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We present an analytic formalism to compute the fluctuating component of the ion{H}{1} signal and extend it to take into account the effects of partial Lyman-$alpha$ coupling during the era of cosmic dawn. We use excursion set formalism to calculate the size distribution of randomly distributed self-ionized regions. These ionization bubbles are surrounded by partially heated and Lyman-$alpha$ coupled regions, which create spin temperature $T_S$ fluctuations. We use the ratio of number of Lyman-$alpha$ to ionizing photon ($f_L$) and number of X-ray photons emitted per stellar baryons ($N_{rm heat}$) as modeling parameters. Using our formalism, we compute the global ion{H}{1} signal, its auto-correlation and power spectrum in the redshift range $10 le z le 30$ for the $Lambda$CDM model. We check the validity of this formalism for various limits and simplified cases. Our results agree reasonably well with existing results from N-body simulations, in spite of following a different approach and requiring orders of magnitude less computation power and time. We further apply our formalism to study the fluctuating component corresponding to the recent EDGES observation that shows an unexpectedly deep absorption trough in global ion{H}{1} signal in the redshift range $15 <z< 19$. We show that, generically, the EDGES observation predicts larger signal in this redshift range but smaller signal at higher redshifts. We also explore the possibility of negative real-space auto-correlation of spin temperature and show it can be achieved for partial Lyman-$alpha$ coupling in many cases corresponding to simplified models and complete model without density perturbations.
The 21-cm signal of neutral hydrogen is a sensitive probe of the Epoch of Reionization (EoR) and Cosmic Dawn. Currently operating radio telescopes have ushered in a data-driven era of 21-cm cosmology, providing the first constraints on the astrophysi
Studying the cosmic dawn and the epoch of reionization through the redshifted 21 cm line are among the major science goals of the SKA1. Their significance lies in the fact that they are closely related to the very first stars in the universe. Interpr
We present an analytic formulation to model the fluctuating component of the HI signal from the epoch of reionization during the phase of partial heating. During this phase, we assume self-ionized regions, whose size distribution can be computed usin
The upcoming radio interferometer Square Kilometre Array (SKA) is expected to directly detect the redshifted 21-cm signal from the neutral hydrogen present during the Cosmic Dawn. Temperature fluctuations from X-ray heating of the neutral intergalact
Upcoming measurements of the 21-cm line of neutral hydrogen will open a new observational window into the early stages of structure growth, providing a unique opportunity for probing large-scale cosmological signatures using the small-scale signals f