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Abstract
We report on the discovery of strong decaHz flaring in the early decay of two out of five outbursts of the accreting millisecond X-ray pulsar SAX J1808.4-3658. The decaHz flaring switches on and, after ~3 days, off again, on a time scale of 1-2 hours. When the flaring is present, the total 0.05-10 Hz variability has a fractional rms amplitude of 20 to 30 percent, well in excess of the 8 to 12 percent rms broad-band noise usually seen in power spectra of SAX J1808 in this frequency range. Coherent 401 Hz pulsations are seen throughout the observations in which the decaHz flaring is detected. We find that the absolute amplitude of the pulsations varies with the flux modulation of the decaHz flaring, indicating that the flaring is caused by an accretion rate modulation already present in the accretion flow prior to matter entering the accretion funnel. We suggest that the decaHz flaring is the result of the Spruit-Taam instability [1]. This instability arises when the inner accretion disk approaches co-rotation. The rotation of the stellar magnetosphere then acts as a propeller, suppressing accretion onto the neutron star. A matter reservoir forms in the inner accretion disk, which episodically empties onto the neutron star, causing flares at a decaHz timescale. A similar explanation was proposed earlier for 1 Hz flaring occurring late in three of five outbursts, mutually exclusive with the decaHz flaring. The 1 Hz flaring was observed at luminosities a factor 5 to 10 below where we see the decaHz flaring. That a different branch of the Spruit-Taam instability could also act at the much higher luminosity levels of the decaHz flaring had recently been predicted by D’Angelo & Spruit [2, 3]. We discuss these findings in the context of the parameters of the Spruit-Taam-d’Angelo model of the instability. If confirmed, after millisecond pulsations, 1 Hz and decaHz flaring would be another diagnostic of the presence of a magnetosphere in accreting low-magnetic field neutron stars.
Highlights
The more common stochastic variability of atoll class neutron star binaries is seen in the power spectra of SAX J1808, and shows very consistent trends in each of the several outbursts [8]
We presented the discovery of decaHz flaring in SAX J1808, a flaring phenomenon which appears in the power spectrum with 20-30% fractional rms and a characteristic frequency of 2-5 Hz
For the 2011 instance we further observed a small increase in flux to be correlated with a decrease in the flaring rms amplitude. These observations suggest the flaring appears only in a very sharply bounded flux window, indicating that the instability region of the underlying mechanism is highly sensitive to mass accretion rate
Summary
During several weeks to months after the main outburst, SAX J1808 enters the so-called flaring-tail, which constitutes to ∼5 day periods of renewed activity at Lx ∼ 1035 erg s−1 separated by by short periods of near quiescence with Lx ∼ 1032 erg s−1 [10, 11] It is at the low luminosities during the tail of renewed activity that the 1 Hz QPO is observed[10]. We present the discovery of yet another flaring phenomenon, observed at the peak of the 2008 and 2011 outbursts We compare this newly discovered decaHz flaring with the 1 Hz QPO, which occurs at an order of magnitude lower luminosity, and discuss the dead-disk accretion instability as a potential common explanation for both phenomena
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