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Register a new userAn Accurate Determination of the Optical Periodic Modulation in the X-Ray Binary SAX J1808.4-3658
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We report on optical imaging of the X-ray binary SAX J1808.4-3658 with the 8-m Gemini South Telescope. The binary, containing an accretion-powered millisecond pulsar, appears to have a large periodic modulation in its quiescent optical emission. In order to clarify the origin of this modulation, we obtained three time-resolved $r$-band light curves (LCs) of the source in five days. The LCs can be described by a sinusoid, and the long time-span between them allows us to determine optical period P=7251.9 s and phase 0.671 at MJD 54599.0 (TDB; phase 0.0 corresponds to the ascending node of the pulsar orbit), with uncertainties of 2.8 s and 0.008 (90 % confidence), respectively. This periodicity is highly consistent with the X-ray orbital ephemeris. By considering this consistency and the sinusoidal shape of the LCs, we rule out the possibility of the modulation arising from the accretion disk. Our study supports the previous suggestion that the X-ray pulsar becomes rotationally powered in quiescence, with its energy output irradiating the companion star, causing the optical modulation. While it has also been suggested that the accretion disk would be evaporated by the pulsar, we argue that the disk exists and gives rise to the persistent optical emission. The existence of the disk can be verified by long-term, multi-wavelength optical monitoring of the source in quiescence, as an increasing flux and spectral changes from the source would be expected based on the standard disk instability model.
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An evolutionary scenario to explain the transient nature and short total duration of the X-ray burst of SAX J1808.4 -- 3658 is proposed. An optical companion of the neutron star (a ``turn-off Main - Sequence star) fills its Roche lobe at the orbital period ($P_{orb}$) $sim$ 19 hours. During the initial high mass--transfer phase when the neutron star is a persistent X-ray source, the neutron star is spun up to a millisecond period. Due to its chemical composition gradient, the secondary does not become fully convective when its mass decreases below 0.3 $msun$, hence a magnetic braking remains an effective mechanism to remove orbital angular momentum and the system evolves with Roche - lobe overflow towards a short orbital period. Near an orbital period of two hours the mass transfer rate becomes so small ($sim$ $10^{-11}msun$/yr) that the system can not continue to be observed as a persistent X-ray source. During further Roche - lobe filling evolution deep mixing allows the surface of secondary to become more and more helium rich. Since the accreted matter is helium rich, it is easy to explain observed short total duration of the burst . This evolutionary picture suggest that radio emission can be observed only at shorter wavelengths. Our model predicts a faster orbital period decay than expected if the orbital evolution is driven only by gravitational wave radiation.
Observations of the accretion powered millisecond pulsar SAX J1808.4-3658 have revealed an interesting binary evolution, with the orbit of the system expanding at an accelerated rate. We use the recent finding that the accreted fuel in SAX J1808.4-3658 is hydrogen depleted to greatly refine models of the progenitor and prior evolution of the binary system. We constrain the initial mass of the companion star to 1.0-1.2 M$_{mathrm{odot}}$, more massive than previous evolutionary studies of this system have assumed. We also infer the system must have undergone strongly non-conservative mass transfer in order to explain the observed orbital period changes. Following Jia & Li (2015), we include mass loss due to the pulsar radiation pressure on the donor star, inducing an evaporative wind which is ejected at the inner Lagrangian point of the binary system. The resulting additional loss of angular momentum resolves the discrepancy between conservative mass transfer models and the observed orbital period derivative of this system. We also include a treatment of donor irradiation due to the accretion luminosity, and find this has a non-negligible effect on the evolution of the system.
The aperiodic X-ray variability in neutron star and black hole X-ray binaries (XRBs), and active galactic nuclei (AGN) shows a characteristic linear relationship between rms amplitude and flux, implying a multiplying-together or `coupling of variability on different time-scales. Such a coupling may result from avalanches of flares, due to magnetic reconnection in an X-ray emitting corona. Alternatively this coupling may arise directly from the coupling of perturbations in the accretion flow, which propagate to the inner emitting regions and so modulate the X-ray emission. Here, we demonstrate explicitly that the component of aperiodic variability which carries the rms-flux relation in the accreting millisecond pulsar SAX J1808.4-3658 is also coupled to the 401 Hz pulsation in this source. This result implies that the rms-flux relation in SAX J1808.4-3658 is produced in the accretion flow on to the magnetic caps of the neutron star, and not in a corona. By extension we infer that propagating perturbations in the accretion flow, and not coronal flares, are the source of the rms-flux relations and hence the aperiodic variability in other XRBs and AGN.
Low-mass X-ray binaries (LMXBs) are a natural workbench to study accretion disk phenomena and optimal background sources to measure elemental abundances in the Interstellar medium (ISM). In high-resolution XMM-Newton spectra, the LMXB SAX J1808.4-3658 showed in the past a neon column density significantly higher than expected given its small distance, presumably due to additional absorption from a neon-rich circumstellar medium (CSM). It is possible to detect intrinsic absorption from the CSM by evidence of Keplerian motions or outflows. For this purpose, we use a recent, deep (100 ks long), high-resolution Chandra/LETGS spectrum of SAX J1808.4-3658 in combination with archival data. We estimated the column densities of the different absorbers through the study of their absorption lines. We used both empirical and physical models involving photo- and collisional-ionization in order to determine the nature of the absorbers. The abundances of the cold interstellar gas match the solar values as expected given the proximity of the X-ray source. For the first time in this source, we detected neon and oxygen blueshifted absorption lines that can be well modeled with outflowing photoionized gas. The wind is neon rich (Ne/O>3) and may originate from processed, ionized gas near the accretion disk or its corona. The kinematics (v=500-1000 km/s) are indeed similar to those seen in other accretion disks. We also discovered a system of emission lines with very high Doppler velocities (v~24000 km/s) originating presumably closer to the compact object. Additional observations and UV coverage are needed to accurately determine the wind abundances and its ionization structure.
We report the discovery of phase shifts between X-ray pulses at different energies in the newly discovered millisecond (ms) X-ray pulsar SAX J1808.4-3658. The results show that low-energy pulses lag high-energy pulses by as much as $sim$0.2 ms (or $sim$8% of the pulse period). The measurements were made in two different ways: (1) computing cross power spectra between different energy bands, and (2) cross-correlating the folded pulse profiles in different energy bands; consistent results were obtained. We speculate that the observed soft lags might be related to the lateral expansion and subsequent cooling of a ``hot spot on the neutron star surface in which the pulsed X-ray emission originates. Also presented is the possibility of producing soft lags via Compton down scattering of hard X-ray photons from the hot spot in the cool surrounding atmosphere. We will discuss possible X-ray production mechanisms for SAX J1808.4-3658 and constraints on the emission environment, based on the observed soft lags, pulse profiles, and energy spectrum.
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