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Register a new userX-Ray Flashes in Recurrent Novae: M31N 2008-12a and the Implications of the Swift Non-detection
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Models of nova outbursts suggest that an X-ray flash should occur just after hydrogen ignition. However, this X-ray flash has never been observationally confirmed. We present four theoretical light curves of the X-ray flash for two very massive white dwarfs (WDs) of 1.380 and 1.385 M_sun and for two recurrence periods of 0.5 and 1 years. The duration of the X-ray flash is shorter for a more massive WD and for a longer recurrence period. The shortest duration of 14 hours (0.6 days) among the four cases is obtained for the 1.385 M_sun WD with one year recurrence period. In general, a nova explosion is relatively weak for a very short recurrence period, which results in a rather slow evolution toward the optical peak. This slow timescale and the predictability of very short recurrence period novae give us a chance to observe X-ray flashes of recurrent novae. In this context, we report the first attempt, using the Swift observatory, to detect an X-ray flash of the recurrent nova M31N 2008-12a (0.5 or 1 year recurrence period), which resulted in the non-detection of X-ray emission during the period of 8 days before the optical detection. We discuss the impact of these observations on nova outburst theory. The X-ray flash is one of the last frontiers of nova studies and its detection is essentially important to understand the pre-optical-maximum phase. We encourage further observations.
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The recurrent nova M31N 2008-12a experiences annual eruptions, contains a near-Chandrasekhar mass white dwarf, and has the largest mass accretion rate in any nova system. In this paper, we present Hubble Space Telescope (HST) WFC3/UVIS photometry of the late decline of the 2015 eruption. We couple these new data with archival HST observations of the quiescent system and Keck spectroscopy of the 2014 eruption. The late-time photometry reveals a rapid decline to a minimum luminosity state, before a possible recovery / re-brightening in the run-up to the next eruption. Comparison with accretion disk models supports the survival of the accretion disk during the eruptions, and uncovers a quiescent disk mass accretion rate of the order of $10^{-6},M_odot,mathrm{yr}^{-1}$, which may rise beyond $10^{-5},M_odot,mathrm{yr}^{-1}$ during the super-soft source phase - both of which could be problematic for a number of well-established nova eruption models. Such large accretion rates, close to the Eddington limit, might be expected to be accompanied by additional mass loss from the disk through a wind and even collimated outflows. The archival HST observations, combined with the disk modeling, provide the first constraints on the mass donor; $L_mathrm{donor}=103^{+12}_{-11},L_odot$, $R_mathrm{donor}=14.14^{+0.46}_{-0.47},R_odot$, and $T_mathrm{eff, donor}=4890pm110$ K, which may be consistent with an irradiated M31 red-clump star. Such a donor would require a system orbital period $gtrsim5$ days. Our updated analysis predicts that the M31N 2008-12a WD could reach the Chandrasekhar mass in < 20 kyr.
Since its discovery in 2008, the Andromeda galaxy nova M31N 2008-12a has been observed in eruption every single year. This unprecedented frequency indicates an extreme object, with a massive white dwarf and a high accretion rate, which is the most promising candidate for the single-degenerate progenitor of a type-Ia supernova known to date. The previous three eruptions of M31N 2008-12a have displayed remarkably homogeneous multi-wavelength properties: (i) From a faint peak, the optical light curve declined rapidly by two magnitudes in less than two days; (ii) Early spectra showed initial high velocities that slowed down significantly within days and displayed clear He/N lines throughout; (iii) The supersoft X-ray source (SSS) phase of the nova began extremely early, six days after eruption, and only lasted for about two weeks. In contrast, the peculiar 2016 eruption was clearly different. Here we report (i) the considerable delay in the 2016 eruption date, (ii) the significantly shorter SSS phase, and (iii) the brighter optical peak magnitude (with a hitherto unobserved cusp shape). Early theoretical models suggest that these three different effects can be consistently understood as caused by a lower quiescence mass-accretion rate. The corresponding higher ignition mass caused a brighter peak in the free-free emission model. The less-massive accretion disk experienced greater disruption, consequently delaying re-establishment of effective accretion. Without the early refueling, the SSS phase was shortened. Observing the next few eruptions will determine whether the properties of the 2016 outburst make it a genuine outlier in the evolution of M31N 2008-12a.
We present a theoretical light curve model of the recurrent nova M31N 2008-12a, the current record holder for the shortest recurrence period (1 yr). We combined interior structures calculated using a Henyey-type evolution code with optically thick wind solutions of hydrogen-rich envelopes, which give the proper mass-loss rates, photospheric temperatures, and luminosities. The light curve model is calculated for a 1.38 M_sun white dwarf (WD) with an accretion rate of 1.6 times 10^{-7} M_sun yr^{-1}. This model shows a very high effective temperature (log T_ph (K) geq 4.97) and a very small wind mass-loss rate (dot M_wind leq 9.3 times 10^{-6} M_sun yr^{-1}) even at the maximum expansion of the photosphere. These properties are consistent with the faint optical peak of M31N 2008-12a because the brightness of the free-free emission is proportional to the square of the mass-loss rate. The model well reproduces the short supersoft X-ray turn-on time of 6 days and turnoff time of 18 days after the outburst. The ejecta mass of our model is calculated to be 6.3 times 10^{-8} M_sun, corresponding to 37% of the accreted mass. The growth rate of the WD is 0.63 times the mass accretion rate, making it a progenitor for a Type Ia supernova. Our light curve model predicts a bright supersoft X-ray phase one or two days before the optical peak. We encourage detection of this X-ray flash in future outbursts.
The Andromeda Galaxy recurrent nova M31N 2008-12a had been observed in eruption ten times, including yearly eruptions from 2008-2014. With a measured recurrence period of $P_mathrm{rec}=351pm13$ 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 2m (Hawaii) discovered the 2015 eruption, estimated to have commenced at Aug. $28.28pm0.12$ UT. The 2013-2015 eruptions are remarkably similar at all wavelengths. New early spectroscopic observations reveal short-lived emission from material with velocities $sim13000$ 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 super-soft 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 $P_mathrm{rec}=174pm10$ d, and we expect the next eruption of M31N 2008-12a to occur around mid-Sep. 2016.
The 2008 discovery of an eruption of M31N 2008-12a began a journey on which the true nature of this remarkable recurrent nova continues to be revealed. M31N 2008-12a contains a white dwarf close to the Chandrasekhar limit, accreting at a high rate from its companion, and undergoes thermonuclear eruptions which are observed yearly and may even be twice as frequent. In this paper, we report on Hubble Space Telescope Space Telescope Imaging Spectrograph ultraviolet spectroscopy taken within days of the predicted 2015 eruption, coupled with Keck spectroscopy of the 2013 eruption. Together, this spectroscopy permits the reddening to be constrained to E(B-V) = 0.10 +/- 0.03. The UV spectroscopy reveals evidence for highly ionized, structured, and high velocity ejecta at early times. No evidence for neon is seen in these spectra however, but it may be that little insight can be gained regarding the composition of the white dwarf (CO vs ONe).
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