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Swift observations and continued spectroscopy/photometry of nova V5856 Sgr trapped in persistent nuclear burning five years past the 2016 outburst

ATel #14884; U. Munari (INAF Padova), N. Masetti (INAF-OAS Bologna), F. M. Walter (Stony Brook Univ.) R. E. Williams (STScI and UCSC), P. Valisa and S. Dallaporta (ANS Collaboration)
on 30 Aug 2021; 07:46 UT
Credential Certification: U. Munari (ulisse.munari@oapd.inaf.it)

Subjects: Optical, Ultra-Violet, X-ray

We have reported earlier (Munari et al. ATel #14804) about Nova V5856 Sgr (= ASASSN-16ma = Nova Sgr 2016 N.4) that in July 2021 we found to be still 9 mag brighter than quiescence five years past the initial outburst, suggesting it is continuing to burn at the surface of the WD. The nova erupted on 2016 October 25 (Stanek et al. 2016, ATel #9669), and was remarkable for the strong emission in gamma-rays recorded by Fermi-LAT (Li et al. 2017, NatAs 1, 697) and its tight correlation with the behavior at optical wavelengths (Munari et al. 2017, MNRAS 469, 4341). The ASASSN light-curve (Shappee et al. 2014, ApJ 788, 48; Kochanek et al. 2017, PASP 129, 104502) shows that around mid-2017 the nova drastically reduced its rate of decline, losing only about 1 mag since then.

We are continuing to collect UBVRI photometry of the nova with the ANS Collaboration telescope ID 0310, low resolution spectroscopy with the Asiago 1.22m telescope and Echelle spectroscopy with the Varese 0.84m telescope. The nova has remained highly stable through July and August 2021, maintaining the spectral appearance already described in ATel #14804 and with constant brightness around the mean values (on the Landolt system):

date UTUBVRI
2021 Jul-Aug13.8113.84513.22012.78212.554

As part of an ongoing monitoring, an Echelle spectrum of the nova was obtained with Chiron high resolution spectrograph on the SMARTS/CTIO 1.5m telescope on 2021 Jul 29, at a higher dispersion and S/N compared to the Varese 0.84m Echelle spectra. The Chiron spectrum confirms that permitted emission lines are composed of a trapezoidal broad pedestal (900 km/s wide at the top, 1500 km/s at the bottom) with superimposed a prominent narrow central spike (FWHM: 60 to 100 km/s) flanked by a narrow absorption (FWHM: 35 to 50 km/s) blue-shifted by 30 to 60 km/s from the narrow emission. The forbidden lines are characterized by a trapezoidal profile slightly wider than for permitted lines (1700 km/s at the base and 1100 km/s at the top) and a much weaker central spike.

Starting at 02:28 UT of 2021 August 25 the source was observed in X-rays and ultraviolet with the instruments XRT and UVOT onboard the Neil Gehrels Swift satellite. The three UVOT filters UVW1, UVM2 and UVW2 were used, with exposure times 431 s, 382 s and 382 s, respectively; V5856 Sgr was detected in all bands at Vega system magnitudes UVW1=13.36+-0.04, UVM2=14.12+-0.04 and UVW2=13.87+-0.04. The simultaneous 1200s XRT observation failed to detect the target, with a 3-sigma limit of 7.6e-3 counts/s (0.3-10~keV)

Assuming a distance d=6.5 kpc and a color excess E(B-V)=0.35 mag (Munari et al. 2017, MNRAS 469, 4341), the energy radiated by the nova over the UVOT and UBVRI photometric bands is 700 Lsun (2.7e36 erg/s). The upper limit to the X-ray luminosity over the 0.3-10 keV range is 0.5 Lsun (1.8e33 erg/s), assuming a bremsstrahlung spectrum with temperature kT=1 keV and a hydrogen column density 2e21 atoms/cm2 (corresponding to the interstellar reddening following the prescription of Predehl & Schmitt 1995, A&A 293, 889). The sustained wind blowing off the WD appears therefore thick enough to absorb locally the X-rays generated by the protracted burning on the surface of the WD.

We thank the Swift Team for the quick approval, scheduling and acquisition of the observation presented in this communication.