TITLE: GCN CIRCULAR NUMBER: 25224 SUBJECT: LIGO/Virgo S190728q: Optical transients from Pan-STARRS coverage during first 2 nights DATE: 19/07/30 16:32:14 GMT FROM: Ken Smith at Queen's University Belfast M. Huber (IfA, Univ. of Hawaii), K. W. Smith (QUB), K. Chambers, A. Schulz (IfA), S. J. Smartt, D. R. Young, O. McBrien, J. Gillanders, S. Srivastav, D. O'Neil, P. Clark, S. Sim (QUB), T. de Boer, J. Bulger, J. Fairlamb, M. Huber, C.-C. Lin, T. Lowe, E. Magnier, R. J. Wainscoat, M. Willman (IfA, Univ. of Hawaii), T.-W, Chen (MPE), A. Rest (STScI), C. Stubbs (Harvard) We report observations of the LALInf skymap of the BBH event s190728q (The LIGO Scientific Collaboration and the Virgo Collaboration, GCN 25208, 25187) with the Pan-STARRS1 telescope (Chambers et al. 2016, arXiv:1612.05560C). Images were taken in the PS1 w and i-bands (Tonry et al. 2012, ApJ 750, 99). Beginning at 2019-07-28 10:08:31 UT (58692.4226) or 3.4hrs after the detection of S190728q, observations started in the w-band in the standard NEO search sequence. At each pointing position a sequence of quads (4 x 45 sec) was taken. This observing sequence ensures exactly the same pointing position for each of the quads. Observations of the map finished on 2019-07-28 14:44:53 UT. On the following night, beginning at 2019-07-29 08:51:02 a series of 45sec i-band dithered images were taken to cover the map. We finished observing at 2019-07-29 10:06:41. In total PS1 covered 105 squ. degrees of the LALInference.offline.fits,gz map of the 90% credible region and we estimate we covered a sky region totalling 92% of the event's localisation likelihood. The images were processed with the IPP (Magnier et al. 2016, arXiv:1612.05240) and difference images were produced using the Pan-STARRS1 Science Consortium 3Pi images as reference frames. Transient candidates were run through our standard filtering procedures, combined with a machine learning algorithm (Wright et al. 2015, MNRAS, 449, 451) were applied and all candidates were spatially cross-matched with known minor planets, and major star, galaxy, AGN and multi-wavelength catalogues (as described in Smartt et al. 2016, MNRAS, 462 4094). We report the following transients which are all within the 90% contour of the LALInference.offline.fits map. They are all supernova like candidates, either offset from a clear galaxy or no obvious host. The majority of these are detected over the 2 nights and do not show any fast fading or rising. The redshifts below are all photometric from SDSS DR15 or GLADE. For reference the redshift range corresponding to the luminosity distance of S190728q (GCN 25208) is approximately 0.14 < z < 0.22. Name TNS Name RA (J2000) Dec (J2000) Disc MJD Disc Mag photoz PS19dxk AT2019lzs 20 42 38.23 +06 05 53.6 58692.43 21.92 w PS19dxc AT2019lzb 20 45 55.34 +02 05 53.5 58692.43 20.62 w 0.464 PS19dxl AT2019lzr 20 47 08.30 +07 16 16.2 58692.43 21.37 w PS19dxb AT2019lzc 20 52 54.72 +02 31 43.6 58692.43 21.69 w 0.228 PS19dwv AT2019lyv 21 00 06.51 +12 16 54.5 58692.53 21.99 w 0.046 PS19dxo AT2019lzv 21 01 02.31 +14 23 48.5 58692.52 20.96 w PS19dxn AT2019lzt 21 01 13.26 +13 29 39.0 58692.53 21.05 w PS19dxf AT2019lzg 21 03 16.16 +14 22 50.9 58692.54 20.91 w PS19dxg AT2019lzf 21 11 56.20 +17 14 27.2 58692.58 21.80 w 0.713 PS19dwz AT2019lyz 21 14 09.78 +13 37 04.8 58692.53 21.58 w PS19dxm AT2019lzu 21 26 42.01 +17 17 25.8 58692.58 21.18 w PS19dxe AT2019lzd 21 29 51.15 +19 10 33.6 58692.58 22.10 w 0.090 PS19dxd AT2019lze 21 33 04.06 +21 25 19.2 58692.58 21.77 w The one exception is PS19dxf, which appears to show a fast rise and decline of 1 mag in 33 minutes in the w-band images of the first night. This is probably an M-dwarf flare, as it is coincident with an uncatalogued, red and faint source in both Pan-STARRS and DECaLs imaging surveys. In Pan-STARRS it is only detected in z. Star-galaxy distinction is not possible, but a galactic M-dwarf flare is the most likely explanation for this transient. Confirmation of DECam-GROWTH objects from Goldstein et al. (GCN 25215) or TNS registered: Name DG Name RA (J2000) Dec (J2000) Det. MJD Mag TNS Name PS19dwt DG19qukmb 20 44 56.55 +07 23 58.6 58692.43 20.57 w AT2019lyc PS19dwy DG19bfhpb 21 05 58.67 +09 58 43.7 58692.53 21.05 w AT2019lvy PS19dxp DG19tneob 21 15 01.04 +14 59 22.7 58692.58 20.37 w AT2019lxc Other known objects we recovered but were previously discovered by Pan-STARRS or ZTF: Name RA (J2000) Dec (J2000) Det. MJD Mag TNS Name PS19dwm 21 34 22.65 +21 20 30.6 58692.58 20.66 w AT2019lxm ZTF19aavndxo 20 47 51.02 +10 17 35.5 58693.38 19.48 i AT2019fyt PS19dxj 21 31 22.91 +20 34 04.4 58692.58 20.95 w AT2019lzo We also note that DG19rxclb as reported in Goldstein et al. (GCN 25215) is an old object (AT2019jvb) discovered by PS1 and registered on TNS on 2019-06-01.