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ASAS-SN Discovery of A Possible Supernova in ESO 253-G003

ATel #6732; T. W.-S. Holoien (Ohio State), S. Kiyota (Variable Star Observers League in Japan), J. Brimacombe (Coral Towers Observatory), Adam Schneider (University of Toledo), Scott Sheppard (Carnegie DTM), J. L. Prieto (Diego Portales; MAS), D. Grupe (Morehead State), K. Z. Stanek, C. S. Kochanek, A. B. Davis, G. Simonian, U. Basu, J. F. Beacom (Ohio State), B. J. Shappee (Hubble Fellow, Carnegie Observatories), D. Bersier (LJMU), D. Szczygiel, G. Pojmanski (Warsaw University Observatory), E. Conseil (Association Francaise des Observateurs d'Etoiles Variables), B. Nicholls (Mt. Vernon Obs., New Zealand), J. Nicolas (Groupe SNAUDE, France)
on 22 Nov 2014; 20:11 UT
Distributed as an Instant Email Notice Supernovae
Credential Certification: Thomas Holoien (tholoien@astronomy.ohio-state.edu)

Subjects: Optical, Supernovae, Transient

During the ongoing All Sky Automated Survey for SuperNovae (ASAS-SN or "Assassin"), using data from the quadruple 14-cm "Brutus" telescope in Haleakala, Hawaii, we discovered a new transient source, possibly a supernova, located near the center of the galaxy ESO 253-G003:

Object       RA (J2000)     DEC (J2000)      Disc. UT Date   Disc. V mag 
ASASSN-14ko  05:25:18.13    -46:00:20.34      2014-11-14.28    17.0 

ASASSN-14ko was discovered in images obtained on UT 2014-11-14.28 at V~17.0 mag. We also detect the object in images obtained on UT 2014-11-07.23 (V~16.4), UT 2014-11-10.22 (V~16.6), and UT 2014-11-13.21 (V~17.0) and marginally detect the object in images obtained on UT 2014-11-04.24 and UT 2014-11-16.28. We do not detect (V>17.0) the object in images taken on UT 2014-11-03.25 and before. Images obtained by S. Kiyota on UT 2014-11-16.46 and by J. Brimacombe on UT 2014-11-19.73 confirm the discovery of the transient. This figure shows the archival DSS image of the host (left) and the S. Kiyota confirmation image (right). The red circle has a radius of 3" and is centered on the position of the transient.

The position of ASASSN-14ko is approximately 0.7" North and 0.5" East from the center of the galaxy ESO 253-G003 (z=0.042489, d=181 Mpc, via NED), giving an absolute V-band magnitude of approximately -19.4 at discovery (m-M=36.29, A_V=0.144). The host galaxy has archival spectra consistent with a Seyfert II AGN.

After discovery of the transient, we were granted a series of Swift space telescope UVOT and XRT observations. Swift data taken on UT 2014-11-16 and UT 2014-11-20 show that in both epochs the central region of the galaxy is significantly brighter in UV wavelengths but has magnitudes consistent with archival host magnitudes in optical filters. We also see evolution in the UV magnitudes between the two epochs (4 days apart), indicating that the source is fading fairly rapidly. Using the UVOT magnitudes and the archival photometry from NED, we construct a spectral energy distribution, shown here. The Swift UVOT Vega magnitudes are reported below:

Filter        2014/11/16 Mag        2014/11/20 Mag 
v                 15.20                 15.16 
b                 15.64                 15.64 
u                 14.85                 14.95 
uvw1              14.73                 14.85 
uvmw              14.64                 14.73 
uvw2              14.64                 14.76 

A follow-up spectrum obtained with the B&C spectrograph on the du Pont 2.5m at LCO shows a strong blue continuum, confirming our Swift photometric observations, as well as emission lines consistent with a Type II Seyfert galaxy.

We also detect X-ray emission in Swift XRT observations of the source, corresponding to flux measurements of (2.85+/-0.8)x10^-16 W m^-2 and (3.1+/-0.7)x10^-16 W m^-2 in our two epochs of data. The X-ray spectrum is consistent with a highly absorbed AGN spectrum with a column density of the order of 10^23 cm^-2 and a luminosity (assuming the redshift of the host) of Lx~3x10^35 W.

Given these observations, we conclude that this is most likely a supernova with a blue continuum projected very near the nucleus of a Seyfert II AGN. However, strong AGN variability, while unusual for a Type II AGN, cannot be excluded. Follow-up observations, particularly spectra, are encouraged.

We thank LCOGT and its staff for their continued support of ASAS-SN. For more information about the ASAS-SN project, see the ASAS-SN Homepage and the list of all ASAS-SN transients.