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IGR J17597-2201 = XTE J1759-220

ATel #156; C. B. Markwardt (U. Maryland & NASA/GSFC) and J. H. Swank (NASA/GSFC)
on 1 May 2003; 14:09 UT
Credential Certification: Craig B. Markwardt (craigm@lheamail.gsfc.nasa.gov)

Subjects: X-ray, A Comment, Binary, Neutron Star, Transient

Referred to by ATel #: 1054

We report that the transient detected by Lutovinov et al. (ATEL #155), IGR 17597-220, is at a position consistent with a transient which has been active since at least 07 Feb 2001, when it was first detected in RXTE PCA galactic bulge scans.

Dedicated PCA scans on 19 May 2001 determined a best fit PCA position to be R.A. = 18h00.0m, Decl. = -22o03m (equinox 2000.0; estimated uncertainty 4'). The XTE designation of XTE J1759-220 was generated based on a preliminary position from the PCA bulge scans.

The X-ray flux has varied erratically between 2 and 17 mCrab (2-10 keV) since detection, with alternating calm and flaring periods. For the past year, the flux has been more stable (3-6 mCrab). Over the time period of the reported INTEGRAL detection (16-26 Apr 2003), the 2-10 keV flux in the PCA scans has been 4.5-7.2 mCrab, with no obvious trend. Before Feb 2001, the X-ray flux was less than approximately 1 mCrab.

In RXTE PCA pointed observations on 01 and 03 June 2001; and 09 and 11 Oct 2001, there was evidence of dipping behavior (~30% dips; ~5 min duration). A period of 1-3 hours is suggested, but highly unconstrained because of the time sampling. No QPOs or pulsations were detected.

The PCA X-ray spectrum was consistent with power law plus soft emission, with photon indices ranging between 1.8 and 2.9. An emission line associated with Fe K, (centroid 6.5-6.7 keV) is present with equivalent widths in the 130-250 eV range.

On 22.843 Apr 2003, during a PCA bulge scan this year, an apparent X-ray burst was detected from a position consistent with the source. Unfortunately, the scan contained only the tail of the burst, and so a reliable peak flux is not yet available.

The combination of these observations suggests that IGR J17597-2201 (= XTE J1759-220) is a neutron star in a short-period binary.