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ATEL # 2996; L.J. Townsend (Southampton), R.H.D. Corbet (NASA/GSFC), M.J. Coe (Southampton), A.B. Hill (Southampton)
on 1 Nov 2010; 17:38 UT
Password Certification: L. J. Townsend (firstname.lastname@example.org)
Subjects: X-ray, Binaries, Neutron Stars, Pulsars
RXTE PCA observations of an ongoing Type II X-ray outburst from the SMC Be/X-ray binary XTE J0103-728 (SXP6.85) reveal a periodicity of 21.86 ± 0.11 days in pulse period measurements obtained between MJD 55435.76 to 55488.64. We interpret this as being due to orbital Doppler modulation of the neutron star spin period. From a fit to the pulse period measurements we derive:
Porbital = 21.86 ± 0.11 days
Eccentricity = 0.26 ± 0.03
Ppulse = 6.8507 +/- 0.0001 seconds (on MJD 55465.49)
Pdot = (8.4 ± 0.5)E-10 s/s
Time of periastron passage = MJD 55457.5 ± 0.4
A plot of the pulse period measurements and best fit model can be found at www.astro.soton.ac.uk/~mjc/sxp6.85.ps
A Swift TOO observation made on 2010-10-26 also detected this source. The XRT gives a position of RA = 1:02:53, dec. = 72:44:31 (J2000) with a 4" error. This is consistent with the position found with XMM (Haberl et al., 2007, ATel #1095), confirming the correct identification of the optical counterpart. We did not detect any pulsations in the Swift light curve, most likely due to the low count rate.
The orbital period we derive is quite different from the periods previously suggested by McGowan et al. (2008, MNRAS, 384, 821) and Galache et al. (2008, ApJS, 177, 189). These authors found that a period of ~112-114 days was present in both the optical and X-ray fluxes at a low level of significance. Thus, if the ~114 day periodicity is physical, it may be a superorbital period, although non-periodic variations on this timescale cannot yet be completely excluded. We note that the combination of a 22 day orbital period and 6.85 s pulse period is consistent with the Be star classification for this system (Corbet, 1986, MNRAS, 220, 1047).