Kinwah Wu

Effects of gravity on the structure of post-shock accretion flows in magnetic cataclysmic variables

We calculate the temperature and density structure of the hot post-shock plasma in magnetically confined accretion flows, including the gravitational potential. This avoids the inconsistency of previous calculations which assume that the height of the shock is negligible. We assume a stratified accretion column with 1D flow along the symmetry axis. We find that the calculations predict a lower shock temperature than previous calculations, with a flatter temperature profile with height. We revise previous determinations of the masses of the white dwarf primary stars, and find that for higher mass white dwarfs there is a general reduction in derived masses when the gravitational potential is included. This is because the spectrum from such flows is harder than that of previous prescriptions at intermediate energies.

High-resolution optical and infrared spectroscopic observations of Cir X-1

We present new optical and infrared (IR) observations of Cir X-1 taken near apastron. Both sets of spectra show asymmetric emission lines. Archival optical observations show that an asymmetric Ha emission line has been in evidence for the past 20 years, although the shape of the line has changed significantly. We present an eccentric (e , 0:7–0:9x86 low-mass binary model, where the system consists of a neutron star orbiting around a (sub)giant companion star of 3–5 M(. We suggest that the broad components of the emission lines arise in a highvelocity, optically thick flow near the neutron star, while the narrow components of the optical and the IR lines arise near the companion star and a heated ejecta shell surrounding the binary respectively. In this model, the velocity of the narrow component reflects the space velocity of the binary; the implied radial velocity (1430 km s after correcting for Galactic rotation) is the highest velocity known for an X-ray binary. Key words: binaries: spectroscopic – stars: individual: Cir X-1 – X-rays: stars. 1 I N T RO D U C T I O N Cir X-1 is one of the most puzzling X-ray binaries known. Like the peculiar systems SS 433 and Cyg X-3, it cannot easily be classified into any of the major categories of X-ray binaries. Indeed, there is even doubt as to whether it is a high-mass X-ray binary (HMXB) or a low-mass X-ray binary (LMXB). Since its discovery in the early 1970s, Cir X-1 has been studied intensively at X-ray wavelengths. The X-ray properties of Cir X-1 were found to differ dramatically each time it was observed (see, e.g., Kaluzienski et al. 1976; Tennant 1988; Tsunemi et al. 1989; Shirey et al. 1996). Periodic modulation of the X-ray flux was found at a period of 16.6 d (Kaluzienski et al. 1976). A radio counterpart was detected (Clark, Parkinson & Caswell 1975), and was found to flare at the same period as the X-ray modulation (Haynes et al. 1978). These flares were initially detected at peak flux levels of .1 Jy; since the 1970s, the flux of the source has decreased dramatically, and it has only occasionally been detected above 50 mJy (Stewart et al. 1991). This radio source is located 25 arcmin from the centre of the supernova remnant G 321.920.3, and is apparently connected to the remnant by a radio nebula (Haynes et al. 1986). Stewart et al. (1993) have imaged arcmin-scale collimated structures within the surrounding nebula, suggesting an outflow from the X-ray binary. Fender et al. (1998) have imaged an arcsec-scale asymmetric jet aligned with these larger structures, raising the possibility that the outflow from the system is relativistic. Recently, Case & Bhattacharya (1998) have revised the estimated distance to G 321.920.3 (and hence to Cir X-1, assuming that they are associated) to 5.5 kpc, which is substantially smaller than the original suggested distance to Cir X-1 of 10 kpc (Goss & Mebold 1977). The discovery of Type I X-ray bursts (Tennant, Fabian & Shafer 1986b) suggests that the compact object is probably a weakly magnetized neutron star. The close association of Cir X-1 with the supernova remnant suggests that the system may be a young (,105 yr old) runaway system from a supernova explosion (Stewart et al. 1993). The optical counterpart to Cir X-1 was identified as a highly reddened star with strong Ha emission (Whelan et al. 1977). This object was later shown to consist of three stars within a radius of 1.5 arcsec, the southernmost of which is the true counterpart (Moneti 1992; Duncan, Stewart & Haynes 1993). The long orbital period and periodic X-ray activity suggested a high-mass system in an eccentric orbit (Murdin et al. 1980); however, the variability of the optical emission, the faintness of the optical counterpart, and several of its X-ray characteristics suggest that the companion is a low-mass star. The lack of spectroscopic studies in the optical band means that most of the fundamental orbital parameters of Cir X-1 have not been determined. Moreover, Cir X-1 shows very different properties q 1999 RAS E-mail: hmj@aaoepp.aao.gov.au 416 H. M. Johnston, R. Fender and K. Wu from time to time. This make it difficult to construct a coherent picture for the system from observations of different wavelengths at different epochs. Here we present new spectroscopic observations of Cir X-1, and use these, together with analysis of archival observations of the system, to suggest a more coherent model for the system. 2 O B S E RVAT I O N S A N D DATA R E D U C T I O N 2.1 New optical observations Cir X-1 was observed on 1997 June 4 using the 3.9-m AngloAustralian Telescope (AAT). The mean orbital phase of the observation was 0.51, calculated according the ephemeris of Stewart et al. (1991). The RGO Spectrograph was used in combination with the TEK 1k CCD in the 82-cm camera and a grating of 270 groove mm in first order, resulting in a dispersion of ,1:08 A pixel21 over a wavelength range 6060–7165 A. The spatial scale was 0.25 arcsec pixel; the spectral resolution, measured from the arc lines, was 5.4 A. A 1.5-arcsec-wide slit was used, oriented north–south so that both Cir X-1 and star 2 of Moneti (1992) were in the slit. The atmospheric seeing was about 1 arcsec. Five 1800-s integrations were taken, interspersed with CuAr arc-lamp exposures, before cloud prevented the acquisition of any more data. The bias and pixel-to-pixel gain variations were removed from each exposure using standard procedures in iraf. Cosmic rays were removed using the method of Croke (1995) to compare adjacent frames. Because of the presence of the nearby confusing star (Moneti’s star 2), special care needed to be taken to measure the flux from our object. At every position along the dispersion direction, we fit two Gaussian profiles, with fixed widths x85FWHM x88 5:6 pixelsx86 and separation (6 pixels), to the skysubtracted frames. The amplitude of these Gaussians was used as the estimate of the flux from Cir X-1 and star 2 at each wavelength. We then determined the wavelength calibration using the CuAr arc lamp exposures. We fitted a low-order polynomial to the arc line wavelengths as a function of pixel number: the rms scatter of the fits was ,1=4 of a pixel. A rough flux-calibration was performed by comparing with the spectrum of the observed flux standard LTT 4364 although, since the night was nonphotometric, this flux calibration should be considered only approximate. 2.2 Infrared observations K-band spectroscopy of Cir X-1 was obtained using the Cryogenic Array Spectrometer/Imager (CASPIR) on the ANU 2.3-m telescope at Siding Spring Observatory on the night of 1997 June 20. The K grism was used with the SBRC 256 256 InSb array, giving a dispersion of 21.5 A pixel over a wavelength range of 1.94–2.49mm. The spatial scale was 0.5 arcsec pixel. A 5-arcsec slit was used, oriented east–west: note that this means that Moneti’s stars 2 and 3 both contributed light in our spectrum. The telescope was nodded by ^12 arcsec along the slit to provide sky frames at the same position as the object. Argon lamp spectra were taken to perform wavelength calibration, and two nearby bright stars (BS 5699 and 5712) were observed in order to remove atmospheric spectral features and perform flux calibration. Standard data reduction procedures were followed, using the local caspir package running in iraf. Bias and dark frames were used to linearize all frames, the sky background was subtracted from the object frames, and pixel-to-pixel variations were corrected. The chip distortion was corrected in order to align the dispersion and spatial directions along rows and columns of the chip; the sky background was then subtracted and spectra extracted. A low-order polynomial was fitted to the argon lines, and these calibrations applied to the object spectra. Flux calibration was achieved by dividing the observed spectra by the spectrum of a nearby mid-G-type star and then multiplying by a model for the absolute flux distribution of the calibrator. Residual terrestrial atmospheric features were then corrected using an early-type star.

Accretion onto Magnetic White Dwarfs

The standard model of stationary and time-dependent accretion onto magnetic white dwarfs are reviewed, with emphasis on the hydrodynamic structure and the emission properties of the post-shock flow. Observational consequences and applications of the model are discussed, and results obtained from our recent calculations are also presented.

Circinus X-1: survivor of a highly asymmetric supernova

We have analyzed the kinematical parameters of Cir X-1 to constrain the nature of its companion star, the eccentricity of the binary and the pre-supernova parameter space. We argue that the companion is most likely to be a low-mass (< 2.0 M_sun) unevolved star and that the eccentricity of the orbit is 0.94 +/- 0.04. We have evaluated the dynamical effects of the supernova explosion and we find it must have been asymmetric. On average, we find that a kick of 740 km/s is needed to account for the recently measured radial velocity of +430 km/s (Johnston, Fender & Wu) for this extreme system. The corresponding minimum kick velocity is 500 km/s. This is the largest kick needed to explain the motion of any observed binary system. If Cir X-1 is associated with the supernova remnant G321.9-0.3 then we find a limiting minimum age of this remnant of 60000 yr. Furthermore, we predict that the companion star has lost 10% of its mass as a result of stripping and ablation from the impact of the supernova shell shortly after the explosion.

An asymmetric arcsecond radio jet from Circinus X-1

In observations with the Australia Telescope Compact Array, we have resolved the radio counterpart of the unusual X-ray binary Circinus X-1 into an asymmetric, extended structure on arcsecond scales. In order to quantify the asymmetry, we have redetermined as accurately as possible both the optical and radio coordinates of the source. The extended emission can be understood as a compact, absorbed core at the location of the X-ray binary and as extended emission up to 2 to the southeast of the core. The arcsecond-scale-extended emission aligns with the larger, more symmetric arcminute-scale-collimated structures in the surrounding synchrotron nebula. This suggests that the transport of mass and/or energy from the X-ray binary to the synchrotron nebula is occurring via the arcsecond-scale structures. The ratio of extended flux from the southeast to that from the northwest of the core is at least 2:1. Interpreted as a relativistic aberration of an intrinsically symmetric jet from the source, this implies a minimum outflow velocity of 0.1c. Alternatively, the emission may be intrinsically asymmetric, perhaps as a result of the high space velocity of the system.

Detection of the optical counterpart of the proposed double degenerate polar RX J1914+24

We have detected the optical counterpart of the proposed double degenerate polar RX J1914+24. The I-band light curve is modulated on the 9.5-min period seen in X-rays. There is no evidence for any other periods. No significant modulation is seen in J. The infrared colours of RX J1914+24 are not consistent with a main-sequence dwarf secondary star. Our ASCA spectrum of RX J1914+24 is typical of a heavily absorbed polar and our ASCA light curve also shows only the 9.5-min period. We find that the folded I band and X-ray light curves are out of phase. We attribute the I-band flux to the irradiated face of the donor star. The long-term X-ray light curve shows a variation in the observed flux of up to an order of magnitude. These observations strengthen the view that RX J1914+24 is indeed the first double degenerate polar to be detected. In this light, we discuss the synchronizing mechanisms in such a close binary and other system parameters.

Radio Emission from GRO J1655?40 during the 1994 Jet Ejection Episodes

We report multifrequency radio observations of GRO J1655-40 obtained with the Australia Telescope Compact Array, the Molonglo Observatory Synthesis Telescope and the Hartebeesthoek Radio Astronomy Observatory at the time of the major hard X-ray and radio outbursts in 1994 August-September. The radio emission reached levels of the order of a few Jy and was found to be linearly polarized by up to 10%, indicating a synchrotron origin. The light curves are in good agreement with those measured with the VLA, but our closer time sampling has revealed two new short-lived events and significant deviations from a simple exponential decay. The polarization data show that the magnetic field is well ordered and aligned at right angles to the radio jets for most of the monitoring period. The time evolution of the polarization cannot be explained solely in terms of a simple synchrotron bubble model, and we invoke a hybrid core-lobe model with a core which contributes both synchrotron and free-free emission and lobes, which are classical synchrotron emitters.

Optical spectroscopy of GX 339-4 during the high—soft and low—hard states — II. Line ionization and emission region

We have carried out observations of the X-ray transient GX 339-4 during its high-soft and low-hard X-ray spectral states. Our high-resolution spectroscopic observation in 1999 April suggests that the H alpha line has a single-peaked profile in the low-hard state as speculated in our previous paper. The He II lambda 4686 line, however, has a double-peaked profile in both the high-soft and low-hard states. This suggests that the line-emission mechanism is different in the two states. Our interpretation is that double-peaked lines are emitted from a temperature-inversion layer on the accretion disc surface when it is irradiatively heated by soft X-rays. Single-peaked lines may be emitted from outflow/wind matter driven by hard X-ray heating. We have constructed a simple plane-parallel model and we use it to illustrate that a temperature-inversion layer can be formed at the disc surface under X-ray illumination. We also discuss the conditions required for the formation of temperature inversion and line emission. Based on the velocity separations measured for the double-peaked lines in the high-soft state, we propose that GX 339-4 is a low-inclination binary system. The orbital inclination is about 15 degrees if the orbital period is 14.8 h.

Measuring the Motion of the Black Hole in GRO J1655–40

We report optical spectroscopic observations of the low-mass X-ray binary GRO J1655-40 during outbursts in 1994 August-September and in 1996 June. The spectra show emission and absorption lines from the accretion disk surrounding the X-ray source, which have enabled us to measure the motion of the black hole candidate in this system. It is the first time among black hole candidates that an emission-line radial velocity curve is seen in phase with the expected motion of the primary. The projected radial velocity semiamplitude determined for the primary (KX = 76.2±7.5 km s-1), combined with estimates of the projected radial velocity semiamplitude of the companion star from previous observations (KC=228.2±3.0 km s-2), yields a 95% confidence limit, derived purely from kinematics, of MX > 5.1 M☉ for the mass of the compact object in GRO J1655-40, confirming its black hole nature.

Perturbative analysis of two-temperature radiative shocks with multiple cooling processes

The structure of the hot downstream region below a radiative accretion shock, such as that of an accreting compact object, may oscillate because of a global thermal instability. The oscillatory behaviour depends on the functional forms of the cooling processes, the energy exchanges of electrons and ions in the shock-heated matter, and the boundary conditions. We analyse the stability of a shock with unequal electron and ion temperatures, where the cooling consists of thermal bremsstrahlung radiation which promotes instability, plus a competing process which tends to stabilize the shock. The effect of transverse perturbations is considered also. As an illustration, we study the special case in which the stabilizing cooling process is of order 3/20 in density and 5/2 in temperature, which is an approximation for the effects of cyclotron cooling in magnetic cataclysmic variables. We vary the efficiency of the second cooling process, the strength of the electron–ion exchange and the ratio of electron and ion pressures at the shock, to examine particular effects on the stability properties and frequencies of oscillation modes.