GCN 44092: EP260321a / AT2026gzf: VLT/X-shooter detection of supernova-like spectral features at z = 0.0344

D. Xu (NAOC), G. Corcoran (UCD), J. An (NAOC), L. Izzo (INAF/OACN), N. Habeeb (Leicester), N. R. Tanvir (Leicester), A. J. Levan (Radboud and Warwick), C. C. Thoene (AbAO), S. Schulze (Weizmann), P. G. Jonker (Radboud), D. B. Malesani (DAWN/NBI), A. Saccardi (CEA/Irfu), A. Martin-Carrillo (UCD), G. Pugliese (API), A. de Ugarte Postigo (LAM), A. L. Thakur (INAF-IAPS) report on behalf of the Stargate collaboration:

We observed the variable point source, AT2026gzf, (Tanvir et al., GCN 44082; see also Lee et al., GCN 44070; Aryan et al., GCN 44081; Ahumada et al., GCN 44084; Liu et al., GCN 44087) associated with X-ray transient EP260321a (Huang et al., GCN 44068) with the X-shooter spectrograph on the ESO VLT UT3 (Melipal) beginning on 2026-03-23 at 00:38 UT (36.1 hr post-trigger). Our observations consisted of 4x600 s exposures, and span the wavelength range 3000-20600 AA.

Bright continuum flux is visible in all three arms of the spectrum. In addition to the blue continuum and narrow emission lines reported for the previous MUSE observation (Tanvir et al., GCN 44082), our X-shooter spectrum shows broad features below 5000 AA, similar to those seen in the early spectra of core-collapse supernovae, and inconsistent with those seen in typical young stellar populations. However, we caution that at this early phase there are limited templates for comparison, combined with some contribution from any underlying cluster light, and a search for matches within GELATO and SNID does not, at this time, yield a firm classification. 

Furthermore, the detection of narrow absorption features due to Ca II at z = 0.0344 conclusively excludes a Milky Way origin for this transient.

We acknowledge excellent support of the ESO observing staff at Paranal, in particular Alonso Luna Ruiz Fernandez, Elyar Sedaghati, Lorena Faundez and Marcela Espinoza.