Fires in the deep: The luminosity distribution of early-time gamma-ray-burst afterglows in light of the Gamow Explorer sensitivity requirements^★,★★

¹ Hessian Research Cluster ELEMENTS, Giersch Science Center, Max-von-Laue-Strasse 12, Goethe University Frankfurt, Campus Riedberg, 60438 Frankfurt am Main, Germany
² Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
³ Department of Physics, George Washington University, Corcoran Hall, 725 21st Street NW, Washington, DC 20052, USA
e-mail: newhite@gwu.edu
⁴ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807, Merate (LC), Italy
⁵ Department of Physics, Lancaster University, Lancaster, LA1 4YB, UK
⁶ Astronomical Institute of the Czech Academy of Sciences (ASU-CAS), Fričova 298, 251 65 Ondřejov, Czech Republic
⁷ Aix Marseille Univ, CNRS, LAM Marseille, France
⁸ Department of Astrophysics/IMAPP, Radboud University, Nijmegen, The Netherlands
⁹ Department of Physics, University of Warwick, Coventry, UK
¹⁰ School of Physics and Centre for Space Research, University College Dublin, Dublin 4, Ireland
¹¹ SNU Astronomy Research Center, Dept. of Physics & Astronomy, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea
¹² DARK, Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen, Denmark
¹³ Osservatorio Astronomico di Capodimonte, Istituto Nazionale di Astrofisica (INAF), Salita Moiariello, 16, 80131 Naples, Italy
¹⁴ Centro Astronómico Hispano-Alemán, Observatorio de Calar Alto, Sierra de los Filabres, 04550 Gérgal, Spain
¹⁵ INAF IASF-Milano, Via Alfonso Corti 12, 20133 Milano, Italy
¹⁶ School of Physics and Astronomy, University of Leicester, University Rd, Leicester, LE1 7RH, UK
¹⁷ Jet Propulsion Lab, 4800 Oak Grove Dr, Pasadena, CA 91109, USA
¹⁸ INAF – Osservatorio di Astrofisica e Scienza dello Spazio, Via Piero Gobetti 93/3, 40129 Bologna, Italy
¹⁹ Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK
²⁰ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monte Porzio Catone (RM), Italy
²¹ Korea Astronomy and Space Science Institute, 776 Daedukdae-ro, Yuseong-Gu, Daejeon 34055, Republic of Korea
²² Space Science Data Center (SSDC) – Agenzia Spaziale Italiana (ASI), Via del Politecnico, 00133 Roma, Italy
²³ Department of Astronomy, University of Maryland, College Park, MD 20742–4111, USA
²⁴ Astrophysics Science Division, NASA Goddard Space Flight Center, 8800 Greenbelt Rd, Greenbelt, MD 20771, USA
²⁵ Department of Astronomy and Astrophysics, The Pennsylvania State University, 525 Davey Lab, University Park, PA 16802, USA
²⁶ Center for Astrophysics and Cosmology, University of Nova Gorica, Vipavska 13, 5000 Nova Gorica, Slovenia
²⁷ Nicolaus Copernicus Superior School, ul. Nowogrodzka 47A, 00-695, Warsaw, Poland
²⁸ Astrophysics Research Center of the Open university (ARCO), The Open University of Israel, PO Box 808, Ra’anana 43537, Israel
²⁹ Department of Natural Sciences, The Open University of Israel, PO Box 808, Ra'anana 43537, Israel
³⁰ Department of Physics and Earth Science, University of Ferrara, Via Saragat 1, 44122 Ferrara, Italy
³¹ INFN – Sezione di Ferrara, Via Saragat 1, 44122 Ferrara, Italy
³² Clemson University, Department of Physics & Astronomy, Clemson, SC 29634, USA
³³ Czech Technical University in Prague, Faculty of Electrical Engineering, Prague, Czech Republic
³⁴ V. P. Engelgardt Astronomical Observatory, Kazan Federal University, Kazan, Republic of Tatarstan, Russia
³⁵ Department of Physics, Northwestern College, Orange City, IA 51041, USA
³⁶ Daegu National Science Museum, 20, Techno-daero 6-gil, Yuga-myeon, Dalseong-gun, Daegu 43023, Republic of Korea
³⁷ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
³⁸ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Bartycka 18, 00-716 Warsaw, Poland
³⁹ Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, PA 19104, USA
⁴⁰ INAF – Osservatorio Astronomico di Cagliari – via della Scienza 5, 09047 Selargius, Italy
⁴¹ Department of Physics, University of the Free State, PO Box 339, Bloemfontein 9300, South Africa
⁴² European Space Agency (ESA), European Space Astronomy Centre, E-28692 Villanueva de la Canñada, Madrid, Spain
⁴³ Department of Physics, University of Bath, Claverton Down, Bath, BA2 7AY, UK
⁴⁴ Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudzia̧dzka 5, 87-100 Toruń, Poland
⁴⁵ Roger Williams University, Bristol, RI, USA
⁴⁶ Institute of Astronomy, National Central University, Chung-Li 32054, Taiwan

Received: 4 October 2023
Accepted: 15 February 2024

Abstract

Context. Gamma-ray bursts (GRBs) are ideal probes of the Universe at high redshift (ɀ), pinpointing the locations of the earliest star-forming galaxies and providing bright backlights with simple featureless power-law spectra that can be used to spectrally fingerprint the intergalactic medium and host galaxy during the period of reionization. Future missions such as Gamow Explorer (hereafter Gamow) are being proposed to unlock this potential by increasing the rate of identification of high-ɀ (ɀ > 5) GRBs in order to rapidly trigger observations from 6 to 10 m ground telescopes, the James Webb Space Telescope (JWST), and the upcoming Extremely Large Telescopes (ELTs).

Aims. Gamow was proposed to the NASA 2021 Medium-Class Explorer (MIDEX) program as a fast-slewing satellite featuring a wide-field lobster-eye X-ray telescope (LEXT) to detect and localize GRBs with arcminute accuracy, and a narrow-field multi-channel photo-ɀ infrared telescope (PIRT) to measure their photometric redshifts for > 80% of the LEXT detections using the Lyman-α dropout technique. We use a large sample of observed GRB afterglows to derive the PIRT sensitivity requirement.

Methods. We compiled a complete sample of GRB optical–near-infrared (optical-NIR) afterglows from 2008 to 2021, adding a total of 66 new afterglows to our earlier sample, including all known high-ɀ GRB afterglows. This sample is expanded with over 2837 unpublished data points for 40 of these GRBs. We performed full light-curve and spectral-energy-distribution analyses of these after-glows to derive their true luminosity at very early times. We compared the high-ɀ sample to the comparison sample at lower redshifts. For all the light curves, where possible, we determined the brightness at the time of the initial finding chart of Gamow, at different high redshifts and in different NIR bands. This was validated using a theoretical approach to predicting the afterglow brightness. We then followed the evolution of the luminosity to predict requirements for ground- and space-based follow-up. Finally, we discuss the potential biases between known GRB afterglow samples and those to be detected by Gamow.

Results. We find that the luminosity distribution of high-ɀ GRB afterglows is comparable to those at lower redshift, and we therefore are able to use the afterglows of lower-ɀ GRBs as proxies for those at high ɀ. We find that a PIRT sensitivity of 15 µJy (21 mag AB) in a 500 s exposure simultaneously in five NIR bands within 1000 s of the GRB trigger will meet the Gamow mission requirements. Depending on the ɀ and NIR band, we find that between 75% and 85% of all afterglows at ɀ > 5 will be recovered by Gamow at 5σ detection significance, allowing the determination of a robust photo-ɀ. As a check for possible observational biases and selection effects, we compared the results with those obtained through population-synthesis models, and find them to be consistent.

Conclusions. Gamow and other high-ɀ GRB missions will be capable of using a relatively modest 0.3 m onboard NIR photo-ɀ telescope to rapidly identify and report high-ɀ GRBs for further follow-up by larger facilities, opening a new window onto the era of reionization and the high-redshift Universe.