Research
In the past two decades, cyclotron emission in polars has been extensively
studied. Currently, there exist many complex models to explain the orbital
modulation of cyclotron harmonics observed in many polars. In older
"Constant-Lambda Models" four global parameters: B, the magnetic field
strength, T, the global temperature of the plasma in the accretion region,
Theta, the viewing angle, and an optical depth parameter were used to fit
observations with much success, accurately describing the wavelength position
of the cyclotron harmonics and the motion of harmonics over the orbital
period, among other things. In the current state of the art, however, the
models are dependent on parameters for which we have little or no direct
observational constraints, such as the thermal profile of each line of sight,
the geometry of the accretion spot, magnetic field structure, and the mass
accretion rate. To determine whether the extra complexity in these new models
is necessary, we will model a number of Polars with both Constant-Lambda as
well as a more complex code which computes the radiative hydrodynamics over
each line of sight in the accretion region, and then does ray tracing to
compute the observed spectrum.
We have obtained low resolution (R ~ 250) infrared spectroscopy of several
magnetic cataclysmic variable stars (polars) with the SPEX instrument at
IRTF.
Our data include phase resolved spectra of EF Eri. It has been shown that the
large photometric variations in the H and K band for this object are due to
cyclotron emission features. Our new data allow us to understand the
photometric variations in the IR band, which are due to the rising and falling
cyclotron emission. For our one component models, we have B=12.6 MG, T=4.5 KeV,
and Log(Lambda) = 5.8. The changing viewing angle over the orbit can be
reproduced by assuming an inclination of 58 degrees, and a magnetic co-latitude
of 6 degrees for the primary accretion spot.
We have spectra of EQ Cet, AM Her, VV Pup, V1432 Aql, XY Ari, ST LMi, AN UMa,
QQ Vul, and HU Aqr as well. EQ
Cet shows a cyclotron feature at 1.05 microns, in addition to the single
optical cyclotron feature at 8200 Angstroms. This implies a lower magnetic
field strength than has been previously derived.
I am currently focusing on phase-resolved (roughly 0.1 phase intervals)
cyclotron spectroscopy of four polars (AN UMa, EF Eri, VV Pup, and EQ
Cet). SPEX covers the I, J, H, and K bands simultaneously, allowing for great
wavelength coverage of all the observed systems. For each object, Constant-
Lambda models were run to compute model cyclotron spectra which were fit to
the data observed by altering four parameters: B, T, Theta, and Lambda, the
"size parameter" of the accretion column.
We find the fits to be satisfactory, although the high temperatures
in EQ Cet (3 Kev) and EF Eri (5 Kev) are a concern. To allow for more
physical setup in the accretion regions in question, we will soon transition
to a structured-shock code based on Fischer and Beuermann (2001), which allows
the effects of variable mass accretion and magnetic fields to be included.
In the future, we will try to resolve cyclotron humps in Intermediate Polars,
which will allow us to place constraints on the magnetic fields of the white
dwarf primaries. Because of obscuration by accretion disks in these systems,
such humps have not been seen yet.
This research is supported by a grant from the New Mexico Space Grant
Consortium (NMSGC).
Meetings
January 2007: American Astronomical Society meeting, Phase-Resolved Infrared Cyclotron Spectroscopy of Polars
R. K. Campbell
January 2005: American Astronomical Society meeting, Low Resolution Infrared Spectroscopy Of Polars
R. K. Campbell & T.E. Harrison
Publications
Spitzer IRS Spectroscopy of Intermediate Polars: Constraints on Mid-Infrared Cyclotron Emission
T. E. Harrison, R. K. Campbell, S. B. Howell, F. A. Cordova, & A. D. Schwope
2007, AJ, in press
Low State, Phase-Resolved IR Spectroscopy of VV Puppis
S. B. Howell, T. E. Harrison, R. K. Campbell, F. A. Cordova, & P. Szkody
2006, AJ, 131, 221
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