Swift Observations of SGR 0418+4729

ATel #2127; J. R. Cummings (NASA/GSFC/CRESST), K. L. Page (U Leicester), A. P. Beardmore (U Leicester), N. Gehrels (NASA/GSFC) on behalf of the Swift Team
on 13 Jul 2009; 13:31 UT
Credential Certification: Jay R Cummings (jayc@milkyway.gsfc.nasa.gov)

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The SGR 0418+5729 (van der Horst et al., GCN #9499, ATel #2077) recently came out of Sun constraint for Swift. A bright, periodic source is found with the XRT. BAT and UVOT do not detect any concurrent emission at the location of the SGR. We have analysed 19 ks of Swift-XRT Photon Counting mode data, collected between 20:52:35 on 2009 July 08 and 07:01:56 on July 10. Using 13.6 ks of XRT Photon Counting mode data and 15 UVOT images, we find an astrometrically corrected X-ray position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue, see Evans et al., http://arxiv.org/abs/0812.3662 for details) of RA, Dec 64.64172, +57.54013 degrees, which is equivalent to RA (J2000): 04h 18m 34.01s Dec (J2000): +57d 32m 24.5s with an error radius of 1.9 arcsec (90% confidence). The mean 0.3-10 keV count rate over the time of the observations is 0.294 ± 0.004 count s-1, corresponding to an observed (unabsorbed) flux of 1.41x10-11 (1.53x10-11) erg cm-2 s-1 over the same energy band. There may be a long-term modulation with a period of a day or more; however, since our data only span 1.5 days, this cannot be well constrained. A spectrum extracted from the entire data set can be best-fitted by an absorbed power-law, with BB kT = 915 +26/-17 eV and NH = (2.1 +0.3/-0.4)x1021 cm-2. An absorbed power-law is a statistically worse fit to the data (chi2 is 100 higher for the power-law fit). The XRT was in Photon-Counting mode, with a 2.5 s timing resolution. Hence determination of the period of the source is imprecise. We determine the period to be 9.078 +- 0.002 seconds (90% confidence), matching the period found for the RXTE source in Gogus et al. (ATel #2076, and as mentioned in ATel #2121, which used part of the data reported here). Further Swift observations of the source are scheduled with the XRT in Window Timing mode in response to a TOO request by Kouveliotou. This circular is an official product of the Swift team.