3 Long-term variability

We have investigated the long-term variability of this object by comparing the magnitudes derived from several Palomar Schmidt surveys. In the Guide Star Catalog GSC-ACT (Lasker et al. 1996-99), which is based on the "Quick-V'' Survey (the epoch for the relevant plate is 1982.6), the GSC V magnitudes for stars 2, 3 and 4 are 0.56 mag too bright with respect to the values in Table 2 and, correcting the catalog value for the variable, we derive V = 12.69, well within the range seen in Fig. 3.

 

 

Table 3: Magnitudes and colors of RX J1643.7+3402 at two different times on 23 September 2000.

V	U-B	B-V	$V-R_{\rm c}$	$R_{\rm c}-I_{\rm c}$
12.58	-0.95	-0.07	+0.02	-0.01
12.65		-0.04	+0.04	+0.02

For the POSS-I survey blue and red plates, the USNO A2 catalog (Monet et al. 1996) gives O = 15.2 and E = 13.2. While the blue magnitude is very much fainter than our values (by 3 mag), the Minnesota APS catalog (Pennington et al. 1993), also based on the POSS-I survey, does not list either the variable or star 2 due to their proximity to the nearby bright star HD 151087. Thus we consider the USNO magnitudes for the variable to be unreliable (this is confirmed by fact that the USNO blue magnitude for star 2 is also too faint (by 1.3 mag), while the values for stars 3 to 8 differ little (by -0.07 mag) from our B magnitudes). In order to remedy this situation, we measured magnitudes for the variable and the stars of Table 2 on the Digital Sky Survey (DSS) POSS-I Red image (Epoch 1954.4) using our aperture photometry software and obtain $R_{\rm c} = 12.7$ for the variable, consistent with our CCD photometry.

For the POSS-II survey, we measured magnitudes for the same stars using the DSS blue (epoch 1989.5) and red (epoch 1993.3) images and obtain B = 13.2 and $R_{\rm c} = 12.7$ for the variable. While the red magnitude is again in good agreement, the object appears fainter on the blue image by 0.6 mag.

We conclude that the brightness of RX J1643.7+3402 in the past does not appear to have undergone large variations relative to the values measured in September 2000.