A preliminary orbital solution for the transient eclipsing pulsar in Terzan 5

ATel #2939; A. Papitto, L. Burderi (Univ. Cagliari), A. D'Ai', T. Di Salvo (Univ. Palermo), S. Motta, T. Belloni (INAF-OAB), A. Sanna (Kapteyn Astr. Inst.), A. Riggio (INAF-OAC)
on 15 Oct 2010; 06:02 UT
Credential Certification: Alessandro Papitto (papitto@oa-roma.inaf.it)

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Considering the observations of the transient source in the globular cluster Terzan 5 (ATEL #2919, #2920, #2922, #2924, #2929, #2932, #2933, #2935, #2937), performed by RXTE between 2010 Oct 13.009 and 14.879 (for a total exposure of 23ks), we could derive a preliminary estimate of the orbital solution of the 11.043847(41) Hz, newly discovered pulsar.

As the ephemerides of the satellite have not been released yet at the time of writing this telegram, we added to the circular model considered to describe the orbit of the pulsar, also a sinusoidal component of period 93.3 +/- 0.7 min to take into account the satellite orbit.

The Doppler shifts affecting the spin frequency of the pulsar indicate the following orbital parameters:
Porb = 21.327 +/- 0.086 hr;
asini/c = 2.46 +/- 0.12 lt-s;
T^* = 55481.783 +/- 0.007 MJD;
where T^* is the epoch of zero mean longitude.

Under the assumption that the orbit is circular, we find that the value of T^*+Porb/4=55482.005(7) MJD, derived according to the proposed orbital solution, is consistent with the mid eclipse epoch of the only eclipse observed so far (Tecl=55482.023320(2) MJD, ATEL #2929). As the considered observations cover only a limited fraction (0.14) of the whole orbital cycle, and the precise ephemerides of RXTE are not applied, more observations will be needed to confirm and refine this solution.

The source shows quite regularly type I X-ray bursts. A total of 25 bursts has been observed since 2010 Oct 13.009, while the recurrence time has decreased from ~1000s to ~700 since the first RXTE observation. The 2-20 keV unabsorbed flux emitted by the source also increased from 1.7(2)E-09 erg cm**-2 s**-1 to 6.60(2)E-09 ergs cm**-2 s**-1. Assuming a distance of 5.5 kpc for Terzan 5, we infer a luminosity of ~1.6E37 erg/s.