Australia Telescope National Facility

Angular Momentum Transfer in the Binary X-ray Pulsar GX 1+4
Greenhill J G , Galloway D K , Murray J R, PASA, 16 (3), 240.

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Spectroscopic evidence on the nature of the neutron star environment

GX 1+4 was observed with the ASCA X-ray satellite on 1994 September 14-15 and spectroscopy of the optical counterpart, V2116 Oph, was carried out at the Anglo-Australian Telescope using the RGO spectrograph on 1994 September 25-26. Photometry during 1994 August to October, using the Mt Canopus, Tasmania and Mt John, New Zealand 1 m telescopes showed little change in the source between the times of the X-ray and optical observations.

The X-ray spectroscopy showed considerable photo-electric absorption in the source region and strong iron line emisssion. The ionisation state of the iron (FeI-FeIV) shows that the $\xi$ -parameter (

$\equiv L_X/nr^2$ , where n is the particle density of the circumstellar matter and r is the path length of the X-rays through this matter)

$\leq 30 \, {\rm erg.cm.s^{-1}}$ . Using the measured values of L_X and $N_H (\sim nr)$ we estimate the characteristic scale of the attenuating matter distribution

$r\geq 3\times10^{12}\,$ cm and

$n\leq 7\times 10^{10}\,{\rm cm^{-3}}$ . The results of the optical spectroscopy were consistent with these conclusions. Using the Balmer line ratios and the calculations of Drake & Ulrich (1980) we estimate the electron density

$n_e \sim 3\times 10^{10} - 10^{11}\,{\rm cm^{-3}}$ and plasma temperature $\sim 20,000K$ in the emission line region. The absence of FeIII and the presence of FeII lines supports this temperature estimate.

Using this information, Kotani et al (1999) propose a model in which the circumstellar matter is gravitationally bound to the neutron star during times of low L_X. This is consistent with the observed $H_\alpha$ line width ( $\sim$ 2 AU) assuming doppler broadening from bound hydrogen at

$r\sim 3\times 10^{12}$ cm. The model provides an unstable negative feedback mechanism leading to large short term fluctuations in L_X when the system is in a low intensity state. Increased accretion raises L_X heating the trapped matter until the thermal velocity exceeds the escape velocity driving off trapped matter and suppressing accretion from the stellar wind. This occurs only at large distances from the neutron star so there is a delay with timescale $\sim$ the orbital period (several months) before accretion begins to decrease. Hence the accretion rate $\dot M$ and L_X will be unstable and variable on timescales of months but relatively stable on longer timescales while the mean L_X is low. If the ram pressure of the M giant wind (or matter transferred by Roche Lobe overflow) becomes much higher than the thermal pressure in the trapped matter it will not be blown off by X-ray heating and the feedback mechanism will not be active. L_X will be larger and dependent only on the rate of mass flow from the M giant. This mechanism requires very special conditions for its operation and is unlikely to apply to systems with supersonic winds as eg in Cen X-3 or Vela X-1.

The Kotani et al (1999) model provides a natural explanation for some aspects of the long term behaviour of GX 1+4. Greenhill & Watson (unpublished report, 1994) collated the results of over 60 published measurements of GX 1+4 between 1971 and 1994. Fig. 1 is their estimate of the time dependence of the 20 keV X-ray flux during this period. Throughout the 1970's L_X was large and relatively stable as expected when the feedback mechanism is not active. Subsequently the source was highly variable on timescales of order months and the mean value of L_X was much lower. We suggest that the feedback mechanism was active during this period. Another prediction is that large X-ray flares will be of shorter duration than smaller flares. Large flares will blow off matter closer to the neutron star and hence more rapidly affect accretion onto it. The pulsed flux history reported by Chakrabarty et al (1997) is qualitatively consistent with this prediction.

**Figure 1:** The time dependence of the 20 keV X-ray flux (
x 10^-4 cm^-2s^-1keV^-1) from 1970 to 1994. The data are from a compilation by Greenhill & Watson (unpublished report, 1994) of over 60 published and unpublished measurements. The filled squares represent positive detections and the open squares are $2 \sigma$ upper limits.
$\begin{figure} \begin{center} \psfig{file=gx.eps,height=7cm} \end{center}\end{figure}$

The model does not provide an explanation for the transition between intensity states. This may be caused by some long term instability in the giant companion. Nor does it make any prediction concerning the direction of angular momentum transfer in this system. We note however that the negative feedback regime with a large diameter shell of low velocity trapped matter may be more conducive to the formation of a contra-rotating disc than the high luminosity regime when the ram pressure of the wind from the giant is higher and the wind extends much closer to the surface of the neutron star.

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