In magnetic CVs, the field is sufficiently strong to channel the accretion flow to the polar region(s) of the white dwarf. Radial accretion along polar field lines is an approximation to this case and is depicted in the center graph of Fig. 12.1. The emission is dominated by thermal emission (mostly hard-ray bremsstrahlung) in the limit
Fig. 12.1 The accretion geometry and the observed radiation components. Left: Boundary layer in nonmagnetic CVs. Center: Cooling flow behind free-standing shock for radial accretion in a magnetic CV with low mass-flow density (i.e., specific accretion rate). Right: Partially and completely buried shocks for intermediate and high mass flow densities, respectively
Fig. 12.1 The accretion geometry and the observed radiation components. Left: Boundary layer in nonmagnetic CVs. Center: Cooling flow behind free-standing shock for radial accretion in a magnetic CV with low mass-flow density (i.e., specific accretion rate). Right: Partially and completely buried shocks for intermediate and high mass flow densities, respectively of low field strength B and high mass flow density m.1 In the shock, the plasma is heated to the shock temperature kTs = (3jUmu/16k)v2f — 55 (M/Mq)1'9 keV, where vff = (2GM/R)1/2 the free-fall velocity . The flow is largely optically thin, but becomes optically thick in the strongest emission lines. Besides thermal X-ray emission, IR/optical cyclotron radiation is an important cooling agent, which dominates for high B and low m. In the cyclotron-dominated regime, the peak plasma temperature drops to only a few kiloelectronvolt and the shock height collapses to small values [2,13,100]. In the non-hydrodynamical regime at m < 10-4b7'6 g cm-2s-1 , with B7 the field strength in units of 10 MG, a shock does not form and the outermost layers of the atmosphere of the white dwarf are directly heated by the kinetic energy of the ions, a situation called bombardment solution [44,99]. Again, the plasma temperature is lower than for a bremsstrahlung-dominated shock.
The stand-off distance of the bremsstrahlung-dominated shock is hsh = 6.0 x 10-19 (2GM/R)3/2/m in cgs-units, which holds as long as the one-dimensional approximation with hsh C R is valid . For M = 0.6Mq, hsh — 4.5 x 107/mcm. At the same time, the bottom of the cooling flow is depressed by the ram pressure to a depth dram — 106 m0'4 cm below the photosphere of the white dwarf [2,15]. As acon-sequence, the entire cooling flow disappears below the level of the photosphere for hsh < dram or m ^ 15g cm-2s-1 . Such buried shocks are schematically depicted in the right-hand graph in Fig. 12.1. Below, we show examples of spectra calculated for different m values (Fig. 12.7). In the buried high-m shocks, the bremsstrahlung flux from the postshock plasma is severely reduced by photoabsorption in the surrounding atmosphere and the corresponding fraction of the accretion luminosity is thermalized and reappears as soft quasi-blackbody emission from the heated atmosphere [2,15,44]. Thermalization as well as heating of the atmosphere by irradiation from tall, low-m shocks [36,45] combine to produce the large excesses of soft over hard X-ray emission observed from many, but by no means from all polars [2,69] (see also Sect. 12.6).
1 The quantity m (cgs unit g cm 2s is also called specific accretion rate.
The emission properties of polars and IPs are basically similar, while the radiative transfer properties differ. Accretion from a disk or ring in IPs causes the accreting matter to form an azimuthally extended curtain, which photoabsorbs soft X-rays over much of the hemisphere as seen from the white dwarf. Broad-band X-ray spectral analyses of IPs suggest that the accretion stream is highly structured and may best be modeled by a range of optical depths present in the line of sight to the source . The narrow accretion stream in polars, on the other hand, blocks soft X-rays only over a very restricted solid angle and allows a largely unrestricted view of the radiation that escapes from the immediate vicinity of the accretion spot.
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