The Structure of the Accretion Disk in the Accretion Disk Corona X-Ray Binary 4u 1822–371 at Optical and Ultraviolet Wavelengths

Abstract

The eclipsing low-mass X-ray binary 4U 1822-371 is the prototypical accretion disk corona (ADC) system. We have obtained new time-resolved UV spectroscopy of 4U 1822-371 with the Advanced Camera for Surveys/Solar Blind Channel on the Hubble Space Telescope and new V -and J-band photometry with the 1.3-m SMARTS telescope at Cerro Tololo Inter-American Observatory. We use the new data to construct its UV/optical spectral energy distribution and its orbital light curve in the UV, V , and J bands. We derive an improved ephemeris for the optical eclipses and confirm that the orbital period is changing rapidly, indicating extremely high rates of mass flow in the system; and we show that the accretion disk in the system has a strong wind with projected velocities -2 -up to 4000 km s −1 . We show that the disk has a vertically-extended, opticallythick component at optical wavelengths. This component extends almost to the edge of the disk and has a height equal to ∼0.5 of the disk radius. As it has a low brightness temperature, we identify it as the optically-thick base of a disk wind, not as the optical counterpart of the ADC. Like previous models of 4U 1822-371, ours needs a tall obscuring wall near the edge of the accretion disk, but we interpret the wall as a layer of cooler material at the base of the disk wind, not as a tall, luminous disk rim.

The Reflares and Outburst Evolution in the Accreting Millisecond Pulsar Sax J1808.4–3658: A Disk Truncated Near Co-Rotation?

“…UV spectra of X-ray binaries can generally be well fitted with continuum models, with emission lines like Mg II sometimes visible, but they usually produce a small amount of flux compared to the continuum in the UV (Hynes et al 1998Hynes & Haswell 1999;Bayless et al 2010). Small irregularities in the UV spectral energy distribution could therefore be due to the presence of prominent emission lines, but UV spectroscopy of X-ray binaries has shown that these usually make only a smallto-negligible difference to the continuum, and hence broadband UV filter fluxes.…”

X-ray spectroscopy of the ADC source X1822-371 withChandraandXMM-Newton

“…Initially, Mason & Cordova (1982) derived an inclination angle between 76 • and 84 • fitting the light curve of X1822-371 in the infrared band. The modulation of the light curve is generally explained with the presence of a geometrically thick disc whose height varies depending on the azimuthal angle and occults part of the X-ray emission (White & Holt 1982;Mason & Cordova 1982;Hellier & Mason 1989;Bayless et al 2010).…”

“…1.8 × 10 −8 M yr −1 for a neutron star of 1.4 M ), suggesting that the mass transfer has to be highly non-conservative, with the neutron star accreting at the Eddington limit and the rest of the transferred mass expelled from the system. Bayless et al (2010), studying the optical and UV data of X1822-371, derived the new optical ephemeris for the source, finding a rapid change of the orbital period with P/Ṗ = (3.0 ± 0.3) × 10 6 yr, similar to the value of the recent X-ray ephemeris of X1822-371 obtained by Burderi et al (2010). Bayless et al (2010) showed that the accretion rate onto the neutron star should be 6.4 × 10 −8 M yr −1 in a conservative mass transfer, suggesting again a highly non-conservative mass transfer.…”

“…Bayless et al (2010), studying the optical and UV data of X1822-371, derived the new optical ephemeris for the source, finding a rapid change of the orbital period with P/Ṗ = (3.0 ± 0.3) × 10 6 yr, similar to the value of the recent X-ray ephemeris of X1822-371 obtained by Burderi et al (2010). Bayless et al (2010) showed that the accretion rate onto the neutron star should be 6.4 × 10 −8 M yr −1 in a conservative mass transfer, suggesting again a highly non-conservative mass transfer. Finally they fitted the optical light curve of X1822-371, concluding that the disc has a vertically extended component that is optically thick at the optical wavelengths with a height of 0.5 times the accretion disc radius.…”

“…Recently, Iaria et al (2011) extended the work of Burderi et al (2010), including a larger sample of data, and combined the optical/UV data from Bayless et al (2010) to obtain an updated ephemeris of X1822-371. Iaria et al (2011) obtaineḋ P orb = 1.59(9) × 10 −10 s/s from the combined X-ray/optical/UV data set, consistent with the previous measurements.…”