  We consider scenario of merger of two stellar mass black holes surrounded by
an accretion disk. Due to emission of gravitational waves, the mass of the
central object decreases and accretion disk experiences perturbation.
Calculations show that the main consequence of this disturbance is formation of
a shock wave propagating from the center of the disk to its periphery. Light
curve is computed and duration of the flash is estimated under assumption that
the flash terminates when the luminosity returns to the initial value.
  It is shown that, if the total mass of the merging binary is 55 M_sol (like
in the event GW170814), the flash produced by the shock will increase
bolometric luminosity of the disk by 4--6 orders of magnitude, up to 10^{45}
erg/s (absolute stellar magnitude -23.8^m). With account of the distance to the
source (540 Mpc) and for reasonable assumptions on the parameters of the
accretion disk, it turns out that the apparent magnitude of the flash at the
maximum of the spectral flux density should be 12.8^m -- 14.2^m, while duration
of the flash --- few minutes.
  The main part of the shock radiation flux is emitted in the X-ray and
gamma-ray ranges. In the spectral band of the EPIC instrument of the XMM-Newton
observatory or the telescope eROSITA of the Spectrum-RG observatory (0.3--10
keV), luminosity will increase by 3--4 orders of magnitude, up to 10^{44}
erg/s, corresponding to the apparent stellar magnitude about 17^m. Luminosity
is at maximum in the observational band of the IBIS instrument of the INTEGRAL
observatory (20 keV -- 10 MeV) and will be 10^{44} -- 10^{45} erg/s,
corresponding to the apparent flux 10^{-4} photons per cm^2/s/keV at the
wavelength ~100 keV. From the far UV to the longer wavelengths, the brightening
is virtually absent.
