We present the long-term optical spectroscopic observations on the Be/X-ray binary A0535+26 from 1992 to 2010. Combining with the public V-band photometric data, we find that each giant X-ray outburst occurred in a fading phase of the optical brightness. The anti-correlation between the optical brightness and the H$\alpha$ intensity during our 2009 observations indicates a mass ejection event had taken place before the 2009 giant X-ray outburst, which might cause the formation of a low-density region in the inner part of the disk. The similar anti-correlation observed around 1996 September indicates the occurrence of the mass ejection, which might trigger the subsequent disk loss event in A0535+26.
Be/X-ray binary represents a major subclass of high mass X-ray binary in our Galaxy (Liu et al. 2006). A neutron star moves in a wide and eccentric orbit around the Be star, which is a non-supergiant rapid-rotating B-type star and ever shows Balmer emission lines at least once in its life (Porter & Rivinius 2003). There are two different disks in Be/X-ray binary: a circumstellar disk around Be star and an accretion disk around neutron star (see Reig (2011) and the reference therein).
A0535+26 was found by Ariel V in 1975 during a giant X-ray outburst (Rosenberg et al. 1975). Its optical counterpart was identified as the Be star HD 245770 with a classification of O9.7 IIIe (Giovannelli & Graziati 1992). A neutron star with a 103-s spin period (Caballero et al. 2007) moves around the primary star in an eccentric orbit (e = 0.47 ± 0.02) (Finger et al. 1994). A 111.1-day orbital period was found from the periodic Type I X-ray outbursts around the periastron passage of the neutron star. Since the first giant outburst in 1975, another six giant X-ray outbursts were observed in 1975April, 1980 October (Nagase et al. 1982(Nagase et al. ), 1983 June (Sembay et al. 1990(Sembay et al. ), 1989 March (Makino et al. 1989), 1994February (Finger et al. 1994), 2005 May (Tueller et al. 2005;Coe et al. 2006;Caballero et al. 2008), and 2009December (Wilson-Hodge et al. 2009;Reynolds & Miller 2010), respectively.
The long-term variability of optical and IR photometry was discussed by Hao et al. (1996), Clark et al. (1998), Clark et al. (1999), Haigh et al. (1999), Lyuty & Zaitseva (2000), and Haigh et al. (2004). The variability of Hα emission line is related with the physical changes in the circumstellar disk around Be star. A disk loss event took place in the system in 1998 and the Hα line changed from emission to absorption (Haigh et al. 2004;Grundstrom et al. 2007). Recent short-term and long-term optical spectral variabilities were reported by Moritani et al. (2010).
In this paper, we present our optical spectroscopic and photometric observations on A0535+26. Combining with the public data of the V-band magnitude and the Hα equivalent widths (EWs), we discuss the optical variability of the system and unveil the nature of X-ray outbursts.
We obtained optical spectra of HD 245770 with the 2.16 m telescope at Xinglong Station of National Astronomical Observatories, China (NAOC), from 1992 to 2010. The optical spectroscopy with an intermediate resolution of 1.22 Å pixel -1 was made with a CCD grating spectrograph at the Cassegrain focus of the telescope. We took both blue and red spectra covering the wavelength ranges 4300-5500 and 5500-6700 Å, respectively, at different times. All spectra were reduced with the IRAF 1 package. The data were bias-subtracted and flatfield corrected, and the cosmic rays were removed. Helium-argon spectra were taken in order to obtain the pixel-wavelength relation. To improve this relation, we also used the diffuse interstellar bands 6614 and 6379 Å observed in the spectra. The short-term and long-term variability of the emission line profile will be discussed in another paper (Liu et al. 2011). In this paper we only concentrate on the long-term evolution of the intensity of the Hα line in the spectra of A0535+26. All the spectra have been normalized to neighboring continuum. The EW of the Hα line has been measured selecting a continuum point on each side of the line and integrating the flux relative to the straight line between the two points using the procedures available in IRAF. The measurements were repeated five times for each spectrum and the error estimated from the distribution of obtained values. The EW(Hα) typical error is within 3%. This error arises due to the subjective selection of the continuum.
On 2010 October 13, we performed the systematic Johnson-Cousin UBV RI photometric observations on A0535+26 with the 80cm Tsinghua-NAOC Telescope (TNT) at Xinglong Station of NAOC. TNT is an equatorial-mounted Cassegrain system with a focal of f /10, made by AstroOptik, funded by Tsinghua University in 2002 and jointly operated with NAOC. The telescope is equipped with the Princeton Instrument 1340×1300 thin backilluminated CCD. The photometric data reduction was performed using standard routines and aperture photometry packages in IRAF, including bias subtraction and flat-field correction. In order to calibrate the instrumental magnitudes of A0535+26, we also observed a sample of Landolt standards in UBV RI bands. The average UBV RI magnitudes of A0535+26 are, 8.6±0.03, 9.6±0.03, 9.2±0.02, 8.9±0.02 and 8.5±0.02, respectively.
In order to study the optical line variability before, during, and after the 2009 giant Xray outburst, we plot the representative spectra of Hα and HeI λ 6678 during our 2007-2010 observations in Figure 1. The Hα emission line shows an asymmetric single-peaked profile during our 2007 observations. The peak intensity of the Hα became stronger during the 2008 obser
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