GCN Circular 8166

SGR 0501+4516: Spin Down Rate and Inferred Dipole Magnetic Field

Peter M. Woods (Dynetics), Ersin Gogus (Sabanci University), Chryssa
Kouveliotou (NASA/MSFC), report on behalf of a larger collaboration:

We continued monitoring the new Soft Gamma Repeater SGR 0501+4516
(Barthelmy et al. 2008, GCN # 8113, Palmer et al. 2008, ATel # 1678)
using our SGR ToO Program with RXTE. We acquired a total exposure of 40
ks in 13 RXTE pointings performed over a time span of ~7.5 days. The
time spacing between each pointing ranged between 0.1 - 0.9 d. We detect
numerous (between 1 to 50) short bursts in observations before 2008
August 28. We clearly detect the 5.76 s pulsations reported by Gogus et
al. 2008 (ATel # 1677) in the 2-10 keV PCA data. We employed a quadratic
spline fit to phase residuals and measured the spin period, P =
5.762067(2) s and the spin-down rate, Pdot = 1.5(5) x 10^-11 s/s. Note
that our spin period measurement is consistent with that reported by
Mangano et al. (2008, ATel # 1682). Assuming that the neutron star slows
down via magnetic dipole radiation, we infer a dipole magnetic field of
B = 3 x 10^14 G, confirming the new SGR as a new magnetar candidate.