La Política Social en el neoliberalismo y el deslinde progresista

There are several systems th a t straddle th e IP /p o la r classification in th a t they show characteristics of b o th classes. These include IPs w ith polar-like soft X-ray or EUV com ponents, low field polars, and asynchronous polars.

Recently, several IPs have been discovered w ith a soft X -ray/E U V emission com­ ponent th a t is distin ct from th e hard X -ray com ponent, as is typically observed in polars. These include PQ Gem (Mason et al. 1992), R X J05 5 8 .0+5353 (Haberl et al. 1994) and RX J0512.2-3241 (Burwitz et al. 1996). All three systems were dis­ covered by RO SA T: PQ Gem was identified during th e All Sky Survey and the other two during the R O S A T galactic plane survey. The soft com ponent in these systems could arise if the accretion flow were highly collim ated (resulting in small accretion regions on th e w hite dwarf, as in polars) or if th e absorbing colum n were unusually low. For example, PQ Gem is observed in a direction th a t was identified by Paresce (1984) as having a very low interstellar absorbing colum n (Mason et al. 1992). A nother soft X-ray source identified during th e R O S A T galactic plane survey, R X J1 9 1 4.4+2456, was tentatively classified as a soft IP by H aberl & Motch (1995). This has been called into question by C ropper et al. (1998) and Ramsay et al. (1999b). T he system shows only one period in its X-ray and infrared light curves, and lacks a hard brem sstrahlung com ponent (m aking an IP interpretation less likely). RX J1914.4+2456 is probably a helium -transferring phase-locked polar w ith an extrem ely short orbital period, th e first of its kind to be identified.

T he field estim ates for th e three polarized IPs are sim ilar to the field strengths of some of the low-field polars, such as V2301 O ph (1 H 1752+081). This system has a prim ary w ith a field strength of only ~ 7 MG, m easured from Zeeman split absorption lines (Ferrario et al. 1995). O th er polars w ith field strengths th a t overlap the range of field strengths deduced for IPs include R X J1 9 5 7.1-5738 (B ~ 16 MG:

C H A P T E R 1. S C IE N T IF IC C O N T E X T 43

T hom as et al. 1996) and E F Eri, which has two separate cyclotron emission regions of field strengths 16.5 MG and 21 MG (Ferrario, Bailey &; W ickram asinghe 1996).

There are four polars th a t are slightly asynchronous: BY Cam (e.g. Mason, L iebert k Schm idt 1989), V1500 Cyg (e.g. Schm idt k Stockm an 1991), V1432 Aql (R X J1 9 4 0 .2-1025, e.g. W atson et al. 1995) and R X J2115-5840 (e.g. Schwope et al. 1997b; R am say et al. 1999)7. These system s have been called ‘near-synchronous’ (e.g. Ram say et al. 1999), since their spin and orb ital periods differ by only a few per cent (unlike th e IPs, where the orbital period is typically ~ 10 tim es the spin period). Changes in the overall behaviour are observed on th e b eat period between th e spin and th e orbital period. For example, th e G inga light curves of BY Cam altern ate between flaring and pulsing behaviour over its 14.5 day b eat cycle (Ishida et al. 1991). T he circular polarization variations in RX J2115-5840 over its 7 day beat cycle change from negative excursions, through zero net polarization, to positive excursions (Ram say et al. 1999). The model th a t has been proposed to explain these observations is th a t of ‘pole-sw itching’ (e.g. M ason et al. 1998): during the sp in -o rb ital period beat phase, the orientation of th e m agnetic field changes slowly w ith respect to the incoming accretion stream , and th e stream is directed onto the geom etrically preferred pole a t each phase. Accretion thus altern ates from one pole to th e other once per b eat period. This pole-switching m odel is very sim ilar to th a t proposed for th e discless IP R X J1712-2414 (Buckley et al. 1997), except th a t RX J1712-2414 has a b eat period of 1003 s, as opposed to several days.

T he near-synchronous system s m ust be in a state where th e accretion torque is larger th a n the synchronization torque, or where this has been the case in the recent past. Clues regarding the origin of the increased accretion torque are provided by V1500 Cyg, which experienced a nova o u tb u rst in 1975: it is possible th a t th e spin-

7Recently, the membership of V1432 Aql in the class of near-synchronous systems has been questioned by Mukai (1998), who proposes that the system could be an IP with a discless or hybrid accretion geometry. He argues that the data are insufficient to distinguish between the two models.

up of the white dwarf is a direct result of the nova explosion (the details of the