Two ~35 day clocks in Hercules X-1: evidence for neutron star free precession

Abstract

We present evidence for the existence of two ∼35 day clocks in the Her X-1/HZ Her binary system. ∼35 day modulations are observed 1) in the Turn-On cycles with two on-and two off-states and 2) in the changing shape of the pulse profiles which re-appears regularly. The two ways of counting the 35 day cycles are generally in synchronization. This synchronization did apparently break down temporarily during the long Anomalous Low (AL3), which Her X-1 experienced in 1999/2000, in the sense that there must have been one extra Turn-On cycle. Our working hypothesis is that there are two clocks in the system, both with a period of about ∼35 days: precession of the accretion disk (the less stable "Turn-On clock") and free precession of the neutron star (the more stable "Pulse profile clock"). We suggest that free precession of the neutron star is the master clock and that the precession of the accretion disk is basically synchronized to that of the neutron star through a feedback mechanism in the binary system. However, the Turn-On clock can slip against its master when the accretion disk has a very low inclination, as is observed to be the case during AL3. We take the apparent correlation between the histories of the Turn-Ons, of the Anomalous Lows and of the pulse period evolution, with a 5 yr quasi-periodicity, as evidence for strong physical interaction and feedback between the major components in the system. We speculate that the 5 yr (10 yr) period is due either to a corresponding activity cycle of HZ Her or a natural ringing period of the physical system of coupled components. The question of whether free precession really exists in neutron stars is very important for understanding matter with supra-nuclear density.

Long-term change in the cyclotron line energy in Hercules X-1

“…precession of the neutron star? -and the mechanism of generating the varying pulse profiles and the varying spectra -continua and CRSF (see Section 8 and discussions in Staubert et al 2009Staubert et al , 2013Vasco et al 2013). …”

“…2. The main peak of the pulse profile moves to later pulse phases with increasing 35 d phase (Staubert et al 2009). This is also true for the 30-45 keV profiles -the energy range which includes the CRSF (these profiles are shown in Fig.…”

Revealing the broad iron Kαline in Cygnus X-1 through simultaneousXMM-Newton, RXTE, and INTEGRAL observations

“…Warped accretion disks often lead to long-term quasiperiodic modulations which are observable in the X-ray lightcurves (Clarkson et al 2003a), such as the 35 d cycle of Her X-1 (e.g., Staubert et al 2009) or the variable long-term period in SMC X-1 (Clarkson et al 2003b;Trowbridge et al 2007). The well-known 150 d or 300 d quasi-periodicity of Cyg X-1 has been associated with a similar warp (Priedhorsky et al 1983;Kemp et al 1983Kemp et al , 1987Brocksopp et al 1999;Zdziarski et al 2002Zdziarski et al , 2011Benlloch et al 2004, and references therein).…”

A Suzaku view of cyclotron line sources and candidates

“…Between outbursts they can show years long quiescence periods. While the CRSF sources listed in the second block of Table 1 are persistent X-ray emitters they are very variable on all observed time scales, reflecting stellar wind, magnetosphere (see, e.g., [5,6] for Vela X-1), or accretion disk structure (see, e.g., [7] for Her X-1). Fig.…”