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Evidence for the first period derivative of SGR 0418+4729

ATel #2209; G. L. Israel (INAF-OA Roma), P. Esposito (INAF-IASF Milano), D. Gotz (Irfu/SAp/CEA Saclay) and N. Rea (CSIC-ICE Barcelona) report on behalf of a larger collaboration
on 23 Sep 2009; 18:23 UT
Credential Certification: GianLuca Israel (gianluca@mporzio.astro.it)

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

We observed SGR 0418+4729 with XRT in PC mode on 2009 September 20-22 (for a total of about 17ks of effective exposure time). By merging these dataset with those already obtained by RXTE and Swift since June 2009, we were able to infer a phase-coherent timing solution also including a first period derivative component (see ATel \#2151 and \#2152 for previous upper limits on it).

According to our preliminary analysis the period and first period derivative of SGR 0418+4729 are: P=9.0783876(3)s and Pdot= 3.0(9) x 10^-13 s/s (1 sigma uncertainties; epoch 54993.0 MJD). Correspondingly, the 3sigma interval for the first period derivative ranges between 4 x 10^-14 and 5.6 x 10^-13 s/s.

Therefore, the magnetic field strength of the dipolar component is approximately (assuming a neutron star radius of 10km and a mass of 1.4 solar masses) Bd = 5(1) x 10^13 Gauss (1sigma c.l.), still in the magnetar range.

Further observations are needed during the next months in order to better constrain the timing parameters (and the magnetic field strength).