Discovery of 1-5 Hz flaring at high luminosity in SAX J1808.4-3658

Abstract

We report the discovery of a 1--5~Hz X-ray flaring phenomenon observed at $>30$~mCrab near peak luminosity in the 2008 and 2011 outbursts of the accreting millisecond X-ray pulsar SAX J1808.4--3658 in observations with the \textit{Rossi X-ray Timing Explorer}. In each of the two outbursts this high luminosity flaring is seen for $\sim$3 continuous days and switches on and off on a timescale of 1--2~hr. The flaring can be seen directly in the light curve, where it shows sharp spikes of emission at quasi-regular separation. In the power spectrum it produces a broad noise component, which peaks at 1--5~Hz. The total 0.05--10~Hz variability has a fractional rms amplitude of 20\%--45\%, well in excess of the 8\%--12\% rms broad-band noise usually seen in power spectra of SAX J1808.4--3658. We perform a detailed timing analysis of the flaring and study its relation to the 401~Hz pulsations. We find that the pulse amplitude varies proportionally with source flux through all phases of the flaring, indicating that the flaring is likely due to mass density variations created at or outside the magnetospheric boundary. We suggest that this 1--5~Hz flaring is a high mass accretion rate version of the 0.5--2~Hz flaring which is known to occur at low luminosity ($<13$~mCrab), late in the tail of outbursts of SAX J1808.4--3658. We propose the dead-disk instability, previously suggested as the mechanism for the 0.5--2~Hz flaring, as a likely mechanism for the high luminosity flaring reported here.

12 pages, 8 figures, 3 tables, Accepted for publication in ApJ

References in corpus (5)

• Thermonuclear (type-I) X-ray bursts observed by the Rossi X-ray Timing Explorer
• Accretion to Magnetized Stars through the Rayleigh-Taylor Instability: Global Three-Dimensional Simulations
• Broad relativistic iron emission line observed in SAX J1808.4-3658
• A decade of timing an accretion-powered millisecond pulsar: The continuing spin down and orbital evolution of SAX J1808.4-3658
• Swift observations of SAX J1808.4-3658: monitoring the return to quiescence

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• The evolution of a newborn millisecond magnetar with a propeller-recycling disk
• On the curious pulsation properties of the accreting millisecond pulsar IGR J17379-3747
• Quasi-periodic pulse amplitude modulation in the accreting millisecond pulsar IGR J00291+5934