Swift/XRT observations of SGR 1627-41's flux decay

ATel #1555; P. Esposito (INAF-IASF Milano), G. L. Israel (INAF-OA Roma), R. Starling (U. of Leicester) D. Palmer (LANL), N. Gehrels (GSFC), S. Barthelmy (GSFC), S. Zane (UCL/MSSL), A. Tiengo (INAF-IASF Milano), N. Rea (U. of Amsterdam), A. De Luca (INAF-IASF Milano), D. Gotz (CEA Saclay), S. Mereghetti (INAF-IASF Milano)
on 3 Jun 2008; 12:33 UT
Credential Certification: Andrea Tiengo (tiengo@iasf-milano.inaf.it)

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The soft gamma-ray repeater SGR 1627-41 re-activated on May 28th 2008 with a series of hard X-ray bursts detected by Swift/BAT (Palmer et al., GCN #7777, ATEL #1548), RXTE/PCA (Woods et al., ATEL #1549) and Konus-Wind (Golenetskii et al., GCN #7778). Its persistent X-ray emission was detected in the same day by Swift/XRT at the highest flux level ever observed from this source (Palmer et al. GCN #7777, ATEL #1548).

Five further 2 ks long Swift/XRT observations in Photon Counting mode were performed starting on May 29th (about 30 hours after the first burst detected by BAT). In all pointings SGR 1627-41 is significantly detected in the 0.3-10 keV energy range and the following results are obtained:

2008-05-28 12:58:14-13:31:27 UTC: 0.067 ± 0.003 counts/s (GCN #7777)

2008-05-29 14:47:17-16:30:39 UTC: 0.015 ± 0.003 counts/s
2008-05-30 11:35:15-14:58:56 UTC: 0.010 ± 0.003 counts/s
2008-05-31 08:49:41-10:37:58 UTC: 0.007 ± 0.002 counts/s
2008-06-01 05:32:11-07:34:56 UTC: 0.008 ± 0.002 counts/s
2008-06-02 08:37:17-10:35:57 UTC: 0.011 ± 0.003 counts/s

(1-sigma c.l. uncertainties are reported above).

A spectrum of all Photon Counting (PC) mode data (11.4 ks of data, including the data from the observation on May 28th, GCN #7777) can be modelled with an absorbed power-law of Gamma = 1.5^+1.0_-0.7 and N_H = (11±5) ×10²² cm^-2 (1-sigma c.l.). This corresponds to a count rate to observed flux conversion of 1 count/s = 1.8×10^-10 erg cm^-2 s^-1 (0.3-10 keV).

The source has therefore significantly faded between the first and second XRT observations, separated by about 26 hours, during which time bursting activity was still being detected. The source then appears to have settled at a nearly constant flux level of about 2×10^-12 erg cm^-2 s^-1, which is a factor 20 above the quiescent level measured by XMM-Newton in September 2004 (Mereghetti et al. 2006, A&A 450, 759).

We note that no other bursts from SGR 1627-41 have been reported since May 28 at 18:27 UT (GCN #7778).

Further X-ray observations are scheduled (GCN #7787).

We thank the Swift Operations Duty Scientist and Flight Operations Team for performing this ToO.