Población encuestada

LS I +61 303 has a nearby visual companion that we also recorded in the spectra made in 2000 December. This companion star has a separation of 1100._{2 and a position angle}

of 167◦_{from LS I +61 303 according to coordinates listed in the Guide Star Catalog II}4

(where the companion is designated 0404701738). The companion is approximately 3.5 mag fainter in the optical than LS I +61 303. Our average spectrum shows a very broad Hαabsorption line, no HeIλ6678, a few weak metallic lines, and an interstellar line at 6613 ˚A that is identical in appearance to its counterpart in the spectrum of LS I +61 303. We compared the spectrum to several in the atlas by Valdes et al. (2004), and we tentatively classify the star as type A2 V. The radial velocity of the star from the Hα line is₋40_±2 km s−1_.

It is possible that this companion is an outlying member of the Cas OB6 associ- ation as is LS I +61 303 (Mirabel, Rodrigues, & Liu 2004). The companion’s radial velocity is comparable to that of other association members (Hillwig et al. 2006) in- cluding the nearby multiple star HD 16429 (McSwain 2003), and the similarity of the

IR magnitudes suggest that the star is closer than Cas OB6. Suppose the absolute

K-band magnitude for a A2 V star is MK = 1.2±0.6 (Cox 2000) and that the ex-

tinction is A(K) = 0.36E(B −V) = 0.3 (Howarth 1983; Fitzpatrick 1999). Then for the 2MASS magnitude of K = 11.55±0.02, the estimated distance is 1.0±0.4 kpc, which would place the star in the foreground of the Cas OB6 stars at a distance of 1.9 kpc (Hillwig et al. 2006). Thus, unless the 2MASS magnitudes and/or the spec- troscopic estimate of absolute magnitude are wrong, this companion is probably a chance optical alignment rather than a neighbor of LS I +61 303.

The Be X-ray Binaries

HDE 245770 = A 0535+26 and X

Persei

Abstract

We1 _{present results from an Hα monitoring campaign of the Be X-ray binary systems}

HDE 245770 = A 0535+26 and X Per. We use the Hαequivalent widths together with adopted values of the Be star effective temperature, disk inclination, and disk outer boundary to determine the half-maximum emission radius of the disk as a function of time. The observations of HDE 245770 document the rapid spectral variability that apparently accompanied the regeneration of a new circumstellar disk. This disk grew rapidly during the years 1998 – 2000, and then slowed in growth in subsequent years. The outer disk radius is probably truncated by resonances between the disk gas and neutron star orbital periods. Two recent X-ray outbursts appear to coincide with the largest disk half-maximum emission radius attained over the last decade. 1_{Grundstrom, E. D.; Boyajian, T. S.; Finch, C.; Gies, D. R.; Huang, W.; McSwain, M. V.;} O’Brien, D. P.; Riddle, R. L.; Trippe, M. L.; Williams, S. J.; Wingert, D. W.; Zaballa, R. A. - published as Grundstrom et al. (2007b)

accretion from the disk. We find that the Hα half-maximum emission radius of the disk surrounding X Per reached a size about six times larger than the stellar radius, a value, however, that is well below the minimum separation between the Be star and neutron star. We suggest that spiral arms excited by tidal interaction at periastron may help lift disk gas out to radii where accretion by the neutron star companion becomes more effective.

6.1 Introduction

Be X-ray binaries (BeXRBs) consist of rapidly rotating B-stars with neutron star companions (Coe 2000). The B-star primaries lose mass into an outflowing circum- stellar disk, and if the disk reaches a radius comparable to the periastron separation, then disk gas accreted by the neutron star can power a significant (and often tran- sient) X-ray source (Okazaki & Negueruela 2001; Okazaki et al. 2002). The disk gas is detected through observations of emission lines, an optical/infrared excess, and a net linear polarization caused by starlight scattered in the disk (Porter & Rivinius 2003). Be star disks are inherently time variable and can develop and disappear on timescales of years to decades (Underhill & Doazan 1982; Hubert & Floquet 1998). Thus, we expect that the X-ray accretion fluxes will vary on similar timescales. Here we present a joint study of the disk Hα emission flux and X-ray flux variations in two of the best studied BeXRBs, HDE 245770 = A 0535+26 and X Persei. We discuss similar co-variations of Hαemission and X-ray flux in Chapters 3 and 5 for the super- giant binary LS I +65 010 and the BeXRB microquasar LS I +61 303, respectively,

and in a companion paper on the black hole binary Cyg X-1 = HDE 226868 (Gies et al. 2003).

The remarkable Be star HDE 245770 (and X-ray counterpart A 0535+26) is one of the prototype objects of the BeXRB class. The primary star of HDE 245770 has a classification of O9.7 IIIe (Giovannelli & Graziati 1992) or B0 IIIe (Steele et al. 1998), but the luminosity class is still somewhat controversial as many of the luminosity criteria are based on spectral lines that are affected by disk emission (Wang & Gies 1998). The orbital period from peaks in the X-ray flux is approximately 110 d (Hutchings 1984; Coe et al. 2006), and a comparison of the times of maxima between the first and most recent epochs suggests a period of P = 109.96_±0.05 d. The only reliable estimate of the other orbital elements comes from X-ray timing of the pulsar spin period during three outbursts in 1993 (Finger et al. 1994; Bildsten et al. 1997), and this solution indicates that the orbital eccentricity is e= 0.47±0.02. The radial velocity curve of the Be star has an amplitude that is smaller than or comparable to the velocity fluctuations introduced by emission contamination and other factors, so only upper limits are available for the Be star’s semiamplitude and the system mass ratio, M2/M1 < 0.12 (Wang & Gies 1998). The star has a long and rich history of

X-ray, optical, and infrared observations (Motch et al. 1991; Giovannelli & Graziati 1992; Clark et al. 1998; Negueruela et al. 1998; Haigh et al. 2004; Coe et al. 2006). Lyuty & Za˘itseva (2000) present a remarkable record of the light curve variations back to the year 1898, and they show that after a quiescent stage in the first part of the century the star brightened by some 40% in the early 1970s. The circumstellar disk that appeared then provided the gas to power the X-ray source at the time coincident with the dawn of X-ray astronomy.

The emission from the disk of HDE 245770 in the form of optical and infrared continuum light and Balmer emission lines displays large temporal variations in the

argue that the disk IR flux tends to hover around three different brightness levels, and they suggest that these correspond to distinct disk radii that are defined by resonances between the disk gas and neutron star orbital periods. Okazaki & Negueruela (2001) developed a model to predict the resonant disk truncation radii in BeXRBs, and Haigh et al. (2004) found that the model predictions are consistent with the IR magnitude jumps. Coe et al. (2006) extended this analysis with additional JHK photometry and measurements of the Hα emission line. The Hα emission variations (Clark et al. 1998; Haigh et al. 1999; Lyuty & Za˘itseva 2000; Haigh et al. 2004; Coe et al. 2006) indicate that the disk almost totally disappeared in 1998 but rebounded to its former strength over the next few years.

The binary system X Per (HD24534; B0 Ve, Lyubimkov et al. 1997) is the brightest and perhaps most famous member of the BeXRB class. X Per has displayed time variable optical emission lines since the earliest photographic spectrograms were made at the beginning of the last century (Cowley et al. 1972). The long term changes in the spectral appearance are reviewed by Roche et al. (1993) and Clark et al. (2001). In a seminal paper, Clark et al. (2001) describe the photometric and spectroscopic variations observed over the period 1987 through 2001, and during this time the star made a remarkable transformation from Be to B and back again to Be. Clark et al. (2001) discuss how these variations are related to the structural properties of the circumstellar disk. The orbital elements were determined by Delgado-Mart´ı et al. (2001) by careful timing observations of the X-ray pulsar (pulse period_≈835 s) using

theRossi X-Ray Timing Explorer (RXTE)satellite. The orbital period isP = 250.3 d

Both HDE 245770 and X Per are among some hundred objects regularly observed with theRXTEAll-Sky Monitor (ASM) instrument (Levine et al. 1996), and here we address the issue of how their disk size variations are related to the observed X-ray fluxes. We present our recent observations of their Hαemission lines in_§6.2. We then show in §6.3 how the Hα emission strength is related to the disk radius, and we use these relations in§6.4 to document how the disk radius variations relate to theV-band and X-ray light curves. The disk radii appear to attain limits that are probably related to resonant truncation radii (Okazaki & Negueruela 2001), and we discuss in §6.5 how such limits affect the mass transfer process in these binaries. Finally_§6.6 offers a reassessment of the orbital motion of X Per based upon radial velocity measurements of UV spectra from theInternational Ultraviolet Explorer (IUE) satellite, from which we derive limits on the binary’s mass ratio and inclination.