Coronal structure inferred from eclipses

I now review selected results from analyses of X-ray eclipses, and, in subsequent subsec- tions, of rotational modulation and Doppler measurements. Some important parameters are summarized in Table 3.

11.10.1. Extent of eclipsed features

Some shallow X-ray eclipses in tidally interacting binary systems of the RS CVn, Algol, or BY Dra type have provided important information on extended coronal structure. For example, Walter et al. (1983) concluded that the coronae in the AR Lac binary components are bi-modal in size, consisting of compact, high-pressure (i.e., 50–100 dynes cm−2) active regions with a scale height < R_∗, while the subgiant K star is additionally surrounded by an extended (2.7R∗) low-pressure corona. This view was supported by an analysis of ROSAT data by Ottmann et al. (1993). In the analysis of White et al. (1990), the association of different regions with the binary components remained ambiguous, and so did the coronal heights, but the most likely arrangement again required at least one compact region with p > 100 dynes cm−2 and favored an additional extended, low-pressure coronal feature with p ≈ 15 dynes cm−2and a scale height of≈ R_∗. Further, a hot component pervading the entire binary system was implied from the absence of an eclipse in the hard ME detector on EXOSAT. Culhane et al. (1990) similarly observed a deep eclipse in TY Pyx in the softer band of EXOSAT but a clear absence thereof in the harder band, once more supporting a model including an extended, hot component. From an offset of the eclipse relative to the optical first contact in Algol, Ottmann (1994) estimated the height of the active K star corona to be ≈ 2.8R.

Table 3. X-ray coronal structure inferred from eclipses and rotational modulation

Star Spectrum Extendeda nb_e Compacta nb_e Referencec height (R_∗) height (R_∗) AR Lac G2 IV+K0 IV ≈ 1 0.29 0.01 4–6 1 AR Lac G2 IV+K0 IV 1.1–1.6 0.2–0.8 0.06 >5 2 AR Lac G2 IV+K0 IV 0.7–1.4 0.3–0.8 0.03–0.06 6–60 3 AR Lac G2 IV+K0 IV ≈ 1 0.12–1.8 – – 4 Algol B8 V+K2 IV 0.8 ... – – 5 Algold B8 V+K2 IV – – _∼< 0.5 _∼> 9.4 6 Algold B8 V+K2 IV – – 0.1 _∼< 3 7 TY Pyx G5 IV+G5 IV ≈1–2 0.02–3 – – 8 XY UMa G3 V+K4 V – – ≤ 0.75 ... 9 VW Cepd K0 V+G5 V 0.84 5 – – 10 αCrB A0 V+G5 V – – _∼< 0.2 _∼< 3 11 αCrB A0 V+G5 V – – _∼< 0.1 0.1–3 12 EK Dra dG0e – – _∼< 0.2 _∼> 4 13 YY Gem dM1e+dM1e – – 0.25–1 0.3–3 14 V773 Taud K2 V+K5 V ≈ 0.6 ≥ 20 15

Notes.aExtended structures of order R_∗, compact structures significantly smaller. b_{Electron density in 10}10_cm−3_{for extended and compact structures, respectively.}

c_{References: 1 Walter et al. (1983); 2 White et al. (1990); 3 Ottmann et al. (1993); 4 Siarkowski} et al. (1996); 5 Ottmann (1994); 6 Schmitt and Favata (1999); 7 Schmitt et al. (2003); 8 Pre´s et al. (1995); 9 Bedford et al. (1990); 10 Choi and Dotani (1998); 11 Schmitt and Kürster (1993); 12 Güdel et al. (2003b); 13 Güdel et al. (1995); 14 Güdel et al. (2001a); Skinner et al. (1997). d_{Refers to observation of eclipsed/modulated flare.}

White et al. (1986) inferred, from the absence of X-ray dips or any modulation in the light curve of Algol, a minimum characteristic coronal scale height of 3R_(i.e., about 1R_∗). Similar arguments were used by Jeffries (1998) for the short period system XY UMa to infer a corona that must be larger than 1R_unless more compact structures sit at high latitudes. This latter possibility should in fact be reconsidered for several observations that entirely lack modulation. Eclipses or rotational modulation can be entirely absent if the active regions are concentrated toward one of the polar regions. This possibility has found quite some attention in recent stellar research.

Detailed studies of light curves that cover complete binary orbits with ASCA raise, however, some questions on the reliability of the derived structure sizes. Unconstrained iterations of the light curve inversion algorithm do converge to structures that are ex- tended on scales of R_∗(Fig 16); but constrained solutions exist that sufficiently represent the light curves with sources no larger than 0.3R_∗(Siarkowski et al. 1996). Nevertheless, detailed studies of the imaging reconstruction strategy and the set-up of initial conditions led Pre´s et al. (1995) to conclude that X-ray bright sources do indeed exist far above the surfaces in the TY Pyx system, most likely located between the two components. The latter configuration includes the possibility of magnetic fields connecting the two stars. Interconnecting magnetic fields would draw implications for magnetic heating through

0 2 4 6 8 10 count rate 0.9 1.0 1.1 1.2 1.3 time (JD − 2451659.0) −0.2 −0.1 phase0.0 0.1 0.2 YY Gem 2000/04/24 a eclipse 0.0 0.5 1.0 1.5 2.0 2.5 3.0 count rate 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 time (d) −0.5 −0.4 −0.3 −0.2 −0.1 phase0 0.1 0.2 0.3 0.4 0.5 AR Lac 1993/06/01−3 b

Fig. 16. Two examples of eclipses and the corresponding coronal image reconstructions. From top to bottom: Light curve of the YY Gem system (from Güdel et al. 2001a, observation with XMM-Newton EPIC); light curve of the AR Lac system (after Siarkowski et al. 1996, observation

with ASCA SIS); reconstructed image of the coronal structure of, respectively, YY Gem (at phase 0.375) and AR Lac (at quadrature). The latter figure shows a solution with intrabinary emission. (The light curve of AR Lac is phase-folded; the actual observation started around phase 0; data and image for AR Lac courtesy of M. Siarkowski.)

reconnection between intrabinary magnetic fields, as was suggested by Uchida and Saku- rai (1984, 1985) and was also proposed from radio observations of the RS CVn-type binary CF Tuc (Gunn et al. 1997), the Algol-type binary V505 Sgr (Gunn et al. 1999), and the pre-cataclysmic binary V471 Tau (Lim et al. 1996). The X-ray evidence remains ambiguous at this time, and alternative X-ray methods such as Doppler measurements (Ayres et al. 2001b) have not added support to this hypothesis (Sect. 11.14).

11.10.2. Structure and location of coronal features

The eclipse light curves often require asymmetric, inhomogeneous coronae, with bright features sometimes found on the leading stellar hemispheres (e.g., Walter et al. 1983; Ottmann et al. 1993; Ottmann 1994), but often also on the hemispheres facing each other (Bedford et al. 1990; Culhane et al. 1990; White et al. 1990; Siarkowski 1992; Siarkowski et al. 1996; Pre´s et al. 1995). This may, as mentioned above, have important implications for intrabinary magnetic fields. For the dMe binary YY Gem, Doyle and Mathioudakis (1990) reported a preferred occurrence of optical flares also on the two hemispheres facing each other, and this hypothesis has been supported by the timing of X- ray flares (Haisch et al. 1990b). However, X-ray image reconstruction from light curves does not require any emission significantly beyond 2R_∗(Güdel et al. 2001a, Fig. 16): interconnecting magnetic fields are thus not supported in this case. Similarly, a deep eclipse observed on XY UMa (in contrast to an observation reported by Jeffries 1998) places the eclipsed material on the hemisphere of the primary that faces the companion, but judged from thermal loop models, the sources are suggested to be low-lying (Bedford et al. 1990). A concentration of activity on the inner hemispheres could, alternatively, be induced by tidal interactions and may therefore not require any interconnecting magnetic fields (Culhane et al. 1990).

Eclipse modulation also confines the latitudes b of the eclipsed material. For AR Lac, b= 10◦− 40◦(Walter et al. 1983, and similarly in White et al. 1990, and Ottmann et al. 1993). Bedford et al. (1990) infer−30◦≤ b ≤ +30◦from a deep eclipse on XY UMa. Most active regions on the dMe binary YY Gem (dM1e+dM1e) are concentrated around±(30◦− 50◦)(Güdel et al. 2001a), in good agreement with Doppler imaging of surface active regions (Hatzes 1995). The confinement of the inhomogeneities leads to the somewhat perplexing result that these most active stars reveal “active” X-ray filling factors of no more than 5–25% despite their being in the saturation regime (Sect. 5; see White et al. 1990; Ottmann et al. 1993).

11.10.3. Thermal properties of coronal structures

The presence of distinct compact and extended coronae may reflect the presence of different thermal structures – in fact, the distinction may simply be due to different scale heights if magnetic fields do not constrain the corona further. The average radial density profile of YY Gem with a scale height of≈ [0.1 − 0.4]R_∗derived from eclipse reconstruction is in good agreement with the pressure scale height of the one component of the plasma that dominates the X-ray spectrum (Güdel et al. 2001a). Compact sources are often inferred with size scales comparable to solar active regions. Based on such arguments, the extended structures are more likely to be associated with the hottest persistent plasma in active binaries (Walter et al. 1983; White et al. 1990; Rodonò et

al. 1999, but see Singh et al. 1996a and Siarkowski et al. 1996 for alternative views). Inferred pressures of up to >100 dynes cm−2make these regions appear like continuously flaring active regions (Walter et al. 1983; White et al. 1990). Alternatively, they could contain low-density, slowly cooling gas ejected from large flares. This view would be consistent with radio VLBI observations. The latter have mapped non-thermal flares that expand from compact cores to extended ( 1R∗) halos where they cool essentially by radiation (Güdel 2002 and references therein). Conjecture about different classes of thermal sources is again not unequivocal, however. Ottmann et al. (1993) and Ottmann (1994) found equivalent behavior of soft and hard spectral components during eclipses in AR Lac and Algol, respectively, and argued in favor of a close spatial association of hot and cool coronal components regardless of the overall spatial extent. This issue is clearly unresolved.