X-ray spectroscopy of the γ-ray brightest nova V906 Car (ASASSN-18fv). Issue 3 (20th July 2020)

• Record Type: Journal Article
• Authors: Sokolovsky, Kirill V; Mukai, Koji; Chomiuk, Laura; Lopes de Oliveira, Raimundo; Aydi, Elias; Li, Kwan-Lok; Steinberg, Elad; Vurm, Indrek; Metzger, Brian D; Kawash, Adam; Linford, Justin D; Mioduszewski, Amy J; Nelson, Thomas; Ness, Jan-Uwe; Page, Kim L; Rupen, Michael P; Sokoloski, Jennifer L; Strader, Jay
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 497:Issue 3(2020)
• Page Start: 2569
• Page End: 2585
• Publication Date: 2020-07-20
• Subjects: white dwarfs -- novae, cataclysmic variables; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/staa2104
• Languages: English
• ISSNs: 0035-8711

Abstract

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 ( N H ≈ 2 × 10 23 cm −2 ), the thermal X-ray luminosity (from a 9 keV optically thin plasma) is just ∼2 per cent 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 ( N H > 10 25 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 d 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 supersoft phase in the X-ray light curve, which, after considering the absorption effects, may point to a low mass of the white dwarf hosting the nova.