The Black Hole Binary V4641 Sagitarii: Activity in Quiescence & Improved Mass Determinations

1.5 MB·

by  Rachel K. D. MacDonald

Draftversion January20,2014
PreprinttypesetusingLATEXstyleemulateapjv.5/2/11
THE BLACK HOLE BINARY V4641 SAGITARII:
ACTIVITY IN QUIESCENCE & IMPROVED MASS DETERMINATIONS
Rachel K.D. MacDonald, Charles D. Bailyn, Michelle Buxton, Andrew G. Cantrell1, Ritaban Chatterjee2,
Ross Kennedy-Shaffer3
DepartmentofAstronomy,YaleUniversity,POBox208101,NewHaven,CT06520-8101, USA
Jerome A. Orosz
4
DepartmentofAstronomy,SanDiegoStateUniversity,SanDiego,CA92182,USA
1
0 and
2 Craig B. Markwardt, Jean H. Swank
AstrophysicsScienceDivision,GoddardSpaceFlightCenter,NASA,Greenbelt, MD20771,USA
n
Draft version January 20, 2014
a
J
ABSTRACT
6
1 We examine ∼10 years of photometric data and find that the black hole X-ray binary V4641 Sgr
has two optical states, passive and active, during X-ray quiescence. The passive state is dominated

] by ellipsoidal variations and is stable in the shape and variability of the light curve. The active state
R is brighter and more variable. Emission during the active state varies over the course of the orbital
S periodandisredderthanthe companionstar. Theseoptical/infraredstateslastforweeksormonths.
. V4641 Sgr spends approximately85% of X-ray quiescence in the passive state and 15% in the active.
h
We analyze passive colors and spectroscopy of V4641 Sgr and show that they are consistent with a
p
reddened B9III star (with E(B−V) = 0.37±0.19) with little or no contribution from the accretion
-
o disk. We use X-ray observations with an updated ephemeris to place an upper limit on the duration
r of an X-ray eclipse of < 8.3◦ in phase (∼1.6 hours). High resolution spectroscopy yields a greatly
st improved measurement of the rotational velocity of the companion star of Vrotsini=100.9±0.8 km
a s−1. Wefitellipsoidalmodelstothepassivestatedataandfindaninclinationangleofi=72.3±4.1◦,
[ a mass ratio of Q = 2.2±0.2, and component masses for the system of MBH = 6.4±0.6 M⊙ and
1 M2 = 2.9±0.4 M⊙. Using these values we calculate an updated distance to V4641 Sgr of 6.2±0.7
kpc.
v
0 Subject headings: black hole physics, stars: individual: V4641 Sgr, X-rays: binaries
9
1
4 1. INTRODUCTION and timing properties.
. X-ray binaries (XRBs) are binary systems in which a V4641 Sgr is a low-mass X-ray binary (LMXB) and a
1
softX-raytransient,withaB9IIIcompanionstar. Itwas
0 companionstaristransferringmasstoacompactobject,
discoveredasanX-raysourceindependentlyinFebruary
4 leading to accretion-driven X-ray outbursts. XRBs can
of1999by bothBeppoSAX (in’t Zand et al.1999, 2000)
1 be classified into two broad categories: high-mass and
and Rossi X-ray Timing Explorer (Markwardt et al.
: low-mass, referring to the ratio of the mass of the com-
v 1999). It was discovered in radio in September of 1999
panionstartothecompactobject. Ingeneral,high-mass
i (Hjellming et al. 2000), shortly after undergoing two X-
X XRBstransfermasstotheircompactobjectviaastrong
rayflares. Althoughconfusedforatimewiththevariable
stellarwind,andlow-massXRBstransfermasswhenthe
r starGMSgr,itstrueopticalcounterpartwasdetermined
a companion star overflows its Roche lobe. The compact
tobeavariablestardiscoveredbyGoranskij(1978),now
object in the system can be either a neutron star or a
designated V4641 Sgr (Orosz et al. 2001).
black hole. One subset of XRBs is referred to as soft
V4641Sgris unusualinseveralcharacteristics,includ-
X-raytransientsorsimplyX-raytransients;thesecanbe
ing its companionstar and its X-rayoutbursts. Its com-
either high-mass or low-mass. A typical soft X-ray tran-
panion star is the most massive, brightest, and bluest of
sient has short X-ray outbursts driven by episodic ac-
thegalacticLMXB companionstars. Thishasledto the
cretion, which rise quickly and decay exponentially, and
system being classified as a high mass X-ray binary by
longerX-rayquiescentperiodsduringwhichtheluminos-
some (for example, Chaty et al. 2003; Revnivtsev et al.
ityisdominatedbythecompanionstar. X-rayoutbursts
2002; Pandey et al. 2007). However, since the mass
inblackholeXRBs arecharacterizedbyseveraldifferent
transfer occurs via Roche lobe overflow, and since the
X-ray“states”,whicharedifferentiatedbytheir spectral
companion star is not more massive than the black hole
[email protected] in the system, we maintain the label of LMXB in this
1The Blake School, 511 Kenwood Pkwy, Minneapolis, MN work. The X-ray outbursts observed in this system do
55403, USA not follow the standard soft X-ray transient fast-rise-
2DepartmentofPhysics,PresidencyUniversity,86/1College
exponential-decay pattern. Rather they are more like
Street, Kolkata-700073, WB,India
3Hunter College High School, 71 E 94th St, New York, NY flares, with a fast rise and an equally fast decay — for
10128, USA example,anX-rayoutburstin1999SeptemberforV4641
2
Sgr rose to 12 crab in 8 hours and faded to 0.1 crab in
less than 2 hours (Hjellming et al. 2000). No good ex- Table 1
ObservingseasonsforV4641Sgrphotometry
planationforthis incongruousoutburstbehaviorhasyet
been found.
Year OpticalData IRData
Although known primarily for their different X-ray
states, black hole XRBs also exhibit variability in the 2001 Mar27–Oct12 Mar27–Sep5
optical and infrared. Optical or infrared data taken 2002 Feb18–May30 Feb22–Sep25
2003 Apr8–Nov14 Apr8–Nov14
during X-ray quiescence is often used to determine bi-
2004 Mar17–Oct5 Mar23–Oct31
nary parameters of the system, such as mass function, 2005 Oct8–Nov5 Feb12–Nov4
mass ratio, and inclination angle, and then to deter- 2007 May5–Sep30 May5–Sep29
2008 Feb29–Nov9 Feb29–Nov9
mine the mass of the compact object in the system
2009 Feb23–Sep5 Feb22–Sep1
(Orosz et al. 2001; Kreidberg et al. 2012, and references 2010 Feb14–Oct31 Feb14–Oct31
therein). This analysis frequently assumes that if a sys-
tem is in X-ray quiescence it is unchanging at all wave-
only infrared data were taken. The source was only ob-
lengths — thatis, that the only variationsin the system
servedinV,I,J,&K during2001,andwasnotobserved
are due to orbital changes in viewing angle.
atallin2006. DuringlateJune2010,thesourcewasob-
Studies have shown that this is not always true (see
served for several hours at a time on three consecutive
Cantrell et al. 2008, 2010, and references therein), and
nights.
that many systems display variability which cannot be
Some images were rejected based on visual inspec-
explained by the ellipsoidal variations alone. In particu-
tion. For the rest of the data, three field stars were
lar,Cantrell et al.(2008)showedthatA0620-00actually
used to check for bad nights: a night was rejected if
hasthreedistinctopticalstatesinwhichitcanexist(des-
the three stars’ raw magnitudes were, on average, more
ignated “active”,“passive”,and “loop”), all while being
than1.5standarddeviationsfainterthantheirmeanval-
quiescent in X-rays. The optical passive state is when
ues. A few observations were also removed from this
variations are solely due to orbital changes, while the
dataset upon discovering that the source had gone into
active and loop states describe periods when there is a
X-ray outburst (for example, 2003 August 5-8, as re-
heightened accretion flow producing brighter and more
ported in Buxton et al. (2003), Bailyn et al. (2003), and
variable emission.
Rupen et al. (2003)). All photometric data presented
The next logical issue is to determine whether A0620-
here were obtained while the source was quiescent in X-
00 is unique in having multiple optical states in quies-
rays.
cence, or whether this is commonplace among X-ray bi-
IndividualobservationsofV4641Sgrweretakeninthe
naries. Herewebegintoanswerthisquestionbyexamin-
followingmanner. Fortheopticalbands,threeexposures
ing one of the optically-brightestquiescent X-ray binary
in each band were taken during each observation. In-
systems, V4641 Sgr. We find, by examining approxi-
frared data were taken in sets of five dithered exposures
mately ten years of photometric data, that this source
in each band. In June, 2010, when we observed several
also exhibits passive and active states during X-ray qui-
hours in a row on three nights, data were again taken in
escence. The passive state is dominated by the bright
sets ofthree exposuresfor the opticalandfive exposures
companion star, with little contribution from the accre-
for the infrared.
tiondisk,whiletheactivestateshowsmoreemissionand
All data reduction and photometry was done using
more variability, presumably due to an increase in the
standard IRAF4 tasks. For optical data, each expo-
accretion flow in the system. In §2 we describe our ob-
sure was reduced and had photometry done separately.
servations,in§3wedefinethedifferentopticalstatesand
Photometric data from each set of three exposures in
discusstheirpropertiesinV4641Sgr,in§4wedetermine
a band were then averaged to produce one data point
the inclination angle of the system and the component
per band per night/observation. For infrared data, each
massesby modeling the systemusing only optically pas-
setoffiveditheredexposureswassky-subtracted,shifted
sivedata,andin§5wediscussourresultsandoffersome
to a common coordinate system, and then summed to
conclusions.
produce one exposure for each band each night. Pho-
tometrywasdoneonthis combinedexposuretoproduce
2. OBSERVATIONS one data point per band per night/observation. In June
2.1. Photometry 2010 when multiple observations were taken throughout
several nights, both optical and infrared exposures were
We have optical and near-IR photometry from the
obtainedandprocessedinthe samemannerasdescribed
ANDICAM instrument on the 1.0m & 1.3m telescopes
above. Altogether this gives 565 points in B, 704 in V,
operated by the SMARTS consortium, located at the
671 in I, 695 in J, 591 in H, and 648 in K.
CerroTololoInter-AmericanObservatory(Buxton et al.
Optical differential photometry was calibrated using
2012). Theinstrumentwaslocatedonthe1.0mtelescope
observations of standard stars (Landolt 1992) taken on
priorto2003,andwasthenmovedtothe1.3mtelescope,
the same instrument by the SMARTS consortium. For
where it is still operating to the present day. Observa-
270 photometric observations taken between 2003 and
tions were taken every night or two from 2001 to 2010,
2005, calibrated magnitudes for V4641 Sgr in each fil-
usually from March until October, although this varied
from season to season. Details of the observation dates
4 IRAF is distributed by the National Optical Astronomy Ob-
can be seen in Table 1. Most of the time observations
servatory, whichisoperated bytheAssociationofUniversitiesfor
were in six bands (B, V, I, J, H, & K), although oc- ResearchinAstronomy(AURA)undercooperativeagreementwith
casionally due to instrumental problems only optical or theNationalScienceFoundation.
V4641 Sgr: Activity in Quiescence & Improved Masses 3
ter were calculated using photometric zero points, color themwithamodelspectrumforastarwithT =10,500
eff
correctionterms,andextinctioncorrectiontermsderived K and logg = 3.5 (Orosz et al. 2001) using the IRAF
from these standard star observations. Using these 270 task fxcor. These Doppler-shifted spectra were then
observations,anaveragecalibrationfactorwascalculated summedto produce one composite spectrum(see Figure
forthedifferentialphotometryineachfilter,andthisfac- 3), which was used to examine the Balmer absorption
tor was then applied to all observations of V4641 Sgr to lines (see §3.2.2).
convertthemtomagnitudesinthestandardsystem. The Echelle spectra of V4641 Sgr were obtained in ser-
infrareddifferentialphotometry was calibratedby deriv- vice mode using the UVES spectrograph on the second
inganaveragecalibrationfactorfromtheknown2MASS 8.2m VLT telescope at Paranal Observatory (European
magnitudes of six of the IR comparison stars. Finding Southern Observatory), on 2001 June 18, 19, and 22 as
chartsshowingV4641Sgrandthecomparisonstarsused part of the ESO program 67.D-0384(A). The slit width
for differential photometry and calibration are shown in was set to 1.2 arcseconds, yielding a resolving power of
Figure 1. Coordinates and magnitudes of comparison R ≈ 33,000. UVES is a double-armed instrument, and
stars are shown in Table 2. we obtained four 2460-secondexposures in the standard
Differential photometric errors were calculated from “DIC1346+580”settingandfour2460-secondexposures
field stars by plotting the standard deviations of their in the standard “DIC2 437+860”setting. By combining
differential photometry (calculated over ∼ 300 obser- thesetwosettings,weobtainedessentiallycompletespec-
vations) against their calibrated magnitudes. We fit a tralcoveragefrom3000˚A to 10,000˚A. IRAF tasks were
fourth-order polynomial to these data, found the mini- used to process these CCD frames to remove the elec-
mum of the curve, and extended that constant numeri- tronicbias,applytheflat-fieldcalibration,andtoextract
cal value out to brighter magnitudes as the error on the andwavelengthcalibratethespectra. Thesignal-to-noise
differentialphotometry. Typicaldifferentialphotometric ratiointhecombinedDoppler-correctedUVESspectrum
errorsforthebrightermagnitudeswere: 0.032maginB, was 100 or more per pixel over most of the wavelength
0.020 mag in V, 0.020 mag in I, 0.036 mag in J, 0.034 range.
mag in H, and 0.033 mag in K. See Figure 2 for the
3. ACTIVE & PASSIVE OPTICAL STATES
plotsofstandarddeviationagainstcalibratedmagnitude
and for the errors on fainter magnitudes. Cantrell et al. (2008) defined active and passive states
Calibration errors were calculated by taking the stan- forA0620-00inthefollowingmanner: inthepassivestate
darderrorofthemeanofthecomparisonstars’calibrated all observed variability is consistent with orbital vari-
magnitudes over the number of photometric nights used ations and photometric errors; in the active state, the
for the calibration. These calculated calibration errors source is typically both brighter and more intrinsically
averaged0.006mag for the opticalbands and 0.025mag variablethan the passivestate for an extendedperiod of
for the infrared. There are undoubtedly systematic er- time. If the companion star in a system is bright, the
rors involved in the photometric calibration, concerning passive state is dominated by the orbital changes of the
theexactmatchbetweenourfiltersandthoseofthestan- companionstaralone. Iftheaccretiondiskcontributesa
dardsystem. Wetookthesesystematicerrorstobeafew significantfractionoflight,thepassivestateisdominated
hundredthsofamagnitude,givingfinalcalibrationerrors by the orbital changes of the combined disk/companion
of ≈ 0.02 mag for the optical bands and ≈ 0.04 mag for star system. These states persist on timescales of weeks
the infrared. Total photometric errors on the calibrated or months.
magnitudes in each band are therefore: 0.04 mag for B, In order to determine whether V4641 Sgr has active
0.03 mag for V and I, and 0.05 mag for J, H, and K. and passive periods similar to those of A0620-00, the
light curve was first folded on the orbital period, which
was taken from Orosz et al. (2001) (see Figure 4). This
2.2. Spectroscopy
revealed a clear “lower envelope” to the data, which re-
We obtaineda totaloftwelveopticalspectra ofV4641 sembledpure ellipsoidalvariations. This ellipsoidalvari-
SgrfromRCSPEConthe1.5mSMARTStelescope. RC- ation (or “passive envelope”) needed to be removed be-
SPEC is a single-slit grating spectrograph, and gratings fore further analysis of other non-ellipsoidal variability
used for these data covered approximately 3700 ˚A to could take place.
5400 ˚A. Data were taken in 2009 and 2010, over six Although we used the orbital period given in
nights (see Table 3 for details). Spectra were taken in Orosz et al. (2001), we used a value for T slightly dif-
0
setsofthreeexposures,attwodifferenttimesduringeach ferent from theirs. Specifically, when we folded our pho-
night. Each exposure was reduced individually, and the tometric data on the Orosz et al. (2001) orbital period
spectrum was extracted and wavelength-calibrated us- and T , we found that both maxima and both minima
0
ing standard IRAF tasks. Following extraction, each set occurredearly by 0.03 in phase. We therefore adopted a
of three spectra was summed in order to increase the value of T =2452423.647±0.01 as our photometric T ,
0 0
signal-to-noise. These summed spectra were then nor- in order to fix this slight inconsistency.
malized to their continua. In 2010 spectra were also
3.1. Definitions of States
taken for three comparison stars, with spectral types
(B8III,B9III,andA0III)bracketingthatwhichhasbeen The passive envelope was computed for each filter as
proposed for V4641 Sgr (B9III). The data reduction, follows: magnitude data were binned in phase, with 20
spectrum extraction, wavelength calibration, and nor- equal-width bins total over the range of phase from zero
malization were done in exactly the same manner as for to one. In each bin, the mean and standard deviation
V4641 Sgr. All individual normalized spectra of V4641 of the magnitude points were calculated, all points out-
Sgrwerealsoshiftedtozerovelocitybycross-correlating side of one standard deviation were discarded, and the
4
Figure 1. Finding charts showing field near V4641 Sgr in both optical and infrared bands. Left panel: Optical (B band, here) finding
chart,showingV4641Sgrandcomparisonstarsusedforopticaldifferentialphotometry. Boxoutlinedwithdashedlineshowsapproximate
size and location of the IR field (shown in the other panel). Right panel: Infrared (K band, here) finding chart, showing V4641 Sgr and
comparisonstarsusedforIRdifferentialphotometry.
mean of the remaining points was calculated. This sec-
ond, more restrictive, mean was defined to be the loca-
8
1.
HIP 88270 (B9III)
6
1.
B
4
10 1.
0.
a
0.00 V d Spectr 1.2
e
0.10 maliz 1
or V4641 Sgr
0.00 I N 0.8
n
o
viati 0.10 0.6
e
d D 0.00 J 0.4
r
a
nd 10
Sta 0. 3700 3900 4100 4300 4500 4700 4900 5100 5300
0
0.0 H Wavelength (Angstroms)
0 Figure 3. Normalizedspectra taken onthe SMARTS1.5mtele-
1
0. scope. Shown are V4641 Sgr and a comparison B9III star, HIP
88270(offsetby+0.8units).
0
0
0. K
tionofthepassiveenvelopeinthatbin. Splineinterpola-
0
0.1 tionconnectedthesemeanstogethertoproduceasmooth
curve. We note that this method works here because of
0
0.0 the large number of passivedata points comparedto ac-
19 18 17 16 15 14 13 12 11 10
tive; in a source with more active points than passive,
Calibrated Magnitude
thismethodwouldfail. Theresultsfromthe V bandare
shown in Figure 4; the passive envelope is shown as a
Figure 2. Standard deviation of differential magnitudes plotted
against calibrated magnitudes, for field stars in each band. A thick curve overlying the data. This smooth curve was
fourth-order polynomial was fit to the distribution, and the mini- subtractedfromallthe data,thus removingthe periodic
mumofthatpolynomialwasextendedouttobrightermagnitudes orbital signature from each light curve. It is important
inordertodeterminethetypicalerrorforthedifferentialphotome-
tonotethatthis “passiveenvelope”curveisnotamodel
try. Horizontallinesindicatetheseminimumvaluesforeachband.
—itdoes notmakeanyassumptions aboutanyphysical
V4641 Sgr: Activity in Quiescence & Improved Masses 5
Table 2
Magnitudes ofcomparisonstars
OpticalComparisonStars
Star RA Dec B V I
(J2000) (J2000) (mag) (mag) (mag)
1 18:19:21.0 -25:24:10.0a 17.15±0.16 16.24±0.17 15.26±0.16
2 18:19:20.8 -25:24:07.7b 17.72±0.20 16.05±0.17 14.36±0.14
3 18:19:20.0 -25:24:18.3b 16.86±0.14 15.82±0.19 14.71±0.15
9 18:19:27.2 -25:25:08.4a 16.47±0.16 14.50±0.17 11.53±0.17
10 18:19:26.2 -25:24:59.3a 15.81±0.13 14.42±0.16 13.01±0.13
11 18:19:22.0 -25:24:52.9a 17.54±0.21 16.54±0.23 15.49±0.20
12 18:19:25.1 -25:24:37.4a 14.62±0.12 13.41±0.15 12.17±0.13
13 18:19:25.9 -25:24:18.2a 17.14±0.15 15.80±0.16 14.40±0.14
14 18:19:24.9 -25:24:15.2a 17.33±0.17 15.96±0.21 14.52±0.15
15 18:19:22.6 -25:24:07.5a 15.53±0.14 13.56±0.16 11.11±0.15
16 18:19:23.5 -25:23:47.2b 17.38±0.21 16.15±0.21 14.65±0.38
17 18:19:24.1 -25:23:33.3b 17.03±0.19 15.39±0.19 13.68±0.16
18 18:19:22.9 -25:23:40.5b 17.66±0.19 16.04±0.18 14.44±0.15
19 18:19:23.1 -25:23:34.6b 16.50±0.14 14.95±0.17 13.34±0.14
20 18:19:22.6 -25:23:38.0b 17.01±0.16 15.56±0.16 14.14±0.14
21 18:19:17.5 -25:23:25.0a 17.11±0.17 15.25±0.18 13.29±0.15
22 18:19:16.9 -25:23:19.3a 16.11±0.14 14.84±0.17 13.56±0.14
23 18:19:16.0 -25:23:27.4a 16.29±0.14 15.28±0.17 14.26±0.14
24 18:19:18.2 -25:24:00.0a 17.56±0.17 16.59±0.18 15.53±0.17
25 18:19:16.7 -25:24:02.2a 15.29±0.13 14.54±0.16 13.70±0.14
26 18:19:19.2 -25:24:13.6b 17.24±0.16 15.86±0.20 14.35±0.15
27 18:19:18.3 -25:24:15.9a 14.52±0.12 14.07±0.15 13.64±0.13
28 18:19:17.2 -25:24:38.1a 15.54±0.14 13.86±0.16 11.94±0.20
InfraredComparisonStars
Star RA Dec J H K
(J2000) (J2000) (mag) (mag) (mag)
1 18:19:21.0 -25:24:10.0a 14.73±0.33 14.34±0.29 14.26±0.22
2 18:19:20.8 -25:24:07.7b 13.08±0.24 12.33±0.19 12.16±0.20
3 18:19:20.0 -25:24:18.3b 13.97±0.27 13.50±0.22 13.37±0.17
4 18:19:20.9 -25:24:16.7b 14.56±0.27 13.88±0.21 13.75±0.19
5 18:19:21.3 -25:24:23.3b 13.97±0.28 13.28±0.22 13.14±0.28
6 18:19:23.2 -25:24:30.8b 13.85±0.25 13.44±0.21 13.36±0.25
7 18:19:21.7 -25:24:35.7b 14.39±0.27 13.61±0.21 13.43±0.24
8 18:19:21.3 -25:24:53.0b 14.07±0.25 13.41±0.21 13.26±0.23
Note. — Star numbers are in accordance with the finding charts in Figure
1. Our infrared magnitudes are consistent with the 2MASS catalog within our
photometricerrors.
a RA&DecarefromtheUSNO-B1.0Catalog.
b RA&Decarefromthe2MASSPointSourceCatalog.
characteristics of the system, it is an empirical descrip- showedmuchmoreintrinsic scatteratalltimes, andwas
tion only. judgednotto be areliable measureofactivity by itself).
We examined the entire data set after the passive en- Final results of this analysis, showing when V4641 Sgr
velope was subtracted (as an example, see the V band was in each state, are shown in Table 4.
andJ banddata in Figure 5). These lightcurvesclearly Light curves in all six bands, folded on orbital period,
showedthatmostofthedataineachbandfellinafairly are shown in Figure 6. There are significant differences
narrow range clustered around zero. The different opti- betweenthe bands. Inparticular,the difference between
calstatesweredefinedbyfirstdrawingthresholdlineson the primary and secondary minima is largest in B and
the plots. The threshold for the optical bands was 0.04 smallest in K. Overall scatter among data is largest in
mag, while the threshold for the infrared bands was 0.1 K andsmallestinB. Thedifferencebetweenthepassive
mag. Periods of time when most data were below the data and the brightest active data is largest in K and
threshold were classified as passive for that band, while smallest in B.
those times when most data were above the threshold, The optical passive state for V4641 Sgr is remarkably
and where the scatter between points was larger, were stable. The folded light curve does not change shape
classifiedasactiveforthatband. Toavoidspuriousclas- over time, unlike A0620-00, which changes shape over
sifications, we required that data be classified as active timescalesofmonthsoryears(Cantrell et al.2010). This
intwoormorebandsinordertodefine aparticulartime contrastingbehaviorislikelyduetothedifferencesinthe
period as active. When available, data from all optical companion stars in these two systems. V4641 Sgr has a
bands (B, V, & I) wereexaminedin orderto find active B subgiantas its companion,whichis largeandbrighter
periods. WhenonlyIRdataweretaken,J &H bandsto- than the accretion disk. It therefore takes large changes
getherwereusedtofindactiveperiods(theK banddata inbehavioroftheaccretiondisktochangethetotalemis-
6
Table 3
Datesofspectralobservations, forV4641Sgrandspectralcomparisonstars
Date Objects
2009Aug13 V4641Sgr
2010Jun27 V4641Sgr,HIP88270(B9III), HIP93132(B8III), HIP105779(A0III)
2010Jun28 V4641Sgr,HIP88270, HIP93132,HIP105779
2010Jun29 V4641Sgr,HIP88270, HIP93132,HIP105779
2010Jun30 V4641Sgr,HIP88270, HIP93132,HIP105779
2010Jul2 V4641Sgr,HIP88270, HIP93132,HIP105779
Note. —AllspectraweretakenwhenV4641Sgrwasinthepassiveopticalstate.
sion from the system. A0620-00,on the other hand, has
a K dwarf as its companion, which is small and cooler Table 4
Datesofpassive&activestates,
than the accretion disk. Changes in that accretion disk
2001–2010
can more easily dominate the emission from the system.
Based on the data we examined, during X-ray quies- Dates State
cence V4641 Sgr spends approximately 85% of the time
2001Mar27–2001Oct12 Passive
in the optical passive state and 15% of the time in the
2001Oct13–2002Feb17 ···
optical active state. It should be noted, however, that 2002Feb18–2002Jun6 Passive
we do not have complete time coverage over the span of 2002Jun7–2002Jun26 ···
our data (2001-2010), and additional active and passive 2002Jun27–2002Jul9 Active
2002Jul10–2002Sep25 Passive
states could have been missed.
2002Sep26–2003Apr7 ···
We constructed color-magnitude diagrams in order to 2003Apr8–2003Jul28 Passive
determine if there was an intermediate “loop” state in 2003Aug1–2003Aug2 Active
2003Aug13–2003Nov14 Passive
V4641 Sgr, as was found in A0620-00 by Cantrell et al.
2003Nov15–2004Mar16 ···
(2008). ThisloopstateinA0620-00wascharacterizedby 2004Mar17–2004Jun19 Passive
being intermediate between passive and active in both 2004Jun20–2004Jul19 Active
luminosity and scatter, and by the presence of clockwise 2004Jul24–2004Oct31 Passive
2004Nov1–2005Feb11 ···
loop patterns traced out by successive data points in a
2005Feb12–2005Nov6 Passive
color-magnitude diagram. For V4641 Sgr, the relatively 2005Nov7–2007May4 ···
long active periods in 2007 and 2010 were examined for 2007May5–2007May15 Passive
2007May17–2007Jul25 Active
2007Aug4–2007Sep30 Passive
2007Oct1–2008Feb28 ···
3
3. 2008Feb29–2008Sep15 Passive
1 2008Sep24–2008Nov8 Active
2008Nov10–2009Feb21 ···
3.4 + + 2009Feb22–2009Mar10 Active
1 2009Mar21–2009Sep5 Passive
+ + + +
V Magnitude 13.813.713.613.5 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ tbdsheueeitrsmwintegyteopneepistoouhsowrdfceesfacerbp2e2Ntt2err0has0ee0saeo1s.1eh0sscc00t9iesaaltvetnavFiJnSa.estveuitesoebe—eilpepfisrpa1o1ep.6oa4sd4Sincein–.Nehl-rd–ytesi22Losod200siwie0de1ndi1ctn1ch00.eaeot0esin.hFOanlJsTmtee3cuiwbacrtfihalbeoferu1ao31onkl3r1dbo3suetodrtldeV-pehnm“fiao4“oPAnwtat6la·itoc·gt4s·hdetoi·sn·1oaria·ipvnvnviteta””eeSssugsdwrteaedrtdeeoi.aefsgorunanomdt
9 ++ ++
3. + +
1
+ + 3.2. The Passive State
0.0 0.5 1.0 1.5 2.0 3.2.1. Photometric Study
Phase We compared the passive colors of V4641 Sgr to the
colorsofalateBstar. Forthecalculationsinthissection,
Figure 4. V band V4641 Sgr light curve, folded on orbital pe- we used the median magnitude of the passive data near
riod. The entire SMARTSphotometric dataset for the V band is
phase0.0(secondaryminimum)ineachband. Secondary
includedhere. Afterdetermination ofactiveandpassivestates as
describedinthetext,thelightcurveisplottedheretoshowpassive minimum was chosen as that is the phase of the orbit
data (black plus symbols), active data (filled circles, green in the when the star is between the observer and the accretion
onlineversion),andthepassiveenvelope(thick curve,magentain disk. We are therefore looking directly at the back side
theonlineversion)whichwassubtractedfromthedatainorderto
ofthe companionstar,whereit looksmostlikea normal
analyzetheaperiodicvariabilitypresentinthesystem. Pointsare
plottedthroughtwoorbitalperiodsforclarity. Atypicalerrorbar spherical star.
forthedataisshowninthelowerrightcorner. To compare our source to a B star, we examined the
V4641 Sgr: Activity in Quiescence & Improved Masses 7
2 2
0. 2001 0. 2001
− −
02 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ +++++++++++++ 02 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ +++++
0. 2002 0. 2002
− −
0 +++++++++++++++++++++++++++++++++++++++++++++++++++++ +++++ 0 ++++++++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++++++++++++++++++++++++++++
0.2 2003 0.2 2003 +
− −
0 ++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 0 ++++++++++++++++ +++++ ++++ ++ ++++++++++++++++++++++++++ +++++++++++++++++++++
+
2 2
0. 2004 0. 2004
Light0− ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ +++++++++++++++++++++++++++++++++++++++++ Light0− ++++++++++++++++++++++++++++++++++++++++++++++ +++++++++++++++++++++++++++++++++++++++++++++++++++++
VbandExcess0−0.2−0.2 22000057 +++++++++++++++++++ JbandExcess0−0.2−0.2 22000057 +++++++ + ++ + +++++++++++++++++++++++++++++
0 +++++++ +++++++++++++++++++++++++++++++++++ 0 ++++ ++++++++++++++++++++++++++++++++
2 2
0. 2008 0. 2008
0− + +++++++++++++++++++++++++++ ++ ++++++++++++++++++++++++++ ++++++++ 0− + ++++++++++++++++++++++++++++ + ++++++++++++++++++++++++ ++++++++
2 2
0. 2009 0. 2009
− − +
02 +++++++++++++++++++++++++++++ ++++++++++++++++++ 02 +++++++++++++++++++++ ++++ ++++++++++++++++
0. 2010 0. 2010
− −
0 ++++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++++++++++ 0 +++++++++++++++++++++++++++++++++++++++++++ +++++++++++++++++++++
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Figure 5. ExamplelightcurvesforV4641Sgr,aftersubtraction ofthepassiveenvelope. Leftpanel showstheV bandlightcurve; right
panel shows the J band light curve. Notice that most of the data fallwithin anarrow range near zero inboth panels (±∼0.04mag, for
V band; ± ∼0.1 mag, for J band). Periods of time when most of the data were above this narrow range and when the overall scatter
washigher(inatleast2ofthe6bands),weredefinedtobe“active”. Passivetimeperiodsaremarkedbyblackplussymbols,activetime
periodsaremarkedbyfilledcircles(greenintheonlineversion). Typicalerrorbarsforthedataareshowninthelowerrightcornerofeach
panel.
spectral energy distributions (SEDs) of V4641 Sgr and (2003) use ≈ 0.25, and Lindstrøm et al. (2005) find
of a generic reddened B9III star. Using the colors of an E(B−V) could be as high as 0.4−0.6. Given the rela-
unreddened B9III star from Wegner (1994) as a starting tivelylargeerrors,thesevaluesareallbroadlyconsistent
point, we reddened the SED of a generic comparison B with that from the BEAM calculator.
star, and normalized it by matching its reddened color Wecalculatedextinctioncoefficientsintheotherbands
to the median passive B−V color of V4641 Sgr. Figure usingA /A =a(x)+b(x)/R andtherelationsfora(x)
λ V V
7 shows the SEDs of both V4641 Sgr and the reddened andb(x) giveninO’Donnell(1994) forthe opticalbands
B9IIIstar,withseveralpossiblevaluesforthereddening. and in Cardelli et al. (1989) for the infrared bands. The
ItisclearthatinthepassivestateV4641Sgrisconsistent errors on the dereddened absolute magnitudes of V4641
with a reddened B star for a range of E(B−V) values Sgr are dominated by the error in E(B−V), and come
from 0.32 to 0.38. to 0.79 mag in B, 0.60 mag in V, 0.36 mag in I, 0.18
Recently Gonzalez et al. (2011) published reddening mag in J, 0.12 mag in H, and 0.08 mag in K. The final
and metallicity maps of the Galactic Bulge regionwhich dereddened SED for V4641 Sgr is shown in Figure 8,
havebothhigherresolutionandlargercoverageareathan and highlights the differences between the median total
previous studies. They have provided an online calcula- (passive & active together) SED, the passive-only SED
tor, the Bulge Extinction And Metallicity (BEAM) Cal- and the active-only SED.
culator5, that allows one to retrieve the reddening value
foranypartoftheirmappedregion. Thisonlinecalcula- 3.2.2. Spectroscopic study
torgivesavalueofE(J−K)=0.1927±0.1005forV4641
We analyzed the SMARTS optical spectra of V4641
Sgr, which converts to E(B−V)=0.37±0.19 using the
Sgr to determine the fraction of light coming from the
relationsofCardelli et al.(1989). Thisisconsistentwith
accretion disk in this system. All of our spectra were
the values we found by reddening a generic B star, and
taken when V4641 Sgr was in the passive optical state.
we adopt this value for the reddening along the line of
Becausethecompanionstarinthissystemisverybright,
sight to V4641 Sgr.
we expected the spectroscopic data to be dominated by
Thereddeningalongthelineofsighttothissourcehas
the companion star but to possibly contain a small, es-
previously been quoted differently in several sources in
sentiallyconstant,amountofdisklight. Thisisquitedif-
the literature: Orosz et al. (2001) use E(B−V) = 0.32
ferent from other X-ray binary systems, which can have
±0.1,in’t Zand et al.(2000)use0.24±0.1,Chaty et al.
significant contributions (30−60%) from the accretion
disk at all times (Cantrell et al. 2010; Filippenko et al.
5 http://mill.astro.puc.cl/BEAM/calculator.php 1995).
8
4
13. B J 2.4
1
6
3. 6
1 2.
1
14.214.013.8 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 3.213.012.8
+ + + + 1
+ +
4
14. V H 2.2
1
2
3.
1 4
2.
e + + 1
Magnitud 13.813.613.4 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 3.012.812.6
+ + 1
0
4.
1 2
2.6 I K 12.
1
4
2.8 12.
3.413.213.01 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 13.012.812.6
1
0.0 0.5 1.0 1.5 2.00.0 0.5 1.0 1.5 2.0
Phase
Figure 6. V4641 Sgr light curves in all six optical and infrared filters, folded on the orbital period. The entire SMARTS photometric
dataset is included here. The filter is indicated in upper left corner of each panel. Note that vertical axis is different for each panel,
althoughtherangeinmagnitudesisthesameforall(eachverticalaxisspans1.0mag). Panelsareshownsothatthesecondaryminimum
islocatedatapproximatelythesameverticalpositionineachplot. Afterdeterminationofactive&passivetimeperiods,differentsymbols
were chosen to markactive and passive states: plus symbols are passive and filled circles (green inthe online version) are active. Points
areplottedthroughtwoorbitalperiodsforclarity. Typicalerrorbarsareshowninthelowerleftcornerofeachpanel.
We calculated the fraction of the light due to the ac- lines in the comparison B star, therefore, allowed us to
cretion disk in V4641 Sgr by analyzing the equivalent determine whether there was any emission from the ac-
widthsoftheBalmerabsorptionlines. Bstarsareknown cretion disk contaminating these spectral features.
tohavedeepBalmerabsorptionlines,asoursourcedoes. After examining the spectra of both V4641 Sgr and
Accretion disks have continuum emission and also often our comparison B9III star HIP 88270 (see Figure 3), we
double-peaked emission lines, either of which can dilute found that our spectra allowed the possibility of a small
the absorption features from the companion star. Mea- contribution from the disk (up to 15% at a 2σ level),
suringthe equivalentwidthofthe BalmerlinesinV4641 but were basically consistent with little to no emission
Sgr and comparing to the equivalent width of the same from the accretion disk. This is plausible during the
V4641 Sgr: Activity in Quiescence & Improved Masses 9
are consistent with those of a solitary late B star.
B V I J H K
3.3. The Active State
2.8 OnceopticalstatesforV4641Sgrweredefinedandthe
1 passive envelope was subtracted from each data point,
0 weexaminedtheresultingresiduals,hereafterdesignated
13. “excess light”. Plotting the excess light folded on the
orbital period (see Figure 9) revealed that the active
2
3. state excess light was not consistent throughout the or-
1
de bital period. Rather, the active light was brighter near
nitu 3.4 phase 0.5 (primary minimum), and fainter near phase
g 1
Ma 0.0 (secondary minimum). This increase in brightness
6 E(B−V) values at primary minimum is likely related to changes in the
3.
1 0.31 viewingangleofthesystemthroughoutthebinaryorbit.
0.32
Primaryminimum is the point in the orbit when the ac-
8 0.33
13. 0.34 cretion disk is between the observer and the companion
0.35
0.36 star. At this phase, we could be seeing more emission
4.0 0.37 from processes originating in the accretion disk, such as
1 0.38
0.39 irradiation of the face of the companion or an accretion
diskwind. Giventhatthesystemisatamoderatelyhigh
4380 7980 12500 16500 22000 inclinationangle(see§4.4),thesetypesofemissionmight
Wavelength (Angstroms) be partially blockedduring the rest of the orbitalperiod
by the rather large companion star.
Figure 7. V4641 Sgr spectral energy distribution (SED), com- The small cluster of passive points at phase 0.5 which
pared to reddening a B9III star, with several different possible arefainterthanallotherdataineachbandarepuzzling.
reddening values. The solidblack line is the V4641 Sgr SED; the
They are too faint to be explained by ellipsoidal vari-
dotted and dashed lines (shown in different colors in the online
version)arethereddenedBstarSEDs. ThereddenedBstarSED ations — attempting to fit those faintest points while
wasnormalizedbymatchingtheB−VcolorofV4641Sgr. calculating the inclination angle of the system leads to
angles >90◦. They could be explained by an infrequent
passive state, given the large intrinsic luminosity of the slighteclipseofthecompanionstarbytherelativelydark
companionstarinthissystemandthefactthatthecolors accretion disk, however we do not have enough of these
very faint data to draw firm conclusions.
We investigated emission from the accretion flow in
B V I J H K
V4641 Sgr by examining the colors of the excess light
2 during active states. We selected B−H, to explore the
2. Median
1 Passive largest separation between blue and red possible with
Active these data. Although K band is obviously redder than
2.3 H, K band data displayed much more intrinsic scatter
1
at all times, making it more difficult to draw firm con-
clusions using that band. Figure 10 shows the results
4
12. of this investigation. In the left panel we plot excess
e light in the H band against excess light in the B band,
d
nitu 2.5 while in the right panel we plot B magnitude of the ex-
ag 1 cess light againstB−H color. The active data represent
M
heightenedemissionfromtheaccretionflowinthesystem
2.6 (which, although hotter than the passive accretion flow,
1
isstillcoolerthanthecompanionstar),whilethepassive
data represent emission dominated by the blue compan-
7
2. ion star. Therefore the excess light emission is redder
1
during an active state than it is during a passive state.
8 This contrasts with A0620-00, where the active light is
2.
1 bluerthanitscompanion,aKstar(Cantrell et al.2008).
4380 7980 12500 16500 22000 This raises the possibility that the accretion flow in the
twosystems existsatsimilartemperaturesandproduces
Wavelength (Angstroms)
similaremission. Differences incolorbetweenV4641Sgr
Figure 8. DereddenedspectralenergydistributionsofV4641Sgr, and A0620-00 could simply be attributed to their very
showing the differences between the optical states. The solid line different companion stars.
labeled“Median”isthemedianofalldata(passiveandactiveto- In order to explore potential secular changes in the
gether). The dashed line (blue in the online version) is passive
accretion disk of V4641 Sgr, we examined the B−H
data only,and thedotted line(redinthe onlineversion)isactive
data only. Note that the active state is always brighter than the coloroftheexcesslightovertime. TheB−H colorofthe
passive, and that this difference increases toward the red. Dered- excess light, plotted against orbital phase (see left panel
deningerrorsaredominated bytheerrorinE(B−V);finalerrors ofFigure11),doesnotseemtoshowobvioustrendsover
on these dereddened magnitudes are 0.79 mag in B, 0.60 mag in
thecourseoftheorbit. Thissuggeststhatchangesinthe
V,0.36maginI,0.18maginJ,0.12maginH,and0.08magin
K. accretiondisk during anorbitaffectthe opticalemission
10
5 5
0. B J 0.
− −
4 4
0. 0.
− −
3 3
0. 0.
− −
+ +
2 + + 2
0. 0.
− −
10.0−0.1 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 10.0−0.1
0. + + + + + + 0.
+ +
5 5
0. V H 0.
− −
4 4
0. 0.
− −
t
h 3 3
g 0. 0.
− −
i
L
2 + + 2
s −0. + + + + −0.
Exces 0.10.0−0.1 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 0.10.0−0.1
5 5
0. I K 0.
− −
4 4
0. 0.
− −
3 3
0. 0.
− −
+ +
0.2 + + + + 0.2
− + + −
0.10.0−0.1 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 0.10.0−0.1
+ +
0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0
Phase
Figure 9. V4641 Sgr light curves as in Figure 6, except that here the passive envelope has been subtracted in each filter. The entire
SMARTS photometric dataset is included here. The filter is indicated in the upper left corner of each panel. Note that the vertical axis
here is the same for each panel. Plus symbols are passive data and filled circles (green in the online version) are active data. Points are
plotted through two orbital periods for clarity. Typical error bars are shown in the upper right corner of each panel. Notice the change
in activity throughout the orbital period — activity is highest near phase 0.5 (when the accretion disk is between the observer and the
companion star), andlowestnear phase 0.0(when the companion starisbetween the observer andthe accretion disk). Thesmallcluster
of passivepoints at phase0.5which arefainter than allother data ineach band mayindicate infrequent slighteclipses ofthe companion
starbytheaccretiondisk. Seetextformorediscussionofthesefeaturesofthedata.