Peremennye Zvezdy (Variable Stars) 28, No. 7, 2008 Received 15 Juny; accepted 15 July.
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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. |
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.
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.
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). |
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 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.
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.
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 |
Figure 2 displays variations of the J and K brightness of Cyg X-1 versus t