|
Accreting compact objects are unique laboratories for the study of physics under extreme conditions; conditions inaccessible to Terrestrial experimenters. The radiating plasmas have temperatures greater than 107-8 K, higher those found in the core of the Sun, magnetic fields greater than 107 G, more than 10 times larger than even the largest transient fields developed in Terrestrial laboratories, fluid speeds approaching the speed of light, and relativistic gravitational fields. |
I have worked on
neutron star and white dwarf x-ray binary systems
(for further information on some aspects of this research
).
From hereon, I discuss my
work on the white dwarf systems known as the AM Herculis
(AM Her) objects.
The AM Her systems are cataclysmic variables (CVs) which contain strongly
magnetic white dwarfs, B* = 7 MG to 230 MG. The observable
characteristics of the AM Her systems are influenced strongly by the
magnetic field. The magnetic field enforces field-aligned
accretion from somewhere inside the inner Lagrangian point of the system
to one or both of the magnetic poles of the
white dwarf and usually leads to white dwarfs rotating synchronously with
their orbital motions. The bulk of the high energy emission from AM Her
systems is thought to arise in radiative shocks formed as the accreting
material merges onto the white dwarf. Recently, however, it has also become
apparent that the accretion stream can be a significant source of
light in the UV and optical in some systems creating the possibility for
studies of the large scale geometry of the accretion flows in the AM Her
systems. Eclipsing systems are natural laboratories for the study of the
flows in the AM Her systems. In eclipsing systems, constraints can be
placed on the properties of the emission regions. We have studied the
theoretical properties of the radiative shocks and performed
x-ray and optical observational studies of several AM Her systems,
eclipsing and noneclipsing systems.
My theoretical work originally centered on the the calculation of the
continuum x-ray spectra of accreting white dwarfs (eg., Imamura, Durisen, Lamb,
& Weast 1987). However, with the discovery by Middleitch in the 1980's of
rapid quasi-periodic oscillations in several AM Her systems, my
interests shifted to the temporal properties of
radiating shock waves. We performed time-dependent simulations of radiative
shocks, in general, and white dwarf radiative shocks, in particular.
Our studies elucidated the stability properties and observable features of
the oscillatory shock instability discovered by Langer, Chanmugam, &
Shaviv (1982) so that detailed comparisons to the
QPOs could be performed.
Our calculations included the most extensive set of physics
considered. The effects of unequal electron and
ion temperatures, electron thermal conduction, and viscosity, and the
cooling processes of e-i and e-e bremsstrahlung, Compton cooling,
cyclotron cooling, and atomic line cooling were included
(Imamura & Wolff 1990; Imamura, Rashed & Wolff 1991;
Imamura, Aboasha, Wolff, & Wood 1996). In
addition, we considered the
effects of steady and nonsteady accretion flows (Wolff, Wood, & Imamura
1991; Wood, Imamura, & Wolff 1992; Wolff, Wood, & Imamura 1994). Our
theoretical work was completed in the mid 1990's.
Several predictions of the calculations are roughly consistent with the
optical QPOs, e.g., the oscillation time scale and the fact that the
QPOs appear to arise in the vicinity (if not in) the shock region on the
white dwarf in several systems (Middleditch, Imamura, Wolff, & Steiman-Cameron
1991; Imamura, Middleditch, Steiman-Cameron, Scargle, Whitlock, Wolff, &
Wood 1993; Middleditch, Imamura, & Steiman-Cameron 1997). However,
because the dominant component of the shock emission
falls in the x-ray, the applicability of the shock model could not be
reliably tested. The recent launch of the Rossi X-ray Timing Explorer
(RXTE) has allowed us to start comparing the predictions of the shock
model to the x-ray data of several AM Her systems. Unfortunately, the
x-ray data is not yet of high enough signal-to-noise to allow definitive
tests of the shock model in general (Wolff, Wood, Imamura, Middleditch,
& Steiman-Cameron 1999). However, the indications are that the shock
model is not appropriate for the observed QPOs in VV Puppis and V834
Centauri (Imamura, Steiman-Cameron, & Wolff 1999).
It is conceivable that given the large amount of observing time to be
devoted to selected AM Her systems by the Unconventional Stellar Aspect
(USA) experiment on a recently launched ARGOS satellite will allow more
stringent tests of the shock model to be performed.
In parallel to the theoretical work on radiative shocks, an observational
survey of eclipsing AM Her systems is being carried out. The
x-ray data are obtained using the Rossi X-ray Timing Explorer and the
optical data obtained at the Cerro Tololo Inter-American Observatory
(CTIO). Detailed studies of the eclipsing AM Her systems UZ Fornacis
(UZ For, Imamura & Steiman-Cameron 1998), and V2301 Ophiucus
(V2301 Oph, Steiman-Cameron & Imamura 1999; Parker, Imamura, Wolff,
& Steiman-Cameron 1999) have already been performed. Studies of the
systems WW Horologi and HU Aquari are underway. Preliminary results from
our survey are that there is strong evidence for
significant optical emission from the accretion streams in both UZ For and
V2301 Oph, which allows mapping of the topology of the magnetically
controlled accretion flows, and the x-ray emission regions and (therefore
accretion flows) are not point sources but, rather, are spread by >
45o-50o in longitude on the surfaces of the white
dwarfs.
For example, consider V2301 Oph. V2301 Oph is bright in both the
optical and x-rays. This, when
coupled with its eclipsing nature, makes it an ideal testbed for
theories of the large scale topology of AM Her flows and the radiative
shocks in AM Her systems. There are total eclipses in both the x-ray
and optical light curves. The x-ray light curves and eclipses are
consistent with a dominant hot spot and a secondary hot spot. The
dominant hot spot is not a point source; it covers about 50o in
longitude on the surface of the white dwarf (similar results are
obtained for other AM Her systems which we studied which had strong
x-ray emission, e.g., BL Hyi [Wolff, Wood, Imamura, Middleditch, &
Steiman-Cameron 1999]). The x-ray spectra is
consistent with that of a radiative shock (as also true for
BL Hyi [Wolff, Wood, Imamura, Middleditch, & Steiman-Cameron 1999] and
V834 Cen [Imamura, Steiman-Cameron,
& Wood 1999]). We further argue that the x-ray light curve and
eclipse shape also suggest that the accretion occurs in a sheet-like
geometry rather than in a columnar geometry. The optical light curves
and eclipses are consistent with emission from the white dwarf
photosphere, an extended emission region which sits above the surface
of the white dwarf, and the x-ray heated face of the
companion star.
The primary emphases for my future work on the AM Her systems will be
the continued study of eclipsing systems and
the calculation of the x-ray line spectra of white dwarf radiative shocks.
In anticipation of the upcoming launches of the x-ray spectroscopy
missions (the Advanced X-ray Astronomy Facility AXAF and the European
mission XMM) we have begun to modify our numerical shock code to include
the calculation of x-ray line spectra of white dwarf radiative shocks
(Wolff, Wood, Imamura, Middleditch, & Steiman-Cameron 1999; Imamura,
Steiman-Cameron, & Wolff 1999).
With the large expected resolving power of AXAF and XMM, detailed
theoretical line spectra will be needed in order
to fully utilize the data. The PCA detectors of the RXTE
satellite has energy
resolution of ~ 16 % at 6 keV the rough energy of
the Kalpha line complexes of highly ionized iron, the strongest x-ray
spectral features observed in accreting white dwarf systems.
I have worked on
neutron star and white dwarf x-ray binary systems
(for further information on some aspects of this research
).
From hereon, I discuss my
work on the white dwarf systems known as the AM Herculis
(AM Her) objects.
The AM Her systems are cataclysmic variables (CVs) which contain strongly magnetic white dwarfs, B* = 7 MG to 230 MG. The observable characteristics of the AM Her systems are influenced strongly by the magnetic field. The magnetic field enforces field-aligned accretion from somewhere inside the inner Lagrangian point of the system to one or both of the magnetic poles of the white dwarf and usually leads to white dwarfs rotating synchronously with their orbital motions. The bulk of the high energy emission from AM Her systems is thought to arise in radiative shocks formed as the accreting material merges onto the white dwarf. Recently, however, it has also become apparent that the accretion stream can be a significant source of light in the UV and optical in some systems creating the possibility for studies of the large scale geometry of the accretion flows in the AM Her systems. Eclipsing systems are natural laboratories for the study of the flows in the AM Her systems. In eclipsing systems, constraints can be placed on the properties of the emission regions. We have studied the theoretical properties of the radiative shocks and performed x-ray and optical observational studies of several AM Her systems, eclipsing and noneclipsing systems.
My theoretical work originally centered on the the calculation of the continuum x-ray spectra of accreting white dwarfs (eg., Imamura, Durisen, Lamb, & Weast 1987). However, with the discovery by Middleitch in the 1980's of rapid quasi-periodic oscillations in several AM Her systems, my interests shifted to the temporal properties of radiating shock waves. We performed time-dependent simulations of radiative shocks, in general, and white dwarf radiative shocks, in particular. Our studies elucidated the stability properties and observable features of the oscillatory shock instability discovered by Langer, Chanmugam, & Shaviv (1982) so that detailed comparisons to the QPOs could be performed. Our calculations included the most extensive set of physics considered. The effects of unequal electron and ion temperatures, electron thermal conduction, and viscosity, and the cooling processes of e-i and e-e bremsstrahlung, Compton cooling, cyclotron cooling, and atomic line cooling were included (Imamura & Wolff 1990; Imamura, Rashed & Wolff 1991; Imamura, Aboasha, Wolff, & Wood 1996). In addition, we considered the effects of steady and nonsteady accretion flows (Wolff, Wood, & Imamura 1991; Wood, Imamura, & Wolff 1992; Wolff, Wood, & Imamura 1994). Our theoretical work was completed in the mid 1990's.
Several predictions of the calculations are roughly consistent with the optical QPOs, e.g., the oscillation time scale and the fact that the QPOs appear to arise in the vicinity (if not in) the shock region on the white dwarf in several systems (Middleditch, Imamura, Wolff, & Steiman-Cameron 1991; Imamura, Middleditch, Steiman-Cameron, Scargle, Whitlock, Wolff, & Wood 1993; Middleditch, Imamura, & Steiman-Cameron 1997). However, because the dominant component of the shock emission falls in the x-ray, the applicability of the shock model could not be reliably tested. The recent launch of the Rossi X-ray Timing Explorer (RXTE) has allowed us to start comparing the predictions of the shock model to the x-ray data of several AM Her systems. Unfortunately, the x-ray data is not yet of high enough signal-to-noise to allow definitive tests of the shock model in general (Wolff, Wood, Imamura, Middleditch, & Steiman-Cameron 1999). However, the indications are that the shock model is not appropriate for the observed QPOs in VV Puppis and V834 Centauri (Imamura, Steiman-Cameron, & Wolff 1999). It is conceivable that given the large amount of observing time to be devoted to selected AM Her systems by the Unconventional Stellar Aspect (USA) experiment on a recently launched ARGOS satellite will allow more stringent tests of the shock model to be performed.
In parallel to the theoretical work on radiative shocks, an observational survey of eclipsing AM Her systems is being carried out. The x-ray data are obtained using the Rossi X-ray Timing Explorer and the optical data obtained at the Cerro Tololo Inter-American Observatory (CTIO). Detailed studies of the eclipsing AM Her systems UZ Fornacis (UZ For, Imamura & Steiman-Cameron 1998), and V2301 Ophiucus (V2301 Oph, Steiman-Cameron & Imamura 1999; Parker, Imamura, Wolff, & Steiman-Cameron 1999) have already been performed. Studies of the systems WW Horologi and HU Aquari are underway. Preliminary results from our survey are that there is strong evidence for significant optical emission from the accretion streams in both UZ For and V2301 Oph, which allows mapping of the topology of the magnetically controlled accretion flows, and the x-ray emission regions and (therefore accretion flows) are not point sources but, rather, are spread by > 45o-50o in longitude on the surfaces of the white dwarfs.
For example, consider V2301 Oph. V2301 Oph is bright in both the optical and x-rays. This, when coupled with its eclipsing nature, makes it an ideal testbed for theories of the large scale topology of AM Her flows and the radiative shocks in AM Her systems. There are total eclipses in both the x-ray and optical light curves. The x-ray light curves and eclipses are consistent with a dominant hot spot and a secondary hot spot. The dominant hot spot is not a point source; it covers about 50o in longitude on the surface of the white dwarf (similar results are obtained for other AM Her systems which we studied which had strong x-ray emission, e.g., BL Hyi [Wolff, Wood, Imamura, Middleditch, & Steiman-Cameron 1999]). The x-ray spectra is consistent with that of a radiative shock (as also true for BL Hyi [Wolff, Wood, Imamura, Middleditch, & Steiman-Cameron 1999] and V834 Cen [Imamura, Steiman-Cameron, & Wood 1999]). We further argue that the x-ray light curve and eclipse shape also suggest that the accretion occurs in a sheet-like geometry rather than in a columnar geometry. The optical light curves and eclipses are consistent with emission from the white dwarf photosphere, an extended emission region which sits above the surface of the white dwarf, and the x-ray heated face of the companion star.
The primary emphases for my future work on the AM Her systems will be
the continued study of eclipsing systems and
the calculation of the x-ray line spectra of white dwarf radiative shocks.
In anticipation of the upcoming launches of the x-ray spectroscopy
missions (the Advanced X-ray Astronomy Facility AXAF and the European
mission XMM) we have begun to modify our numerical shock code to include
the calculation of x-ray line spectra of white dwarf radiative shocks
(Wolff, Wood, Imamura, Middleditch, & Steiman-Cameron 1999; Imamura,
Steiman-Cameron, & Wolff 1999).
With the large expected resolving power of AXAF and XMM, detailed
theoretical line spectra will be needed in order
to fully utilize the data. The PCA detectors of the RXTE
satellite has energy
resolution of ~ 16 % at 6 keV the rough energy of
the Kalpha line complexes of highly ionized iron, the strongest x-ray
spectral features observed in accreting white dwarf systems.
|
|
|
|
|
|
|
|