Annual modulation in the intraday variability of quasar 0917+624 due to interstellar scintillation

The quasar 0917+624 has been one of the best-studied intraday variable radio sources. However, debate continues as to whether the underlying cause is intrinsic or extrinsic. Much previous work has assumed the intraday variability (IDV) to be intrinsic, which implies an extraordinarily compact source for the radio emission; in contrast, an extrinsic variation due to interstellar scintillation (ISS) implies a relatively larger source diameter, although at the smaller end of the range expected for relativistic jet models. Kraus et al. reported a marked slowing of the IDV at a wavelength of 6 cm in 1998 September and suggested that a change in the source was responsible. However, here we show that the slowing is consistent with the annual modulation in the scintillation timescale expected for ISS, under the assumption that the scattering medium moves with the local standard of rest (LSR). The ISS timescale is governed by the ISS spatial scale divided by the Earth's velocity relative to the scattering plasma. It happens that in the direction of 0917+624, the transverse velocity of the Earth with respect to the LSR varies widely, with a deep minimum in the months of September-November. Hence, the slowing of the IDV in 1998 September strongly suggests that ISS rather than intrinsic variation of the source is the dominant cause of the IDV.