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X-ray Binaries
Long Term Intensity Variations in X-ray Binaries
The availability of a multi-mission archive spanning many years has
been central to a number of important insights concerning the
properties of X-ray binaries. Both high mass and low mass systems
often show X-ray modulations on periods longer than the orbital period
of the system. The mechanism that produces such super-orbital
periods is still uncertain. Several processes have been proposed : a
precessing accretion disk, neutron star precession, and feedback in
the mass transfer process. The presence of a third member in some
systems has also been suggested. Since these periods are often months
to years, it is essential to monitor sources over many years. The
HEASARC archive has proven to be essential for such studies.
Wijnands et al. (1996, ApJ 473, L45) found a tentative ~78 day period in
RXTE/ASM data from Cyg X-2 and then confirmed this
period by examining archival data from the Vela-5B and Ariel V
all-sky monitor data sets. The ratio of super-orbital to orbital period in Cyg
X-2 is similar to the high mass systems (e.g.. LMC X-4). This,
combined with the fact that Type I burst activity is not correlated
with the long term modulation suggests that mass transfer feedback is
probably not responsible for the periodic flux modulation; more likely
a precessing accretion disk is responsible (Wijnands et al. 1996).
More recently, Paul et al. (2000, ApJ 528, 410) have used
archival RXTE, Ginga, Ariel V, Vela-5B and HEAO-1 data to investigate
the long term intensity variations of Cyg X-2 and also LMC X-3. They find
that more than one periodic or quasi-periodic component is almost
always present in Cyg X-2 at 40 and 69 days but the 78 day period was
not present in a longer RXTE ASM dataset.
These results, as well as others (e.g. Kong et al. 1998, New
Astron. 3, 301), highlight the unique strengths of easily accessible
archival data: the ability to confirm or reject tentative
identification of modulation periods in accreting binaries; the
ability to make more sensitive post facto searches of older data
sets when significant periods are discovered in new data; and perhaps
most importantly, to maintain a long, ever increasing temporal
baseline for variability studies.
Accreting Pulsars
Long term monitoring of the spin frequency of binary X-ray pulsars has
led to a number of new insights into accretion onto magnetized neutron
stars. The capability of BATSE to monitor the frequencies of a sample
of bright pulsars led to the realization that torque reversals are
common and that the magnitudes of the spin-up and spin-down
torques are almost the same and moreover, are consistent with those
expected from simple theoretical estimates for magnetized
neutron stars (see Bildsten et al. 1997, ApJS 113, 367; and references
therein).
BATSE data continue to be a valuable resource for the study of
pulsars. For example, Cagdas & Baykal (2000, A&A 353, 617) have used
archival BATSE data to investigate the relationship between X-ray flux
and pulse frequency in three high mass binary pulsars, Vela X-1,
GX301-2, and OAO 1657-415. They find a strong correlation between
specific angular momentum and torque. This, and the lack of any
correlation between the pulse frequency derivative and flux, supports the
idea that the accretion geometry changes continuously in a manner
consistent with hydrodynamic simulations (Blondin et al. 1990, ApJ 356,
591).
Paul et al. (1997, A&A 320, L9) used BATSE archival data to
study the relationship between torque and luminosity in the X-ray
pulsar GX 1+4. They find a positive correlation. However, changes in
the pulse period were found to lag the flux changes by ~6
days. Since this is much longer than any characteristic time-scale in
the vicinity of the neutron star, they argue that the delay may be due
to internal properties of the neutron star, perhaps a manifestation of
the crust-core coupling time. The availability of the BATSE data in the
HEASARC archive, along with scientific expertise to maintain them,
will ensure many more such studies.
BATSE archival data for GX1+4 from Pereira et al.
(1999)
Evidence for a ~304-day periodicity in the spin history of the
accretion-powered pulsar GX1+4 that is most probably associated with
the orbital period of the system has been obtained using archival data
from BATSE (Fig. 8). Attempts over the last 20 years to find the
orbital period of GX 1+4 by Doppler shifts of the pulsar pulse timing
or optical lines had been inconclusive. For the X-ray timing
measurements, the accretion torque magnitude is much larger than the
expected orbital Doppler shifts, and the torque fluctuations have
significant power at the time-scales comparable to the expected binary
period (Chakrabarty 1996). Pereira et al. (1999, ApJ 526, L105) have
derived a period for GX1+4 of 303.8 +- 1.1 days, making it the widest
known low-mass X-ray binary system by more than an order of
magnitude. A likely scenario for this system is an elliptical orbit in
which the neutron star decreases its spin-down rate (or even exhibits
a momentary spin-up behavior) at periastron passages due to the higher
torque exerted by the accretion disk onto the magnetosphere of the
neutron star.
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Last modified: Monday, 19-Jun-2006 11:24:57 EDT
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