Title:	First Results from HERA Phase II
Authors:	Abdurashidova, Zuhra Adams, Tyrone Aguirre, James E Baartman, Rushelle Barkana, Rennan Berkhout, Lindsay M Bernardi, Gianni Billings, Tashalee S Bizarria, Bruno B Bowman, Judd D Breitman, Daniela Bull, Philip Burba, Jacob Byrne, Ruby Carey, Steven Chandra, Rajorshi Sushovan Chen, Kai-Feng Choudhuri, Samir Cox, Tyler DeBoer, David R Acedo, Eloy de Lera Dexter, Matt Dhandha, Jiten Dillon, Joshua S Dynes, Scott Eksteen, Nico Ely, John Ewall-Wice, Aaron Fagnoni, Nicolas Fialkov, Anastasia Furlanetto, Steven R Gale-Sides, Kingsley Garsden, Hugh Gorce, Adelie Gorthi, Deepthi Halday, Ziyaad Hazelton, Bryna J Hewitt, Jacqueline N Hickish, Jack Huang, Tian Jacobs, Daniel C Josaitis, Alec Kern, Nicholas S Kerrigan, Joshua Kittiwisit, Piyanat Kolopanis, Matthew Lanman, Adam La Plante, Paul Liu, Adrian Ma, Yin-Zhe MacMahon, David H E Malan, Lourence Malgas, Cresshim Malgas, Keith Marero, Bradley Martinot, Zachary E McBride, Lisa Mesinger, Andrei Mirocha, Jordan Mohamed-Hinds, Nicel Molewa, Mathakane Morales, Miguel F Muñoz, Julian B Murray, Steven G Nikolic, Bojan Nuwegeld, Hans Parsons, Aaron R Pascua, Robert Patra, Nipanjana Pochinda, Simon Qin, Yuxiang Rath, Eleanor Razavi-Ghods, Nima Riley, Daniel Rosie, Kathryn Santos, Mario G Singh, Saurabh Storer, Dara Swarts, Hilton Tan, Jianrong Thélie, Emilie van Wyngaarden, Pieter Wilensky, Michael J Williams, Peter K G Zheng, Haoxuan The HERA Collaboration
Issue Date:	2-Feb-2026
Publisher:	The Astrophysical Journal
Citation:	The Astrophysical Journal, 2026, Vol. 998 (1), AR No. 33
Abstract:	We report the first upper limits on the power spectrum of 21 cm fluctuations during the Epoch of Reionization and Cosmic Dawn from Phase II of the Hydrogen Epoch of Reionization Array (HERA) experiment. HERA Phase II constitutes several significant improvements in the signal chain compared to Phase I, most notably resulting in expanded frequency bandwidth, from 50–250 MHz. In these first upper limits, we investigate a small two-week subset of the available Phase II observations, with a focus on identifying new systematic characteristics of the instrument, and establishing an analysis pipeline to account for them. We report 2σ upper limits in eight spectral bands, spanning 5.6 ≤ z ≤ 24.4 that are consistent with thermal noise at the 2σ level for k ≳ 0.6–0.9 h Mpc−1 (band dependent). Our tightest limit during Cosmic Dawn (z > 12) is 1.13 × 106 mK2 at (k = 0.55 h Mpc−1, z = 16.78), and during the EoR (5.5 < z < 12), it is 1.78 × 103 mK2 at (k = 0.70 h Mpc−1, z = 7.05). We find that mutual coupling has become our dominant systematic, leaking foreground power that strongly contaminates the low-k modes, resulting in the loss of modes from k = 0.35 to 0.55 compared to Phase I data.
Description:	Open Access.
URI:	http://hdl.handle.net/2289/8617
ISSN:	1538-4357
Alternative Location:	http://doi.org/10.3847/1538-4357/ae2d54
Copyright:	2026 IOPscience
Appears in Collections:	Research Papers (A&A)

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