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Swift detection of the second period derivative in the timing evolution of SGR0501+4516

ATEL # 1837; G.L. Israel (INAF-OA Roma), P. Woods (Dynetics), S. Dall'Osso (INAF-OA Roma), P. Esposito (INAF-IASF, Milan), N. Rea (U. Amsterdam), C. Kouveliotou (NASA-MSFC), E. Gogus (Sabanci Univ.) V. Mangano, P. Romano (INAF-IASF Palermo), D. Palmer (LANL), N. Gehrels (GSFC), L. Stella (INAF-OA Roma), S. Zane (MSSL), on behalf of a larger collaboration
on 11 Nov 2008; 0:14 UT
Password Certification: GianLuca Israel (gianluca@mporzio.astro.it)

Subjects: Soft Gamma-ray Repeaters

The Swift satellite has been monitoring SGR0501+4516 (ATel #1676, #1677, #1678, #1682, #1683, #1688, #1691, #1692 and #1824) since the first burst detected by the BAT on board Swift on 22nd August 2008.

After about two months since the BAT event, we detect an additional component with respect to the phase-coherent timing solution previously reported which included only the period and its first time derivative (ATel #1692; P=5.7620699(4)s and Pdot=5(1) x 10^-12 s/s; 1 sigma uncertainties). Based on out preliminary analysis the new timing solution is (reference epoch 54701.0 MJD):

P=5.7620690(1) s

Pdot=7.4(1)E-12 s/s

Pdotdot=-4.3(1.1)E-19 s/s^2

The second time derivative Pdotdot is detected at about 4 sigma confidence level. The negative sign of Pdotdot implies that the spin-down is decreasing on a characteristic timescale of about half a year. This implies a transient increase of the spin-down above the secular trend, towards which the source is now recovering. We note that timing components of similar strength and evolution timescale were detected in other AXPs and SGRs following the occurrence of glitches (Dall'Osso et al. 2003, ApJ, 599, 485; Dib et al. 2008, ApJ 673, 1044). This finding suggests that a similar event might have connected to the burst and/or outburst behavior displayed by the source in August 2008.

We finally note that, even assuming that the secular spin-down was an order of magnitude smaller then currently measured, our findings still imply a magnetic field strength of the dipolar component in the range 7 x 10^13 < Bd < 2 x 10^14 Gauss (assuming a neutron star radius of 10km and a mass of 1.4 solar masses).


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