The near-infrared counterpart of 4U 1636-53

0.11 MB·

by  D. M. Russell

Astronomy&Astrophysicsmanuscriptno.jan06-accepted (cid:13)c ESO2012
January9,2012
⋆
The near-infrared counterpart of 4U 1636–53
(ResearchNote)
D.M.Russell1,2,K.O’Brien3,T.Mun˜oz-Darias2,4,P.Casella4,5,P.Gandhi6 andM.G.Revnivtsev7
1 AstronomicalInstitute‘AntonPannekoek’,UniversityofAmsterdam,P.O.Box94249,1090GEAmsterdam,theNetherlands
2 InstitutodeAstrof´ısicadeCanarias(IAC),V´ıaLa´cteas/n,LaLagunaE-38205,S/CdeTenerife,Spain
e-mail:[email protected]
3 DepartmentofPhysics,UniversityofCalifornia,SantaBarbara,California93106,USA
e-mail:[email protected]
2 4 SchoolofPhysicsandAstronomy,UniversityofSouthampton,Southampton,HampshireSO171BJ,UK
1 e-mail:[email protected]
0 5 INAF-OsservatorioAstronomicodiRoma,ViaFrascati33,I-00040MonteporzioCatone(Roma),Italy
2 e-mail:[email protected]
6 ISAS,JapanAerospaceExplorationAgency,3-1-1Yoshinodai,chuo-ku,Sagamihara,Kanagawa229-8510,Japan
n
e-mail:[email protected]
a
J 7 SpaceResearchInstitute,RussianAcademyofSciences,Profsoyuznaya84/32,117997Moscow,Russia
e-mail:[email protected]
6

Received????,2011;accepted????,2011
]
E
ABSTRACT
H
. Context.Theoptical counterpart of theneutron star X-raybinaryand wellknown X-rayburster, 4U 1636–53 (=4U 1636–536 =
h
V801Ara)hasbeenwellstudiedforthreedecades.Howevertodate,noinfraredstudieshavebeenreported.
p
Aims.Ouraimsaretoidentifyandinvestigatethenear-infrared(NIR)counterpartof4U1636–53.
-
o Methods.Wepresentdeep,KS-band(2.2µm)imagingoftheregionof4U1636–53takenwiththeInfraredSpectrometerAndArray
r Camera (ISAAC) on the Very Large Telescope. Archival optical and UV data are used to infer the 0.2−2.2µm spectral energy
t distribution(SED).
s
a Results.One star islocatedat coordinates α = 16:40:55.57, δ = −53:45:05.2 (J2000; 1σ positional uncertainty of ∼ 0.3 arcsec)
[ whichisconsistentwiththeknownopticalpositionof4U1636–53; itsmagnitudeisKS = 16.14±0.12.Thisstarisalsodetected
in the 2MASS survey in J-band and has a magnitude of J = 16.65±0.22. Under the assumption that the persistent emission is
2 largelyunvarying,the0.4−2.2µmde-reddenedSEDcanbedescribedbyapowerlaw;F ∝ ν1.5±0.3,withsomepossiblecurvature
ν
v (F ∝ ν.1.5)at0.2−0.4µm.TheSEDcanbeapproximatedbyablackbodyoftemperature∼ 27000K.Thisistypicalforanactive
ν
9 low-massX-raybinary,andtheemissioncanbeexplainedbytheouterregionsofa(likelyirradiated)accretiondisc.Wetherefore
3 interpretthisK -bandstarastheNIRcounterpart.
S
8
1 Keywords.Stars:neutron–X-rays:binaries–stars:infrared
.
9
0
1 1. Introduction radiated accretion discs which also dominate at optical wave-
1 lengths(e.g.,vanParadijs&McClintock1995).
Low mass X-ray binaries (LMXBs) are interacting binaries
:
v where a low mass donor is transferring material onto a neu-
Xi tron star or a black hole. In order to transport the excess of Even in the near-infrared(NIR), where the companion star
angular momentum outwards, an accretion disc is formed. In could have an important contribution, the disc emission seems
r
a the disc, the gravitational potential energy is transformed into dominantinbrightsystems.Forinstance,intheprototypicalper-
mainly X-ray radiation and kinetic energy, and temperatures sistent neutron star, Sco X–1, which given its relatively long
approach ∼ 108 K. The mass transfer rate supplied by the orbital period (∼ 19 h) should have a large, evolved compan-
donor star, M˙ , is driven by the binary/donor evolution and ionstar,nospectralfeaturefromthedonorhasbeendetectedin
2
mass transfer rates M˙ > M˙ ∼ 10−9M kg yr−1 (where M theNIR(Bandyopadhyayetal.1997).Inmostpersistentneutron
2 crit ⊙ ⊙
is the mass of the Sun in kg) result in persistently bright X- starX-raybinaries,spectralandtemporalstudieshavefavoured
ray sources (Kingetal. 1996). There are ∼ 200 such bright anX-rayheatedaccretiondiscastheoriginoftheNIRemission
(L ≃ 1036 −1038erg s−1) LMXBs in the Galaxy and most of (e.g., 4U 1705–440;Homanetal. 2009). Compact jets produc-
X
them harbour neutron stars as implied by the detection of pul- ingsynchrotronemissiontypicallydominatetheradioemission
sationsandX-raybursts resultingfromnuclearburningcaused (Migliari&Fender2006)andtheirspectracanextendtohigher
bytheaccumulationofHydrogenandHeliumontheirsurfaces. frequencies. In the NIR, high amplitude flares from GX 17+2
Theyshowenergyspectradominatedbyemissionfromtheirir- (Callananetal. 2002), a synchrotron spectrum in 4U 0614+09
(Migliarietal.2010)andvariablelinearpolarizationinScoX–
⋆ Based on observations collected at the European Southern 1 (Russell&Fender2008) have suggesteda strong infraredjet
Observatory,Chile,underESOProgrammeID085.D-0456(D). contributioninthesepersistentneutronstarX-raybinaries.
1
Russelletal.:TheNIRcounterpartof4U1636–53(RN)
pixelscaleis0.1478′′pixel−1.The24averageimageswerethen
aligned and stacked using IRAF to produce a deep image. The
totalon-sourceexposuretimeofthecombinedimageis3180s.
TheK -bandseeingasmeasuredfromthecombinedimagewas
S
0.6arcsecandconditionswereclear.Thecamerawasrotatedat
an angle of127◦ to maximise the numberof referencestars on
thearray.ThecombinedimageispresentedinFig.1andcanbe
usedasadeep,highresolutionK -bandfindingchart.
S
3. Sourceidentification
Within the combinedimage, six stars listed in the Two Micron
All Sky Survey (2MASS; Skrutskieetal. 2006) are detected,
which were used to calculate the world coordinate system and
achieve K -bandfluxcalibration.DAOPHOT in IRAF is used to
S
perform point-spread-function (PSF) photometry on the com-
binedimage.Allcomparisonstarsweresuccessfullyfittedbya
singlePSFconsistentwiththeimagePSF;noblendswereiden-
tified.Thereisonestarconsistentwiththebestopticalposition
of 4U 1636–53, as listed in the USNO-B1 digital sky survey
(α = 16:40:55.61,δ = −53:45:05.1;J2000,withanerrorcircle
Fig.1.VLT/ISAACdeep,highresolutionK -bandfindingchart of0.339′′),shownasawhitecircleinFig.1.Wederiveaposition
S
for 4U 1636–53.The optical position (USNO-B1 0.339′′ error of α = 16:40:55.57,δ = −53:45:05.2(J2000) for the infrared
circle) is indicated by a white circle and the infrared (ISAAC) counterpartof4U1636–53,witha1σpositionaluncertaintyof
position is shown by two black lines. The two positions agree ∼0.3arcsec.Otheropticalcoordinatesof4U1636–53inthelit-
within1σerrors.Thesix2MASSstarsareindicatedbydiagonal erature(Jerniganetal.1977;Liuetal.2001;Samusetal.2003)
lines. arealsoconsistentwiththeUSNO-B1andISAACpositions.No
preciseX-raycoordinatesof4U1636–53fromChandraorSwift
observationsexistintheliterature.
Oneoftheclassicalneutronstar systemsis4U1636–53(=
Somefaint,blendedstarslie just 1.5′′ from4U 1636–53in
4U 1636–536 = V801 Ara). It is an X-ray burster, which has
theISAACimage,butnoneareconsistentwiththeopticalposi-
beenextensivelystudiedinX-rayandopticalregimesformore
tion. Theabovesourceis the onlycandidatecounterpartin our
thanthreedecades(e.g.,Pedersenetal.1982).Ithasa3.8hor-
K -band image. All stars within a 3′′ radius of the LMXB are
bitalperiod(vanParadijsetal.1990;Gilesetal.2002)pointing S
≥1.2magnitudesfainterinK -bandthanthisproposedcounter-
to a relatively faint, late type companionstar. The spectrum of S
part.NofaintblendedstarsarevisibleaftersubtractingthePSF
thesystemfromX-raytoopticalwavelengthsseemstobedomi-
ofthecounterpart.
natedbytheemissionofitsbrightaccretiondisc,thecompanion
Fromtheknownmagnitudesofthe2MASSstarswithinthe
staronlybeingdetectedbyusingemissionlinesarisingfromre-
processing of the strong X-ray emission (L ∼ 1037−38erg s−1) field,wemeasurethemagnitudeof4U1636–53duringthetime
X of our observations to be K = 16.14 ± 0.12. The errors are
initsinnerhemisphere(Casaresetal.2006). S
dominatedbysystematicerrorsderivedfromthesix2MASSstar
Todate,nodetectionsof4U1636–53havebeenreportedat
magnitudes.4U1636–53isdetectedwithasignal-to-noiseratio
wavelengths longer than the optical regime. At radio frequen-
(S/N)of145.ThelimitingmagnitudeintheimageisK ∼19.3
cies, upper limits of both the persistent and burst fluxes were S
(i.e.starsbrighterthanthisaredetectedatthe3σlevel).
presentedinThomasetal.(1979).Herewepresentthefirstde-
The counterpartwe have found is not listed in the 2MASS
tectionofthenear-infraredcounterpartof4U1636–53.Together
point source catalogue. On inspection of the 2MASS images,
with available optical and UV data, we construct the intrinsic
the counterpartis notvisiblein K and H-bands,butappearsas
0.2–2.2µmspectralenergydistribution(SED).
afaintsourcein J-band.PerformingPSFphotometryonthe J-
band2MASSfield,weobtainamagnitudeof J = 16.65±0.22
2. VeryLargeTelescopeobservations for 4U 1636–53; the S/N is 7. The aforementioned faint stars
within∼ 2′′ ofthesourcemaycontributeupto∼ 10−30%of
The data were acquired with the Infrared Spectrometer And
thefluxmeasured,althoughthePSFfittingmethodshouldhave
ArrayCamera(ISAAC)ontheEuropeanSouthernObservatory
removed the majority of this excess flux (which is offset from
(ESO)8-mclassVeryLargeTelescopeUT3(Melipal)on2010-
thecentralPSFposition).This2MASSobservationwasmadeon
05-2102:23–03:17UT(MJD55337.118±0.019)underESO
1999-06-1804:27UT.Inthe2MASSH-bandimage,theupper
ProgrammeID085.D-0456(D).Twenty-fourdatacubes,eachof
limit to the magnitudeof 4U 1636–53is H > 15.76. The new
exposuretime 132.5 s (comprising 530×0.25 s individualex-
VLT and 2MASSdetectionsof 4U 1636–53are listed in Table
posures) were obtained of the region of 4U 1636–53 in high
1.
time resolution (FastJitter) mode using the K -band filter cen-
S
tred at 2.16µm. The brightest object in the field has < 1000
countsineach0.25sexposure,sonon-linearitiesarenotacon-
4. SwiftUVOTobservations
cern. A small (up to 6′′) telescope offset was applied between
cubes. Average images were made of each cube, removingthe Optical–ultravioletobservationsof4U1636–53weremadewith
skygeneratedfromamedianofthesurroundingaveragedcubes. the UltraViolet/Optical Telescope (UVOT; Romingetal. 2005)
The field of view of each ISAAC image is 40′′ ×40′′ and the on board the Swift satellite and the data are publicly available.
2
Russelletal.:TheNIRcounterpartof4U1636–53(RN)
at the ISAAC-derived position was adopted, which excludes a
neighbouringstar6′′awayfrom4U1636–53;nostarsappearto
10
contaminatethefluxof4U1636–53withintheextractionradius.
ThemeanmagnitudesandrangeineachfilteraregiveninTable
1.
5. Intrinsicspectralenergydistribution
We de-reddenthe K , J-bandandUVOTmagnitudesusingthe
K H J I R V B U W1 M2 W2 S
1 s knowninterstellarextinctiontowards4U1636–53;AV = 2.5±
0.3mag(derivedfromopticaldata;Lawrenceetal.1983)apply-
Jy ingtheextinctionlawofCardellietal.(1989).Wealsotakeop-
m
y; ticalmagnitudesfromLawrenceetal.(1983)andPedersenetal.
sit (1982);U = 17.92±0.06,B= 18.47±0.03,V = 17.89±0.03,
n
e R = 17.46 ± 0.03, I = 16.77 ± 0.03, and de-redden them
d
x in the same manner. Using the de-reddened fluxes (adopting
u
Fl the standard conversion zeropoints for optical Johnson filters,
and 2MASS IR filters from the Explanationary Supplement of
0.1 Skrutskieetal. 2006) we construct the intrinsic NIR–optical–
UVSED ofthepersistent(non-burst)emissionfrom4U1636–
53.
VLT, 2MASS TheSED,whichspansoneorderofmagnitudeinfrequency,
Pedersen et al. 1982 ispresentedinFig.2.Theobserved,reddenedfluxesareplotted
Lawrence et al. 1983
in addition to the de-reddened data. The optical (I to U-band)
Swift UVOT
Power law fit: α ∼ 1.5 SED from Lawrenceetal. (1983) and Pedersenetal. (1982),
blackbody T=27,000 K which has small errors, has a spectral index of α = 1.5±0.4,
data not de-reddened where F ∝ να.Theerroronαisdominatedbytheuncertainty
0.01 uncertainty due to extinction ν
in the extinction. This spectral index is typical for active low-
14 14.2 14.4 14.6 14.8 15 15.2 massX-raybinaries(bothneutronstarandblackholesystems;
log (Frequency; Hz) e.g.,Hynes2005;Russelletal.2007)andisconsistentwiththe
outerregionsofablueaccretiondisc.Shihetal.(2011)showed
Fig.2.Intrinsic(de-reddened)UV–optical–NIRspectralenergy that the optical emission is correlated with the soft X-ray flux;
distributionof4U1636–53(top)andobserved(reddened)fluxes theopticallightisdominatedbytheirradiateddisc(X-rayrepro-
(lower,crosses).Thesystematicerrorsduetouncertaintiesinthe cessingonthediscsurface).
extinctionaredemonstratedbythedottederrorbars. The K , H-band and J-band magnitudes correspond to de-
S
reddenedfluxdensitiesofF = 0.30±0.03mJy,F < 0.83
ν,KS ν,H
Table 1.New NIR–optical– UV magnitudesof 4U1636–53. mJy and Fν,J = 0.67 ± 0.14 mJy, respectively. The optical–
TheSwift UVOTphotometryis takenbetween2005-02-13and infrared (KS to U-band) SED (neglecting the UVOT data) can
2010-06-10. be fitted with a power law with spectral index α = 1.5 ± 0.3
(solid line in Fig. 2). This is similar to the optical spectral in-
dex, and the K -band and J-band fluxes are consistent with an
S
Filter λ1 S/N ———-Magnitude———- Days2 extrapolation of the accretion disc spectrum. It is unlikely that
(Å) mean brightest faintest IR emission lines (e.g. Bandyopadhyayetal. 1997) could con-
tributea significantfractionofthe observedIRfluxes.Thereis
K 21590 145 16.14 16.14(12) 16.14(12) –
S some possible curvaturein the spectrum at the higher frequen-
J 12350 7 16.65 16.65(22) 16.65(22) –
v 5468 28 17.94 17.94(6) 17.94(6) 1 cies, apparent in the UVOT data, and the whole SED can be
b 4392 21 18.67 18.67(8) 18.67(8) 1 approximated by a blackbody at a temperature of ∼ 27 000 K
u 3465 27–43 17.82 17.63(7) 17.95(8) 5 (dotted line in Fig. 2). The curvature implies the blackbodyof
uw1 2600 10–24 18.35 18.16(9) 18.67(15) 4 the irradiated disc spectrum may peak at around 1900 Å, sim-
um2 2246 7–15 18.95 18.61(13) 19.31(20) 5 ilar to some other LMXBs (Hynes 2005). However,the curva-
uw2 1928 7–34 19.20 18.53(10) 19.61(21) 4
ture (and even more so the spectral index of the power law)
is sensitive to the uncertainty in the extinction, and the optical
1The central wavelength of the filter in Angstroms; 2The number of counterpart varies by ∼ 0.6 mag amplitude (a factor of ∼ 1.7
datesthesourcewasobservedwithUVOT. in flux) over weeks to months (e.g., Shihetal. 2011). We de-
tect variability in our UVOT data over several years (Table 1).
Quasi-simultaneous optical–infrared data are required to accu-
UVOT observed4U 1636–53on 11 datesbetween 2005-02-13 rately measure the spectralindex, and uncertaintieswill be de-
and2010-06-10.Onmostdates,oneortwoofthesixUVOTfil- creasedoncetheextinctiontowardsthesourceismeasuredmore
terswereused.Theimagedataofeachfilteroneachdatewere accurately.
summed using uvotimsum. Photometry of the source in indi- Thereisnoevidenceforthecompanionstar,orsynchrotron
vidual sequences was derived with uvotsource. 4U 1636–53 emission from the jet (if the system has a jet) to dominate the
is clearly detected at the position derived above, in all six fil- infraredflux.Bothofthesecomponentsareexpectedtobered-
ters(5500−1900Å).Anextractionregionofradius3′′ centred der than the irradiated disc component, and would produce an
3
Russelletal.:TheNIRcounterpartof4U1636–53(RN)
infraredexcessabovethediscspectrum.Sincethespectrumbe- van Paradijs, J. & McClintock, J. E. 1995, in Optical and Ultraviolet
tweenJandK -bandsisblue(α∼1.5),thesecomponentsmake ObservationsofX-rayBinaries,eds,Lewin,W.H.G.,vanParadijs,J.,van
S
a negligible contribution to J-band. In the most extreme sce- denHeuvel,E.P.J.(Cambridge,CambridgeUniv.Press),58
nario, the disc could have a steep spectrum, α = 2.0 between
J and K -band,andthereforethe disc couldbe asfaintas0.18
S
mJyinK -band(0.12±0.03mJyfainterthantheobservedflux).
S
Ifthiswerethecase,thejetorcompanioncouldcontributeupto
46%ofthe K -bandflux.Thesecalculationsagainneglectpos-
S
sible variability,since the NIR observationswere made on dif-
ferent dates. Simultaneous optical–infrared observations could
constrainthelevelofstarorjetemissionmoreprecisely.
6. Conclusions
We have identified the near-infraredcounterpartof the neutron
starbursterLMXB4U1636–53.Itsmagnitudesonthedatesob-
servedareK =16.14±0.12(VLT/ISAAC), J =16.65±0.22
S
(2MASS). The intrinsic infrared–optical–UV spectrum of the
persistent (non-burst) emission is consistent with a blackbody,
likelyfromthe irradiatedsurfaceoftheaccretiondisc. We find
noevidenceforemissionfromothercomponentsthatmaybeex-
pectedtocontribute,suchasthedonorstarorsynchrotronemis-
sionfromajet,althoughsimultaneousopticalandinfrareddata
areneededtoconstrainthesecontributionsfurther.
Acknowledgements. This research was partly supported by a Netherlands
Organisation forScientific Research (NWO)Veni Fellowship andpartly bya
Marie Curie Intra European Fellowship within the 7th European Community
Framework Programme under contract no. IEF 274805. Theresearch leading
totheseresultshasreceived fundingfromtheEuropeanCommunitysSeventh
Framework Programme (FP7/2007-2013) under grant agreement number ITN
215212 -Black Hole Universe-. Partially funded by the Spanish MEC under
the Consolider-Ingenio 2010 Program grant CSD2006-00070: First Science
withtheGTC(http://www.iac.es/consolider-ingenio-gtc/).TMDacknowledges
support by ERC advanced investigator grant 267697-4PI-SKY. PC acknowl-
edges funding via a EU Marie Curie Intra-European Fellowship under con-
tract no. 2009-237722. MR is supported by grants MD-1832.2011.2, RFBR
10-02-00492 and by Dynasty foundation. This publication makes use of data
products from the Two Micron All Sky Survey, which is a joint project of
the University of Massachusetts and the Infrared Processing and Analysis
Center/California InstituteofTechnology,fundedbytheNationalAeronautics
andSpaceAdministrationandtheNationalScienceFoundation.
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4