Abstract

1
Modeling of High/Soft State Flares in Cygnus X-1
Abstract We present first modeling results of the
rapid spectral variability of flares in the X-ray
binary Cygnus X-1 in the high/soft state. The
coupled radiation transfer and electron
heating/cooling problem was solved with a fully
time-dependent 2-D Monte-Carlo/Fokker-Planck
code. Starting with an initial soft state model
consisting of an optically thick accretion disk
sandwiched by a hot corona, we modeled a high
energy flare through an impulsive energy release
in that corona. This flare could be
representative of a reconnection event of
magnetic field lines anchored in the disk. We
found that such a scenario provides a good fit to
the rapid (millisecond timescales) spectral
evolution recently observed in Cyg X-1.
Justin Finke (Ohio U.) and Markus Böttcher (Ohio
U.)
Two Dimensional Monte Carlo/Fokker-Planck Code
Figure 1 An artists conception of an X-ray
binary. Courtesy NASAs HEASARC web page.
Figure 5 Light curve during high/soft flare.
Black crosses are RXTE observations with error
bars, red line is simulated with the following
parameters mdot0.05, a0.1, tflare0.02 s,
(DT/T)²7.
Figure 3 Simulation geometry illustration from
Böttcher, Jackson Liang (2003).
What are X-ray binaries? Often times, a normal
star will orbit near a compact object with
intense gravity (a black hole or a neutron star).
Matter of the outer layers of the normal stars
may be transferred to the compact object. This
material will not fall onto the it right away it
will slowly circle the object first, like water
going down a drain, forming an accretion disk.
Viscosity causes the plasma in this disk to heat
up to T 107 K, which then emits blackbody
radiation in the X-rays. X-ray spectra for these
objects consist of a disk blackbody and a hard
power law tail, with variability on the order of
milliseconds to months.

• A cylindrical series of vertical and radial
  zones was used to represent the corona. Each
  zone has its own set of variables (temperature,
  density, magnetic field, etc.) see Figure 3.
  Blackbody photons are injected from below,
  representing the accretion disk.
• A Monte Carlo technique is used to simulate
  photon transport the electron evolution was
  calculated with the locally isotropic
  Fokker-Planck equation.
• In each time step, photon scattering and the
  plasmas evolution are calculated photons that
  stay in the simulation volume are stored for the
  next step, while the ones that leave the volume
  are stored in an event file and used to
  calculate light curves and spectra.

Figure 6 Simulated Cyg X-1 spectra before
(black) during (red) and after (green) the flare.

• Cygnus X-1
• Cygnus X-1 is the first-discovered,
  most-observed X-ray binary. Consists of a
  normal O9 supergiant star and a 10 solar mass
  black hole.
• Has been observed in two major states the
  low/hard state, and the high/soft state (see
  Figure 2).
• In the low/hard state, Cyg X-1 has a lower
  luminosity, and emits mostly harder (i.e., more
  energetic) X-rays. In the high/soft state, Cyg
  X-1 has a higher luminosity, and emits mostly
  softer (i.e., less energetic) X-rays.
• Cyg X-1 spends most of its time in the low/hard
  state.

Simulations of Cyg X-1 Millisecond Flares

• Powerful millisecond flares were discovered in
  Cyg X-1 in the low/hard and high/soft states by
  Gierlinski Zdziarski (2003).
• A possible cause of the flare a magnetic field
  reconnection event in the corona, which increases
  the temperature in an active region for a few
  milliseconds.
• A flare in the high/soft state has been
  successfully simulated with the 2-D MC/FP code.
• Our flare simulation involved a sudden increase
  in proton temperature in part of the corona. The
  electron temperature responds to this, as shown
  in Figure 4.
• Observed in the high/soft flare were an increase
  in luminosity as well as a hardening of the
  spectrum, both of which were reproduced in our
  simulation.
• Figure 5 shows the observed and simulated light
  curves. Figure 6 shows the spectrum before,
  during, and after the flare clearly there is a
  hardening. This is also seen in Figure 7 which
  shows the spectral index, a measure of hardness.

Figure 6 observed (black) and simulated (red)
spectral indices (G) during the flare. F(E)
E-G the lower G, the harder the spectra.
Figure 2 Energy Spectra and illustration of (a)
the low/hard state and (b) the high/soft state.
Courtesy Zdziarski Gierlinski (2004).

• The Future?
• Simulations of other observed flares in Cyg X-1,
  especially those in the low/hard state.
• Develop a more efficient, versatile,
  parallelized Monte Carlo/Fokker-Planck code.

• Spectra of X-ray Binaries
• Soft blackbody photons are thought to come from
  the disk, while the hard power law tail is
  thought to be from photons Compton up-scattered
  in optically thick corona.
• In the low/hard state, Cyg X-1 is thought to
  have a large corona, and a less extended
  accretion disk.
• In the high/soft state, it is thought to have a
  smaller corona and a more extended accretion disk.

References Böttcher, M., Jackson, D. R., Liang,
E.P., 2003, ApJ, 586, 389 Gierlinski, M.,
Zdziarski, A. A., 2003, MNRAS, 343,
L84 Zdziarski, A. A., Gierlinski, M., 2004,
PThPh, in press (astro-ph/0403683)
Figure 4 coronal electron temperature profiles
(a) before (b) during and (c) after the flare.
Contours are labeled in keV.