Spectral and temporal changes associated with flux enhancement in 4U 1626-67

Abstract

4U 1626− 67 is an accretion powered X-ray pulsar that shows remarkably stable X-ray luminosity above a time-scale of hours and gradual intensity variation on a time-scale of a few years. Unlike the other high magnetic field binary X-ray pulsars, the spin-up or spin-down rate of the neutron star is also very stable on time-scales up to several years. Here, we report a significant increase in the X-ray intensity in the long-term Rossi X-ray Timing Explorer (RXTE) All Sky Monitor light curve of 4U 1626− 67. Similar enhancement in the X-ray flux has also been detected in the Swift Burst Alert Telescope light curve. The increase in the X-ray flux took place over a long period of about 100 d and there appear to be two episodes of flux enhancement. We have investigated the spectral and timing features of 4U 1626− 67 during its current state of enhanced flux emission with data obtained from the Proportional Counter Array and the High-Energy X-ray Timing Explorer on board RXTE. We report the detection of a torque reversal to spin-up in 4U 1626− 67. The source has entered a new spin-up phase with a spin-up rate of 4.02(5) × 10−13 Hz s−1 . The present spin-up rate is almost half of the earlier spin-up and spin-down trends. A significant excess in soft X-ray photon emission is observed during the enhanced flux state, which is similar to the energy spectrum obtained during the spin-up era of the pulsar before 1990. 4U 1626− 67 is a unique accretion powered X-ray pulsar in which quasi-periodic oscillations have been consistently observed over the past ∼20 years. During the recent observations, however, we did not detect a quasi-periodic oscillation at frequencies as seen in earlier observations. Instead, we report detection of a significant broadening in the wings of the 130 mHz peak and a change in the shape of the continuum of the power spectrum. These results indicate that the flux enhancement is not a simple case of increased mass accretion rate, but there is also a change in the accretion geometry in the vicinity of the neutron star.