The Orbital Period of 4U 1210-64

ATel #1861; R. H.D. Corbet and K. Mukai (UMBC, CRESST/NASA GSFC)
on 4 Dec 2008; 23:37 UT
Credential Certification: Robin Corbet (corbet@gsfc.nasa.gov)

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The nature of 4U 1210-64 (= 1ES 1210-646) is not yet clear. Revnivtsev et al. (2007, ATEL #1253) propose that 4U 1210-64 may be an intermediate polar while Masetti et al. (2008, A&A in press, arXiv:0811.4085v1) find that the source is a high-mass X-ray binary.

The RXTE ASM light curve of 4U 1210-64 obtained between MJD 50,087 and 54,797 shows that the source has exhibited different states with relatively low or high flux. We identify three main states: a high state followed by a low state and then another high state. During the interval from MJD 50,087 to 50,780 ("epoch 1") the mean flux was 0.62 +/- 0.01 cts/s (8.2 mCrab). Between MJD 50,780 to 53,500 ("epoch 2") the mean flux was 0.15 +/- 0.01 cts/s (2 mCrab). From MJD 53,500 to 54,797 ("epoch 3") the mean flux was 0.45 +/- 0.01 cts/s (6 mCrab). The errors are statistical and systematic errors during all three epochs are expected to be approximately 0.1 cts/s.

A power spectrum of the ASM light curve from only the high flux states (i.e. excluding epoch 2) shows a highly significant modulation at a period near 6.7 days. This period is expected to be the orbital period of the system. In order to parameterize the modulation in a simple way we fitted a sine wave to the light curve and obtained a period of 6.7106 +/- 0.0006 days and epoch of maximum flux = MJD 54,006.0 +/- 0.2. However, when the folded light curve is examined, the modulation is not strictly sinusoidal. Maximum and minimum fluxes occur at phases of approximately 0.93 and 0.56 respectively. The shape of the folded light curve near minimum suggests the possibility that it is caused by an eclipse, although additional observations are needed to confirm this.

An RXTE PCA observation of 4U 1210-64 was performed starting on 2008-12-04 at 00:33 UT (MJD 54,804.02), near the time of predicted orbital maximum, with a 10ks exposure. In a preliminary spectral analysis of the PCU 2 data, a good fit (reduced chi2 = 1.15) was obtained over the 2.5 - 20 keV range using a power law continuum (photon index 1.41) with an exponential cut-off (Ecut = 6.0 keV, Efold = 5.7 keV) and an Fe K line at 6.56 keV (equivalent width = 300 eV). The inferred flux is 3.5e-10 ergs/cm2/s, roughly corresponding to 1.1 cts/s with the ASM (14 mCrab).

Both intermediate polars and HMXB neutron star systems are expected to exhibit pulsations. We therefore searched for pulsations in the PCA light curve. None were detected in a period range of 0.06 to 1,000s. For periods longer than 1,000 s a search for pulsations is complicated by the variability on timescales of hours seen in the light curve which gives a strong red noise component to the power spectrum.

Further observations of this source over the 6.7 day cycle at X-ray and optical wavelengths are encouraged.