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Swift observations of Nova Scuti 2017

ATel #10636; N. P.M. Kuin (UCL/MSSL), K. L. Page (U. Leicester), S. C. Williams (U. Lancaster), M. J. Darnley (Liverpool Johns Moore U.), T. J. Nelson (U Pittsburg), J. Osborne (U Leicester)
on 13 Aug 2017; 14:55 UT
Credential Certification: Paul Kuin (npkuin@gmail.com)

Subjects: Ultra-Violet, X-ray, Nova

Referred to by ATel #: 10725

Nova Sct 2017 (ASASSN-17hx/ASASSN-17ib) went into eruption on 2017-06-19.41 (hereafter taken as Day 0), as reported on the ASAS SN transient page (see also ATEL #10523,#10524) Spectroscopic confirmation (ATEL #10527,#10542) observed emission in H and He I, and in N II as well as Fe II. The ASASSN light curve data show a peak of V = 8.75 on 2017, July 30, day 40.867.

Swift UVOT started observing on 2017, June 30, day 10.86, in both the UV grism and UVW2. The UV brightness was UVW2 (1928Å) = 13.60 ± 0.05. The spectrum showed a significant 2175 Å dip from interstellar extinction, and using the Cardelli et al. law we derive E(B-V) = 0.8 ± 0.1. The day 10.86 spectrum shows N III] 1750Å in emission, N II] 2143Å, possibly a P-Cygni profile for Mg II 2800Å, with a an edge velocity of -4900 km/s, while the line width (FWZI) of the N III] 1750Å and squarish Mg II 2800Å profiles are of order 4600 km/s. Neutral Mg I 2850Å is much narrower with a FWZI of about 2600 km/s. The spectrum around the location of C III 1909 and O II 2471 was badly affected by zeroth order contamination, and a weak feature is seen at C II 2323Å.

Subsequent UVW2 = 13.86 mag photometry on day 26.8 shows a slightly lower brightness, but on day 51 the UVW2 = 12.80 mag, brighter than before, and may indicate a delayed maximum in the UV. The UVW2 band includes the C III] 1909Å line, so the brightening may partly be due to that line. UVM2 (2246 Å)= 15.45 ± 0.05 on day 42.6, but we do not have comparable data in UVW2 so we cannot say if the Fe-curtain had started yet.

We obtained another good UV spectrum on 2017-08-09, day 51, and a large Fe-curtain absorption can be seen to be present in the 1800-3800Å part of the spectrum where the minimum depletion is reached around 2600Å being a factor of 10 depleted in the flux (after correction for the reddening). The optical emission lines in our spectrum have strengthened when compared to day 10.86, with the blended Fe II lines of multiplets 47 and 48 clearly present, consistent with ATEL #10558. In the UV we see two unidentified emission lines at 2985 and 3285 Å, as well as two emissions lines that may tentatively be identified as [Fe V] 3445 Å, and [Fe VI] 3662 A. Possibly, due to the large Fe curtain absorption these "lines" are regions of lesser absorption instead, and just appear as emission lines.

No X-ray emission has yet been detected from ASASSN-17hx with Swift XRT. In the 10.9 ks of data collected between day 10.86 and 51.47, we place an upper limit of 1.2x10-3 count s-1 on the 0.3-10 keV XRT count rate.