Simbol-X capability of detecting the non-thermal emission of stellar flares

0.07 MB·English

by  C. Argiroffi

Mem.S.A.It.Vol.,1
(cid:13)c SAIt 2004 Memoriedella
Simbol-X
apability of dete
ting the
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non-thermal emission of stellar (cid:13)ares
0
0
C.Argiroffi1,2,G.Micela2,andA.Maggio2
2

n
a 1 Dip.diScienzeFisicheedAstronomiche,Universita` diPalermo,PiazzadelParlamento
J 1,90134Palermo,Italy,e-mail:[email protected]
6 2 INAF-OsservatorioAstronomicodiPalermo,PiazzadelParlamento1,90134Palermo,
1 Italy,e-mail:[email protected], [email protected]

]
h
Abstract.Weinvestigatethecapabilityofdetecting,withSimbol-X,non-thermalemission
p
duringstellarflares,anddistinguishingitfromhotthermalemission.Wefindthatflarenon-
-
o thermalemissionisdetectablewhenatleast∼20ctsaredetectedwiththeCZTdetectorin
r the20−80keVband.ThereforeSimbol-Xwilldetectthenon-thermalemissionfromsome
st of theX-raybrightestnearby stars,whetherthethermalvs. non-thermal relation,derived
a forsolarflares,holds.
[

1
v 1. Introduction sented the detection of non-thermal emission
2 inthehardX-raysduringastellarflare.
Stellar flares are phenomena where the mag-
3 The study of the flare non-thermal emis-
5 neticfieldrapidlyreleaseslargeamountofen- sion allows to study: 1) the depicted scenario
2 ergy. It is supposed that: 1) magnetic recon-
for stellar flares, and hence whether and how
. nections generate non-maxwellian population
1 the thermal emission is caused by the non-
offastparticles;2)theseparticleshitandheat
0 thermal particles; 2) the flare energy balance,
8 thechromosphericplasma,evaporatingit, and since only a small amount of released energy
0 producingnon-thermalemission (hard X-rays
islostbythermalradiation.
: and γ-rays); 3) the evaporated chromospheric
v We investigate the Simbol-X capability
plasma fills coronal structures and generates
i of detecting non-thermal emission associated
X thermal emission (mostly in the UV and soft
withstellarflares,anddistinguishingitfroma
r X-raybands). hot thermal one. Results about this issue are
a In solar data a power-law spectrum, com-
importantalsoforotherastrophysicalsubjects
patible to the bremsstrahlungemission of fast
where non-thermal emission is likely present
particleshittingathicktarget,isobserved(i.e.
(galaxyclusters,SNR).
Hudson&Ryan 1995). This hard power-law
emissionprecedesthesoftX-rayandUVther-
mal emission.Informationon stellar emission
2. Method
in the hard X-ray band is very poor, since
mostX-rayobservationsarededicatedonlyto Weadoptthefollowingprocedure:1)wesim-
thesoftband.RecentlyOstenetal.(2007)pre- ulateMPDandCZTspectraadoptingasmodel
the superposition of a thermal and a non-
Sendoffprintrequeststo:C.Argiroffi thermalcomponent;2) we assume as nullhy-
2 C.Argiroffietal.:Detectingthenon-thermalemissionofstellarflares
Table1.Modelparameters
Name D EM T logLa
XT
(pc) (1050cm−3) (MK) (erg/s)
ProxCen 1.3 5 20 27.5
ADLeo 4.7 10 20 27.7
EVLac 5.0 2×102 20 28.9
ABDor 14.9 2×102 20 29.0
HR1099 29.0 1×103 20 29.6
PMSstars 150 5×104 20 31.3
aLXTiscomputedinthe1.55−12.4keVband.
pothesis that the simulated spectra can be de-
scribed by thermal components only; hence Fig.1. One of the model adopted for simulating
we fit simultaneously MPD and CZT spectra flarespectra.
adoptinga2-T opticallythinplasmaasmodel;
3) we assume thatthe null hypothesisof only
thermal emission is rejected, and hence non-
thermal emission is detected and recognized,
when the EMof the hottestcomponentis sig-
nificantlygreaterthanzeroanditstemperature
islargerthan300MK.
ToproducesimulatedMPDandCZTspec-
tra we consider typicalflares of nearby active
stars. We are not interested in time resolved
spectroscopy, hence we consider all the pa-
rametersas time-averagedoverthe flaredura-
tion. In all cases we consider a flare duration
of 20ks. For each star we produce different Fig.2.Luminosityofthethermalandnon-thermal
simulatedspectra:wekeepfrozenthethermal componentfortheflaresampleanalyzed.Grayarea
component,and vary the normalizationof the marksthesolarflarerange.
non-thermalcomponent.Withthischoicefora
fixedthermalemissioncomponentweareable nizedwithSimbol-Xobservationswhetherthe
to check the amount of non-thermalemission CZTdetectorcollectsmorethan∼ 20photons
neededtodetectit. inthe20−80keVband.
We use the mekal XSPEC model to sim- For each simulated spectrum we compute
ulate the thermal component (Table 1 con- the thermal and non-thermal luminosity. We
tains the parameters of the thermal emission compare the results obtained with those de-
fortheselectedstellartargets).Wesimulatethe rivedfromsolarflaresbyIsolaetal.(2007,see
non-thermalemission using the brokenpower Fig.5).Wefindthatforsomeoftheconsidered
lawXSPECmodel(bknpower).Weassume:a flaresthenon-thermalemissioncanbedetected
photonindexof3forE < E0,inordertohave withSimbol-Xifthermalvs.non-thermalsolar
a negligible contribution of the non-thermal flarerelationisvalidalsoathighluminosities.
componentat low energy;a breakpoint E =
0
10keV; a photon index of -2 for E > E .
0
References
Figure1showsoneoftheexploredmodels.
Hudson,H.&Ryan,J.1995,ARA&A,33,239
Isola, C., Favata, F., Micela, G., & Hudson,
3. Resultsandconclusions
H.S.2007,A&A,472,261
Inspectingallthesimulatedflareswefindthat: Osten,R.A.,Drake,S.,Tueller,J.,etal.2007,
non-thermal emission is detected and recog- ApJ,654,1052