AstroNote 2026-180

AT2026obc was first reported in AstroNote 2026-176 as a rapidly rising, potentially extragalactic blue transient, brightening by ~0.8 mag over ~2 days since discovery.

A follow-up spectrum was obtained at MJD 61194.5 using the Next Generation Palomar Spectrograph (NGPS; AstroNote 2024-340) on the Palomar 200-inch Hale Telescope. The spectrum displays narrow H and K Calcium Fraunhofer lines in absorption, as well as narrow H-alpha emission, at a redshift of z = 0.0195. At this redshift, the spectrum also shows strong, narrow P-Cygni features corresponding to C II, C III and C IV with an absorption component at ~1300 km/s, along with a lack of strong H or He lines. We therefore classify this event as a nearby SN Icn.

We note that the spectrum displays weak narrow hydrogen Balmer emission lines, as well as possibly He II 4686, at z = 0. These may be artefacts from the reduction process or contamination from nearby foreground objects. We also note that our measured redshift of z = 0.0195 conflicts with the previously published redshift of the event’s host candidate 2MFGC 823 (z = 0.0374 ± 0.0007; Huchra et al. 1999); the cause of this discrepancy is currently unknown.

We also obtained ugriz photometry using IO:O on the Liverpool Telescope at MJD 61194.2. This showed the transient had not significantly brightened or faded since the previous ZTF P48 detection ~0.75 days prior, implying that the transient may have reached peak brightness after a short rise. We measure magnitudes of u = 14.70 ± 0.07, g = 14.89 ± 0.04 and r = 15.07 ± 0.05. At a redshift of z = 0.0195, this implies an absolute magnitude of M_u = -20.46 ± 0.07 after correcting for MW extinction (E(B-V) = 0.10) and applying a simple 2.5log(1+z) K-correction, as well as rest-frame colours of g - r ~ -0.3 and u - g ~ -0.3 (after MW extinction correction).

We note that this is the closest SN Icn in over 7 years, since SN 2019jc (z = 0.01948; TNS Classification Report No. 12409), and encourage further follow-up as the event's visibility improves. We intend to continue further photometric and spectroscopic follow-up of 2026obc as it evolves.

Based on observations obtained with the Samuel Oschin Telescope 48-inch at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under Award #2407588 and a partnership including Caltech, USA; Caltech/IPAC, USA; University of Maryland, USA; University of California, Berkeley, USA; University of Wisconsin at Milwaukee, USA; Cornell University, USA; Drexel University, USA; University of North Carolina at Chapel Hill, USA; Institute of Science and Technology, Austria; National Central University, Taiwan, and OKC, University of Stockholm, Sweden. Operations are conducted by Caltech's Optical Observatory (COO), Caltech/IPAC, and the University of Washington at Seattle, USA.

Based on observations made with the Liverpool Telescope operated on the island of La Palma by Liverpool John Moores University in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias with financial support from the UK Science and Technology Facilities Council.

NGPS is a new medium-resolution, single-object, long-slit optical spectrograph on the Palomar 200-in Hale Telescope at Palomar Observatory. NGPS replaces the Double Beam Spectrograph (DBSP; Oke & Gunn 1982), the workhorse Palomar 200-in optical spectrograph for four decades. NGPS is designed to be over a factor of three higher throughput than DBSP.  NGPS is a partnership between Caltech, Peking University, the National Astronomical Observatories of the Chinese Academy of Sciences, and Jet Propulsion Laboratory. The NGPS team acknowledges generous support from Caltech, the National Science Foundation (2018866), the Heising Simons Foundation (2020-1865), Strategic Priority Research Program of the Chinese Academy of Sciences (XDB0550100), and the National Natural Science Foundation of China (11927804).