High cadence photometry of SN 2009ip

ATel #4439; J. L. Prieto (Princeton), J. Brimacombe (Coral Towers Observatory), A. J. Drake (Caltech)
on 30 Sep 2012; 05:57 UT
Credential Certification: Jose L. Prieto (jose@obs.carnegiescience.edu)

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The surprising spectroscopic changes in the latest reported outburst (Drake et al., ATel #4334) of the LBV/SN impostor SN 2009ip led Smith & Mauerhan (ATel #4412) to conclude that it had most likely transitioned to a SN explosion by Sep. 15, 2012 (see also Burgasser et al., ATel #4431; Vinko et al., ATel #4435). Although initial photometric monitoring found no clear evidence of brightening between Sep. 11-24 (Margutti et al., ATel #4414; Martin et al., ATel #4416), J. Brimacombe (ATel #4423) reported a dramatic brightening at red wavelengths of ~2.5 mag, reaching SN luminosities, observed between Sep. 23.6 and 25.5. This was confirmed at other wavelengths (Margutti et al., ATel #4425; Leonard et al., ATel #4430).

We report a re-analysis of the photometric observations of SN 2009ip obtained by J. Brimacombe (ATel #4423) as well as new CCD imaging. The data were obtained between UT Sep 23.6 and Sep 28.5 with 0.3m (R-band filter) and 0.4m (IR filter; >700 nm) telescopes at the Coral Towers Observatory (Cairns, Australia). The initial report had used nightly stacks to estimate the SN magnitudes. Here we treat each 900 sec exposure independently (73 images in IR, 45 in R) and use difference imaging to estimate SN 2009ip fluxes. The photometric zeropoints for the R and IR data were estimated using Sloan r' and i' magnitudes of 5 stars in the field obtained from the AAVSO Photometric All-Sky Survey , converted to standard R and I magnitudes. The light curves of SN 2009ip are shown here. The high cadence and number of observations allow us to clearly resolve the very rapid brightening that most likely started between Sep. 23.65 and 24.45. Between Sep. 24.45 and 24.70 (6hr period) it brightened by 0.6 mag, but this rate of brightening significantly decreased between Sep. 24.7 and 25.4 as the light curve started to turn over. Assuming a linear increase in mag we estimate a brightening of 0.19 mag/day using data between Sep. 26.4 and 28.5, a factor of ~13 slower (in mag) than during the night of Sep. 24. We note that during the full brightening episode these observations are consistent with no color changes and a blue continuum (R-I ~ 0).

The spectroscopic observations have shown that SN 2009ip had broad (~10,000 km/s) velocity components as early as Aug. 26 (ATel #4435) and until at least Sep. 23 (see Mauerhan et al. 2012, arXiv:1209.6320), but the lines became much narrower by Sep 26-27 (Smith & Mauerhan, ATel #4427; ATel #4431; ATel #4435). This change in spectroscopic properties has been interpreted as the start of strong ejecta-CSM interaction in a type IIn SN, and its timing is consistent with the "turn-on" observed in our high cadence photometry. If this is the case, then we might expect to see a very interesting evolution of the light curve as the fast SN shock interacts with the material ejected in recent eruptions.

SN 2009ip light curve