GCN Circular 34041

Daniel A. Perley (LJMU), Igor Andreoni (JSI/UMD/NASA-GSFC), Kareem El-Badry (CIT), Ilaria Caiazzo (CIT), Michael Coughlin (UMN), Mansi Kasliwal (CIT), and Zach Vanderbosch (CIT) report:

We observed the fast optical transient ZTF23aaoohpy/ATLAS23msn/AT2023lcr
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 (Swain et al., GCN 34022
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; Gompertz et al., GCN34023
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; Kumar et al., GCN34025
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; Perley et al, GCN 34031
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; Fulton et al., AstroNote 2023-179) with the Low Resolution Imaging Spectrometer (LRIS) at W. M. Keck Observatory.  Observations started on 2023 June 20 at 07:22 UT.  The exposure time was 2x1200s and 3x800s in the blue and red arms of LRIS, respectively.  Observations employed the 600/4000 blue grism and 600/7500 red grating, providing continuous wavelength coverage from 3140 to 8784 Angstroms.  Data were reduced using LPipe (Perley, 2019).

The spectrum shows a simple continuum, well-fit by a simple power-law of f_lamba~lambda^-1.  The signal-to-noise ratio is about 20 per resolution element (although lower blueward of 4000 Angstroms).  We detect clear (but weak) absorption lines at observer-frame wavelengths of 5688 and 5683 Angstroms which we attribute to the redshifted MgII(2796,2803) doublet at z=1.0272.  Weak absorption from FeII 2344 and FeII 2383 at a consistent redshift is also detected.  We also detect a possible intervening MgII absorber at z=0.7795.  No other lines are apparent in the spectrum.

Based on this information, we propose z=1.0272 as the redshift of this event.  While strictly this redshift is only a lower limit, the absence of any higher-redshift absorption features suggests that a higher-redshift origin is unlikely.  The lack of Lyman-alpha absorption over the spectral range imposes a redshift upper limit of z<1.6.  In either case, the spectroscopy confirms this to be an intrinsically very luminous, fast-rising event with a power-law SED characteristic of synchtrotron radiation, similar in nature to GRB afterglows.