[vsnet-grb-info 23823] LIGO/Virgo S190930t: Candidates from ATLAS observations and constraints on AT2019rpr and AT2019rpn

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  25922
SUBJECT: LIGO/Virgo S190930t: Candidates from ATLAS observations and constraints on AT2019rpr and AT2019rpn
DATE:    19/10/02 08:40:20 GMT
FROM:    Stephen Smartt at Queen's U/Belfast  <s.smartt at qub.ac.uk>

S. J. Smartt, S. Srivastav, K. W. Smith (QUB), T.-W. Chen (MPE), D. R.
Young, M. Fulton, (QUB) L. Denneau, H. Flewelling, A. Heinze, J.
Tonry, H. Weiland (IfA, Univ. Hawaii), A. Rest (STScI), B. Stalder
(LSST), C. Stubbs (Harvard), O. McBrien, M. Dobson, J. Gillanders, D.
O'Neil, P. Clark, S. Sim (QUB)

We report observations of the Bayestar skymap of the NSBH event
S190930t (The LIGO Scientific Collaboration and the Virgo
Collaboration, GCN 25876) with the ATLAS telescope system (Tonry et
al. 2018, PASP, 13, 164505). ATLAS is a twin 0.5m telescope system on
Haleakala and Mauna Loa employing two filters - cyan and orange. While
carrying out the primary mission for Near Earth Objects, we can adjust
the schedule rapidly to point at LVC gravitational wave skymaps. The
survey provides coverage from declination -40 to +80 every 2 nights to
typical depths of 19.5 mag in the o-band.

In this case we did not perturb the normal ATLAS sky survey footprint,
given the large area of the Bayestar skymap. We estimate coverage
of the 90% contour of 2451 square degrees and 14% of the total
probability between MJD 58756.607097 and 58757.555924 (for reference,
S190930t is reported with a discovery time 58756.60703339205 from the
FITS header of bayestar.fits.gz).

Normal survey operations provided a sequence of quads (4 x 30 sec) at
each pointing position. The images were processed with the ATLAS
pipeline and reference images subtracted from each one. Transient
candidates were run through our standard filtering procedures,
combined with machine learning algorithms (e.g. Wright et al. 2015,
MNRAS, 449, 451). Candidates were spatially cross-matched with known
minor planets, and star, galaxy, AGN and multi-wavelength catalogues
(as described in Smartt et al. 2016, MNRAS, 462 4094; Stalder et al.
2017, ApJ, 850, 149).

We have reported all candidates to the IAU Transient name server.
Candidates of note are : 

Name      | ATLAS Name | RA (J2000)  | Dec (J2000) | Disc. MJD | Disc Mag | Notes
AT2019rpq | ATLAS19wzl | 08:59:22.13 | -00:53:53.6 | 58756.61  |  17.81 o | 1. 
AT2019rpi | ATLAS19wym | 07:41:17.77 | +03:37:18.6 | 58756.61  |  18.85 o | 2. 
AT2019rog | ATLAS19wxr | 20:20:47.61 | +15:05:03.1 | 58757.29  |  16.77 o | 3.  
AT2019rpt | ATLAS19xas | 08:23:32.96 | +21:20:24.7 | 58757.57  |  17.60 o | 4.
AT2019rpj | ATLAS19wyn | 22:39:20.79 | +31:29:30.0 | 58753.49  |  19.93 o | 5.

Notes on objects : 

1. Host probably 2MASS 07411836+0337242; a J=14.60 mag galaxy found in
the 2MASS XSC catalogue. It's located 5.65" S, 8.93" W, but could also
be associated with a fainter galaxy which is closer. No previous
observations in last 30 days due to RA and solar conjunction.

2. No host detected. No previous observations in last 30 days due to RA
and solar conjunction. 

3. Very fast decline, 1 mag in 30 minutes. No host star in Pan-STARRS1 3Pi. Decline is
similar to M-dwarf flare decline rate. 

4. Host is PGC023546: transient is located 26.77" S, 17.48" W (11.7
Kpc) from the galaxy centre. A host z=0.018 implies a distance of
74Mpc. The distance modulus m - M = 34.46, implies M > -17, which is
SN like.

5. Detected also by ZTF in GCN 25899. We have Clear detections 3 days
before S190930t. Host is AGC321427. Transient is located 21.00" S,
16.40" W (17.2 Kpc) from the galaxy centre. A host distance of 133.5
Mpc(z=0.030). Has been confirmed as type II SN (Karambelkar et al. GCN 25921). 

Comments on other objects reported: 

AT2019rpr (MASTER OT J205529.78+220725.5) reported by Lipunov et al. GCN 25900
is a recurring variable/outbursting object in ATLAS and likely a Galactic CV. 

AT2019rpn (ZTF19acbpqlh) has no previous detections in ATLAS. We
observed the field on 58754, 58755, 58756, 58757, 58758 and there are
no clear >5sigma detections above o ~ 19.5. The object is not rapidly
rising (Tan et al. GCN 25916). Karambelkar et al. (GCN 25921) report
possible broad H-alpha. Stacking of the ATLAS data is underway.