Recent timing studies on RXTE observations of 4U 1538-52

¹ Physics Department, Middle East Technical University, 06531 Ankara, Turkey e-mail: altan@astroa.physics.metu.edu.tr
² Department of Electrical and Electronics Engineering, Başkent University, 06530 Ankara, Turkey e-mail: inam@baskent.edu.tr
³ Physics Department, Süleyman Demirel University, 32260 Isparta, Turkey e-mail: elif@astroa.physics.metu.edu.tr

Received: 30 November 2005
Accepted: 2 April 2006

Abstract

The high mass X-ray binary pulsar 4U 1538-52 was observed between July 31 and August 7, 2003. Using these observations, we determined new orbital epochs for both circular and elliptical orbit models. The orbital epochs for both orbit solutions agreed with each other and yielded an orbital period derivative  yr^-1. This value is consistent with the earlier measurement of  yr^-1 at the level and gives only an upper limit to the orbital period decay. Our determination of the pulse frequency showed that the source spun up at an average rate of  Hz s^-1 between 1991 and 2003.