arxiv: 2604.21666 · v1 · submitted 2026-04-23 · 🌌 astro-ph.CO · gr-qc

Recognition: unknown

Impact of Primordial Black Hole population on 21 cm observables at high redshift

Atrideb Chatterjee , Barun Maity , Koushiki

Authors on Pith no claims yet

Pith reviewed 2026-05-08 14:12 UTC · model grok-4.3

classification 🌌 astro-ph.CO gr-qc

keywords 21 cm signalprimordial black holescosmic dawnreionizationX-ray heatingJWSTpower spectrumAGN

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The pith

Primordial black holes that seed early AGNs can make the 21-cm global signal shallower and suppress its power spectrum during cosmic dawn.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper extends a semi-numerical reionization model to include X-ray heating from primordial black holes that may seed the AGNs recently observed by JWST at high redshifts. It computes both the global 21-cm signal and its power spectrum from redshift 30 to 5. A sympathetic reader would care because the 21-cm line is a primary tool for mapping the early universe, and new JWST data requires updating predictions before observations begin. The central finding is that PBH X-ray output reduces the depth of the absorption feature and lowers the expected fluctuation power at cosmic dawn. Different assumed mass distributions for the black holes produce noticeably different signals.

Core claim

By incorporating the X-ray contributions from PBHs into the explicitly photon-conserving SCRIPT framework along with standard cosmic dawn physics, the model predicts that this additional heating shallows the global 21-cm absorption trough and reduces the amplitude of the 21-cm power spectrum. The choice of PBH mass function, while obeying observational constraints, leads to significantly different 21-cm predictions.

What carries the argument

The extended SCRIPT semi-numerical code that adds PBH X-ray heating to the reionization and cosmic dawn calculations.

If this is right

The global 21-cm signal reaches a shallower minimum than in models without PBHs.
The 21-cm power spectrum amplitude is suppressed during the cosmic dawn epoch.
Predictions for the 21-cm observables depend strongly on the assumed primordial black hole mass function.
Self-consistent modeling from z=30 to z=5 becomes possible within one framework.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

Future 21-cm observations could constrain the abundance or mass distribution of primordial black holes if the heating effect is isolated from other sources.
Models that ignore early AGN activity may overestimate the contrast of the 21-cm signal and thus the detectability with instruments like HERA or SKA.
Alternative seeding mechanisms for high-redshift AGNs would produce different heating histories and 21-cm signatures.

Load-bearing premise

That the AGNs seen by JWST at redshifts above 6 are seeded by primordial black holes whose X-ray output can be added directly without other dominant heating channels at those epochs.

What would settle it

A detection of a global 21-cm absorption feature deeper than the PBH-inclusive predictions at redshifts around 15-20 would indicate that the assumed PBH heating is too strong or that the seeding assumption does not hold.

Figures

Figures reproduced from arXiv: 2604.21666 by Atrideb Chatterjee, Barun Maity, Koushiki.

**Figure 1.** Figure 1: Mass functions obtained for Different PBH models at z ∼ 5 along with the recent JWST estimates (Matthee et al. 2024; Taylor et al. 2025). The magenta, green, and blue lines correspond to power-law, log-normal, and critical mass functions, respectively. Further, the black points with error bars are from Matthee et al. (2024), whereas the red ones are taken from Taylor et al. (2025). Since we do not add the … view at source ↗

**Figure 2.** Figure 2: Redshift evolution of the global X-ray emissivity, Ly-α background flux and Ionizing photon production rate for different PBH models along with the SF galaxies. The black, magenta, green, and blue lines respectively denote SF, lognormal, powerlaw and critical PBH seeded systems as mentioned in the legends. 25 20 15 10 5 Redshift (z) 75 50 25 0 25 50 Tb [m K] SF+lognormal SF+powerlaw SF+critical SF-only 25 … view at source ↗

**Figure 3.** Figure 3: The redshift evolution of the 21-cm global signal and the power spectrum (computed at k ∼ 0.1 h cMpc−1 ) corresponding to different models studied in this paper. The legends in the left panel show the colour scheme for various models, whereas the legends in the right panel show the upper limits obtained from different observations. power-law, and critical to compute key quantities relevant to the 21-cm sig… view at source ↗

**Figure 4.** Figure 4: Effect of varying f PBH X,esc in the lognormal mass function. As mentioned in the legend, the dashed magenta line shows the scenario with f PBH X,esc = 1.0, whereas the solid magenta line represents the case with f PBH X,esc = 1.0. Further, the black line shows the SF-Only scenario. as current observations suggest that these early AGNs produce very weak radio emission compared to AGNs observed in the local… view at source ↗

read the original abstract

The 21-cm signal, one of the most promising probes of the high-redshift Universe, has traditionally been modelled without accounting for the effects of active galactic nuclei (AGN) in the pre-JWST era, primarily due to the lack of observational evidence for AGNs at z > 6. However, following the discovery of several AGNs at redshifts as high as z ~ 10 by JWST, it has become imperative to incorporate the impact of these early AGNs when predicting the 21-cm signal. Supposing that these AGNs are seeded by primordial black holes (PBHs), we study their impact with a semi-numerical model setup. Specifically, we extended the explicitly photon-conserving reionization framework, SCRIPT, including essential cosmic dawn physics and PBH contributions. This enables us to compute both the global signal and the power spectrum of the 21-cm line over the redshift range z ~ 30 - 5 within a self-consistent framework. Building on this setup, we then investigate the impact of different PBH mass functions (obeying current observational constraints) on the resulting signal. The X-ray heating from PBHs can substantially make the depth of the global 21-cm signal shallower and suppress the expected power amplitude during cosmic dawn. We also find that the choice of mass function plays a crucial role in shaping the 21-cm signal, and can, in fact, lead to significantly different predictions.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

This extends SCRIPT to add PBH X-ray heating assuming JWST AGNs are PBH-seeded, showing shallower global 21cm absorption and lower power amplitude, but the outputs track that assumption directly.

read the letter

The paper's central move is to take the JWST high-redshift AGN detections, assume they are seeded by primordial black holes, and fold the resulting X-ray output into the existing SCRIPT reionization code. It then runs the extended model across z~30-5 to produce both the global 21cm signal and the power spectrum for several observationally allowed PBH mass functions. The main reported effect is that the extra heating shallows the absorption trough and reduces power-spectrum amplitude during cosmic dawn, with the choice of mass function producing visibly different outcomes.

Referee Report

2 major / 1 minor

Summary. The paper extends the SCRIPT semi-numerical reionization framework to incorporate X-ray heating and ionization from primordial black holes (PBHs) under the assumption that JWST-observed AGNs at z>6 are PBH-seeded. It computes the global 21-cm signal and power spectrum over z~30-5 for several PBH mass functions consistent with current constraints, reporting that PBH X-rays make the global absorption trough shallower and suppress power amplitude during cosmic dawn, with the mass function choice leading to significantly different predictions.

Significance. If the seeding assumption holds, the work demonstrates the sensitivity of high-redshift 21-cm observables to early AGN/PBH X-ray contributions and provides a self-consistent extension of an existing photon-conserving code. The explicit exploration of multiple mass functions is a strength, as it shows how this choice alters signal depth and power without introducing new free parameters beyond those already constrained observationally.

major comments (2)

[Model description and implementation] The central results (shallower global signal and suppressed power) rest on normalizing PBH X-ray output directly to the observed high-z AGN luminosity function under the fixed premise that these AGNs are PBH-seeded. No section quantifies the effect of scaling this contribution down (e.g., if only a fraction of AGNs are PBH-seeded or if stellar/mini-halo sources dominate the X-ray budget at z~10-20), leaving the headline claim vulnerable to overcounting.
[Results and discussion] The manuscript describes the SCRIPT extension and resulting signals but supplies no equations for the PBH X-ray emissivity, no validation tests (e.g., recovery of standard 21-cm results when PBH term is set to zero), and no error bars or convergence checks on the reported changes in signal depth and power amplitude.

minor comments (1)

[Abstract] The abstract states that the model includes 'essential cosmic dawn physics' but does not enumerate the additional processes (e.g., Lyman-alpha coupling, IGM temperature evolution) beyond the PBH term.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their detailed and constructive report. The comments highlight important aspects of model transparency and robustness that we will address in the revised manuscript. Below we respond to each major comment in turn.

read point-by-point responses

Referee: [Model description and implementation] The central results (shallower global signal and suppressed power) rest on normalizing PBH X-ray output directly to the observed high-z AGN luminosity function under the fixed premise that these AGNs are PBH-seeded. No section quantifies the effect of scaling this contribution down (e.g., if only a fraction of AGNs are PBH-seeded or if stellar/mini-halo sources dominate the X-ray budget at z~10-20), leaving the headline claim vulnerable to overcounting.

Authors: We agree that the headline results are presented under the assumption that the JWST-observed high-z AGNs are entirely PBH-seeded, as stated in the abstract and introduction. To address the concern about potential overcounting, we will add a dedicated subsection in the discussion that explores the effect of scaling the PBH X-ray contribution by a factor f (0 < f ≤ 1), where f represents the fraction of AGNs that are PBH-seeded (or equivalently, the relative contribution if stellar sources dominate). This will include explicit calculations and figures showing the global 21-cm signal and power spectrum for f = 0.1, 0.5, and 1.0 while holding the PBH mass function fixed. We believe this addition directly mitigates the vulnerability noted by the referee. revision: yes
Referee: [Results and discussion] The manuscript describes the SCRIPT extension and resulting signals but supplies no equations for the PBH X-ray emissivity, no validation tests (e.g., recovery of standard 21-cm results when PBH term is set to zero), and no error bars or convergence checks on the reported changes in signal depth and power amplitude.

Authors: We acknowledge these omissions in the current draft. In the revised version we will: (i) insert the explicit analytic expressions for the PBH X-ray emissivity (derived from the observed AGN luminosity function convolved with the chosen PBH mass function) into Section 2; (ii) add a validation subsection demonstrating that setting the PBH term to zero recovers the standard cosmic-dawn and reionization signals previously published with the original SCRIPT code; and (iii) report error bars on the global signal and power-spectrum differences based on the standard deviation across multiple simulation realizations, together with a brief convergence study with respect to grid resolution and simulation volume. These changes will be placed in the methods and results sections respectively. revision: yes

Circularity Check

0 steps flagged

No circularity: results are computed outputs from model extension under external assumptions

full rationale

The paper explicitly supposes that JWST AGNs at z>6 are PBH-seeded, extends the SCRIPT framework with PBH X-ray terms, adopts PBH mass functions from external observational constraints, and computes the resulting 21-cm global signal and power spectrum. These computed changes in depth and amplitude are generated by the extended model rather than fitted to 21-cm data or defined into the inputs by construction. No self-definitional steps, fitted inputs renamed as predictions, or load-bearing self-citation chains appear in the derivation; the central claims remain independent of the target observables.

Axiom & Free-Parameter Ledger

1 free parameters · 2 axioms · 0 invented entities

The central claim rests on the assumption that JWST AGNs are PBH-seeded and on the addition of their X-ray heating into an existing reionization code; no new particles or forces are introduced beyond standard PBH populations.

free parameters (1)

PBH mass function parameters
Different mass distributions obeying current observational bounds are tested; specific functional forms and normalizations are chosen by hand to explore impact.

axioms (2)

domain assumption AGNs at z greater than 6 are seeded by primordial black holes
Explicitly stated as the supposition enabling inclusion of PBH X-ray contributions in the model.
domain assumption SCRIPT framework accurately captures photon conservation and cosmic dawn physics when PBH heating is added
The paper extends an existing code without reporting new validation against independent benchmarks.

pith-pipeline@v0.9.0 · 5566 in / 1513 out tokens · 52900 ms · 2026-05-08T14:12:42.452354+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

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