New measurement of orbital and spin period evolution of the Accretion Disk Corona source 4U 1822-37
Abstract
4U 1822-37 is a Low Mass X-ray Binary (LMXB) system with an Accretion Disk Corona. 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 timescale for a change in the orbital period of 4U 1822-37. We have derived an orbital period Porb = 0.23210887(15) d, which is changing at the rate of · Porb = 1.3(3) x 10-10 d d-1 (at T0 = MJD 45614). The timescale for a change in the orbital period is Porb/ · Porb of 4.9(1.1) x 106 yr. We also report the detection of 0.59290132(11) s (at T0 = MJD 51975) X-ray pulsations from the source with a long term average · Pspin of -2.481(4) x 10-12 s s-1, i.e., a spin-up time scale (Pspin/ · Pspin) 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 · Porb 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 108 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 years by ? 40% 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.