Discovery of SN 2005gj-like supernova CSS120327:110520-015205

A.J. Drake (Caltech), D. Garcia-Alvarez (IAC/GTC), S.G. Djorgovski, A.A. Mahabal, M.J. Graham, R. Williams (Caltech); J. Prieto (Princeton); M. Catelan (PUC Chile); E. Christensen (Gemini Obs); S.M. Larson (LPL/UA) plus the GTC and VSOP teams.

On March 27.2UT CRTS discovered transient event CSS120327:110520-015205 (hereafter CSS120327) in data taken by the Catalina Sky Survey (CSS) Schmidt telescope. Upon inspection of SDSS DR8 data a faint host galaxy was found with magnitudes u=20.7, g=19.7, r=19.3, i=19.1 and z= 18.9. The rising lightcurve of the event showed that the event began around 2012 Jan. 23 UT. Based on the available evidence the transient was classified as a likely supernova and posted to CRTS transient webpages and SkyAlert on March 27.7UT.

Follow-up observations were taken with the Palomar 1.5m telescope in g,r,i and z on March 29.13UT and showed that the event had colours g-r=0.53, r-i=0.26 and i-z=0.04. For nearby supernovae such colours are most consistent with a core collapse events (see Poznaski et al. 2002 figure 8 and 9). Additional CSS observations of the object were taken on Apr. 15UT The combined CSS data, Figure 1, appear to show some evidence for variability near peak brightness, although additional analysis is required to constrain variability of the host galaxy.

Figure 1. The flux lightcurve of event CSS120327. Here a combined template image has been subtracted from individual images to determine the flux of the source. Left: CSS data for the location of the transient over the past 1700 days. Right: photometry for the event.

Spectroscopic observations were taken with the Gran Telescopio CANARIAS (GTC) on April 17th with OSIRIS using the R1000B grism (range 370-785 nm). The spectrum shows features of type-Ia/Ic SNe along with broad and narrow Balmer features associated with type-IIn.

Based on narrow lines in the spectrum the supernova is at redshift z=0.09. After correcting for foreground reddening of Av=0.17, based on Schlegel et al. (1998, ApJ, 500, 525) exinction maps, the peak observed luminosity of the event is Mv~-20.5. This is significantly brighter the normal type-Ia events and consistent with supernovae such as SN 2005gj. The spectrum is consistent with events that were previously noted as type-Ia's interacting with the CSM. For example SN 2002ic and SN 2005gj as well as hypernova candidate SN 1997cy (Germany et al. 2000). In Figure 2, we show the spectrum along with that of SN 2005gj from Aldering et al. (2006). The difference between the two spectra appear to be due to the presence of [OII], [OIII] and [S II] emission from the host galaxy. The observed peak luminosity is also very similar to that of SN 2005gj (Aldering et al. 2006) and SN 1997cy (Germany et al. 2000).

Figure 2., Spectroscopic comparison of CSS120327 with known CSM-interacting type-Ia supernova SN 2005gj.

The variability near peak may be a sign of the interaction between supernova shock and a clumpy CSM. However, more detailed analysis is required to determine whether the variability may be caused by underlying variability, such as caused by an AGN in the host galaxy.

Summary

Based on the spectroscopic features and photometric lightcurve of CSS120327 it is a supernova like SN 2002ic, SN 2005gj and SN 1997cy. Further analysis and follow-up is required to investigate the evolution of the source and the variability of its host.