Constraining the redshifted 21-cm signal with the unresolved soft X-ray background

Abstract

We use the observed unresolved cosmic X-ray background (CXRB) in the 0.5 − 2 keV band and existing upper limits on the 21-cm power spectrum to constrain the highredshift population of X-ray sources, focusing on their effect on the thermal history of the Universe and the cosmic 21-cm signal. Because the properties of these sources are poorly constrained, we consider hot gas, X-ray binaries and mini-quasars (i.e., sources with soft or hard X-ray spectra) as possible candidates. We find that (1) the soft-band CXRB sets an upper limit on the X-ray efficiency of sources that existed before the end of reionization, which is one-to-two orders of magnitude higher than typically assumed efficiencies, (2) hard sources are more effective in generating the CXRB than the soft ones, (3) the commonly-assumed limit of saturated heating is not valid during the first half of reionization in the case of hard sources, with any allowed value of X-ray efficiency, (4) the maximal allowed X-ray efficiency sets a lower limit on the depth of the absorption trough in the global 21-cm signal and an upper limit on the height of the emission peak, while in the 21-cm power spectrum it sets a minimum amplitude and frequency for the high-redshift peaks, and (5) the existing upper limit on the 21-cm power spectrum sets a lower limit on the X-ray efficiency for each model. When combined with the 21-cm global signal, the CXRB will be useful for breaking degeneracies and helping constrain the nature of high-redshift heating sources.

The rise of the first stars: Supersonic streaming, radiative feedback, and 21-cm cosmology

“…While the uncertainties are still large, it now seems that the IGM was most likely heated by X-ray sources with a hard spectrum ( § 6.3 and § 7.4), a possibility not considered until recently; in this case, the cosmic heating transition produces a clear minimum on small scales, but a weak heating peak remains on the largest scales that are larger than the typical distance traveled even by hard X-rays. Continuing with Figure 41, the Lyα peak occurs in this example at z = 18 − 20, and (generally in the case of late heating) it is both the strongest and highest-redshift signal from the first stars (In the case of a soft X-ray spectrum, the heating peak is somewhat higher than the Lyα peak [261]). We note that additional theoretical uncertainties result from the complexity of the astrophysics during early times, including substantial transitions in the basic character of star formation expected due to various types of stellar feedback such as supernova outflows, LW radiation, and metal enrichment.…”

“…Indeed, a clear observational indication that this feature corresponds to a cosmic milestone is that the minima at all k > 0.5 Mpc −1 should occur at essentially the same redshift (namely the true redshift of the heating transition); lower wavenumbers correspond to larger scales than the typical X-ray mean free path, leading to a more complicated evolution and to minima delayed to lower redshifts (see also Figure 41). More generally, observations of the 21-cm power spectrum over a broad range of wavenumbers will clearly probe the X-ray spectrum of the sources of cosmic heating [92,260,261,130].…”

“…However, the difference is visually striking, in that the map for the hard spectrum is strongly suppressed in terms of both the typical value of T b and the typical size of its fluctuations. From [261].…”

Analysing the 21 cm signal from the epoch of reionization with artificial neural networks

“…During the Cosmic Dawn, X-ray heating of the IGM plays an important role. Thus, it is important to incorporate X-ray heating models and study their impact on the 21 cm (Baek et al 2010;Mesinger et al 2013;Ewall-Wice et al 2016;Fialkov et al 2017). Recently, (Cohen et al 2016) explored the behaviour of the 21cm global signal in a large parameter space from z = 40 to z = 6.…”

A Space-based Observational Strategy for Characterizing the First Stars and Galaxies Using the Redshifted 21 cm Global Spectrum

“…DARE can measure x X,III with 15% uncertainty (see Figure 8). Further modeling plus multi-wavelength observations (e.g., the cosmic X-ray background; Fialkov et al 2017) may help to better constrain the identity of the universe's first X-ray sources, whether they be black hole X-ray binaries, hot gas in star-forming galaxies, or proto-quasars.…”