Spectral State Transitions in Aquila X-1: Evidence for “Propeller” Effects

Abstract

Aquila X-1 is a soft X-ray transient source and emits type I X-ray bursts. A spectral state transition was observed with RXTE during its outburst decay in 1997 February and March. Its 10-30 keV and 5-10 keV count rate ratio increased suddenly when its luminosity was between 4 and ergs s Ϫ1 , assuming a 2.5 kpc 35 12 # 10 distance. Spectral fitting with a model composed of a blackbody and a power-law component showed that its blackbody component decreased and that the power-law component became much harder significantly and simultaneously. We interpret this transition to be caused by the centrifugal barrier, more commonly known as the "propeller" effect. We thus infer that the magnetic field strength of the neutron star is around G, if the 8 1 # 10 neutron star spin period is 1.8 ms. Similarly, we infer the neutron star magnetic field strength in another soft Xray transient Cen X-4 is about G. We also propose a unified scheme for spectral state transitions in soft 9 2 # 10 X-ray transients, from soft high state to hard low state and further to quiescent state. With this scheme accretion onto neutron star may take place even during the propeller regime.

Constraints on the neutron star magnetic field of the two X-ray transients SAX J1808.4–3658 and Aql X–1

“…Menou et al (1999) have presented a propeller scenario that still allows for partial residual accretion if accretion in quiescent neutron star SXTs occurs via a quasi-spherical Advection Dominated Accretion Flow (ADAF), rather than a thin disk. This allows some material to accrete near the poles, bypassing the centrifugal barrier (see also Zhang et al 1998). However, only a very small fraction (if any) of the total accretion rate in quiescence is expected to accrete onto the neutron star surface (Menou et al 1999), consistently with the fact that no coherent pulsations are detected during quiescence.…”

Constraining the properties of neutron star crusts with the transient low-mass X-ray binary Aql X-1

“…Decay times of τ ≃1.7-1.8 days were also reported for the 1997 and 2010 outbursts of Aql X-1 . Such similarities may be expected if the rapid decay is caused by the onset of a propeller mechanism (e.g., Campana et al 1998bCampana et al , 2014Zhang et al 1998;Asai et al 2013). Alternatively, the rapid decay may correspond to draining of the accretion disc on a viscous time scale (e.g., Fridriksson et al 2010; Armas Padilla et al 2013).…”

Transient Low-Mass X-Ray Binaries in Quiescence

“…Finally, if a magnetic field plays an important role in the geometry of quiescent accretion, one might expect some asymmetries that would produce X-ray pulsations. This was not observed in the recent fading of Aql X-1 (Campana et al 1998b;Zhang et al 1998a), where stringent limits on the pulsed fraction of the emission were placed (≤1.2% rms variability, 95% confidence; Chandler & Rutledge 2000) at a time believed to be just at the onset of the propellor for a 10 8 G field. What this implies about the magnetic field strength of Aql X-1 is still unknown.…”