Abstract
ABSTRACT The Andromeda Galaxy recurrent nova M31N 2008-12a had been observed in eruption 10 times, including yearly eruptions from 2008 to 2014. With a measured recurrence period of days (we believe the true value to be half of this) and a white dwarf very close to the Chandrasekhar limit, M31N 2008-12a has become the leading pre-explosion supernova type Ia progenitor candidate. Following multi-wavelength follow-up observations of the 2013 and 2014 eruptions, we initiated a campaign to ensure early detection of the predicted 2015 eruption, which triggered ambitious ground- and space-based follow-up programs. In this paper we present the 2015 detection, visible to near-infrared photometry and visible spectroscopy, and ultraviolet and X-ray observations from the Swift observatory. The LCOGT 2 m (Hawaii) discovered the 2015 eruption, estimated to have commenced at August 28.28 ± 0.12 UT. The 2013–2015 eruptions are remarkably similar at all wavelengths. New early spectroscopic observations reveal short-lived emission from material with velocities ∼13,000 km s−1, possibly collimated outflows. Photometric and spectroscopic observations of the eruption provide strong evidence supporting a red giant donor. An apparently stochastic variability during the early supersoft X-ray phase was comparable in amplitude and duration to past eruptions, but the 2013 and 2015 eruptions show evidence of a brief flux dip during this phase. The multi-eruption Swift/XRT spectra show tentative evidence of high-ionization emission lines above a high-temperature continuum. Following Henze et al. (2015a), the updated recurrence period based on all known eruptions is days, and we expect the next eruption of M31N 2008-12a to occur around 2016 mid-September.
Highlights
Novae are powerful eruptions resulting from a brief thermonuclear runaway (TNR) occurring at the base of the surface layer of an accreting white dwarf (WD; see Schatzman 1949, 1951; Gurevitch & Lebedinsky 1957; Cameron 1959; Starrfield et al 1972, 2008; 2016; José & Shore 2008; José 2016, for recent reviews)
The earliest spectroscopic observations of M31N 2008-12a prior to the 2015 eruption were obtained by the William Herschel Telescope (WHT) 1.27 days after the 2014 eruption (DHS15)
The 2015 eruption of M31N 2008-12a was discovered independently by dedicated monitoring programs utilizing the Swift orbiting observatory and the Las Cumbres Observatory Global Telescope Network (LCOGT) 2 m (Hawaii) on 2015 August 28.41 UT and 28.425, respectively, with preeruption non-detections constraining the time of the eruption to 2015 August 28.28 ± 0.12 UT
Summary
Novae are powerful eruptions resulting from a brief thermonuclear runaway (TNR) occurring at the base of the surface layer of an accreting white dwarf (WD; see Schatzman 1949, 1951; Gurevitch & Lebedinsky 1957; Cameron 1959; Starrfield et al 1972, 2008; 2016; José & Shore 2008; José 2016, for recent reviews). The shorter recurrence periods are driven by a combination of a high-mass WD and a high mass accretion rate Such high accretion rates are typically driven by an evolved companion star, such as a Roche lobe overflowing sub-giant star (SG-novae; the U Scorpii type of RNe) or. A major result of this work was the discovery of strong correlations between various observable parameters, indicating that novae with a faster visible decline tend to show a shorter SSS phase with a higher temperature (Henze et al 2014b). This is consistent with the trends seen in Galactic novae (see Schwarz et al 2011). X-ray (ROSAT) X-ray (ROSAT) X-ray (Chandra) Visible (Miyaki-Argenteus) Visible (PTF) Visible (Miyaki-Argenteus) Visible (ISON-NM) Visible (Miyaki-Argenteus) Visible (iPTF); UV/X-ray (Swift) Visible (LT); UV/X-ray (Swift) Visible (LCOGT); UV/X-ray (Swift)
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