Rays from Intermediate Polars

The defining feature of IPs is a coherent pulsation at a period Ps shorter than the orbital period Porb [62, this reference contains also a list of confirmed IPs]. This short period usually dominates at X-ray energies and is interpreted as the spin period

wavelength (A)

Fig. 12.3 Chandra LETG spectra of WZ Sge in superoutburst (top panel) and SS Cyg in outburst (bottom panel). The data are shown in black and the model spectrum in grey with the strongest lines indicated (from [54])

wavelength (A)

Fig. 12.3 Chandra LETG spectra of WZ Sge in superoutburst (top panel) and SS Cyg in outburst (bottom panel). The data are shown in black and the model spectrum in grey with the strongest lines indicated (from [54])

of the white dwarf. This interpretation is convincingly confirmed when reprocessed optical/ultraviolet radiation at the beat period Pb = (Ps-1 - POTb)-1 is also detected (see [66,93] for further possible periodicities). The magnetic nature of the accreting white dwarf is taken for granted, although in most cases a direct observational proof is missing, with the exception of the few systems found to be polarized at red or near IR wavelengths. An evolutionary link to polars has long been claimed and is likely since synchronization is bound to take place in some systems as their orbital separation decreases with time [62, and references therein].

A comprehensive study based on EXOSAT results [61] summarized the knowledge we had of the spin modulated X-ray emission of IPs before the ROSAT era and demonstrated the importance of partial absorption, X-ray reflection from the white dwarf surface, self occultation, and a structured emission flow. Subsequent pulse profile studies at different photon energies have greatly added to our understanding of the complex accretion and emission geometries. With ROSAT, the link to polars was strengthened by the discovery of about a dozen soft X-ray emitting IPs [25]. The X-ray spectra of some of these closely resemble those of polars, consisting of an optically thin thermal hard X-ray and an optically thick quasi-blackbody soft X-ray component. The superior sensitivity of Chandra and XMM-Newton permits detailed spin-phase resolved spectroscopy of these objects. Figure 12.4 shows the LETG wavelength-integrated light curve of the IP PQ Gem and in Fig. 12.6, later, we compare the mean LETG spectra of PQ Gem and the prototype polar AM Her [8]. Both spectra display a thermal hard X-ray component and a superimposed soft component. For PQ Gem, the latter can be approximated by a 40 eV blackbody absorbed by a column density of 1.3 x 1020 cm-2, for AM Her, more than a single bb-component

0.817

0.817

Fig. 12.4 Chandra LETG soft X-ray light curve of the intermediate polar PQ Gem. The 50000 s exposure covers 2.7 orbital periods and shows the individual 833 s pulsations (from [8, and private communication by V. Burwitz])

Fig. 12.4 Chandra LETG soft X-ray light curve of the intermediate polar PQ Gem. The 50000 s exposure covers 2.7 orbital periods and shows the individual 833 s pulsations (from [8, and private communication by V. Burwitz])

and a complex absorber [63] is needed. The OVII 21.6 - 22. lA complex is nicely resolved in the original LETG data [8].

Near the white dwarf, accretion in IPs is quasiradial and the central graph in Fig. 12.1 applies. Information on the structure of the region emitting the thermal component may be gathered by detailed emission line studies. The Chandra HETG spectra of cooling flows exhibit a continuum and strong lines of H- and He-like ions of the abundant elements, the Fe K-shell lines, and the entire Fe L-shell complex. The line spectra of the little absorbed IPs EX Hya and V603 Aql can be nicely modeled by cooling flows with near-solar abundances. The same is true for the nonmagnetic CVs U Gem and SS Cyg in quiescence, indicating the similarity of the hard X-ray emission regions in the two types of objects [58]. The line spectra of the more strongly internally absorbed IPs, V1223 Sgr, AO Psc, and GK Per, on the other hand, are basically different in that they are dominated by lines produced by photoionization [58]. The spectral resolution of the grating spectrometers on board of Chandra and XMM-Newton allows for the first time detailed plasma diagnostic studies of the emission regions in the brighter X-ray emitting CVs. Mauche et al. [57] have studied the density, temperature, and photoionization dependency of various line ratios and found the FeXXII 7(11.92 A)//(11.77 A) line ratio to be particularly useful for CV studies with a critical density of about 5 x 1013 cm-3 and a low sensitivity to temperature and photoionization. Figure 12.5 shows a small section of the Chandra MEG spectrum of EX Hya, which contains the two FeXXII lines (along with an FeXXIII line at 11.74 A). The 7(11.92 A)/7(11.77 A) line ratio implies a density in the emission region of ne = 1.0 ±2 5 x 1014 cm-3 at a temperature of Te ~ 1.2 x 107 K [57]. If one combines this result with the shock temperature for the 0.5MQ white dwarf [5, and references therein], one finds a preshock mass flow density of ^3 x 10-3 g cm-2s-1, a key plasma parameter characterizing the accretion flow that produces the FeXXII lines in EX Hya. The example demonstrates the potential of the plasma-diagnostic tools now available.

Observations with the Chandra HETG can easily resolve the H-like, He-like, and the fluorescence lines of the Fe Ka emission line complex and are on the verge of

Fig. 12.5 Detail of the Chandra HETG spectrum of EX Hya in the vicinity of the FeXXII 3 ^ 2 lines, binned to 0.005 A (from [57])

resolving the He-like resonance, intercombination, and forbidden lines, as well as the dielectronic satellite lines in the brighter IPs including EX Hya [32]. The absence of Doppler shifts in these lines suggests that they are emitted from near the bottom of the cooling flows. Hence, the flows are not buried in the photosphere of the white dwarf, consistent with the small value of m found above for EX Hya. Observations of EX Hya with the Chandra HETG have also, for the first time, allowed to measure the orbital motion of the white dwarf in a CV using X-ray emission lines [31].

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