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Swift-XRT detection of x-ray pulsations from IGR J18173-2509 and SWIFT J2138.8+5544

ATel #2354; E. Nichelli, G. L. Israel (INAF OA Roma), A. Moretti, S. Campana (INAF OA Brera), S. Bernabei (INAF OA Bologna), E. Mason (ESO Chile), D. Götz (Irfu/SAp/CEA Saclay), L. Stella (INAF OA Roma) on behalf of a larger collaboration
on 17 Dec 2009; 16:13 UT
Credential Certification: GianLuca Israel (gianluca@mporzio.astro.it)

Subjects: Optical, X-ray, Binary, Cataclysmic Variable

Referred to by ATel #: 3205

As part of a systematic search for periodicities in the Swift-XRT public light curves (energy range 0.5-10 keV) performed with the ad hoc developed software Swift Automatic Timing ANAlysis of Serendipitous Sources at Brera And Roma Astronomical Observatories (SATANASS @ BAR), we discovered two new X-ray pulsators. A periodic signal at about 830.70 +/- 0.02 s (uncertainties are at 1 sigma c.l.; power spectrum detection significance of > 5 sigma) was detected in the X-ray flux of IGR J18173-2509, an INTEGRAL source recently classified as a Cataclysmic Variable (CV) through optical spectroscopy (Masetti et al. 2009, A&A, 495, 121). The source was observed by Swift-XRT between 2008 March 5th and 8th, for a total effective exposure time of 13700 s, at a level of 0.08 count/s. The analysis of Chandra archival observation carried out on May 2008 (5000s of effective exposure time) including the position of IGR J18173-2509, showed a strong signal at about 1660s, suggesting that the true period of the source is the latter, twice that detected in Swift. The 1660s modulation was double peaked and energy dependent, with a pulsed fraction (semi-amplitude of the modulation divided by the mean source count rate) of about 50-percent for the (dominant) second harmonic. The source flux has been already reported in literature by Landi et al. (2008, ATel #1437) and Tomsick et al. (2009, ApJ, 701, 811) at a level of 1.3 x 10-11 erg cm-2 s-1 (2 - 10keV) and 8.5 x 10-12 erg cm-2 s-1 (0.3 - 10keV), respectively. For the estimated distance of 330 pc (Masetti et al. 2009) this translates to a luminosity Lx = (1 - 1.6) x 1032 erg s-2.

We also detected 990 s period pulsations (significance of 9 sigma) from a serendipitous source, named SWIFT J2138.8+5544 which was found in the field of view of GRB 050422. Swift-XRT took 11 pointings of the source between 2005 April 22nd and 2005 May 6th, for a total effective exposure time of about 200 ks. The source was observed at a level of about 0.007 count/s. The signal had a barycentric period of 989.167 +/- 0.001 s (1 sigma c.l.) and had a nearly-sinusoidal shape with an 85-percent pulsed fraction. The spectrum was well fitted by two absorbed black bodies with kT_1 = 2.0 +/- 0.1 (1 sigma c.l.) keV and kT_2 = 0.29 +/- 0.08 keV (1 sigma c.l.), NH = 6 x 1021 cm-2 and a flux of 1.6x 10-12 erg cm-2 s-1. The galactic coordinates (l = 98.1 b = 2.4) imply a position nearly on the galactic plane, far away from the galactic centre, and a distance ranging from 1 kpc to 10 kpc, translating to a luminosity from 0.02 to 2 x 1034 erg s-1. The 2.4arcsec accurate Swift XRT resulted in the identification of two optical candidate counterparts, the brightest (R.A. = 21 38 49.91 Dec. = +55 44 05.6, V = 18, K = 14.9) being listed in the IPHAS catalog (Witham et al. 2008, MNRAS, 384, 1277) as an Halpha emitter. We carried out low-resolution spectroscopy for both candidates using the 1.52 m telescope at Loiano Observatory. Data were acquired on 2009 October 26th with an exposure of 3600 s, a spectral coverage between 3400 and 8500 A and a dispersion of 4 A/pix. No relevant spectral features were found for the faintest object, while the brightest one showed several H and He emission lines, making its association with the SWIFT J2138.8+5544 very likely.
Based on these results, the most likely interpretation is that SWIFT J2138.8+5544 be a CV.

First Results from SATANASS @ BAR