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Researchers analyzed GRB 240825A, a gamma-ray burst with a duration near the boundary between short and long GRBs, and found its properties consistent with originating from a massive star collapse (collapsar). However, despite extensive optical and near-infrared observations, no bright supernova was detected down to very faint limits, which contradicts typical expectations for collapsar-origin GRBs. The burst occurred at redshift z=0.659 in a massive, star-forming galaxy, and the researchers conclude the associated supernova was likely either heavily obscured by dust or intrinsically very faint.
Why it matters
This finding challenges standard models of gamma-ray burst classification and progenitor identification, demonstrating that not all long GRBs from massive star collapses produce detectable bright supernovae. It highlights the necessity of combining multiple classification methods and comprehensive multiwavelength observations to accurately determine the physical origins of these extreme cosmic events.
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⚠️ Preprint – Noch nicht peer-reviewed
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Abstract: We present a comprehensive multiwavelength analysis of GRB 240825A, a bright gamma-ray burst (GRB) detected by Fermi and Swift, with a prompt duration ($T_{rm 90}$ $sim$ 4 sec in 50-300 keV in GBM) near the boundary separating short and long GRBs, prompting a detailed investigation into its classification and progenitor. We use classical prompt metrics (duration, minimum variability timescale (MVT), lag, and spectral hardness) and modern classification techniques (machine-learning (ML) based t-SNE, support vector machine, energy-hardness-duration, and $varepsilon equiv E_{gamma,mathrm{iso},52} / E_{p,z,2}^{5/3}$) and find most properties (prompt energetics, placement on the Amati relation, and spectral lag) of GRB 240825A consistent with a collapsar origin. However, extensive late-time optical and NIR follow-up with the 10.4m GTC and 8.4m binocular LBT telescopes reveals no bright supernova (like SN 1998bw) is detected down to stringent limits (e.g., $m_r > 25.0$ mag at 17.59 days), despite a redshift of $z = 0.659$ measured from GTC spectroscopy. Host galaxy SED modeling with Prospector indicates a massive, and star-forming galaxy-typical of collapsar GRB hosts, though with a large offset. We compare these findings with hybrid events like GRB 211211A, GRB 230307A, GRB 200826A, including SNe-GRBs, and conclude that GRB 240825A most likely originated from a massive star collapse, possibly with the associated SN heavily obscured or intrinsically faint. This study emphasizes the need for multiwavelength follow-up and a multi-layered classification to determine GRB progenitors.
Source: Deciphering the Physical Origin of GRB 240825A: A Long GRB Lacking a Bright Supernova