Large brightness increase of V1405 Cas (Nova Cas 2021) to naked-eye visibility

ATel #14614; U. Munari (INAF Padova), P. Valisa and S. Dallaporta (ANS Collaboration)
on 8 May 2021; 07:28 UT
Credential Certification: U. Munari (ulisse.munari@oapd.inaf.it)

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The transient PNV J23244760+6111140 was discovered at unfiltered 9.6mag on 2021 March 18.424 by Yuji Nakamura (Japan) on frames taken with a 135mm/F4.0 lens with a CCD. A nova classification for the transient was proposed in ATel #14471, #14472 based on the presence of emission lines of Balmer series, 4640 blend, He II 4686, and He I, the latter with P-Cyg profiles. Other description of optical spectra, as well as near-IR and X-ray observations were reporter in ATel #14472, #14478, #14482, and #14530.

We are keeping an almost daily monitoring of V1405 Cas with the Varese 84cm + Echelle spectrograph and in BVRI bands with ANS Collaboration telescope ID 0310.

Based on the ANS Collaboration lightcurve, the object remained rather constant in brightness and colors for about 5 weeks, with an initial peak at V=7.529, B-V=+0.546, V-R=+0.495, V-I=+0.785 reached on March 20.154 UT, followed by a very slow decline through a shallow minimum centered at V=7.995, B-V=+0.608, V-R=+0.586, V-I=+0.870 on April 8.789 UT, and then an equally gentle rise back to initial brightness reaching V=+7.495, B-V=+0.606 V-R=+0.621, V-I=+0.960 by April 25.065 UT. The amplitude of the outburst during this initial phase remained rather shorter than expected from a normal nova. This has been spectacularly remedied over the last few days with a fast and large increase to naked-eye brightness: this morning, Apr 8.094 UT, we obtained V=5.740, B-V=+0.648, V-R=+0.463, V-I=+0.817. The sudden and large 1.8mag increase in brightness has been accompanied by only a minimal change of the optical colors.

A similarly flat spectroscopic evolution has accompanied the initial 5 weeks of photometric plateau, with the spectrum dominated by the hotter excitation temperatures than FeII novae have to go through on their way to the cooler conditions characterizing the optical maximum, namely strong Balmer and HeI emission lines flanked by highly variable P-Cyg absorptions, as described in ATel #14476, with FeII emission lines appearing only recently as described in ATel #14577. On the time-series Echelle spectra taken with the Varese 84cm, the earliest and feeble absorption for FeII 42 multiplet appeared on March 27, followed by a first traceable appearance of the P-Cyg emission component only around April 16. During this phase the FeII lines remained relatively weak and concomitant with the persistent presence of HeI emission lines.

The large rise in brightness of the last few days has deeply impacted the optical spectra. The spectrum for this morning April 8.085 UT, shows in addition to Balmer and Paschen lines, the FeII P-Cyg profiles now much stronger and in full swing, will all the usual multiplets present, while all HeI lines have gone. All P-Cyg profiles are rather complex, the emission and absorption components being the result of blended sub-structures. For Hbeta, the emission component can be best described as a Voigt profile characterized by a FWHM~410 km/s, with the strongest absorption located -730 km/s from it. OI 7772 presents two main absorption components, located at -700 and -290 km/s from a still rather feeble emission component.

Overall, the spectrum of V1405 Cas is now finally looking as a textbook example for a FeII nova around maximum brightness.