Table Of ContentMon.Not.R.Astron.Soc.000,1–??(2006) Printed5February2008 (MNLATEXstylefilev2.2)
On the ultra-compact nature of 4U 1822–000
T.Shahbaz1⋆, C.A.Watson2 and H.Hernandez-Peralta1
1Instituto de Astrof´ısica de Canarias, 38200 La Laguna, Tenerife,Spain
2Department of Physics and Astronomy, University of Sheffield, Sheffield, S3 7RH, UK
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a ABSTRACT
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1 Wereportthediscoveryofaperiodicmodulationintheopticallightcurveofthecandi-
3 dateultra-compactX-raybinary4U1822–000.Usingtime-resolvedopticalphotometry
taken with the William Herschel Telescope we find evidence for a sinusoidal modula-
1
tion with a semi-amplitude of 8percent and a period of 191min, which is most likely
v
close to the true orbital period of the binary. Using the van Paradijs & McClintock
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relationforthe absolutemagnitude andthe distance modulus allowingfor interstellar
1
9 reddening, we estimate the distance to 4U1822–000 to be 6.3kpc. The long orbital
1 period and casts severe doubts on the ultra-compact nature of 4U1822–000.
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Keywords: accretion,accretiondisc–binaries:close–stars:individual:4U1822–000
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- 1 INTRODUCTION sizeofthebinaryi.e.theorbitalperiod(foranobservational
o
review see Nelemans & Jonker2006).
r Low-mass X-ray binaries are systems in which a low-mass
t 4U1822–000 was discovered by Uhuru and has since
s companion star transfers material onto a neutron star or
been observed briefly by all the major X-ray missions
a backhole.Mostofthesystemshaveorbitalperiodsofhours
: (Giacconi et al., 1972; Warwick et al., 1981; Wood et al.,
v to days and contain ordinary hydrogen-rich donor stars.
1984;Smale & Lochner1992;Christian & Swank1997)and
i Theminimumorbitalperiodforsystemswithhydrogen-rich
X has been roughly constant in the X-rays with an unab-
donorstarsisaround80min(Rappaport,Joss, & Webbink sorbedfluxof1×10−9ergscm−2s−1inthe0.5–20keVrange
r 1982), however, systems with hydrogen-poor or degen-
a (Christian & Swank1997).IthasafaintV=22opticalcoun-
erate donor stars can evolve to extremely small binary
terpart(Chevalier & Ilovaisky1985)andwasidentifiedasan
separations with orbital periods as short as a few min-
UCXB candidate based on its low X-ray/optical flux ratio.
utes (Nelson, Rappaport, & Joss 1986). Such systems are
Chandra observations show that the X-ray source is coinci-
called ultra-compact X-ray binaries (UCXBs) and have
dentwith theoptical position and therenoevidencefor or-
a range in orbital periods from 11 to 50 minutes (see
bitalmodulationintheX-rays(Juett & Chakrabarty2005).
Nelemans & Jonker 2006)
Wang& Chakrabarty (2004) obtained optical photometry
X-ray spectroscopy has identified several candidate
oftheopticalcounterpartusingthe6.5mMagellantelescope
UCXBs based on similarities with the known UCXB
and detected significant variability on a timescale of about
4U1850–87 where the inferred enhanced neon/oxygen
90mins.Inthisletterwereportonouropticaltime-resolved
ratio is interpreted as being local to the system.
photometry of 4U1822–000.
Juett, Psaltis, & Chakrabarty (2001) thus concluded that
the candidate UCXBs have ultra-short periods and pro-
posed thattheirdonorsstarswereoriginally carbon-oxygen
or oxygen-neon-magnesium white dwarfs. There are 12 sys- 2 OBSERVATIONS AND DATA REDUCTION
tems with known or suggested orbital periods, which in-
cludeatransient pulsar,X-raybustersand millisecond pul- Our optical photometric observations were taken on 2006
sars UCXB candidates have also been identified via their July 23 using the 4.5m William Herschel Telescope on La
low optical-to-X-ray flux ratio and optical faintness. This Palma. Weused theTEK 1024×1024 pixelCCD camera at
is what is expected for the small accretion discs in these aux-portfocus,ACCDbinningof2×2wasusedprovidinga
systems, because the optical emission is dominated by the platescale of0.22 arcsecpixel−1 andanunvignetted(circu-
irradiatedaccretiondiscwhosesurfaceisdeterminedbythe lar)fielddiameterof1.8arcmins.TheJohnsonV-bandfilter
wasusedandthetotalobservationtimelengthon4U1822–
000was approximately 5h.Theexposuretimeofeach indi-
⋆ E-mail:[email protected] vidual frame was 60s and with a readout time of 13s, we
2 T. Shahbaz, C. A. Watson and Hernandez-Peralta
Figure 1. Left:thelightcurveof4U1822–000. Thedashedlineisasinusoidalfitwithaperiodof191min.Thecomparisonstarisalso
shown and is shifted vertically downwards for clarity. The lightcurves of both 4U1822–000 and the comparison star have been binned
to a time resolution of 2.4min. The dashed line is a sinusoidal and constant fit to the 4U1822–000 and comparison star lightcurves
respectively.Right:thelightcurvefoldedonthe191minperiodandbinnedinto25phasebins.Thedashedlineisasinusoidalfittothe
dataandtwoorbitalcyclesareshown.
obtained one image approximately every 1.2min over the either X-ray irradiation of the inner face of the secondary
course of our observations. The conditions were not photo- star and/or a superhump modulation from the accretion
metric because of the calima (Sahara dust), but the seeing disc. Sumperhumps only occur in binaries with extreme
was excellent, ranging from 0.6 to 1.0arcsecs. mass ratios (such as UCXBs), and are due to a precessing,
Weused IRAFfor ourinitial datareduction, which in- eccentric accretion disk (Whitehurst & King 1991), which
cluded bias subtraction using the overscan regions of the haveobservedopticalperiodsafewpercentlongerthanthe
CCD, and flat-fielding using sky flat fields taken during binary orbital period. Thus the 191min period most likely
twilight. The ULTRACAMreduction pipeline software was liesclose totheorbitalperiod ofthebinary,andweassume
thenusedtoobtain lightcurvesfor4U1822–000 andseveral thisto be thecase in the rest of thediscussion.
comparison stars by extracting the counts using aperture van Paradijs & McClintock (1994) determined the ab-
photometry.A variable aperture which scaled with thesee- solutemagnitudeofX-raybinariesandderivedanempirical
ingwasused.Differentiallightcurveswerethenobtainedby relation between the absolute magnitude (MV), orbital pe-
computing the count ratio of 4U1822–000 with respect to riod (P) and X-ray luminosity (LX)
a local standard (non-variablestar). Asa check of thepho-
tometryandsystematicsinthereductionprocedure,wealso MV =1.57−2.27log(Σ); Σ=(LX/LEdd)1/2(P/1hr)2/3 (1)
extractedlightcurvesofacomparison starsimilarinbright- where LEdd is the Eddingtion luminosity (2×1038ergs−1
nesstothetarget.Thephotometricaccuracyof4U1822–000
for a 1.4M⊙ neutron star). Their relation was based on
for each exposure is about 10 percent and agrees with the
the assumption that the optical flux is due to X-ray ir-
scatter of thecomparison star with similar brightness.
radiation of the accretion disc, which means that UCXBs
with short orbital periods are expected to have rela-
tively faint discs. Therefore faint MV values have been
3 RESULTS
used to select UCXB candidates which have MV >3.7
(Nelemans & Jonker 2006). Indeed 4U1822–000 was provi-
Figure1showsthelightcurvefor4U1822–000 which clearly
sionally classified as an UCXB based on its’ faint absolute
exhibits a strong modulation. Given the baseline of our
dataset of 5h, a Lomb-Scargle analysis of the lightcurve magnitude; MV >7.5 assuming a distance of 8kpc (even
does not reveal any significant periods on timescales of less if the distance is 20kpc, MV=5.5) and the lack of hydro-
gen and helium emission lines in it’s low quality VLT opti-
than a few hours. However, fitting the lightcurve with a si-
nusoidal modulation reveals a period of 191±8min and a calspectrum(Nelemans, Jonker, & Steeghs2006).However,
semi-amplitude of 0.083±0.010mags. Although the deter- our estimate of the orbital period of 191min casts severe
doubtson theUCXBnature of 4U1822–000.
mination of the period is biased because our baseline is
Based on our estimate for the orbital period and as-
just less than twice the period, it should be noted that
suming that 4U1822–000 is a persistent X-ray binary, we
Wang & Chakrabarty (2004) also saw a modulation on a
timescale of ∼180min (2×90min; note that they observed can use the van Paradijs & McClintock relation and the
distance modulus to estimate the distance to 4U1822–
half a periodic modulation).
000. The optical magnitude of 4U1822–000 is V=22
(Chevalier & Ilovaisky 1985) and theoptical extinction can
4 DISCUSSION be determined using the empirical relation between the
optical extinction (AV) and the total column density of
Wehavefounda191minsinusoidalmodulation intheopti- hydrogen NH (Gorenstein 1975; Predehl & Schmitt 1995).
cal lightcurve of 4U1822–000 which most likely arises from Using the observed value for NH = 0.97 × 1022cm−2
On the ultra-compact nature of 4U1822–000 3
(Juett & Chakrabarty 2005) we find AV=5.5mag. Using ACKNOWLEDGMENTS
the de-reddended optical magnitudes, the unabsorbed X-
ray flux of 1.1×10−9ergscm−2s−1 (determined using the TSacknowledges supportfrom theSpanishMinistryof Sci-
enceand Technology undertheprogramme Ram´on y Cajal
observed X-ray flux, the power-law index and NH given in
and under the project AYA2004-02646. CAW is supported
Juett & Chakrabarty 2005) and P=191min in conjuction
bya PPARC postdoctoral fellowship.
withequation(1)andthedistancemodulus,wefindthedis-
tance to 4U1822–000 to be 6.3±2.0kpc and MV =2.6+−00..86
(the errors relfect the uncertainties in the van Paradijs &
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does not show any features that may indicate that it is an Wood, K. S., et al. 1984, ApJS,56, 507
UCXB.Furthermore,ourdeterminationofa191minperiod
(whichwebelieveisclosetotheorbitalperiod)castssevere
doubtsontheUCXBnatureof4U1822–000.Giventhis,itis
puzzlingthatnohydrogenorheliumemission linesareseen
intheopticalVLTspectra.However,thiscouldbeduetothe
poor quality of the spectra (Nelemans, Jonker, & Steeghs
2006).
It is thus interesting to make a comparison with the
similarshortperiodXRBs,suchas4U1323-619,EXO0748–
676 and GRMUS Ritter& Kob (2003), with orbital peri-
ods of 2.96hr, 3.82hr and 3.93hr respectively. The optical
lightcurves of these systems, which are all X-ray bursters,
showasinusoidal modulation with partialeclipes/dips con-
sistent with the idea that a large reprocessing region in the
diskisbeingpartiallyobscuredbystructureatthediskedge.
In all these system, broad emission lines are observed. In
contrasttheopticallightcurveof4U1822–000 containsonly
a sinusoidal component, which suggests that system lies at
arelativelylowinclinationangle.However,thisdoesnotex-
plainthelackofemissionlines,whichonewouldexpecttobe
independenton orbital phase for systems at low inclination
angles.Clearlyamoredetailedphotmetricandspectroscopic
study will proveuseful.
