GRB 210704A: a luminous fast blue transient in a GRB afterglow at z=2.34

Pieterse, DLA; Levan, AJ; Ravasio, ME; Rastinejad, JC; van Hoof, APC; Malesani, DB; Sarin, N; Lamb, GP; Martin-Carrillo, A; Nugent, AE; Tanvir, NR; Jonker, PG; Kann, DA; Agüí Fernández, JF; Berger, E; Corcoran, G; Cusano, F; D’Avanzo, P; D’Elia, V; de Ugarte Postigo, A; Dimple; Fong, WF; Fynbo, JPU; Izzo, L; Maiorano, E; Melandri, A; Palazzi, E; Quirola-Vásquez, J; Rossi, A; Rouco Escorial, A

GRB 210704A: a luminous fast blue transient in a GRB afterglow at z=2.34

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Pieterse, DLA ORCID: 0000-0003-3114-2733, Levan, AJ ORCID: 0000-0001-7821-9369, Ravasio, ME ORCID: 0000-0003-3193-4714, Rastinejad, JC ORCID: 0000-0002-9267-6213, van Hoof, APC ORCID: 0009-0005-5404-2745, Malesani, DB ORCID: 0000-0002-7517-326X, Sarin, N ORCID: 0000-0003-2700-1030, Lamb, GP ORCID: 0000-0001-5169-4143, Martin-Carrillo, A ORCID: 0000-0001-5108-0627, Nugent, AE ORCID: 0000-0002-2028-9329, Tanvir, NR ORCID: 0000-0003-3274-6336, Jonker, PG ORCID: 0000-0001-5679-0695, Kann, DA ORCID: 0000-0003-2902-3583, Agüí Fernández, JF ORCID: 0000-0001-6991-7616, Berger, E ORCID: 0000-0002-9392-9681, Corcoran, G ORCID: 0009-0009-1573-8300, Cusano, F ORCID: 0000-0003-2910-6565, D’Avanzo, P ORCID: 0000-0001-7164-1508, D’Elia, V ORCID: 0000-0002-7320-5862, de Ugarte Postigo, A ORCID: 0000-0001-7717-5085 et al (2026) GRB 210704A: a luminous fast blue transient in a GRB afterglow at z=2.34. Monthly Notices of the Royal Astronomical Society, 548 (2). pp. 1-24. ISSN 0035-8711

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Publisher URL: https://doi.org/10.1093/mnras/stag555

Abstract

We present detailed multiwavelength analysis of GRB 210704A: a Fermi Gamma-ray Burst Monitor discovered and Fermi Large Area Telescope (LAT) detected gamma-ray burst (GRB). The burst is dominated by a short ((Formula presented) s) pulse followed by weaker softer emission. We line stack our afterglow spectrum and determine the most likely redshift to be (Formula presented). This is corroborated by the photometric redshift of the extended source underlying the GRB. The spectral energy distribution fit parameters, late-time imaging, as well as the GRB’s energetics, spectral lag, and location point to a collapsar nature. Follow-up observations reveal excess optical/infrared emission with respect to a standard afterglow, peaking around (Formula presented) d (2 d in the rest frame). The excess is extremely luminous ((Formula presented) mag) and rapidly evolving. Strikingly, it resembles the emission seen in recently discovered Einstein Probe fast X-ray transients EP241021a and EP240414a, as well as the population of luminous fast blue optical transients (LFBOTs). This provides a link between these sources and GRBs. Fermi/LAT observations imply a high Lorentz factor, making this a case where LFBOT-like emission is also associated with a powerful successfully launched jet. We model the excess as likely coming from an energetic refreshed shock.

Item Type:	Article
Uncontrolled Keywords:	gamma-ray burst: general; gamma-ray burst: individual: GRB 210704A; supernovae: general; gamma-ray bursts; transients: supernovae; 5101 Astronomical Sciences; 51 Physical Sciences; 5101 Astronomical Sciences; 51 Physical Sciences; 0201 Astronomical and Space Sciences; Astronomy & Astrophysics; 5101 Astronomical sciences; 5107 Particle and high energy physics; 5109 Space sciences
Subjects:	Q Science > QB Astronomy
Divisions:	Astrophysics Research Institute
Publisher:	Oxford University Press
Date of acceptance:	18 March 2026
Date of first compliant Open Access:	3 June 2026
Date Deposited:	03 Jun 2026 14:56
Last Modified:	03 Jun 2026 14:56
DOI or ID number:	10.1093/mnras/stag555
URI:	https://researchonline.ljmu.ac.uk/id/eprint/28726

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