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Title: Orbital evolution measurement of the accreting millisecond X-ray pulsar SAX J1808.4-3658
Authors: Jain, Chetana
Dutta, Anjan
Paul, Biswajit
Keywords: neutron stars—pulsars
individual (SAX J1808.4–3658)
magnetic fields—X-rays:
Issue Date: 2007
Publisher: Indian Academy of Sciences, Bangalore
Citation: Journal of Astrophysics and Astronomy, 2007, Vol.28, p197-206
Abstract: We present results from a pulse timing analysis of the accretion-powered millisecond X-ray pulsar SAX J1808.4–3658 using X-ray data obtained during four outbursts of this source. Extensive observations were made with the proportional counter array of the Rossi X-ray Timing Explorer (RXTE) during the four outbursts that occurred in 1998, 2000, 2002 and 2005. Instead of measuring the arrival times of individual pulses or the pulse arrival time delay measurement that is commonly used to determine the orbital parameters of binary pulsars, we have determined the orbital ephemeris during each observation by optimizing the pulse detection against a range of trial ephemeris values. The source exhibits a significant pulse shape variability during the outbursts. The technique used by us does not depend on the pulse profile evolution, and is therefore, different from the standard pulse timing analysis. Using 27 measurements of orbital ephemerides during the four outbursts spread over more than 7 years and more than 31,000 binary orbits, we have derived an accurate value of the orbital period of 7249.156862(5) s (MJD = 50915) and detected an orbital period derivative of (3.14 ± 0.21) × 10−12 s s−1. We have included a table of the 27 mid-eclipse time measurements of this source that will be valuable for further studies of the orbital evolution of the source, especially with ASTROSAT. We point out that the measured rate of orbital period evolution is considerably faster than the most commonly discussed mechanisms of orbital period evolution like mass transfer, mass loss from the companion star and gravitational wave radiation. The present time scale of orbital period change, 73 Myr is therefore likely to be a transient high value of period evolution and similar measurements during subsequent outbursts of SAX J1808.4–3658 will help us to resolve this.
Description: Open Access
ISSN: E-ISSN: 0250-6335
P-ISSN: 0250-6335
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Copyright: 2007 The Indian Academy of Sciences
Appears in Collections:Research Papers (A&A)

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