Swiftdetection of the super-swift switch-on of the super-soft phase in nova V745 Sco (2014)

Abstract

V745 Sco is a recurrent nova, with the most recent eruption occurring in February 2014. V745 Sco was first observed by Swift a mere 3.7 hr after the announcement of the optical discovery, with the super-soft X-ray emission being detected around four days later and lasting for only ∼ two days, making it both the fastest follow-up of a nova by Swift and the earliest switch-on of super-soft emission yet detected. Such an early switch-on time suggests a combination of a very high velocity outflow and low ejected mass and, together with the high effective temperature reached by the super-soft emission, a high mass white dwarf (>1.3 M ⊙ ). The X-ray spectral evolution was followed from an early epoch where shocked emission was evident, through the entirety of the super-soft phase, showing evolving column density, emission lines, absorption edges and thermal continuum temperature. UV grism data were also obtained throughout the super-soft interval, with the spectra showing mainly emission lines from lower ionization transitions and the Balmer continuum in emission. V745 Sco is compared with both V2491 Cyg (another nova with a very short super-soft phase) and M31N 2008-12a (the most rapidly recurring nova yet discovered). The longer recurrence time compared to M31N 2008-12a could be due to a lower mass accretion rate, although inclination of the system may also play a part. Nova V745 Sco (2014) revealed the fastest evolving super-soft source phase yet discovered, providing a detailed and informative dataset for study.

M31N 2008-12a—THE REMARKABLE RECURRENT NOVA IN M31: PANCHROMATIC OBSERVATIONS OF THE 2015 ERUPTION

“…demonstrated a clear difference between the colormagnitude tracks of eruptions from systems hosting a red giant companion (RG-nova) and those having a sub-giant or mainsequence companion (SG-or MS-nova). The color-magnitude track evolves almost vertically along the line of -= B V 0 ( ) -0.03 (the intrinsic color of optically thick free-free emission; as shown in Figure 7, along with -= B V 0.13 0 ( ) for optically thin free-free emission; also see the discussion in Section 7.2) for RNe harboring a red giant companion, such as V745Sco (2014 eruption, data from Page et al 2015; also see Section 7.6) and RS Oph (1958Oph ( , 1985Oph ( , and 2006 ), respectively. On the other hand, the track goes blueward and then turns back redward near the two-headed arrow, as shown in Figure 7(c), for RNe with a sub-giant or main-sequence companion-for example, USco, CIAquilae, and TPyxidis (data from Pagnotta et al 2015, VSOLJ, and AAVSO/SMARTS, respectively).…”

“…Additionally, while for M31N 2008-12a we speculate, in Section 7.4, that the short recurrence time may require the accretion disk to stay intact, it may not survive the eruption in V745Sco, thus delaying the next eruption by at least the time it takes for accretion to resume again. Page et al (2015) reported that V745Sco showed no variability during the early SSS phase. However, the observed smooth rise to peak flux was exceptionally rapid and was essentially covered by only six observations, which might not have been sufficient to capture variability.…”

“…Blackbody fits to the X-ray spectra indicated a particularly hot source (∼100 eV) compared to the M 31 nova population (see Henze et al 2014b). The SSS phase lasted for only twelve days; at the time, M31N 2008-12a had the fastest SSS turn-on and turn-off ever observed (these were both surpassed by the 2014 eruption of the Galactic RN V745 Scorpii, an RG-nova; see Page et al 2015 and Section 7.6). The X-ray properties pointed to a combination of a high-mass WD and low ejected mass, with the HST data indicating a high mass accretion rate.…”

“…with the peak luminosity reaching (only) the 10th magnitude, some eruptions may have been missed, rendering the inferred recurrence period an upper limit (see Shafter 2016 and references therein for a general Galactic nova completeness discussion). Page et al (2015) presented a comprehensive X-ray and UV analysis of the 2014 eruption of V745Sco. Taking into account the uncertainty of the eruption date, they found that photospheric X-ray emission was detected only 4-5 day after eruption, thereby narrowly surpassing the turn-on timescale of M31N 2008-12a.…”

X-RAY FLASHES IN RECURRENT NOVAE: M31N 2008-12a AND THE IMPLICATIONS OF THE SWIFT NONDETECTION

“…Observations of recurrent novae have shown that they evolve much faster than typical classical novae, which is demonstrated by their very short X-ray turn-on time (duration between the optical peak to the X-ray turn-on, see, e.g., Page et al 2015, for the shortest 4 day case of V745 Sco). By analogy, and without observational support, we suspect that the rising phase of recurrent novae must also be fast.…”