Significant Optical Rebrightening of SN 2010da in NGC 300

ATel #3726; R. Chornock, I. Czekala, & E. Berger (Harvard)
on 31 Oct 2011; 01:20 UT
Credential Certification: Ryan Chornock (rchornock@cfa.harvard.edu)

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We report continuing observations of the dust-enshrouded supernova impostor SN 2010da in NGC 300 (CBET #2289, ATEL #2632, #2633, #2636, #2637, #2638, #2639, #2640, #2642, #2648, #2658, #2660). After the initial optical outburst to an unfiltered magnitude of 16.0 on 2010 May 23 (CBET #2289), this object faded rapidly (ATEL #2640, #2660). We measured i~20.7 mag (AB) on 2011 January 13.10 UT using LDSS on the Magellan/Clay 6.5-m telescope. Further LDSS observations obtained on 2011 October 21.16 UT reveal that the object has now rebrightened by about 2 magnitudes in the optical to i~18.7 mag. This second outburst from the source provides further evidence that the initial event was not a true supernova.

Followup spectroscopy was obtained on 2011 October 22.11 UT with the Magellan Echellette spectrograph (MagE), also mounted on the Magellan/Clay telescope. The moderate-resolution (FWHM~60 km/s) spectrum broadly resembles spectra during the initial outburst (ATEL #2636, #2637), with lines of H I, He I, He II, O I, Fe II, and Ca II displaying complex profiles with multiple velocity components. In addition, weak unresolved emission from the [Ca II] 7291, 7324 doublet is present.

The initial outburst from this source was accompanied by strong X-ray emission (ATEL #2639), so we obtained 10 ks of exposure with the Swift X-ray Telescope (XRT), centered on 2011 October 27.05 UT. No source is detected at the position of SN 2010da, to a limit of < 4e-14 erg/s/cm^2. This upper limit is a factor of ~35 below the earlier detection (ATEL #2639), but is close to the flux level recorded with Chandra on 2010 September 24 UT by Binder et al. (2011, ApJL, 739, L51).

We thank the Swift team for approving our Target of Opportunity request.