Sub-arcsecond position of the the transient 11-Hz X-ray pulsar in the Globular Cluster Terzan 5 derived from a Moon occultation observed by RXTE

ATel #3892; A. Riggio (INAF-OAC, Italy), L. Burderi (Univ. Cagliari, Italy), T. Di Salvo (Univ. Palermo, Italy), A. Papitto (CSIC-IEEC, Spain), E. Egron (Univ. Cagliari, Italy), T. Belloni (INAF OAB, Italy), A. D'Ai', R. Iaria (Univ. Palermo, Italy), M. Floris (CRS4, Italy), S. Motta (INAF/Brera), V. Testa, M. T. Menna (INAF-OAR)
on 28 Jan 2012; 11:02 UT
Credential Certification: Alessandro Riggio (riggio@dsf.unica.it)

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During an RXTE observation of IGR J17480-2446 performed on 2010 Oct 13, the transient X-ray pulsar in the Globular Cluster Terzan 5 (ATels #2919, #2920, #2922, #2924, #2929, #2932, #2933, #2937, #2939,#2940, #2946, #2952, #2958, #3000, #3044, #3264) was serendipitously eclipsed by the Moon for about 511 s (ATel #2946).

Using information on the RXTE position and orbit at the time of the eclipse we have reconstructed the precise position of the Moon at the eclipse ingress and egress as seen from the satellite.

The start and end times of the eclipse were measured down to an accuracy of 0.007 s (1 σ c.l.).

To reach a sub-arcsecond accuracy, we have considered the lunar topography, taking into account lunar libration in order to find the precise locations on the Moon surface were the eclipse ingress and egress took place.

For the lunar topography, we used data (courtesy of JAXA) from the laser altimetry instrument LALT on board the Japanese lunar explorer Kaguya (SELENE)(Araki et al. 2009,2010). The radial precision is < 5m (1 σ c.l.) on a grid spaced of ~2 km at the lunar equator.

Because of parallax effect, the geocentric libration differs from the topocentric libration (where topocentric refers to RXTE position) up to about 1 deg.

We find that the ingress position is at selenographic coordinates (Lon 96.72°, Lat 2.16°), slightly south of the Babcock H Crater, and west of Fox A Crater, close to the Moon rim.

The egress position, at selenographic coordinates (Lon -78.92°, Lat -70.58°), is ~25km west of the Bailly U crater, a lunar impact crater located in the southern limb of the Moon.

We find the following position for IGR J17480-2446:

RA = 17h 48m 04.8245(26) s
DEC = -24° 46' 48.88(4)"

Taking into account all the uncertainties discussed above, and including the error on the Rossi XTE satellite position, we determine an error of ~0.04" (1 σ c.l.).

This position is at 1.5 σ from the Chandra position reported in Heinke et al. 2006, confirming the association made by Pooley at al. 2010 (ATel #2974).

We acknowledge JPL's Horizons online ephemeris generator for providing the Earth and Moon position during the observations, and JAXA for providing Kaguya/LALT data.

In figure 1 the details of the Moon rim as seen by RXTE at eclipse ingress and egress, together with a detail of the intersection zone. The two black circles represent the 1 σ c.l. around the Chandra and RXTE positions, while the gray circle represent the 2 σ c.l. around the RXTE position.

In figure 2 the lunar topography in the estimated points of eclipse ingress and egress are reported.