Imprints of quasar duty cycle on the 21cm signal from the Epoch of
  Reionization
  Quasars contribute to the 21-cm signal from the Epoch of Reionization (EoR)
primarily through their ionizing UV and X-ray emission. However, their radio
continuum and Lyman-band emission also regulates the 21-cm signal in their
direct environment, potentially leaving the imprint of their duty cycle.
  We develop a model for the radio and UV luminosity functions of quasars from
the EoR, and constrain it using recent observations. Our model is consistent
with the z~7.5 quasar from Banados et al 2017, and also predicts only a few
quasars suitable for 21-cm forest observations (10mJy) in the sky. We exhibit a
new effect on the 21-cm signal observed against the CMB: a radio-loud quasar
can leave the imprint of its duty cycle on the 21-cm tomography. We apply this
effect in a cosmological simulation and conclude that the effect of typical
radio-loud quasars is most likely negligible in an SKA field of view. For a
1-10mJy quasar the effect is stronger though hardly observable at SKA
resolution. Then we study the contribution of the lyman band Ly-alpha to
Ly-beta) emission of quasars to the Wouthuisen-Field coupling. The collective
effect of quasars on the 21-cm power spectrum is larger than the thermal noise
at low k, though featureless. However, a distinctive pattern around the
brightest quasars in an SKA field of view may be observable in the tomography,
encoding the duration of their duty cycle. This pattern has a high
signal-to-noise ratio for the brightest quasar in a typical SKA shallow survey.
