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Chandra Location of SGR 0418+5729

ATel #2159; Peter M. Woods (Dynetics), Stefanie Wachter (IPAC/Caltech), Ersin Gogus (Sabanci Univ.) & Chryssa Kouveliotou (NASA/MSFC) report on behalf of a larger collaboration:
on 14 Aug 2009; 19:05 UT
Credential Certification: Peter Woods (Peter.Woods@dynetics.com)

Subjects: X-ray, Neutron Star, Soft Gamma-ray Repeater, Transient, Pulsar

Referred to by ATel #: 2164, 2169, 2244

The recently discovered Soft Gamma Repeater SGR 0418+5729 (GCN #9499, ATEL #2077) was observed with the Chandra HRC-I on 2009 July 12 for an exposure of 24 ks. The brightest X-ray source in the image was clearly identified as the SGR through detection of the 9 sec pulsations. Two additional, uncatalogued X-ray sources were discovered within 3 arcminutes of the telescope boresight, but well separated from the position of the SGR.

No IR or optical counterparts were found in the 2MASS and DSS archives. However, a deep IR frame obtained with the WIRC on the Palomar Hale telescope revealed unambiguous IR counterparts for the two fainter X-ray sources in our Chandra image. The WIRC image was registered to the 2MASS catalog to within 0.1" (1 sigma) and thus allowed the X-ray frame to be aligned with the 2MASS astrometric calibration. A systematic offset in right ascension (0.33") and no offset in declination were found between the X-ray and IR positions of these two sources.

The registered locations of the three X-ray sources are as follows:

    SGR 0418+5729
    RA = 04h 18m 33.867s
    DEC = +57d 32m 22.91s
    CXOU J041819.0+573341
    RA = 04h 18m 19.097s
    DEC = +57d 33m 41.75s
    CXOU J041812.5+573154
    RA = 04h 18m 12.578s
    DEC = +57d 31m 54.99s

The absolute positional uncertainty on the SGR position is 0.35" (95% confidence). We find our position to be consistent with the original Swift XRT location (ATEL #2121), but significantly offset from the refined XRT position (ATEL #2127).

We thank Harvey Tananbaum and the Chandra team for carrying out this ToO observation. We gratefully acknowledge Chris and Dawn Gelino for contributing their WIRC image for registration of the X-ray data.