DRAFT
2024-05-09 16:04:54
Type: Object/s-Discovery/Classification
Bibcode: 2024TNSAN.126....1S
ZTF and Keck/LRIS Observations of the Fast, Blue, and Luminous Optical Transient AT2024hxb
Authors: Jason Sevilla (Cornell), Gokul Srinivasaragavan (UMD), Kaustav Das (Caltech), Anna Y. Q. Ho (Cornell), Jesper Sollerman (OKC), Jacob Wise, Daniel Perley (LJMU), Mansi Kasliwal (Caltech)
Keywords: Transient, Optical, Spectroscopy
Abstract:
We report photometric and spectroscopic observations of a rapidly evolving blue transient with a host-galaxy counterpart, AT2024hxb (ZTF24aalppmi). We obtained an LRIS spectrum, which yielded a redshift of z = 0.203 and showed similarities to young core-collapse supernovae.

ZTF24aalppmi was discovered at a position (J2000) of:

RA = 14:56:41.95

Dec = +16:50:26.51

and reported to the Transient Name Server and designated AT2024hxb (Perley et al. 2024). 

The first ZTF detection was at g = 20.35 +/- 0.18 mag on 2024-05-01 08:09:36 UTC (MJD 60431.34).  The next g-band detection was the peak of the light curve, at g = 19.81 +/- 0.14 mag on 2024-05-02 07:26:24 UTC (MJD 60432.31), implying a fast rise rate of 0.5 mag in 1 day in g-band.  Over the next 3 days, the transient faded by 0.5 mag, implying a fade rate of about 0.15 mag/day. There is a faint (g~r~24 mag) source classified as extended in Legacy Survey DR9 (Dey et al. 2019) 1.1 arcseconds away from AT2024hxb, with photo-z = 0.8 +/- 0.5.

From Schlafly & Finkbeiner (2011), the extinction at this location is E_(g-r) = 0.03.  At peak g-band magnitude, the implied g-r color was g-r = -0.45 +/- 0.2 mag. 

Due to the rapid evolution, blue colors at peak light, and implied extragalactic nature, we identified this as a candidate fast blue optical transient (FBOT).  We obtained a spectrum using LRIS on Keck at 2024-05-08 13:51:11 UTC (MJD 60438.58) six days after peak light.   Detections of Hɑ and [O III] emission lines yielded the spectroscopic redshift of z = 0.203 and an implied absolute peak magnitude (with a cosmological correction) of g = -20.14 mag.  The spectrum shows features reminiscent of young core collapse supernovae, but we did not identify any secure classifications. The spectrum’s closest SNID (Blondin et al. 2007) and NGSF (Goldwasser et al. 2022) matches were to IIP and SLSN-I supernovae, while also being somewhat similar to the spectrum of the Ic-BL SN iPTF16asu (Whitesides et al. 2017) at a similar phase; iPTF16asu was at a comparable redshift of z = 0.187.

We encourage additional follow-up observations of this transient, particularly optical photometry and spectroscopy.

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.

 

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