Hot thermal X-ray emission from the Be star HD119682

0.17 MB·English

by  J. M. Torrejon

Draftversion January15,2013
PreprinttypesetusingLATEXstyleemulateapjv.5/2/11
HOT THERMAL X-RAY EMISSION FROM THE BE STAR HD119682.
J.M. Torrejo´n1, N.S. Schulz2, M.A. Nowak2, P. Testa3 and J.J. Rodes1
1Instituto UniversitariodeF´ısicaAplicadaalasCienciasylasTecnolog´ıas,UniversidaddeAlicante,E03080Alicante,Spain;[email protected]
2MITKavliInstituteforAstrophysicsandSpaceResearch,CambridgeMA02139,USAand
3HarvardSmithsonianCenter forAstrophysics,CambridgeMA02138,USA
Draft version January 15, 2013
ABSTRACT
3
1 We present an analysis of a series of four consecutive Chandra high resolution transmission grat-
0 ings observations, amounting to a total of 150 ks, of the Be X-ray source HD119682 (= 1WGA
2 J1346.5−6255), a member of the new class of γ Cas analogs. The Chandra lightcurve shows signifi-
cant brightness variations on timescales of hours. However, the spectral distribution appears rather
n
stable within each observation and during the whole campaign. A detailed analysis is not able to
a
detect any coherent pulsation up to a frequency of 0.05 Hz. The Chandra HETG spectrum seems
J
to be devoid of any strong emission line, including Fe Kα fluorescence. The continuum is well de-
4
scribed with the addition of two collisionally ionized plasmas of temperatures kT ≈ 15 keV and 0.2
1
keV respectively, by the apec model. Models using photoionized plasma components (mekal) or non

thermal components (powerlaw)give poorer fits, giving support to the pure thermal scenario. These
]
E twocomponentsareabsorbedbya singlecolumnwithN =(0.20+0.15)×1022 cm−2 compatiblewith
H −0.03
H the interstellar value. We conclude that HD119682 can be regarded as a pole-on γ Cas analog.
. Subject headings: stars: individual (HD119682) - X-rays: binaries
h
p
o- 1. INTRODUCTION sourcefromthebackgroundSupernovaRemnantG309.2-
r HD119682(=1WGAJ1346.5−6255)isaBestarwhich 00.6. These authors find that the XMM-Newton
t PN+MOS data can be satisfactorily described by the
s presents copious X-ray emission. A member of the open
a cluster NGC 5281, located at a distance of ∼ 1.3 kpc, addition of two optically thin thermal plasmas, using
[ this Be star displays an X-ray luminosity in the 0.5–10 the mekal model, with kT1 = 1.7±0.3 keV and kT2 =
1 keV energy band of the order of LX ∼ 1032 ergs−1, low 13.0+−22..64 keV respectively, while the fit to the Chandra
3v wrahyenbincaormiepsa(rLedXw∼it1h03t6h−e37tyeprigcsa−l1l)u,mtwinoosoitrideesrsofofBmeaXg-- seprsecttermumpe,reaxtutrraecsteodf kfrTo1m=th0e.9S53+−0A0..32C44ISkeCVCaDndchkipT,2d≥eli1v4-
1 nitude lower than persistent Be X-ray binaries (Reig & keV respectively. Alternatively, the hotter thermal com-
9 Roche,1999)andatleastone orderofmagnitude higher ponentcanbedescribedbyapowerlaw. Thisdegeneracy
2 thantheexpectedX-rayemissionfromisolatedOBstars hasalsobeenfoundinγ Cas(Smithetal. 2004,Lopesde
1. (Bergh¨ofer et al. 1997). With a mass of M =18±1M⊙ Oliveira et al. 2011) and HD110432 (Lopes de Oliveira
and an age of 4±1 Myr, HD119682 seems to be a blue 2007, Torrej´on et al. 2012). S07 present also hints of
0
straggler (Marco et al. 2009), the second (out of the 7 a possible pulsation with P ≃ 1500 s which could be
3
known) gamma Cas analogs that shares this property. caused by the spin of a NS or a WD.
1
Rakowski et al. (2006; R06), using the superb spa- The origin of the X-ray emission in γ Cas analogs is
:
v tialresolutionofChandra aswellasHαimagesfromthe still a mystery. Two hypotheses have been put forward:
i Clay6.5mTelescopeintheMagellanObservatoryatLas
X
Campanas, unambiguously identified the X-ray source • TheX-raybinaryscenarioinwhichaWhiteDwarf
r 1WGA J1346.5−6255 with the OB star HD119682. Us- (WD) accretes matter from the Be donor (Haberl,
a
ingXMM-Newton PN+MOSdata,theseauthorsshowed 1995; Kubo et al., 1998; Owens et al., 1999 for the
that the X-ray spectrum is well described by the addi- caseofγ Cas;Torrej´on&Orr,2001forHD110432).
tion of two optically thin (raymond) thermal plasmas The low potential well of the WD, compared with
with kT = 1.07 ± 0.16 keV and kT = 10.4+2.3 keV thatofaNS,accountsnaturallyforthelowerlumi-
1 2 −1.6 nosity and the thermal nature of the X-ray emis-
respectively. Based on the similarities shared between
sion. This explanation faces several problems. On
HD119682 and γ Cas (early type optical counterpart,
one hand, the X-ray emission shows no signof any
thermal nature of the X-ray emission, logLX/Lbol =
−5.4, etc) R06 classify the source as a γ Cas analog, orbital modulation or spin pulse. On the other
hand, the progenitor of the putative WD should
and argue against other X-ray production mechanisms
havebeenstillmoremassivethantheearlyBcom-
likecollidingwinds(withanunseencompanion)oremis-
sion from a magnetically confined wind (given the very panion (M ∼ 20M⊙). From the evolutionary per-
spective, it is challenging to produce a WD from
high temperature of the hot plasma).
such a massive star; naturally, they would explode
Safi-Harb et al. (2007; S07), in a multifrequecy
as a supernova leaving behind a NS, as is the case
analysis, confirm the identification between 1WGA
J1346.5−6255 and HD119682 and characterize the op- forthe BeX-raybinaries. InthecaseofHD119682
no SN explosionseems to have occurred(Marco et
tical counterpart (B0.5Ve), definitely unrelating this
al. 2009) which argues against the NS companion.
2 Torrej´on et al.
and,consequently,the spectrais notaffectedbypileup1.
TABLE 1 For the lightcurve extractions we add data from all dis-
Journalof Chandra HETG observations.
persed orders (m = −3,−2,−1,1,2,3) from HEG and
ObsID Date texp Ratea MEG. High dispersion orders(m=±3,±2)constitute a
(dd-mm-yyyy) (ks) (c/s) 25 % of the total count rate.
The analysis was performed with the Interac-
tive Spectral Interpretation System (ISIS) v 1.6.1-24
8929 17-12-200800:20:10 28.9 0.039
10835 19-12-200809:32:42 29.7 0.031 (Houck & Denicola 2000). The photoelectric absorp-
10834 20-12-200815:45:04 59.3 0.032 tion has been treated with the tbnew absorption model
10836 21-12-200815:47:03 29.9 0.035 (Wilms et al. 2012), which includes the most up to date
a Zerothorder photoelectric cross sections.
• The standalone star scenario in which the X-rays
are produced by the interaction of the star’s mag- 3. TIMINGANALYSIS
netic field with the circumstellar disk (Robinson The four observations analyzed in this work span five
et al. 2002, Smith & Robinson 2003, Smith et differentdayswithuninterruptedcoveragesrangingfrom
al. 2004). In this scenario, magneto rotational 30 ks to 60 ks. In total they represent 150 ks worth of
instabilities in the inner part of the circumstel- data. This allows us to search for coherent pulsations
lar disk around the Be star (close to the Keple- over a wide range of frequencies with unprecedented de-
rian co-rotation radius) enhance a seed magnetic tail. In the left panel of Fig. 1 we present the Chandra
fieldthroughadiskdynamomechanism(Balbus& lightcurve binned to 2000 s bins. As can be seen, the
Hawley,1998). Thedifferentialrotationofthedisk, source brightness is variable on timescales shorter than
stretch and sever the entangled field lines acceler- thedurationofasingleobservation(ObsID).Duringthe
ating particles which, upon impact on the star’s whole campaign, however, the source remains quite sta-
surface, heat the plasma up to MK temperatures. ble. In order to search for coherent pulsations we have
The finding that the X-ray emission seems to be analyzedthe lightcurvewithamaximumtimeresolution
produced close to the surface of the Be star sup- of 10 s. Subsequently we have created lightcurves with
ports this scenario (Smith et al. 2012, Torrej´on et largertimebinsof100,250and500storeducetheerror
al. 2012). The details of this mechanism, though, bars. In Fig. 1, right panel, we show, as an example,
are far from clear. Why other early type Be stars the normalized Fast Fourier Transform periodogram of
withlargecircumstellardisksdonotproduceX-ray the lightcurveusing250sbins,using the Lomb-Scargle
emission at all is not explained. technique, for each ObsID. The upper plot corresponds
to all four intervals together taking into account the du-
In any case, to understand the true nature of these
ration of the time gaps between each observation. Safi-
systems will have importantconsequences in our picture
Harbetal. (2007)suggestedthepossibilityofa∼1500s
of the structure and evolutionof early type stars and/or
pulsationinthelightcurveofHD119682. InourlastOb-
how binary systems evolve.
sID(10386)thereisindeedapeakatP =1443±17s(and
In this work we present an analysis of a series of four
alsootherat2350±50s). Thesignificanceofthispeak,as
consecutiveChandra HighEnergyTransmissionGratings
givenbytheFalseAlarmProbability(FAP),islow. FAP
(HETG) observations of the Be star HD119682 allow-
is defined as the probability that no periodic component
ing us to study the spectrum at the highest resolution
is present in the data with such a period within the cur-
availabletoday. Thefourobservationsamountto atotal
rent frequency search parameters. In order to compute
of ∼ 150 ks, spread over five days, allowing a sensitive
this significance we have run 103 Monte Carlo simula-
search for pulsations in a wide range of frequencies.
tions of the randomized Y-axis values of the lightcurve
2. OBSERVATIONS and performed the corresponding periodograms. Then
we search for peaks higher than that in the original
We have processed and analyzed the Chandra data
periodogram at any frequency. We have found higher
from the ObsIDs 8929, 10835, 10834 and 10836 avail-
peaks in 20±3 % (two sigma standard error) of the pe-
able at the public archive (PI C.E. Rakowski). In Ta-
riodograms. Therefore, the False Alarm Probability is
ble 1 we show the journal of observations. The Chan-
ratherhigh,∼20%and,consequently,thesignificanceof
dra High Energy Transmission Gratings spectrometer
the peak low. Similar results are obtained irrespectively
(HETG; Canizareset al. 2005)observedthe source dur-
ofthesizeofthetimebin. Howeverneitherofthesepeaks
ing 150 ks, splitted in four intervals. Both HETG in-
arepresentinthepreviousdatasets. Althoughsomelow
struments, the High Energy Gratings (HEG; 0.7-8 keV)
significancepeakscanbeseenineachperiodogram,none
and Medium Energy Gratings (MEG; 0.4-8 keV) were
ofthemremainstable. Inconsequence,inthewholedata
used for the analysis. After filtering the data, the 0.5
set we do not find evidence of any significant period up
to 7.5 keV spectral range showed enough signal-to-noise
to a frequency of 0.05 mHz2).
ratio for the scientific analysis. Both the spectra as well
In order to study the spectral variations of the X-
as the response files (arf and rmf) were extracted us-
ray source we have extracted the Chandra lightcurves
ing the CIAO software (v 4.4). First dispersion orders
in the wavelength intervals 3–6 keV (H), 2–3 keV (M)
(m = ±1) were fitted simultaneously. The extracted
and 0.5–2 keV (S), and have created the correspond-
spectra were binned to match the resolution of HEG
(0.012˚AFWHM) and MEG(0.023˚A FWHM) andhave 1 See The Chandra ABC Guide to Pileup, v.2.2,
aminimumS/Nratioof7. Thepeakcountrateinasin- http://cxc.harvard.edu/ciao/download/doc/pileup-abc.pdf
gle (MEG) gratings arm spectrum is ≈ 0.008 cts/sec/˚A 2 TheNyquistfrequencycorrespondingtothe10stimebin.
Chandra observations of HD119682 3
1
.
0
8929 10835 10834 10836
5
s 0
c/ 0.
0
0 5×104 105 1.5×105
Time (s)
Fig. 1.—Left: Chandra lightcurve, inthe2-25˚Awavelength range, binnedin2000sintervals. Alllightcurves have anarbitrarytime
offsetand,therefore,thetimegapsarenottoscale. Right: Powerspectrumcorrespondingtothe250sbinnedChandra HETGlightcurve.
The1500spulsation(ν≃6.7×10−4 Hz)isonlydetected inObsID10836butisnotdetected neitherintherestoftheObsIDsnorinthe
combinedligthcurve.
ing hardness ratios versus source intensity plots. In Fig. 1
2, we show the variation of the soft/high hardness ra-
tio against the source intensity (0.5–6 keV) using 7000 s
time bins. As can be seen, the sourcedoes not show any 8
spectral correlation with brightness. The average val- 0.
ues of the hardness ratios are 0.43±0.07 (ObsID 8929),
0.48±0.07(ObsID10835),0.53±0.06(ObsID10834)and )
H 6
0.50±0.06(ObsID10836). Theyare,therefore,compat- + 0.
ible within errors. The differences between single values S
of the hardness ratio are not significant either. Individ- )/(
ual errors range between 0.15 and 0.20. In conclusion, H 4
while changes in brightness from bin to bin are, overall, − 0.
S
significantthe hardness ratios are consistent with a con- (
stant value. The source appears rather monochromatic
2
on timescales from hours to days. 0.
4. SPECTRALANALYSIS
According to the previous analysis, the spectra of 0
HD119682 is fairly stable in a time scale of days de- 0.02 0.03 0.04 0.05
spite the variationsin the brightness. Thereforewe have
Counts/s
combinedthe fourobservationsintoasinglespectrumto
increase the signal to noise ratio (S/N). The resulting
spectrum is shown in Fig. 3.
Fig.2.—Chandra hardness-intensityplotusing7000stimebins.
In order to describe the continuum, models consist-
DifferentcolorsrepresentthedifferentObsIDs,withthesamecolor
ing of one component do not give satisfactory results. coding as in Fig. 1. While changes in brightness from bin to bin
For example, an absorbed powerlaw gives a reduced chi are, in general, significant the hardness ratios are consistent with
square χ2 = 1.71 for 88 degrees of freedom (dof) while aconstant value.
r
a single mekal gives χ2 = 1.64 (88 dof). The spectrum The high sensitivity of the Chandra gratings allow us to
r
shows always a pronounced excess below 1 keV which constrainthistemperaturewithunprecedentedaccuracy.
can only be modeled when a second component to the Although there are few energy bins below 1 keV, the as-
model is added. The best fit is attained with the ad- sociated errors (blue crosses) are very small. The ob-
dition of two optically thin collisionally ionized plasmas tained plasma temperature kT = 0.20+0.03 keV is fully
2 −0.02
(apec). The parameters are shown in the fourth col- compatible with the temperature of the cool plasma in
umn of Table 2. The low energy range is fitted by a HD110432derivedfromhighresolutionspectra(Torrej´on
plasma with kT = 0.20+0.03 keV and solar abundance. et al. 2012) and significantly lower than that obtained
−0.02
4 Torrej´on et al.
The parameters are shown in the third column of Ta-
TABLE 2 ble 2. However, in terms of χ2 the two apec model is
Model parametersfor Chandra HETG data.
significantly better. Replacing the collisionally ionized
plasmas (apec) by photoionized plasmas (mekal), gives
Component Parameter Value
slightly worse fits. For example, the two mekal model
PL+APEC 2APEC gives χ2 = 1.38 (86 dof) while powerlaw + mekal gives
r
NHa 0.15+−00..1053 0.20+−00..1053 mχ2rod=el1co.4m9p(o8s6eddboyf).twoInapcoencccluosmiopno,nethnetsfiusllfyavtohreerdmbayl
powerlaw Norm 0.00026+−00..00000054 ... the present data set.
Γ 1.36+−00..1051 ... The most conspicuous characteristic of the spectrum
is the lack of any discernible emission line. In particu-
apec1 Norm ... 0.0018+−00..00000056 lar, no Fe Kα line is seen and even the flux at 6 keV is
kT (keV) ... 15.7+−45..74 below the 7 σ level. There is flux up to 7 keV at the 3σ
level; however, there is certainly not an Fe emission line
apec2 Norm 0.0003+−00..00000031 0.0005+−00..00000012 evenat1σ. Finally,theresidualspresentsomestructure
kT (keV) 0.21+−00..0023 0.20+−00..0032 between 1 and 2 keV that could be due to the presence
of emission lines. Indeed, this is the line rich region for
Fluxb 1.78 1.81 highly ionized plasmas. The HETG spectra of bright γ
χ2 (dof) 1.48(86) 1.35(86) Cas analogs display several strong emission lines in this
r
region(Smithetal. 2004,Fig. 2;,forγ Cas;Torrej´onet
a Inunitsof×1022 cm−2 al. 2012,Fig. 7and8,forHD110432). StronglinesofNe
b Unabsorbed0.5–8keVfluxinunitsof×10−12 ergs−1 cm−2 x Lyα (1.02 keV) and Si xiv Lyα (2.00 keV) as well as
weakerlinesofFexxiv (1.17keV)andMg xiiLyα(1.47
keV) might be expected in this area. However, none of
4 these lines show up in the spectrum of HD119682 with
−
0 EW upper limits of the order of 1 eV.
1
V × The high sensitivity of Chandra HETG at low ener-
e 5
gies also allows us to study the absorption column with
k
s/ −4 unprecedented detail. The analysis of brighter mem-
/ 0
2 1 bers of this class have shown that the different spectral
m × componentsareaffectedby differentabsorptioncolumns
c 2
s −4 (Smith et al. 2004, for γ Cas; LO07 and Torrej´on
on 10 kT=0.20 keV et al. 2012, for HD110432). Here we have tried the
t same strategy. The values of the resulting columns turn
o 5 kT=15.7 keV
h 0− out to be identical, within the errors, for all models.
P 1 Therefore a single photoelectric absorption satisfacto-
×
5 rilydescribes the spectrum ofHD119682. The measured
5 NH =(0.20+−00..1053)×1022 cm−2 is very low, in agreement
with R06 and S07.
χ 0
5. DISCUSSION
5 The spectrum of HD119682 differs from other γ Cas
−
analogs observed at high resolution mainly in that it
0.5 1 2 5
shows no discernible emission lines. Particularly strik-
ing is the fact that no Fe complex is observed. Despite
Energy (keV)
this, the continuum spectral parameters of HD119682
Fig.3.— The ChandraHETG unfolded spectrum of HD119682 are very similar to those of γ Cas (Smith et al. 2004,
showingtherelativecontributionsofthelowandhightemperature Fig. 2) and HD110432 (Torrej´on et al. 2012). Consis-
plasmasrespectively. Alsoshown(inred)isthebestfitmodel.
tently with these previousworks,the correctdescription
of the spectrum is attained with the addition of several
by R06 (kT = 1.07± 0.16 keV; Raymond-Smith) and
thermal components. In the case of HD119682 only two
S07 (kT = 1.7±0.3 keV; mekal). The absorption col-
are needed with characteristic temperatures in the ∼ 2
umn, however, remains essentially compatible (see be-
MK and ∼ 150 MK range. Both components are mod-
low). Therefore, the differences in kT could be due to ified by a single absorption column NX ∼ 0.20×1022
intrinsic long term variability of the source. This vari- H
cm−2. This column would imply an interstellar ex-
ability in the cool plasma temperature seems also to be
present in γ Cas (Lopes de Oliveira et al. 2010) while cess E(B −V) = NHX/6.83× 1021 = 0.29+−00..0031 (Ryter
the hot plasma temperature appears to be more stable. 19963) in good agreement with the interstellar medium
The high energy range requires a second, much hot- (ISM) absorption towards NGC 5281 (Moffat & Vogt
ter apec model with kT = 15.7+4.7 keV. This tempera- 1973, E(B − V) = 0.26 ± 0.03; Marco et al. 2009,
−5.4
tureiscompatiblewithinthe errorswiththatderivedby E(B−V)=0.28±0.02).
R06 and S07. Alternatively, in agreement with previous
works on γ Cas candidates, the high energy range can 3 The errors represent the propagated uncertainty in the mea-
be fit with a powerlaw with a photon index Γ = 1.36. suredNHX
Chandra observations of HD119682 5
HD119682 has been observedseveraltimes both in X- is absorbed by material with a column density compati-
rays (XMM-Newton August 2001 and Chandra Decem- ble with the circumstelar disk. If this is also correct for
ber 2004, R06, S07; December 2008, this work) and in HD119682, then the NX ≈ NISM implies that the sys-
H H
the Hα light (June 2003 and February 2004; R06, S07) temmustbeobservedathighinclination. Thiswouldbe
showing always a strong emission. Although the time consistent with the single peaked emission lines seen in
coverageisstillpoor,the sourceappearstobepersistent the optical spectrum of this object (S07).
in both energy bands. Assuming that the circumstellar R06andS07reporthintsofFeKαemission. However,
envelope(andhencetheHαemission)wasalsopresentat no sign of such emission line (nor any other) is present
the epoch of our Chandra observation, the X-ray source here. Thus, the fluorescence emission from neutral Fe
was not obscured by the disk material. The same would seemstobevariableinthelongterm. Ontheotherhand
be true for the observations analyzed in previous works the0.5-8.5keVX-rayluminosityofthesourcehasvaried
(R06,S07). Thecircumstellardisk(confinedtotheequa- withinamaximumfactorof∼2inaperiodofsevenyears
torial plane) does not intercept the X-ray emission. In (R06, S07, this work). One of the possible sites for the
the framework of the accreting binary system, the com- Kα reprocessing of the X-rays is the circumstelar disk.
pact object (the WD) should not be deeply embedded Smith et al. (2012)find a direct correlationbetween the
into the circumstellar disk; yet it should be accreting absorption column of the hard emission and the EW(Fe
enough material to power persistently the X-ray source Kα) (as well as with the reddening) for γ Cas (Table 5
up to a non negligible LX ∼ 3.8×1032 ergs−1. This ofthatreference)implyingthatmost,perhapsall,ofthe
would require a very special tuning of the astrophysical X-rayemissionis producedbehind the circumstellardisk
parameters of the system. Yet, even though rare, this material. These authors also deduced an inclination of
situation can happen: 4U2206+54 is a low luminosity i=[43±1]oforγ Cas. InthecaseofHD119682,however,
(L ≈1035−36 ergs−1)HMXBwithaNSaccretingfrom the absorption column remains always compatible with
X
the wind of a massive O9.5V star while the X-ray spec- theISMvaluedespitethechangesintheX-raybrightness
trum presents very few (if any) local absorption (Reig of the source and variable Fe Kα emission because the
et al. 2012). Contrary to the case of 4U2206+54, how- circumstellardisk,confinedtotheequatorialplane,never
ever,nopulsationshavebeenfoundforHD119682orany intercepts the X-ray emission.
otherγ Casanalogsofar. Incontrast,within the frame- HD119682 would thus appear as a pole-on γ Cas, an
work of the standalone active star, the single absorption importantbenchmark to explorethe phenomenonwhere
column, compatible with the ISM value, can be simply theplasma,anditsvariations,couldbedirectlyobserved
understood if the system is seen at very high inclina- without any intervening circumstellar material.
tion angles. It has been recently found that the hard The authors acknowledge the constructive criticism
X-raycomponentofthe multi-thermalemissionofγ Cas of an anonymous referee. This work has been sup-
(Smithetal. 2012)andHD110432(Torrej´onetal. 2012) ported by the Spanish Ministerio de Ciencia e Inno-
isverylikelyproducednearthesurfaceoftheBestarand vacio´n (MICINN) through the grants AYA2010-15431
and AIB2010DE-00057.
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