AstroNote 2021-291

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 (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 PS21mbb (AT2021aeba) in the host galaxy SDSS J080844.91+184204.6, at a redshift of z = 0.044, or d = 191 Mpc (from NED). The transient was discovered on MJD 59526.57 (2021-11-08, 13:40:48 UTC) at mag m_w = 21.69 +/- 0.08. It is offset from the host by 2.3" N, 3.5" E (a projected 3.4 kpc radius from the galaxy centre). At this distance the absolute magnitude is M_w = -14.82 +/- 0.17 (assuming m-M = 36.40 and foreground exinction of  A_w = 0.11). A screenshot of the finder is included below.

We followed the object up with Pan-STARRS1 two days later, measuring m_w = 22.30 +/- 0.15 on MJD 59528.56 (2021-11-10, 13:26:24 UTC) with forced photometry. Using forced photometry across both epochs, we measure a fading of 0.3 +/- 0.1 mag per day in the PS1 w-band. Further follow-up were performed on MJD 59529.61 (2021-11-11, 14:36:57 UTC) and MJD 59530.58 (2021-11-12, 13:50:52 UTC) obtaining multicolour (grizy) photometry. Again using forced photometry, we measured m_r = 22.63 +/- 0.13 and m_r = 22.79 +/- 0.16 across the two epochs respectively, and g-r ~ 0.1 and r-i ~ 0.1 on the first multicolour epoch implying that the transient is blue in colour. 

There was no detection at the position of PS21gqd by Pan-STARRS when the field was last visited on MJD 59278 (2021-03-05). Running the transient position through the ATLAS forced photometry server (see Shingles et al. AstroNote 2021-7 for details and Smith et al. 2020, PASP, 132, 085002) 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 3σ limiting magnitudes of m_o > 20.7 on MJD 59525.53 (2021-11-07, 12:43:12. UTC) and m_g > 21.3 on MJD 59526.33 (2021-11-08, 07:55:12 UTC) respectively.

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, however, a shock cooling signature of IIb supernova or a very luminous LBV outburst are also plausible given the transient's apparent blue colour.

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.