X-ray spectroscopy of the γ-ray brightest nova V906 Car (ASASSN-18fv)

Abstract

Shocks in γ-ray emitting classical novae are expected to produce bright thermal and non-thermal X-rays. We test this prediction with simultaneous NuSTAR and Fermi/LAT observations of nova V906 Car, which exhibited the brightest GeV γ-ray emission to date. The nova is detected in hard X-rays while it is still γ-ray bright, but contrary to simple theoretical expectations, the detected 3.5–78 keV emission of V906 Car is much weaker than the simultaneously observed >100 MeV emission. No non-thermal X-ray emission is detected, and our deep limits imply that the γ-rays are likely hadronic. After correcting for substantial absorption (NH ≈ 2 × 1023 cm−2), the thermal X-ray luminosity (from a 9 keV optically-thin plasma) is just ∼2% of the γ-ray luminosity. We consider possible explanations for the low thermal X-ray luminosity, including the X-rays being suppressed by corrugated, radiative shock fronts or the X-rays from the γ-ray producing shock are hidden behind an even larger absorbing column (NH > 1025 cm−2). Adding XMM-Newton and Swift/XRT observations to our analysis, we find that the evolution of the intrinsic X-ray absorption requires the nova shell to be expelled 24 days after the outburst onset. The X-ray spectra show that the ejecta are enhanced in nitrogen and oxygen, and the nova occurred on the surface of a CO-type white dwarf. We see no indication of a distinct super-soft phase in the X-ray lightcurve, which, after considering the absorption effects, may point to a low mass of the white dwarf hosting the nova.

Fermi-LAT Observations of V549 Vel 2017: a Sub-Luminous Gamma-Ray Nova?

“…In V549 Vel, the quasi-simultaneous Swift and Fermi -LAT flux measurements taken on MJD 58072 result in a flux ratio of 2-77%, unambigously showing that the γray luminosity is higher. This, despite the difference in the X-ray energy ranges, is in agreement with the result of the recent Fermi -LAT and NuSTAR concurrent observations of the two novae V5855 Sgr and V906 Car, which yield ∼0.1-1% (Nelson et al 2019) and 2% (Sokolovsky et al 2020;Aydi et al 2020), respectively. However, in the nova shock model of Metzger et al (2014) with standard parameters assumed, one would expect to observe much brighter thermal X-ray emission-10 to 1000 times more luminous than the γ-ray flux-and this contradiction was also found in V5855 Sgr and V906 Car.…”

The onset of jets in classical novae

“…It was independently discovered after this by Stanek et al (2018) using the All Sky Automated Survey for SuperNovae (hereafter ASAS-SN). In this paper, we reference times as days since commencement of brightening ( 2458193 Aydi et al (2020b) suggest a value of 2 = 44 d. Sokolovsky et al (2020b) determined that this classical nova occured on a CO white dwarf, and they concluded that the mass must have been low as there was no distinct supersoft phase detected in the X-ray. Their thorough analysis of the NuSTAR, Fermi/LAT, XMM-Newton and Swift/XRT X-ray data revealed that the nova shell was expelled by +24 d. The white dwarf was found to be of CO-type by Sokolovsky et al (2020b).…”

The onset of jets in classical novae

“…The mass of the white dwarf in V906 Car was estimated at 0.71 +0.23 −0.19 𝑀 through an analysis of the progenitor system by Wee et al ( 2020) using an accretion disc model to understand the light curve. Their decline times of 𝑡 2,𝑣 = 26.2 d and 𝑡 3,𝑣 = 33.0 d point to V906 Car's being a moderately fast classical nova, although Aydi et al (2020b) suggest a value of 𝑡 2 = 44 d. Sokolovsky et al (2020b) determined that this classical nova occured on a CO white dwarf, and they concluded that the mass must have been low as there was no distinct supersoft phase detected in the X-ray. Their thorough analysis of the NuSTAR, Fermi/LAT, XMM-Newton and Swift/XRT X-ray data revealed that the nova shell was expelled by +24 d. The white dwarf was found to be of CO-type by Sokolovsky et al (2020b).…”