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Observations of the SN impostor in NGC 2770 from the 10.4 m GTC

ATel #7409; A. de Ugarte Postigo (IAA-CSIC, DARK/NBI), G. Leloudas (Weizmann, DARK/NBI), C. C. Thoene (IAA-CSIC), S. Geier (GRANTECAN, IAC), A. Tejero (GRANTECAN)
on 17 Apr 2015; 18:10 UT
Distributed as an Instant Email Notice Supernovae
Credential Certification: Antonio de Ugarte Postigo (adeugartepostigo@gmail.com)

Subjects: Optical, Supernovae

Referred to by ATel #: 7514, 7515, 7517, 7541

The 10.4m Gran Telescopio Canarias (+OSIRIS) observed NGC 2770 on 27 March, 9 April and 14 April 2015 in the context of a galaxy study. Within these data, the supernova impostor PSN J09093496+3307204 / SNhunt275 showed an increase in brightness as compared to the observation of 9 February 2015 presented by Elias-Rosa et al. (ATel #7042). The imaging of the first two epochs were obtained with narrow-band filters (of several hundred AA) centred at 4580, 6270 and 7230 AA respectively. The central wavelengths are similar to those of g, r and i bands, and we present below photometry as compared with SDSS stars. However, we caution that there is a systematic error due to the difference in the filters that will dominate the magnitude uncertainty. In the last epoch we used a Sloan r filter.

Epoch (UT) | filter | magnitude (AB) |
20150327.00 | f723/45 (~i) | 18.46 |
20150327.01 | f627/24 (~r) | 18.75 |
20150327.02 | f458/13 (~g) | 19.13 |
20150409.02 | f723/45 (~i) | 18.02 |
20150409.02 | f627/24 (~r) | 18.46 |
20150409.03 | f458/13 (~g) | 18.78 |
20150414.96 | r | 18.20+/-0.05 |

The last epoch can be directly compared to the magnitude measured by Elias-Rosa et al. (ATel 7042) from 9 February 2015 and shows a significant increase of brightness of 1.2 mag. The previous two epochs already show a gradual increase of the brightness happening over the last month. The r-band magnitude of 14 April implies an absolute magnitude of M_r=-14.2 (assuming a distance modulus of 32.4).

On 14 April we also obtained a spectrum of the transient using grism R500B, covering the range between 3500 and 8700 AA at a resolution of R~500. The spectrum is dominated by Ha and other Balmer lines, but we also observe a Na I/He I line at 5930 AA (observed wavelength) and prominent Fe lines between 4950-5400 AA. The Ha line shows an asymmetric profile. As noted by Elias-Rosa et al. (ATel #7042) it consists of 2 components, although the contrast between the narrow and broad component appears less strong in our spectrum and we do not distinguish a P-Cygni profile for the narrow component. The FWHM is 1600-1800 km/s and the line extends to ~5000km/s in the red, while it shows a sharper drop in the blue. In general, our spectrum looks fairly similar to the spectra of SN 2009ip from 25-29 September 2009 shown in Pastorello et al. (2013, ApJ, 767, 1), except the fact that we do not observe a prominent P-Cygni profile in Ha.