Orbital parameters of the high-mass X-ray binary 4U 2206+54
K. A. Stoyanov, R. K. Zamanov, G. Y. Latev, A. Y. Abedin, N. A. Tomov
aa r X i v : . [ a s t r o - ph . S R ] N ov Astron. Nachr. / AN , No. 88, 789 – 792 (2014) /
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Orbital Parameters of the High-Mass X-ray Binary 4U 2206+54 ⋆ K. A. Stoyanov ,⋆⋆ , R. K. Zamanov , G. Y. Latev , A. Y. Abedin , and N. A. Tomov Institute of Astronomy and National Astronomical Observatory, Bulgarian Academy of Sciences, 72 TsarighradskoShousse Blvd., 1784 Sofia, Bulgaria Department of Physics and Astronomy, The University of Western Ontario, London, ON, N6A 3K7, CanadaReceived 2014 Apr 30, accepted 2014 Aug 28Published online 2014 Dec 01
Key words binaries: close – stars: individual: (4U 2206+54) – X-rays: binariesWe present new radial velocities of the high-mass X-ray binary star 4U 2206+54 based on optical spectra obtained withthe Coud´e spectrograph at the 2m RCC telescope at the Rozhen National Astronomical Observatory, Bulgaria in the periodNovember 2011 – July 2013 . The radial velocity curve of the HeI λ orb = 9.568 d and an eccentricity of e = 0.3. These new measurements of the radial velocity resolve the disagreements of theorbital period discussions. c (cid:13)
4U 2206+54 (BD 53 Uhuru satellite (Giacconi et al. 1972). The system also ap-peared in the Ariel V catalogue (Warwick et al. 1981). Themass donor is classified as an O9.5Vp star with a higherthan normal helium abundance, underfilling its Roche lobeand losing mass via a slow but dense stellar wind, υ ∼ − (Rib´o et al. 2006). However, there are some metalliclines typical for a later-type spectrum (Negueruela & Reig2001). The compact object is a neutron star (Torrej´on et al.2004) with spin period P s = 5554 ± ∼ G. If the high magnetic field is con-firmed, then 4U 2206+54 will become the first kind of anaccreting magnetar.Here we present new optical spectroscopy and radial ve-locity measurements of 4U 2206+54. ⋆ based on observations obtained with 2m RCC telescope at RozhenNAO, Bulgaria ⋆⋆ Corresponding author: e-mail: [email protected]
All data reported here are obtained by the 2m RCC tele-scope of the Rozhen National Astronomical Observatory,Bulgaria. 4U 2206+54 is observed between November 2011and July 2013 with the Coud´e spectrograph of the telescope.We used two gratings with resolutions of R = 30 000 and R= 15 000, respectively. The log of observations is given inTable 1.The spectra are reduced in the standard way includingbias removal, flat-field correction, wavelength calibrationand correction for the Earth’s motion. Pre-processings andmeasurements of the radial velocities are performed usingstandard routines provided by IRAF . The spectra obtainedwithin each observational night are processed and measuredindependently.Our spectra include two prominent spectral features -H α and HeI λ α line is dom-inated by emission from the circumstellar disk of the donorstar. We did not use H α line to measure the orbit since itsradial velocity measurements may be strongly affected bychanges in the disk structure. The HeI λ λ The radial velocity curve was modeled with an eccentric or-bital solution. We used the P
HOEBE program (Prˇsa & Zwit- IRAF is distributed by the National Optical Astronomy Observatory,which is operated by the Association of Universities for Research in As-tronomy, Inc., under cooperative agreement with the National SciencesFoundation c (cid:13)
90 Stoyanov, Zamanov, Latev, Abedin & Tomov: Orbital Parameters of 4U 2206+54
Table 1
Observations of 4U 2206+54. Given here are as follows: the ID of the spectrum, MJD of the start of the exposure,the exposure time, the resolution of the grating (R30 for R=30 000 and R15 for R=15 000), the orbital phase folded withP orb = 9.568 d, the radial velocity of the HeI λ spectrum ID MJD -start Exp Time R Orbital V r HeI (O - C)yyyymmddxxx [min] Phase [km s − ] [km s − ]20111108055 2455873.896963 20 R15 0.272 -24.8 -1.820111108056 2455873.912325 20 R15 0.274 -20.3 2.920120706124 2456114.974316 20 R15 0.469 -71.8 -1.520120706125 2456114.989071 20 R15 0.470 -79.8 -9.620120707159 2456115.983374 20 R15 0.574 -94.2 -16.520120707160 2456115.997523 20 R15 0.576 -84.3 -6.620120708024 2456116.885644 20 R15 0.668 -89.3 -13.820120708025 2456116.900287 20 R15 0.670 -81.6 -6.620120709092 2456117.905134 30 R15 0.775 -88.3 -19.420120709098 2456117.955419 30 R30 0.780 -80.2 -11.720120710130 2456118.891829 30 R30 0.878 -81.6 -21.120120830453 2456169.795944 30 R30 0.198 -27.5 -2.620120830454 2456169.816999 30 R30 0.201 -34.2 -9.620120927415 2456197.824752 20 R15 0.128 -54.4 -21.720120927416 2456197.838901 20 R15 0.129 -41.8 -9.420121005639 2456205.765701 30 R30 0.958 -56.6 4.320121005640 2456205.786759 30 R30 0.960 -46.2 16.520121006736 2456206.762868 30 R30 0.062 -51.8 -11.120121006737 2456206.783925 30 R30 0.064 -41.0 -0.520121025107 2456225.733967 30 R30 0.045 -28.5 14.420121025108 2456225.755025 30 R30 0.047 -29.4 13.720121026204 2456226.720280 30 R30 0.148 -30.5 -0.320121026205 2456226.741339 30 R30 0.150 -27.3 2.620121104034 2456235.755836 20 R15 0.092 -37.8 -0.720121104035 2456235.771710 20 R15 0.094 -35.5 1.320130102019 2456294.756720 20 R15 0.259 -9.4 13.020130102020 2456294.770866 20 R15 0.260 -8.1 14.520130123098 2456315.695479 20 R15 0.447 -50.7 15.620130123099 2456315.710368 20 R15 0.449 -55.8 10.820130520065 2456432.934320 20 R15 0.700 -58.8 15.120130521110 2456433.948816 20 R15 0.806 -64.8 1.820130521111 2456433.962972 20 R15 0.808 -61.6 4.820130522136 2456434.907042 30 R15 0.906 -42.4 15.320130522137 2456434.928138 30 R15 0.909 -44.7 12.920130525168 2456437.902431 30 R15 0.220 -23.9 -0.720130525169 2456437.923521 30 R15 0.222 -25.9 -2.820130619206 2456462.970446 20 R15 0.840 -60.9 2.920130718030 2456492.014043 20 R15 0.875 -51.7 8.920130719041 2456492.807739 20 R15 0.958 -48.4 4.320130719044 2456492.827451 20 R15 0.960 -35.9 6.120130723140 2456496.968814 20 R15 0.393 -65.6 -13.520130724170 2456497.786707 20 R15 0.478 -64.2 7.520130724171 2456497.800856 20 R15 0.480 -64.2 7.7 ter 2005) to solve the component parameters and indepen-dently applied the N IGHTFALL program (Wichmann 2006).Both programs are based on the Wilson - Devinney code(Wilson & Devinney 1971). We estimate the errors by com-paring the consistency of the output from the two programs.Using the both possible orbital periods, we estimate the stan-dard deviation of the fit to reveal the proper orbital period. P orb = 9.568 d and P orb = 19.25 d give standard deviationsof the fits of σ =10.3 km s − and σ =40.8 km s − , respectively.For our orbital solution, we took P orb = 9.568 d. In Sect.4.1we discuss the reasons for this choice in more details.For the orbital solution, we adjusted the following orbitalparameters: eccentricity of the orbit e , periastron passage ω ,systemic velocity γ , time of the conjunction passage MJD , c (cid:13) stron. Nachr. / AN (2014) 791 Fig. 1
Examples of HeI λ Table 2
Orbital parameters of 4U 2206+54.
ParameterP orb (d) 9.568*e 0.30 ± ω (deg) 61 .2 ± γ (km s − ) -54.5 ± (HJD-2,450,000) 5871.67 ± K (km s − ) 30.5 ± a sin i (R ⊙ ) 3.76 ± f ( M ) (M ⊙ ) 0.0232 ± σ (km s − ) 10.3 ∗ for the purposes of our orbital solution, we used this value. semiamplitude velocity K , projected separation a sin i ,mass function f ( M ) , and the standard deviation of the fit σ .A set of fixed parameters necessary for the fitting procedureis applied to match the properties of the optical companion.The best-fitting orbital parameters are listed in Table 2. InFig.2 are plotted the radial velocity curve, the best-fittingsolution, and the residuals of the fit. The geometry of theorbit is illustrated in Fig.3. The orbital periods of the Be/X-ray binaries are in the rangefrom ∼
10 d to ∼ orb = 12.7 d (Camero Arranz et al. 2007).The orbital period of 4U 2206+54 is still not well deter-mined. X-ray observations revealed two possible values forthe orbital period: P orb = 9.568 ± orb = 19.25 d(Corbet & Peele 2001; Corbet, Markwardt & Tueller 2007). Fig. 2
Radial velocity curve of 4U 2206+54 ( upperpanel ). The best-fitting solution is overplotted with red line.
The lower panel shows the residuals of the fit.We found P orb = 9.55 ± α )and orbital period for Be/X-ray binaries is explored in de-tails in Reig, Fabregat, & Coe (1997) and Reig (2011). Thesystems with shorter orbital periods show less emission inH α than those with longer orbital periods (Fig.15 in Reig(2011)).We have measured the EW(H α ) in our spectra in order tofind a clue for the real orbital period of 4U 2206+54. Ourminimum and maximum values for the EW(H α ) are 0.51 ˚Aand 3.12 ˚A respectively. Blay et al. (2006) have measured amaximum value of EW(H α ) of 7.3 ˚A. According to Fig.15in Reig (2011), the orbital period of the system should bethe shorter one: P orb = 9.568 d.Moreover, in Fig.4 we plot the radial velocities of the HeI λ orb = 9.568 dis confirmed, 4U 2206+54 will become the Be/X-ray binarywith the shortest orbital period. It will be another addition tothe peculiar features that divert the system from the classicalBe/X-ray binaries. There is a group of Be/X-ray binaries (X Per, GS 0834-430,KS 1947+300, XTE J1543-568, and 2S 1553-542) charac-terized by very low eccentricities: e ≤ c (cid:13)
92 Stoyanov, Zamanov, Latev, Abedin & Tomov: Orbital Parameters of 4U 2206+54
Fig. 3
Orbital geometry of 4U 2206+54 showing the rela-tive orbits of the 10.7 M ⊙ donor star and the 1.4 M ⊙ neutronstar. The relevant phases of the periastron, apastron, and theconjunctions are marked. The center of the mass is indicatedwith a cross.assumed by current evolutionary models (Pfahl et al. 2002).These objects have P orb ≥
30 d.Most of the Be/X-ray binaries have moderately eccen-tric orbits with e ≥ On the basis of radial velocity measurements of the HeI λ e = 0.3, if theorbital period is 9.568 d. We discussed the probability that4U 2206+54 is a Galactic Be/X-ray binary with the shortestorbital period known up today. Acknowledgements.
We thank the anonymous referee for the con-structive comments. This work was supported by the OP “HRD“,ESF and Bulgarian Ministry of Education, Youth and Science un-der the contract BG051PO001-3.3.06-0047.
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