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Register a new userThe long-term evolution of the spin, pulse shape, and orbit of the accretion-powered millisecond pulsar SAX J1808.4-3658
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Jacob M. Hartman
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We present a 7 yr timing study of the 2.5 ms X-ray pulsar SAX J1808.4-3658, an X-ray transient with a recurrence time of ~2 yr, using data from the Rossi X-ray Timing Explorer covering 4 transient outbursts (1998-2005). We verify that the 401 Hz pulsation traces the spin frequency fundamental and not a harmonic. Substantial pulse shape variability, both stochastic and systematic, was observed during each outburst. Analysis of the systematic pulse shape changes suggests that, as an outburst dims, the X-ray hot spot on the pulsar surface drifts longitudinally and a second hot spot may appear. The overall pulse shape variability limits the ability to measure spin frequency evolution within a given X-ray outburst (and calls previous nudot measurements of this source into question), with typical upper limits of |nudot| < 2.5x10^{-14} Hz/s (2 sigma). However, combining data from all the outbursts shows with high (6 sigma) significance that the pulsar is undergoing long-term spin down at a rate nudot = (-5.6+/-2.0)x10^{-16} Hz/s, with most of the spin evolution occurring during X-ray quiescence. We discuss the possible contributions of magnetic propeller torques, magnetic dipole radiation, and gravitational radiation to the measured spin down, setting an upper limit of B < 1.5x10^8 G for the pulsars surface dipole magnetic field and and Q/I < 5x10^{-9} for the fractional mass quadrupole moment. We also measured an orbital period derivative of Pdot = (3.5+/-0.2)x10^{-12} s/s. This surprising large Pdot is reminiscent of the large and quasi-cyclic orbital period variation observed in the so-called black widow millisecond radio pulsars, supporting speculation that SAX J1808.4-3658 may turn on as a radio pulsar during quiescence. In an appendix we derive an improved (0.15 arcsec) source position from optical data.
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Soft phase lags, in which X-ray pulses in lower energy bands arrive later than pulses in higher energy bands, have been observed in nearly all accretion-powered millisecond pulsars, but their origin remains an open question. In a study of the 2.5 ms accretion-powered pulsar SAX J1808.4-3658, we report that the magnitude of these lags is strongly dependent on the accretion rate. During the brightest stage of the outbursts from this source, the lags increase in magnitude as the accretion rate drops; when the outbursts enter their dimmer flaring-tail stage, the relationship reverses. We evaluate this complex dependence in the context of two theoretical models for the lags, one relying on the scattering of photons by the accretion disk and the other invoking a two-component model for the photon emission. In both cases, the turnover suggests that we are observing the source transitioning into the propeller accretion regime.
The Rossi X-ray Timing Explorer has observed five outbursts from the transient 2.5 ms accretion-powered pulsar SAX J1808.4-3658 during 1998-2008. We present a pulse timing study of the most recent outburst and compare it with the previous timing solutions. The spin frequency of the source continues to decrease at a rate of (-5.5+/-1.2)x10^-18 Hz/s, which is consistent with the previously determined spin derivative. The spin-down occurs mostly during quiescence, and it is most likely due to the magnetic dipole torque from a B = 1.5x10^8 G dipolar field at the neutron star surface. We also find that the 2 hr binary orbital period is increasing at a rate of (3.80+/-0.06)x10^-12 s/s, also consistent with previous measurements. It remains uncertain whether this orbital change reflects secular evolution or short-term variability.
We analysed Rossi X-ray Timing Explorer observations of the accretion-powered 401 Hz pulsar SAX J1808.4-3658, in order to precisely determine the source distance. While the fluences for the five transient outbursts observed from 1996 were constant to within the uncertainties, the outburst interval varied signficantly, so that the time-averaged flux (and accretion rate) decreased by around 40%. By equating the time-averaged X-ray flux with the expected mass transfer rate from gravitational radiation, we derived a lower limit on the distance of 3.4 kpc. Combined with an upper limit from assuming that the four radius-expansion thermonuclear bursts observed during the 2002 October outburst reached at most the Eddington limit for a pure He atmosphere, we found that the probable distance range for the source is 3.4-3.6 kpc. The implied inclination, based on the optical/IR properties of the counterpart, is i<~30 degrees. We compared the properties of the bursts with an ignition model. The time between bursts was long enough for hot CNO burning to significantly deplete the accreted hydrogen, so that ignition occurred in a pure helium layer underlying a stable hydrogen burning shell. This is the first time that this burning regime has been securely observationally identified. The observed energetics of the bursts give a mean hydrogen fraction at ignition of <X> approx. 0.1, and require that the accreted hydrogen fraction X_0 and the CNO metallicity Z_CNO are related by Z_CNO approx. 0.03(X_0/0.7)^2. We show that in this burning regime, a measurement of the burst recurrence time and energetics allows the local accretion rate onto the star to be determined independently of the accreted composition, giving a new method for estimating the source distance which is in good agreement with our other estimates.
The accreting millisecond pulsar SAX J1808.4-3658 has shown a peculiar orbital evolution in the past with an orbital expansion much faster than expected from standard binary evolutionary scenarios. Previous limits on the pulsar spin frequency derivative during transient accretion outbursts were smaller than predicted by standard magnetic accretion torque theory, while the spin evolution between outbursts was consistent with magnetic dipole spin-down. In this paper we present the results of a coherent timing analysis of the 2011 outburst observed by the Rossi X-ray Timing Explorer and extend our previous long-term measurements of the orbital and spin evolution over a baseline of thirteen years. We find that the expansion of the 2 hr orbit is accelerating at a rate 1.6E-20 s/s^2 and we interpret this as the effect of short-term angular momentum exchange between the mass donor and the orbit. The gravitational quadrupole coupling due to variations in the oblateness of the companion can be a viable mechanism for explaining the observations. No significant spin frequency derivatives are detected during the 2011 outburst (<4E-13 Hz/s) and the long term spin down remains stable over thirteen years with a rate of approximately -1E-15 Hz/s.
We have observed the millisecond X-ray pulsar SAX J1808.4-3658 on three occasions during its 2000 outburst with the BeppoSAX satellite. The source was highly variable and erratic during this outburst, and by coincidence we obtained data only during times when the source had very low luminosities. During our observations, we detected four faint sources. The source closest to the position of SAX J1808.4-3658 is still ~1.6 away. This source can only be identified with SAX J1808.4-3658 if we assume that the BeppoSAX positional reconstruction is not completely understood. We also reanalyzed a BeppoSAX observation taken in March 1999 when the source was in quiescence and during which the source was thought to have been detected (Stella et al. 2000). Based on the similarities (position and luminosity) of this source with the above mentioned source ~1.6 away from SAX J1808.4-3658, it is possible that they are the same source. If this source is not the millisecond pulsar, then during all BeppoSAX observations of SAX J1808.4-3658 (the 2000 outburst ones and the 1999 quiescent one), the millisecond pulsar was not detected. A reanalysis of the ASCA quiescent data of SAX J1808.4-3658 (Dotani, Asai, & Wijnands 2000) confirms that during this observation the source was securely detected in quiescence. We discuss our results for SAX J1808.4-3658 in the context of the quiescent properties of low-mass X-ray binary transients.
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