Dipole Anisotropy as an Essential Qualifier for the Monopole Component of the Cosmic-Dawn Spectral Signature, and the Potential of Diurnal Pattern for Foreground Estimation

While the importance of detecting the Global spectral signatures of the red-shifted 21-cm line of atomic hydrogen from the very early epochs cannot be overstated, the associated challenges primarily include isolating the weak signal of interest from the orders of magnitude brighter foregrounds, and extend equally to reliably establishing the origin of the {\it apparent} global signal to the very early epochs. This letter proposes a critical dipole test that the measurements of the monopole component of the spectrum of interest should necessarily pass. Our criterion is based on a unique correspondence between the intrinsic monopole spectrum and the differential spectrum as an imprint of dipole anisotropy resulting from motion of observer with respect to the rest frame of our source (such as that of our Solar system, interpreted from the dipole anisotropy in CMBR). More importantly, the spectral manifestation of the dipole anisotropy gets {\it amplified} by a significant factor, depending on the monopole spectral slopes, rendering it feasible to measure. We describe details of such a test, and illustrate its application with the help of simulations. The letter also alludes to a novel model-independent path toward isolating the foreground contribution, using the diurnal pattern readily apparent in drift-scan observations. Such dipole qualifier for the monopole spectrum, when combined with reliable foreground estimation, is expected to pave way for in situ validation of spectral signatures from early epochs, important to presently reported and future detections of EoR signal.