GCN Circular 25356

M. Huber (IfA, Univ. of Hawaii), K. W. Smith (QUB), K. Chambers, A.
Schulz (IfA), S. 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 LALInference skymap of the NSBH event
S190814bv (The LIGO Scientific Collaboration and the Virgo
Collaboration, 25333, 25324) with the Pan-STARRS1 telescope (Chambers
et al. 2016, arXiv:1612.05560
Not found
 
 
C). Images were taken in the PS1 i and z
bands (Tonry et al. 2012, ApJ 750, 99).

Beginning at 2019-08-15 12:40:37 UT (58710.5282) or 15.5hrs after the
detection of S190814bv, observations started in the i-band. We used
the updated LALInference.v1.fits.gz map for pointing coverage.
Observations finished at 2019-08-15 15:13:38 UT.

At each pointing position a dithered sequence of 45 sec i-band and
z-band images were taken. These were combined into a single night
stack, covering the GPC1 camera chip gaps. These dithered sequences
were repeated, with overlaps, to map 18 square degrees of the
LALInference.v1.gz map 90% credible region, corresponding to a summed
probability 89% of the skymap. We did not cover the smaller
probability blob to the south east at DEC=-32.

Conditions were somewhat affected by clouds, and moon, seeing was
around 1.2 - 1.3 arcsec. 5-sigma limiting magnitudes were around i ~
20.8 and z ~ 20.3.

The images were processed with the IPP (Magnier et al. 2016,
arXiv:1612.05240
Not found
 
 
) 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), and already reported transients in the TNS before
S190814by.

After removing these, and requiring detections in BOTH i and z-band
stacks, we were left with two transients. Both of which we discount as
possible counterparts.

Name    | TNS Name  | RA (J2000)  | Dec (J2000) | Disc MJD |  i Mag err  | z Mag err
PS19epf | AT2019noq
Not found
 
 
 | 00 48 47.88 | -25 18 23.4 | 58710.58 |  19.93 0.11 | 20.17 0.16
PS19eph | AT2019nor
Not found
 
 
 | 00 49 51.99 | -24 16 17.7 | 58710.59 |  19.69 0.07 | 19.55 0.07 

PS19epf is within the inner 20% contour. It is located 0.46"S 3.96"E
from the centre of the galaxy PSO J012.1980-25.3064 (r = 18.3 Kron
mag). The host has no measured photometric or spectroscopic redshift.
However there are 4 separate, single night detections in the ZTF
public stream, from Lasair (Smith et al. 2019;
https://lasair.roe.ac.uk/object/ZTF19abkhnce/), across the last 12
days. Hence it is most probably a SN exploding before the GW.

PS19eph is within the inner 10% contour. However it is coincident with
the core of the B=18.67 galaxy 6dF J0049520-241618 at z = 0.436522
from NED, and hence is not likely related to S190814bv.

We do not recover desgw-190814b (AT2019nme
Not found
 
 
). Reported at i=19.33
z=19.39, (58710.278) by Soares-Santos et al. GCN 25336
Not found
 
 
. This is on the
edge of our stack, but we estimate a 3-sigma limit of i~20.6 z~20.2.
If it is real, it implies a very fast fade in i-band of 1 mag in about
8hrs. Deeper follow-up is required, and confirmation from the DECam
team if it is real.