New Torque Reversal and Spin-Up of 4u 1626- 67 Observed by Fermi/GBM and Swift/BAT

After about 18 years of steadily spinning down, the accretion-powered pulsar 4U 1626-67, experienced a torque reversal at the beginning of 2008. For the present study we have used all available Fermi/GBM data since its launch in 2008 June 11 and over 5 yr of hard X-ray Swift/BAT observations (starting from 2004 October up to the present time). This second detected torque reversal is centered near MJD 54500 (2008 Feb 4) and it lasts approximately 150 days. From 2004 up to the end of 2007 4U 1626-67 the spin-down rate decreased at a mean rate of ~ -5.5E-13 Hz s-1 until the source reversed torque again. Since then it has been following a steady spin-up at a mean rate of ~ 5E-13 Hz s-1. In addition, 4U 1626-67 increased its flux simultaneously (a ~2.5 factor). We present detailed long-term timing analysis of this source and a long term spectral hardness ratio study in order to see whether there are spectral changes around this new observed torque reversal.

💡 Research Summary

The accretion‑powered pulsar 4U 1626‑67, a 7.66 s X‑ray pulsar in a low‑mass X‑ray binary with an ultra‑low‑mass companion, exhibited a second torque reversal after roughly 18 years of steady spin‑down. Using all available Fermi Gamma‑ray Burst Monitor (GBM) data from its launch on 2008 June 11 and over five years of Swift Burst Alert Telescope (BAT) hard‑X‑ray monitoring (2004 Oct – 2009), the authors performed a comprehensive timing and spectral study of this event.

Timing analysis of GBM CTIME data (0.256 s resolution, 8 energy channels) required careful background modeling because of the spacecraft’s continuously changing orientation. The authors fitted a combined model that included bright source contributions, Earth occultation steps, and a quadratic spline to capture long‑term background trends. After subtracting this model, residuals were binned in ~300 s intervals and fitted with a Fourier expansion to obtain pulse profiles. Six‑day averaged profiles were then used to determine the pulse frequency via the Yₙ (n = 2) statistic. The resulting frequency history shows a clear reversal centered near MJD 54500 (2008 Feb 4) lasting ≈150 days. Prior to the reversal the spin‑down rate was ≈ ‑4.8 × 10⁻¹³ Hz s⁻¹; after the reversal the source has been spinning up at ≈ +4 × 10⁻¹³ Hz s⁻¹, essentially mirroring the earlier 1990 reversal.

Swift/BAT data (15–150 keV) were processed in a similar fashion. Quadrant count rates were cleaned, background‑subtracted, and pulse profiles generated in 35‑day intervals. The BAT count rate, which traces the hard X‑ray flux, increased by a factor of ~2.5 during the torque reversal, and the spin‑up rate derived from the timing analysis correlates tightly with the BAT flux. This strong flux‑torque correlation suggests that the mass accretion rate (ṁ) increased substantially during the reversal, providing the extra angular momentum needed to flip the torque sign.

Spectral analysis employed simultaneous RXTE/PCA (2.5–20 keV) and HEXTE (18–100 keV) observations taken in March 2008. Two spectral models were fitted in XSPEC: (1) a low‑energy absorption (fixed N_H = 1.3 × 10²¹ cm⁻²), a blackbody, a power‑law, and a high‑energy cutoff around 20 keV, plus a broad Gaussian near 6.5 keV to account for Fe Kα emission; (2) a similar model with a bremsstrahlung component. The inclusion of the iron line significantly improved χ², confirming its presence. The blackbody temperature (~0.7 keV) and normalization imply an emitting area of ~9 × 10¹² cm² (assuming a distance of 10 kpc), larger than a simple polar cap or inner‑disk region, indicating a more complex emission geometry, possibly involving both the accretion column base and the inner disk edge.

Hardness‑intensity analysis combined BAT (15–50 keV) count rates with RXTE/ASM (1.5–12 keV) rates to form a hardness ratio (HR). The HR shows a clear transition from a hard to a softer spectrum during the torque reversal, consistent with the increase in soft X‑ray flux and the spectral fits that indicate a stronger blackbody component. This spectral softening is interpreted as a change in the inner disk radius or magnetospheric coupling, which modifies the temperature distribution of the emitting plasma.

The authors place 4U 1626‑67 in context with other torque‑reversing pulsars such as Her X‑1, Cen X‑3, GX 1+4, and OAO 1657‑415. Unlike many high‑mass systems where wind accretion dominates, 4U 1626‑67’s ultra‑low‑mass companion feeds a persistent, geometrically thin accretion disk, making it a cleaner laboratory for studying angular‑momentum transfer. The observed simultaneous changes in spin frequency, X‑ray flux, and spectral hardness strongly support a scenario where variations in the mass accretion rate alter the magnetosphere‑disk interaction, leading to a reversal of the net torque.

In summary, the paper delivers a robust, multi‑instrument, long‑term observational campaign that captures the full phenomenology of the 2008 torque reversal in 4U 1626‑67. The results provide quantitative evidence that torque reversals in low‑mass X‑ray binaries are driven by changes in the accretion rate and the resulting re‑configuration of the disk–magnetosphere coupling, offering valuable constraints for theoretical models of angular‑momentum exchange in accreting neutron stars.