Imprints of quasar duty cycle on the 21-cm signal from the Epoch of Reionization

Quasars contribute to the 21-cm signal from the epoch of reionization (EoR) primarily through their ionizing ultraviolet (UV) and X-ray emission. However, their radio continuum and Lyman-band emission also regulate 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 recent discovery of the quasar J1342+0928 at redshift similar to 7.5, and also predicts only a few quasars suitable for 21-cm forest observations (similar to 10 mJy) in the sky. We exhibit a new effect on the 21-cm signal observed against the cosmic microwave background: 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 Square Kilometer Array (SKA) field of view. For a similar to 10-mJy 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.