandmiddlepanels:likeFig.3,thefilledcirclesaretheaver-
agemeasuredradiospectrumofHR7355.Thegreyareasaretheenvelope
of thethermal plasma trapped withintheinner magnetosphere of
oftheaverages fromsimulatedradiospectra corresponding tothemodel
HR7355.Theadoptedradialdependencefortheplasmatempera-
solutionswithδ=2.5(toppanel),andwithδ=2(middlepanel).Inthe
bottompanel,thestandarddeviationsoftheobservations(continuouslines) tureanddensityarerespectively:n=n0r−1 andT =Teffr,hence
andsimulations (dashedlines corresponding toδ=2.5,anddottedlines thethermalpressure(p=kBnT)isconstantinsidetheinnermag-
correspondingtoδ=2)arecompared.Thethicklinescorrespondtotheto- netosphere. Insteadystate p= pram,where pram isthewindram
talintensity(StokesI),thethinlinestothecircularlypolarizedfluxdensity pressure.InthebottompanelofFig.8thegreyarearepresentsthe
(StokesV). thermalpressureoftheplasmatrappedintheinnermagnetosphere.
Thosesolutionsthatdonotsatisfytheaboveequalityconditioncan-
notbeconsideredvalid.TheaverageAlve´nradiithatarephysically
6 THEMAGNETOSPHEREOFHR7355 plausiblearelistedinTable4.Thecorresponding windmassloss
rate,thedensityofthewindattheAlve´nradius,theaveragethermal
6.1 Radiodiagnostic
temperatureoftheplasmatrappedwithintheinnermagnetosphere,
Analysisofmodelsolutionsfortheobservedmulti-wavelengthra- as well as the corresponding emission measure are also listed in
dio light curves of HR7355, respectively for the Stokes I and V, Table4.
can be used to constrain the physical conditions of the magneto- In the case of a dipolar shaped magnetosphere (see Fig. 4),
sphere of this hot star. Thethermal electron density at the stellar theradiativelydrivenstellarwindcanfreelypropagateonlyfrom
surface(n0)iswellconstrained.Wefoundacceptablelightcurves the northern and southern polar caps. As a consequence, the ac-
forAlfve´nradiigreaterthan10R∗.Theothertwomodelfreepa- tual mass loss rate (M˙act) is a fraction of M˙. The fraction of the
rametersaredegenerate, namely thenon-thermal electrondensity wind that freely propagates can be estimated from the ratio be-
(nr) and the length of the current sheet (l). The product of these tweenthetwopolarcapsareaandthewholesurface.Thepolarcaps
twoparametersisthecolumndensityofrelativisticelectronsatthe areaisderivedfromtherelationdefiningthedipolarmagneticfield
Alfve´nradius. Wefound that the column densityisafunction of line:r=RAcosλ2 (whereλisthemagneticlatitude).Infact,the
RA,themathematicalrelationship,obtainedbyfittingtheseparam- pointwherethefieldline,withagivenRA,crossesthestellarsur-
eters,isprovidedinTable3,andpicturedinFig.7. faceindividuatesthelatitudeofthepolarcap.ThevaluesofM˙act,
The value of the Alfve´n radius is related to the wind of listedinTable4,areingoodagreementwiththoseobtainedfrom
HR7355.InTrigilioetal.(2004)wecomputedR giventhemag- theUVspectralanalysesofHR7355(seeSec.2)andotherB-type
A
netic field strength, the wind mass-loss rate, its terminal velocity starswithsimilarspectraltypes(Prinja1989;Oskinovaetal.2011;
(v∞),thestellarradius,andtherotationperiod.Inthepresentanal- Krticka2014)
ysis, we reverse this approach: given v∞ and the rotation period, The indirect evaluation of the linear extension of the radio
we estimate the mass-loss rates (M˙) of HR7355 that are com- emitting region is also useful for estimating the brightness tem-
patible with the values of RA listed in Table 3. We assume two perature of HR7355. The average flux densities for HR7355 are
values of wind terminal velocity that are reasonable for a main ≈15.5mJy,inthefrequencyrange6–44GHz.Assumingthemean
sequence B type star (Prinja 1989; Oskinovaetal. 2011; Krticka equatorial diameter of the Alfve´n surface (31 R ) for the source
∗
2014):v∞=500and1000[kms−1].Fig.8showsthevaluesofM˙, size,thecorresponding brightness temperatureisTbril≈3×1010
and thecorresponding pressure, asafunction of R . Thehighest [K]. The above estimate reenforces the conclusion that the radio
A
valuesofR needalowwindmass-lossrate. emissionfromHR7355hasanon-thermalorigin.
A
The model simulation provide an estimate for the density Itisinstructivetocomparetheresultsobtainedfromtheanal-
MNRAS000,1–??()
ThemagnetosphereofHR7355 9
Figure9.Radialdependenceofthemagneticfieldstrengthintheequatorial
planeofHR7355(continuousline),andCUVir(dashedline).Theranges
oftheAlfve´nradiiofthetwostarsandthecorresponding magneticfield
strengthsareindicated.
Figure8.Toppanel: values ofthewindmass-lossrate corresponding to
therangeofAlfve´nradii derived bythe modelsimulations ofthemulti-
wavelengthradiolightcurvesofHR7355.ThevaluesofM˙ havebeende- Keplercorotationradii(RK=(GM∗/ω2)1/3)arerespectively:1.3
R for HR7355, 1.9 R for CUVir. Comparing the above esti-
rivedassumingtworeasonablevaluesforamainsequenceB-typestarwind ∗ ∗
terminalvelocity.Bottompanel:windpressureattheAlfve´nradius.Dot- matedvaluesofRKwiththeaverageAlfve´nradii,respectively15.5
tedline refers tothecasev∞=500[kms−1],dashed linetov∞=1000 for HR7355, and 14.5 [R∗] for CUVir.The ratio RA/RK for the
[kms−1].Thegreyareaindicatestherangeofpressuresfromthethermal HR7355 CM magnetosphere is ≈11.9, versus ≈7.6 in the case
plasmatrappedwithintheinnermagnetosphere. of CUVir.Theabove estimationhighlights thatHR7355 ischar-
acterized by a larger magnetospheric volume maintained in rigid
co-rotationcomparedwiththatofCUVir.
Table4.DerivedparametersofHR7355 We also compare the magnetic field strength at the Alfve´n
radius for both stars. The two stars have similar rotation periods
v∞ <RA> v∞ <RA> (≈0.52 d), but HR7355 has a larger size, and a stronger polar
[kms−1] [R∗] [kms−1] [R∗] magneticfieldstrength (11.6kG versus3.8kG,Kochukhovetal.
500 14 1000 17 2014).Theradialdependence forasimplemagneticdipoleatthe
<M˙aMc˙t[>M⊙[Myr⊙−1y]r−1] 14..52××1100−−1110 02..75××1100−−1110 iesqupalotottreiadlipnlaFniegi.s9d.eTschreibreadngbeysBoefqt=he1/a2llBowp(eRd∗/RrA)3v[aGluaeuss,s]g,iavnend
nw(RA)[cm−3] 2.0×106 0.4×106 inunitsofsolarradii,areshownforbothstars.Thecorresponding
<T>[MK] 0.16 0.19 magneticfieldstrengthsarederived.Fig.9makesclearthatthecur-
EM[1055cm−3] 2.22 2.72 rentsheet regionofHR7355ischaracterized byamagneticfield
strengthofroughlydoublethevalueofthecaseforCUVir.From
a purely qualitative point-of-view, it is reasonable to assume the
non-thermalaccelerationprocessoperateswithinathickermiddle
ysisofradioemissionfromHR7355andfromtheApstarCUVir
magnetosphereforHR7355.
conductedusingsimilarapproach(Letoetal.2006).Forexample,
At the distances of the two analyzed stars, D=236 pc for
the wind electron density number at the Alve´n surface, nw(RA), HR7355andD=80pcforCUVir,theirradioluminositiesarere-
hassimilarvaluesforbothstars.Theestimatedcolumndensityof
spectively,≈1018[ergs−1Hz−1],obtainedusingtheaverageradio
therelativisticelectronsattheAlve´nradiusliesintherange3.2–
fluxdensitymeasuredinthispaper;and≈3×1016[ergs−1Hz−1],
4.6×1014 [cm−2]inthecaseofCUVir,versusacolumndensity
usingthemeanof themeasuredfluxdensitieslistedinLetoetal.
thatrangesbetween1.9×1015and3.0×1015[cm−2]forHR7355.
(2006).Asdiscussedabove,thetwostarsarecharacterizedbydif-
ThisisevenhigherinthecaseoftheHR7355modelsolutionswith
ferentradioemittingvolumes.ForHR7355thenon-thermalelec-
δ=2.5:thederivedrangeis1.1–1.8×1016 [cm−2].Forthecase
tronsalsotravelwithinmagnetosphericregionsathighermagnetic
ofδ=2,columndensityofthenon-thermalelectronsforHR7355
field strength. Using a model for gyrosynchrotron emission, the
ishigherbyaboutanorderofmagnitudeascomparedtoCUVir.
magneticfieldstrengthdirectlyaffectstheobservedradiofluxden-
Under the reasonable assumption that HR7355 and CUVir
sitylevel(Letoetal.2006).Takingintoaccountthevariousphys-
havesimilarnon-thermalaccelerationefficiencies,thehighernon-
icaldifferences,weareabletoexplainqualitativelywhyHR7355
thermal electron column density of HR7355 could be explained
isabrighterradiosourceascomparedtoCUVir.
if it is characterized by a more extended acceleration region
as compared to CUVir. The magnetosphere of HR7355 is big-
ger than CUVir, with R∗ = 3.69 versus 2.06 R⊙ for CUVir 6.2 X-raydiagnostic
(Kochukhovetal.2014),andsothelinearsizeoftheacceleration
region (l) will be consequently wider for HR7355. Furthermore, TheX-rayfluxofHR7355inthe0.2–10keVbandmeasuredbythe
theB2typestarHR7355hasahigherstellarmasscomparedtothe XMM-Newtonis≈1.6×10−13ergcm−2s−1(seeTable5).Thisis
ApstarCUVir,6versus3.06M (Kochukhovetal.2014).Their ordersofmagnitudehigherthatmaybeexpectedifplasmaemitting
⊙
MNRAS000,1–??()
10 P.Leto et al.
Table 5. The X-ray spectral parameters derived from the XMM-Newton
EPIC observations of HR7355 assuming two-temperature CIE plasma
(apec) (tbabs) model and apec+powerlaw model, both models corrected
fortheinterstellar absorption. Thevalueswhichhavenoerrorhavebeen −V1 01
frozenduringthefittingprocess.Thespectralfitscorrespondingtothethe e 0.
k
apec+powerlawmodelareshowninFig.10. −s1
s
nt
NHa [1020cm−2] 3.2 cou −03
Twotemperaturethermalmodel ed 1
z
kT1[keV] 0.9±0.2 mali
EkTM21[k[e1V05]1cm−3] 43..49±±10..98 nor −104
EM2[1051cm−3] 46.2±4.9
hkTi≡∑ikTi·EMi/∑iEMi[keV] 3.6
Fluxb[10−13ergcm−2s−1] 1.6 0.5 1 2 5
Energy (keV)
Thermalpluspower-law(A(E)=KE−α)model
Figure10.XMM-NewtonPN(uppercurve),andMOS1andMOS2(lower
kT1[keV] 1.0±0.1
EM1[1051cm−3] 6.5±1.5 curves)spectraofHR7355witherrorbarscorresponding to3σwiththe
bestfitthermalpluspower-lawmodel(solidlines).Themodelparameters
α 1.7±0.1
K[photonskeV−1cm−2s−1at1keV] (1.9±0.2)×10−5 areshowninTable5.
Fluxb[10−13ergcm−2s−1] 1.7
Lb [ergs−1] 1.1×1030
X First, the observed spectrum in 0.2-10.0keV band was fit with a
logLX/Lbol −6.5 thermaltwo-temperaturespectralmodelthatassumesopticallythin
LX/Lν,rad[Hz] 1.1×1012
plasmaincollisionalequilibrium.Thefitisstatisticallysignificant,
a correspondtotheISMhydrogencolumndensity with reduced χ2 =0.72 for 88 degrees of freedom. The model
b dereddened;inthe0.2–10keVband fitparametersareshowninTable5.Thetwo-temperaturecompo-
nentsarewellinaccordwiththevalueslistedbyNaze´etal.(2014).
The thermal plasma is extraordinarily hot, with the bulk of the
inradioregimewouldbesolelyresponsiblefortheX-raygenera- plasmaatatemperature3.6keV(40MK).Thisissignificantlyhot-
tion–usingtheaveragetemperatureandemissionmeasurelisted ter than usually found in magnetic B-stars (Oskinovaetal. 2011;
inTable4,theexpectedX-rayfluxisonly≈10−15ergcm−2s−1. Naze´etal.2014;Ignaceetal.2013;Oskinovaetal.2014).
Thus,acoldthermalplasmacomponent responsible fortheradio In the framework of the MCWS model, the wind plasma
emissionalonecannotexplaintheobservedX-raysfromHR7355. streamsthatcollideatthemagneticequatorgiverisetoashockthat
Comparing the X-ray and the radio emission of HR7355 to heatstheplasma.Hence,themaximumtemperaturefollowsfroma
thatoflate-typestarsrevealssignificantdifferences(seeTable5). Rankine-Hugoniotconditionandcannotexceedavaluedetermined
HR7355violatestheempiricalrelationcouplingtheX-rayandra- bythemaximumstellarwindvelocity.
dio luminosities of magnetically active stars (LX/Lν,rad ≈1015.5 In the analysis presented in Sect.6.1, we assumed two dis-
Hz, Gue¨del&Benz 1993; Benz&Gue¨del 1994), which is valid tinct wind velocities, 500kms−1 and 1000kms−1, that encom-
among stars distributed within a wide range of spectral classes pass values plausible for a main sequence B-type star (Prinja
(from F to early M stars). This is clear evidence that the physi- 1989; Oskinovaetal. 2011; Krticka 2014). Using Eq.10 from
calmechanismsfortheradioandX-rayemissionsoperatinginan ud-Doulaetal.(2014),weestimatethemaximumplasmatempera-
early B-star like HR7355 are distinct from coronal mechanisms turesthatcanbeproducedviaamagneticallyconfinedwindshock
operatingintheintermediate- andlow-massmainsequence stars. for these two wind speeds; these temperatures are respectively:
Yet,somewhatsurprisingly,thedeviationoftheearlytypeHR7355 3.5MK and 14MK – significantly lower than that deduced from
fromtheGue¨del–Benzrelationissimilartothatforthestarsatthe theX-rayspectralanalysis.
bottomofthemainsequence–theultracooldwarfswithspectral Thisled us toconclude that the assumption of the hard part
type later than M7 (Bergeretal. 2010; Williams,Cook&Berger ofX-rayspectrumbeingproducedbythehotthermalplasmaisnot
2014;Lynchetal.2016).Theseimportantsimilaritiesbetweenac- realistic.Therefore,asanextstep,weattemptedtofittheHR7355
tiveultracooldwarfsandastronglymagneticBstarindicatethat spectra with an absorbed power-law model, however no satisfac-
radioandX-rayemissionintheirmagnetospheresmaybeproduced toryfitcouldbeobtained.Finally,wefittheobservedX-rayspec-
byrelatedphysicalmechanismsandprovideusefulhintsforthelat- trumbycombining thermalandpower-law models.Theresulting
ter. fit,correspondingtothepower-lawplusthermalmodel,isshownin
AccordingtotheMCWSmodel,thethermalplasmarespon- Fig.10.Ahigh-qualityfitwithreducedχ2=0.725for89degrees
sibleforX-rayemissionsfrommagneticB-typestarsisproduced of freedom was obtained. Based on spectral fitting, the 2T ther-
bystellarwindstreamscollidingatthemagneticequator.Theradio malmodelhasnopreferenceoveramodelthatcombinesthermal
wavelengthsareinsteadsensitivetoonlythecoldthermalplasma andpower-law(non-thermal)components. Themodel fitparame-
that accumulates at the higher magnetic latitudes. Consequently, ters are shown in Table 5. The temperature of the thermal X-ray
the X-ray emission provides a different set of constraints on the plasma in this combined model, ≈10 MK, is easier to reconcile
physicalconditionsinthemagnetosphereofHR7355. withatypicalwindvelocityofaB2Vstar.Amorecomplexmodel
Wehaveanalyzedthearchival XMM-Newtonmeasurements. involvingtwotemperaturesplusapower-law,canalsobefittothe
MNRAS000,1–??()