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GCN Circular 20382

Subject
SUBJECT: LIGO/Virgo G268556: ATLAS17aeu - an unusual transient within the skymap
Date
2017-01-06T19:09:41Z (7 years ago)
From
S. J. Smartt at Queens U Belfast <s.smartt@qub.ac.uk>
J. Tonry, L. Denneau, A. Heinze, B. Stalder, H. Weiland (IfA),
C. W. Stubbs (Harvard), K. W. Smith, S. J. Smartt, (QUB), D. R. Young
(QUB), A. Rest (STScI), K. C. Chambers (IfA), M. Coughlin (Harvard),
M. E. Huber (IfA), D. E. Wright (QUB), H. Flewelling, E. A. Magnier,
A. S. B. Schultz, C. Waters, R. J. Wainscoat (IfA)

Following on from GCN 20377, we have processed the ATLAS data
beginning at 57758.296 (2016-01-05.296 UT) which started at a pointing
centre of RA=80, DEC=-30, and moved north and east across the northern
sky lobe to RA=170,80.

We find one transient that appears to be new (and recovered old
ones). This object has some unusual characteristics that warrant
further attention

ATLAS17aeu 09:13:13.89 +61:05:33.6 (138.30789 +61.09267) J2000

The position is within the inner 16% probability contour.  This object
is not spatially coincident with any star or host galaxy.

The lightcurve faded by 0.85 mag over 1.176hrs  

  MJD           c mag  err 
 57758.4129676  18.05 0.09   
 57758.4144595  18.18 0.1    
 57758.4267173  18.22 0.1    
 57758.4419066  18.58 0.13   
 57758.4469072  18.45 0.11   
 57758.4479226  18.34 0.11   
 57758.454986   18.39 0.11   
 57758.4619614  18.90 0.18   

(the cyan mag is effectively an average of g and r). 

The object has now fallen below the detection limit (orange filter; 
o ~ 19) on the night of 57759. 

Such a fast fade is unusual if the object is extragalactic. 

The obvious immediate explanations for this object are 
1) It is not a real astrophysical transient. We consider this unlikely. 
2) The photometric errors are under-estimated and it is flat. We consider
this unlikely, and the errors to be reliable. 
3) It is a Galactic CV. But the fading is too fast for typical CVs.
4) It is an M-dwarf flare. But the fading is slow for a typical
M-dwarf (which are usually between 2 and >5 mags per hour)

We note that there is no faint point source in the Pan-STARRS 3Pi
stacked sky (Chambers et al. 2017, arXiv:1612.05560, and
http://panstarrs.stsci.edu).  There is no counterpart in the MPC,
Vizier, WISE, various CV and stellar catalogues.

The object is 23 arcsec from the galaxy SDSS J091312.36+610554.2 which
has a spectroscopic redshift of z = 0.199 +/- 0.00004.  This implies a
luminosity distance of 990 Mpc (for Ho=69).  If ATLAS17aeu is related,
it would be at a projected distance of 75kpc from the Galaxy.

This luminosity distance is consistent with the BAYESTAR 3D volume
rendering (Singer et al. 2016, ApJ 829, L1) of G268556 which suggests
a 10-90 percentile probability of 520-1010 Mpc in this direction.

If it were associated with the z = 0.199 +/- 0.00004 galaxy, then it 
would be unusually luminous (M_g ~ -21.5). 

While a Galactic origin is still the most probable, it remains to be
confirmed. We encourage a search of any contemporaneous data taken at
the same time we detected this object and deep imaging of the position
to recover any host object (star or galaxy).
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