Jeffrey Alan Pedelty

The X-Ray System R Aquarii: A Two-sided Jet and Central Source

We report Chandra ACIS-S3 X-ray imaging and spectroscopy of the R Aquarii binary system that show a spatially resolved two-sided jet and an unresolved central source. This is the first published report of such an X-ray jet seen in an evolved ∼2–3 M, stellar system. At keV, the X-ray jet extends to both the northeast E ! 1 (NE) and southwest (SW) relative to the central binary system. At keV, R Aqr is a pointlike source 1 ! E ! 7.1 centered on the star system. While both 3.5 cm radio continuum emission and X-ray emission appear coincident in projection and have maximum intensities at ∼7 .5 NE of the central binary system, the next strongest X-ray component is located ∼30 SW of the central binary system and has no radio continuum counterpart. The X-ray jets are likely shock-heated in the recent past and are not in thermal equilibrium. The strongest SW X-ray jet component may have been shocked recently since there is no relic radio emission as expected from an older shock. At the position of the central binary, we detect X-ray emission below 1.6 keV consistent with blackbody emission at K. There is also a prominent 6.4 keV feature, a possible fluorescence or collisionally 6 T ∼ 2 # 10 excited Fe Ka line from an accretion disk or from the wind of the giant star. For this excitation to occur, there must be an unseen hard source of X-rays or particles in the immediate vicinity of the hot star. Such a source would be hidden from view by the surrounding edge-on accretion disk. Subject headings: binaries: symbiotic — circumstellar matter — radio continuum: stars — stars: individual (R Aquarii) — stars: winds, outflows — white dwarfs — X-rays: general

Spatial Resolution of the R Aquarii Binary System

We report continuum VLA observations at 7 mm that have resolved the stellar components in the R Aqr binary system. R Aqr was simultaneously probed in both a 50 MHz bandpass containing lineless continuum emission associated with the hot companion/accretion disk and a 3.125 MHz bandpass containing the spectral line v = 1, J = 1-0, SiO maser emission associated with the long-period-variable (LPV) envelope. The offset between the two stars is 55 ± 2 mas with a position angle of ~18° ± 2° relative to the LPV, providing the first data point for a subsequent monitoring program to determine precisely the binary orbit that is suspected to be highly elliptical and have a period of ~44 yr. We evaluate these first observations in the context of constraints placed on the orbital geometry of the system and obtain a geometrical distance of ~200 pc to R Aqr. We also report spectral line VLA observations at this same epoch that confirm that the SiO maser spots have a ringlike morphology, as previously reported by other investigators using the VLBA.

Outer Jet X-Ray and Radio Emission in R Aquarii: 1999.8 to 2004.0

Chandra and VLA observations of the symbiotic star R Aqr in 2004 reveal significant changes over the 3-4 year interval between these observations and previous observations taken in with the VLA in 1999 and with Chandra in 2000. This paper reports on the evolution of the outer thermal X-ray lobe jets and radio jets. The emission from the outer X-ray lobe jets lies farther away from the central binary than the outer radio jets and comes from material interpreted as being shock-heated to 106 K, a likely result of collision between high-speed material ejected from the central binary and regions of enhanced gas density. Between 2000 and 2004, the northeast (NE) outer X-ray lobe jet moved out, away from the central binary, with an apparent projected motion of 580 km s-1. The southwest (SW) outer X-ray lobe jet almost disappeared between 2000 and 2004, presumably due to adiabatic expansion and cooling. The NE radio-bright spot also moved away from the central binary between 2000 and 2004, but with a smaller apparent velocity than the NE X-ray-bright spot. The SW outer lobe jet was not detected in the radio in either 1999 or 2004. The density and mass of the X-ray-emitting material is estimated. Cooling times, shock speeds, pressure, and confinement are discussed.

R Aquarii: Evidence for Differential Rotation of the SiO Maser Shell

We previously reported Very Large Array and Berkeley-Illinois-Maryland Association (BIMA) array observations that suggested rotation of the SiO maser shell surrounding the long-period variable (LPV) in the R Aquarii binary system. In the present Very Long Baseline Array (VLBA) work, we report high spatial and spectral resolution observations of the v = 1, J = 1-0, SiO maser line that confirm our previous result and further suggest that the LPV maser shell is undergoing differential rotation. The 8-34 yr range of rotational periods resulting from differential rotation of the maser shell contains the ~18 yr period reported previously. The velocity structure of the VLBA data suggests a rotation symmetry axis oriented at a position angle of ~150°. The differential rotation model can be envisioned as a series of nested thin spherical shells that have a common rotation axis; each thin shell is characterized by its radius, r, with the innermost shell rotating fastest and the outermost shell slowest, in accordance with an equatorial plane velocity law of the form v ∝ 1/2. We find that q ≈ 1.09 is necessary to approximate the VLBA data, suggesting that the differential rotation is approximately Keplerian.

Discovery of Rapid Hard X-Ray Variability and New Jet Activity in the Symbiotic Binary R Aquarii

Two Chandra observations of the R Aqr symbiotic binary system taken 3.3 yr apart show dramatic changes in the X-ray morphology and spectral characteristics in the inner 500 AU of this system. The morphology of the soft X-ray emission has evolved from a nearly circular region centered on the binary system to an hourglass shape that indicates the formation of a new southwest jet. Synchrotron radiation from the new jet in contemporaneous VLA radio spectra implies the physical conditions in the early stages of jet development are different from those in the more extended outer thermal jets known to exist for decades in this system. The central binary source has two X-ray spectral components in each of the two epochs, a soft component and a highly absorbed hard component characterized by T ~ 108 K if fit with a thermal plasma model. The spectrum hardened considerably between 2000.7 and 2004.0, primarily due to increased flux above 5 keV, suggesting a change in the accretion activity of the white dwarf on a timescale of a few years or less. Point-source Fe K emission is detected at the position of the central binary system in both observations. While the earlier observation shows evidence of only a single emission peak near Fe Kα at 6.4 keV, the later observation shows a more complex emission structure between 6 and 7 keV. Finally, we have discovered a modulation in the hard X-ray flux with a period of 1734 s at a 95% confidence level in the 2004 observation only. The modulation potentially arises from standing shocks in an accretion column, and we have explored the possibility that the white dwarf in R Aqr is analogous to the magnetic white dwarfs in Intermediate Polars.

R Aquarii: Constraints on the Rotational Period of the Long-Period Variable

We report Very Large Array (VLA) observations taken in 1996 November and 1998 May of the v = 1, J = 1-0, SiO maser line and BIMA array observations taken in 1999 December and 2000 February of the v = 1, J = 2-1, SiO maser line associated with the long-period variable (LPV) in the R Aquarii binary system that suggest rotation of the maser shell. From these interferometric data cubes, we determine that the maser shell rotation axis is approximately northeast-southwest, thus aligning approximately with the direction of the R Aqr jet; the sense of the maser shell rotation is such that northwest is approaching and southeast is receding; the period of rotation is ~17 yr. Alternatively, co-adding 72 time series spectra of the v = 1, J = 1-0, SiO maser line obtained during the period 1984 July-1990 May with a single-dish antenna, we constructed a composite spectral emission envelope that shows the LSR velocity limits of maser emission over this epoch. From this composite spectral emission envelope and Very Long Baseline Array observations in 1996 February of the v = 1, J = 1-0, SiO maser line, which show the maximal spatial extent of the maser shell, we obtain a shell rotation period of ~18 yr, which is in excellent agreement with the VLA and BIMA array results and represents the maximum rotation period of the LPV if corotating with the maser shell. On the other hand, we obtain a minimum rotation period for the LPV of ~5 yr if the LPV supplies material to the maser shell under the constraint of conservation of angular momentum. The ~5-18 yr range for the rotational period of the LPV determined here and the ~18 yr rotational period for the hot companion determined by previous investigators suggest that tidal effects at successive periastron passages in the R Aqr binary system are tending to synchronize these stellar rotational periods to the orbital period of ~44 yr.