Swift UVOT UV spectrum and photometry of V3890 Sgr

ATel #13072; Paul Kuin (MSSL/UCL), Matt Darnley (LJMU), Jeremy Drake(CfA), Marina Orio (U. Wisconsin), Julian Osborne, Kim Page (U. Leicester), Bob Williams (STScI), Chick Woodward (U. of Minnesota), Jan-Uwe Ness(ESA), Steve Shore (U. Pisa), Fred Walter (Stonybrook)
on 3 Sep 2019; 14:15 UT
Credential Certification: Paul Kuin (npkuin@gmail.com)

Tweet

Swift UVOT took a 418s UV grism spectrum of V3890 Sgr at 2019-09-02.07 about 5.67d after the estimated eruption time of 2019-08-27.40 (Strader et al, ATEL#13047). A second UV spectrum was taken at 2019-09-02.73 at a different roll angle allowing the identification of contamination from zeroth orders from field stars. In the 1700-4000 Angstrom region, a number of emission lines are seen. Prior to measuring the lines a reddening correction E(B-V) = 0.59 (Munari and Walter, ATel #13069) was applied to the spectrum.

The strongest line is from Mg II 2800Å, followed by NIII] 1750Å, a blend of Si III] and C III] peaking at 1908Å, and a broad feature centred at 2625Å which is possibly due to Fe II a6D. The Si III] is affected by a zeroth order.

R. Gonzalez-Riestra (1992A+A___265___71G) identified the 2630Å feature with [Mg VII] emission in the spectra estimated to be taken on day 18 and 26 in the 1990 eruption. The same broad feature can be seen in the IUE spectra from nova V745 Sco in 1989.

The FWZI of the Mg II doublet is 110Å. Subtracting the wavelength difference of the components we get a related velocity FWZI = 11,000 km/s.

We cannot base a classification for the nova of CO or ONe based on the UV-grism spectrum. Considering that after a less than 24 days the nova ejecta will not dominate the spectrum since it will mix with the red giant wind, the best time to determine abundances from the spectrum would be from the earliest spectra. (Assuming turbulent mixing speed is larger than the wind speed of 50 km/s). Key will be to examine the blue-shifted emission from the ejecta in C, O, and Ne to determine abundance ratios.

Standard UVOT photometry was possible using the uvm2 band from 3 days after eruption. This shows the brightness decreasing from uvm2=11.0 on day 3.1 to uvm2=11.8 on day 5.9. During this time the decay rate is seen to decrease.

Swift observations are continuing. We are grateful to Brad Cenko, the Swift PI, for approving these observations and to the Swift operations team for planning the observations.