Optical and Swift X-ray/UV observations of the 2020 flare of the symbiotic nova V1413 Aql

ATel #14187; U. Munari (INAF Padova), N. Masetti (INAF Bologna), G. L. Righetti, S. Dallaporta and P. Valisa (ANS Collaboration)
on 17 Nov 2020; 16:37 UT
Credential Certification: U. Munari (ulisse.munari@oapd.inaf.it)

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The faint symbiotic star V1413 Aql (= AS 338) turned into a bright symbiotic nova in 1981, reaching a peak V=10.3 in 1983, and remaining much brighter that in quiescence ever since, with a light-curve punctuated every 434 days by deep eclipses of the burning WD by the M5III companion (Munari 1992, A&A 257, 163). The high ionization conditions that prevailed in quiescence (HeII, [FeVII]; Allen 1984, Proc.A.S.A 5, 369), where replaced in outburst by emission lines of a much lower ionization degree while a strong A-type continuum overwhelmed at optical wavelengths the TiO absorption bands of the M5III giant.

We have been closely monitoring V1413 Aql over the last thirty years, collecting UBVRI photometry with ANS Collaboration telescopes and low- and high-resolution spectra with the Asiago 1.22m and 1.82m telescopes. In particular we have been closely watching the evolution of the large flare that characterized V1413 Aql in 2020. The three orbital cycles preceding it (from early 2016), were characterized by a light-curve much less regular than ever seen before.

The flare begun in early June, 2020 when the star was stable around B=14.31, B-V=+0.96, V-R=+1.02, and V-I=+2.26 since the exit from the last eclipse in Dec 2019. The rise in brightness of V1413 Aql was smooth and linear to a maximum reached on Aug 27 at B=11.17, U-B=-0.08, B-V=+0.75, V-R=+0.57, and V-I=+1.20. The decline quickly follow, again very smooth and linear, with the star transiting through B=11.99, U-B=+0.01, B-V=+0.88, V-R=+0.72, and V-I=+1.43 on Nov 15.

On the rise toward maximum, Balmer and low excitation emission lines like FeII were slow to respond, while HeI displayed P-Cyg absorption. At maximum HeI, [OIII] and [NeIII] emission lines were no longer present, and Balmer and FeII were flanked by deep P-Cyg absorptions, with terminal velocities in excess of -700 km/s for Hbeta, plus a second weaker and narrower absorption at -150 km/s from the emission peak. A trapezoidal emission pedestal appeared on Balmer lines, shrinking from 1530 km/s width at maximum brightness to 950 km/s on our last Echelle spectrum for Nov 9.

The fast and massive outflow traced by the P-Cyg absorptions and the appearance of a broad pedestal to Balmer lines prompted for X-ray/UV observations of V1413 Aql to be carried out. On August 19, August 26, September 2 and September 9, 2020 we monitored with weekly cadence the source using the Neil Gehrels Swift Observatory. These 'snapshot' observations lasted about 1700, 1900, 700 and 400 s, respectively, and were performed in X-rays with the XRT instrument and in ultraviolet with UVOT. They cover the transit of V1413 Aql through maximum brightness in the evolution of the 2020 flare.

We do not detect any signal in the 0.3-10 keV X-ray band down to 3-sigma limit count rates 1.9e-3, 1.7e-3, 6.1e-3 and 1.5e-2 cts s-1, corresponding to fluxes of 8.2e-14, 7.3e-14, 2.6e-13 and 6.5e-13 erg cm-2 s-1 assuming an X-ray spectral emission modeled using WebPIMMS with a bremsstrahlung of temperature kT = 10 keV and no Galactic absorption. The X-ray non detection is in agreement with the absence of high ionization emission lines in the spectra. V1413 Aql is instead well detected in the UVM2 band (lambda_eff = 2246 Angstroms) with Vega magnitudes 13.62+-0.04, 13.80+-0.04, 13.72+-0.04 and 13.70+-0.04 in the four pointings. For comparison the U-band magnitude we measured on the same dates was 11.04, 11.04, 11.08 and 11.14, with an average UVM2-U=2.64 mag.