We are carrying out the "Pan-STARRS Search for Kilonovae", a focused search for intrinsically faint transients, or rapidly evolving transients in galaxies that are closer than 200 Mpc in the ongoing Pan-STARRS Near-Earth Object surveys (see Smartt et al. AstroNote 2019-48 for details). We will release AstroNotes for all these candidates and encourage spectroscopic classification or photometric follow-up.
In normal survey mode, Pan-STARRS1 takes 4 x 45s exposures in w, i or z each night, reaching 5 sigma magnitudes of w<22, i<21 and z<20.5 in each frame (see Chambers et al. 2016, 1612.05560). The w-band filter is a broad g+r+i composite.
We report the discovery of an intrinsically faint transient PS21gqd (AT2021qvw) in the host galaxy CGCG 454-019, at a redshift of z = 0.025, or d = 111 Mpc (from NED). The transient was discovered on MJD 59386.55 (2021-06-21, 13:14:16 UTC) at mag m_w = 21.40 +/- 0.16. It is offset from the host by 11.3" S, 4.0" W (a projected 6.2 kpc radius from the galaxy centre). At this distance the absolute magnitude is M_w = -14.0 +/- 0.2 (assuming m-M = 35.23 and foreground exinction of A_w = 0.14).
We followed the object up with Pan-STARRS1 three days later, measuring m_w = 22.12 +/- 0.26 on MJD 59389.56 (2021-06-24.56) with stacked forced photometry. Using forced photometry across both epochs, we measure a fading of 0.25 +/- 0.09 mag per day in the PS1 w-band. A screenshot of the Pan-STARRS flux (uJy) lightcurve and finder are included below.
There was no detection at the position of PS21gqd by Pan-STARRS when the field was last visited on MJD 59209. Running the transient position through the ATLAS forced photometry server (see Shingles et al. AstroNote 2021-7, Smith et al. 2020, PASP, 132, 085002 for details) and the ZTF forced-photometry service (Masci, F. et al. 2018, PASP, 131, 995.) revealed no obvious detection of a point source either historically or recently. This suggests the transient is likely intrinsically faint and not an old, fading supernova. Recent ATLAS and ZTF non-detections place limiting magnitudes of m_o > 20.1 on MJD 59383.54 and m_g > 20.2 and m_r > 19.5 on MJD 59385.4. A shock cooling signature of IIb supernova is possible, but it is more than 1 mag fainter than known IIb SNe cooling curves at their minimum.
The luminosity and decline rate are consistent with the locus of viable kilonova models from Nicholl et al. (2021 arXiv:2102.02229). We measure the model decline rates for the PS1-w passband and transform them into a probability density function of decline rate versus absolute magnitude by applying a Gaussian Kernel Density Estimation. PS21gqd falls well within the limits of these kilonova model contours.
Further photometric follow-up and spectroscopic classification are strongly encouraged.
The discoveries from this program are a byproduct of the Pan-STARRS NEO survey observations. Operation of the Pan-STARRS1 and Pan-STARRS2 telescopes is primarily supported by the National Aeronautics and Space Administration under Grant No. NNX12AR65G and Grant No. NNX14AM74G issued through the SSO Near-Earth Object Observations Program. Data are processed at Queen's University Belfast enabled through the STFC grants ST/P000312/1 and ST/T000198/1.
We made use of data from the Asteroid Terrestrial-impact Last Alert System (ATLAS) project: (Tonry et al. 2018, PASP, 130f4505), ATLAS forced photometry server and the ZTF forced-photometry service.
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