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

Abstract

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.

8 pages, 3 figures, Accepted in ApJL

References in corpus (6)

• Five-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Data Processing, Sky Maps, and Basic Results
• An absorption profile centred at 78 megahertz in the sky-averaged spectrum
• Charting the Parameter Space of the Global 21-cm Signal
• Local Voids as the Origin of Large-angle Cosmic Microwave Background Anomalies I
• Exploring cosmic origins with CORE: effects of observer peculiar motion
• Dipole of the Epoch of Reionization 21-cm signal

Cited by in corpus (9)

• Data Analysis for Precision 21 cm Cosmology
• Formulating and critically examining the assumptions of global 21-cm signal analyses: How to avoid the false troughs that can appear in single spectrum fits
• Cosmic Reionization May Still Have Started Early and Ended Late: Confronting Early Onset with CMB Anisotropy and 21 cm Global Signals
• Simulation-Based Inference of the sky-averaged 21-cm signal from CD-EoR with REACH
• Probing the global 21-cm background by velocity-induced dipole and quadrupole anisotropies
• Prospects for probing dark matter particles and primordial black holes with the Hongmeng mission using the 21 cm global spectrum at cosmic dawn
• Effects of observer peculiar motion on the isotropic background frequency spectrum: from monopole to higher multipoles
• Matter Dipole and Hubble Tension due to Large Wavelength Perturbations
• Predictions for the diffuse cosmic dipole at radio frequencies from reionization imprints