  The transformation of powerful gravitational waves, created by the
coalescence of massive black hole binaries, into electromagnetic radiation in
external magnetic fields is revisited. In contrast to the previous calculations
of the similar effect, we study the realistic case of the gravitational
radiation frequency below the plasma frequency of the surrounding medium. The
gravitational waves propagating in the plasma constantly create electromagnetic
radiation dragging it with them, despite the low frequency. The plasma heating
by the unattenuated electromagnetic wave may be significant in a hot rarefied
plasma with strong magnetic field and can lead to a noticeable burst of
electromagnetic radiation with higher frequency. The graviton-to-photon
conversion effect in plasma is discussed in the context of possible
electromagnetic counterparts of GW150914 and GW170104.
