AstroNote 2026-109

We report observations of a rapidly-evolving luminous blue transient, AT2026iqx (ZTF26aarjkso), with a host-galaxy counterpart. 

AT2026iqx was discovered at a position (J2000) of:

RA = 08:24:14.07

Dec = +00:41:35.80

which corresponds to a galactic latitude of 21 degrees.

The first ZTF detections were on 2026-04-07 UTC (MJD 61137)  at g =  19.03 +/- 0.05 mag, r = 19.44 +/- 0.07 mag, and i = 19.57 +/- 0.16 mag.  [1] measures an extinction of E(B-V) = 0.03 mag, which yields an extinction-corrected color of g-r = -0.44 +/- 0.10 mag.   The next day, the transient brightened in r and i-band by 0.15 mag and plateaued in g-band.  The transient then started to fade over the next eight days in all bands. 

We observed the transient with the Large Monolithic Imager (LMI) on the Lowell Discovery Telescope on 2026-04-16 05:15:59 UTC (MJD 61147.7), about 10.5 days after the first ZTF detection. Data were reduced using mirar (https://github.com/winter-telescope/mirar, https://doi.org/10.5281/zenodo.10888436), and we measure a brightness of 21.46 +/- 0.08 in r and 21.76 +/- 0.06 in g.  This marks an evolution to a red extinction-corrected color of  g-r = 0.27 +/- 0.10 mag and gives a fade rate of 0.2 mag/day over ten days.

The transient has a 2’’ offset from a bright galaxy with a Legacy Survey spectroscopic redshift of 0.1 [2].

This motivated us to take Gemini South GMOS spectra of the transient and nearby galaxy.  The transient’s spectrum revealed broad features that led to its classification as a Type Ic-BL supernova.  The galaxy spectrum confirmed a spectroscopic redshift of z=0.0997 +/- 0.0001.  The transient’s peak g-band absolute magnitude at this redshift would be M_g = -19.35.  

 

We triggered a Target-of-Opportunity observation with the Follow-up X-ray Telescope (FXT) on board the Einstein Probe. We obtained a 2.3 ks observation and do not detect a source at 3-sigma significance when analyzing the data from both the A and the B modules. Our 90%-confidence upper limit on the flux is 7.75e-14 erg/s/cm^2; at the distance of this transient, that corresponds to an upper limit on the X-ray luminosity of 2.1e42 erg/s. This is in line with the evolution of a non-GRB-associated Ic-BL supernova.

Due to its fast light curve and color evolution reminiscent of iPTF16asu, we encourage follow-up observations of this Type Ic-BL supernova.

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

[2] Dey, A., Schlegel, D. J., Lang, D., et al. 2019, AJ, 157, 168

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.