Observations of X-ray flares and associated MHD oscillations in star XI Boo

0.47 MB·English

by  J. C. Pandey

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a Observations of X-ray flares and associated MHD oscillations in
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star ξ Boo
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J.C.Pandey∗ andA.K.Srivastava

] Aryabhatta Reearch Institute of Observational Sciences,
R
Manora Peak, Nainital 263139, India
S ∗E-mail: [email protected]
. www.aries.res.in
h
p
- Using data from observations made with XMMNewton, we present an X-ray
o
analysis of two flares observed in ξ Boo. The flare loop parameters are de-
r
t rived usingvarious loop models including state-of-the-art hydrodynamic flare
s
model.Thelooplengthsderivedfortheflaringloopstructurearefoundtobe
a
[ less than the stellar radius. The exponential decay of the X-ray light curves,
and timeevolution of theplasmatemperature andemissionmeasurearesim-

1 ilar to those observed in compact solar flares. The X-ray light curve of post
v flarephaseisinvestigatedwithwaveletanalysis.Waveletanalysisclearlyshow
7 oscillations of the period of 1019 s. Using the observationally estimated loop
2 length, densityandmagnetic field,the theoreticallyderivedoscillationperiod
1 forfast-kinkmodeapproximatelymatches withthe observationallyestimated
3 period.Thisisthefirstlikelyobservational evidence offundamental fast-kink
. modeofmagnetoacousticwavesinthestellarloopsduringthepost-flarephase
1
ofheightenedemission.
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1 Keywords: Star;X-ray;flares;coronae; MHDoscillations
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: 1. Introduction
v
i
X Sudden enhance and gradual decay in the intensity of a star is the most
r known feature of a flare observed in it. During the flare large amount of
a
energyisreleasedinashortintervaloftime.Flaresareknowntobeaman-
ifestation of the reconnection of magnetic loops, accompanied by particle
beams, chromospheric evaporation, rapid bulk flows or mass ejection, and
heatingofplasmaconfinedinloops.Stellarflarescouldberadiatingseveral
orders of magnitude more energy than a solar flare. The flare produced by
the stellar sources (e.g. RS CVn and BY Dra types) are usually detected
in UV and X-rays. Flares from these stars present many analogies with
the solarflares.However,they alsoshow significantdifferences,suchas the
amount of energy released. Modeling the dynamic behavior of X-ray flares
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allows us to constrain the properties of magnetic loops in ways that can-
not be done from analysis of quiescent coronae that provide only a spatial
and temporal average over some ensemble of structures. Therefore, study
ofstellarflaresisavaluabletoolforunderstandingstellarcoronae,asthese
are dynamical events, which therefore embody different information than
available from static, time-averagedobservations (e.g.,1,2,34) .
With the advent of HINODE/STERIO the study of oscillations in the
solarcoronabecamethemostinterestingnowadays.Thewavesinthesolar
coronahavebeen explainedby magneto-hydrodynamic(MHD) oscillations
incoronalloops.5 MHD wavesandoscillationsinthe solarandprobablyin
thestellaratmospheresareassumedtobegeneratedbycouplingofcomplex
magnetic field and plasma. These MHD waves and oscillations are one of
the important candidates for heating the solar/stellar coronae and accel-
erating the supersonic winds. In the Sun, magnetically structured flaring
loops, anchored into the photosphere, exhibit various kinds of MHD oscil-
lations(e.g.fastsausage,kinkandslowacousticoscillations).StellarMHD
seismology is also a new developing tool to deduce the physical properties
of the atmosphere of magnetically active stars,and it is basedon the anal-
ogy of the solar coronal seismology.6 One of the most exciting aspects of
”coronal seismology” is that it potentially provides us with the capability
fordeterminingthemagneticfieldstrengthinthecorona,7aswellas,inthe
stellar case, with information on otherwise unresolved spatial scales, e.g.,
flare loop lengths.
In this article, we have studied the X-ray flares and associated MHD
oscillationsin the post flarephase of the starξ Boo. ξ Boo is a nearby(6.0
pc)visualbinary,comprisingaprimaryG8dwarfandasecondaryK4dwarf
withanorbitalperiodof151yr.Intermsoftheouteratmosphericemission,
UV and X-ray observations show that the primary dominates entirely over
thesecondary.8 Recently,usingtheChandraobservationsWood&Linsky9
show that ξ Boo primary contributes 88.5% of total X-ray emission.
2. Observations and data reduction
ξ Boo was observed with XMM-Newton satellite using the EPIC MOS
detectors and Reflection Grating Spectrometer (RGS) on 2001 January 19
at 19:25:06 UT for 59 ks. The details on the XMM-Newton satellite and
its detectors are given in the references10 and.11 The data were reduced
using the Science Analysis System version 8.0 with updated calibration
files.Details ofdatareductionis givenin.2 The stellarseparationofxiBoo
primary and xi Boo secondary is 6.3 arcsec, which more than the spatial
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resolution of XMM-Newton. Therefore, we can safely consider that ξ Boo
primary is observed by XMM-Newton.
3. X-ray light curves
The MOS light curve of ξ Boo in the energy band 0.3-10 keV is shown in
Figure 1. Two flare like features, marked as F1 and F2 are clearly seen.
The quiescent state in the light curve is marked by Q. A higher emission
marked by U is also seen in the X-ray light curve. Both flares are found to
be 2.6 times more energetic than that of the quiescent state. A flux of
∼
2 10−11 erg cm−2s−1 was observed during the quiescent state of ξ Boo.
×
Thee-foldingdecaytimesoftheflaresF1anF2werefoundtobe4396 333
±
and9885 362s,respectively.However,the e-foldingrisetimes werefound
±
tobe3169 259and3351 244sfortheflaresF1andF2,respectively.The
± ±
duration of flares F1 and F2 were 8.4 and 11.3 ks. From the light curve,
it is seen that before ending the flare F1 the flare F2 starts. Similar loop
systemshavebeenobservedinaflareontheSun(e.g.so-calledBastilleDay
flare12),andinastellaranaloguedMestarProximaCentauri.4Inthesetwo
events, a double ignition in nearby loops was observed or suggested, and
the delay between the ignitions appears to scale with the loop sizes. Both
flares are appeared to be a long decay flare (τ 1 h).
d
≥
Fig.1. X-raylightcurveofthestarξBoodetectedbyMOS1,verticallinesaredifferent
timesegments.
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4. Evolutions of flares
In order to trace the spectral changes during the flares, we have analysed
the spectra of the different time intervals shown in Fig. 1. Flare F1 and
F2 have binned into five and six intervals, respectively. The spectra corre-
sponding to different time segments associated with flares F1 and F2 are
shown in Fig. 2 (a) and 2 (b), respectively. The quiescent state spectra
are also shown by solid circles in each Fig. 2 (a) and 2(b) for comparison.
The quiescent state spectra of ξ Boo were fitted with a single (1T) and
two (2T) temperature collisional plasma model known as APEC,13 with
variableelementalabundances.Only2Tplasma modelwithvariableabun-
dance(Z <0.23Z⊙)werefoundacceptable.The temperatureandemission
measure(EM)werefoundtobe 0.20 0.05keVand15 6 1051 cm−3 for
± ± ×
cool component, and 0.57 keV and 12 1 1051 cm−3, respectively. The
± ×
luminosity during the quiescent state was found to be 5 1028 erg s−1.
×
1
malized counts s keV−1−1 0.11 RDDDD11234 malized counts s keV−1−1 0.010.1 RDDDDDU256789
nor Q nor Q
0.01 10−3
0.5 1 2 5 0.5 1 2 5
Energy (keV) Energy (keV)
(a) (b)
Fig. 2. (a) Spectra of different time segments during the flare F1, and (b) Spectra of
differenttimesegmentsduringtheflareF2.Thequiescentstatespectraisalsoshownby
solidcirclesineachFigure(a)and(b)
To study the flare emission only, we have performed 1T spectral fits of
the data, with the quiescent emission taken into account by including its
best-fitting 2T model as a frozen background contribution. This is equiva-
lent to consider the flare emission subtracted of the quiescent level, allows
us to derive one effective temperature and one EM of the flaring plasma.
The abundances were keptfix to that of the quiescentemission. Figs.3 (a)
and3 (b) show the temporalevolutionof the temperature andcorrespond-
ing EM of the flares. Both the temperature and EM show the well-defined
trends i.e. the changes in the temperature and the EM are correlated with
the variations observed in the light curves during the flares. The hardness
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ratio can reveal the information about temperature variations. The hard-
ness ratio also varied in the similar fashion to their light curves (see also.2
1.6 8.0 7.20 ζζ == 02..7130 ++//-- 00..3400 ((FF12))
7o T (10 K) 01..82 51-3EM (10 cm) 246...000 0log T (K) 677...901000 F1
0.4 F2
20000 Time (s )40000 20000 Time (s )40000 25.70 0.5 lo2g5 .E8M0 (cm-3)25.90
(a) (b) (c)
Fig.3. (a)Evolutionoftemperature,and(b)emissionmeasureduringtheflareF1and
F2and (c) the densitytemperature diagram, whereEM1/2 has been usedas a proxyof
density.
5. Determinations of density using RGS spectra
Figs.4 (a),4(b) andFig.4(c) showthe He-liketriplets fromO viiduring
thequiescentstate,flarestateF1andflarestateF2+U.Linefluxesandpo-
sitions were measured using the XSPEC package by fitting simultaneously
the RGS1 andRGS2spectra witha sumofnarrowGaussianemissionlines
convolvedwith the response matrices of the RGS instruments.The contin-
uum emissionwas described using Bremsstrahlungmodels at the tempera-
tures of the plasma components inferred from the analysis of EPIC-MOS1
data. The emission measure derived from the analysis of the EPIC data
was used to freeze the continuum normalization.The detailed study of the
RGS spectra was done by Nordon & Behar.14 Therefore, for the present
purpose, we give the line fluxes of He-like triplets from O vii in Table 1.
r
normalized counts s Å−1−1 0.050.10.150.2 r i f normalized counts s Å−1−1 0.050.10.15 i f normalized counts s Å−1−1 0.050.10.150.2 r i f
0 21.5 Wavel2e2ngth (Å) 22.5 0 21.6 W21a.8velength (Å2)2 22.2 0 21.5 Wave2le2ngth (Å) 22.5
(a) (b) (c)
Fig.4. RGSspectraofξBooduringquiescentstateQ(a),flarestateF1(b),andflare
stateF2(c).
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Table1. ObservedOviilinefluxesduringtheQ,F1andF2+U
states
Quiescent(Q) Flare(F1) Flare(F2+U)
λ(˚A) Fluxa λ(˚A) Fluxa λ(˚A) Fluxa
22.08(f) 2.1±0.2 22.07 2.3±0.2 22.08(f) 2.2±0.3
21.78(i) 0.8±0.2 21.77 0.9±0.2 21.77(i) 0.9±0.2
21.58(r) 3.0±0.3 21.56 3.5±0.3 21.57(r) 3.4±0.3
Note:a Measuredfluxin10−4 photons/cm2/s
The He-like transitions, consisting of the resonance line (r) 1s2 1S
0
- 1s2p 1p , the intercombination line (i) 1s2 1S - 1s.2p 3P1, and forbid-
1 0
den line (f) 1s2 1S - 1s.2s 3S are density- and temperature-dependent.
0 1
The intensity ratio G = (i+f)/r varies with temperature and the ratio R
=f/i varies with electron density due to collisional coupling between the
metastable 23S upper level of forbidden line and the 23P upper level of
intercombination line. In the RGS wavelength, the O vii lines are clean,
resolved and potentially suited to diagnose electron density and tempera-
ture (see Fig. 4). These lines at 21.60˚A(r), 21.80˚A(i) and 22.10˚A(f) have
beenusedtoobtaintemperatureanddensityvaluesfromtheG-andR-ratio
usingCHIANTIdatabase(version5.2.1;15).TheG-ratioasfunctionoftem-
perature and intensity ratio R as a function of density are shown in Figs.
5(a) and 5(b), respectively. It is clear from Fig. 5(a) the temperature was
found tobe 2 MK.Therefore,forthe representativetemperaturearound
∼
2MK,wederiveelectrondensitiesofn =1.3+1.2 1010 cm−3 forquiescent
e −0.5×
state (Q) and1.0+1.5 1010 cm−3 during flare F1 and 1.6+1.2 1010 cm−3
−0.8× −0.8×
for flare state (F2+U). The electron densities were found well with in a σ
level for each three states. These densities are found to be similar to that
of solar conditions.
6. Loop modeling
In a star, flares cannot be resolved spatially. However, by an analogy with
solar flares and using flare loop models, it is possible to infer the physical
sizeandmorphologyofthe loopstructuresinvolvedinastellarflare.Based
on quasi static radiative and conductive cooling during the early phase of
the decay. Haisch16 suggested an approachto model a loop. Given an esti-
mateoftwomeasuredquantities,theemissionmeasure(EM)andthedecay
timescale of the flare (τ ), this approach leads to the following expression
d
for the loop length (L ):
ha
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O VII line ratio relative to 21.602 Å O VII line ratio relative to 21.807 Å
1.5 3
Ratio (Energy)1.0 Ratio (Energy)2
0.5 1
2212..810071ÅÅ
22.101Å
0.0 0
5.5 6.0Log10 (Elect6ro.5n temperatur7e.0 [K]) 7.5 8 9 Log1100 (Electron d1e1nsity [cm−31]2) 13
(a) (b)
Fig.5. (a)TheratioG=(i+f)/rofthesummedintensitiesoftheOViiintercombi-
nationandforbiddenlinesovertheintensityoftherecombinationline(λ=21.60˚A)and
(b) intensity ratio R = f /i of the OVii forbidden (λ = 22.12 ˚A) and intercombination
(λ = 21.80 ˚A) lines as a function of electron density, calculated using the CHIANTI
database.
L (cm)=5 10−6(EM)1/4τ3/4 (1)
ha × d
After Haisch16 various stellar flare models came up. These are (i) two-
ribbon flare method,17 (ii) quasi-static cooling method,18 (iii) pure radia-
tioncoolingmethod,19 (iv)riseanddecaytimemethod,20 and(v)hydrody-
namicmethod,2122Thetwo-ribbonflaremodelassumesthattheflaredecay
is entirely driven by heating released by magnetic reconnection of higher
and higher loops and neglects completely the effect of plasma cooling. The
other four methods are based on the cooling of plasma confined in a single
flaring loop. The hydrodynamic model includes both plasma cooling and
the effect of heating during flare decay. Reale et al.21 presented a method
to infer the geometrical size and other relevant physical parameters of the
flaringloops,basedonthe decaytimeandonevolutionoftemperatureand
theEMduringtheflaredecay.Thethermodynamicloopdecaytimecanbe
expressed23 as
3.7 10−4L
τ = × (2)
th
√T
max
where L is the loop half-length in cm, and T is the flare maximum
max
temperature (K) calibrated for EPIC instruments as,
T =0.13T1.16 (3)
max obs
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T is the maximumbest-fit temperature derivedfromsingle temperature
obs
fitting of the data. The ratio of the observedexponential light curve decay
time τ to the thermodynamic decay time τ can be written as a function
d th
whichdependsontheslopeζ ofthedecayinthedensity-temperatureplane.
For the XMMNewton EPIC spectral response the ration τ /τ = F(ζ) is
d th
given as22
τ 0.51
d
=F(ζ)= +1.36 (for 0.35<ζ 1.6) (4)
τ ζ 0.35 ≤
th
−
Combining above equations the expression for semi-loop length is
τ √T
d max
L=L = (5)
hyd 3.7 10−4F(ζ)
×
Alternatively, Reale22 derive the semiloop length from the rise phase
and peak phase of the flare as
T5/2
L =3 105/2t max (6)
hyr × M T2
M
T and t are temperature and time at which density peaks.
M M
Based on magneto-hydrodynamic simulations Shibata & Yokoyama24
assumed that, to maintain stable flare loops, the gas pressure of the evap-
orated plasma must be smaller than the magnetic pressure
B2
2nkT (7)
≤ 8π
where B is the minimum magnetic field necessary to confine the flaring
plasma in the loop. The derived equations for B and L are
p
EM −1/5 n 3/10 T 17/10
e
B=50 G (8)
1048 cm−3 109 cm−3 107 K
(cid:18) (cid:19) (cid:16) (cid:17) (cid:18) (cid:19)
EM 3/5 n −2/5 T −8/5
L =109 e cm (9)
p 1048 cm−3 109 cm−3 107 K
(cid:18) (cid:19) (cid:16) (cid:17) (cid:18) (cid:19)
The averagetemperaturesofthe loops,usually lowerthanthe realloop
maximum temperatures, are found from the spectral analysis of the data.
According to equation (3), the loop maximum temperatures for the flare
F1andF2arefoundtobe18.9 0.8and25 2MK,respectively.Thepeak
± ±
valuesoftemperaturearesimilartothosefoundinthesolarflares.However,
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Table2. Loopparametersobtainedfromvariousloopmodels.
Flare Lha Lhyd Lhyr Lp B ne
(1010cm) (1010cm) (1010cm) (1010cm) (G) (1010cm−3)
F1 2.2±0.2 1.9±0.8 1.7±0.2 5.7±1.4 18.3±3.4 1.0+−10..58
F2 3.8±0.2 7.9±0.6 3.6±0.8 3.6±0.9 29.4±6.0 1.6+−10..28
thepeakemissionmeasurewasfoundtobeanordermorethanthosefound
inthesolarflares.Thepathofdensity-temperature(n-T)diagramisshown
in Fig. 3(c). The EM1/2 was taken as a proxy of density. The solid lines
represent the best linear fit to the corresponding data, providing the slope
ζ. The values of ζ for the flare F1 and F2 were found to be 0.7 0.3 and
±
2.1 0.4,respectively.ThisindicatesthattheflareF1isdrivenbythetime-
±
scale of the heating process,whereas sustained heating is negligible during
the decay of the flare F2.
The semi-looplengths derivedfromabovemethods aregivenintable2.
The loop length derived for flare F1 is consistent using three methods (see
equations(1),(5)and(6)).However,the looplengthderivedfrompressure
balance method ( see equation 9) was found to be 3 times more than
∼
other three methods. The value of ζ for the flare F2 is outside the domain
of the validity of the hydrodynamic method (see equation5), therefore, the
looplengthderivedfromhydrodynamicmethodbasedondecayphasemay
not be actual one. The loop length derived from other three methods (see
equations(1),(6)and(9))is found tobe consistentata value of3.6 1010
×
cm.Themagneticfieldderivedusingtheequation(8)is18.9and29.4Gauss
for the flares F1 and F2, respectively.
The total energy rate is defined as H = E V, where E =
H H
×
10−6T3.5 L−2 erg s−1 cm−325 heating rate per unit volume and V =
max
EM/n2 volume of the loop. Therefore, the total energy rate was deter-
mined to be 5.4 1030 and 9.8 1030 ergs s−1 for the flare F1 and F2,
× ×
respectively. The peak X-ray luminosity during the flare F1 and F2 was
found to be 1.02 1029 and 1.11 1029 ergs s−1, which is only 2 and 11 %
× ×
of the total energy rate, respectively.
7. Search for coronal oscillations
We have used the wavelet analysis IDL code ”Randomlet” developed by
E. O’Shea. The program executes a randomization test,2627 which is an
additional feature along with the standard wavelet analysis code28 to ex-
amine the existence of statistically significant real periodicities in the time
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series data. The advantage of using this test is that it is distribution free
or nonparametric, i.e., it is not constrained by any specific noise model
such as Poisson, Gaussian, etc. Using this technique, many important re-
sultshavebeenpublishedbyanalyzingapproximatelyevenlysampleddata
(e.g.,,27,2930).ThewaveletpowertransformsoftheUpartoftheX-raylight
curveofcadence40sareshownintheleftpanelofFig.6,wherethedarkest
regions show the most enhanced oscillatory power in the intensity wavelet
spectrum. The crosshatched areas are the cone of influence (COI), the re-
gion of the power spectrum where edge effects, due to the finite lengths
of the time series, are likely to dominate. The maximum allowed period
from COI, where the edge effect is more effective, is 7363 s. In our wavelet
analysis, we only consider the power peaks and corresponding real periods
(i.e., probability > 95%) below this threshold. The period with maximum
power detectable outside the COI is 2883 s with a probability of 94% and
the repetition of only 3 cycles in the time series. However, another peak is
visible at a period of 1019 s with a probability of 99% and the repetition
of 11 cycles. Therefore, only the periodicity of 1019 s satisfies the recently
reported stricter criteria of O’Shea & Doyle,31 i.e., at least four cycles of
repetition over the lifetime of the oscillations. The right panel in Fig.6
shows the global wavelet power spectra of the time series from which the
statisticallysignificantperiod(1019s)isselected.Waveletanalysiswasalso
performedin the decayphase ofthe flareF2, andno significantperiodicity
was found.
Fig.6. ThewaveletpowerspectrumoftheUpartofX-raylightcurveofξ Boo.