New measurement of orbital and spin period evolution of the accretion disc corona source 4U 1822−37

Abstract

4U 1822-37 is a low mass X-ray binary (LMXB) system with an accretion disc corona. It is one of the very few LMXBs that shows narrow X-ray eclipses and small amplitude pulsations of the neutron star. The X-ray eclipse is an excellent reference for measurement of orbital evolution of the binary, and we have obtained 16 new mid-eclipse time measurements of this source during the last 13 years using X-ray observations made with the RXTE-PCA, RXTE-ASM, Swift-XRT, XMM-Newton and Chandra observatories. These, along with the earlier known mid-eclipse times have been used to accurately determine the time-scale for a change in the orbital period of 4U 1822-37. We have derived an orbital period P orb = 0.23210887(15) d, which is changing at the rate of P orb = 1.3(3) × 10 -10 d d -1 (at To = MJD 45614). The time-scale for a change in the orbital period is P orb /P orb of 4.9(1.1) x 10 6 yr. We also report the detection of 0.59290132(11) s (at T 0 = MJD 51975) X-ray pulsations from the source with a long-term average P spin of -2.481(4) x 10 -12 s s -1 , i.e. a spin-up time-scale (P spin /P spin ) of 7578(13) yr. In view of these results, we have discussed various mechanisms that could be responsible for the orbital evolution in this system. Assuming the extreme case of conservative mass transfer, we have found that the measured P orb requires a large mass transfer rate of (4.2-5.2) x 10- 8 M ⊙ yr -1 which together with the spin-up rate implies a magnetic field strength in the range of ( 1-3) x 10 8 G. Using the long-term RXTE-ASM light curve, we have found that the X-ray intensity of the source has decreased over the last 13 yr by ~40 per cent and there are long-term fluctuations at time-scales of about a year. In addition to the long-term intensity variation, we have also observed significant variation in the intensity during the eclipse. Variation was also seen in the pulse profile, which could be due to changes in the accretion geometry. We briefly discuss the implications of these results on our understanding of the properties of the neutron star and the accretion geometry in this source.