We present photometry results of the search for the orbital period of the low mass X-Ray binary (LMXB) system 4U 1556-60. The orbital period is the first parameter needed before the other geometric parameters and physical properties of a binary system can be determined. The optical counterpart for the X-ray source of 1556-60 was identified by Charles et al. (1979), and subsequent observations failed to detect significant periodicity in optical variations. Smale (1991) proposed a possible period of 9.1hrs (0.3807 days) but added that a further independent study would be needed for confirmation.
The data obtained for this project were obtained on three separate runs, the first with the CTIO 1.5 meter telescope from April 29 to May 5, 1997, and the second and third with the CTIO 0.9 meter telescope from May 22 to 29, 2000 and from June 17 to 23, 2000. We have 170 400s V filter exposures from the 1997 run, and we have 174 450-900s V and 122 600-900s I exposures from the combined runs from 2000. We observed the source to vary between 0.2 and 0.6 magnitudes nightly as well as a larger time scale brightening visible in the 2000 light curve (Figure 2). The time coverage and size of our data sets are both improvements over previous published observations, and should be capable of revealing the orbital period if photometrically possible. Another goal of this project was to correlate simultaneous X-ray observations taken with the RXTE satellite over two days of the 1997 run, but there was not sufficient temporal overlap in the optical and X-ray data sets to perform the correlation.
We used the CLEAN program to search for periods in the data separated into two sets from the two epochs. We normalized the data from 2000 to the mean value to remove contributions from the overall brightening observed which we believe to be unrelated to the orbital period. The CLEAN program performs a fourier analysis of the data and iteratively subtracts spectral contributions due to sampling, such as gaps in the data and the finite width of the data set itself, and unfortunately, our light curve hints that the orbital period could be close to 1 day, which is confounded by the earth's rotation.
Analysis of the two data sets with CLEAN did not provide a definitive result for the orbital period of 1556-60. We did find candidate periods of 0.46229 days and 0.43993 days for the 1997 and 2000 data, respectively, and the data folded on those periods are presented in Figures 3 and 4. Neither data set provided any evidence for the 0.3807 day period proposed by Smale (1991). There are many factors that could complicate the search for the orbital period of this system. First, the inclination angle of the system could be low, greatly decreasing the amplitude of optical variations. Second, there are is a large nightly scatter in the light curve that could be caused by X-ray flickering of the accretion disk and nonuniform heating of the normal star due to the disk. We conclude that further photometry will not be able to detect the orbital period of 4U1556-60, and that spectroscopic radial velocity measurements are needed for further investigation of the system.