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ATEL # 2576; C. B. Markwardt (U. Maryland & NASA/GSFC), T. E. Strohmayer (NASA/GSFC), J. H. Swank (NASA/GSFC), D. Pereira (WYLE & NASA/GSFC), E. Smith (WIS & NASA/GSFC)
on 20 Apr 2010; 5:39 UT
Distributed as an Instant Email Notice (Request for Observations)
Password Certification: Craig B. Markwardt (firstname.lastname@example.org)
Subjects: X-ray, Request for Observations, Binaries, Neutron Stars, Pulsars, Transients
RXTE continues to perform pointed observations of the accreting millisecond X-ray pulsar SWIFT J1749.4-2807 (ATEL #2548, #2565, #2567), known to be in a binary system with 8.82 hr orbital period (ATEL #2568, #2569).
The RXTE PCA X-ray light curve shows a steady decline, at a rate of about 1 mCrab per day, with two clear dips in the X-ray light curve. When the X-ray light curve is folded on the X-ray orbital period (ATEL #2569), the dips occur near orbital longitude 90 degrees (as measured from the ascending node). Longitude 90 is superior conjunction, where an earth observer might expect the neutron star to be eclipsed by the secondary. The light curve profiles during these dips are more or less flat, and we interpret them as X-ray eclipses of the neutron star by its companion star. X-ray pulsations are not detectable during the putative eclipses, further solidifying this interpretation.
RXTE observations cover two separate eclipse egresses, but no ingresses. However, we can assume by construction of the pulse timing model that eclipses are centered on longitude 90. In that case, we determine the X-ray eclipse duration to be approximately 5.5% of the orbital period, or
Tdur = 1700 sec
Assuming Roche lobe overflow, the mean density of the companion star is consistent with a sun-like star.
This is the first known eclipsing accreting millisecond X-ray pulsar, out of fourteen known systems. The eclipse and pulse timing will enable much better constraints on the geometry of the system than for non-eclipsing systems. Measurement of the radial velocity of the secondary, or a spectroscopic mass would also enable an accurate neutron star mass measurement.
RXTE is planning further observations. We will attempt to concentrate on the time of eclipse. Observations to determine the spectral type of the secondary and its radial velocity curve are strongly encouraged.