Spectroscopy and photometry of nova V5856 Sgr, still 9 mag brighter than quiescence five years past the initial outburst

ATel #14804; Ulisse Munari (INAF-Padova), Robert Williams (STScI and UCSC), Paolo Valisa and Sergio Dallaporta (ANS Collaboration)
on 22 Jul 2021; 15:59 UT
Credential Certification: U. Munari (ulisse.munari@oapd.inaf.it)

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Nova V5856 Sgr (= ASASSN-16ma = Nova Sgr 2016 N.4) erupted on 2016 Oct 25 (Stanek et al. 2016, ATel #9669), peaked at B=6.25, V=5.90, I=5.59 on Nov 8, and after a second maximum at V=6.51 reached seven days later, it entered a fast decline characterized by t(2)=6.5 and t(3)=10.5 days (Munari et al. 2017, MNRAS 469, 4341). The progenitor was not detected in quiescence by OGLE (I>22 mag; Mroz et al. 2016, Atel #9683), implying an outburst amplitude >16 mag in I-band. While passing through the secondary maximum, the nova emitted strongly in the gamma-rays as recorded by Fermi-LAT (Li et al. 2017, NatAs 1, 697).

We have recently re-observed V5856 Sgr and found that it is still very bright five years after the eruption: in I-band the nova is currently 7.0 mag down from maximum and at least 9.5 mag brighter than in quiescence. We have obtained the following photometry on the Landolt system with ANS Collaboration telescope ID 0310:

date UT	U	B	V	R	I
2021 Jul 11.942	13.80	13.862	13.212	12.824	12.603
2021 Jul 14.920		13.875	13.223	12.788	12.541

The ASASSN light-curve (Shappee et al. 2014, ApJ 788, 48; Kochanek et al. 2017, PASP 129, 104502) confirms that around mid-2017 the nova drastically reduced its rate of decline, losing only about 1 mag since then.<\p>

A low resolution spectrum (3200-8000 Ang, 2.3 Ang/pix) of V5856 Sgr was obtained on 2021 July 10 with the Asiago 1.22m telescope, and Echelle spectra (4300-8700 Ang, res. power 20,000) were recorded on 2021 July 18 and 19 with the Varese 0.84m telescope. They are characterized by a blue continuum (in agreement with the low reddening affecting the nova and the blue colors of the above photometry) with superimposed strong emission lines primarily from [OIII], Balmer, HeI, [OII], [NII], 4640 blend, and HeII. The integrated flux of some of them is /p>

line	flux(erg/cm2 s Ang)
Halpha + [NII]	5.42e-12
Hbeta	1.10e-12
HeI 5876	3.13e-13
[OIII] 5007	7.16e-12
[OII] 7325	6.50e-13
[NII] 5755	5.21e-13

In our spectra H and He emission lines possess a prominent narrow central spike (FWHM from 60 to 100 km/s) superposed on a broad Gaussian pedestal having FWHM=1050 km/s. The central spike is absent in forbidden lines, which are characterized by a broad component with FWHM=1200 km/s and a more rectangular shape. Inspecting the SMARTS database (http://www.astro.sunysb.edu/fwalter/SMARTS/NovaAtlas/v5856sgr/v5856sgr.html) reveals that the central peak to H and HeI lines is present over the whole 2017 Jul 22 to 2019 May 27 period the nova was monitored, and always absent in forbidden lines.

The fact that the nova is still so bright five years past the initial eruption suggests that nuclear burning could still be going on the surface of the central WD, and the broad component observed in the emission lines probably form in a sustained wind blowing off the WD. The narrow component is instead probably associated with the central binary and/or the innermost circumstellar regions, and repeated Echelle observations will be valuable to ascertain if the narrow component display variation in radial velocity.