INTEGRAL probes the luminosity distribution of Galactic X-ray binaries
The X-ray luminosities of accreting compact object binaries are known
to follow a broken power-law distribution, spanning for at least six
orders of magnitude in luminosity. Most of the sources are thus expected
to be faint and, therefore, hard to observe. This makes it hard to constrain
the parameters of the luminosity distribution (or X-ray luminosity function,
XLF), which is of key importance for understanding the origin and evolution
of X-ray binaries (XRBs).
The XLF is usually derived from the observed flux distribution of X-ray
binaries with known distances. Since only a fraction of the entire
population is normally observed, the derived XLF must then be corrected
for incompleteness effects. Measuring distances is not trivial, so the
sample of XRBs with known distances is rather small, which consequently
implies large statistical uncertainties for the derived XLF parameters.
To overcome this problem, it is proposed to model the observed flux
distribution of XRBs, rather than their luminosity distribution. This
allows to substantially increase the size of the object sample. The fluxes
of high-mass and low-mass X-ray binaries (HMXBs, LMXBs, respectively)
derived from INTEGRAL/ISGRI 9-year survey are used as an observational
reference, which is the most sensitive hard X-ray survey to date. To model
the observed flux distribution, Monte-Carlo calculations are used to simulate
the expected flux distribution from a synthetic XRB population. The animation
in the top panel shows an example of the spatial distribution for simulated
HMXB (blue) and LMXB (red) populations and their X-ray luminosity coded with
the size of the symbol. For each artificial source, the flux which would be
observed by INTEGRAL at Sun location (yellow point), can be easily calculated
and the resulting flux distribution can be compared with observations. The
parameters of the simulated XRB populations can then be adjusted so that the
simulated flux distribution is consistent with observations.
Two plots at the bottom show the resulting range of the allowed XLF parameters
(hatched area) for LMXBs (left) and HMXBs (right). Existing estimates, published
in the literature, are also shown for reference.
Reference:
"Population of the Galactic X-ray binaries and eRosita"
Doroshenko et al., accepted in A&A
(arXiv:1405.0802)
