Adv. Space Res. Vol. 16, NO. 3, pp. (3)103-(3)106, 1995 Copyright Q 1995 COSPAR

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OBSERVATIONS OF AM HER STARS USING ROSAT

G. Rarnsay, K. 0. Mason and M. Cropper Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RHS 6NT, U.K.

ABSTRACT

We show that the degree of excess soft X-ray emission measured in AM Her stars during their high intensity state is correlated with the magnetic field strength of the accretion region and the radius at which material couples on to the magnetic field lines of the white dwarf. AM Her in an intermediate intensity state, however, does not follow the same correlation and shows an apparent soft X-ray d&c&my. ST LMi in a low intensity state exhibits a bremsstrahlung temperature that is over an order of magnitude lower than in the high state. One interpretation of these results is that only a small fraction of the accretion region accretes at a rate sufficiently high for a shock to form in the intermediate state so that the observed blackbody flux severely underestimates the true bolometric flux. In the low state, no shock at all forms over the accretion region.

INTRODUCTION

AM Her stars are close binaries in which the white dwarf has a sufficiently strong magnetic field (-lo-50MG) to synchronize its rotation with the binary orbital motion. Mass transfer occurs through R.oche lobe overflow from the red dwarf secondary star onto the white dwarf. In the so- called ‘standard’ model, it is assumed that the accreted matter shocks at some height above the photosphere of the white dwarfproducing hard X-ray bremsstrahlung radiation and optical-infrared cyclotron flux in the post-shock region /l/. Assuming that radiation is emitted isotropically in the shocked accretion column, half of this emission will be directed towards the white dwarf. If all of this flux is absorbed and re-radiated as soft X-rays, we would expect equal contributions to the total energy output from the soft X-ray component and the combined hard X-ray and cyclotron components. Previous observations (e.g. /2/, /3/) h ave shown that the flux from the soft X-ray component far exceeds that predicted by the standard model: this is known as the soft X-my excess. Although the problem of the soft X-ray excess is well known, its overall incidence has until now remained unclear. It was to determine the incidence of the soft X-ray excess that we have observed a number of AM Hers using ROSAT.

OBSERVATIONS OF SYSTEMS IN A HIGH STATE

The energy balance ratio &,,bol/(L~b,~+L~) was determined for 17 AM Her systems which were in a high accretion state ( LK,,~, Ltb,w and L, are the bolometric blackbody, thermal bremsstrahlung and cyclotron luminosities respectively). This quantity is plotted in Figure 1 as a function of the orbital period of the binary. A soft X-ray excess is seen in the majority of the systems, but some are consistent with the standard model. More importantly, however, a correlation was found /4/ between the energy balance ratio and the magnetic field strength, and also the coupling radius r. (the radius at which the accretion stream couples onto the field lines on the white dwarf) which itself is a function of the magnetic field strength. The sense of the correlation is such that the soft X-ray excess increases with increasing magnetic field strength and increasing r, (c.f. figure 1). (One system which does not fit this trend is BY Cam, but there is reason to believe that the white dwarf rotation in this system is not exactly synchronised with the orbital period which may complicate the accretion flow 141).

(3)103

(3)104 (3. Ranlsny et al.

100.00

g

AM Her

0.01 50 100 150

Period (mins)

Figure 1: The energy balance ratio L&/(&b + L,-+) plotted as a function of the orbital period, where the dotted line is the energy balance ratio predicted by the standard model where X-ray scattering has been taken into account.

EK UMa 100.00 b) ----EKU$ -1

0 10 20 30 40 50 60 70 Magnetic field (MG)

o.olt....~ 1 1o’O 10”

Coupling radius (cm)

Figure 2: The energy balance ratio L&/(&b + Lcye) plotted as a function of: a) the magnetic field strength of the accretion region and b) the coupling radius.

A proposed explanation of the soft X-ray excess stems from the fact that the accretion stream in AM Hers is highly inhomogenous and consists of blobs of material with a range of length and density /5/. Dense blobs will penetrate the white dwarf photosphere before being shocked, and their kinetic energy will thus be directly thermalised, contributing to the soft X-ray emission, but not to the hard X-ray or cyclotron components. By adjusting the fraction of dense blobs in the accretion stream it is possible, in principle, to account for the spread in values seen for the soft X-ray excess.

Although it is perhaps not surprising that the magnetic field strength would influence the physical characteristics of the accretion stream, it is more difficult to determine how this might work in quantitiative terms. One factor which may affect the fraction of dense blobs in the accretion stream is the length of time that material in the accretion stream spends travelling along the magnetic field lines - this is related to rC. However, in current models of the accretion stream /6/, the dense blobs are formed due to the Rayleigh-Taylor instability when the magnetic field pinches the accretion stream. These blobs are then shredded by the Kelvin-Helmholtz instability as they travel along the magnetic field lines. We might expect that for larger values of T,, the chance of the blobs being completely evaporated is greater and hence we would observe low soft X-ray excesses

~lxervations of AM Her Stars ITsing ROSAT (3105

100.00 -0 l1 10.00 +

-I % 1.00 Y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

bz -I 5: 0.10

0.01 0.01 0.10 1.00 10.00

LSofi (1030 ergs s")

Figure 3: The energy balance ratio for AM Her stars plotted as a function of the intrinsic Lb,&, where the parentheses (I) and (II) denote the high and intermediate level systems respectively.

for large values of rc. This is in contrast with our observations.

OBSERVATIONS OF SYSTEMS IN AN INTERMEDIATE STATE

When AM Her was observed on 1991 Sept 14 its mean flux was lower by a factor of ~100 compared to its mean flux on 1991 Apr 12. When BL Hyi and WW Hor were observed during 1991 Apr-May and 1992 July 21 respectively, their mean fluxes were lower by a factor of ~20 and ~5 compared to their mean fluxes at epochs when they were considered to be in a high state from EINSTEIN and EXOSAT data. (We exclude the bright flares seen in the data of BL Hyi /4/). However, both AM Her and BL Hyi have also shown lower mean fluxes in past epochs than measured using ROSAT. This suggests that these systems were in an intermediate accretion state when observed using ROSAT.

Using the observed high/intermediate state soft X-ray flux ratio as a guide to the relative accretion rate, we determined T, for these systems (T= 0: h). When plotted in the T,, L~,a/(L~b,bd + L,+) plane, AM Her lies in the lower right hand quadrant of figure 2b. It does not follow the correlation found for systems in a high level. The values obtained for T, in WW Hor and BL Hyi were relatively unconstrained compared with AM Her, and as such, are consistent with the correlation found for high state systems. AM Her shows a spectrum that has a much lower energy balance ratio for its magnetic field strength than would be expected from Figure 2b. Indeed, it shows a soft X-ray deficiency compared with the standard model. This implies another accretion mechanism must be at work.

OBSERVATIONS OF SYSTEMS IN A LOW STATE

In addition to our observations of systems in high and intermediate states, we made a long (44ksec) observation of ST LMi in a low accretion state. The signal-to-noise ratio was sufficiently high that the bremsstrahlung temperature (KZ’ tt, could be well constrained. ST LMi was found to show ) kTtb N 0.5 f 0.1 keV: an order of magnitude lower than the high state. This is in good agreement with /5/ who predict that for low accretion rates (Ai 510-l g crne2 s-l) a shock will not form and the accreting material cools through Coloumb interactions. They predict the temperature of the accreting material will be much lower compared with the high state (kTtb NlkeV as opposed to N30keV). For a given accreting surface area, the lower accretion accretion rate will also result in a lower blackbody temperature in the low state compared with the high state (kZ’& NlOeV compared with N30eV, using typical values for k in a low state). For other systems which we observed in a low state (MR Ser and RE1313-32), the exposure was much lower than obtained for ST LMi and did not give well constrained values for bT,b.

JASR 16:3-H

(31106 G. Rammy et al.

We fitted a combined absorbed blackbody and thermal bremsstrahlung model to the ST LMi data as we did for the high state data and fixed the blackbody temperature at kT%=30eV (typical for the high level). Using the upper limit to the blackbody flux, we found the fractional size of the area emitting as a blackbody (f~) is over an order of magnitiude less compared to the high level.

DISCUSSION

To investigate these results further, we plot &,,t,&/(&b,bd + Lw) as a function of L&,u in figure 3 for systems in both high and intermediate intensity states. There is a clear correlation: as Lu,w increases so does L&,I&/(Ltb,&i + Lcyc). On the other hand, there is no correlation between &b,w and hb,bd/(Ltb,bd + J&C). 0 ne interpretation is that as the accretion rate increases, the majority of the additional radiation is emitted in soft X-rays. From this, we would infer, based on the model of /5/, that the number of dense filaments increases as the accretion rate increases.

For systems in an intermediate state, a different mechanism from that postulated for systems in a high state is possible. The current model of the accretion region is one of an extended arc with varying accretion rates along its length /7/. If we assume that in a high state, the accretion rate is suiIicient for a shock to form over all parts of the accretion region, then in an intermediate state, it is possible that only a small part of the accretion region has a sufficient accretion rate for a shock to form. In the intermediate state perhaps only a small fraction of the accretion region has an accretion rate sufficiently high for the thermalised radiation to be hot enough to be observable using ROSAT. In the remainder of the accretion region, the thermalised radiation will have a cooler blackbody temperature and the soft component will not be observable as a separate component since the photon8 will be absorbed by the interstellar medium. This will result in an apparently much lower value for the observed .&,,~/&,W ratio as seen in the observation of AM Her in an intermediate state. If we were able to observe such low temperature radiation, we would expect this ratio to be much greater. Indeed, for a spot of contant size, we would expect AM Her in an intermediate state to show an energy balance ratio comparable to that of systems in a high state.

REFERENCES

1. Lamb, D. Q. & Masters, A. R., 1979, ApJ, 234, L117 2. Heise, J. et al. 1985, A&A, 148, L14 3. Osborne, J. P., et al. 1986, MNRAS, 221,823 4. Ramsay, G., Mason, K. O., Cropper, M., Watson, M. G. & Clayton, K. L., 1994, MNRAS, in press 5. Kuijpers, J. % Pringle, J. E., 1982, A&A, 114, L4 6. Liebert, J. & Stockman, II. S., 1985, In CVs and LiUXBs, ed. Lamb & Patterson, Rediel Pub- lishing 7. Wickramasinghe, D. T., 1988, In Polarized Radiation of Circumstellar Origin, p.1, ed Coyne et al., Vatican Obs.

ACKNOWLEDGEMENTS

GR would like to thank the SERC/PPARC for a 8tUdent8hip and KOM the Royal Society of London for support. This paper is based partially on data extracted from the ROSAT data archive in Leicester.