Supernova AT2025ulz Detected in Host Galaxy | Gemini North Telescope
A representative color image of the field around AT2025ulz, the candidate counterpart to gravitational wave event S250818k that was first reported on August 18, 2025, by the LIGO-Virgo-KAGRA (LVK) collaboration. The host galaxy of AT2025ulz is at the center of the image. Clumpy star forming regions can be seen across the galaxy. The transient is visible close to the center of the host, though clearly offset from the nucleus. Early data suggested that AT2025ulz may be a kilonova, making it a promising candidate for the counterpart to S250818k. However, further investigation showed that it is a typical Type IIb supernova with early shock-cooling emission.
A Type IIb supernova is a supernova that occurs in the core-collapse phase of a massive star. These supernovae are characterized by the presence of hydrogen in their spectra. This distinguishes them from Type Ia supernovae that lack hydrogen. Type IIb supernovae typically arise from stars that have exhausted their nuclear fuel and have lost their outer layers, leading to a rapid collapse of their cores. They are often observed in binary systems and are known for their strong helium absorption lines in their spectra.
A kilonova (also called a macronova) is a transient astronomical event that occurs in a compact binary system when two neutron stars (BNS) or a neutron star and a black hole collide. The kilonova, visible over the weeks and months following the merger, is an isotropically expanding luminous afterglow of electromagnetic radiation emitted by the radioactive decay of r-process nuclei synthesized by—and then ejected from—the initial cataclysmic event. It is understood that the merger of two compact objects are a strong source of gravitational waves (GW).
This image was taken with the Gemini Multi-Object Spectrograph (GMOS) on the Gemini North telescope, one half of the International Gemini Observatory, funded in part by the U.S. National Science Foundation and operated by NSF NOIRLab. The data are reported in a paper titled “AT2025ulz and S250818k: Investigating early time observations of a subsolar mass gravitational-wave binary neutron star merger candidate” to appear in The Astrophysical Journal Letters.
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Acknowledgment: PI: B. O'Connor (Carnegie Mellon University)
Image Processing: J. Miller & M. Rodriguez (International Gemini Observatory/NSF NOIRLab), M. Zamani (NSF NOIRLab)
#NASA #Astronomy #Space #Science #Supernovae #Stars #NeutronStars #BlackHoles #Kilonovas #AT2025ulz #GravitationalWaves #Galaxies #StellarMergers #Cosmos #Universe #InternationalGeminiObservatory #GeminiNorthTelescope #NOIRLab #NSF #AURA #STEM #Education
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