A Search for the Orbital Period of LMXB X0614+091

Abstract



We present extensive new photometry of the low mass X-ray binary (LMXB) X0614+091. Discovered in 1974, its X-ray properties have identified it as a member of the so-called Atoll sources. Atoll sources are thought to have main sequence mass donors and relatively short (< 8 hours) orbital periods. Early observations of the faint (V=18.5) optical counterpart revealed an unusual spectrum that only showed the NIII/CIII 4640/50 emission complex. LMXB spectra usually always also exhibit HeII 4686 emission. Previous variability searches detected a 5.2 day modulation in X-rays (Marshall & Millit 1981) and a ~10 day periodicity in the optical (Machin et al. 1990). If either one of these modulations reflects the orbital period, mass transfer via Roche lobe overflow would require a giant secondary, inconsistent with the observed optical properties of the source. An alternative cause for the variations is precession of the accretion disk in the system, though it is not clear how these two periods can be reconciled.

We obtained V band observations of X0614 with the queue scheduled YALO telescope at CTIO between UT 1999 Dec 10 and 2000 Jan 31 in order to confirm the reported ~10 day variability and search for an orbital period. Comparison with Rossi X-ray Timing Explorer (RXTE) All Sky Monitor (ASM) data reveals that some of the optical observations occurred during a rare state of decreased X-ray variability of the source. Comparison of optical observations from the periods of high and low X-ray variability shows several differences. First, optical variability increases during the period of high X-ray variability, with an increase in the amplitude of variability from 0.1 mag to 0.25 mag. In addition, this increased optical variability seems to be over longer time scales than during the X-ray constant period. Second, the average optical magnitude decreases during the period of high X-ray variability, while the average X-ray magnitude increases.

We used the CLEAN algorithm to remove the sampling window and search for periodicities. No convincing periodicities were found in the complete optical lightcurve. Due to the differences in optical variability during the high and low X-ray variability states the data was split into two sets for further analysis. For the low X-ray variability state a peak in the power spectrum occured at 1.06 days. For the high X-ray variability state the highest peak in the power spectrum occurs at 1.11 days. Figure 4 shows a strong periodicity at this peak. No evidence of a 5.2 day periodicity was found. While the complete data set does not show a 9.8 day periodicity, the optical lightcurve of just the high X-ray variability state folded on 9.8 days reveals a possible periodicity. No comparable (~10 day) periodicity was detected in the low X-ray variability state.

Figures:



Figure 1: Optical (red circles) and X-ray (blue circles) lightcurve of X0614+091 showing anticorrelation of optical and X-ray brightness. A comparison star (black circles) is also plotted. JPEG (40 kb) or Postscript (59 kb)

Figure 2: Optical (open circles) and X-ray (solid circles) lightcurve of X0614+091 showing rare state of decreased X-ray variability. JPEG (45 kb) or Postscript (57 kb)

Figure 3: Optical light curve of X0614+091 during decreased X-ray variability state, folded on a period of 1.06 days. JPEG (23 kb) or Postscript (26 kb)

Figure 4: Optical light curve of X0614+091 during increased X-ray variability state, folded on a period of 1.11 days. JPEG (23 kb) or Postscript (28 kb)

Figure 5: Optical light curve of X0614+091 during increased X-ray variability state, folded on a period of 9.8 days. JPEG (22 kb) or Postscript (28 kb)

RXTE ASM data provided by the ASM/RXTE teams at MIT and at the RXTE SOF and GOF at NASA's GSFC




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