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M31N 2017-05b (Gaia17bjg) may be a symbiotic nova eruption

ATel #10520; S. C. Williams (Lancaster), M. J. Darnley (LJMU), K. Hornoch (Ondrejov), A. W. Shafter (SDSU)
on 22 Jun 2017; 22:07 UT
Credential Certification: Steven Williams (scw@astro.ljmu.ac.uk)

Subjects: Optical, Nova, Transient, Variables

M31N 2017-05b (Gaia17bjg, AT2017ehm) was discovered by Gaia on 2017 May 23.53 UT as a brightening of an existing source by 0.7 mag, to mag 19.3 (see Gaia observations). We obtained photometry with IO:O on the 2-m Liverpool Telescope (LT; Steele et al. 2004) on 2017 June 16.14 UT and found:

u' = 18.6 +/- 0.3 
B = 17.58 +/- 0.09 
V = 17.17 +/- 0.04 
r' = 17.04 +/- 0.04 
i' = 16.99 +/- 0.05  
On June 19.14 UT, M31N 2017-05b had faded to V = 18.29 +/- 0.04. We obtained a low-resolution spectrum using SPRAT (also on the LT, resolution R ~ 350; Piascik et al. 2014) on June 21.18 UT. The spectrum shows strong, narrow Balmer emission lines (Hα, Hβ, Hγ, Hδ). A number of other emission lines are also visible, including He I (4471, 5876, 6678, 7065 Å), He II (4686 Å) and Fe II (5169, 5198, 5317 Å). The spectrum shows many similarities to V407 Cyg around two weeks after outburst.

Comparing LT imaging to archival imaging of M31 from Massey et al. 2006 shows the transient is coincident with a V = 20.70, B-V = 1.47 stellar source. Ignoring extinction, the absolute magnitude of the outburst is MV = -7.3, and it is significantly bluer than in quiescence, at B-V = 0.41.

Taken together these observations imply M31N 2017-05b may be a symbiotic nova eruption in M31 (of a similar kind to V407 Cyg, RS Oph, T CrB for example). We will continue to follow this object and encourage further observations, particularly higher resolution spectroscopy.