Weakly Magnetized Neutron Stars

Neutron stars with weak magnetic fields (B — 108-10G) are all found in LMXB. They are most likely old systems with lifetimes that have allowed the originally strong field of the NS to decay (for a discussion on magnetic field decay see [9]). The low mass and the corresponding long development time scale of the companion are in agreement with this picture. Most weakly magnetized NS do not show regular pulsations but rather other types of variability behavior such as X-ray bursts, quasi periodic oscillations (QPOs) and strong low and high frequency noise. However, the Bursting Pulsar, the new class of accreting ms X-ray pulsars and those bursters showing ms burst oscillations or kHz twin QPOs are examples of weakly magnetized NS in which the spin of the NS is displayed.

15.6.1 Z-and Atoll-Sources

Even though X-ray bursters are the largest subgroup of LMXB, we start with a smaller group of objects, some of which are also bursters, with highly interesting spectral and timing properties. They are divided into two types:

- High luminosity (>1037 ergs-1) Z-sources (6 objects), showing quasi periodic oscillations (QPOs)

- Low luminosity (<1037 ergs-1) Atoll-sources (18 objects), mostly the ones showing X-ray bursts. Several of them are now known to also show low frequency QPOs

Both types show strong low and high frequency noise components. The Z/Atoll-classification, based on the correlated spectral and timing behavior of a sample of bright LMXB observed with EXOSAT, was introduced by [22]. For reviews on aperiodic variability in X-ray sources (BH and LMXB) see [38,39]. Figure 15.7 shows

Z SOURCE

Z SOURCE

G.75

G.85

G.9G

G.75

G.85

G.9G

ATOLL SOURCE

ATOLL SOURCE

G.9G G.95 l.GG 1.G5 1.1G 1.15

SOFT COLOR

Fig. 15.7 Color/color diagrams (top) and Power Spectral Densities (bottom)) for Z-sources (left) and for Atoll-sources (right). Soft color is (3-5 keV)/(1-3 keV) and hard color is (6.5-18 keV)/ (5-6.5 keV) (after [2])

l in2

1G-31G-21G-1 1G 1G1 1G

l m2

FREQUENCY (Hz)

Fig. 15.7 Color/color diagrams (top) and Power Spectral Densities (bottom)) for Z-sources (left) and for Atoll-sources (right). Soft color is (3-5 keV)/(1-3 keV) and hard color is (6.5-18 keV)/ (5-6.5 keV) (after [2])

color/color diagrams1 (CD) and power spectral distributions forZ-sources (left) and Atoll-sources (right). The instantaneous state of an individual Z- or Atoll-source is characterized by its current position in the CD. The physical reason is most likely the mass accretion rate M, changes of which simultaneously affect the X-ray spectrum and the X-ray variability. The arrows in Fig. 15.7 indicate the direction in which the accretion rate is thought to increase. So, a source never jumps from one position to another in such a diagram, but always moves along the track according to luminosity. It seems that Z-sources generally have higher magnetic field (109—10 G), higher mass accretion rate and and longer orbital periods (>10h) than Atoll-sources. For the very large amount of details with respect to spectral and timing behavior within the various branches in the CD (which cannot be discussed here) see e.g., [38,41].

15.6.1.1 The Physics of QPOs

Regarding the physics behind QPOs, many models were proposed ( [38] and references therein, [40]). A recent comprehensive review on Rapid X-ray varibility is given in [39]. For moderate accretion rates the so-called beat frequency model has been widely accepted. In this model the QPO frequency is identified with the beat frequency vB between the frequency of Kepler rotation vK of the material at the inner edge of the accretion disk and the spin frequency of the NS vS: vB = vK — vS. The idea is that "clumps" of material can enter the magnetosphere preferentially at certain points (e.g., near the magnetic poles). If vS and vK are different the favorable situation occurs with the beat frequency between these two. Even though the relationship between the position in the CD and M is not really known, certain constraints on the magnetic field and the spin period of the NS can be obtained by applying standard accretion torque theory. Spin periods between 3 and 20 ms were predicted in this way [20]. It took another 6 years until the first accreting ms pulsar with coherent pulsations of 2.5 ms was finally discovered by RXTE in 1998 (see below).

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