GCN Circular 20459

M. E. Huber (IfA), K. C. Chambers (IfA), T.-W. Chen (MPE),
S. J. Smartt, (QUB), K. W. Smith (QUB), D. R. Young, D. E. Wright,
E. Kankare, R. Kotak, C. Inserra (QUB), M. Coughlin (Harvard),
L. Denneau, H. Flewelling, A. Heinze, E. A. Magnier (IfA), A. Rest
(STScI), B. Stalder (IfA), A. S. B. Schultz, C. W. Stubbs (Harvard)
J. Tonry, C. Waters, R. J. Wainscoat, H. Weiland (IfA)

Further to our Pan-STARRS campaign (Smartt et al, GCN 20410
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 ) and
classification of transients (Chambers et al. GCN 20437
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 ) we report
spectra of two further objects at the cores of galaxies.

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PS17fr - possible luminous nuclear SN or TDE
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PS17fr was discovered coincident with the nucleus of an r=18.36 galaxy
(SDSS J081152.10+252521.3), with a photometric redshift z=0.199.  The
magnitude on the first epoch MJD=57758.433 (from the difference image)
was i=19.2, rising 0.6 mag in 3 days.

A spectrum with the UH2.2m + SNIFS (R~1300, 3200-10000A) was taken on
MJD=57997 and shows broad H-alpha, H-beta and He I 5876. The line
profiles are asymmetric. The strong H-alpha has a width at half
maximum of 5200 km/s, but is either multicomponent or significantly
asymmetric with a red excess. Assuming the centroid of the peak
velocity is representative of the host and transient, then 
z = 0.1862 +/- 0.0003.

This implies an absolute magnitude of M_r = -21.1. The object may be a
a superluminous supernova of type II or IIn.  We note that it is
within the volume defined by the LALInference 3D map, at a luminosity
distance of 918 Mpc (Ho=69, Omega_M=0.3, Omega_Lam=0.7). 

As this is a nuclear transient, AGN activity is possible. But we see
no strong [N II], [OIII] or [O II]. A weak feature close to, but not
exactly at the expected position of [O III] 5007 is visible in the
SNIFS data. Another spectrum is required to confirm. 
A tidal disruption event is also a possible explanation.

In summary this is an unusually luminous transient within the LIGO
skymap and at a distance consistent with the central 80% of the
probability distribution. Further multi-wavelength follow-up is
encouraged to determine its nature.
Coords : 08:11:52.11 +25:25:21.4 (J2000)

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PS17er - probable AGN/Seyfert 2 variability
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PS17er is also a rising transient (now at i=20) at the core of the
r=19.2 galaxy SDSS J084157.26+405507.6. A spectrum with Gemini-N+GMOS
(R400 grating, 4491-8778 A) on 57764.4223666319 shows it to be at a
redshift of z=0.4064 from the [O III] 4959/5007 Angs, implying M_g =
-21.3.  These lines have a resolved width of 700 km/s, and there is a
broader (4000 km/s) H-beta line in emission. 

The ratio Log([OIII]/Hb) ~ 0.8, is normal for an AGN.  In Pan-STARRS
regular survey mode, we have seen various detections of i ~ 20
variability. Hence we consider this as likely ongoing Seyfert 2
activity, similar to episodes previously detected in PSST (Huber et
al. 2015, ATel 7153).