AstroNote 2026-113

We report observations of a rapidly-evolving luminous blue transient, AT2026jjc (ZTF26aarbnwe), with a faint host-galaxy counterpart. 

AT2026jjc was discovered at a position (J2000) of:

RA = 16:46:02.45

Dec = +30:55:42.13

which corresponds to a galactic latitude of 39 degrees.

At this location, [1] measures an extinction of E(B-V) = 0.03 mag, and we report extinction-corrected photometry in the following. 

The first ZTF detection was made on 2026-04-02 09:41:45 UTC (MJD 61132)  at g =  19.52 +/- 0.21 mag. The optical light curve reached its peak one day later at g =19.03 +/- 0.20 and r = 19.27 +/-0.12. This yields an extinction-corrected color of g-r = -0.24 +/- 0.23 mag at peak.   The optical emission plateaued for the next three days, then began to fade at a rate of 0.1 mag/day over the next ten days, consistent with a Type Ibn supernova. We obtained optical follow-up observations with the IO:O instrument on the Liverpool telescope around 20 days after peak light (MJD 61153), and confirmed the transient’s continued optical fading.

The transient’s fast optical light curve and likely extragalactic origin motivated us to take Gemini South GMOS spectra of the transient and nearby galaxy.  The transient’s spectrum revealed several strong He I emission lines that led us to classify it as a Type Ibn supernova.   The emission lines corresponded to a redshift of z = 0.099 +/- 0.004, which would imply an extinction-corrected g-band absolute magnitude of M_g = -19.34. This is in line with the templates for Ibn provided in Hosseinzadeh+2017 [2], which peak at around -19.5. 

Given the early luminous blue emission, we triggered a Target-of-Opportunity observation using the Follow-up X-ray Telescope (FXT) on board the Einstein Probe. We obtained a 3 ks observation (mid-time MJD 61148.60405, roughly 16 days after the first ZTF detection), and do not detect a source at this position. Our 90% CL upper limit on the flux (after analyzing data from both the A and the B modules) is 3.1 x 10^-14 erg/s/cm^2; at the redshift of this transient, this corresponds to a limit on the X-ray luminosity of 8.19 x 10^41 erg/s. This is in line with the early X-ray flux reported for SN 2022ablq, another luminous Type Ibn supernova [2].

[1] Schlafly, E. F., & Finkbeiner, D. P. 2011, ApJ, 737, 10

[2] C. Pellegrino et al 2024 ApJ 977 2

ZTF is supported by the National Science Foundation under Grant No. AST-2034437 and a collaboration including Caltech, IPAC, the Weizmann Institute for Science, the Oskar Klein Center at Stockholm University, the University of Maryland, Deutsches Elektronen-Synchrotron and Humboldt University, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, Trinity College Dublin, Lawrence Livermore National Laboratories, and IN2P3, France. Operations are conducted by COO, IPAC, and UW.

Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).

We thank the Gemini Observatory staff for assistance in rapidly obtaining these observations.