AT 2016dah and AT 2017fyp: the first classical novae discovered within a
  tidal stream
  AT2016dah and AT2017fyp are fairly typical Andromeda Galaxy (M31) classical
novae. AT2016dah is an almost text book example of a 'very fast' declining, yet
uncommon, Fe II'b' (broad-lined) nova, discovered during the rise to peak
optical luminosity, and decaying with a smooth broken power-law light curve.
AT2017fyp is classed as a 'fast' nova, unusually for M31, its early decline
spectrum simultaneously shows properties of both Fe II and He/N spectral types
- a 'hybrid'. Similarly, the light curve of AT2017fyp has a broken power-law
decline but exhibits an extended flat-topped maximum. Both novae were followed
in the UV and X-ray by the Neil Gehrels Swift Observatory, but no X-ray source
was detected for either nova. The pair were followed photometrically and
spectroscopically into their nebular phases. The progenitor systems were not
visible in archival optical data, implying that the mass donors are main
sequence stars. What makes AT2016dah and AT2017fyp particularly interesting is
their position with respect to M31. The pair are close on the sky but are
located far from the centre of M31, lying almost along the semi-minor axis of
their host. Radial velocity measurements and simulations of the M31 nova
population leads to the conclusion that both novae are members of the Andromeda
Giant Stellar Stream (GSS). We find the probability of at least two M31 novae
appearing coincident with the GSS by chance is ~1%. Therefore, we claim that
these novae arose from the GSS progenitor, not M31 - the first confirmed novae
discovered in a tidal steam.
