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

TL;DR

This study reports a long-term flux enhancement in the X-ray pulsar 4U 1626-67, analyzing spectral and timing changes, including a new spin-up phase and altered power spectrum features, indicating significant accretion state transitions.

Contribution

It provides the first detailed analysis of spectral and timing variations during a prolonged flux enhancement in 4U 1626-67, revealing new insights into its accretion dynamics and spin behavior.

Findings

Detected a long-term flux increase over 100 days with two episodes.

Observed a new spin-up phase with a reduced rate.

Identified spectral softening and broadening of the 130 mHz peak.

Abstract

4U 1626-67 is an accretion powered X-ray pulsar that shows remarkably stable X-ray luminosity above hours timescale and gradual intensity variation on a few years timescale. Here we report a significant increase in the X-ray intensity in the long term RXTE-ASM light curve of 4U 1626-67. Similar enhancement in the X-ray flux has also been detected in the Swift-BAT light curve. The increase in the X-ray flux took place over a long period of about 100 days and there appears 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 the Rossi X-ray Timing Explorer. The source has entered a new spin-up phase with a spin-up rate of 4.02(5) $\times$ 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. We also 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.