ASAS-SN Discovery of an Unusual Nuclear Transient in PGC 043234
ATel #6777; J. Jose, Zhen Guo, Feng Long, G. Herczeg, S. Dong (KIAA), T. W.-S. Holoien (Ohio State), J. L. Prieto (Diego Portales; MAS), D. Grupe (Morehead State), B. J. Shappee (Hubble Fellow, Carnegie Observatories), K. Z. Stanek, C. S. Kochanek, A. B. Davis, G. Simonian, U. Basu, J. F. Beacom (Ohio State), D. Bersier (LJMU), J. Brimacombe (Coral Towers Observatory), D. Szczygiel, G. Pojmanski (Warsaw University Observatory)
on 2 Dec 2014; 02:21 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Thomas Holoien (tholoien@astronomy.ohio-state.edu)
Subjects: Optical, AGN, 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 near the center of the galaxy PGC 043234:
Object RA (J2000) DEC (J2000) Disc. UT Date Disc. V mag
ASASSN-14li 12:48:15.23 +17:46:26.22 2014-11-22.63 16.5
ASASSN-14li was discovered in images obtained on UT 2014-11-22.63 at V~16.5 mag. We also detect the object in images obtained on UT 2014-11-30.65 (V~16.7), UT 2014-11-29.61 (V~16.8), UT 2014-11-27.62 (V~17.0), and UT 2014-11-11.65 (V~15.8). We do not detect (V>16.7) the object in images taken on UT 2014-07-13.25 and before. We do not have a later non-detection as the field has only recently become observable from behind the Sun. Confirmation images obtained on 2014-11-28.62 with the LCOGT 1-m telescope at McDonald observatory confirm that central region of the host galaxy shows increased flux over the archival SDSS data (see here).
The measured position of ASASSN-14li in the LCOGT g-band image is approximately 0.04" South from the measured position of the center of the galaxy PGC 043234 (z=0.0206, d=90.3 Mpc, via NED), a galaxy in the Coma supercluster. The position of ASASSN-14li is consistent with the center of the host galaxy. At the redshift of the host, the transient would have an absolute V-band magnitude of approximately -18.4 (m-M=34.78, A_V=0.07) at discovery.
After discovery of the transient, we were granted a series of Swift space telescope UVOT and XRT observations. Swift data taken on UT 2014-11-30 show that the central region of the galaxy is significantly brighter in the UV and bluer wavelengths. Using 5" aperture magnitudes measured in the Swift UVOT images, the LCOGT follow-up images, and the archival SDSS images, we construct a spectral energy distribution, shown here. The SED confirms that the source shows strong emission at shorter wavelengths.
ASASSN-14li was oberved by the Swift X-ray Telescope on 2014-11-30 for a total of 2.9 ks. The X-ray spectrum appears to be very soft and can be fit with a blackbody spectrum with a temperature equivalent energy of 66+/-3 eV with the absorption column density fixed to the Galactic value (1.64e20 cm-2). With this model the flux in the 0.2-2.0 keV range is (1.81+/-0.10) e-14 W/m2.
The archival SDSS spectrum of the host is not consistent with that of an AGN. A follow-up spectrum of the nuclear region of the host galaxy obtained on 2014-11-30.6 with the SNIFS IFU spectrograph mounted on the UH 2.2m telescope shows a broad H-alpha emission feature at the redshift of the host with FHWM ~ 9000 km/s and stronger emission at shorter wavelengths. A comparison between the archival spectrum and the follow-up spectrum highlighting this evolution can be seen here.
Based on these observations and the position of the transient, we believe ASASSN-14li is consistent with a tidal disruption event. However, an unusual increase in AGN activity could also be an explanation, and additional follow-up observations are highly 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.
