Orbital evolution measurement of the accreting millisecond X-ray pulsar SAX J1808.4-3658 |
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| Authors: | Chetana Jain Anjan Dutta Biswajit Paul |
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| Institution: | (1) Department of Physics and Astrophysics, University of Delhi, Delhi, 110 007, India;(2) Raman Research Institute, Sadashivnagar, C.V. Raman Avenue, Bangalore, 560 080, India |
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| 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. |
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| Keywords: | Stars: neutron stars pulsars: individual (SAX J1808 4-3658) magnetic fields X-rays: binaries accretion |
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