[vsnet-grb-info 23125] LIGO/Virgo S190728q: Optical transients from Pan-STARRS coverage during first 2 nights

[GCN Circulars]

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  <k.w.smith at qub.ac.uk>

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.