Swift XRT follow-up of the transient AXP 1E1547.0-5408 in outburst

ATel #1763; P. Esposito (INAF/IASF-Milan), G. L. Israel (INAF-OAR), N. Rea (U. Amsterdam), H. A. Krimm (CRESST/GSFC/USRA), D. M. Palmer (LANL), N. Gehrels (GSFC), S. D. Bart helmy (GSFC), S. Mereghetti, A. Tiengo (INAF/IASF-Milan), D. Gotz (CEA-Saclay)
on 4 Oct 2008; 19:31 UT
Credential Certification: GianLuca Israel (gianluca@mporzio.astro.it)

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We report on the preliminary results of the first Swift-XRT observations of the transient AXP 1E1547.0-5408 (Krimm et al. GCN 8311, 8312). We analyzed the observation performed on 2008, October 3rd (orbit 1 : UT 9:31 - 9:51, orbit 2 : UT 10:54 - 11:28, and orbit 3 : UT 13:34 - 12:53) in PC mode, and on 2008, October 3rd-4th (UT 12:53 - 01:37) carried out in WT mode.

Preliminary spectral study of these observations found the source at a persistent flux >50 times with respect to its quiescent level, and with a spectrum significantly harder than the pre-burst one.

Fitting simultaneously the pre-burst and the post-burst spectra with an absorbed power-law gives the following parameters (with an N_H = (5.1±0.6)×10^22 cm^-2; 90% c.l. are reported).

-- Pre-bursts (XRT July-Sept 2008):
powerlaw PhoIndex 5.4±0.5
2-10 keV observed flux 8×10^-13 ergs cm^-2 s^-1

-- First orbit (2008-10-03 UT 9:31 - 9:51):
powerlaw PhoIndex 1.9±0.3
2-10 keV observed flux 7×10^-11 ergs cm^-2 s^-1

-- Second orbit (2008-10-03 UT 10:54 - 11:28):
powerlaw PhoIndex 2.9±0.3
2-10 keV observed flux 5×10^-11 ergs cm^-2 s^-1

-- Third orbit (2008-10-03 UT 13:34 - 12:53):
powerlaw PhoIndex 2.6±0.3
2-10 keV observed flux 5×10^-11 ergs cm^-2 s^-1

-- WT data (2008-10-03 UT 12:53 - 2008-10-04 UT 01:37):
powerlaw PhoIndex 3.2±0.2
2-10keV observed flux 4×10^-11 ergs cm^-2 s^-1

We note that the time interval of the first orbit includes a number of bursts detected by BAT (GCN 8318).

Timing analysis was performed with data carried out in WT mode after having removed bursts and corrected the photon arrival times to the barycenter of the Solar System. Pulsations at about 2s were clearly detected. A refined period measurement was obtained by means of a phase fitting technique, and corresponds to a best-fit value of P=2.071336(3)s (1 sigma c.l.). The pulse shape is mainly sinusoidal with a background-subtracted pulsed fraction of about 25±1%. Period, pulsed fraction and pulse shape are significantly different from those reported in literature (P=2.069961s , PF=4%; Camilo et al. 2008, ApJ, 679, 681; Halpern et al. 2008, ApJ, 676, 1178).

Further Swift GO observations are planned in order to monitor the new outburst of 1E1547.0-5408 during the next weeks.