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ATEL # 2568; T. Belloni (INAF-OAB, Italy), L. Stella (INAF-OAR, Italy), E. Bozzo (ISDC, Switzerland), G. Israel (INAF-OAR, Italy), S. Campana (INAF-OAB, Italy)
on 16 Apr 2010; 18:09 UT
Distributed as an Instant Email Notice (Transients)
Password Certification: Tomaso Belloni (email@example.com)
Subjects: X-ray, Binaries, Neutron Stars, Transients
Referred to by ATEL #: 2569, 2576
We analyzed the spin period evolution of Swift J1749-2807 during four RXTE observations, the two reported in ATel #2567 and ATel #2565, plus two observations beginning at April 15 21:42UT and April 16 4:57UT. The spin frequency was observed to vary between 1035.6 and 1036.3 Hz (not barycentered). A preliminary fit with a sinusoid yields an orbital modulation period of 3.19x10^4 seconds, corresponding to 8.86 hr. The amplitude of the frequency modulation is 0.39 Hz and the mean frequency is 1035.94. This corresponds to a projected semimajor axis of 1.9 light seconds. With these values, we obtain a mass function of 0.05 solar masses. Assuming mass transfer driven by Roche Lobe overflow, a mass of about 1 solar mass is derived for the companion star. Further RXTE observations are planned.