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Detection of Orbital Period of EXO 1722-363

ATel #179; C. B. Markwardt (U. Maryland & GSFC) & J. H. Swank (GSFC)
on 21 Aug 2003; 02:29 UT
Credential Certification: Craig B. Markwardt (craigm@lheamail.gsfc.nasa.gov)

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

In regards to Lutovinov et al.'s report of activity from EXO 1722-363 (ATEL #178), the source has been detected in RXTE PCA scans of the galactic center since Feb 1999. Ginga observations showed it to have a 413 s pulse period. (It was designated GPS 1722-363 by Tawara et al. 1989, PASJ, 41, 473.) Between Feb 1999 and 17 Aug 2003, the PCA scans measured a 2-10 keV flux range of 0-8 mCrab with random sampling of the pulse phase. The bulge scan observation of Aug 20.51 detected an increased 2-10 keV flux of about 15 mCrab.

From 1999 to 2003, there is a 9.7 day modulation of the X-ray flux, with apparent total eclipses in the 2-10 keV band. A fit of a simple eclipse model to the flux history is consistent with the following timing ephemeris for the eclipse centers:

T = T0 + N * Porb

where T0 = 51988.91 +/- 0.04 MJD, Porb = 9.737 +/- 0.004 d, and N is an integer. The formal eclipse duration at minimum flux is 1.79 +/ 0.09 d. RXTE PCA pointed observations in 1998 show that the X-ray pulsations are clearly eclipsed at approximately the expected times. This reported orbital period is consistent with the lower limit of 9 days set by Takeuchi et al (1990, PASJ, 42, 287) by pulse timing with Ginga.