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GOTO identification and broadband modelling of the counterpart to the SVOM GRB 250818B

Fuente: arXiv

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Auteurs principaux:	Belkin, S., Lamb, G. P., Ackley, K., Wortley, M. E., McGee, S., Schroeder, G., Shrestha, M., Gompertz, B. P., Galloway, D. K., Starling, R., Fong, W. -f., Laskar, T., Liu, C., Gordon, A. C., Pankov, N., Volvach, A. E., Volvach, L. N., Shein, A., Pozanenko, A., Dyer, M. J., Lyman, J., Ulaczyk, K., Steeghs, D., Dhillon, V. S., O'Brien, P., Ramsay, G., Noysena, K., Kotak, R., Breton, R. P., Nuttall, L. K., Pollacco, D., Awiphan, S., Casares, J., Chote, P., Chrimes, A., Eyles-Ferris, R., Godson, B., Irawati, P., Jarvis, D., Julakanti, Y., Kelsey, L., Kennedy, M. R., Killestein, T., Kumar, A., Levan, A., Littlefair, S., Magee, M., Mandhai, S., S'anchez, D. Mata, Mattila, S., McCormac, J., Mkrtichian, D., Moran, S., Mullaney, J., O'Neill, D., Patel, M., Pu, K., Pursiainen, M., Sahu, A., Sawangwit, U., Stanway, E., Sun, Y., Warwick, B., Wiersema, K.
Format:	Preprint
Publié:	2026
Sujets:	High Energy Astrophysical Phenomena
Accès en ligne:	Acceder al recurso
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_version_	1866917280914341888
author	Belkin, S. Lamb, G. P. Ackley, K. Wortley, M. E. McGee, S. Schroeder, G. Shrestha, M. Gompertz, B. P. Galloway, D. K. Starling, R. Fong, W. -f. Laskar, T. Liu, C. Gordon, A. C. Pankov, N. Volvach, A. E. Volvach, L. N. Shein, A. Pozanenko, A. Dyer, M. J. Lyman, J. Ulaczyk, K. Steeghs, D. Dhillon, V. S. O'Brien, P. Ramsay, G. Noysena, K. Kotak, R. Breton, R. P. Nuttall, L. K. Pollacco, D. Awiphan, S. Casares, J. Chote, P. Chrimes, A. Eyles-Ferris, R. Godson, B. Irawati, P. Jarvis, D. Julakanti, Y. Kelsey, L. Kennedy, M. R. Killestein, T. Kumar, A. Levan, A. Littlefair, S. Magee, M. Mandhai, S. S'anchez, D. Mata Mattila, S. McCormac, J. Mkrtichian, D. Moran, S. Mullaney, J. O'Neill, D. Patel, M. Pu, K. Pursiainen, M. Sahu, A. Sawangwit, U. Stanway, E. Sun, Y. Warwick, B. Wiersema, K.
author_facet	Belkin, S. Lamb, G. P. Ackley, K. Wortley, M. E. McGee, S. Schroeder, G. Shrestha, M. Gompertz, B. P. Galloway, D. K. Starling, R. Fong, W. -f. Laskar, T. Liu, C. Gordon, A. C. Pankov, N. Volvach, A. E. Volvach, L. N. Shein, A. Pozanenko, A. Dyer, M. J. Lyman, J. Ulaczyk, K. Steeghs, D. Dhillon, V. S. O'Brien, P. Ramsay, G. Noysena, K. Kotak, R. Breton, R. P. Nuttall, L. K. Pollacco, D. Awiphan, S. Casares, J. Chote, P. Chrimes, A. Eyles-Ferris, R. Godson, B. Irawati, P. Jarvis, D. Julakanti, Y. Kelsey, L. Kennedy, M. R. Killestein, T. Kumar, A. Levan, A. Littlefair, S. Magee, M. Mandhai, S. S'anchez, D. Mata Mattila, S. McCormac, J. Mkrtichian, D. Moran, S. Mullaney, J. O'Neill, D. Patel, M. Pu, K. Pursiainen, M. Sahu, A. Sawangwit, U. Stanway, E. Sun, Y. Warwick, B. Wiersema, K.
contents	Rapid localisation and follow-up of gamma-ray bursts (GRBs) increasingly rely on low-latency triggers from new missions coupled to wide-field robotic optical facilities. We present the discovery and multi-wavelength follow-up of GRB 250818B, detected by the Space Variable Objects Monitor (SVOM) and localised optically by the Gravitational-wave Optical Transient Observer (GOTO). We compile and homogenise X-ray, optical/NIR, and radio data to build broadband light curves and spectral energy distributions. The afterglow is unusually luminous for a nominal short GRB, lying on the bright end of the short-GRB population in X-rays and optical and among the most luminous high-redshift short-GRB afterglows in the radio. MeerKAT detects the source at 3.1 GHz, while ALMA provides deep higher-frequency limits. Keck/LRIS spectroscopy shows continuum and metal absorption (Fe II, Mg II, Mg I), giving $z=1.216$. Synchrotron forward-shock modelling favours a constant-density medium and strongly prefers refreshed (energy-injection) emission, well described by a two-component jet with $E_{K,iso} \sim 4\times10^{52}$ erg, $n_0 \sim 3.6$ cm$^{-3}$, $θ_j \simeq 0.10$ rad ($\sim 5.7$ deg), and $p \simeq 1.64$. The host association is ambiguous: the nearest LS DR10 galaxy candidate ($r_{AB} \sim 24.7$) is offset by $\sim 4$ arcsec ($\sim 34$ kpc) with chance-alignment probability $P_{cc} \sim 0.2$, and current imaging does not exclude a fainter, near-coincident host. SED fitting of the candidate host suggests a low-mass galaxy. GRB 250818B highlights the power of rapid wide-field counterpart identification in the SVOM era, while host-association uncertainty can still limit offset-based interpretation.
format	Preprint
id	arxiv_https___arxiv_org_abs_2602_16559
institution	arXiv
publishDate	2026
record_format	arxiv
spellingShingle	GOTO identification and broadband modelling of the counterpart to the SVOM GRB 250818B Belkin, S. Lamb, G. P. Ackley, K. Wortley, M. E. McGee, S. Schroeder, G. Shrestha, M. Gompertz, B. P. Galloway, D. K. Starling, R. Fong, W. -f. Laskar, T. Liu, C. Gordon, A. C. Pankov, N. Volvach, A. E. Volvach, L. N. Shein, A. Pozanenko, A. Dyer, M. J. Lyman, J. Ulaczyk, K. Steeghs, D. Dhillon, V. S. O'Brien, P. Ramsay, G. Noysena, K. Kotak, R. Breton, R. P. Nuttall, L. K. Pollacco, D. Awiphan, S. Casares, J. Chote, P. Chrimes, A. Eyles-Ferris, R. Godson, B. Irawati, P. Jarvis, D. Julakanti, Y. Kelsey, L. Kennedy, M. R. Killestein, T. Kumar, A. Levan, A. Littlefair, S. Magee, M. Mandhai, S. S'anchez, D. Mata Mattila, S. McCormac, J. Mkrtichian, D. Moran, S. Mullaney, J. O'Neill, D. Patel, M. Pu, K. Pursiainen, M. Sahu, A. Sawangwit, U. Stanway, E. Sun, Y. Warwick, B. Wiersema, K. High Energy Astrophysical Phenomena Rapid localisation and follow-up of gamma-ray bursts (GRBs) increasingly rely on low-latency triggers from new missions coupled to wide-field robotic optical facilities. We present the discovery and multi-wavelength follow-up of GRB 250818B, detected by the Space Variable Objects Monitor (SVOM) and localised optically by the Gravitational-wave Optical Transient Observer (GOTO). We compile and homogenise X-ray, optical/NIR, and radio data to build broadband light curves and spectral energy distributions. The afterglow is unusually luminous for a nominal short GRB, lying on the bright end of the short-GRB population in X-rays and optical and among the most luminous high-redshift short-GRB afterglows in the radio. MeerKAT detects the source at 3.1 GHz, while ALMA provides deep higher-frequency limits. Keck/LRIS spectroscopy shows continuum and metal absorption (Fe II, Mg II, Mg I), giving $z=1.216$. Synchrotron forward-shock modelling favours a constant-density medium and strongly prefers refreshed (energy-injection) emission, well described by a two-component jet with $E_{K,iso} \sim 4\times10^{52}$ erg, $n_0 \sim 3.6$ cm$^{-3}$, $θ_j \simeq 0.10$ rad ($\sim 5.7$ deg), and $p \simeq 1.64$. The host association is ambiguous: the nearest LS DR10 galaxy candidate ($r_{AB} \sim 24.7$) is offset by $\sim 4$ arcsec ($\sim 34$ kpc) with chance-alignment probability $P_{cc} \sim 0.2$, and current imaging does not exclude a fainter, near-coincident host. SED fitting of the candidate host suggests a low-mass galaxy. GRB 250818B highlights the power of rapid wide-field counterpart identification in the SVOM era, while host-association uncertainty can still limit offset-based interpretation.
title	GOTO identification and broadband modelling of the counterpart to the SVOM GRB 250818B
topic	High Energy Astrophysical Phenomena
url	https://arxiv.org/abs/2602.16559