Improved Swift-XRT position of SAX J1806.5-2215
ATel #3219; P. A. Evans (U. Leicester), P. Romano (INAF-IASFPA), J. P. Osborne (U. Leicester), J. A. Kennea (PSU), M. Del Santo (INAF/IASF-Roma), L. Sidoli (INAF/IASF-Milano)
on 14 Mar 2011; 19:25 UT
Credential Certification: Phil Evans (pae9@star.le.ac.uk)
Subjects: X-ray, Neutron Star, Transient
The neutron star transient/X-ray burster SAX J1806.5-2215 has been
localised by Swift (ATEL #3202) and Chandra (ATEL #3218), with
inconsistent positions.
The Swift result was obtained by using the UV/Optical telescope field
stars to correct the XRT astrometric frame (see Goad et al. 2007, A&A,
476, 1401; Evans et al 2009, MNRAS, 397, 1177). Examining the results of
the process in this case we note that the astrometric corrections
derived for each Swift observation show significant scatter. The UVOT
images show elongated stellar images in several observations, implying
that the attitude of the Swift spacecraft was not perfectly determined
on-board, thus invalidating the astrometric corrections at these times.
The UVOT image appears most stable during the observation with ID
00035714005, and we generated the enhanced Swift-XRT position using
only this observation. We find a position:
RA = 18h 06m 32.19s (= 271.63412 deg)
Dec = -22d 14' 16.5" (= -22.23791 deg)
with an uncertainty of 2.2" (radius, 90% confidence).
This position is 0.7" from the Chandra position in ATEL #3218,
in good agreement with that position.
Note that the problems encountered in the original analysis are rare,
and we regularly monitor the calibration of the position enhancement
process. Details of this calibration procedure are given in Goad et al.
and Evans et al.
