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Swift discovery of 328s coherent pulsations from the HMXB IGR J17200-3116

ATel #3205; E. Nichelli, G. L. Israel (INAF OA Roma), A. Moretti, S. Campana (INAF OA Brera), D. Götz (Irfu/SAp/CEA Saclay), L. Stella (INAF OA Roma) on behalf of a larger collaboration
on 3 Mar 2011; 11:04 UT
Credential Certification: GianLuca Israel (gianluca@mporzio.astro.it)

Subjects: X-ray, Binary, Neutron Star, Pulsar

As part of a systematic search for coherent periodicities in the Swift XRT X-ray light curves of relatively bright (more than 200 photons each) serendipitous sources (currently more than 2500; see also Nichelli et al. 2009, ATel #2354), we detected a highly significant (>10sigma) coherent signal in the X-ray light curve of IGR J17200-3116, corresponding to about 328s.

IGR J17200-3116 is a persistent X-ray source first discovered by ROSAT, and then detected by INTEGRAL during an ultra deep survey of the Galactic Center (Revnivtsev et al. 2004, AstL, 30, 382). Masetti et al. (2006, A&A, 459, 21) classified this source as an High Mass X-ray Binary (HMXB) through optical spectroscopy of the counterpart.

The source was first observed by Swift-XRT between 2005 October 26th and 27th, with a total exposure time of 11.4ks (count rate of 0.16 +\- 0.01 counts/s). A strictly periodic signal at about 326.28(1)s (all uncertainties are preliminary and at 1 sigma c.l.) was detected in the power spectrum of the X-ray light curve. The modulation was non-sinusoidal, with pulsed fractions (semi-amplitude of the modulation divided by the mean source count rate) of about 35 % and 20 % for the first and the second harmonic, respectively. The spectrum was well fitted by an absorbed power law with Gamma = 1.0(1), NH = 1.6(3) x 1022 cm-2 and a flux of 1.6 x 10-11 erg cm-2 s-1 (1 - 8 keV band).

The signal could not be confirmed in a ~5ks-long Chandra snapshot carried out on September 2007 owing to poor statistics. We asked and obtained Swift ToO pointings on October 2010 and February 2011 which allowed us to confirm the presence of the pulsations. During February 2011 observations we obtained a refined phase-coherent timing solution with a best period of P = 328.182(3)s and an upper limit to the first period derivative of about 1.5 x 10-8 s/s (with a > 3 sigma significance). The latter value is consistent with the long-term period derivative of about 1.2 x 10-8 s/s inferred from the comparison of the 2005 and 2010/2011 periods.

We collected INTEGRAL archival pointings taken from August 2003 to August 2006 and folded the source's light curve to the best period inferred from Swift. We found that, in the 18 - 30 keV band, IGR J17200-3116 emission is also modulated at the 328 s period, with a pulsed fraction of about 25 - 30 per cent. The 18 - 100 keV flux is about 2 x 10-11 erg cm-2 s-1. The 7 years INTEGRAL light curve of IGR J17200-3116 does not suggest the presence of outbursts or strong variability.

First Results from SATANASS @ BAR