Longterm Periodicities in Xray Binaries

1
Longterm Periodicities in X-ray Binaries Will
Clarkson1,?, Phil Charles1, Malcolm Coe1, Silas
Laycock1,2 1 Department of Physics and Astronomy,
Southampton University, SO17 1BJ, UK 2
Harvard-Smithsonian Center for Astrophysics,
Cambridge MA 02138, USA
? wic_at_astro.soton.ac.uk
Abstract A significant number of X-ray binaries
are now known to exhibit long-term periodicities
on timescales of 10 - 100 days.
Several physical mechanisms have been proposed
that give rise to such periodicities, one of
which is warping and precession of the
accretion disk. Recent theoretical work predicts
the stability to disk warping of X-ray binaries
as a function of the mass ratio, binary
radius, viscosity and accretion efficiency, and
we attempt here to confront this theory with
observations of a sample of neutron star X-ray
Binaries.

• 2. Superorbital Periodicities
• 15 sources show variations on timescales of ten
  days or more. Persistently bright sources
• Source Mdonor1 Porb2 Plong2
• 1. Her X-1 2.2 1.70 35
• 2. LMC X-4 14.8 1.41 30
• 3. SMC X-1 10.8 3.89 60
• 4. Cyg X-2 0.48 9.84 78
• 1Solar Masses 2days

• 4. Dynamic Power Spectrum
• Power spectrum is plotted as a function of time

• 3. Accretion Disk Warping
• Precession of disk warp ? longterm X-ray
  variation at the observed timescale
• Force out of disk plane needed to maintain warp.
  Three possibilities
• Reprocessed emission from accretion disk
  (radiation-driven warping e.g. Petterson 1977)
  
• Accretion disk wind (e.g Schandl Meyer 1994)
• Magnetic field of donor or ns / disk (e.g Murray
  et al. 2002)

150

• 7year RXTE/ASM dataset
• Sliding 400d window, 50d overlap between windows
• Power spectrum computed for each window
• LS power re-plotted as grayscale

LS Power
0
Period (d)
1000
25
2002
Time
1996
Period (d)
5. Warp Stability Predictions
6. Observed Behaviour vs. Predictions
Mode 1 higher

• DPS ? classification of observed warping
  behaviour
• Observed behaviour commensurate with the
  predicted stability region of each source

Mode competition
3
Stable mode 0
4
Intermediate Instability
2
1
Her X-1
LMC X-4
Warping not permitted
Binary Separation
(Figure from Ogilvie Dubus, 2001
Cyg X-2
Stable mode 0
Stable mode 0
Mode 1 higher
M2/ M1

• Binary parameters ? warp behaviour prediction
  (Wijers Pringle 1999, Ogilvie Dubus 2001)

Precession Frequency
SMC X-1
2002
Mode competition

• Clarkson, W.I., Charles, P.A., Coe, M.J.,
  Laycock, S., Tout, M., Wilson-Hodge, C., 2003
  MNRAS 339, 447
• Clarkson, W.I., Charles, P.A., Coe M.J., Laycock
  S., 2003 MNRAS in press, astro-ph/0304073
• Murray, J.R., Chakrabarty, D., Wynn, G.A.,
  Kramer, L., 2002 MNRAS 335, 247
• Ogilvie, G.I., Dubus, G., 2001 MNRAS 320, 485
• Petterson, J., 1977 ApJ 218, 783
• Schandl, S., Meyer, F., 1994 AA 289,149
• Wijers, R.A.M., Pringle J.E., 1999 MNRAS 308, 207

1996
Binary Radius
Period (d)
1000
25
Radiation-driven warping correctly predicts the
nature of observed X-ray superorbital
periodicities, for our selection of bright
neutron star XRB..