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CSS100217:102913+404220 a Luminous SN or Tidal Disruption Event?

ATEL # 2544; A.J. Drake, A.A. Mahabal, S.G. Djorgovski, M.J. Graham, R. Williams (Caltech); V. Mohan, S. Ravindranath (IUCAA); J. Prieto, L.C. Ho (OCIW); L. Kewley (IFA); A. Myers (UIUC); M. Catelan (PUC); E. Christensen (Gemini Observatory); E.C. Beshore, S.M. Larson (LPL/UA)
on 10 Apr 2010; 1:48 UT
Distributed as an Instant Email Notice (Request for Observations)
Password Certification: Andrew J. Drake (ajd@cacr.caltech.edu)

Subjects: Optical, Request for Observations, Black Holes, Nova, Supernova, Transients
Referred to by ATEL #: 2554

We report the discovery and follow-up of an extremely luminous optical transient discovered by the Catalina Real-time Transient Survey (CRTS) on Feb 17.3UT.

IDDisc. DateRADecDisc. Mag Redshift
CSS100217:102913+404220 2010-02-17T07:16:31 10:29:12.56 40:42:20.0 16.2 0.148

The SDSS spectrum shows strong emission lines of with width 450km/s at redshift z=0.15. Ho and Kim (2010) find log[OIII/H_beta] = 0.14, log[NII/H_alpha] = -0.81, log[SII/H_alpha] =-1.46 and log[OI/H_alpha] =-1.42. This clearly places the host galaxy within the locus of star-forming galaxies on the BPT diagram (see Kauffmann et al. 2003), rather than AGN. However, Ho and Kim note that the galaxy spectrum also exhibits significant flux from a broad (2000km/s) component suggesting of the presence of a AGN with the values best matching a NLS1. The unusual placement on the BPT suggests the object exhibits a combination of star-formation and AGN behaviour.

Follow-up g,r,i,z photometry was performed with the P60 telescope on Mar 02UT and Mar 11UT giving g=16.2, r=15.8, i=16.0, z=15.9. Subtracting the galaxy contribution and correcting for the foreground extinction, we find the optical transient has the following absolute magnitudes Mg=-22.8, Mr=-23, Mi=-23.2, Mz=-23.3. We obtained SWIFT ToO time on April 6th which gives UVW1= -23.1, UVM2= -23.2, UVW2=-23.0, U=-23.5, B=-22.7, V =-23.0. The object appears to be marginally detected in the SWIFT XRT x-ray image.

Spectra were obtained with the IGO 2m+IFOSC on Feb 19th and April 5th, and with the Palomar 5m+DSBS of Mar 16UT and resemble the initial SDSS host spectrum. However, the continuum and Balmer and HeI (5876) line have clearly increased in strength by a factor of ~5. In contrast the [OII], [NII], and [OIII] lines remain at the same strength as in the SDSS spectrum. The presence of narrow Balmer emission is typical for type IIn supernova and the timescale of the luminosity increase is also consistent with such events. The line widths have not clearly increased beyond the SDSS spectrum. However, subtraction of the SDSS spectrum from the P200 spectrum appears show evidence for a very broad component. If this event is truly due to a type IIn supernova, it is a magnitude more luminous than the brightest ever seen (SN 2008fz, SN 2008es). High resolution images could potentially reveal the presence of a supernova offset from the galaxy's nucleus.

An alternate possibility is that the event is due to tidal disruption flare. Ulmer (1997) note that for very luminous tidal disruption flares can produce a very blue continuum as well as emission lines. Since the transient event is extremely bright, blue, and near the center of a luminous galaxy it may well be a tidal disruption event. The increase in continuum and Balmer emission with little sign of evolution in line widths, as well as a tentative X-ray detection, appears in agreement with this.

We request follow-up observations in all wavelengths.
Further details are presented here: http://crts.caltech.edu/CSS100217.html

We thank the Swift PI Neil Gehrels for approving the requested ToO, and the Swift staff for planning and executing the observations.
We also thank M. Kim for the decomposition of the SDSS spectrum.


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