Gamma-ray observations of Nova Sgr 2015 No. 2 with INTEGRAL

Abstract

Context. INTEGRAL observed Nova Sgr 2015 No. 2 (V5668 Sgr) around the time of its optical emission maximum on 21 March 2015. Studies at UV wavelengths showed spectral lines of freshly produced 7Be. This could also be measurable in gamma rays at 478 keV from the decay to 7Li. Novae are also expected to synthesise 22Na which decays to 22Ne, emitting a 1275 keV photon. About one week before the optical maximum, a strong gamma-ray flash on timescales of hours is expected from short-lived radioactive nuclei such as 13N and 18F. These nuclei are β+-unstable, and should yield emission of up to 511 keV, but this emission has never been observed from any nova. Aims. The SPectrometer on INTEGRAL (SPI) pointed towards V5668 Sgr by chance. We use these observations to search for possible gamma-ray emission of decaying 7Be, 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 extracted 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 7Be 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 7Be and 22Na mass depend on the uncertainties of the distance to V5668 Sgr: the 7Be mass is constrained to less than 4.8 × 10−9 (dkpc−1)2 M⊙, and the 22Na mass to less than 2.4 × 10−8 (dkpc−1)2 M⊙. For the 7Be mass estimate from UV studies, the distance to V5668 Sgr must be greater than 1.2 kpc (3σ). During the three weeks before the optical maximum, we find 23 burst-like events in the ACS rate, of which 6 could possibly be associated with V5668.