A Refined Orbital Solution and the Transient Pulsar in Terzan 5 is Not Eclipsing

ATel #2946; T. E. Strohmayer (NASA/GSFC), C. B. Markwardt (NASA/GSFC), D. Pereira (GSFC/Wyle IS), E. A. Smith (GSFC/Wyle IS)
on 15 Oct 2010; 21:34 UT
Credential Certification: Tod Strohmayer (tod.strohmayer@nasa.gov)

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We report results of recent RXTE observations of the new bursting and pulsing transient in Terzan 5 (ATels #2919, #2920, #2922 ,#2924, #2929, #2932, #2933, #2937, #2939, #2940). Barycentric pulse frequency measurements from eight orbits of data spanning 00:13 on 2010-10-13 to 2010-10-15 at 10:53 UTC are consistent with a circular orbit with parameters;

Orbital Period = 21.252 +- 0.02 hrs; Pulsar frequency = 11.044893 +- 8.7 x 10-6 Hz; vsini = 62.13 +- 0.46 km/sec; asini = 2503.12 +- 19 lt-ms; Epoch of T90 = 55482.8883 +- 0.0013 MJD (TDB)

The epoch of T90 based on this solution is approximately 47 minutes earlier than the mid-eclipse point (ATEL #2929), which is inconsistent if the orbit is indeed circular (ATEL #2939). Fits with an eccentric orbit do not improve the fit, suggesting the orbit is circular to the accuracy of the present data. Moreover, an eclipse ephemeris derived from the above solution suggests that an eclipse should have been observed in the orbit beginning near 2010-10-14 18:30 UTC, but none was seen. These considerations raised concerns about the interpretation of the eclipse event. It subsequently came to our attention that the Moon was near to the position of the transient during the epoch of the putative eclipse. Closer study confirms that the event originally identified as an eclipse intrinsic to the new transient is in fact a lunar occultation of the source, and not an eclipse. RXTE continues to observe this interesting source.