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The recurrent symbiotic nova V745 Sco exploded on 2014 February 6 and was observed on February 22 and 23 by the Chandra X-ray Observatory Transmission Grating Spectrometers. By that time the supersoft source phase had already ended, and Chandra spectra are consistent with emission from a hot, shock-heated circumstellar medium with temperatures exceeding 10 7 K. X-ray line profiles are more sharply peaked than expected for a spherically symmetric blast wave, with a full width at zero intensity of approximately 2400 km s −1 , an FWHM of 1200 ± 30 km s −1 , and an average net blueshift of 165 ± 10 km s −1 . The red wings of lines are increasingly absorbed toward longer wavelengths by material within the remnant. We conclude that the blast wave was sculpted by an aspherical circumstellar medium in which an equatorial density enhancement plays a role, as in earlier symbiotic nova explosions. Expansion of the dominant X-ray-emitting material is aligned close to the plane of the sky and is most consistent with an orbit seen close to face-on. Comparison of an analytical blast wave model with the X-ray spectra, Swift observations, and near-infrared line widths indicates that the explosion energy was approximately 10 43 erg and confirms an ejected mass of approximately 10 −7 M e . The total mass lost is an order of magnitude lower than the accreted mass required to have initiated the explosion, indicating that the white dwarf is gaining mass and is a Type Ia supernova progenitor candidate.

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.

Context. INTEGRAL observed Nova Sgr 2015 No. 2 (V5668 Sgr) around the time of its optical emission maximum on March 21, 2015. Studies at UV wavelengths showed spectral lines of freshly produced 7 Be. This could be measurable also in gamma-rays at 478 keV from the decay to 7 Li. Novae are also expected to synthesise 22 Na which decays to 22 Ne, emitting a 1275 keV photon. About one week before the optical maximum, a strong gamma-ray flash on time-scales of hours is expected from short-lived radioactive nuclei, such as 13 N and 18 F. These nuclei are β + -unstable, and should yield emission up to 511 keV, but which has never been observed from any nova. Aims. The spectrometer SPI aboard INTEGRAL pointed towards V5668 Sgr by chance. We use these observations to search for possible gamma-ray emission of decaying 7 Be, and to directly measure the synthesised mass during explosive burning. We also aim to constrain possible burst-like emission days to weeks before the optical maximum using the SPI anticoincidence shield (ACS), i.e. at times when SPI was not pointing to the source. Methods. We extract spectral and temporal information to determine the fluxes of gamma-ray lines at 478 keV, 511 keV, and 1275 keV. Using distance and radioactive decay, a measured flux converts into the 7 Be amount produced in the nova. The SPI-ACS rates are analysed for burst-like emission using a nova model light-curve. For the obtained nova flash candidate events, we discuss possible origins using directional, spectral, and temporal information. Results. No significant excess for the 478 keV, the 511 keV, or the 1275 keV lines is found. Our upper limits (3σ) on the synthesised 7 Be and 22 Na mass depend on the uncertainties of the distance to V5668 Sgr: The 7 Be mass is constrained to less than 4.8 × 10 −9 (d/kpc) 2 M , and the 22 Na mass to less than 2.4 × 10 −8 (d/kpc) 2 M . For the 7 Be mass estimate from UV studies, the distance to V5668 Sgr must be larger than 1.2 kpc (3σ). During three weeks before the optical maximum, we find 23 burst-like events in the ACS rate, of which six could possibly be associated with V5668.