AstroNote 2022-149

During the night of 2022 June 30 (UT), the Zwicky Transient Facility (ZTF) detected a new optical transient, internally designated ZTF22aaqytjb, at the following position (J2000):

RA, Dec = 15:11:58.02 +32:50:29.6 (227.991738, +32.841560)

The transient was discovered as part of the Caltech one-day cadence survey. The magnitude at the time of first detection (JD 2459760.7382) was g = 21.24 +/- 0.38 mag (all magnitudes AB, not corrected for Galactic extinction). The next g-band detection (two nights later) showed that the transient had brightened by two magnitudes (g=19.20 +/- 0.12 mag). We saved the source to the TNS and it was assigned the name AT2022pff. 

By 2022 July 4, the transient had begun fading in g-band. Due to the fast overall evolution (time above half-max of 5 days) the source was flagged during daily scanning for fast transients. Follow-up imaging with the Liverpool Telescope (LT) over the next few nights showed that the transient had persistently blue colors while fading. 

An SDSS galaxy 4.6 arcseconds away from the transient has a photometric redshift of z=0.119 +/- 0.0549. The photometric redshift range corresponds to a range in peak absolute magnitude of M_g = -18 mag to M_g = -20 mag. 

The duration, possible high peak luminosity, and persistent blue colors of the optical light curve were similar to that of the X-ray and radio-loud transient AT2020mrf (Yao et al. 2022, arXiv:2112.00751). So, on 2022 July 8 we obtained a ToO spectrum with GMOS on Gemini North (program ID GN-2022A-Q-127; PI Ho). The source was close to the moon at the time, and the relatively low S/N precluded the identification of any features. 

On 2022 July 14, we obtained a ToO observation with the Swift X-ray telescope (target ID 15263). No emission was detected at the position of the transient, with an upper limit on the count rate of 5.47e-03 ct/s. 

On 2022 July 19, we obtained a spectrum of the host galaxy with the Nordic Optical Telescope (NOT) with possible weak emission lines at a redshift of z=0.0905.

On 2022 July 21, we obtained a ToO spectrum of the transient and host galaxy with LRIS on Keck (Project Code 2022A_C242, PI Kasliwal). The host-galaxy spectrum shows emission lines consistent with z=0.091. The transient spectrum shows a broad emission feature at 6400 AA consistent with He I at that redshift, as well as several absorption features consistent with He I. We therefore conclude that the likely classification is Type Ibn. 

At this redshift, the peak luminosity (M_g = -18.88 +/- 0.12 mag) is fainter than that of AT2020mrf (M_g = -20 mag), and the upper limit on the X-ray luminosity is approximately 4.8E42 erg/s (assuming a flux to count rate conversion factor of 4E-11). The upper limit is fainter than the X-ray luminosity of AT2020mrf, and close to the X-ray luminosity of the similar transients AT2018cow (Rivera Sandoval et al. 2018, MNRAS, 480, L146; Margutti et al. 2019, ApJ, 872, 18; Ho et al. 2019, ApJ, 871, 73) and AT2020xnd (Ho et al. 2022, ApJ, 932, 116) at a similar epoch. 

Due to the optical luminosity, the spectral features, and the lack of detected X-ray emission, we conclude that AT2022pff is likely to be a fast Type Ibn SN. However, given the redshift and relatively unconstraining X-ray upper limit we cannot rule out an engine-powered outflow similar to that observed in AT2018cow and AT2020mrf. 

We thank the Gemini-North, Swift, NOT, and Keck staff for their assistance scheduling and executing the ToO observations.

Based on observations obtained with the Samuel Oschin Telescope 48-inch and the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. 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.