Peremennye Zvezdy

Peremennye Zvezdy (Variable Stars) 28, No. 7, 2008

Received 15 Juny; accepted 15 July.

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THE INFRARED VARIATIONS OF CYG X-1 (V1357 CYG), 1995-2007

O.G. Taranova, V.I. Shenavrin, A.E. Nadjip

Sternberg Astronomical Institute, 13, University Ave., Moscow 119992, Russia. E-mail: taranova@sai.msu.ru


We present the results of our long-term (13 years) J and K photometry of the X-ray binary Cyg X-1. The object's JK variability amplitudes were less than 0 2. The J and K orbital light curves are appreciably asymmetric in quadratures. The secondary minimum is deeper, in comparison to the primary, and it is probable that the star becomes hotter at secondary minima. The ratio of average radii of the supergiant in quadratures and in minima is 1.02 if the temperature of the optical component does not change during its orbital motion. We estimate the interstellar extinction as and the distance to the star, as  kpc. The luminosity and radius of the optical component are respectively and . The JK variations of the X-ray binary Cyg X-1 in 1995-2007 can be explained with orbital ellipsoidal variations of the optical component (hot supergiant HD 226868), accompanied with long-term JK variations on a time scale of 11.5 years with an amplitude of 006-007. In 1995-2007, the JK brightness fluctuations probably had a 294-day periodicity with an amplitude of no more than 003-005.

1. INTRODUCTION

Cyg X-1 is one of the best-studied X-ray sources since its discovery in 1965 (Bowyer et al. 1965). Webster & Murdin (1972) identified the source with the O9.7Iab supergiant HD 226868 and found an orbital period of 5.6 days. On the basis of the analysis of data from the X-ray satellite UHURU, together with radio and optical observations, Tananbaum et al. (1972) suggested that the invisible component of the X-ray binary Cyg X-1 was a black hole. Optical variations of the supergiant were investigated by Lyuty (1972) and Lyuty et al. (1973). They calculated parameters of the binary, confirmed that the source Cyg X-1 was the first black-hole candidate, and explained variations of the optical component with its ellipsoidal shape, changing aspect during orbital motion.

Since then, the orbital period (5.6) was reliably identified in the binary's brightness variations and the existence of several periods (from dozens of days to hundreds of days; see, for example, Kemp et al. 1978; Priedhorsky et al. 1983) was suspected.

Despite extensive studies of Cyg X-1 in the recend decades, the IR variability of the source was reliably established only in the early 1990s (Leahy & Ananth 1992). The cited authors discovered fluctuations of the J and K brightness with an amplitude of 03 during the orbital motion and explained the near-IR radiation from the system as due to the accretion disk.

The first cycle of our JK photometry of Cyg X-1 was performed in 1984-1994 (Nadzhip et al., 1996). From the analysis of these observations together with optical photometry of Cyg X-1, we concluded that the variations of the binary's J and K brightness were ellipsoidal.

The results of our JK photometry of Cyg X-1 acquired in 1995-2007 are presented in this paper.

2. OBSERVATIONS

Our JK photometry of Cyg X-1 was performed at the 1.25-m telescope of the Crimean Laboratory of the Sternberg Astronomical Institute using a photometer with a liquid-nitrogen-cooled InSb detector. The photometer was mounted at the Cassegrain focus of the telescope; the exit aperture was . The star BS 7615 from the Johnson et al. (1966) catalog served as a photometric standard. The results of our photometry of Cyg X-1 between June, 1995 and September, 2007 and their uncertainties are presented in Table 1.

3. RESULTS AND DISCUSSION

Fig. 1. The 1984–2007 JK light curves and J-K color curve of Cyg X-1. The straight lines are linear fits. The solid and dotted curves are fits to eighth-order polynomials and their confidence intervals. The lines 1, 2, and 3 show approximal time intervals of the maximum (line 1) and minimum (line 2) in the optical brightness and in the X-ray source activity (line 3).

Characteristic Features of the JK Brightness Variations of Cyg X-1 in 1995-2007

Variations of the J and K magnitudes and J–K color of Cyg X-1 are presented in Fig. 1 for the whole time interval covered with our observations (1984-2007). The observations of 1984-1994 are plotted as asterisks; circles are observations of 1995-2007. The straight lines are linear fits of the observations. The results of our J and K photometry of 1984-1994 were discussed earlier (Nadzhip et al. 1996). The following qualitative features of the J and K magnitude and J–K color variations are apparent from Table 1 and Fig. 1:

It is easily apparent from the JK light curves (Fig. 1) that the brightness of Cyg X-1 decreased during 1990-1994, then it increased to a maximum near 2000 and again decreased to 2003. We attempted to fit the J and K brightness fluctuations observed in 1984-2007 to polynomials of different order (from 2 to 9). As a result, the narrowest confidence intervals were found for curves of the eighth order, they are shown in Fig. 1 as solid curves (the dashed lines are boundaries of the confidence intervals). The periodic component having a time scale of 4000-4300 days ( years) and an amplitude of $\sim 0\hbox{$.\!\!^{\rm m}$}06{-}0\hbox{$.\!\!^{\rm m}$}.07$ in the K filter is visible well enough.

Fig. 2. The 1995–2007 JK light curves of Cyg X-1 folded with the orbital periods. The broken lines are for averaged values.

In 1998-2004, an activity increase of the X-ray source was accompanied with a decrease of the binary's optical activity (Karitskaya et al. 2006). In the near-IR range, on the contrary, the activity of the X-ray source probably caused some increase both of the IR brightness and of the color temperature. It is clearly seen in Fig. 1, where the lines 1, 2, and 3 mark the approximate time intervals of the maximum (line 1) and minimum (line 2) in the optical brightness and in the X-ray source activity (line 3).

In other words, we can assume that there is a "long-term" component of the JK radiation with a variability time scale of about 11.5 years in the observed JK brightness variations of Cyg X-1 . The "long-term" component and the activity of the binary's X-ray component are probably interrelated.

Table 1. The JK photometry of V1357 Cyg (Cyg X-1), 1995-2007

JD J   K   JD J   K  
(2440000+)         (2440000+)        
9885.521 7.091 0.005 6.673 0.012 11774.338 7.010 0.005 6.624 0.007
9885.528 7.083 0.006 6.667 0.007 11774.345 7.005 0.005 6.600 0.007
9905.538 7.034 0.008 6.609 0.010 11775.354 7.015 0.005 6.606 0.007
9969.287 7.072 0.018     11776.392 7.017 0.006 6.591 0.003
9969.295 7.059 0.014     11776.400 7.014 0.006 6.595 0.004
9969.300 7.062       11777.382 7.025 0.007 6.619 0.005
11063.339 6.972 0.011 6.549 0.010 11777.390 7.050 0.006 6.618 0.007
11063.346 6.991 0.012 6.549 0.009 11778.310 7.066 0.005 6.657 0.003
11065.339 6.995 0.014 6.566 0.011 11779.395 7.015 0.009 6.591 0.007
11065.346 7.002 0.012 6.561 0.012 11780.367 7.047 0.006 6.620 0.008
11066.347 7.007 0.018 6.574 0.009 11780.368 7.036 0.010 6.622 0.007
11066.355 7.012 0.017 6.574 0.010 11782.363 6.973 0.009 6.593 0.016
11067.264 6.995 0.015 6.577 0.011 11782.370 7.005 0.011 6.599 0.009
11067.270 6.997 0.014 6.597 0.012 11802.298 6.987 0.007 6.547 0.007
11067.277 7.009 0.013 6.603 0.009 11802.305 6.984 0.007 6.557 0.005
11068.356 6.975 0.015 6.549 0.017 11824.283 7.006 0.007 6.591 0.004
11068.363 6.981 0.011 6.551 0.015 11832.202 7.041 0.005 6.636 0.006
11069.374 7.011 0.017 6.584 0.014 11834.298 7.035 0.006 6.626 0.008
11069.380 6.997 0.015 6.572 0.008 11850.258 7.022 0.006 6.614 0.007
11069.387     6.576 0.012 11864.178 7.035 0.004 6.634 0.007
11072.316 7.005 0.008 6.589 0.014 11865.270 7.075 0.007 6.673 0.010
11072.323 7.017 0.008 6.595 0.012 11866.208 7.020 0.005 6.607 0.006
11383.403 6.989 0.006 6.584 0.006 11867.152 7.045 0.007 6.649 0.003
11383.410 7.000 0.008 6.584 0.014 11887.139 7.023 0.010 6.616 0.006
11383.417 6.989 0.006 6.578 0.013 12125.448 7.034 0.009 6.655 0.012
11384.466 6.980 0.009 6.580 0.008 12127.417 7.014 0.010 6.627 0.012
11384.474 6.986 0.010 6.589 0.006 12127.425 7.029 0.015 6.618 0.013
11385.422 7.020 0.009 6.589 0.029 12153.366 7.060 0.009 6.657 0.008
11385.430 7.019 0.018 6.612 0.013 12153.374 7.043 0.008 6.657 0.010
11387.445 6.941 0.007 6.524 0.012 12154.396 6.994 0.008 6.614 0.009
11387.452 6.943 0.009 6.536 0.013 12154.403 7.023 0.009 6.582 0.007
11393.362 6.960 0.013 6.558 0.012 12158.336 7.042 0.011 6.663 0.009
11393.370     6.564 0.011 12163.365 7.027 0.008 6.631 0.006
11420.377 6.993 0.004 6.584 0.017 12188.251 6.994 0.008 6.590 0.008
11420.384 7.008 0.005 6.592 0.012 12209.205 7.061 0.002 6.637 0.003
11421.372 6.978 0.007 6.563 0.010 12209.210 7.041 0.004 6.631 0.003
11421.380 6.962 0.008 6.560 0.010 12450.456 7.035 0.013 6.611 0.012
11451.304 6.977 0.008 6.579 0.007 12454.463 6.975 0.011 6.563 0.007
11451.310 6.966 0.012 6.573 0.007 12455.434 7.028 0.008 6.625 0.015
11452.290 6.928 0.008 6.525 0.005 12456.448 7.044 0.018    
11452.297 6.927 0.008 6.519 0.008 12458.444 7.045 0.008 6.644 0.018
11453.288 6.996 0.008 6.575 0.009 12458.450 7.061 0.010 6.661 0.010
11454.319 7.002 0.010 6.585 0.010 12514.377 7.032 0.010 6.623 0.011
11455.326 6.989 0.006 6.594 0.013 12514.385 7.031 0.011 6.627 0.010
11456.255 6.971 0.018 6.577 0.009 12867.288 7.085 0.007 6.648 0.009
11738.444 7.048 0.009 6.635 0.007 12867.297 7.074 0.007 6.652 0.005
11738.450 7.049 0.009 6.645 0.009 13962.376 7.045 0.006 6.651 0.008
11741.431 7.073 0.010 6.652 0.013 14023.319 7.023 0.009 6.604 0.008
11742.418 7.043 0.007 6.645 0.006 14337.372 7.057 0.008 6.655 0.004
11744.390 7.055 0.011 6.661 0.008          

The average J and K magnitudes and J–K color indices of Cyg X-1 for some intervals of our observations (1995-2007, 1984-1994, and 1984-2007) are collected in Table 2. The standard errors (se) for average magnitudes, maximal and minimal magnitudes, and numbers of averaged values (N) are given. It follows from Table 2 that the source became brighter (its JK brightness increased) and hotter (its J–K color index decreased) during 1995-2007, compared to 1984-1994.

Table 2. The mean JK magnitudes and J–K color index for several intervals of observations of Cyg X-1 (1984-2007)

Parameter Mean se Max Min  
1995-2007
J 7.021 0.004 6.93 7.15 110
K 6.606 0.004 6.52 6.68 107
J–K 0.412 0.001 0.37 0.44 105
1984-1994
J 7.046 0.005 6.91 7.22 83
K 6.612 0.005 6.44 6.70 73
J–K 0.433 0.004 0.35 0.59 70
1984-2007
J 7.032 0.003 6.91 7.22 193
K 6.608 0.003 6.44 6.70 180
J–K 0.420 0.002 0.35 0.59 175

Variations of JK Magnitudes and J–K Color of Cyg X-1 in the Course of Orbital Motion

Figure 2 displays variations of the J and K brightness of Cyg X-1 versus t