Swift UVOT photometry and spectra from V1674 Her

ATel #14736; Paul Kuin (UCL/MSSL), Sumner Starrfield (U Arizona), Marina Orio (U Wisconsin, INAF-Padova), Kim Page (U Leicester)
on 23 Jun 2021; 16:41 UT
Credential Certification: Paul Kuin (npkuin@gmail.com)

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Nova Her 2021 (V1674 Her, TCP J185730.95+165339.6) was discovered on the rise on 2021 June 12.537 by Seji Ueda (CBAT "Transient Object Followup Reports") peaking soon thereafter at magnitude 6. The first Swift UVOT observation was taken on June 13.48 in UVM2. The nova was at that time too bright for most observation modes of the Swift Ultraviolet and Optical telescope, but the UVM2 filter is most affected by the interstellar reddening and using the readout streak in the UVM2 image, a magnitude of UVM2=10.05+/-0.1 (Vega) was found. On June 13.96 a UV grism spectrum was obtained with the peak of the brightest lines exceeding the calibration limit. This was followed by more grism spectra at days: June 14.41, June 15.49, June 15.88, and June 18.66. A dip around 2170 A is consistent with the reported reddening E(B-V)=0.55 (Munari et al., ATEL #14704).

Prior to June 18th, the spectra show deep absorptions of the Fe II UV lines, and comparison to IUE spectra of nova V838 Her (1991) show a great similarity in features. Therefore some peaks in the UV spectrum may not be due to emission lines but the large absorption of cooler outer layers. There are several peaks in the uv spectrum that decrease in size between the 13th and 16th of June. Detailed modeling is needed for understanding those spectra.

The spectrum of June 18th no longer shows signs strong Fe II absorption lines. Emission lines may be identified with the Si III]/C III] blend at 1900A, N II] at 2143A, Fe II at 2344A, Al II] at 2660A and Mg II 2800A, with an unidentified line at 2530A.

The first two spectra show P-Cygni profiles in the Balmer lines up to H6 indicating high densities. High velocity absorptions show up as multiple dips in H6 and H-epsilon up to about 6500 km/s. A strong Mg II 2800 A line is present in all the spectra. The FWHM of Mg II 2800A is 6500 km/s, consistent with earlier reports. In the early spectra He II lines are seen at 5411A, 4687A, and 3203A; the latter becomes markedly weaker with time. There is no clear sign of forbidden lines yet in these spectra.

In ATEL #14720 Mroz et al. report a 8.4 minute period, suggesting this is an intermediate Polar system. The flat-topped line profiles reported (ATEL #14723, Woodward et al.) and rapid variability suggest the ejecta are structured in cone or jet-like geometry. The different velocities and excitation support structured ejecta.

These observations would not have been possible without the support of the Swift operations team, and Swift leadership who approved prompt observations.